Investigation of Low Temperature Creep Behaviour of PM Steels

Artikel i vetenskaplig tidskrift, 2014

The automotive industry accounts for almost 70% of the total use of water atomized steel powder for powder metallurgical structural components. Nowadays, such components are increasingly used for high demanding applications, where good tolerances and high mechanical properties are combined. However, it has been found that some PM-steel components at low temperature (100-150°C) and high static loading may experience dimensional instability. Hence, high performance diffusion-alloyed powder grade was investigated for low temperature creep/relaxation at 120°C and 20 kN tensile loading (corresponding to 90% of the yield strength of the material). The materials investigated were sinter-hardened and subsequently tempered at different temperatures. Characterization using different techniques (optical microscopy, dedicated testing, X-ray analysis, hardness testing, etc.) was carried out before and after creep testing and it was revealed that each kind of sample exhibited creep/relaxation behaviour correlated to the tempering temperature. The results were compared to test results of components under similar conditions and a good correlation between the test bars and components were found. Moreover, it was found that selecting proper tempering considerably lowered the creep/relaxation response. Hence, the dimensional instability at high static loading conditions for the studied powder metallurgical could be reduced.