大颗粒碳化物对GCr4Mo4V钢旋转弯曲疲劳的影响

Influence of large carbides on rotating bending fatigue of GCr4Mo4V steel

以“真空感应+电渣重熔”工艺制备的退火态热轧直条高温轴承钢GCr4Mo4V为试验材料,经过1120℃保温30 min油淬以及3组530℃保温2 h的回火处理后,开展960、1000、1040、1080 MPa四个应力水平的旋转弯曲疲劳试验,得到S-N曲线并计算出中值疲劳极限为686 MPa,对旋弯疲劳试样断口形貌分析表明,GCr4Mo4V轴承钢旋转弯曲断口由近表面起裂源、裂纹扩展区、应力撕裂区3个特征区域构成,起裂源距试样表面约240μm,中心为粒径范围为16.93~53.94μm的大颗粒碳化物,裂纹在大颗粒碳化物与基体界面处形核,并逐渐向试样中心扩展,最终在扭矩作用下将试样撕扯断裂;数值分析表明,大颗粒碳化物粒径D与疲劳寿命对数lg(N)呈线性关系,经线性拟合后得到的数学关系式为lg(N)=-0.053 77D+7.326 82,由此指明了GCr4Mo4V轴承钢大颗粒碳化物的极限细化是进一步实现轴承钢长寿化的关键举措。

Annealed hot rolled high temperature bearing steel GCr4Mo4V manufactured by “VIM+ESR” was heated at 1120 ℃ for 30 min before being oil quenched, and then the specimen was tempered at 530 ℃ for 2 h followed with air cooling for 3 times. The rotating bending fatigue experiment was carried out on the heat treated specimen under stress levels of 960, 1000, 1040 and 1080 MPa, respectively. The S-N curve was obtained via the experimental data. A median fatigue limit of 686 MPa is calculated accordingly. Analysis of the fracture morphology indicates that the rotating bending fracture of GCr4Mo4V bearing steel consists of 3 typical zones, which are near surface crack-initiated site, crack propagation zone and stress tearing zone. The distance between the crack initiated site and specimen surface is about 240 μm, and at the center of the crack-initiated site exists large size carbide, the particle size of which distributes in the range of 16.93-53.94 μm. The crack nucleates at the boundary between large size carbide and the matrix, and gradually propagates towards the center of the specimen;in the end, the specimen is teared into two pieces by the torsion stress. The mathematical analysis shows that, the size of the large carbides D shows a perfect linear relation with the logarithm of fatigue life N, and the mathematical relation is lg(N)=-0.053 77D+7.326 82 according to the fitting results. And it is clear that the extreme refinement of the large carbides is absolutely a significant way for life prolonging of GCr4Mo4V bearing steels.