Microstructure And Wear-resistance Of Fe-C-Cr-Nb-B Open-arc Hardfacing Alloys

According to grinding abrasion theory, the increasing material hardness cangreatly improve abrasion resistance when the ratio of the material hardness and thehardness of the abrasive grains is above0.5-0.8. For example, the alloy hardness up to60HRC can easily satisfy it when quartz sands are acted as abrasive particles. For thesereasons, Niobium-containing components such as Fe-Nb were selected to add into thealloy with high hardness based on Fe-C-Cr-B hardfacing alloy system and NbC phaseswere in-situ precipitated to markedly increase the bulk hardness, and so upgraded theabrasion resistance. A series of Fe-C-Cr-Nb-B hardfacing alloys with the bulk hardnessof67-68HRC were successfully prepared to make sure that the workpiece can keepoperating smoothly and permanently in aggressive abrasive wear conditions such asmining, metallurgical industry.Fe-C-Cr-Nb-B hardfacing alloys were deposited on low carbon steel Q235substrates by the method of flux-cored wire self-shielded open arc welding. Themicrostructure, micro-area chemical composition, property as well as abrasionresistance and wear mechanism of hardfacing alloys were investigated by opticalmicroscopy (OM), X-ray diffractometer (XRD), Rockwell hardmeter andmicro-hardness tester, scanning electron microscopy (SEM) together with energydispersive spectrometer (EDS).Firstly, the effects of Fe-Nb on the microstructure and abrasion resistance ofFe-C-Cr-B hardfacing alloys were studied. The experimental results show that, with theincreasing addition of Fe-Nb powders, the microstructure of the deposited alloyschanges from a eutectic texture into a hypoeutectic one following by the precipitationof primary NbC phases. Its bulk hardness increases from64HRC to69HRC. For thoseNbC grains, the shape of dot-like, strip-like, block-like and tree-like is respectivelyobserved form the fusion-line interface to the deposited metal surface. Themicro-hardness of those NbC grains gradually rises up to2290HV0.1. Wear mass lossand surface worn morphology of hardfacing alloys indicate that pro-precipitated NbCgrains significantly improve their abrasion resistance and their abrasion resistance isdetermined by the size, the distribution and the volume fraction of NbC phases. Thepart of Fe-Nb powders was replaced by Fe-V ones during the test, it showed that, NbCgrains refined slightly but became more dispersed, while excessive V content can make the C atomic concentration and the hardness of the matrix decrease, these matrix werepreferentially abraded due to low resistance, so its wear mass loss ΔM firstly decreasedand then increased.Then, keeping Fe-Nb powder constant, the effect of boron-containing componentssuch as B4C, Fe-B on the welding processing properties and the microstructure as wellas the abrasion resistance of Fe-C-Cr-Nb hardfacing alloys were investigated. Theresults showed that, B4C and Fe-B had improved its self-shielded performance and theshapes of weld beads. With the increase of B content, its bulk hardness increases from45.5HRC to68HRC, austenite transform into martensite. The wear loss ΔM decreasedgradually was mainly due to the increasing intercrystalline M23(C,B)6volume fraction.The dominanting abrasion mechanism changes from the micro-cutting into thecoupling micro-cutting and micro-fracturing gradually.Finally, keeping Fe-Nb and boron-containing component constant, the effects ofFe-Mo and Fe-Cr powder on the microstructure and the abrasion resistance ofFe-C-Cr-Nb-B hardfacing alloys were analyzed. The experimental results showed that,with increasing of Fe-Mo content, the volume fration of the eutectic (α-Fe+M3(B,C))decreased and their size reduced, but the volume fraction of intercrystalline M23(C,B)6raised, it led that the wear loss ΔM decreased. Due to the reason that Mo is a solutionstrengthening element in chief, excessive Mo induces the increasing of the volumefraction of γ-Fe, and then the bulk hardness and the wear mass loss ΔM keepunchanged with the increasing addition of Fe-Mo powders, but it is observed thesurface tend to more uniformly wear. It indicates that only a small amount of Fe-Mopowder can improve the abrasion resistance of hardfacing alloys. Moreover, with theincrease of Fe-Cr powders, the size of cellular matrix and intercrystalline tree-like orreticular carbide M7C3, M23(C,B)6were decreased, NbC grains become more dispersed,the wear resisting framework which is made up of intercrystalline hard phases getsincreasingly intensive. It enhanced the inhibition for the plunging into the surface ofabrasive particle, so the bulk hardness increased and ΔM decreased.