Steel Castings Handbook. Welding Of High Alloy Steel Castings

STEEL CASTINGS HANDBOOK Supplement 7 Welding of High Alloy Castings Steel Founders' Society of America 2004 Welding of high alloy steel castings by E. A. Schoefer Consultant Steel Founders’ Society of America Edited by M. Blair Steel Founders’ Society of America 1. Introduction Iron-base and nickel-base high alloys - by definition those containing eight percent or more of another element - are widely used for construction of industrial process equipment that must resist the deteriorating effect of a corrosive of high temperature environment. Both wrought and cast forms of such alloys may be welded during the manufacture of finished components so the weldability of the alloys often is a matter of concern to the user. The same welding processes are applied to wrought and cast products and, in general, similar techniques and practices are employed. Differences between wrought and cast alloys in chemical composition and microstructure, however, influence the welding characteristics of each form and must be given consideration. In addition, the high alloys differ markedly from carbon and low alloy steels in physical properties such as electrical resistance, thermal expansion and thermal conductivity. It is essential, therefore, to employ procedures allowing for all these factors when welding high alloy castings. 1.1 All the casting alloys have equal or better weldability than the corresponding wrought alloys, but there are variations from grade to grade in the ease with which satisfactory welds are obtained. The low-carbon, austenitic grades usually are considered easier to weld than high-carbon austenitic or straight-chromium ferritic or martensitic types. Nevertheless, each of the standard alloy compositions can be welded successfully in the foundry. Using information derived from the extensive research of Alloy Casting Institute and Steel Founders’ Society of America, the foundryman often is able to tailor the composition balance especially to provide the optimum weldability. Accordingly, castings are readily welded into fabricated structures and welding is considered a regular part of the foundry production process. 1.2 Welding is used a procedure for upgrading casting quality during the course of manufacture through improvement of surface conditions, or by elimination of shrinkage voids. It is also used for producing large or complex assemblies where the size of the completed structure precludes production as a one-piece castings, or where total quality will be improved by dividing the structure into simpler components which can later be welded int an integral assembly. 1.3 Welds properly made do not impair high alloy castings with respect to their corrosion resistance or their mechanical properties from sub-zero to elevated temperatures. 1 Proven welding techniques that are procedurally correct and metallurgically sound involve consideration of the following factors: a. Characteristics of the alloy type b. Choice of filler material c. Preparation of the weld cavity or joint d. The weld process to be used e. Preweld and postweld heat treatment f. Methods of demonstrating weld quality All of these topics will be covered in subsequent sections of this discussion or in the accompanying welding procedure descriptions. 2. Properties of the alloy types At the outset it is necessary to review the microstructures and the physical and