Stainless steel is a general term encompassing both stainless steel and acid-resistant steel. Stainless steel refers to steel that resists corrosion from weak media such as the atmosphere, steam, and water. Acid-resistant steel, on the other hand, resists corrosion from chemically aggressive media like acids, alkalis, and salts. The primary distinction between "stainless iron" and standard stainless steel lies in their nickel content. How can they be differentiated?
"Stainless iron" is technically a subset of stainless steel, with common grades including 409, 410, 430, and 444. These belong to the martensitic and ferritic stainless steel families and are magnetic (attracted to magnets).
Stainless steel (also known as stainless acid-resistant steel) is defined by its ability to resist corrosion from atmospheric or chemical media. It is important to note that stainless steel is not entirely rust-proof; rather, its corrosion behavior varies depending on the environment. Based on their microstructure, commonly used stainless steels are classified into three types: martensitic, ferritic, and austenitic.
a. Martensitic Stainless Steel
Common martensitic stainless steels contain 0.1–0.45% carbon and 12–14% chromium. They are chromium-based steels, typically referred to as Cr13 series. Representative grades include 1Cr13, 2Cr13, 3Cr13, and 4Cr13. These steels are generally used to manufacture components that require both load-bearing capacity and corrosion resistance, such as valves, pump parts, and certain stainless tools.
To enhance corrosion resistance, the carbon content in martensitic stainless steel is strictly controlled, usually below 0.4%. Lower carbon content improves corrosion resistance, while higher carbon content increases strength and hardness but also promotes the formation of chromium carbides, which reduces corrosion resistance. Consequently, 4Cr13 offers superior strength and hardness compared to 1Cr13, but its corrosion resistance is inferior.
Grades 1Cr13 and 2Cr13 resist corrosion from atmospheric and steam environments and are often used as corrosion-resistant structural steels. To achieve optimal comprehensive properties, they undergo quenching followed by high-temperature tempering (600–700°C) to form tempered sorbite, making them suitable for turbine blades and boiler tube fittings. Conversely, 3Cr13 and 4Cr13, with their higher carbon content and relatively lower corrosion resistance, are treated with quenching and low-temperature tempering (200–300°C) to form tempered martensite. This yields high strength and hardness (up to HRC 50), making them ideal for tool applications such as medical instruments, cutting tools, and hot oil pump shafts.
b. Ferritic Stainless Steel
Common ferritic stainless steels contain less than 0.15% carbon and 12–30% chromium. They are also chromium-based steels, with typical grades including 0Cr13, 1Cr17, 1Cr17Ti, and 1Cr28. Due to their lower carbon and higher chromium content, these steels maintain a single-phase ferrite microstructure from room temperature up to high temperatures (960–1100°C). Their corrosion resistance, ductility, and weldability are superior to those of martensitic stainless steels. High-chromium ferritic stainless steels exhibit strong resistance to corrosive media, with corrosion resistance increasing alongside chromium content.
The addition of titanium refines grain size, stabilizes carbon and nitrogen, and improves toughness and weldability. Since ferritic stainless steels do not undergo phase transformations during heating and cooling, they cannot be strengthened through heat treatment. If grain coarsening occurs during heating, cold plastic deformation and recrystallization must be applied to refine the microstructure and improve properties. Prolonged exposure to temperatures between 450–550°C causes embrittlement, known as "475°C embrittlement," which can be eliminated by heating to approximately 600°C followed by rapid cooling. Additionally, long-term heating at 600–800°C leads to the precipitation of a hard and brittle sigma (σ) phase, causing σ-phase embrittlement. Rapid cooling from above 925°C can induce intergranular corrosion susceptibility and brittleness due to significant grain coarsening. These issues pose serious challenges for welded joints; however, the former can be mitigated through short-term tempering at 650–815°C. Although weaker than martensitic stainless steels, ferritic grades are primarily used for corrosion-resistant components and are widely employed in the nitric acid and nitrogen fertilizer industries.
c. Austenitic Stainless Steel
Adding 8–11% nickel to steel containing 18% chromium produces austenitic stainless steel, with 1Cr18Ni9 being the most representative grade. The addition of nickel expands the austenite phase field, resulting in a metastable single-phase austenite structure at room temperature. Thanks to their high chromium and nickel content and single-phase austenite structure, these steels offer greater chemical stability and superior corrosion resistance compared to chromium-only stainless steels, making them the most widely used category today.
In the annealed state, 18-8 type stainless steel exhibits an austenite + carbide microstructure. The presence of carbides significantly impairs corrosion resistance; therefore, solution treatment is typically applied. This involves heating the steel to 1100°C followed by water quenching, dissolving carbides into the high-temperature austenite. Rapid cooling then retains a single-phase austenite structure at room temperature.
Colloquially, "stainless iron" refers specifically to ferritic and martensitic stainless steels, distinguishing them from the more corrosion-resistant and universally used austenitic stainless steels.
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