Quantitative evaluation of porosity effects in sintered and heat treated high performance steels

Abstract

Many heavy duty components suitable for production by powder metallurgy (PM), such as transmissions and also safety parts, require mechanical properties characteristic of high density. Therefore, the ability to produce PM parts of high density has the potential to expand the use of sintered components beyond its present limitations. The purpose of the present study was to investigate the features of pores, to establish quantitative correlations under different sintering conditions and to compare the effects of heat treatments such as case hardening on the pore distribution in the as sintered state. An approach involving quantitative analysis of the upper 10% of the pore size distribution was also employed because these bigger pores have the greatest influence on mechanical properties (especially the dynamic ones). Pore area, maximum length and roundness were analysed for compacts of several densities. A comparison of average values was made to detect differences in pore characteristics among densities, sintering temperatures (1120 and 1250°C) and heat treatments. Results are discussed on the basis of tensile strength values. It has been possible to establish relationships between pore populations and sintering conditions or heat treatment. In general, high velocity compaction was found to give smaller and rounder pores than conventional pressing. Higher sintering temperatures produced more rounded pores, but there was no obvious effect of small variations in sintering temperature during processing on the pore characteristics. Carbonitriding and similar heat treatments can produce closing of surface pores and promote diffusion, leading to slight changes in the inner pore distribution.

Keywords:

• HIGH DENSITY STEEL
• POROSITY EVALUATION
• QUANTITATIVE ANALYSIS