Steel Microstructures: Transformations, Heat Treatments, and Surface Hardening
Steel Microstructures and Transformations
Pearlite: Eutectoid decomposition of austenite into alternating layers of ferrite and cementite. The cooling rate determines the fineness of the pearlite (fine or coarse). Bainite: A mixture of ferrite needles with cementite precipitation at the edges. High cooling rates favor bainite over pearlite. Austenite transforms from FCC to BCC. Shear deformation within the austenite leads to the dissemination and precipitation of supersaturated ferrite. Bainite exhibits both hardness and toughness. Two types exist: superior and inferior.
Martensite: A supersaturated solid solution of carbon in alpha-iron. It is obtained from quenching. It has a tetragonal crystal structure. Transformation occurs without diffusion, leading to a loss of toughness and increased hardness. TTT diagrams show the processing zones of austenite into pearlite, bainite, and martensite. These zones can be modified by: carbon content, alloying elements. Elements like Mn, Ni, B, and N retard the transformation of pearlite and bainite. Gamma-forming elements like Ni and Mn lower the A3 and Ae1 lines. Alpha-forming elements like Ti, Mo, Si, and W accelerate the transformation of pearlite and bainite. The Ac1 curve: the higher the grain size, the later the transformations of pearlite and bainite. Grain size: as grain size decreases, the transformation temperature decreases. A larger grain size increases the austenitic temperature. Lower temperatures promote austenite homogeneity.
Heat Treatments
Annealing: Softens steel, relieves internal stresses, and regenerates the microstructure. The heating temperature is chosen based on the type of annealing, followed by slow cooling. Homogenization: Aims to equalize the chemical composition by reducing differences. It involves holding at high temperatures for extended periods to promote diffusion. This process can lead to grain growth. Regeneration: Regenerates the microstructure and reduces grain size. It involves heating to 50°C above the austenitization temperature (A3 or Acm). This process relieves internal stresses. It is not recommended for hypereutectoid steels. Isothermal Annealing: Involves partial or total austenitization. Globularization or Spheroidization: Aims to produce globular cementite. It is used for hypereutectoid steels. Two methods: 1) Heating and cooling within a temperature range around A1. 2) Heating slightly above A1, holding for a long time, and then cooling slowly. Standard Annealing: Reduces grain size and homogenizes the microstructure, relieving internal stresses. It involves heating 50°C above the austenitization temperature and cooling in still air. Total austenitization is essential. Fine pearlite is obtained after annealing, resulting in smaller grains and improved microstructure for hypoeutectoid steels. This leads to greater tensile strength and elastic limit compared to hardened steel.
Tempering
The objective is to reduce the stresses produced during quenching, which cause increased hardness and brittleness. The result is improved toughness and decreased hardness. We start with tetragonal martensite, heat to a temperature always lower than A1, and hold to allow carbon to diffuse within the network, followed by cooling to ambient temperature. Martensitic transformation during tempering: starting from ambient temperature to 200°C. Transformation from tetragonal to cubic. Step 1: Temperature < 200°C: the tetragonal structure begins to lose carbon, forming cubic martensite and epsilon carbide. Step 2: Temperature approximately 200°C: the retained austenite transforms into bainite or martensite. This stage is associated with secondary hardening. Step 3: Temperature > 200°C: the cubic part of martensite and epsilon carbide continue to lose carbon, forming cementite. This results in less hardness and more toughness.
Quenching
Quenching: The objective is to obtain martensite. The heating temperature should be adequate, followed by appropriate cooling. A larger amount of martensite results in greater hardness. Total austenitization = hypoeutectoid: ferrite + pearlite = ferrite + austenite. Partial austenitization = hypereutectoid: austenite + cementite = cementite. The surface cools rapidly, resulting in high hardness and brittleness. The interior expands and tends to crack if cooling is too fast. Factors to consider: effect of size, cooling rate, and TTT curves. Carbon content: low carbon content shifts the curves to the right, delaying transformations. Alloying elements: shift the curves to the right, except for cobalt. Improved hardness not only facilitates quenching. Isothermal Quenching: The aim is to transform austenite at a constant temperature. 1. Martempering: Homogenizes temperature and prevents cracking. Transfers austenite to martensite at 100%. Continuous cooling to a temperature slightly above the martensite start temperature, followed by a cooling rate faster than the critical rate. Then, cool to ambient temperature. Used for thick pieces. 2. Austempering: Total transformation to bainite. Starts with austenitization, followed by rapid cooling to the bainite formation temperature and holding until transformation is complete. Results in good hardness and toughness. 3. Mixed Isothermal Quenching: Involves a partial transformation of austenite to bainite and martensite. Involves continuous cooling to the bainite formation region, holding until transformation is complete, and then cooling to ambient temperature. The remaining austenite transforms into martensite. Surface Quenching: Allows quenching only the surface of a piece, obtaining martensite at a certain depth. The difference from other methods is the heating time, which determines the depth of the austenitized zone. The necessary hardness is only achieved on the surface.
Surface Treatments
Designed to increase hardness and wear resistance of the surface while maintaining a tough and plastic core. Used for shafts subjected to surface wear. Surface hardening treatments include thermochemical treatments (carburizing and nitriding). Thermochemical Treatments: Involve changes in chemical composition through the diffusion of elements (C, N, B, S…) into the material. More expensive. Carburizing: Diffusion of carbon into the surface of low-carbon steel. Surface: pearlite + cementite. Interior: pearlite + ferrite. Involves forming, carburizing, quenching, and tempering. Carbon content should not exceed 0.9% because it weakens the surface layer. Good hardness is achieved with higher carbon content. Increased grain size, hardenability, and base steel strength make the process more expensive. Carburizing steels: low alloy steels with Cr and Mo (to avoid carbon networks); Ni and Mn (to avoid residual austenite and distortions during quenching). Nitriding: Designed to achieve surface hardness. Involves introducing nitrogen atoms into the microstructure, obtained from ammonia. Alloying elements (Al, Cr, Mo, V…) are used to form nitrides. Nitriding temperature is between 590-600°C (to avoid grain growth) and requires less time than carburizing. Involves quenching and tempering after nitriding. Controlled impurities are essential. The core has high toughness. A hard surface is achieved without quenching. The layer is thinner, fragile, and easily dislodged.