Metals, Materials, Processing and Characterization

It takes two: Welding using laser beam with electron beam

2011-04-10T08:20:00.000-07:00

Laser Safety Classification

2010-04-15T10:58:00.000-07:00

Class 1: A Class 1 laser is considered safe based upon current medical knowledge. This class includes all lasers or laser systems which cannot emit levels of optical radiation above the exposure limits for the eye under any conditions inherent in the design of the laser product. There may be a more hazardous laser embedded in the enclosure of a Class 1 product, but no harmful radiation can escape the enclosure.Class 2: A Class 2 laser or laser system must emit a visible laser beam. Class 2 laser light is too dazzling to stare into for extended periods. Momentary viewing is not considered hazardous since the upper radiant power limit on this type of device is less than the Maximum Permissible Exposure for momentary exposure of 0.25 seconds or less. Intention extended viewing, however, is considered hazardous. Class 3: A Class 3 laser or laser system can emit any wavelength, but it cannot produce a diffuse (not mirror-like) reflection hazard unless focused or viewed for extended periods at close range. It is also not considered a fire hazard or serious skin hazard. Any continuous wave (CW) laser that is not Class 1 or Class 2 is a Class 3 device if its output power is 0.5 W or less. Since the output beam of such a laser is definitely hazardous for intra-beam viewing, control measures center on eliminating this possibility.Class 4: A Class 4 laser or laser system is any that exceed the output limits (Accessible Emissions Limits, AELs) of a Class 3 device. As would be expected, these laser may be either a fire or skin hazard or a diffuse reflection hazard. Very stringent control measures are required for a Class 4 laser or laser system. Ultrafast lasers fall into this classification.
Ultrafast lasers possess safety hazards not associated with light from conventional sources. Among potential injuries, eyes are the most vulnerable. Damage to the retina cannot be repaired and does not heal by itself.Why is eye safety so critical? Compare the output of femtosecond lasers with the radiation that the human eye can receive from the sun on a clear day. The sun’s image on the retina is approximately 160 microns. This yields a radiation density of around 30 W/cm2. A typical Ti:Sapphire femtosecond oscillator is capable of producing intensity in excess of 50 GW/cm2, while an amplifier will easily exceed 1014 W/cm2. Obviously, very weak laser reflections, or even scattering from rough surfaces, can be extremely bright - dangerously bright - to the eyes.You are urged to read carefully the safety section found in the user manual of any laser product you use and to strictly adhere to those instructions.

iClad: A Wonderful Laser Cladding Head

2010-04-15T10:20:00.000-07:00

In the race against time, the specialists at Stork Gears & Services BV, Rotterdam, have a global reputation in sectors like the shipbuilding, petrochemical, wind power, and steel industries where the maximum uptime of efficient gearboxes and associated drive systems is of crucial importance. As quickly as possible, the experts at Stork repair damaged or failed gearbox-related components, rebuilding them or replacing them with new parts. In this line of work, laser cladding is used as a tried-and-tested technology to prevent wear-out and unwanted material changes. The team led by Jelmer Brugman, head of Stork's laser cladding department, successfully applies this technique on more than 20 gear repair or modification jobs per month. However, in about 10% of the cases, inaccessible internal surfaces could not be treated with conventional laser processing heads. Until about a year ago, there was no alternative but to replace the damaged parts with new ones. The solution to this problem is iClad, a laser processing head developed by Pallas GmbH the treatment of internal contours from a diameter of 26 mm and to a depth of 600 mm. By using this innovative processing head, the Dutch company has opened up new possibilities for the repair of critical components. Compared with the previously unavoidable effort involved in such cases, several weeks of repair time can be saved. In the maritime industry, in power stations, or wind power plants, gearboxes and gearwheels have to operate reliably over long periods of time under the hardest of conditions. In many different sectors, corrosion, abrasion, and erosion put such drive systems under extreme pressure. This results in broken or damaged gearwheels, leading to unplanned downtimes or, in the worst case, complete failure. Since last year, however, Stork Gears & Services has been successfully using the new iClad processing head for the preventative maintenance, modification, and repair of such heavily stressed drive systems. Now, both damaged and new components can be optimally coated with low distortion using a 2-kilowatt diode laser. The surface of the component and the powder jetsprayed onto it are melted by the laser and joined together metallurgically with minimal dilution. The previously available processing heads were only deployable for internal contours with an access aperture of at least 100 mm diameter or were limited to the treatment of freely accessible surfaces. The iClad processing head, developed by the company Pallas in collaboration with the Fraunhofer Institute for Laser Technology (ILT), can be used from a diameter of 26 mm and to a depth of 600 mm for the complete range of laser cladding tasks. Whether hardening, alloying, or coating, even in the tightest of internal spaces, iClad renders the most demanding internal surface treatments possible. For Brugman, laser cladding with iClad has significant advantages. Using the new processing head, highly strained critical components that were previously inaccessible can now be optimized or repaired with a coating specifically designed to meet their respective stress loads. As one of the first industrial-scale users, Stork Gears & Services deployed the newly developed processing head, a prototype of iClad with integrated collimation as a special feature. This was a fixed head with a working depth of 500 mm for the internal coating of 50 mm bores. In addition to gearwheels and clutches, gearbox casings are also treated using the new technology. On average, Stork deals with three gearbox repairs per month. One-piece gearbox casing models used to pose a real problem, as laser cladding by means of the standard processing heads was not an option. Before iClad was deployed, the alternative, the production of a completely new casing, used to take several weeks. Now, the filigree processing head for laser cladding of internal contours makes it possible to save time on the repair of a damaged gearbox and thus to reduce the duration of downtimes, a factor of great market relevance. So far, Stork has successfully used iClad on about 20 jobs – most of them demanding single-piece projects, i.e. not serial production. The new technology has proven itself both for repairs and for the production of new components. After just one year of deployment, the slender optical processing head for the laser treatment of hard-to-access inner contours has already significantly enlarged Stork's scope of service. Previously inaccessible surfaces and structures of critical components are now being successfully equipped in record time with specific coatings to meet the challenges of their wear-intensive daily lives.

