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Welding-heat-resistingnickel and cobalt alloys:SOLUTIONS with Effective, Powerful Advice
Welding-heat-resisting materials is important for Hot Service Applications. Although different, Nickel and Cobalt base alloys are sometimes grouped together, when considering welding. That is because they are used for heat resistant and for corrosion resistant applications. In wrought form they present no special welding problems, with due attention. However very special high temperature cast alloys used for gas turbine engine blades were known to be unweldable. In spite of that, local repair attempts of such blades, were recently performed successfully. Drop by drop edge build up was done by laser beam or electron beam. But that requires very special equipment and utmost attention. Welding by usual means (GTAW, GMAW) should never be attempted. That is because their elevated creep properties (resistance at high temperature), which make them so useful in the first place, would be severely affected. Known also as Superalloys, Nickel and Cobalt base alloys, are the most important of the heat resisting class. There are a few Iron base alloys displaying also some high temperature (although inferior) resistance.
What is important in Welding-heat-resisting alloys? What are heat resisting alloys?... Where are they used? What are the characteristics required, even besides Welding?
See also our pages on Welding Nickel and Welding Cobalt.
Are they difficult to weld?... In general Welding-heat-resisting of weldable alloys of this class is not difficult. These are some of the questions that should concern anyone considering Welding-heat-resisting alloys. The identification of the material to weld and of its condition is extremely important for selecting the process and the filler material. One should make an effort to obtain the Specification identifying the material, or at least its commercial name. How can one know which material one has to work with?
Iron base alloys... ...for heat resisting applications were not considered steels. But newer conventions may have changed that habit. See PWL#114, section 2. Their composition is complex and includes important percentages of nickel and chromium, with other elements added to provide special properties. Welding-heat-resisting iron base alloys can be a challenge, depending on the specific type. See PWL#112, PWL#113 and PWL#114 mentioned above. and Nickel or Cobalt base alloys... Other heat resisting materials include also alloys whose most important constituent (called the base metal) is nickel or cobalt, while their composition is modified for special purposes by the addition of other different elements. Alloying base metals as above with various elements produces different classes of materials. Those which derive their properties exclusively from their composition and are not susceptible to improvement by heat treatment are designated as hardened by solid solution. Welding-heat-resisting materials of this type is easily performed.
Other classes, called hardenable by solution and precipitation (or aging) processes, respond to heat treatment because of subtle reactions that modify their microstructure, taking place while heating and cooling. Welding-heat-resisting hardenable alloys is more difficult (if at all permitted) and due precautions should be taken.
A different class of materials is called more properly that of corrosion resisting alloys. They have however a large set of properties in common with those discussed above. These are designed to resist attack by aggressive chemicals, with or without the influence of superposed heat. Welding-heat-resisting alloys... As a general rule Welding-heat-resisting materials should be performed in their most ductile condition, often designated as the annealed or the solution treated condition. Nickel is a ductile metal. As a base metal it is used for its remarkable resistance to heat and corrosion. As an alloying element it is used for modifying the properties of the alloys involved, like steels and stainless steels. It should be remembered that properties modification is achieved through micro-structural changes obtained by the combined action of
In particular they can develop elevated resistance to stress under heat, both in cast or wrought form, when alloyed and treated as needed. It should be appreciated that the highest properties are developed by suitable balancing different, often contrasting, tendencies. Fusion welding is a drastic, if local, process that changes the chemical composition, by introducing a new and different cast structure. Moreover the intense heat deeply affects the micro-structures present by annealing them, and tending to over-age the precipitates, if any. (Note: Over-aging means increasing their size over the optimum for best properties. Over-aging is due to excessive temperature or excessive time at heat). Therefore, as in cast turbine blades mentioned above, welding is likely to destroy the best properties. Nickel base materials are selected for their corrosion resistance and elevated temperature properties, with suitable heat treatments. Although covered by Specifications, they are know mostly by commercial names. Alloys hardened by Solid Solution are readily welded in annealed condition. Precipitation hardenable alloys in wrought form are welded in solution treated condition, followed by heat treatment as required. Specific designations and compositions can be found in general Handbooks and in Manufacturers' publications. Of the weldable Nickel base wrought alloys, a few names are listed hereafter: See Welding Nickel. Cobalt too is a ductile metal. It is used as a major alloying element for a wide selection of special purpose materials. As a base metal, alloyed with other elements, its major property is the ability to resist oxidation and scaling at elevated temperature, although developing only limited strength at high temperature. Cobalt base materials have somewhat different compositions depending if they are in Cast or Wrought form. Some of the Cobalt base Cast alloys are known by the following names: Common Cobalt base Wrought alloys are:
