Effects Of TiC On The Microstructure And The Properties Of High Chromium Hardfacing Layers

The Preparation of wear parts with flux cored wire open arc self-shielded arcwelding has a good economy, reliability and high deposition efficiency, so it has beenwidely used in the manufacture of wear parts such as concrete pipes, coal grindingroller. However, the currently commercially available high-chromium alloys are sobrittle that the cracks are easy to initiate and flake off. It results in the failure of thosewear parts maily for the reason of the apperance of excessive dendritic-like ornetwork-like carbides. In view of this situation, the optimization of a serie ofFe-C-Cr-Si-Ti system flux-cored wire composition was employed to improve themorphology of carbides. Morever, the buffer and transition layers on base metals werealso used to reduce the welding residual stress of the hardfacing alloy layers.Firstly, the effects of carbon component added with different mode on themicrostructure and the properties of high chromium surfacing alloy were researchedby optical microscopy （OM）, scanning electron microscopy （SEM）, X-ray diffraction（XRD） and micro-hardness tester. Due to the dilution effect of the base metald, meshor dendritic carbide volume fraction significantly reduced until it disappears with theaddition of TiC. But when TiC increased to5%, the weld surface residue was purifieddeterioration in process performance. The results of wet sand rubber wear tests andthe analysis of worn morphology indicated that with the content of TiC increased, thewear-resistance of the alloy surfacing improvement,4%was the best choice, and wearmorphology analysis showed that the dominating wear mechanism of open arcsurfacing alloy was micro-spalling.In view of open arc welding condition, TiC particles were heated to decomposeinto Ti and C atoms, Ti atoms were so easy to oxide or solid solute in the matrix andform excessive slag on the surface.The effects of up to16%TiC particles on themicrostructure and properties were researched by trying selection of submerged arcwelding method to deposited high chromium alloy layer. The results showed that,with increasing TiC content, the number of primary M₇C₃has increased and becomemore dispersed, but the size was reduced. The eutectic （-Fe+M3（B, C）） volumefraction decreased, while the secondary of M₇C₃phase particles increased. And thewear loss decreased firstly and then increased and finally decreased, It was due to thatthe primary M₇C₃phase particles decreased primally, and then the secondary M₇C₃phase particles increased. In particular, when10%TiC is added, hexagonal primary M₇C₃aggregates are appeared in the hardfacing alloys, which were significantlydifferent from the conventional high chromium cast iron. It was carefully studied tobe dirently originated from the primary M₇C₃grains contained in high carbonferrochrome particles.Secondly, due to the open arc surfacing welding was a non-equilibrium rapidsolidification process, different component granularity and adding mode meansdifferent metallurgical reactivity, which affected phase morphology significantly. Thecomponent granularity effects of carbon-containing component such as graphite, B₄Cand Fe-Si on the open arc surfacing alloy microstructure and wear resistance as wellas surface residue slag were studied in this paper. The results showed that ultrafinegraphite was easy to oxide into such as CO or CO₂gas, which enhanced the protectiveeffect on molten weld pool at the open arc condition. The transition coefficient ofcoarse graphite particles was much higher and strenghthen the weld alloys, but it ledto more heavy welding spatter and the self-generated protection gas was insufficient.The results showed that the comprehensive effect was the best when thecarbon-containing component was set as （4%silver flake+4%mud） graphite.Combined carbon component B₄C was easy to decompose at elevated temperature,the released atoms such as B can intensify self-deooxidization of the melts andimprove the fluidity, which provide a better contour of the weld beads. Carbon atomscan be oxidized to form a self-shielded gas and reduce the residue on the surface ofweld beads. The experimental results also showed fine ferrosilicon particles couldimprove the fluidity of the melts and raised the size of the primary M₇C₃phases.Therefore, the selection of appropriate component is very important for enhanceingthe deoxidation ability of open arc surfacing alloy and optimizing the morphology ofthose primary carbides.Finally,20Cr2Mn12NiMoN plastic buffer layer and transition layer between thebase metals and the wear layer containing TiC particles were prepared. The analysison the microstructre such as the interface and the hardness curve showed that it canreduce the dilution effect on wear-resisting alloys at one hand. It was favourable forthe stability of hardfacing wear-resist layer. On the other hand, the hardnessdistribution gradient of the whole hardfacing layers can be effectively reduced andrefrained from the concentration of the residual stress.