Welding Trends For The 21st Century

2009-11-01T00:19:00.000-07:00

Lighter Metals Will Push Low-Frequency TIG Welding
Collision shop welders will be dealing with lighter metals and new technology as the auto industry advances into the new millennium. In addition to keeping up with day-to-day concerns, there will be other changes to deal with as today's popular metal inert gas (MIG) systems will likely face increasing competition from tungsten inert gas (TIG) welding and resistance spot welding.
There is a trend to do a lot more replacement work and less repair. I see a lot more parts that are being classified structural or HSLA (high-strength, low-alloy). Straightening alloy steel just isn't done anymore, although a lot of that has been done with the malleable steel in the past.
Chesterland Auto Body is a complete service and collision shop servicing all makes. There are eight employees in the shop, including two paint teams working in one downdraft and one cross-flow room. All welding work is done on three units: two Hansen hand-helds and one Lincoln AC/DC Stick Welder. Every job is MIG welded at Chesterland and it is expected to continue that for a long time. There is no more brazing or acetylene welding done anymore. Everything is MIG. The Hansen units are 160 amp units; the Solar is 175 amps.
One of the things paramount to doing a good job is proper use of a good weld-through primer. It's really important to stay on top of the technology to keep the seams from corroding. You don't want to get sued 15 years from now because the car started coming apart at the seams.
In the 1970s, unitized body construction became the rage, saving weight by using thinner material. When high-strength steel became widely available, its use forced body shops to go to MIG systems to handle the welds. Smaller and less expensive 110 amp machines became commonplace.
MIG or "squirt-gun" welding grew in popularity over the past 15 years as shop owners were attracted by its quick training time and ease of use. The downside with MIG is the difficulty controlling MIG's high deposition rate with the electrode wire and the time required to grind down welds. Still, it remains the most popular aftermarket repair welding technique.
More recently, because of the high cost of high-strength steel, auto makers have begun going back to mild steel, using thicker steel in the car's body construction. The added weight is being offset by more fuel-efficient engines, however, it changes the repair strategy. As the structural panels on the newer cars become heavier, it requires more amps to do the job. The upper panels generally remain lighter.
Toyota, Honda and similar manufacturers now insist on a minimum 200 to 250 amp power supply. Another typical difficulty is with feed problems. A good feeder costs more than a MIG welder.
Cost, both for equipment and training, is probably the main factor that has kept resistance spot welding equipment out of many shops. All car manufacturers use resistance spot welds, therefore shops should consider them for any application where high-strength steel is being fused or where a factory-look weld is required. Resistance spot welding will let a technician work four times faster than MIG. The grinding task associated with MIG will disappear. Why should one fill up and then have to grind down a weld?
Today, resistance spot welding has about 1 percent of the market. That will change over the next 24 months as repair shops find themselves dealing more with high-strength steel. MIG will not go away, he concedes, but will be used for butt welding or structural welds in places where a resistance spot welder cannot reach.
TIG welding is another leading contender for handling the new materials shops will encounter. While not a new technique, as the auto industry moves to lighter metals in construction, TIG promises to play a larger role in welding shops. Wyatt Swaim, a consultant for Lincoln Electric who is best known for his work at the Indianapolis 500, is a big proponent of TIG, especially in lightweight, high-strength applications. "TIG is versatile ... it can do aluminum, steel, stainless steel or titanium. It is much like gas welding but with more precise control, lower metal distortion, more precise disposition of material and not a lot of after-welding cleanup," Swaim said. As manufacturers move to lighter vehicles, shops will face the need to handle metals like aluminum and titanium.
Aluminum Is ComingAs part of the move to lightweight cars, many manufacturers are looking at aluminum. Ford already has paraded an all-aluminum Taurus at several shows. Chesterland Auto is set up for aluminum with the wire and argon gas. "We don't get into a lot of aluminum yet," Mader said, adding that he expects more down the road.
"I haven't seen it in the lower-end vehicles. It's more in the top-of-the-line cars like Mercedes and Jaguar," Mader said. "Occasionally you'll see it in a first-run hood on the Fords, but the replacement will be steel." He adds that aluminum also can be found in bumper components.
Ron Kuehn, business development manager for the Inter-Industry Conference on Auto Collision Repair (I-CAR), agrees that the market is headed to aluminum. "It is still the next evolution of the unibody program," he said. To this point, he feels MIG will be the tool of choice for welding aluminum because of the high-frequency nature of TIG. "High-frequency welding machines and cars' on-board computers are not compatible," Kuehn said.
Kuehn points out that plasma cutters had the same problem when they first came out in the early 1980s. The industry quickly went from big machines to smaller, low-frequency units. Similarly, emerging TIG technology may make it possible to do low-frequency TIG welding on aluminum.
Neal Borchert, TIG product manager for Miller Electric Manufacturing, Appleton, Wis., agrees that TIG's biggest problem is the high-frequency interference with computers. Miller has a low-frequency unit that sells at $7,700, a price tag that puts them beyond the reach of the typical collision shop. Called the Aerowave, it can operate at single-phase or three-phase, with or without high frequency.
In Europe, high frequency is frowned on for a number of reasons, including technical ones. In the U.S. market, however, there is a constraint on primary power input because single-phase is used everywhere. In Europe, three-phase is typical. For that reason, the largest market potential for a low-frequency unit is overseas. In the United States, the strongest interest in low-frequency TIG units comes from the racing world, where $12,000 aluminum blocks and $10,000 testers are commonplace.
However, emerging technology generally drops in price as demand increases. Borchert says there will be a full-featured, downsized version of the Aerowave on the market later this year at a lower price than today's Aerowave.
Naslund points out that much of the aluminum fraction in today's street cars is not in the panels, but in the castings for the engine and gear box. While the typical car has only 17 pounds of aluminum in the panels, he agrees that aluminum is the metal of the future. He notes that electric cars, gaining popularity particularly in California where he works, will be all aluminum when they finally hit the market in five or 10 years. Resistance spot welding can be used for aluminum, although the technique requires using a larger nugget.
Swaim agrees that electric cars are coming and that they will drive aluminum body panels into the general market. However, he says titanium material is also showing up. "Titanium has gone down in price and you'll start to see more of it in car parts," Swaim said. "Technicians need to understand the metal and how to shield for it."
Corrosion is always a consideration - remember, it induced DeLorean to use stainless steel in his car bodies. Swaim forecasts a Mercedes-quality electric car will be built with titanium body parts. "Don't be surprised if you see titanium panels, rather than aluminum," he said. "It is non-corrosive and salt resistant. Titanium will be the metal of choice since it is so tough." TIG technology works well with titanium, he adds.
Training is the key to proper use of TIG equipment, Swaim says. It will take about a half-semester of night school classes to make a technician proficient with TIG. That's a lot of training compared to the several hours required to get a technician working with MIG or an AC/DC buzz box.
"One day's worth of training with TIG will just get a technician frustrated," Swaim said. "They'll put the tools down and say they'd just as soon grind." But Swaim says such workers, and shop owners, are missing a bet. "I'd absolutely recommend TIG, and I do all processes," he said. "When it comes to working with thin materials, TIG is the only way to go."
A TIG system such as Lincoln Electric's Square Wave TIG 175 will run about $1,300, including accessories. Shipped as a complete package, all a welder has to do is connect the power, add gas and go to work. It provides single range, continuous control from 12 to 175 amps (although operation around 30 to 45 amps will handle most body shop jobs).
An upgrade kit to convert AC/DC to TIG is available for $249. Added to the $350 cost of a typical buzz box, a shop can get into TIG for about $600.
"There is no 25-pound spool of wire needed," Swaim points out. "You'll find you were using about five times as much material with MIG as you do with TIG. Plus, there is five times less grinding with TIG." Where TIG shines is in its focused hot spot. The heat-effective zone with a TIG weld is between 1/16 inch and 1/8 inch, contrasted with 1/2 inch for a typical MIG weld.
Some of the money saved on materials and grinding will have to be spent up front in training. Swaim is working with the Lincoln Foundation on a video that will teach proper TIG technique. It should be available in the fall of 1997 at a nominal cost through suppliers or from Lincoln Electric in Cleveland.
I-CAR has offered several classes on aluminum welding. "The guys interested in being leaders, being in the forefront of the field, will be ready for aluminum when it comes," Mader concludes.
Miller Electric announced in January a package for its Econotig TIG welder, including a heavy-duty current and contactor foot control for industrial-type uses. The TIG/Stick package produces small, high-penetration TIG weld beads with no spatter and can be used on any weldable metal 18 gauge to 3/16 inch thick. In the Stick mode, it can weld 3/32-inch and 1/8-inch electrodes.
Miller also offers the Millermatic 185, a MIG welder with 30 to 185 amps power and the ability to weld stainless steel and aluminum, according to product manager David Anderson. While I-CAR generally looks for 200 amps when welding aluminum, the complete 185 package, at $1,199, is versatile enough to provide intermittent support around a shop. Anderson also notes the SpoolMatic 30A spool gun is handy for aluminum repair and use with other metals. It uses a two-pound spool, not the usual 30-pound spool, making it versatile for occasional jobs.
Safety ConcernsAnother major concern in welding shops is safety. Many manufacturers now coat metals with toxic materials to prevent rusting in such susceptible areas as the rocker panels, according to Hansen. "The result is more toxic fumes," he said. "The guys complain that the smoke is making them sick."
Shop managers like Mader agree that they have encountered some vapors as they burn through the coatings. However, Mader says that it has not been a problem for them.
Both steel and aluminum panels can produce toxic fumes that should be filtered away from the workplace. Hansen recommends the articulating MIG arms that his firm produces as a practical solution. "The trend is to be cleaner, bigger, more professional," he says. "The goal is to eliminate sloppy work spaces."
Hansen's MIG arm has a smoke extraction system that sucks fumes through the welding gun, up the hose and out the arm. The fumes are either vented to the outside or trapped in a filtering system. Hansen acknowledges that there are other solutions, as well. Audi, in its European operations, has developed a special overhead gantry system for their repair bays. "The goal is the same - to get hoses and other things off the floor," he said.
Naslund agrees that vapors can be a problem. He points out that new cars are dipped in vats and receive an electro-coating of zinc. The E-coating generally includes some plastic, but it is the zinc that can cause trouble for the worker. "The auto aftermarket shops are one of the only industries that weld zinc," Naslund said. "The manufacturers weld first, then apply the zinc."
Naslund, whose firm deals in resistance spot welding equipment, says resistance spot welding - unlike MIG welding - does not take off the E-coating because it can weld right through the coating. "The machine that creates more heat will create more fumes," he said. Since the resistance spot weld creates less heat, Naslund says it is less hazardous.
TIG also has advantages over MIG. Because TIG uses an inert process with argon gas, it does not put off as much smoke, Swaim notes. The more focused heat-effective zone also means there is less zinc material burned off.
Hansen says that even such a minor improvement as using a central vacuum system, rather than a broom, can make the shop more productive and safer.
Naslund cautions against destroying the film between two panels since that is the likely spot for corrosion to start.
The bottom line, of course, is to have the safest possible weld for both the vehicle owner and for the technician working on the car.

