Improvement of Nuclear Castings by Application of Hot Isostatic Pressing (HIP). Final Report

摘要

A three-part team of foundries assess coal the benefits of hot-isostatic processing (HIP), a pressure heat treatment, as applied to cast stainless steel (CF-8 and CF-8M) valve components destined for nuclear power plant service. The basic goal was to establish the level of quality improvement obtained when pores, tears, and other flaws are eliminated from these castings. Improvement was expected and achieved in mechanical properties, inspectability, and weld quality. Improved properties enhanced reliability of the HIP components and the power station in which they are installed. A treatement of 1093 exp 0 C (2000 exp 0 F) at 103 MPa (15,000 psi) closed and bonded hot tears and gas porosity in all castings evaluated where the as-cast defect was isolated from the high pressure argon used to cause densification. Section thickness in excess of 7.6 cm (3 in.) were successfully healed. As-cast defective zones possessed tensile properties equal to sound metal after HIP. No microstructural degradation was noted and ultrasonic transmission was generally improved after the HIP upgrade. A substantial fraction of the defectsected defects present in as-cast stainless steel are connected to the surface by inter-granular paths. Experiments confirmed that these paths permit argon to fill the connected defects and no closure can occur. Based on the results of this invetigation, three developments were recommended to assist full implementation of HIP to valves and other steel castings: a sealing method to isolate all flaws from the surface, additional property data from healed gas porosity zones, and a statistical mechanical property base to support code acceptance.