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The austenitization process of alloy steel during heating consists of four stages: the formation of austenite nuclei, their growth, the dissolution of carbides, the homogenization of austenite, and the growth of austenite grains. The entire austenitization process is closely related to carbon diffusion, and alloying elements influence austenitization by altering the rate of carbon diffusion and the phase transition temperature. The effect of alloying elements on the formation rate of austenite: When alloying elements are added to steel, they affect the formation rate of austenite by altering the temperature at which austenite forms and the diffusion rate of carbon within the steel. Among them, Co and Ni can increase the diffusion rate of carbon in austenite, thereby accelerating the formation of austenite ； Si, Al, and Mn have little effect on the diffusion rate of carbon in austenite; therefore, they have almost no impact on the formation rate of austenite ； Carbide-forming elements such as Cr, Mo, W, Ti, and V have a strong affinity for carbon, which hinders the diffusion of carbon in steel and slows down the formation of austenite. Effect of alloying elements on the dissolution of residual carbides: After austenite formation, some undissolved carbides remain in alloy steels. The stability of various carbides varies, and as a result, their dissolution temperatures and rates also differ. For example, chromium carbides can dissolve in large quantities at temperatures above 850°C, while tungsten and molybdenum carbides begin to dissolve significantly only at temperatures above 950°C. Carbides of vanadium, titanium, and niobium require temperatures above 1050°C to dissolve. To increase the austenite content of the alloy and fully utilize the effects of the alloying elements, it is necessary to raise the quenching heating temperature of the steel. Sometimes, the heating temperature for quenching exceeds the critical temperature by several dozen degrees to even several hundred degrees; for example, the quenching heating temperature for high-speed steel W18Cr4V is 1280°C–1300°C, which is several hundred degrees higher than the critical temperature (the A1 point temperature of 820°C). The effect of alloying elements on the homogenization of austenite: Due to the different contents of alloying elements in ferrite and carbides, when austenite is first formed, the distribution of carbon and alloying elements within it is uneven. Austenite homogenization requires the diffusion of carbon and alloying elements ； However, the diffusion rate of alloying elements is very slow; even at a temperature of 1000°C, it is only a few thousandths or a few percent of the diffusion rate of carbon atoms. Furthermore, the presence of carbide-forming elements also reduces the diffusion rate of carbon in austenite. Therefore, it takes much longer to homogenize the austenite in alloy steel than in carbon steel. The effect of alloying elements on austenite grain growth can be divided into 4 cases. (1) Elements that strongly inhibit the growth of austenite grains include V, Ti, Nb, Zr, Al, etc. They readily form stable carbides, nitrides, oxides, and the like, which mechanically hinder grain growth. For example, the carbides formed by elements such as V, Ti, Nb, and Zr have high stability; even when heated to high temperatures, some of these undissolved carbides remain at the grain boundaries, strongly preventing the migration of austenite grain boundaries. Although Al is not a carbide-forming element, it can form high-melting-point Al₂O₃ and AlN particles with O and N in steel; these particles are distributed at the grain boundaries and act to prevent the migration of austenite grain boundaries. (2) Elements that moderately inhibit the growth of austenite grains include W, Mo, Cr, etc. Since they can form stable carbides with carbon, their stability is somewhat lower than that of V, Ti, Nb, Zr, etc., but they can still mechanically inhibit the growth of austenite grains, albeit with a slightly weaker effect compared to V, Ti, Nb, Zr, etc. (3) Elements that can prevent the growth of austenite grains and possess this effect include Si, Co, Ni, Cu, etc., which are non-carbide-forming elements. Regarding the mechanism by which they hinder grain growth, it is believed that the addition of such elements to steel can reduce the energy of grain boundaries, thereby slowing down the growth of austenite. (4) Elements that promote the growth of austenite grains include P, Mn, C, etc. Some believe that these elements can increase grain boundary energy and enhance the tendency for austenite grains to grow, while others think that when P, Mn, C, etc. dissolve into austenite, they reduce the bonding forces between metal atoms, thereby increasing the self-diffusion coefficient of iron atoms and facilitating grain growth.