M50NiL轴承钢淬回火组织特征及拉压疲劳裂纹的萌生

Microstructure and tension and compression fatigue crack initiation of quenched and tempered M50NiL bearing steel

通过拉压疲劳试验及微观组织观察研究了M50NiL轴承钢淬回火组织特征及对疲劳裂纹萌生的影响作用。结果表明,试验钢的组织主要为板条马氏体,少量δ-铁素体及M_(2)C、MC碳化物;扫描电镜观察发现1070℃淬回火试样δ-铁素体及临近的聚集球状M_(2)C碳化物,δ-铁素体位于晶界处,尺寸主要为4~8μm,钢中碳化物尺寸最大为2.66μm;1100℃淬回火试样δ-铁素体相界存在大量片状的M_(2)C碳化物,尺寸在8μm以上的δ-铁素体数量增加,最大为17.51μm;1070℃淬回火试样的疲劳极限为713MPa,主要起裂源为Al_(2)O_(3)、Al_(2)O_(3)-CaO夹杂物,循环次数随夹杂物尺寸的增大而降低,且Al_(2)O_(3)-CaO夹杂更易引起试样的疲劳断裂;1100℃淬回火试样的疲劳极限为707MPa,主要起裂源为δ-铁素体组织缺陷,起裂源于8μm以上δ-铁素体的相界处,疲劳寿命受应力及δ-铁素体尺寸影响。

The tensilecompression fatigue test and microstructure observation were used to study the influence of the structure of M50NiL bearing steel on the fatigue crack initiation after quenching and tempering.The results show that the structure of the test steel is mainly lath martensite,a small amount ofδ-ferrite,M_(2)C and MC carbides.Scanning electron microscope observation revealed that theδ-ferrite and adjacent aggregated spherical M_(2)C carbides of the sample quenched at 1070℃,δ-ferrites are located at the grain boundaries with the sizes of 4-8μm.The biggest size of carbides is 2.66μm.There are a large number of flaky M_(2)C carbides in theδ-ferrite phase boundary of the 1100℃quenched sample,and the number ofδ-ferrites with a size of 8μm or more increases,and the maximum size is 17.51μm;the tensilecompressive fatigue limit of the sample quenched at 1070℃is 713MPa,and the main sources of cracking are Al_(2)O_(3)and Al_(2)O_(3)-CaO inclusions cracking.The number of cycles decreases with the increase of the size of inclusions,and Al_(2)O_(3)-CaO inclusions are more likely to cause fatigue fracture of the specimen.The fatigue limit of 1100℃sample is 707MPa.The main source of cracking isδ-ferrite structure defect.The cracking originates from the phase boundary ofδ-ferrite are above 8μm.Fatigue life is affected by stress and the size ofδ-ferrites.