Effect Of Boron On Structures And Mechanical Properties Of High Strength Spring Steels

The effect of boron element on microstructures and mechanical properties of a kind of 2000 MPa-grade medium-carbon steel for high strength spring was studied by using optical microscope, X-ray diffraction analysis, TEM, SEM, Bauschinger torsion test and notched tensile sustained load delayed fracture test et al.The microstructural observation of tested steels shows that the structural changes during tempering are fundamental the same as that of medium and high carbon steels. With the increase of tempering temperature, the precipitation of (-iron carbide, the decomposition of remained austenite and the replacement of (-iron carbide by cementite and cementite coarsening and spheroidization take place. However, owing to high silicon content in tested steels, the dissolving of (-carbide and the transformation of (-iron carbide to cementite is delayed considerably during early stages of tempering. Therefore, when the tested steels were tempered at temperatures below 350℃, hardness and tensile strength decreases slowly, whereas yield strength increases, with the increasing of temper temperature. However, hardness, tensile strength and yield strength all decrease continuously when the tested steels were tempered at temperatures above 350℃. Boron element has little influence on both strength and plasticity of tested steels, but it does have a beneficial effect on increasing impact toughness.It was found that the trace boron could enhance the hardenability of tested steels markedly and the difference of hardenability is not significant for the tested steels containing different amount of boron. The results of Bauschinger torsion test shows that the Bauschinger hysteresis loop area, which had been confirmed to represent the sag resistance of spring steels, has a linear relationship for all the steels used, i.e. sag resistance increases linearly with increasing hardness. However, there does not have obvious difference of Bauschinger hysteresis loop area of the tested steels containing different boron content. Delayed fracture resistance of the tested steels increases with the increase of boron content, which are also higher than that of conventional steel 60Si2Mn at same strength level. It was also found that the decarburization depth and oxidation weightlessness obtained from isothermal and equi-time heating treatment decrease with the increase of boron content. The above effects of trace boron on the mechanical properties of the tested steels mainly relate to the priority segregation of boron to prior austenite grain boundaries. Being a typical grain boundary active element, the segregation of boron in grain boundary could result in the decrease of austenite grain boundary energy, the restraining of the segregation of impurity elements such as phosphorus at the grain boundaries, and therefore enhance grain boundary strength, and delay grain boundary diffusion.