FABRICATING NICKEL ALLOYS TO AVOID STRESS RELAXATION CRACKING

摘要

Nickel-chromium alloys for service at elevated temperatures are generally solution annealed prior to service to coarsen their grain size to enhance their high temperature strength and resistance to creep and rupture. Alloys for high temperature are often supplied from the producing mill as solution annealed. However, this large grain condition is not optimum for fabrication as it can compromise weldability and the large grain material is subject to stress relaxation cracking when annealed after heavy cold forming. Ideally these alloys are supplied after a lower temperature heat treatment that anneals the product but avoids substantial grain growth. Such fine grain material is more readily formed and much more resistant to stress relaxation cracking when solution annealed after forming. While the example of alloy 800HT is used to demonstrate the advantage of this processing sequence, the technique can be applied to essentially any alloy or stainless steel that is to be used for applications at highly elevated temperatures.