Ten Frequently Asked TIG Welding Questions

2009-11-01T00:11:00.000-07:00

Experienced welders know that without the right information, it’s easy to sacrifice quality, lose time, and generally become frustrated with TIG welding. And while there is merit in learning by trial and error, if you want to move toward precision TIG welding, getting answers to 10 very basic questions can ease the transition.
1. Should I use an air-cooled or water-cooled torch?
For low-amperage applications, an air-cooled torch cooled by shielding gas works well. These torches are simple to operate and require minimal setup. For high-amperage applications, you can still use an air-cooled torch, but the cable and torch must be much heavier and may be cumbersome to manipulate.
A water-cooled torch, which circulates water through the torch and cable, works equally well, but requires additional equipment and maintenance. These torches use clean, de-ionized water with filters that prevent contaminants from entering the cooling supply or the inner-diameter tube of the torch. You also may need to use additives to prevent algae growth.
Whether you choose a water-cooled system depends on your willingness to invest in additional equipment, as well as additional time and money for maintenance. Many welders, however, prefer these systems because the smaller torch configuration provides better maneuverability and reduces operator fatigue.

2. When should I use a gas lens?
A gas lens should be used when your application requires increased shielding gas coverage. The gas lens reduces turbulence and provides lengthier, undisturbed gas flow and allows you to move the nozzle farther away from the workpiece while still keeping the arc or weld puddle in view. Using a larger nozzle with a gas lens—which consequently produces a larger blanket of shielding gas—can help when welding on materials such as stainless steel and titanium.
A gas lens also allows more direct and broader gas coverage on tight joints, such as an inside corner, where access can be limited. In critical applications that have potential for atmospheric contamination, a gas lens can help to reduce the likelihood of weld discontinuities.

3. What are the causes of an unstable arc, and how can I remedy them?
Using the wrong size tungsten, whether in AC or DC applications, is one of the more common causes of an unstable arc. If the tungsten is too large for the amperage, the arc may rotate around the end of the tungsten. Conversely, if the tungsten is too small for the amperage, the current can melt the electrode and cause an erratic arc. To remedy either of these conditions, match your welding current to the tungsten size recommended by the manufacturer.
Contaminated tungsten—caused by debris on the base metal, oxidation from inadequate shielding gas, or gas impurities from a leak—also can cause an unstable arc in both AC and DC applications. To resolve this problem, replace or regrind the tungsten, make sure the base metal is clean, or increase the shielding gas flow after making sure all your hoses are intact and leak-free.

4. How do I prevent tungsten contamination and discoloration?
Allowing the tungsten to touch the weld pool is one of the most common causes of contamination. This problem can be resolved by moving your torch farther away from the workpiece, which in turn lengthens the arc. Touching the filler metal to the tungsten also can be a source of contamination, but there is no clear solution to this problem other than practice; trial and error will determine your best technique.
If you feel your technique is not the culprit of tungsten contamination or discoloration, check to see that you have adequate gas flow, and allow adequate postflow time as well. Allow several seconds of postflow—about 1 second for every 10 amps of weld current. Finally, you may want to consider using a power supply that offers high-frequency starts if you think using the scratch-start method is causing the contamination.

5. What are the causes and solutions for excessive electrode consumption?
Using too much current on a given application is a major cause of excessive electrode consumption and is most easily solved by increasing the tungsten size, changing the type of tungsten you are using, or decreasing the amperage.
Using the wrong polarity also can result in excessive tungsten consumption. During AC welding, for example, using more electrode-positive current may provide more cleaning action, but it also subjects the tungsten to more current and thus consumption. Instead, it is best to set the power source more toward electrode negative on the balance control to minimize the amount of current and time spent on the electrode.
Using an incorrect or contaminated shielding gas can lead to high electrode consumption. Be sure to use pure argon, and check for leaks in the hoses, either from cracks or loose fittings.

6. What causes porosity in a TIG weld bead, and how can I prevent it?
A loose hose or torch component is a primary cause of porosity. It is remedied by tightening the fittings. If you cannot find loose hose connections by doing a visual check, you might want to place the hose in soapy water until you find the leak and tighten the fittings accordingly.
Using the wrong shielding gas or one that has impurities also can lead to porosity. This is remedied by using pure argon after using nitrogen to purge the line of air and any condensation that may have accumulated. Drafts from fans or open doors also can lead to porosity, so be sure that your working environment is well-isolated from drafts or use a gas lens to provide better gas coverage.
To prevent porosity caused by inadequate shielding gas flow, follow the recommended flow rate, which is approximately 10 to 20 cubic feet per minute (CFM), depending on the application. Doing so helps ensure quality welds.

7. What type of tungsten should I use?
There are several tungsten choices for TIG welding, including 2 percent thoriated tungsten, 2 percent ceriated tungsten, and 1.5 percent lanthanum tungsten.
Two percent thoriated tungsten is a good choice when using a DC power source because it maintains a pointed shape, resists melting, and has a high current-carrying capacity. When welding thin aluminum—0.09 in. or less—thoriated tungsten is also a good alternative to pure tungsten because it creates a more focused arc. It is recommended that thoriated tungsten be used in a properly ventilated area and measures be taken during preparation to capture dust from grindings.
Two percent ceriated tungsten is a good alternative to thoriated tungsten and provides good arc starts at low currents, along with greater arc stability. This type of tungsten is recommended for aluminum AC welding with an inverter-based power source.
A 1.5 percent lanthanum tungsten is most commonly used for applications in which long weld times and multiple arc restarts are necessary.8. What is the proper procedure for cleaning base metals?The base metal should be free of all contaminants, including dirt, paint, and oil. Wipe the base metal with a cloth or scrape it with a wire brush dedicated for use on a particular material. Before welding on aluminum, in particular, you need to remove oxides with a stainless steel brush manually; using a power brush is not recommended as it can re-embed contaminants into the metal. You can also use a caustic solution to clean aluminum. If you are considering using this method, your local welding distributor is the best resource to provide you with product options. You can also discuss several scraping methods with your distributor if you do not want to use chemicals.

9. How do I solve high-frequency-interference problems?
Malfunctioning electrical equipment, such as computers, telephones and radios, is often a sign that you are experiencing high-frequency interference from your welding power source.
To remedy such high-frequency interference, start by verifying that the power source is grounded according to the installation instructions provided in the operator’s manual. Keep your torch cables and work cables as short as possible, and place them close together. Physically separating your welding equipment from devices that may experience interference is also an option, but doing so can be time-consuming and space-prohibitive.
If all else fails, you could switch to an inverter-based power source that provides a high frequency for arc starting only.

