Austenitization of steel

Principle of Austenitization

• Phase transformation principle: When steel is heated, the internal structures such as ferrite and pearlite will gradually transform into austenite through the redistribution of carbon atoms in the iron crystal lattice.
• Temperature influence: The austenitization temperature is usually between ac₁ (lower critical point) and ac₃ (upper critical point). However, the critical point temperature varies with the composition of steel. For example, the carbon content, types and contents of alloying elements all affect the critical point.

Austenitization Process (Taking Eutectoid Steel as an Example)

• Formation of austenite nuclei: Austenite nuclei usually preferentially form at the phase interface between ferrite and cementite, because the carbon atom concentration at the phase interface is uneven, and the energy is high and the atomic arrangement is irregular, which easily meets the conditions of concentration fluctuation, energy fluctuation and structure fluctuation required for nucleation. In addition, the boundaries of pearlite domains and ferrite mosaic blocks can also be nucleation sites.
• Growth of austenite nuclei: After heating to the austenite phase region, the diffusion rate of carbon atoms accelerates at high temperature, and iron atoms and alloy atoms can also diffuse fully. Austenite nuclei continue to grow towards ferrite and cementite through the diffusion of carbon atoms.
• Dissolution of residual cementite: Because the composition and structure of ferrite are closer to austenite, ferrite disappears first during the growth of austenite nuclei, while the residual cementite continues to dissolve with the extension of holding time until it all disappears.
• Homogenization of austenite composition: After all cementite dissolves, the carbon concentration in austenite is not uniform, and the carbon content in the original cementite region is higher. At this time, it is necessary to undergo long - term holding or continue heating to allow carbon atoms to diffuse fully, so as to obtain austenite with uniform composition.

Austenitization of Hypoeutectoid Steel and Hypereutectoid Steel

• Hypoeutectoid steel: In addition to the basic process of austenite formation of eutectoid steel mentioned above, when heated above the ac₁ temperature, the proeutectoid ferrite needs to dissolve gradually until it is heated above the ac₃ temperature to be completely transformed into austenite.
• Hypereutectoid steel: When heated above the ac₁ temperature, the proeutectoid cementite (secondary cementite) needs to dissolve gradually, and only when heated above Accm (upper critical point of hypereutectoid steel) can all cementite dissolve and a single austenite structure be obtained.

Factors Affecting Austenitization

• Heating temperature and time: The higher the heating temperature, the faster the atomic diffusion rate, the faster the austenitization speed, and the shorter the time required for formation; At a certain temperature, the longer the holding time, the more uniform the austenite composition, but too long holding time will lead to grain growth.
• Heating rate: The faster the heating rate, the shorter the incubation period, the higher the temperature at which austenite begins to transform and ends to transform, and the shorter the time required for transformation. During rapid heating, the increase in the nucleation rate of austenite is greater than the growth rate, and fine austenite grains can be obtained.
• Alloying elements: Elements such as cobalt and nickel will accelerate the austenitization process; Elements such as chromium, molybdenum and vanadium slow down the austenitization process; Elements such as silicon, aluminum and manganese basically have no effect on the austenitization process. Since the diffusion rate of alloying elements is much slower than that of carbon, the heat treatment heating temperature of alloy steel is generally higher and the holding time is longer.
• Original structure: When the cementite in the original structure is lamellar, the austenite formation rate is faster than that of granular cementite; The smaller the interlamellar spacing of cementite, the more phase interfaces, the higher the nucleation rate, and the faster the transformation rate; The granular pearlite in the spheroidizing annealing state has few phase interfaces, so the austenitization rate is the slowest.

Practical Application of Austenitization

• Heat treatment: Austenitization is a key step in steel heat treatment. Different subsequent cooling methods (such as quenching, normalizing, annealing, tempering, etc.) will cause austenite to transform into different structures, so as to obtain the required mechanical properties. For example, quenching steel after austenitization can obtain martensite structure and improve the hardness and strength of steel; Normalizing treatment can refine grains and improve the machinability of steel, etc.
• Pressure processing: Steel ingots, steel billets and steel products are generally heated to above 1100°C for austenitization. At this time, austenite has good plasticity and low yield strength, which is convenient for plastic processing such as forging and rolling to make parts or finished products of various shapes.