With the code case approval for Hot Isostatic Pressed 316L there has been much focus on the performance of the powder metallurgical material and how it compares to conventional forged or cast material. Owing to the very fine micro structure of the HIPed material it generally displays a higher strength compared to the conventional material and surpasses the requirements for UNS S31603. However there is a discrepancy in the materials behavior regarding impact toughness. The conventional material exhibits only little drop in impact toughness with decreasing temperature. The HIPed material however, exhibits a drop in impact toughness below a certain temperature.
It is well known that the oxygen content influences the impact toughness of the HIPed materials. This is likely a result of the precipitation of small oxide particles in the microstructure. These small oxide particles are often precipitated along Prior Particle Boundaries (PPB) and can have a negative effect on the impact toughness of the HIPed and heat treated material. Furthermore, the HIP-parameters used for consolidating the powder can also greatly influence the mechanical properties of the material. This study has investigated the mechanical properties of three batches of HIPed 316L and conventional rolled bar material by the use of tensile testing and instrumented impact testing for separating crack initiation energy and crack propagation energy. Tensile testing has been performed in situ in SEM to study the influence of the oxides and the fine microstructure on the failure of the material during tensile loading. In addition to this the influence HIP parameters on mechanical properties of the material have been investigated.
This study highlights the differences seen between the HIPed and conventional material with regards to impact toughness, tensile strength as well as microstructure. Furthermore, the results clearly show a clear dependence between the HIP parameters and the mechanical properties of the HIP and heat treated material.