Hot isostatic pressing or HIP has been used for diffusion bonding, casting densification, and powder consolidation. Continuous advances in HIP equipment design have allowed increasingly rapid cooling, recently reaching a point where true high-pressure gas quenching is now possible within the HIP unit. This capability further enables the integration of a heat treat and HIP processing. Within the heat treat industry, high pressure gas quenching has been an area of significant development, however, where typical high pressure gas quenching equipment offers quench pressures up to 15 or 20 bar, common HIP pressures are 1000 bar or higher. The ability to quench from HIP pressures appears to offer heat treat options not previously available. This paper examines ultra-high pressure gas quenching (from 1500 bar) within the HIP unit from a heat treating point of view using AISI 4140 steel, a well characterized, medium hardenability alloy, comparing the properties and microstructure of ultra-high pressure gas/HIP quenched steel to conventional water and oil quenched results.

Transient liquid phase diffusion bonding was used to join stainless steel 304 with pure copper and aluminum foils as interlayers. The bonding process was conducted in a vacuum furnace at various temperatures and diffusion times. The joints were analyzed using optical and scanning electron microscopy, energy dispersive spectrometry, and microhardness measurements. Results indicated that the hardness of the bonds formed with the copper interlayer in a vacuum was higher than those formed with the aluminum interlayer. The poor mechanical properties of the bonds were attributed to the formation of intermetallic compounds within the bond region. Prolonged holding of the parent alloy at the bonding temperature likely led to complete isothermal solidification. The diffusion of the main elements from the interlayers into the base metal at bonding temperatures was the primary factor influencing the microstructural evolution of the joint interface. Selecting an appropriate bonding temperature to achieve the maximum concentration of melting point depressants depended on the duration of isothermal solidification. To assess the corrosion resistance of the joints, Tafel tests were conducted in a 3.5% NaCl solution. The presence of eutectoid γFe + eutectic Cu + Cr and Fe-Al intermetallics was detected at the interface of the joints bonded with copper and aluminum interlayers, respectively. The highest microhardness was observed in the diffusion zone, with hardness values gradually decreasing as the distance from the joint increased. The joints involving stainless steel and copper exhibited crevice corrosion due to the galvanic couple between the stainless steel and copper. Additionally, pitting occurred due to intergranular stress corrosion cracking on the copper surface.