10. What are the cause and solution for arc rectification?
Arc rectification occurs when the surface oxide of a nonferrous metal acts as a barrier, making it more difficult for electrons to flow from the workpiece to the tungsten than from the tungsten to the workpiece. Excessive arc noise, unstable weld pools, or a weld pool that appears to dry up are all signs of arc rectification.
You can either increase your travel speed or decrease the amperage for the application. Another option is to adjust the balance control on your power source toward electrode negative, which provides more penetration.

Welding of Pure or Low Alloy Materials: Difficulties and Solutions

2008-10-12T08:45:00.000-07:00

Pure metallic materials or low alloyed materials find many engineering applications and has many potential applications. For example aluminum alloys find extensive applications as thermally conducting fuel cladding materials in nuclear research reactors. Another important material is TZM (Titanium Zirconium Molybdenum); which contains more than 99.5% Mo. There are many applications where pure or nearly pure (or low alloy) materials need to be welded and welding of these materials is not so easy. So what are the problems and what could be potential solutions.
Difficulties:
1. High thermal conductivity:
Pure or nearly pure metallic materials exhibit high thermal conductivity. Therefore, the heat deposited at the joint line runs away quickly from the joint and thus entire component gets heated up at the cost of fusion of the material near the joint. Whether high energy density process like Laser Beam Welding or Electron Beam Welding will help? It helps to some extent but there are other problems.
2. High reflection:
The metallic materials which exhibit high thermal conductivity have high reflectivity as well and this acts as double whammy for laser beam welding of such materials – First, they will reflect most of the energy and then they will conduct away whatever was absorbed. But this is not all there is one major metallurgical issue.
3. Narrow liquidous:
Low alloy materials have narrow liquidous and therefore melt solidifies quickly leading to entrapment of gases / vapors and thus formation of porosity.
So what are the solutions?
Solutions:
1. Twin Spot Laser Beam Welding:
In this technique, the laser beam is focused at two spots – one leading and another trailing. Now many things can be varied to suit the purpose like power of the leading and the trailing beam and the distance between the two. The leading spots serves the purpose of melting the materials surface for promoting the absorption of the trailing beam and the trailing beam penetrates deep and prolongs the melt life and thus enables the release of the entrapped gas / vapor and thus minimizes occurrence of porosity.
2. Laser Arc Hybrid Welding:
Here the arc creates a shallow melt skin and the falls in this molten skin leading to deep penetration. Arc welding has longer melt life and thus occurrence of porosity is minimized. Additionally, arc welding has larger gap tolerance between the work pieces and therefore edge preparation is reduced to that extent.

Materials Processing - Diode lasers test their mettle in surface treatment

2008-09-13T03:53:00.000-07:00

Most everyone has heard the old adage, “If it ain’t broke, don’t fix it.” But history has repeatedly demonstrated that technology advances can often improve a process or outcome we didn’t even realize needed fixing. (Then it’s just a matter of waiting until it becomes affordable.)

Advances in high-power diode lasers are a case in point, especially when it comes to materials processing. During the past 20 years, multimode diode-laser bars and individual single-emitter diode devices have achieved increasingly higher output powers and better power-conversion efficiencies, allowing semiconductor lasers to evolve from the scientific arena into true industrial tools. In fact, interest in direct-diode materials processing has been a key factor in the development of high-power diode lasers. While diode lasers are still a few years away from being practical ablation or cutting tools for heavy metals, they are gaining traction in materials-processing applications in which beam quality and brightness are not critical to the outcome, such as surface treatment.

Surface treatment is one of the most efficient uses of laser energy and one of the most controllable heating processes when working with metal components. Laser-based techniques such as heat treating, cladding, alloying, and welding have become well-established in the automotive, aerospace, energy, defense, and machine-tool industries for applications ranging from increasing wear resistance of turbine blades to improving corrosion resistance and performance in car engines. Historically, these applications have been served by Nd:YAG and CO₂ lasers, both of which are well-accepted materials-processing tools. Displacing these lasers would require a solution that brings not only operational but financial advantages.

This is where high-power diode lasers come in. While engineers continue to work out a few remaining kinks-such as how to offset the thermal issues that arise in a compact package as it churns out 3 to 4 kW of power in a single shot-the diode laser offers a number of advantages for industrial surface-treatment applications as compared to the Nd:YAG or CO₂ laser. In particular, the shorter wavelength of the diode laser enables much better absorption of the laser energy, leading to an overall lower power requirement for surface treatment applications. A diode-laser cladding system can typically perform cladding with half as much laser power as a CO₂ laser (see table). The diode laser typically has 25% to 30% wall-plug efficiency, compared to about 10% for the CO₂ and the Nd:YAG laser. In addition, the diode laser can be fiber-delivered, which makes it more attractive for automated applications. Also, there is no need to precoat the metal to increase absorption-a necessity with the CO₂.

Electrical energy costs
	CO₂	Nd:YAG (diode pumped)	Diode
Required laser power	5 kW	3 kW	3 kW
Average wall-plug efficiency	10%	10%	30%
Approximate electrical power consumption of the lasers	50 kW	30 kW	10 kW
Electrical power cost per hour @0.09$/kWh	4.50 $/h	2.70 $/h	0.90 $/h
Source: Fraunhofer

“Most laser-cladding operations use the CO₂ laser, but there are inherent disadvantages compared to the diode laser or other lasers with the same wavelength as the diode,” said Eric Stiles, laser-division manager at the Fraunhofer Center for Coatings and Laser Applications (Plymouth, MI). “But the diode laser is interesting for applications like cladding because of the low cost for kilowatt power. A 3 to 4 kW diode-laser system can do the same work as a 6 to 8 kW CO₂-laser system because a lot of the energy of the CO₂ is lost. With the diode you get much better absorption with a lot of materials, especially at lower power intensities where CO₂ absorption is poor.”

Pros and cons

Despite these advantages, however, diode lasers still need further refinement in terms of being reliable enough for intensive industrial applications like laser cladding, a process in which laser energy is used to melt or weld a metallic or ceramic powder onto a substrate to create a wear- or corrosion-resistant layer on a metal component (see Fig. 1). While the current generation of high-power diode lasers has resolved many of the reliability issues that plagued earlier generations, random failures remain a problem when a diode-laser system is used in high-volume production applications, particularly when pulsing is required. Thus, while the overall cost of an industrial diode-laser system should be lower, the need to replace the diodes more frequently disrupts production cycles and increases overall cost of ownership.

“The main specificity of this market [materials processing] is the operating regime,” Franck Leibreich, director of marketing at Newport’s Spectra-Physics Lasers Division (Mountain View, CA). “Long micropulses-one second on, one second off-is the most difficult operating regime for the diode because it stresses the diode very much, which leads to random failures. So we and others are working to make the lasers more robust by focusing on the interaction between the soldering material and the heat sink in order to compensate for the coefficient of expansions. The goal is to optimize the thermal exchange between the materials.”

Brightness is another factor. According to Leibreich, the need to transform inherently low-brightness, highly asymmetric diode lasers have led to the development of several important beam-shaping and beam-combining technologies (see www.laserfocusworld.com/articles/250394). As a result, diode-laser devices that produce 4 kW from a 600-µm-diameter-core fiber are now being used in applications such as cladding and annealing.

“As diode lasers continue to evolve, we can predict that there are two areas where this technology will grow in attraction for industrial applications,” said Phillip Anthony, manager, macro business unit at Rofin-Sinar (Plymouth, MI). “The life of the diodes and diode bars will get longer, and as this happens the long-term cost of ownership becomes less of an issue. Second, a lot of time and energy is being spent on improving the beam quality, which should lead to becoming more realistic for fiber delivery. In the end this will be driven by the cost of ownership and the predictable life of the diode that can be measured in tens of thousands of hours.”

