Qualification of Hot Isostatic Pressing Processes

Hot isostatic pressing (HIP) is a process in which material is subjected to controlled temperature and isostatic gas pressure, such as argon gas.  The theory behind this process is to create a hydrostatic pressure in the vessel high enough so that any internal porosity (not surface connected) will collapse. While the effectiveness of the HIP process may be evaluated with actual product, the population may not contain “worst-case” defects. It is therefore of greater interest to use a test sample designed to simulate a worst-case scenario with an exaggerated internal void.  The test sample is made of the alloy of interest, for example cast CoCrMo (ASTM F75) with a counter- bored hole in the center, and a wrought CoCrMo (ASTM F1537) cylindrical pin that is vacuum welded after press fit into the hole.  A cavity is created within the counter-bore between the bottom of the pin and the surrounding base. This cavity size is designed to be approximately 5 or 10 times larger than the smallest detectable void (based on radiography sensitivity of 2-2T) such that this test sample represents a worst-case internal void. An appropriate sample size of the test samples are then subjected to extreme and nominal HIP processing conditions, distributed throughout the HIP vessel. Subsequent solution annealing or other standard thermal processing may be performed. Test samples are then evaluated for closure of the internal void by radiography and residual porosity analysis may be performed by metallographic examination.