# Effect of Austenitizing on the Microstructure and Mechanical Properties of Gray Cast Iron
## 3 Results and Discussion  
Figure 2 shows the graphite morphology of the specimen. In the figure, type A graphite (flaky graphite with a non-directional and uniform distribution) and a small amount of type F graphite (primary star-shaped or spider-shaped graphite) can be observed. The non-directional and uniformly distributed structure of type A graphite contributes to the improvement in the mechanical properties of gray cast iron. Its uniform distribution can also enhance the wear resistance. Type F graphite can effectively disperse stress, thereby improving the toughness of the specimen. This combination of graphite morphology is beneficial to the wear resistance and toughness of gray cast iron castings [29]. Through an analysis of the microstructure using Image-Pro Plus software (Version 6.0), it was observed that the as-cast microstructure is a typical structure with a pearlite content  
Figure 1: Dimension diagrams of tensile specimens and friction and wear specimens, and a diagram of the isothermal quenching heat treatment process. Figure 2: Graphite morphology of the as-cast specimen (**a**) under 100 times magnification; (**b**) partial enlarged view. of 80 (the proportion of pearlite accounts for 75% to 85%). As shown in Figure 3, which displays the microstructures of different specimens, the matrix structure transitions to ferrite and ausferrite after heat treatment. With increasing heat treatment temperature, austenitization progresses more completely. Furthermore, higher austenitizing temperatures enhance the solid solubility of carbon in austenite. This occurs because the lattice interstices of austenite expand at elevated temperatures, facilitating carbon atom dissolution into the austenitic matrix, thereby increasing carbon dissolution capacity. Consequently, as temperatures rise, the proportion of ausferrite in the matrix structure progressively increases, while the content of ferrite decreases. As shown in Figure 4, the ausferrite content increases from 38.6% in At 810 to 81.2% in At 850, and finally reaches 98.1% in At 900. It can be observed that the ausferrite content first shows a linear increase, and the increasing trend is quite obvious. However, between 850 \({}^{\circ}\)C and 900 \({}^{\circ}\)C, the increasing trend of the ausferrite content slows down, which indicates that complete austenitization has occurred at 900 \({}^{\circ}\)C.  
Figure 4: Austenite content of gray cast iron specimens at different austenitizing temperatures. Figure 3: Microstructure diagrams of different specimens. (**a**) Hi zt; (**b**) At 810; (**c**) At 850; (**d**) At 900. Figure 5 shows the mechanical properties of different gray cast iron specimens. The as-cast specimen exhibits a tensile strength and elongation corresponding to grade HT 150. For the heat-treated specimens, both the tensile strength and elongation first increase and then decrease. The tensile strength of At 810 is 253 MPa. With the increase in ausferrite, At 850 shows the best tensile strength, reaching 332 MPa. However, At 900 shows relatively poor tensile strength and elongation. This is because at higher temperatures, the diffusion ability of atoms is enhanced and atoms between grains can more easily cross the grain boundaries, leading to the gradual growth of grains. According to the Hall-Petch relationship [30], the smaller the grain size, the greater the number of grain boundaries, and the stronger the hindrance to dislocations. As a result, the slip of dislocations between grains is restricted, which enhances the strength of the material. However, austempering at high temperatures is more likely to cause stress concentration and the gray cast iron material has the characteristic of stress concentration. The high temperature heat treatment magnifies this characteristic. Therefore, At 900 exhibits poorer tensile properties. Generally, there is a positive correlation between hardness and wear resistance.