The real world

While efforts are under way to improve the reliability and lifetimes of diode lasers, some companies and organizations are already demonstrating the efficacy of this technology for surface treatment. At Fraunhofer, for example, Stiles and his colleagues have developed a new cladding process that utilizes a 3 kW Rofin-Sinar direct diode laser and a coaxial powder-feeding nozzle. It was initially tested in the oil industry, in which new wear-protective hard coatings were developed, tested, and applied to a number of down-hole drilling tools.

On the commercial front, Nuvonyx (Bridgeton, MO) and Laserline (Mülheim-Kärlich, Germany) have both had some success with their diode-laser cladding systems (see Fig. 2). Nuvonyx, founded in 1998 by former employees from the McDonnell/Douglas Laser Systems Division, has been a pioneer in the commercial introduction of high-power direct-diode and fiber-coupled laser systems for industrial applications in aerospace and defense. In 2005 Nuvonyx was acquired by ICx Technologies, which then acquired Thales Laser Diode (Orsay, France), which has since become Nuvonyx Europe. The company’s newest product, introduced in June 2006, is a fiber-coupled diode-laser system with an optical power density of more than 1 MW/cm², which the company says is an order of magnitude higher than any other commercially available system operating at a single wavelength.

Laserline’s expertise is also in fiber-delivered high-power diode lasers up to 6 kW. According to Klaus Kleine, vice president of U.S. operations, Laserline is beginning to see diode-laser cladding systems replacing CO₂ and Nd:YAG lasers in industrial applications, particularly in the United States. While the aerospace and power-plant industries continue to account for the bulk of laser-cladding applications, Kleine says there is a big push under way in other industries and that the compactness, efficiency, and affordability of the diode-laser systems should begin to attract more customers.

“The automotive applications are much more prevalent in Europe, which is also at the forefront of installed diode-laser systems for surface treatment,” he said. “A lot of these advances are being pushed by diesel-engine technology; the goal (in Europe) is to make diesel engines cleaner and much more fuel efficient via surface treatment.”

Slow Industrial Acceptance of Laser Cladding

2008-09-11T10:28:00.000-07:00

Laser cladding appears to require a dedication to the specialty to make it successful

Laser cladding is one of several ways in which a wear- or corrosion-resistant surface layer can be applied to repair or extend the life of components. Because industrial lasers are a controllable heat source, the laser cladding process is characterized by good control of the heat input to the workpiece. This results in lesser dilution of the coating layer (mixing of the coating with the substrate) and lesser distortion of the workpiece. Other methods, such as submerged arc welding or shielded gas metal arc welding, require significantly higher heat input to the part, greater dilution, the potential of greater distortion of the part, and a rougher surface that requires grinding if a smooth surface is required. Non-welding methods, flame spraying and plasma spraying, produce coatings that are mechanically rather than metallurgically bonded to the surface. Generally speaking, coatings produced by these methods are thinner than coatings produced by laser or arc welding processes.

By the early 1980s, laser cladding was identified as a process with a significant edge over the various competing processes for depositing wear- and corrosion-resistant coatings. The three main suppliers of high-power industrial lasers (Spectra Physics Industrial Lasers, Avco Everett Metalworking Lasers and the United Technologies Research Center, later United Technologies Industrial Lasers), none of whom currently exist, had developed an in-house expertise at laser cladding and were willing to address the needs of customers. A patent, #3,952,180 issued to Avco, said to be the dominant patent in this technology, was made available to all, on a non-royalty basis, to advance the technology. Still, the applications by-and-large did not become popular. Lists of locations where laser cladding was being used in the mid-1980s are not very different from current lists prepared today. Of the more than 150,000 industrial laser systems sold since lasers were invented, less than 100 are performing laser cladding operations.

Why has laser cladding seen so little market penetration? Here we consider only laser cladding for surface modification and not the extension of this process (called direct metal deposition or laser engineered net shaping (LENS) by different manufacturers) to building up solid objects by laser powder consolidation. This article is also limited to the installed base of lasers in North America, which are most familiar to the author; however, it is believed that laser surface modification practices in general have seen much wider applications on other continents.

Laser cladding is a production application in cladding of steam turbine blades, for manufacture of gas turbine blades on both sides of the Atlantic, for repair of gas turbine blades, for hard facing of oil field valves, hard-banding of drill rods, and surfacing of automotive and diesel valves. Water walls for power plants, paper mill components, and components of earth-moving equipment are being laser clad. Military components currently being laser clad include catapult launchers and drive shafts.

In the automotive industry, the valves of a non-commercial vehicle will be subject to 300 to 500 million thermal and mechanical cycles in an automobile's lifetime, and an exhaust valve in particular is subject to a stream of hot gas that may contain soot particles and corrosive constituents. In the early days of laser technology, numerous research facilities showed pictures of laser-clad valves in their publications and literature. How many of these laser systems for cladding automotive valves were put into production? To the best of this author's knowledge, only two, and these are not in North America. The chief reason for this lack of market penetration is the availability of somewhat less expensive equipment for coating valves using plasma transferred arc (PTA) welding, which produces deposits with the low levels of dilution. Deposits produced by laser welding do not have a demonstrated advantage over those produced by PTA, so it is difficult for manufacturers to justify the extra expense.

This is an example in which high technology has been applied to a process typically considered low technology. Because lasers are relatively expensive, it is easy to justify using robotic control and computer-controlled manipulators to make the best use of the controllable heat source. Manufacturers are reluctant to apply the same degree of controlled manipulation to inexpensive heat sources. But a highly controlled motion system applied to the plasma arc welding process has allowed repeated, high-quality deposits to be put onto valves, so little advantage is seen to using the laser process.

Numerous suppliers and repair shops in the airborne gas turbine industry around the world use lasers for reinforcement of the shroud interlocks and repair of blade tips. Competing processes for these applications include gas tungsten arc welding (GTAW) for the shroud interlocks and both GTAW and plasma arc welding for the blade tips. Many manufacturers and blade repair installations have laser equipment installed. Reasons why the laser process is finding increasing success in these applications include the fine control of the heat input in the laser process, which leads to accurate powder deposition at locations where it is needed.

In this successful application there is still considerable room for market growth, even though it is perceived as one of the more sophisticated of the industrial laser applications, requiring special skills. There are a limited number of people in North America who have developed these skills, and a limited number of equipment suppliers. One vendor of laser cladding equipment has sold a system into a shop where it was the first piece of equipment with computer-controlled (CNC) manipulators.

The laser cladding process equipment is usually applied to high-value products, consequently potential users require samples to be produced before they will commit a considerable amount of money to equipment purchase. But demand for the equipment is low, and equipment vendors cannot afford to have a laser cladding machine in their application laboratories or showrooms for occasional sales. The vendor must also have machine tool operators skilled in the laser cladding process as well as tooling to process a variety of part shapes in order to produce parts for potential customers. Some suppliers have provided one or two machines each, while only a few major vendors that specialize in this area have provided the largest amount of cladding equipment currently in use.

Successful laser cladding requires an accurate and controllable method of applying filler metal at the leading edge of the weld pool created by the laser beam. Use of an uncontrolled or poor method of adding filler metals results in poor utilization of the often expensive filler metal. This lack of control can be tolerated in high-heat-input arc welding processes or cladding with a multi-kilowatt laser because the filler metal inevitably gets absorbed into the larger weld pool. Laser cladding for repair of detail in components such as turbine blades requires more control in the powder deposition, which has lead to the development of highly controlled methods of adding the filler metal.

Sandia National Laboratories has developed a computer-controlled wire feeder that allows controlled deposition of wire into a weld pool; this design of wire feeder has been commercialized by Alabama Laser Systems (Munford, GA). Several suppliers of powders have developed powder feeders with a higher degree of control than possible with powder feeders used for other applications.

