Study On Electroslag Surfacing Process And Microstructure Properties Of High Chromium Cast Iron Hardfacing Layers

High-chromium cast iron?HCCI?contains a large amount of high-hardness M₇C₃type carbides.It has excellent wear resistance and is widely used in harsh working conditions such as petrochemicals,mining machinery,and iron and steel metallurgy.However,the HCCI has low toughness,poor mechanical processing performance and weldability,which limits the application.Combining HCCI with low-alloy steel?LAS?and utilizing the high wear resistance of HCCI and excellent plastic toughness and processability of LAS can effectively overcome the disadvantages of HCCI and extend the life of workpiece.In this paper,HCCI hardfacing is deposited on LAS by electroslag surfacing to form bimetallic material.Through the experiments,the influence of process parameters on the stability of the electroslag surfacing process and the forming quality of the surfacing layer was systematically studied;a special sintered flux suitable for HCCI electroslag surfacing was developed;The temperature field and thermal cycle characteristics and its effect on the microstructure and performance was studied;The effect of V alloying on the microstructure and properties of HCCI hardfacing layers was studited;A heat treatment method that can significantly improve the wear resistance of electroslag surfacing hypoeutectic HCCI hardfacing layer was developed.The main research results are as follows:The heat input of electroslag surfacing is large.When the HCCI hard surface is deposited on the LAS D32 by electroslag surfacing,the temperature distribution of the workpiece is uniform and cracks can be avoided;The heat input has a great influence on the quality of electroslag HCCI hardfacing layer.If the heat input is too high,the LAS substrate will be over-melted,and if the heat input is too low,slag inclusion and unfusion defects will occur at the interface.When the energy density of heat input is8.5⁹.5W/mm²,the electroslag surfacing method can effectively compound the high-thickness CCI hardfacing layer on the LAS.The surfacing metal width and fusion ratio are mainly determined by the voltage parameters.Under the appropriate surfacing heat input,moderately reducing the surfacing voltage will help reduce the fusion ratio,obtain uniform microstrucutures and properties.When the composition of CaF₂-CaO-Al₂O₃ sintered flux is 40%CaF₂,20³0%Al₂O₃,17²5%CaO,5¹5%SiO₂,2⁶%MgO,the conductivity,viscosity and surface tension of liquid slag pool are 3.3045?^-1·cm^-1,0.083 Pa·s and 338.456 Nm^-1,respectively.The electroslag surfacing process is stable,the hardfacing layer has no defects,the slag detachability is good,and the process performance is optimal.When surfacing the HCCI hardfacing layer with flux-cored welding wire,the transition coefficients of alloy elements such as C and Cr in the flux-cored welding wire are 83%and 91%respectively,which is suitable for the HCCI electroslag surfacing.During electroslag surfacing,the HAZ of LAS is wider and the time of T_800-500is more than 200 s,so brittle martensite can be avoided in the HAZ.There is an austenite zone with a width of about 50?m at the interface between the HCCI hardfacing and the LAS.The microstructure of the electroslag HCCI hardfacing layer is uniform,and the hardfacing layer has good wear resistance.The 3.6C-20Cr-Fe-?0².32?V HCCI hardfacing layers were deposited on low alloy steel by electroslag surfacing.The 3.6C-20Cr-Fe hardfacing layer contains primary carbides and eutectic.Increasing V wt.%in the hardfacing layer,primary carbides are decreasing by increasing eutectic along with martensite formation.For 1.50 wt.%of V,the microstructure contains a lot of eutectic and a little of martensite.For 2.32 wt.%of V,primary austenite formed,the microstructure is primary austenite,eutectic and a little of martensite.In the V alloyed hardfacing layers,V has strong affinity with carbon than chromium,hence V can replace a part of Cr in M₇C₃ and?Cr,Fe,V?₇C₃ type carbides are formed.When the V is 2.32 wt.%,(Cr_0.23V_0.77)C carbides present in the hardfacing layer.The hardness and wear resistance are improved by increasing V from 0 to 1.50 wt.%.However,when the V is 2.32 wt.%,the primary austenite has reduced the hardness and wear resistance of hardfacing layer.Hypoeutectic HCCI hardfacing layer is developed by Electroslag surfacing.When the heat treatment temperature is 440?,the microstructure of hardfacing samples exhibite little change;When heat treatment temperature increases from 480 to 520?,precipitation of secondary carbides M₂₃C₆ and local martensite transformation in austenite matrix has hardened the hardfacing layer;However,the decarburization in martensite and nucleation of pearlite at 600?weakes the hardness of hardfacing metal significantly.During the sliding process,micro-cracks appeares on the wear surface and extendes into the hardfacing metal;the crack-face oxidizes for the temperature raising caused by repeated sliding.The cracks extension and oxidation on crack-face cause metal stripping and continuous wear of HCCI hardfacing layer.The wear rate of HCCI hardfacing decreases first,reaching the minimum for heat treatment at 520?and then increases with the increase of heat treatment temperature.The variation of friction coefficient shows inverse variation trend to wear rate.