Effect of diffusion bonded Cu and Ni on sinter hardening response of Mo prealloyed PM steel

Through utilization of hardenable powder in combination with accelerated post sintering cooling rates, sinter hardening offers good manufacturing economy as well as unique combination of mechanical properties. In order to obtain desired mechanical properties, the microstructure can be manipulated to form the required amount of martensite and bainite using suitable alloying system. The aim of the current study is to outline the sinter hardening response of Astaloy Mo based PM steels. For this purpose and to elucidate the effect of the alloying, together with Astaloy Mo (prealloyed with 1.5% Mo), Distaloy DH (diffusion bonded with 2 wt. % Cu) and Distaloy DC (diffusion bonded with 2 wt. % Ni) were examined. For each alloy tensile samples were manufactured by sintering at 1120°C for 30 minutes and cooling at a rate of 2.5 °C/s (within 800 to 300 °C). Afterwards, the tensile specimens were subjected to a tempering treatment at 200 °C for 60 minutes. Distaloy DH and Distaloy DC with two different levels of admixed graphite content (i.e. 0.55 and 0.7 wt. %), and Astaloy Mo admixed with 0.55 wt. %C have been investigated for tensile properties, hardness, microhardness, porosity and residual stresses. Metallographic examination using optical microscopy along with scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDX) has been performed in an attempt to relate the results obtained from the mechanical testing to the microstructure of the alloys. Regarding tensile properties, Distaloy DH was seen to be slightly better than Distaloy DC while Astaloy Mo possessed the lowest values. Increase in graphite content did not contribute to a significant increase in tensile strength; however, it resulted in a decrease in elongation for both Distaloy DC and Distaloy DH. These findings were related to the fully bainitic microstructure of Astaloy Mo as compared to a bainitic-martensitic microstructure of Distaloy materials. A correlation between the microhardness and the effect of distribution of alloying elements has been proposed, with which the microstructure development can be explained. Regarding surface residual stresses, Astaloy Mo had a compressive surface stress state, while for the other materials it was tensile but with very low values. For all the materials analyzed, porosity analysis revealed that, for a particular size range or pores their shape was similar. In conclusion, the investigation presents a comparative study on the effect of addition of 2 wt. % Cu and Ni (to Astaloy Mo) on the microstructure and the resulting influence on the final mechanical properties