Research On The Microstructure And Abrasion Wear Of Open Arc Austenitic Hardfacing Alloys

Austenitic hardfacing alloys with high strength and excellent toughness are suited to be used for the wear-resisting materials of machine parts in the impact working conditions. The addition of those strong carbide forming elements such as Nb, V and Ti lead to the in-situ precipitation of MC-type carbides in autenite grains and it result into the refinement of austenite matrix and the improvement of the toughness. In this paper, Cr-Mn-Nb-W-V-Si-Ti complex austenitic wear-resisting hardfacing alloys on Q235 A base metals were deposited by the method of flux-cored wire self-shielded open arc welding.The effects of the addition of WC particles, rare earth ferro-silicon alloys, hardfacing current and post weld heat treatment on the microstructure and the abrasion resistance were investigated by optical microscopy(OM), X-ray diffractometer(XRD), scanning electron microscopy(SEM), energy dispersive spectrometer(EDS) as well as wet sand rubber wear tests.Firstly, the effects of WC particle on on the microstructure and the wear resistance were investigated. The results show that with increasing WC content in metal-cored wire, the size of austenitic grain decreases and the volume fraction of intergranular complex carbides increases. The in-situ(Nb,Ti,V)C grains precipitated from primary γ-Fe phase as well as the residual WCx grains play a role of dispersion strengthening on the intra-crystalline of γ-Fe.(Nb,Ti,V)C and M6C(M=Fe, Cr, Mn, V, W) grains in the cellular γ-Fe inter-granular colonies intercepted intergranular M7C3 phases in reticular or dendritic shape and make them refine and discontinuous,which can improve the toughness of hard-facing alloys and decrease the effect of intergranular carbide increasing. The results of hardness measurement and abrasion testing showed that the wear mass loss of these austenitic hard-facing alloys with the bulk hardness of only 40-47 HRC were lower than those high-chromium cast irons. With the increasing of WC particles, the microhardness gaps between intra-austenitic grains and inter-austenitic grains decrease markedly and tend to uniformly wear on surface, which can improve the wear resistance.Secondly, the effects of medium carbon ferromanganese(MCFM) and rare earth ferro-silicon alloys(REFSA) on the microstructure and the abrasion resistance of Cr-Mn-Nb-W-V-Si-Ti complex open arc hardfacing alloys were studied. The results show that with increasing MCFM content in metal-cored wire, the macrohardness rises up continuously and the wear mass loss is firstly reduced and then increases. At the addition of 17.5% MCFM, the grains of γ-Fe phases are the finest and it behaves the best the abrasion resistance. With the increasing of REFSA content, the grains of γ-Fe phases are refined due to the increasing volume fraction of in-situ precipitated(Nb,Ti,V)C grains and residual WCx grains in the intra-crystalline of those γ-Fe matrix. The distributing states of intergranular M7C3 carbides change from the reticular or dendritic shapes into discontinuous or isolated ones and their volume fraction is reduced. At the addition of 6% REFSA, the volume fraction of γ-Fe phases is the highest and it behaves the best the abrasion resistance. Then, γ-Fe phases become coarse and the abrasion resistance deceases.Then, the effects of hardfacing current on the microstructure and abrasion resistance of those open arc hardfacing austenitic alloys were investigated. With the increasing hardfacing current value, the results show that the macrohardness rises up firstly and then decreases and the abrasion resistance is improved firstly and then descended. γ-Fe phases become coarse and the volume fraction of intergranular carbides and in-situ precipitated(Nb,Ti,V)C grains increase with the increasing current value. At 350 A of hardfacing current, the volume fraction of γ-Fe phases is the highest and nd it behaves the best the abrasion resistance.Finally, post weld heat treatment on these hardacing austenitic alloys is accomplished. With the increasing of heat treatment temperature, the volume fraction of γ-Fe phases increases firstly and then drops dowm, but the volume fraction of intergranular carbides behave a reverse law. At 1150℃, the volume fraction of γ-Fe phases is the highest but their abrasion resistance behaves the best.The above results all show that the in-situ(Nb,Ti,V)C grains precipitated from primary γ-Fe phase as well as the residual WCx grains play a key role on the abrasion resistance. The dominating wear mechanism of hardfacing austenitic alloys is micro-cutting and they adapt to be used in the abrasive particle working condition with certain impact load.