Modelling and simulation of densification and σ-phase precipitation in PM duplex steel AISI 318LN during hot isostatic pressing

Abstract

Following the development of hot isostatic pressing (HIP) with integrated rapid cooling technology, it is now possible to combine consolidation of encapsulated powder and subsequent heat treatment in a single process step in a pressure vessel. Using this technology, a near-net-shape product with target microstructure can be achieved. However, qualified final products still require traditional expensive and time-consuming trial-and-error tests. To eliminate these costly trial-and-error tests and achieve the required final properties of the products, it is urged to develop a combined numerical model to precisely predict the geometrical changes of the component during the HIP process and the evolution of significant microstructural features. This is realised in this work exemplarily by using duplex stainless steel AISI 318LN. This steel was selected due to requirements coming from applications regarding precise complex shaped near-net-shape components combined with high strength and toughness by avoiding brittle σ-phase precipitation. In this study, an existing finite element model for the densification in the HIP process was supplemented by a model for σ-phase precipitation kinetics based on microscopical characterisation. Moreover, the temperature dependent heat transfer coefficient (HTC) was implemented in the developed model to simulate the evolution and the distribution of the σ-phase during cooling. The numerically simulated final component dimensions and σ-phase volume fractions were verified by comparing with experimental data obtained from specimens and components of various shapes produced by both conventional HIP process and HIP with integrated rapid cooling. The high consistency between experimental and numerical results validates the accuracy of the developed simulation approach.