Study On Effect Of Heat Treatment On Sulfides In High Sulfur Steel And Its Tribological Property

In traditional theory, sulfur was considered as a harmful element because it can incur defects, which could result in reduction of mechanical, welding and corrosion properties of the steel. However, recent studies lead to new insight into the effect of the sulfur in the steel. For one thing, sulfur could improve the cutting performance of the steel; for the second, sulfur could form sulfide with other elements, which could improve the wear property of the steel. Wear is one of the primary causes that give rise to consumption of materials and energy, therefore, it is necessary to conduct research on High Sulfur steel (HS for short) and broaden its applicationThe substrate of HS steel before heat treatment is mainly compounded of slice-alike pearlite and ferrite that is distributed on the grain boundary, and the sulfide groups scatter in the substrate, in flocculent, near-spherical, long, and spindle shapes. The distribution of sulfide groups in initial HS steel is uneven, moreover, bearing capability and wear resistance of initial HS steel are poor, which make it unsuitable for the situation of high speed and heavy load.In the experiment, HS steel samples were quenched at 800℃, 820℃, 840℃, 860℃, 880℃, 920℃respectively. Cracks and overburning occurred when the temperature exceeded 880℃. After quenching, microstructure of substrate mainly is martensite and residual austenite. After tempering,the substrate of HS steel is mainly consisted with tempered martensite and residual austenite. The hardness of HS steel decreases after tempering, but still surpasses that of initial HS steel. The sulfide groups were analyzed quantitively through Image Pro Plus, which showed that size of sulfide group increases after quenching and decreases after tempering, while the variation of the number is on the opposite. The specimen, which was quenched at 860℃and then was tempered at 200℃, contains smallest size of sulfide groups that are most evenly dispersed.In this thesis, basing on the ternary system of Fe-Cr-C, pseudobinary phase diagram of FeS-MnS, binary phase diagram of Fe-S and Fe-C, including major elements in High Sulfur steel, I conducted the solidification process of HS steel, which agreed well with actual metallography, distribution of alloying elements; in heat treatment, the changing of sulfide groups mainly due to the diffusion of C.In frictional test, the friction coefficient and wear loss of initial HS steel specimen drops in different degrees after heat treatment. The specimen, whose quenching temperature is at 860℃, exhibits lowest friction coefficient, friction variance as well as wear rate. The wear loss of HS steel specimens after heat treatment is positively correlated with the variance rather than hardness of specimens. Frictional process of HS steel is very complex, with multiple wearing forms: abrasive wear, mild adhesion wear, corrosive wear and fatigue wear.The tensile strength of HS steel reduces from 525 Mpa to 505 Mpa after quenching at 860℃and tempering, and impact energy of HS steel decreases from 2.9 J to 2.6 J after heat treatment, which demonstrates that the mechanical property of HS steel varies little after heat treatment.