Transformations of steel during heating: Factors affecting the rate of austenite transformation

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1. Effect of heating temperature  ① The higher the heating temperature, the faster the formation of austenite. This is because a high heating temperature (i.e., a large superheat) results in a rapid increase in both the nucleation rate and growth rate of austenite, as well as an enhanced ability of atoms to diffuse, which facilitates the dissolution of cementite and the transformation of ferrite.        ① The heating temperature must be above point AC1 for pearlite to transform into austenite. A transformation can only begin after a period of incubation, and the higher the temperature, the shorter this incubation period.         ② The higher the transformation temperature, the faster the formation of austenite, and the shorter the time required for the transformation. This is caused by two reasons. On the one hand, the higher the temperature, the greater the free energy difference between austenite and pearlite, which results in a stronger driving force for transformation. On the other hand, higher temperatures accelerate atomic diffusion, thereby speeding up the redistribution of carbon and the alteration of the iron lattice; as a result, nucleation and growth of austenite, dissolution of residual cementite, and homogenization of austenite all occur more rapidly. It can be seen that the same austenitization state can be achieved either by heating at a lower temperature for a longer period of time, or by heating at a higher temperature for a shorter period of time. Therefore, when formulating the heating process, the effects of temperature and time must be taken into full consideration.  2. Effect of heating rate During continuous heating, the heating rate has a significant impact on the austenitization process. The faster the heating rate, the greater the superheat of the pearlite, and both the starting temperature AC1 of the transformation as well as the ending temperature increase. However, the incubation period for the transformation becomes shorter, meaning that less time is required for the transformation to occur.  3. Influence of chemical composition On the one hand, the higher the carbon content in steel, the faster the formation of austenite. This is because as the carbon content increases, the amount of cementite also increases accordingly; the surface area of the interfaces between ferrite and cementite rises, thereby increasing the sites for austenite nucleation and raising the austenite nucleation rate. At the same time, an increase in the amount of carbides reduces the diffusion distance of carbon. Moreover, as the carbon content in austenite increases, the diffusion coefficients of carbon and iron atoms increase, thereby accelerating the growth rate of austenite.  4. Influence of the original microstructure Under identical chemical compositions, as the dispersion degree of carbides in the original microstructure increases, not only do more phase interfaces between ferrite and cementite appear, but this also increases the nucleation rate of austenite ； Furthermore, the reduction in the interlayer spacing of pearlite leads to an increase in the carbon concentration gradient within austenite, all of which contribute to an increased growth rate of austenite. Therefore, the finer the original structure of the steel, the faster the formation of austenite. Austenite grain size and its influencing factors: The austenite structure formed in steel after heating, particularly the size of the austenite grains, has a significant impact on the structure and properties of the steel after cooling transformation. Generally speaking, the finer the austenite grains, the higher the strength, the better the plasticity, and the higher the impact toughness of the steel after heat treatment. However, too high an austenitization temperature or an excessive holding time at high temperatures will cause the austenite grains in the steel to grow. Great care must be taken during heat treatment to prevent the coarsening of austenite grains. The grain size of austenite can be expressed in terms of initial grain size, actual grain size, and intrinsic grain size, among others. 1. Initial grain size: When steel is heated, at the moment when pearlite just transforms into austenite, the size of the austenite grains is referred to as the initial grain size. The grains are generally quite small at this stage; as the temperature rises or the heating time increases, the grains grow larger.  2. Actual grain size      After the initial grains of austenite are formed, if the temperature is further increased or maintained above the critical point, the grains will grow spontaneously. The higher the temperature and the longer the time, the larger the grains grow. The size of the austenite grains obtained under each specific heating condition is referred to as the “actual grain size” of austenite. That is, the actual grain size refers to the actual size of the austenite grains obtained when heating under specific conditions. It directly affects the properties of steel after cooling. 3. Inherent grain size is an indicator that reflects the tendency of austenite grains to grow when steel is heated. Steels in which austenite grains tend to grow large are called \"intrinsically coarse-grained steels,\" whereas those in which the grains remain small are called \"intrinsically fine-grained steels.\" As the heating temperature increases, the austenite grains in the essentially coarse-grained steel continue to grow and gradually become coarser. Essentially, fine-grained steels are different: when heated below a certain temperature, the austenite grains grow very slowly, remaining finely grained. However, above a certain temperature, the grains grow rapidly and suddenly become coarser. The temperature at which this grain begins to grow rapidly is called the “grain coarsening temperature”. In essentially fine-grained steels, the grains do not tend to grow when heated below the grain-coarsening temperature; the size of austenite grains and the factors affecting it ； 1. Effects of heating temperature and holding time The higher the heating temperature and the longer the holding time, the coarser the austenite grains become. 2. Effect of heating rate The faster the heating rate, the greater the superheat, and the higher the actual temperature at which austenite is formed, allowing for the development of fine initial grains. 3. Influence of the chemical composition of steel When all carbon is dissolved in austenite, as the carbon content in austenite increases, the tendency for grain growth increases.      Alloying elements such as Ti, Zr, V, Nb, and Al, when they form dispersed and stable carbides and nitrides, hinder the movement of grain boundaries by being distributed at those boundaries, thereby preventing the growth of austenite grains and contributing to the formation of steel with fine grains. Mn and P are elements that promote the growth of austenitic grains.