Influence of Heat Treatment on Mechanical Properties and Microstructure of Low Alloy Wear Resistant Cast Steel (1)
HU Zhi-jun, ZHAO Xiao-peng, YANG Yi-tao
(School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China)
Abstract： The influences of normalizing and tempering temperature on mechanical properties and microstructure of low alloy wear-resistant cast steel were investigated by using a vacuum arc furnace in lab situations. The results show that normalizing can refine the grain and increase hardness. Hardness increases with the increase of normalizing temperature from 890 ℃ to 920 ℃, while exceeding.
normalizing temperature from 920 ℃ to 940 ℃ make the hardness of the steel decrease. The steel was tempered at 500, 530, 560, 590, 620, and 650 ℃, respectively, after normalized at 890 ℃, a rule of controlling mechanical properties of the steel can be obtained.
Key words：Low alloy wear-resistant cast steel; vacuum melting; physical simulation; microstructure; mechanical properties
To improve the properties of steel castings, we can control and optimize alloy elements’ content in steel castings and change the heat treatment process to obtain excellent comprehensive properties. Normalizing is an effective way to adjust the microstructure and improve the strength and toughness of micro-alloyed cast steel. In the case of cast steel containing a small amount of alloying elements, the segregation of the alloying elements and the insufficient dissolution of carbides in the austenite caused by the casting process results in the variable composition of the austenite; it isn’t easy to give full play to its strength and toughness. The effect of normalizing temperature and tempering temperature after normalizing on the microstructure and properties of low alloy wear-resistant cast steel was studied to improve the comprehensive mechanical properties of cast steel; it provides a reliable basis for the optimization of the heat treatment process of low alloy wear-resistant steel castings in practical production.
1.Experimental materials and methods
The experimental material is the low alloy wear-resistant cast steel, which is improved by the previous experiment. The main components are 0.25 ~ 0.34 c, 0.76 ~ 0.84 SI, 1.22 ~ 1.33 MN, 0.24 ~ 0.25 Mo. The button ingot sample of about 110g was produced in a small vacuum arc furnace. PMI-MASTER Pro Mobile Spectrometer analyzed the components. The value of AC3 was measured by DIL805A thermal expansion fast phase change meter with wire cut quasi 4mm × 10mm specimen. The polished surface was corroded by 4% nitric acid and alcohol, and the microstructure was observed by Nikon MA100 inverted metalloscope. In order to simulate the microstructure of the steel castings, the button ingots melted in a vacuum were cooled after holding at 950 °C for 2 hours.
2.Results and analysis
2.1 thermal expansion curve and heat treatment process parameters
The thermal expansion curve is shown in figure 1. AC1 and AC3 are 715 °C and 840 °C, respectively, and the normalizing austenitizing temperature is 890 °C, 900 °C, 910 °C, 920 °C, 930 °C, and 940 °C, respectively, and the heating and holding time is 40 min. The specimens were then annealed at 500 °C, 530 °C, 560 °C, 590 °C, 620 °C and 650 °C for 2 hours before Air Cooling.
2.2 Relationship between hardness and normalizing temperature
The hardness of the HBS specimen before normalizing is 192. The influence of normalizing temperature on the hardness of casting is shown in Fig. 2. It can be seen that the hardness of the castings increases after normalizing, and decreases first and then increases with normalizing temperature changing from 890 °C to 940 °C and reaches the lowest at 920 °C.
2.3 Histological observation after normalizing
Fig. 3 shows the microstructure changes of low alloy wear-resistant cast steel before and after normalizing. It can be seen that the microstructure is refined, and the ferrite is increased after normalizing. With the increase of normalizing temperature, the supercooling degree increases, the nucleation of PROEUTECTOID ferrite increases, the hardness decreases, and the solid solubility of MN and SI in austenite increases with the increase of normalizing temperature, the increase of solid solubility of solute atoms in austenite will increase the hardness of steel, so the hardness rises.
2.4 Comparison of hardness at different tempering temperatures
After annealing at 950 °C, the button ingots with higher hardness were normalized at 890 °C and tempered at different temperatures. The hardness changes were shown in Fig. 4. Except for tempering at 530 °C, the hardness of the samples tempered at other temperatures decreased to some extent, and the hardness decreased with the increase of tempering temperature.