Effect of Tempering Process on Microstructure and Properties of ZG70Cr2MnNiSi Cast Steel

The ZG70Cr2MnNiSi cast steel is a kind of low alloyed structural steel, which is often used in general engineering and construction applications due to its good mechanical and corrosion resistance properties. To further improve the mechanical properties of the steel, a tempering process can be employed. During the tempering process, the retained austenite in the steel is converted to martensite and its microstructure is refined, leading to an increased strength and ductility of the steel. In this article, the effects of the tempering process on the microstructure and properties of ZG70Cr2MnNiSi cast steel will be discussed in detail.
    
Microstructure and Properties of ZG70Cr2MnNiSi Steel
The ZG70Cr2MnNiSi cast steel comprises of the following chemical composition, expressed in weight percentage: 0.50-0.70% Carbon (C), 1.80-2.00% Manganese (Mn), 1.00-1.50% Nickel (Ni), 1.00-1.50% Silicon (Si), and the rest is Iron (Fe). As shown in Figure 1(a), the microstructure of the steel is characterized by a ferrite matrix, with a dispersed austenite and/or pearlite particles. The ferrite has a bainitic and/or lath structure, and its grain size is related to the cooling rate of the steel during solidification.
    
Microstructure of ZG70Cr2MnNiSi cast steel (a); microstructure of ZG70Cr2MnNiSi steel after tempering at 550°C (b).
    
As a result of its chemical composition, the mechanical properties of the steel are usually low. The strength and hardness of the steel can be improved through heat treatment processes such as austenitizing and tempering.
    
Effect of Tempering on microstructure and properties
The tempering process is commonly used to refine the microstructure of steels and increase the toughness of the material. If a ZG70Cr2MnNiSi cast steel is tempered at a temperature range of 200-550°C, the retained austenite will be transformed into martensite (Figure 1b) and a tempering layer will form around the ferrite grains. As a result, the grain size of the ferrite is refined, and the hardness and strength of the steel will be increased. The toughness of the steel will also be improved due to the higher ductility of the martensite.
 
In conclusion, the tempering process has a positive effect on the microstructure and properties of the ZG70Cr2MnNiSi cast steel. The retained austenite is transformed into martensite, which refines the ferrite grain size, increases the strength and hardness of the steel, and improves its toughness. Therefore, the tempering process can be an effective way to increase the performance of ZG70Cr2MnNiSi cast steel.

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