What is the hardening mechanism of martensitic stainless steel

In materials science and metallurgical engineering, martensitic stainless steel has attracted considerable attention for its unique hardening ability. Understanding its hardening mechanism is crucial for optimizing material properties and guiding heat treatment processes. The hardening of martensitic stainless steel is essentially a complex process in which metastable austenite undergoes a diffusionless phase transformation during rapid cooling (quenching) to a supersaturated solid solution, namely martensite.

Austenite: Preparation Before Quenching
The quenching process begins with heating. Martensitic stainless steel is heated to a sufficiently high temperature, typically between 850°C and 1050°C, to completely or largely transform its internal structure into austenite. Austenite is a solid solution with a face-centered cubic (FCC) structure. At this high temperature, the carbon and chromium atoms in the alloy are fully dissolved in the austenite lattice. Austenite exhibits good plasticity but relatively low hardness, preparing the structure for subsequent quenching.

Quenching: A Critical Phase Transformation
Quenching is the core step in achieving hardness. When steel is rapidly cooled from the austenitizing temperature, carbon atoms do not have sufficient time to diffuse out of the crystal lattice. Due to the rapid drop in temperature, the face-centered cubic (FCC) lattice of austenite becomes unstable. To adapt to the low-temperature conditions, the lattice must transform. However, the carbon atoms are unable to diffuse and become "trapped" in the new lattice structure. This rapid, diffusion-free lattice restructuring leads to the transformation of austenite to martensite.
Martensite has a body-centered tetragonal (BCT) lattice structure. Compared to the FCC structure of austenite, the BCT lattice is "stretched" along the c-axis by carbon atoms, while being compressed along the a- and b-axes. This lattice distortion creates significant internal stress, which is the fundamental reason for martensite's high hardness. Imagine, at a microscopic level, the countless trapped carbon atoms act like nails, preventing movement between the lattice layers, thereby significantly increasing the material's hardness and strength.

Characteristics and Influencing Factors of the Martensitic Transformation
The martensitic transformation has several notable characteristics:
Diffusionlessness: This is the most fundamental difference between the martensitic transformation and traditional diffusion-type phase transformations. Carbon and alloying atoms undergo almost no long-distance diffusion, resulting in an extremely rapid phase transformation, complete in less than a second.
Shear Mechanism: The phase transformation occurs through the coordinated shearing of atomic layers. The lattice reconfiguration acts like a pair of scissors, with one atomic layer sliding and pulling adjacent atomic layers with it. This shearing process creates the lamellar or flaky structure unique to martensite.
Time-Independent Phase Transformation: The martensitic transformation temperature (Ms) and the martensitic finish temperature (Mf) are key factors in determining whether a phase transformation occurs. Phase transformation begins immediately below the Ms point, and ends below the Mf point. The extent of the phase transformation is dependent solely on the final cooling temperature and is independent of the duration of the phase transformation at that temperature.

Many factors influence the hardening effect, but two are the most important:
Carbon content: Carbon is the most important hardening element in martensitic stainless steel. The higher the carbon content, the greater the lattice distortion of the martensite formed after quenching, and the higher the hardness. For example, 440C stainless steel has extremely high hardness due to its high carbon content.
Alloying elements: In addition to carbon, alloying elements such as chromium, molybdenum, and vanadium are also crucial. They lower the martensitic transformation temperature (Ms) and increase hardenability. Hardenability refers to the ability of steel to form martensite from the surface to the core during quenching. By dissolving into austenite, these alloying elements delay the formation of diffusion phases such as pearlite and bainite, providing a longer "window" for the martensitic transformation.

Tempering: Balancing Hardness and Toughness
Martensite after quenching is extremely hard, but it also exhibits significant internal stresses and high brittleness, making it difficult to use directly. Therefore, tempering is necessary. Tempering involves reheating the quenched steel to a temperature below the Ms point and holding it at that temperature for a period of time. The purpose of tempering is to release internal stresses and improve the material's toughness while maintaining a high hardness. During the tempering process, supersaturated carbon atoms precipitate from the martensite lattice, forming fine carbides dispersed throughout the ferrite matrix. This precipitation strengthening mechanism allows the material to maintain high strength while improving toughness. Different tempering temperatures produce different microstructures and properties. For example, low-temperature tempering (approximately 150-250°C) primarily maintains high hardness, while high-temperature tempering (approximately 500-650°C) significantly improves toughness and ductility, but reduces hardness.