Classification of Stainless Steels (by Room-Temperature Microstructure)

Stainless steels are a category of high-alloy steels based on the Fe-Cr, Fe-Cr-C, and Fe-Cr-Ni alloy systems, must contain a minimum of 10.5% chromium by mass. Steels with this minimum chromium content form a passive inert oxide layer on their surface, which protects the underlying metal from oxidation and corrosion in air free of corrosive media. On this basis, adding specific amounts of alloying elements such as Ni, Mo, Cu, Nb, Ti, and W enables the steel to exhibit properties like resistance to specific corrosion, high-temperature oxidation resistance, or certain high-temperature strength—these characteristics have led to the development of various types of stainless steels.

As indispensable corrosion-resistant structural materials in the modern industrial system, the core of performance differences among stainless steels lies in their crystalline microstructure at room temperature—a microstructural feature determined by the content of alloying elements (e.g., Cr, Ni, C) and subsequent heat treatment processes. The industry generally classifies stainless steels into five major categories based on room-temperature microstructure, with significant differences in compositional design, strengthening mechanisms, key properties, and application scenarios:

I. Martensitic Stainless Steels (α’ Phase)

Microstructural and Compositional Characteristics
• Crystal Structure: After “quenching + tempering”, it consists of body-centered tetragonal (BCT) martensite (α’) phase; the annealed state contains a small amount of ferrite, and it is strongly magnetic.
• Compositional Design: Medium chromium content (Cr: 11.5%–18%), medium-to-high carbon content (C: 0.08%–1.2%, carbon content determines hardness), no nickel, and other alloying elements account for less than 2%–3%.

Key Properties
• Advantages: High strength, high hardness, excellent wear resistance, and good fatigue resistance.
• Limitations: Poor corrosion resistance (only better than ordinary carbon steel; prone to rust in humid environments) and poor weldability.

In the 1950s, to improve the weldability of martensitic stainless steels, the carbon content was reduced to below 0.07%. A certain amount of nickel was added to enable martensitic transformation, forming a new steel series. With the development of refining technology and its application in stainless steel production, the carbon content in steels can be reduced to below 0.03%. By optimizing the steel composition as needed, a series of super martensitic stainless steels (e.g., 00Cr13Ni2Mo, 00Cr13Ni5Mo) were developed, with significantly improved weldability.

II. Ferritic Stainless Steels (α Phase)

Microstructural and Compositional Characteristics
• Crystal Structure: At room temperature, it consists of a single body-centered cubic (BCC) ferrite (α) phase, and it is strongly magnetic.
• Compositional Design: Medium-to-high chromium content (Cr: 12%–30%), low/no nickel (Ni ≤ 1%, significant cost advantage), low carbon content (C ≤ 0.12%); some grades add Ti/Nb (0.1%–0.3%) to inhibit carbide precipitation, and high-chromium grades add Mo (1%–3%) to enhance corrosion resistance.

Key Properties
• Advantages: Low cost, good thermal conductivity, and excellent resistance to chloride-induced stress corrosion cracking.
• Limitations: High brittleness and poor weldability.

Based on steel purity (especially carbon and nitrogen impurity content), ferritic stainless steels are divided into ordinary and high-purity types. Ordinary ferritic stainless steels have shortcomings such as low-temperature and room-temperature brittleness, high notch sensitivity, tendency to intergranular corrosion, and poor weldability. In contrast, high-purity ferritic stainless steels have extremely low carbon and nitrogen contents (0.015%–0.025%), exhibiting both good service performance and weldability (e.g., 03Cr17Mo, 01Cr27Mo).

III. Austenitic Stainless Steels (γ Phase)

Microstructural and Compositional Characteristics
• Crystal Structure: At room temperature, it consists of a single face-centered cubic (FCC) austenite (γ) phase, and it is non-magnetic.
• Compositional Design: Core composition of high chromium (Cr: 18%–25%) + high nickel (Ni: 8%–25%), low carbon content (C ≤ 0.12%; ultra-low carbon grades: C ≤ 0.03%) to avoid intergranular corrosion; some grades add Mo (2%–3%) to improve pitting corrosion resistance, or Ti/Nb to achieve carbon stabilization (e.g., 321, 347).

Key Properties
• Advantages: Optimal general corrosion resistance, excellent ductility, outstanding weldability, and good low-temperature toughness.
• Limitations: Relatively low strength; prone to intergranular corrosion at high temperatures (requires improvement via Ti/Nb stabilization or ultra-low carbon content); high cost.

IV. Duplex Stainless Steels (α+γ Phases)

Microstructural and Compositional Characteristics
• Crystal Structure: At room temperature, it has a duplex structure of “ferrite (α) + austenite (γ)”; the typical phase ratio is approximately 50:50 (allowable range: 30%–70%), and it is weakly magnetic.
• Compositional Design: High chromium content (Cr: 21%–27%), medium nickel content (Ni: 3%–10%); mandatory addition of Mo (1.5%–4%) and N (0.08%–0.25%)—N expands the austenite region and improves strength, while Mo significantly enhances pitting corrosion resistance; low carbon content (C ≤ 0.03%); reduced Ni/Mo usage results in lower cost than high-alloy austenitic stainless steels.

Key Properties
• Advantages: Balanced “strength + corrosion resistance”—high tensile strength; higher Pitting Resistance Equivalent Number (PREN) corresponds to better corrosion resistance (PREN ≈ 34 for 2205; PREN ≈ 45 for 2507, with chloride pitting resistance far exceeding that of 316L); excellent resistance to stress corrosion cracking (SCC).
• Limitations: Stringent welding process requirements; moderate low-temperature toughness.

V. Precipitation Hardening Stainless Steels (PH Steels)

Microstructural and Compositional Characteristics
• Crystal Structure: They are Fe-Cr-Ni alloys containing precipitation-hardening elements (Cu, Al, Ti, Nb) and are strengthened via heat treatment. Based on microstructure, they are divided into three categories: martensitic precipitation hardening stainless steels, semi-austenitic precipitation hardening stainless steels, and austenitic precipitation hardening stainless steels.
• Compositional Design: Medium chromium content (Cr: 15%–17%), medium nickel content (Ni: 3%–7%); addition of precipitation-hardening elements (Cu: 3%–5%, Al: 0.7%–1.5%, Ti: 0.1%–0.5%); low carbon content (C ≤ 0.07%).

Key Properties
• Advantages: Ultra-high strength; corrosion resistance close to that of austenitic stainless steels (atmospheric corrosion resistance equivalent to 304); combines high strength and lightweight properties.
• Limitations: Complex aging process; high cost.

In general, most stainless steels have good weldability, but appropriate process measures are required to avoid cold/hot cracking, embrittlement, and deterioration of properties such as intergranular corrosion resistance and stress corrosion resistance. However, stainless steels designed for excellent machinability (e.g., 303, 416, 416Se, 430F, 430FSe with high S, high P, or Se content) and high-carbon stainless steels (e.g., 440 series) are extremely difficult to weld.