What Is Martensitic Steel?
Martensitic steel refers to a group of stainless steel alloys characterized by a martensite crystal structure at room temperature.
Containing chromium as a primary element along with iron, it falls under chromium-based stainless steel, similar to ferritic stainless steel. Notable examples include SUS403 and SUS410, which comprise 13% chromium.
Applications of Martensitic Steel
Due to its relatively low chromium content, which is crucial for passivity formation, martensitic steel offers less corrosion resistance compared to other stainless steel types, limiting its use in corrosive environments. However, it finds extensive applications thanks to its affordability, strength, and heat resistance. Common uses include mechanical structural components like shafts and bolts, and molds for plastic injection molding. Furthermore, variants such as SUS440 and SUS420, recognized for their hardness post-quenching and tempering, are frequently chosen for cutlery, including knives, and medical instruments like scalpels.
Properties of Martensitic Steel
While properties can vary across different grades, a shared feature of martensitic steel is ferromagnetism, attributed to its body-centered cubic lattice structure. This is in contrast to austenitic stainless steel, which is non-magnetic due to its face-centered cubic lattice. The strength of martensitic steel is enhanced by quenching, and its wear resistance and toughness are further improved by tempering. The presence of a relatively high carbon content in austenitic stainless steel enables the formation of a hardened microstructure through high-temperature quenching.
Generally, martensitic steel undergoes quenching and tempering, with the resulting properties dependent on the tempering temperature. For instance, low-temperature tempering at 150 to 200°C enhances wear resistance, whereas high-temperature tempering at 600 to 750°C followed by rapid cooling boosts toughness. Conversely, tempering at around 475°C can lead to decreased ductility and toughness, a phenomenon known as 475°C embrittlement.
Structure of Martensitic Steel
Martensitic steel, containing approximately 11-18% chromium, exhibits a single-phase austenite structure or a dual-phase structure with minor ferrite at high temperatures. The carbon, soluble in austenite at elevated temperatures, transforms into a martensitic microstructure upon rapid cooling post-quenching. The fundamental composition includes 13% chromium and 0.2% carbon, with typical examples being SUS410 and SUS420J2, containing 11.50-13.00% chromium and up to 0.15% carbon, and SUS420J2, with 12.00-14.00% chromium and 0.26-0.40% carbon, respectively. These are also referred to as 13Cr steel or the 13Cr series.
How to Select Martensitic Steel
Choosing the appropriate stainless steel—martensitic, ferritic, or austenitic—depends on the application, considering the variations in properties with different crystal structures.
1. Martensitic Stainless Steel
Primarily used for blades and cutting parts, martensitic steel is hardened through quenching.
2. Ferritic Stainless Steel
Optimal for building interiors, automotive components, and commercial kitchens. It resists stress corrosion cracking without hardening by quenching, offering great economic value.
3. Austenitic Stainless Steel
Known for its strength, ductility, toughness, and heat resistance, this alloy is non-magnetic and highly recyclable. Attention must be paid to stress corrosion cracking.