Nickel and Cobalt base alloys are two heat-resisting welding materials that are often grouped together because they are used for similar purposes. From heat resistance to corrosion resistance, Nickel and Cobalt base alloys, also known as the super-alloys, are the most important of the heat-resisting class.
Heat-resisting alloys are metals developed to withstand the severe conditions prevailing in service at elevated temperatures where other more common materials would fail.
What characteristics are required even when not welding heat-resisting materials?
Heat resisting applications are not considered steels because their behavior is similar to other types of materials. Their composition is complex and includes important percentages of nickel and chromium, with other elements added to provide special properties.
Other heat resisting materials include alloys whose base metal is nickel or cobalt, while their composition is modified by the addition of other different elements for special purposes.
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 from this type is 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 S-816.
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 bases of this type are A-286 and Incoloy 901. Typical Nickel base is Waspaloy.
A different class of materials, with a large set of properties in common with those discussed above, are called corrosion resisting alloys, designed to resist attack by aggressive chemicals, with or without the influence of superposed heat.
When considered as base metals, 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.
As a general rule welding heat-resisting materials should be performed in their most ductile condition, often designated as the annealed or 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, with adequate heat treatments. Although covered by specifications, they are known 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. Of the weldable wrought alloys names we can list a few hereafter: Hastelloy B, C, C276, N, X, Inconel 600, 601, 625, Rene 41.
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 on whether they are in cast or wrought form. Some of the cast alloys are known by the following names: HS 21, X 40 (Stellite 31), G 34, Mar M 509, and FSX 414.
Common Wrought Alloys Include: S 816, L605 (HS 25), HS 188, Mar M 918, and G 32 B. All of 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.
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 combuster 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 is 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 of the materials.
Gas Tungsten Arc Welding heat-resisting alloys is best for thin sections. It is good practice to have 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.
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. In necessary, re-solution and precipitation (aging) treatment should immediately follow welding. Shielded Metal Arc Welding is used sometimes for solid solution strengthened heat resisting alloys, but is not used for solution and precipitation hardened ones.
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. Cleaning the workpiece and the filler metal before welding heat-resisting alloys is critical. Cracks of any type, in the weld or in the base metal are never admitted. Joint design should avoid stress concentration and multi-axial stresses. High heat input producing large residual shrinkage stresses can also be a cause for cracks.
High energy welding of heat-resisting alloys presents various levels of weldability depending on the measure of restraint the parts undergo during welding. Even nickel cast alloys, which normally exhibit very low weldability, can be electron beam welded for non demanding applications.
If you are interested in automating a nickel or cobalt welding application, contact our staff at 740-251-4312 or fill out a contact form and our engineers can work out the best options for your process requirements.