What elements determine stainless steel grades?

The key elements that determine stainless steel grades are chromium, nickel, carbon, molybdenum, manganese, nitrogen, and iron. The proportions of these elements — along with minor alloying additions like titanium, niobium, and silicon — define the grade and its corresponding series such as 200, 300, 400, and duplex categories.

Major Alloying Elements and Their Influence

• Chromium (Cr): The most essential element (10.5–30%) that provides stainless steel’s signature corrosion resistance by forming a protective oxide layer. Higher chromium content leads to increased resistance against oxidation and scaling.
• Nickel (Ni): Enhances toughness, ductility, and corrosion resistance, especially in acidic environments. It also stabilizes the austenitic structure in the 300-series grades (e.g., 304 with 18% Cr, 8% Ni).
• Carbon (C): Adds hardness and strength but reduces corrosion resistance when excessive. Low-carbon variants (suffix “L”, e.g., 316L) prevent carbide precipitation during welding.
• Molybdenum (Mo): Improves resistance to pitting and crevice corrosion caused by chlorides. Found in marine-grade steels like 316 (2–3% Mo).
• Manganese (Mn) and Nitrogen (N): These elements substitute nickel in 200-series steels. They enhance strength and austenite stabilization while reducing cost.
• Titanium (Ti) and Niobium (Nb): Added to stabilize carbon and prevent intergranular corrosion, especially in welded structures (e.g., 321 and 347 grades).

Major Families and Grade Structure

Stainless steels are classified into five main categories based on their microstructure and alloying composition :

The specific grade numbers (e.g., 304, 316, 430) refer to the relative proportions of these key elements as standardized by organizations such as the SAE, ASTM, and ISO.