Liquid phase sintering, heat treatment and properties of ultrahigh carbon steels

Abstract

Thermo-Calc modelling was employed to predict liquid phase amounts for Fe–0·85Mo–(0·4–0·6)Si–(1·2–1·4)C in the temperature range of 1285–1300°C and such powder mixes were pressed and liquid phase sintered. In high C steels, carbide networks form at the prior particle boundaries, leading to brittleness, unless the steel is heat treated. To assist the break-up of these continuous carbide networks, 0·4–0·6% silicon, in the form of silicon carbide, was added. After solution of processing problems associated with the formation of CO gas in the early part of the sintering cycle, and hence large porosity, densities in excess of 7·75 g cc⁻¹ were attained. A spheroidising treatment resulted in microstructures having the potential of producing components, which are both tough and suitable for sizing to improve dimensional tolerance. Yield strengths up to 410 MPa, fracture strengths up to 950 MPa and strains up to 16% were attained.

Keywords:

• Ultrahigh carbon PM steels
• Phase diagram modelling
• Liquid phase sintering
• Spheroidisation

The authors wish to thank The School of Engineering, Design and Technology at The University of Bradford for making facilities available for this work and are grateful to Höganäs (Kent, UK) for supplying the Astaloy 85 Mo base powder. We are particularly grateful to Ms Linda Maude for her technical assistance.