Welding of Pure or Low Alloy Materials: Difficulties and Solutions

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

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

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

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

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

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

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.