All these have around 20% Chromium and some carbide forming elements like Niobium, Tantalum, Zirconium, Vanadium. Carbon content is 0.25-1.0% for casting alloys and 0.05-0.4% for wrought alloys. See Welding Cobalt. RESOURCES
Looking for more Online Reference Links? Click on Welding Resources When considered as base materials the above, nickel and cobalt, have a set of interesting properties. These make them useful for elevated temperature applications because of their heat resistance, especially for gas turbines, furnace accessories, hot chemical processing systems, and also for corrosion resisting applications. Cleaning is important... Before Welding-heat-resisting alloys, cleaning of the workpiece and of the filler metal is a most important consideration. All major welding processes are applicable, excluding possibly the oxyacetylene method that is not recommended because the use of fluxes introduces complications which are not present with other techniques. FRICTION WELDING can be used for Welding-heat-resisting materials. For those alloys gaining their properties through solution and precipitation hardening, one must be aware of the influence of the weld heat on the properties in the immediate vicinity of the joint. If the outcome of locally reduced strength is not objectionable, there are no other limiting considerations. RESISTANCE WELDING, both spot and seam is widely used for Welding-heat-resisting alloys. In particular many heat resisting sheet metal items, like combustor liners, flame holders and many other elements of modern gas turbine engines. Also for other hot working machine parts, spot and seam welding are used as production or repair procedures, much the same as done with more current stainless steels. It should be noted that in many cases, ARC Welding-heat-resisting alloys can produce cracking, especially in those hardened by solution and precipitation heat treatments, during welding or during heat treatment. That is why much attention should be paid to avoid cracking by developing suitable procedures. All of the arc welding processes can be used, but some are better suited than others depending on the thickness being welded.
Gas Tungsten Arc Welding-heat-resisting alloys is used for thin sections. It is good practice to have the fixtures holding the elements fitted with a back up copper bar with tiny holes abutting in a groove through which a thin stream of argon is provided. Filler metal compositions for Welding-heat-resisting alloys should be compatible with that of the base metal. Also ductility should be considered as to provide maximum freedom from cracking when considering the dilution ratio of filler to base metal. An Article on Filler Metals for Heat Resisting Alloys was published in the issue #10 of Practical Welding Letter for June 2004. To read the article click on PWL#010. A Contribution on "Biocompatible Materials" was published in Practical Welding Letter issue No. 22 for June 2005. To see the article click on PWL#022. In certain extreme cases, Heat Resisting metallic materials are simply not adequate to provide useful service life to implements and parts. In those cases one should probably consider the application of non-metallic High Performance Ceramics. An Article describing the development of High Performance Ceramics was published in Issue 28 of Practical Welding Letter for December 2005. An Article on Gas Turbines Welding and other Processes was published (7) in the Issue 33 of Practical Welding Letter for May 2006. An Article on Advances in Thermal Barrier was published (11) in Issue 153 of Practical Welding Letter for May 2016. An Article on Modern High Temperature Testing was published (11) in Issue 155 of Practical Welding Letter for July 2016.
For receiving regularly the Letter as it is published, click on Subscription. Before Welding-heat-resisting precipitation hardenable alloys, they should be relieved of all forming or bending stresses by a suitable process annealing heat treatment. This should be preferably done in a vacuum or controlled atmosphere furnace in order to prevent oxidation. Re-solution and precipitation (aging) treatment should immediately follow welding as required. Shielded Metal Arc Welding is used sometimes for solid solution strengthened heat resisting alloys, but is not used for solution and precipitation hardened ones. Also Brazing of heat resisting alloys can be performed, generally in vacuum furnaces, as needed for complex assemblies, with suitable heat resisting alloy brazing filler metals. Controlling defects... Of the defects likely to appear in this type of Welding-heat-resisting alloys, porosity is controlled by proper cleaning before welding and removing of surface contamination. Cracks of any type, in the weld or in the base metal are never admitted. Joint design should avoid stress concentration and multiaxial stresses. High heat input producing large residual shrinkage stresses can also be a cause for cracks. HIGH ENERGY Welding-heat-resisting alloys is commonly used. Different alloys in different conditions present various levels of weldability depending also on the measure of restraint to which the parts are subject during welding. Even nickel cast alloys, which normally exhibit very low weldability, can be successfully electron beam welded for non demanding applications.
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