Several different styles of nozzles have been developed for applying the beam and the powder together at the workpiece. Accurate deposition of the powder inevitably means the powder nozzles must be close to the weld pool and, hence, must be water cooled. Nozzles have been developed at Imperial College in the United Kingdom, Sulzer Innotec in Switzerland, Fiat Automotive in Turin, IREPA laser in France, the Laser Institute in Canada, Battelle Columbus Laboratories, Huffman Corporation, and General Electric Aircraft Engines in the US. The proprietary nature of these designs and lack of information available to small users has been another factor in slowing the acceptance of the laser cladding process. Many users of the laser cladding process have developed their own nozzle design, but there is little technical information available to assist in this process.

Not all powders of nominally identical composition are suitable for the laser cladding process. Some powders prepared by gas atomization contain a fine porosity. Because the laser process is a low-heat-input, high-cooling-rate welding process, there is insufficient time available for the porosity to escape from the weld pool. A ground surface prepared with such a powder is found to contain fine porosity, which may frustrate attempts to introduce the laser cladding process. This is avoided by careful evaluation of powder suppliers.

A final reason for the slow acceptance of the laser cladding process is the perception that laser cladding is a process requiring more sophistication than other laser applications. Laser cladding has been economically profitable for companies in which there has been a champion who will sponsor the adoption of the technologies. Other more established processes such as laser cutting don't need a champion because the economics of these technologies will speak for themselves, and companies generally have the necessary expertise in-house to adopt the new process. Laser cladding requires the company to have three sets of skills, the CNC skills, the welding skills and the laser skills. The laser skills can be provided by the equipment vendor in a short course, as in the case of cladding, but the other skills must be provided in-house or by hiring outside technical people.

Laser cladding knowledge has been developed by a relatively scarce supply of "champions." For example, personnel from Westinghouse R&D assisted in the founding of a job shop, which is now a successful Praxair operation. A graduate from Imperial College established the laser cladding technology at Quantum Laser Systems, which is now a part of the Honeywell-Allied Signal organization. Personnel from IREPA laser in France established the Gremada Industries laser operation. By contrast, a number of smaller shops have attempted to add laser cladding to their operations. But unless they have a full-time champion to provide the expertise and bring in a lot of business, the attempts to provide all services for all customers appear to be less successful than that seen in the shops such as Gremada and Praxair that specialize in the laser cladding operation. In general, laser cladding appears to require a dedication to the specialty to make it successful, and companies, either processing shops or equipment vendors that attempt to add laser cladding to their diverse capabilities do it much less successfully than companies that specialize in it.

Microstructure of Materials

2008-08-26T08:49:00.000-07:00

Most of the materials around us are polycrystalline materials. Properties of these materials which are relevant to humanity like mechanical strength, hardness, ductility, fracture toughness, creep resistance, fatigue resistance, corrosion resistance etc. to name a few are strongly dependent on microstructure of polycrystalline materials. Therefore, it is useful to understand microstructure of materials. In this article microstructure of a polycrystalline material will be elaborated in a manner that a reader with a common knowledge will be able to comprehend it.
All the materials are made of atoms. These atoms combine (bond) together either similar atoms or dissimilar atoms or both to form large aggregates. The name of these large aggregate of atoms – depends on how these atoms have bonded together. Some atoms combine to form molecules and some atoms combine to form crystals. Molecules are much smaller units and combine together to form either crystals or non-crystalline polymers. Let us move ahead with crystals. When atoms (or molecules) combine together to form a periodic structure; this structure is known as crystals. The basic unit that repeats to form the entire crystal is the lattice. A crystal displays different kinds of symmetries and symmetry of a crystal also profoundly affects its properties. To provide some examples iron crystals can be either face centered cubic (fcc) or body centered cubic (bcc) depending upon the temperature. What is interesting to note that fcc iron is non-magnetic while the bcc iron is magnetic.
Now these crystals are the grains of a polycrystalline material. When large number of these crystals comes together to large aggregate what we have is the polycrystalline material that we use in everyday life. When two crystals come together is highly probable that their orientation will not match and this results in formation of a boundary between two crystals (grains) in a polycrystalline material. This is what is known as grain boundary. In most of the cases the size of these crystals or grains is too small to be seen by naked eye and a microscope is need to see these grains and the boundary between them that’s why the structure is known as microstructure (something that can be seen by a microscope). Another reason why these are called as microstructure is that size of these crystals (grains) is of the order of a few tens of micrometer. Besides size, another important feature of a crystal (grain) is its morphology or shape it could be nearly spherical, or cylindrical or dendritic (tree like) or lenticular (lens like) etc.
Depending upon the chemistry and processing condition a polycrystalline material may be composed of chemically and morphologically different kind of crystals (grains) in varying proportions. Besides, there may be intentional or unintentional flaws like dislocations, porosity, inclusions etc. As these flaws also affect property of a material significantly, nowadays a new trend has emerged to include these also a constituents of microstructure.
Therefore, when one talks of microstructure of a material he is essentially talking about the size, shape and relative proportion of the different phases including the flaws. Lets us elaborate this with an example of steel, let us take mild steel.Microstructure of mild steel consists of two different phased – Ferrite (small amount of interstitial elements like C, N etc) dissolved in bcc iron and Cementite (a compound of iron and carbon – Fe3C). Now let us talk about the morphology of these phases. It depends on the processing condition. But the microstructure I am talking about is a near equilibrium microstructure. The two phases are distributed in the following manner. There are two kinds of regions – one consists of only ferrite grains (of nearly spherical size) and the other region consists of alternate layers of ferrite and cementite lamellae. This structure consisting of alternate lamellae of ferrite and cementite is known as pearlite. Thus one can say that microstructure of mild steel consists of ferrite and pearlite.
How this microstructure can be altered / modified to a different kind of microstructure will form part of the next article that will talk about heat treatment of materials.

Welding “Grade 91” Alloy Steel - Ferritic Martensitic Steel

2008-08-23T00:39:00.000-07:00

One of the materials that have spread through the piping and boiler industry recently is an alloy, referred to in various specifications as “T-91, “P-91, “F-91” and “Grade 91.” This is a specially modified and heat treated 9% chromium, 1% Molybdenum, Vanadium enhanced (9Cr-1MoV) steel that performs quite well at elevated temperature – usually 1000°F and higher. It was first used in the mid-1980s and has “picked up steam” since then. If you are going to weld or fabricate Grade 91 alloys, beware! These are not your father’s chrome-moly steels!

Development of Grade 91 began in 1978 by Oak Ridge National Labs for the breeder reactor and further developed by other researchers since then. Other grades such as grade 92, 23, 24, 911 and others are also under development, and the guidelines in this paper should be followed for those materials until the industry gains experience that may dictate other practices.

Since P/T-91 is modified with vanadium, nickel, aluminum, niobium and nitrogen, it develops very high hardness. Tramp residual elements in this steel, such as phosphorous, sulfur, lead, tin, copper, antimony and other elements will segregate to the grain boundaries during solidification of the weld, and, since the weld metal is very hard, it will crack quite easily. It is, therefore, very important to use low residual filler metal.

For SMAW, E9015-B9 electrodes are preferred. EXX15 type electrodes have no extra iron powder in the coating like EXX18 electrodes, eliminating one source of contaminants. While electrode manufacturers have recently improved awareness of the need to make clean E9018-B9 electrodes, if you occasionally get crater cracks (also known as “solidification anomalies” and “rogue weld metal’), the filler is not low in residuals and you should send it back (or at least get some good stuff). Look carefully for crater cracks, and keep in mind that one batch of electrodes from a manufacturer may crack and another batch not cracks. Keep an eye on it. Two trade names of electrodes and filler that have low residuals are Metrode Chromet 9B9 electrode and Euroweld 9CrMoV wire. The wire is suitable for GTAW, GMAW and SAW (with a suitable flux, such as Lincoln MIL800H, Lincoln 882, Thyssen Marathon 543, Bavaria-Schweisstechnik WP380. Welding Grade 91 using FCAW requires even more care since many FCAW wires do not provide adequate toughness at 70°F (the lowest hydrostatic test temperature permitted by ASME); the only FCAW wire that consistently provides more than 20 Ft-lbs absorbed energy at 70°F is Metrode’s Supercore F91.

