Cr15-high Chromium Cast Iron Sintered By Two-stage Supersolidius Liquid Phase Sintering And Welded With Low Carbon Steel By Gas Tungsten Arc Welding

High chromium cast iron(HCCI)is widely used in cement,mining,electric power,construction machinery and other industries because of its excellent wear resistance and good toughness.The supersolidius liquid phase sintering(SLPS)can provide a unique thermodynamic condition for microstructure evolution of HCCI in comparison with the casting process,which improves the nucleation rate of carbides,refines the grains and obtains fully densified products.Sintered HCCI has double strength&impact toughness in comparison with cast HCCI,and the hardness of as-sintered HCCI can meet the hardness requirement of hardened cast HCCI.However,one-stage SLPS with a fixed sintering temperature still has the problems of narrow sintering window and mechanical property's sensitivity to temperature uneven distribution in a sintering furnace,which limit the application of SLPS in HCCI fabrication.In this study,a two-stage SLPS process for HCCI was proposed,the common one-stage SLPS was divided into the high-temperature stage(HTS)and the low-temperature stage(LTS).The sample was firstly sintered at a HTS for a short time to be densified efficiently,and then held at LTS for a long time to hinder microstructure coarsening and let carbides fully precipitated.After microstructure observation and testing mechanical properties of two-stage sintered HCCI,the following conclusions were drawn:(1)The full densification could be realized at HTS without LTS.The sintering window of two-stage sintered SLPS is 20?(1225?1245?).There were mainly M₇C₃carbides,martensite and small amount of retained austenite in the alloy prepared by two-stage sintered SLPS.The alloy's microstructure was gradually coarsened with temperature or holding time increasing in the range of 1225?1245?and 5?15min at HTS respectively.With the introduction of LTS,the carbides fully precipitate and the content of residual austenite increases.The microstructure coarsening rate of two-stage sintered SLPS was significantly lower than that of one-stage sintered SLPS.(2)The hardness and bending strength of two-stage sintered SLPS increased firstly and then decreased with the increase of temperature at HTS,and decreased with the extension of holding time,and its impact toughness was quite stable with the average of 12.10J/cm²as temperature changed.The hardness,impact toughness and bending strength of two-stage sintered SLPS HCCI are better than those of one-stage sintered SLPS HCCI at the same sintering temperature.(3)The alloy without LTS had higher martensite content and finer grains,so it's hardness and bending strength were higher,impact toughness was low.The introduction of LTS made the grain grow up,the content of retained austenite increased,and the carbide fully precipitated,which made the hardness lower but still maintain above 61HRC.The hardness and impact toughness of the alloy increased with the increase of low temperature,but the bending strength decreased first and then increased.(4)The hardness,impact toughness and bending strength of HCCI sintered at1235?×10min+1190?×80min are 11.93J/cm²,61.60HRC,2279.4MPa,respectively.The relative wear resistance of the specimen under 1,2 and 3J/cm²impact energy for 1h is 5.87,4.80 and 5.21,respectively.High carbon equivalent and sensitivity to stress concentration cast HCCI lead to its poor weldability,and the welding research and preparation of HCCI composite parts are limited.Sintered HCCI has fine microstructure,low sensitivity to stress concentration as well as double mechanical properties in comparison with cast HCCI,which improve its weldability.In this paper,welding joint between sintered HCCI and LCS was prepared by multi-pass manual gas tungsten arc welding(GTAW),and the effects of welding current on the microstructure evolution and mechanical properties of the weld joint were systematically investigated.The following conclusions were drawn:(1)The weld joint could metallurgically bond without obvious defects.The fusion zone(FZ)of the joint consisted of austenite and tempered martensite,and there was a single austenite columnar-crystal zone(SAZ)in the FZ near the LCS side and a columnar crystal zone(CCZ)near the HCCI side.Coarsen carbides with branches were distributed along matrix's grain boundary in CCZ.(2)The tensile strength of joint reached the peak of 538.1MPa at 140A,which was95.3%and 97.4%of that of HCCI and LCS,respectively.Samples welded at 140A and 150A showed ductile fracture with necking in LCS side.As welding current decreased,FZ with low alloy content became the weakest zone of the joint,and the fracture appeared there.When welding current increased,the CCZ of HCCI got wider and became the weakest zone.(3)The microhardness of the joint welded at 140A decreased from HCCI side to LCS side in horizontal direction,while presented M-type in vertical direction due to repeated tempering and alloying element diffusion/segregation.