Net-Shape Consolidation of Water-Atomised and Gas-Atomised Steel Powder Towards Full Density

Licentiatavhandling, 2022

Powder metallurgy (PM) provides resource- and cost-efficient routes to create near-net shape products compared to conventional metal fabrication techniques. To improve the mechanical performance of PM steels, it is a prerequisite to enhance the density of the material. Utilizing cold isostatic pressing (CIP) process allows to isostatically compact and eliminate the otherwise needed organic lubricants used for the powder compaction resulting in homogeneous distribution of density. To reach full density, HIP is an attractive final stage and so in particular if the HIP can be done capsule-free directly on a sintered component. Hence, combining CIP, sintering and capsule-free HIP means that full density processing of novel products can be achieved. Chromium-alloyed PM steels have been developed with the benefit of lower cost owing to the lesser use of alloying elements such as Ni, Cu and Mo. PM steel powder grades contain Fe-rich oxide mostly covering the surface of powder particles. It is of outermost importance to sinter the material prior to final capsule-free HIP to largely remove the surface oxides to eliminate possible transfer of oxygen to trigger added formation of Cr-oxide during the sintering. The test materials used in this thesis study are water-atomised steel powder grades prealloyed with 1.8%Cr and admixed 0.3% graphite and with 2%Ni and gas-atomised Vanadis 4E tool steel powder grade. While Cr-alloyed PM steel powder grade is isostatically pressed directly from virgin powder to densify the green compacts, granules of Vanadis 4E were needed for the this and produced before CIP. Results show that after sintering at high temperatures either in reducing or vacuum conditions for water-atomised steel powder grade, most of the surface pores are eliminated and nominally full bulk density is achieved after capsule-free HIP. A novel approach is also explored investigated to sinter the Cr-alloyed PM steel in HIP furnace and followed by capsule-free to approach high densification. Microstructures of these compacts sintered at 1250 °C revealed the complete closure of surface pores, after which densification to nominally full bulk density has been achieved by the final capsule-free HIP stage. Following the route of CIP, sintering and capsule-free HIP, nominally full density is also achieved for the Vanadis 4E tool steel when CIP compacting the powder in granulated condition.