Study On Microstructure And Properties Of High Chromium Tungsten Cast Iron

By means of microstructure observation, X-ray Difractometer (XRD), Scanning Electron Microscopy (SEM) with Energy Dispersive Spectrometer (EDS), rockwell hardness test, impact-toughness test, abrasive wear test and observation of the wear surface and the as-cast morphology of impact fracture, the effect of W and Mo on the microstructure, mechanical properties and wear resistance were researched. The reasonable content of W and heat treatment technology were obtained. Compared the hardenability of high chromium molybdenum cast iron with that of high chromium tungsten cast iron, the effect of W and Mo on the hardenability was investigated. And the RE-Nb compound modificator of Cr26 high chromium tungsten cast iron was studied.Experimental results show that the as-cast microstructure of high chromium tungsten cast iron consists of austenite, martensite and carbides. M₇C₃, M₂₃C₆ are the main types of chromium carbide. The distribution of W is almost uniform in carbide and matrix. The types of W carbide contain WC_1-x, W₆C_2.54 and CW₃. The macro-hardness of high chromium tungsten cast iron increases because of higher hardness of matrix and carbides due to the tungsten addition. The abrasive loss decreases with the increment of W. And the impact toughness increases until 3% W. The hardness increases from 55.75HRC to 58.75HRC and impact toughness increases from 9.43 J/cm² to 11.18 J/cm² with 1.03% W addition. The abrasive loss is 3.8 mg, which is 54.8% lower than that of the sample with no W addition.The microstructure of high chromium tungsten cast iron consists of martensite, retained austenite and carbides after heat treatment. There is no change in types of chromium carbide after heat treatment. The types of W carbide contain WC_1-x,W₆C_2.54,CW₃ and W₂C. The heat treatment hardly has any effect on the distribution of tungsten in carbide and matrix. The hardness and wear resistance increase as the destabilization temperature increases from 950℃to 1050℃. The impact toughness decreases with the temper temperature increasing from 250℃to 450℃. And the hardness increases until 550℃. The reasonable heat treatment technology of high chromium tungsten cast iron is destabilized at 1050℃and tempered at 250℃. In this heat treatment condition, the hardness and impact toughness of the sample with 1.03% W addition are 61.8HRC and 7.9J/cm², respectively. And the abrasive loss is 2.7mg, which is 66.7% lower than that of the sample with no W addition.Compared the effect of Mo with W on the hardenability of high chromium cast rion, the 60×120×120mm sample with Mo addition is full hardening destabilized at 950℃, of which the hardness in the core is 60—62HRC. The 60xl20xl20mm sample with no W and Mo addition is full hardening destabilized at 1050℃, of which the hardness in the core is 60⁶1HRC. When the destabilized temperature is up to 1100℃, the hardness of the sample with 3.34% W addition is 61—62HRC. Mo has a strong effect in increasing the hardenability of high chromium cast iron. However, Due to the W dissolving in the matrix partly, the diffusion activation energy of carbon and alloy elements in the matrix decreases while quenching, so that the matrix of high chromium tungsten cast iron has high carbon and alloy elements content, which leads to a lower Ms temperature than that of the sample with no W addition. Therefore, high chromium tungsten cast iron is less likely to transform to martensite upon air-cooling to room temperature.The study of RE-Nb compound modificator of Cr26 high chromium tungsten cast iron shows that both RE and Nb can refine austenite and carbide. The reasonable addition of RE-Nb compound modificator is 0.4% rare-earth ferrosilicon alloy + 1.2% ferroniobium (25%RE of rare-earth ferrosilicon alloy, 65% niobium of ferroniobium). With the reasonable addition of modificator, the hardness increases from 55HRC to 58.3HRC and the impact toughness increases from 5.3 J/cm² to 9.42 J/cm². After destabilized at 1050℃and tempered at 250℃, the hardness and impact toughness are 63.45 HRC and 7.21 J/cm², respectively.