The hot isostatic processing characteristics of 70/30 cupronickel castings.

70/30 cupronickel designated in the Naval Engineering Standard NES 824 is associated with high integrity cast components. However this specification restricts the use of welding to recover castings if they are to be wetted by sea water. Hot isostatic pressing is considered to be an alternative recovery process. The mechanisms of pore closure and the processing parameters for successful recovery have been investigated together with their effect on microstructural and mechanical property characteristics. In addition models have been identified which accurately predict pore closure rates in cupronickel castings.Hot isostatic pressing trials were carried out using processing temperatures within the range 850°C and 1025°C and argon gas pressures of 45MPa to 145MPa. Two time, temperature, pressure procedures were investigated; successive cycles of short duration and single continuous cycles. The investigations were carried out on 70/30 cupronickel (NES 824) specimens that contained artificially introduced porosity in the form of closed internal pores and surface-connected porosity: the latter required encapsulation prior to processing. These investigations have revealed that for the removal of surface-connected porosity, successive cycles of short duration were more effective in closing large pores than a single cycle of the same total duration. In addition elongated pores of small diameter were removed more quickly than pores of equivalent volume porosity but smaller aspect ratio. Shorter recovery times were required for the consolidation of internal porosity in comparison with surface-connected porosity indicating that encapsulation restricts the densification process.Initially the rate of densification during HIPping was rapid suggesting that plastic flow was occurring.Subsequent densification probably by viscous flow mechanisms occurred at a much lower rate once the sustain conditions had been reached. This fall off in the densification rate applied in particular to encapsulated castings.The optimum processing conditions for the successful densification of 70/30 cupronickel castings are short sustain times of 45 minutes, a temperature of 950°C and argon gas pressures within the range 83-103MPa.Hot isostatic processing significantly increases the ductility of 70/30 cupronickel castings without a reduction in strength and has no adverse effects on corrosion resistance.