Formation Mechanism And Control Method Of In Situ Sulfur Based Self-lubricating Phases In Self-lubricating Steel

The sulfur based self lubricating steel is a kind of functional alloy steel with in-situ self lubrication phase. This kind of steel has high temperature resistance, self lubrication, high wear resistance, low cost and other advantages. The morphology, size and distribution of the self-lubricating phase in the self-lubricating steel have important influence on its lubrication and wear resistance. However sulfur in iron often reacts with Fe and forms FeS. The FeS reactant is mainly bulk phase, and mainly distributes nearby grain boundaries. This results in the weakness of grain boundaries, separation of the matrix and stress concentration, which seriously reduces the toughness and strength of the steel and the forging quality of the steel. Therefore, how to effectively control the number, shape, size and distribution of the self lubricating phase can be the key factor for the preparation of good self lubrication and wear resistance and good mechanical properties.In this paper, main contents focus on the followings:the formation mechanism of the sulfur-based self-lubricating phase in the self-lubricating steel, the micro structure of sulfur based self lubricating steel, the influence and mechanism of solid solution treatment and aging treatment on the shape and size of self lubricating phase, as well as the mechanical properties, normal temperature and high temperature friction and wear properties. Some research was carried out to reduce the content of FeS, therefore reducing the harmful effect to the steel.The X-ray diffraction, X-ray photoelectron spectroscopy and were employed to study the micro structure and formation mechanism of the self-lubricating phase. The results showed that the content of S element is mainly concentrated in the metal sulfide self-lubrication phase. The metal sulfides in the self lubrication steel were mainly FeS, CrS, MoS2 and some multi-component sulfides. The generation sequence of sulfide in steel was CrS> FeS>MoS2. So FeS can be replaced by adding Cr. The method of increasing the content of Cr, Mo and the reaction temperature or the pressure, can lead to the reducing of the content of FeS in self-lubricating phase.Optical microscope, scanning electron microscope analysis and mathematical statistics analysis were used to study the microstructure of sulfur based self-lubricating steel. The solid solution treatment temperature and time as well as the aging treatment parameters were determined. The mechanism of solid solution treatment and aging treatment was also discussed. The results showed that solid solution treatment and aging treatment had important influence on the shape and size of self-lubricating phase. The best solid solution treatment parameters were 930℃ and 40 minutes. The best aging treatment parameters were 550℃ and 60 minutes. Self-lubricating wear-resistant steel in solid solution treatment began to unsaturated phase, self-lubricating phase reduce the overall size and shape more rounded. In solid solution treatment reaches saturation, because of the dynamic equilibrium and balance concentration, size will increase slightly, small size will decrease slightly. After the aging treatment, the number of the small self-lubricating phase was increased, and the self-lubrication distribution was more uniform. The mechanism of the size and shape changing of the self-lubricating phase belonged to the diffusion controlled dissolution and precipitation mechanism.Tensile tests showed that the annealed steel had such mechanical properties as 455MPa ultimate tensile strength,3.6% the elongation,2.23J/cm2 impact toughness and HRC33 hardness.After quenching and tempering, the mechanical properties changed to 385MPa tensile strength,3.4% elongation,2.70J/cm2 impact toughness, and HRC60 hardness.Normal temperature friction and wear test results showed that the friction coefficient of the sulfur based self-lubricating steel was slightly higher than that of the 663 tin bronze, but its wear rate was only 1/43～1/39. High temperature friction and wear test results showed that the friction coefficient of the sulfur based self-lubricating steel was slightly higher than that of the 663 tin bronze, but the wear resistance of the sulfur system was 20 times more than that of the 663 tin bronze. In normal temperature friction and wear test and high temperature friction and wear test, the average friction coefficient of sulfur based self lubricating steel would decrease with the increase of temperature, load and rotating speed.