The performance of Grade 91 welds depends entirely on having the correct chemical analysis in the weld metal; therefore, it is highly recommended that filler metals be purchased with test reports showing actual chemical analysis for the specific heat/lot combination that one has purchased. In addition, a minimum carbon content of 0.09%, a minimum niobium content of 0.03%, and minimum nitrogen of 0.02% should be specified to ensure adequate creep strength in the weld metal.

A slightly lower niobium level can be accepted with flux cored wire if titanium is added; titanium is an effective substitute for niobium, but the titanium should not exceed 0.010% since titanium will also combine with nitrogen, reducing nitrogen’s effectiveness as a creep strength enhancer.) In addition, the sum of Mn + Ni should not exceed 1.5%. Manganese and nickel depress the lower transformation temperature, and as it exceeds 1.5%, the transformation temperature drops below 1450ºF, narrowing the range in which heat treatment can be done safely. In addition, the Mf temperature goes down, increasing the possibility of retained austenite after PWHT. When using SAW, a basic flux is preferred since other flux types will burn out carbon and permit elevated oxygen and nitrogen levels reducing the strength and toughness of the weld metal. Since this is a highly-hardenable alloy, it is subject to hydrogen cracking. Purchase of E9015-B9-H4 electrode is recommended. The “H4” designation indicates that the electrode exhibits less than 4 ml of hydrogen per 100 grams of weld metal. This is truly a very low hydrogen electrode – exactly what is best for welding highly-hardenable steel like Grade 91. Even with diffusible hydrogen control of the electrodes, it is recommended that the electrodes be stored in heated portable rod boxes at the welding location rather than just distributed in the normal fashion. SAW wire/flux combinations and FCAW wire should be ordered with “-H4” designations also, although FCAW wire may not be available except as H-8.

Preheat and interpass temperature are very important. A range of 400 to 550°F is recommended, after welding is completed, the joint should be allowed to cool slowly to at least 200°F after welding is completed to be sure that all the austenite has been transformed to martensite prior to postweld heat treatment (PWHT). If this is not done, there is risk of martensite formation after PWHT; this will result in hard, brittle welds. For the metallurgists out there, the Mf temperature is above 212°F, varying some with the grain size. The welding technique is also important. Since a wide, flat bead is best, a slight weave technique and high travel speed should be specified. Ropy beads are bad since tall, narrow beads tend to crack.

Concave beads should also be avoided, particularly with SAW. Bead thickness should not exceed 1/8 in. for SMAW and FCAW to promote tempering of previous passes. These conditions of welding should specified in the WPS to provide correct guidance to welders, not to give them a hard time. Be sure that your welders have been trained regarding these special requirements and that they comply with them.

Finally, postweld heat treatment is required for Grade 91 steels, regardless of what construction codes may permit. The holding range should be 1375 to 1425°F for a minimum of 2 hours. Even on small superheater tubes, a long time at temperature PWHT temperature is necessary to form the required weld structure, to ensure adequate toughness during hydrostatic testing and to ensure adequate service life. The lower transformation temperature can be as low as 1450°F; if this temperature is exceeded during PWHT, the weld should be allowed to cool to below 200°F followed by reheat treating or the condition of the joints should be evaluated by hardness testing. Refer to AWS D10.10, Recommended Practice for Local Heating of Welds in Piping and Tubing, for excellent direction on locating and attachment of thermocouples, the extent of insulation needed, heating coils arrangement, etc. if local heating (preheat, postweld baking, PWHT, etc) is going to be done.

It should be noted that the Codes specify an upper temperature limit of 1425°F (775°C) for PWHT of Grade 91 type materials. If this temperature is exceeded (as sometimes may happen due to errant thermocouples, power surge or temporary insanity by the operator), the effect on Grade 91 type materials is dramatic and warrants evaluation. For most materials, including your father’s Cr-Mo steels, excursions above the lower transformation temperature have little consequence since the material properties return to practically their original condition when the material cools down from such an excursion. The worst case scenario for the old Cr-Mo steels - the WPS might have to be requalified since the lower transformation temperature was exceeded. However, high-performance Cr-Mo steels develop their properties via normalizing and tempering; this results in the precipitation of the carbides that give these materials their superior elevated-temperature performance characteristics. If the lower transformation temperature is exceeded (depending on the chemical makeup of the alloy, it can be as low as 1450°F), the carbide matrix is destroyed and the material loses its elevated temperature strength. Since it is not possible to reform the normalized and tempered microstructure using local heating (there is always a temperature gradient beyond the band that is being normalized that contains damaged material), it is necessary to cut out and replace the weld joint, including a minimum of 3 inches of base metal on each side of the joint that was overheated. Alternative solutions include normalizing and tempering of the entire assembly and justifying use of the weld based on properties of the material in the annealed condition (e.g., using the allowable stress values for Grade 9 instead of those for Grade 91 for the joint and surrounding material). Good luck on getting an engineer to agree to the latter.

When joining the high-chromium alloys to lower-chromium alloys or carbon steel, carbon in the lower-chromium steels will migrate to the higher-chromium steel during PWHT. This will result in a soft zone in the lower chromium steel. The higher the PWHT temperature and the longer the joint is held at PWHT temperature, the more diffusion occurs and the bigger the soft zone becomes. While the minimum PWHT temperature for welds involving Grade 91 welded to itself have been raised, the PHWT temperature for dissimilar joints remains at 1300°F (705°C) to minimize this undesirable effect. To minimize the size of the soft zone, do not heat treat dissimilar metal joints at temperatures much over minimum, and don’t hold them at temperature longer than is required by the applicable Code.

After PWHT, the weld hardness should be in the range of 200 to 275. Hardness up to 300 Brinnell may be accepted, but any hardness over 300 is an indication of inadequate PWHT. SMAW and SAW weld metal will exhibit higher hardness when compared to GTAW and FCAW. Hardness below175 indicates overheating of the joint, and such joints should either be replaced or the part should be normalized and tempered.

Do not perform hardness tests that will leave deep impressions in the surface of thin tubes. When performing hardness tests, it is important to prepare the surface properly, particularly for HAZ readings. Since the base metal may have a layer of decaraburization on the surface, about 1/32 inch of metal should be removed by grinding, and that should be followed by polishing to a 120 grit finish. This preparation will also make readings more consistent and should also be followed when measuring the hardness of the weld metal.

Grade 91 can be hot bent using furnace heating or induction heating between 1600 and 2000°F, but the low end of this range is preferred. Pipes that are hot bent should be given a full-furnace normalizing heat treatment at 1900 to 1950°F for 30 minutes per inch of wall thickness, air cooled to below 200°F and tempered in the PWHT range of 1375 to 1425°F for 1 hour per inch of thickness. Cold bent pipe should be given a stress-relieving heat treatment at the above tempering temperature for 15 minutes per inch of wall thickness.

Another strange phenomenon with Grade 91 is that it is subject to stress-corrosion cracking in the as-welded condition. The media that causes this has not been identified as yet, and it does not happen for several days after the weld has cooled to ambient, but it does happen. It also appears to occur if the joint is allowed to get damp, such as might occur if the part gets cold and moisture condenses on it; keeping joints that have not been given PWHT warm and dry seems to preclude this cracking. Of specific concern is shop-fabricated pipe which may get moved around in a shop for a few days before heat treatment. With Grade 91, heat treat welds as soon as practical after welding.

