17NiCrMo6 and 20CrMnTi difference: 17NiCrMo6 better toughness carbon content is different, different alloying elements, different mechanical properties. Thermal process is also different. Front performance superior to some, the latter transmission gear, differential gear widely used. 2 chemical composition of carbon (C): 0.15-0.20, silicon (Si): ≤0.40, manganese (Mn): 0.40-0.60, sulfur (S): ≤0.035, phosphorus (P): ≤0.035,, chromium (Cr) : 1.50-1.80, nickel (Ni): 1.40-1.70, molybdenum (Mo): 0.25-0.35, 3 Analysis and Prevention Measures 1. Introduction In 1997, a factory for the production of early Maanshan supporting bar mill, rolling mill gear process, the material is 17NiCrMo6 steel gear after carburizing slow cooling cracks, cracks in order to find out why, we analyzed and discussed at the Chinese Academy of Sciences expert guidance and help. The main two reasons for the slow cooling causes cracks crack is nitrided layer phase change caused by uneven cooling process. Chunks of cementite and continuous network carbide, the diffusion layer in the presence of diffusion layer microstructure of three layers, the outermost layer of lower bainite and network carbide; the middle is quenched martensite and lower shell s body and mesh carbide; the third layer is lower bainite and ferrite, hardness test table outside to the inside. Examined site carburized layer base material outer surface of the intermediate layer transition layer hardness (HL) 420.433.458 513.501.479 492.479.414 318.337.307 phase change by following influencing factors: 2.1 Influence of temperature due to the solubility of carbon in ferrite in small (up to about 0.025%), while in the austenitic state, carburizing temperature is higher, the diffusion of carbon in which the larger the coefficient, the greater the speed of both carburization. But the temperature is not too high, otherwise the carburizing equipment life significantly decreased or damage, and the temperature is too high for too long can cause tissue infiltration layer of coarse carbide grade ultra-poor defects. Typically used in the actual production of 900 ℃, 930 ℃ carburizing. 2.2 carbon concentration and carbon potential slow cold cracking carburizing time related. In the early carburizing, due to poor carbon surface of the workpiece, the ability to accept highly activated carbon atoms, carburizing speed, low carbon potential of the furnace at this time, you need to furnace through a lot of penetration, in order to maintain the furnace carbon potential, but also with the specific amount of stove relevant at this time if you can not replenish penetration may cause carburizing time is too long, the carbon concentration profile concave defects such as, but not too strong, it may appear a lot of mesh carbonation matter can not be eliminated. When the carbon content of the surface rising carbon potential continue to establish circumstances, should be gradually reduced to join penetration agents, carburizing into the diffusion stage, if at this time remains a large dose of penetration, it is necessary to form a surface mesh carbonized things, the strength of the diffusion layer decreases, fragility, especially the decrease of tensile strength, slow cooling to prevent cracking quite negative. 2.3 carburizing time when the carburizing temperature, carbon potential in the future to determine, carburizing time depends on effective case depth, the longer carburizing time, the deeper hardened layer, whereas the more shallow. Workpiece 17NiCrMo6 steel layer at 10-15μm, if the diffusion of control is not good, time is too short, it may result in the diffusion layer of carbon concentration profile too steep, after the slow cooling process, the formation of slow cooling cracks. 2.4 The slow cooling rate of the slow cooling is generally performed well in the cooling, the cooling rate should be more slowly than air-cooled, so as to get a more balanced organization. If for some reason, so slow cooling rate equivalent to air cooling rate, the results will appear in the slow cooling cracks. The results also show that when the carbon content of the carburized layer reaches the surface above the eutectoid composition, hardenability nitrided layer is not exactly the same, in particular the slow cooling rate, uneven phase transition occurs, the middle layer Markov hematocrit larger body, the surface tensile stress due to surface deterioration, can not afford a big pull and cracking. Three measures to prevent the slow cooling crack through the above analysis, produce slow cooling cracks first condition is nitrided layer there are a lot of lumps and carbide network, making performance degradation; the second is the occurrence of phase transitions uneven diffusion layer. Preventive measures are: first, to avoid generating a large number of network carbide infiltration layer. For this 17CrNiMo6 containing Cr, Mo strong carbide forming elements of steel, carburizing carbon potential is not too high, especially to the diffusion period, must take down the carbon potential of about 0.9% C, and maintain a certain time, to prevent produce carbides. In addition, to avoid the intermediate layer martensite. Slow cooling effect is good, the general organization is relatively balanced, no uneven phase transition, but more humid due to the cooling wells, large water cooling speed and cracks. If the ambient temperature in winter is relatively low, the workpiece loading furnace less, although it is well in the cooling, the cooling rate is still very fast, but also prone to slow cooling cracks.