Full Description
Precipitation-hardenable nickel-base alloys possess good strength, ductility, and corrosion resistance from cryogenic temperatures to above 1600F. These properties are obtained through the addition of many alloying agents including chromium, cobalt, aluminum, titanium, molybdenum, columbium, boron, and zirconium. Mechanical properties may be varied over a wide range through manipulation of the solution treating and aging procedures and alloy content. The principal strengthening phase is gamma prime (a precipitate based upon the Ni
Al compound), but its composition and kinetics will vary with alloy content and it may include titanium, columbium, and possibly molybdenum. Many other phases, most notably the carbides, are also found. Carbide-phase morphologies exert control over elevated-temperature ductility; however, the morphologies and compositions of the carbides depend upon the alloy, temperature of formation, and prior history. As a result of the great strengths and complexities of the alloys, welding and postweld heat treating have proved to be expensive problems. The chief difficulties are microfissuring (during welding) and strain-age cracking (during postwelding heat treatment). Porosity, hot cracking, and lack of fusion are also encountered unless extra precautions are taken. Microfissuring, or intergranular cracking (probably occurring on cooling after welding) is encouraged by increasing grain size as well as lead, sulfur, phosphorus, zirconium, boron, and rare earth elements. Magnesium, manganese, and silicon are currently under investigation as additions for reducing the tendency towards microfissuring. These fissures cannot generally be detected nondestructively. Strainage cracking increases from alloy to alloy with increasing hardener content and decreases with increasing postweld heating rate. Recent studies with one alloy have disclosed the role of the heat-treating atmosphere in crack initiation. However, it is not known if this is generally applicable. Many users are endeavoring to establish reliable testing procedures that would be of value in both studying and screening material. The methods of interest include hot-ductility testing, testing simulated heat-affected zones, and modifications of the weld circle-patch and the Varestraint tests. Insufficient correlation has been developed between aged parent-metal mechanical properties and weldability.
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