The effects of physical and mechanical properties of surface hardening treatments at ferrous based P/M parts

In this study, the effects of boronizing, shot peening+boronizing or boronizing+shot peening on the mechanical properties of low carbon P/M parts were investigated. Boronizing treatment was carried out at 950 °C for 4 hours for all treatment combinations. The S110 and S230 steel balls with HRC 44 - 45 hardness were used in shot peening process.To determine peening intensity, Almen A sheets were used. The determined peening intensities were applied to samples. The saturation ratio was 100 % in shot peening process. Samples used in this study are produced using 96 % Höganas ASC 100.29 grade iron powder, 0,2 % C, 3 % Cu and 0,8 % Zn - Stearate by weight. A series of mechanical tests including tensile, three point bending, wear, fatigue, microhardness and layer tickness determination were applied to samples. Then, inner structural properties, wear and crack surfaces of the samples were examined by SEM. The XRD analyses were carried out to determine new compounds on the material surface. Wear and microhardness distrubution graphs and S - N line graphs were drawn. The comparative effects of boronizing, boronizing+shot peening or shot peening+boronizing on fatigue behavior of the samples were investigated. Wear resistance was significantly improved by boronizing treatment. The boronizing+shot peening or shot peening+boronizing treatments reduced wear resistance more than boronizing treatment alone. Microhardness was found to be 2400 HV with boronizing treatment. In the light of metalographic observations, shot peening damaged bor layer if used after boronizing treatment. A single layer Fe2B phase was observed in groups 2, 3, 4 and 5 whereas a double FeB and Fe2B phase was observed in groups 6, 7 and 8. A double layer phase is not a desirable property due to layer separation on the material surface. The peening intensity should be kept low to avoid structural damages on the surface if boronizing is applied with shot peening and boronizing should be applied after shot peening to decrease density increases on the surface. Moreover, a double layer phase on the material surface should be avoided.