Towards Establishment of Weldability Testing Standards for Solidification Cracking

Abstract

The establishment of standards for weldability testing would be desirable to both industrial and academic research laboratories. This would have the obvious advantage of allowing data to be reliably compared between different research labs. But making decisions regarding standards requires some careful thought and agreement on (i) how test parameters affect test results, (ii) what exactly needs to be measured, and (iii) how test results should be interpreted and reported. Our depth of understanding on these points has matured significantly over time and while there is not always universal agreement, it is at least possible to identify factors important to standards. This paper examines these factors, including: 1. Welding Parameters. When comparing different alloys having different thermal characteristics, the use of constant welding parameters (common practice) will result in variable weld penetration and weld pool shape. This can influence grain shape and mushy zone size, which can result in inequitable weldability comparisons. 2. Restraint. Welding on test coupons having different dimensions can affect restraint, which will influence the strain fields around a moving weld pool. Variation in test fixtures may also affect restraint. High restraint does not always result in higher crack susceptibility. 3. Travel Speed. Use of high travel speed gives tear-drop shaped weld pools that are more susceptible to cracking, and it reduces the size of the mushy zone. However, high speed can also result in grain refinement in certain alloys, giving improved weldability. Speed can also shift the location of compression/tension regions behind the weld pool. 4. Rate of Loading. Extrinsic tests, such as the Varestraint Test, involve the controlled application of a strain during welding. The rate of loading, relative to the weld travel speed, can influence crack length due to crack extension during loading. 5. Cracking Index. Selection of an appropriate cracking index is required for data analysis. Quantification of crack length and brittle temperature range are common indexes used for comparison. Threshold strain and critical strain rate are additional indexes. How well these indexes actually represent weldability remains unclear. This paper will examine and quantify these issues in detail, thus providing the reader with a comprehensive appreciation of all things that must be considered when preparing a standardized procedure for weldability testing.