Contamination with sulfur-bearing compounds (cutting fluids, lubricants, markers, etc.) will cause transgranular cracking, so they should not be used around Grade 91 in the as-welded condition. Hot bend pipe must be renormalized and tempered since hot bending destroys the structure that gives Grade 91 its enhanced creep properties. Cooling rates after hot bending are typically too slow to achieve a uniform martensitic microstructure, or, when induction heating is used, there is a temperature gradient that contains sections of pipe that are over tempered. The same thing happens if one performs local normalizing and tempering of a bend. Normalize and temper any hot bends, and that includes the entire assembly, not just the bend area.

Local heating (such as during square-up) using a rosebud or other heating torch is acceptable, but the temperature must be monitored and not ever allowed to exceed 1300ºF in order to stay below the lower transformation temperature and out of danger of over tempering the pipe. Such heating should be limited to making small changes in dimensions; if large changes are needed, pipe should be cut and rewelded.

Cold deformation (such as occurs during cold bending) should be limited to not more than 10% strain in the metal, and if this limit is exceeded, the part should be renormalized and tempered. The strain formula is % strain = 100 r/R where R is the radius of the bend and r is the radius of the pipe.

For dissimilar metal welds between Grade 91 and lower chrome-moly steels, use filler metals that match either the lower chromium grade or the Grade 91. A large difference in chromium causes a gradient that pulls carbon out of the lower-chromium steel causing a depleted carbon band in the lower-chromium side of the joint and a higher-carbon band in the Grade 91 side of the joint. This happens during welding and cannot be avoided; however, has been shown to have no significant effect on creep behavior when heat treatment time and temperature are not excessive. The width of the band can be increased by excessively high or long PWHT temperatures, so PHWT should be done following typical requirements for the lower-chromium steel rather than those of the Grade 91. Use regular carbon grade filler metal, not low carbon grades.

For welds to austenitic stainless, use nickel-based filler metals ENiCrFe-2, ENiCrFe-3 or ERNiCr-3. If the stainless is a stabilized grade or a low-carbon grade, the completed joint can be given standard PWHT. If the stainless is not a stabilized or low-carbon grade, the P/T-91 side of the joint should be buttered with at least 1/4” of nickel-alloy weld metal and heat treated in the normal fashion. The buttered and heat treated end can then be welded to the stainless steel using nickel alloy filler metal without preheat or PWHT. Don’t even think of welding nickel overlay with stainless -- the weld metal will be fully austenitic and crack.

Preheating to 300°F is recommended when thermal cutting or using carbon or plasma arc gouging.

A NOTE OF CAUTION: When performing hydrostatic testing, there is always danger of brittle failure due to the presence of flaws and inadequate toughness of the metal. Weld metal can be somewhat unpredictable in toughness due to variations in welding techniques by different welders and the possibility of weld discontinuities. Accordingly, Grade 91 should be hydrostatically tested at 70°F (19°C) or higher to be sure that the weld metal is above 15 ft-lbs or 15 mils lateral expansion. This is usually adequate toughness to ensure failure by leak-before-break rather than brittle failure

High Power Lasers – CO2 Laser

2008-06-21T06:07:00.000-07:00

In this section the important features of high power lasers for job shop applications will be discussed. Only those lasers which carry out materials processing on macroscopic scale will be discussed in this section. Other high power lasers for high precision and microscopic / nano scale processes will be discussed in different blog.
Presently there are four lasers in the category of high power lasers which can carry out processing of materials on macroscopic scale. These lasers are CO2 Laser, Nd-YAG Laser, Fiber Laser and High Power Diode Laser (HPDL). The important features of these lasers will be briefly discussed in the following sections.
CO2 laser
This has been and continues to be the main work horse as far as laser materials processing is concerned. The major positive factor favoring this laser is the kind of power it can deliver. It can deliver 5, 10, 15, 20, 25 kW comfortably. There are CO2 lasers with 45 kW as well. This laser offers a range of beam qualities. Very high quality beam to carry out cutting and welding; besides higher order and relatively poor beam qualities to carry out surface hardening, surface alloying and cladding can be produced with relative ease in case of CO2 lasers to suit the final application. The very high power lasers like those with 20, 15, 20, 25 kW and more power are used mainly for cutting and welding of thick plates. The major user of this class of lasers is the ship building industry. Those lasers with 1 to 10 kW powers are used for welding, cutting and surface modifications like transformation hardening, surface alloying and cladding.
CO2 laser has evolved a lot since its invention. Different versions of this laser are there in use depending upon who the manufacturer is / was. This laser is generally with folded cavity with very highly reflecting cavity mirrors – highly polished OFHC (Oxygen Free High Conductivity) copper mirrors coated with gold and mirror. The lasing medium is a mixture of CO2, N2 and He. This is made to flow either axially (along axis of the cavity) or in transverse direction by means of a blower. The high frequency gas discharge is carried out for pumping the lasing medium. The lasing medium is re-circulated to cool it so as to sustain the discharge. The gas pressure is ~ 30 Torr. Mostly high power lasers are transverse flow.
It is relevant to discuss the state of art in this class of laser. The state of art in this class of laser is “Fast Axial Flow Laser”. This has been developed by Trumpf Lasers, Germany. In this laser system the cavity is a folded cylinder of rectangular shape in two layers. The amplification takes place in both the layers which are connected by folding mirrors. The lasing media flows along the axis of the cylindrical cavity at very fast speed by means of blower rotating at 45000 rpm. The reason why the cavity is folded and two – layered is to increase the amplification length and thus to be able to extract high powers. By making the flow of lasing media fast and axial the quality of the laser beam is very good and it can be used to produce very good quality cut and welds. The folding mirrors in this laser are coated dielectric mirrors and not the metallic mirrors as is the case with other versions of CO2 lasers.
Buyers Beware!!
Many readers will either like to buy or may recommend buyers for CO2 lasers. While you will find many manufacturers in the list of Ads by google in this blog page; I feel it fair to put my perspective.
There are two classes of manufacturer – Primary manufacturers and Assemblers. While Primary manufacturers develop a laser system with bottoms up approach and manufacture all the major and critical components. The assemblers simply design the system; get the components manufactured form different sources and assemble them before selling it to gullible buyers. These assemblers hardly provide any after sell support and many times shut their shop after selling a couple of machines and thus leaving the buyers in lurch after the ill designed system keeps on faltering.
To protect your interest I must list a couple of primary manufacturers. Some really good manufacturers are –
TRUMPF LASERS from Germany
ROFIN-SINAR from Germany
JK LUMONICS from United Kingdom
These are some really good manufacturers and one can buy a laser system from them even blindly. In case you like to explore for manufacturers from USA; read the Ads by Google on this page or search on the google search engine; links for the same is provided on this page for your help. In case I get some good name, I will share the same with you all in my subsequent blogs.
In case you really want to know about some Assemblers – MLI, Israel is one such name. Any company claiming to a manufacturer from India or China on this date is nothing but an assembler. So be careful. Be careful even when you are buying from Russia or East European countries.
In case you need any more information do not hesitate to write on this page. That you will be responded is my guarantee.

Laser the Light Fantastic

2008-06-11T08:05:00.000-07:00

Invented by Maiman in 1960; this fantastic beam of light has emerged as very useful beam of energy for carrying out a diverse set of materials processing activities like transformation hardening of steel, remelting, surface alloying, cladding and welding of metallic and composite materials and cutting of literally any material. Sophiticated activities like micromachining, thin film depostion are also carried out using high power pulsed laser systems.
The fundamental principal behind laser beam generation are "Population Inversion" and "Stimulated Emission". These mechanisms were proposed by none less than Albert Einstein himself as early as 1917.; who can be rightly termed as the enlightened mind going by the fact that all of his major works have strong connection with light.
For materials processing community a laser is nothing but an intense beam of light capable of heating, melting and evaporating literally any material. Besides, there are many tangible and intangible benefits of laser materials processing which will be discussed in subsequent blogs.