Effect of aluminizing and austempering processes on structural, mechanical and wear properties of a SSF ductile iron

The aim of this study is to investigate the effect of aluminizing process on structural, mechanical and tribological properties of GJS 600-10 solution strengthened ferritic (SSF) ductile iron before and after austempering heat treatment. The as-cast samples were coated by hot-dip aluminizing (HDA) at 750 °C for 5 min. For austempering heat treatment, the as-cast and aluminized samples were austenitized at 975 °C for 2 h and austempered (ADI) at 330 °C for 1 h, which were referred as ADI and HDA + ADI samples, respectively. Structural characterizations made by scanning electron microscopy (SEM), energy dispersive x-ray spectrometry (EDX) and x-ray diffraction (XRD) analysis revealed that single HDA process produced a coating of FeAl₃ and Fe₂Al₅ layers with an Al topcoat on the surface. By the application of austempering after HDA, as-aluminized coating layer transformed into Fe₂Al₅ + FeAl₂, FeAl and α-Fe with Al₂O₃ scale on the surface. Tensile tests conducted at room temperature indicated that single HDA does not significantly affect the mechanical properties of the as-cast ductile irons, whereas application of austempering after HDA decreases strength and increases ductility in comparison to ADI samples. Dry sliding wear tests performed against Si₃N₄ ball at room temperature showed that HDA samples exhibited much worse wear resistance than the as-cast samples. However, wear resistance was increased about 30% when austempering is applied after HDA. Formation of various oxides on the worn surfaces of the samples was studied by Raman spectroscopy and EDX analysis. Test results were comparatively evaluated on the basis of surface and subsurface characteristics of the investigated ductile irons, and showed that increased wear resistance can be obtained when HDA is followed by austempering for the ductile iron used in this investigation.