METALLURGICAL PROBLEMS: WELDING CRACKS, LACK OF FUSION AND GAS IN 1025 STEEL AND HEAT TREAT QUENCH CRACKS IN 4140 STEEL

摘要

This paper reviews two different investment casting problems with the same cause: heat treatment atmosphere control. In the first, a customer who welds 1025 Steel investment castings into assemblies found sporadic gas, lack of fusion, and cracks in the welds. There followed a large amount of testing and process changes that were often pointed in the wrong direction. The true cause of the problem, high nitrogen in the outer .003" cast surface, was proposed, measured and corrected. The high nitrogen source was an ammonia leak in a heat treatment furnace used for both atmosphere normalizing and nitriding operations. The second problem: quench cracks occurring during a hardening quench and temper operation in 4140 steel. A small percentage of parts cracked. Many foundry and heat treatment variables were evaluated and analytical work, equipment modifications and heat treat trials were performed. The primary root cause was a thin layer of oxide introduced in the austenitizing phase of the heat treatment operation.