The Effects Of Inclusions To The Properties Of 51CrV4 Spring Steel

In this paper, four kinds of 51CrV4 whose components are in line with the European standard requirements were as the object of study. Effect of the inclusions in steel to the mechanical properties of steel was studied. By controlling the contents of impurity elements such as Al, S, O, steels with different types and contents of inclusions were produced. Phase transformation character of the four kinds of 51CrV4 was investigated. CCT curves were measured by using the equipment of Formastor-F. According to European standard DIN 50602, the types and grades of inclusion were assessed. The conventional mechanical properties of four steels were tested, such as impact, tensile, hardness. The fatigue properties of two steels in which the types and contents are both different were measured by using the Shimadazu USF-2000 ultrasonic fatigue test machine.The results of phase transformation character show that the Ac3 and Ac1 of 1# steel are respectively 754℃and 714℃; the Ac3 and Ac1 of 2# steel are respectively 750℃and 715℃; the Ac3 and Ac1 of 3# steel are respectively 780℃and 720℃; the Ac3 and Ac1 of 1# steel are respectively 766℃and 669℃. When the cooling rate is faster than 5℃/s, the microstructure of four steels are all martensite. The Ar3 and Arl are all increased when the heating rate increases from 0.05℃/s up to 50℃/sThe results of conventional mechanical properties testing show that when the content of inclusion is under the K3=62.5, the hardness value decline with the content of inclusions increased. The tensile properties has nothing to do with the content of sulfide inclusion; With the content of inclusion which is thin and long increased, the impact properties of the material of 51CrV4 decreased.The results of Utlrasonic fatigue testing show that the traditional fatigue limit of JK exist, and its value is 675MPa, but that of 1# does not. Under the projection, the fatigue limit of 1# is 432.4MPa. It's obvious that the fatigue limit of JK is higher than that of 1#. Because the size of hard inclusion of JK is smaller than its critical size of 4.05μm, the origin of fatigue fracture is in surface matrix. But the size of the oxide inclusion of 1# is much large than its critical size of 4.13μm, so the origin of fatigue fracture is the inclusions; The content of sulfide in JK is higher than that in 1#, but there's no fatigue fracture in which the origin is the sulfide inclusion, so it shows the fatigue limit does not depend on the content of sulfide.