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Welding-heat-resistingnickel and cobalt alloys:SOLUTIONS with Effective, Powerful Advice
Welding-heat-resisting alloys, welding Superalloys, welding nickel alloys, cobalt alloys, iron base alloys, solid solution strengthened, welding corrosion resisting alloys, solution and precipitation heat treatments, welding processes, welding precautions, welding techniques, oxide removal, welding information, welding links, welding tips, improving welding results, joining questions needing answers: these are some of the items developed in this Site for the benefit of interested readers. Welding-heat-resisting materials like Nickel and Cobalt base alloys is a subject which often in certain contexts, groups together these alloys because, although different, 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 should never be welded in order not to impair their elevated creep properties (resistance at high temperature) which make them so useful in the first place. Known also as Superalloys, Nickel and Cobalt base alloys, are the most important of the heat resisting class, although there are a few Iron base alloys displaying some high temperature resistance.
What is in here for me? What is important in Welding-heat-resisting alloys? A basic knowledge of the different types and heat treatment conditions, of their weldability, and on the necessary steps to avoid crack formation. What are heat resisting alloys?... They are metals developed to withstand the severe conditions prevailing in service at elevated temperatures where other more common materials would fail. Where are they used? What are the characteristics required even when not Welding-heat-resisting materials? Are they difficult to weld?... In general Welding-heat-resisting weldable alloys of this class is not difficult, provided that a basic understanding of compositions and of conditions is available. These are some of the questions that should concern anyone considering specializing in 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? Click here for the needed information. Iron base alloys... ...for heat resisting applications are not considered steels because their behavior is much more similar to that of other types of materials briefly described in the following. 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. 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 in easily performed. Typical Iron base of this type is called N-155 (or Multimet). Typical Nickel Base are Inconel 600 and Hastelloy X. Typical Cobalt base are L-605 (or HS-25) and
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. Typical Iron base of this type are A-286 and Incoloy 901. Typical Nickel base is Waspaloy. Here Welding-heat-resisting is more difficult (if at all permitted) and due precautions should be taken. A different class of materials, having however a large set of properties in common with those discussed above, is called more properly that of corrosion resisting alloys, 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 metallic element used for alloying steels and stainless steels, and as such it modifies the properties of the alloys involved. As a base metal it is used for its remarkable resistance to heat and corrosion. In particular it can develop elevated resistance to stress under heat, both in cast or wrought form, when alloyed and treated as needed. Nickel base materials are selected for their corrosion resistance and elevated temperature properties, whith adequate 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 wrought alloys names we can list a few hereafter: 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.
All these have around 20% Chromium and some carbide forming elements like Niobium, Tantalum, Zirconium, Vanadium. Carbon content NEW 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 which 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 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 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 and of other hot working machine parts are spot and seam welded 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 and of such ductility 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 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. To read the article (in Section 7) click on PWL#028. An Article on Gas Turbines Welding and other Processes was published (7) in the Issue 33 of Practical Welding Letter for May 2006. To read it click on PWL#033.
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, possibly 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 but 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 electron beam welded for non demanding applications.
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