Novel approaches for achieving full density powder metallurgy steels

Doctoral thesis, 2019

Powder metallurgy (PM) is one of the most resource-efficient methods for manufacturing structural components with complex shapes. The utilisation of the metal powder to shape the components allows to minimise material waste and increase energy efficiency. However, with increased usage of PM parts in high-performance applications, there is a demand for components that can withstand extreme loading conditions with properties being equivalent or better than those of their wrought counterparts. The PM steel components fabricated through press and sinter route, even with all their advantages, have limitations due to the presence of residual porosity. Hence, it is desirable to reach full density to meet the highest performance demands. This study covers different powder consolidation approaches for water atomised steel powder with the aim of reaching near full density. This is achieved through the following processes: cold isostatic pressing (CIP) followed by sintering, liquid phase sintering (LPS), double pressing-double sintering (DPDS). These approaches were complimented by capsule free hot isostatic pressing (HIP) to reach full density.

Densification and subsequent enhancement of mechanical properties are to a certain extent directly connected to the successful removal of the surface oxide layer, covering the metal particles. This behaviour is especially critical in the case of powder pre-alloyed with oxygen-sensitive elements as chromium. The hydrogen in the sintering atmosphere reduces most of the surface iron oxide layer and any oxide residues are transformed into more stable oxides rich in Cr and Mn. Vacuum sintering provides oxide reduction through the formation of better local microclimate in the pores. When the powder is encapsulated and processed using HIP, the initial surface oxide is transformed into stable oxide particles that decorate the particle boundaries. Based on these results a model of oxide transformation during powder consolidation is proposed with regards to the alloy composition, powder properties and processing conditions.

In order to realise full density, CIP is utilised for consolidating iron powder and Cr-Mo pre-alloyed water atomised powder to reach a relative density of around 95% in sintered state to attain surface pore closure. This allows for subsequent HIP without capsule to reach full density. In case of Mo pre-alloyed powder, the LPS approach utilising Ni-Mn-B master alloy was established for enhanced sintering and densification. The best mechanical properties were then obtained with 0.12 wt.% of boron that allowed reaching as-sintered relative density of up to 96%. In addition, pore free surface was obtained after sintering that enabled capsule-free HIP to reach full density. Through the DPDS process, a pore free surface could also be achieved, which enabled reaching full-density through the subsequent HIP. Even though fine powder showed better densification, the density gradient in the compact persisted from the first pressing is there as the low-density region i.e., neutral zone, in the middle of the compact even after second pressing and HIP. Hence, optimisation during the first pressing is necessary to avoid this phenomenon.

All the above approaches represent different methods of achieving full density and selection of the appropriate method depends on the required geometry, alloy composition and hence resulting properties, number of components, cost, etc. Based on the analysis of the different methods it can be concluded that the combination of the tailored alloy concepts and consolidation techniques allows manufacturing of complex-shaped full-density PM components for high-performance applications.