Are you still puzzled about the difference between stainless steel and stainless iron? How to distinguish? After reading this, you will know

Stainless steel is generally a general term for stainless steel and acid-resistant steel. Stainless steel refers to steel that is resistant to weak media such as atmosphere, steam, and water corrosion, while acid-resistant steel refers to steel that is resistant to chemical corrosive media such as acid, alkali, and salt corrosion. The difference between stainless steel and stainless steel lies in the composition of nickel.

Stainless iron is a type of stainless steel made by processing recycled scrap iron, lead, steel, and other materials through secondary furnace treatment and desintering. Its models include 409, 410, 430, 444, which belong to martensitic and ferritic stainless steels. It has magnetic properties when used as a magnet. Austenitic stainless steels include 201, 202, 304, 321, 316L, etc.

Stainless steel (also known as stainless acid-resistant steel) refers to steel that can resist corrosion from atmospheric or acidic chemical media. Stainless steel is not rust-free, but its corrosion behavior varies in different media. There are many types of stainless steel with different properties, and they have gradually formed several major categories in the development process.

According to organizational structure, it can be divided into four categories: martensitic stainless steel (including precipitation-hardened stainless steel), ferritic stainless steel, austenitic stainless steel, and austenitic-ferritic duplex stainless steel;

According to the main chemical composition of steel or some characteristic elements in steel, it can be classified into chromium stainless steel, chromium-nickel stainless steel, chromium-nickel-molybdenum stainless steel, low-carbon stainless steel, high-molybdenum stainless steel, high-purity stainless steel, etc;

According to the performance characteristics and applications of steel, it can be classified into nitric acid resistant stainless steel, sulfuric acid resistant stainless steel, pitting corrosion resistant stainless steel, stress corrosion resistant stainless steel, high-strength stainless steel, etc.

According to the functional characteristics of steel, it can be classified into low-temperature stainless steel, non-magnetic stainless steel, easy cutting stainless steel, superplasticity stainless steel, etc.

The commonly used classification method currently is based on the structural characteristics and chemical composition of steel, as well as a combination of both. Generally divided into martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, duplex stainless steel, and precipitation hardening stainless steel, or into two categories: chromium stainless steel and nickel stainless steel.

a. Martensitic stainless steel

The commonly used martensitic stainless steel has a carbon content of 0.1-0.45% and a chromium content of 12-14%, belonging to chromium stainless steel, usually referring to Cr13 type stainless steel. Typical steel grades include 1Cr13, 2Cr13, 3Cr13, 4Cr13, etc. This type of steel is generally used to make various valves, pumps, and other parts that can withstand loads and require corrosion resistance, as well as some stainless tools.

In order to improve corrosion resistance, the carbon content of martensitic stainless steel is controlled within a very low range, generally not exceeding 0.4%. The lower the carbon content, the better the corrosion resistance of the steel, while the higher the carbon content, the higher the carbon content in the matrix, resulting in higher strength and hardness of the steel; The higher the carbon content, the more chromium carbides are formed, and its corrosion resistance becomes worse. It is not difficult to see that the strength and hardness indicators of 4Cr13 are better than those of 1Cr13, but its corrosion resistance is not as good as 1Cr13.

1Cr13 and 2Cr13 have the ability to resist corrosion from atmospheric, steam and other media, and are often used as corrosion-resistant structural steels. In order to achieve good comprehensive performance, quenching and high-temperature tempering (600-700 ℃) are often used to obtain tempered martensite, which is used to manufacture turbine blades, boiler tube accessories, etc. However, 3Cr13 and 4Cr13 steels have relatively poor corrosion resistance due to their higher carbon content. Through quenching and low-temperature tempering (200~300 ℃), tempered martensite is obtained, which has high strength and hardness (HRC up to 50). Therefore, they are often used as tool steels to manufacture medical devices, cutting tools, hot oil pump shafts, etc.

b. Ferritic stainless steel

The commonly used ferritic stainless steel has a carbon content of less than 0.15% and a chromium content of 12-30%, which also belongs to chromium stainless steel. Typical steel grades include 0Cr13, 1Cr17, 1Cr17Ti, 1Cr28, etc. Due to the corresponding decrease in carbon content and increase in chromium content, the microstructure of steel remains a single-phase ferrite structure when heated from room temperature to high temperature (960-1100 ℃). Its corrosion resistance, plasticity, and weldability are all superior to martensitic stainless steel. For high chromium ferritic stainless steel, its ability to resist medium corrosion is strong, and its corrosion resistance further improves with the increase of chromium content.

Adding titanium to steel can refine grain size, stabilize carbon and nitrogen, and improve the toughness and weldability of the steel. Ferritic stainless steel cannot be strengthened by heat treatment methods because it does not undergo phase transformation during heating and cooling. If the grain coarsens during the heating process, only cold plastic deformation and recrystallization can be used to improve the structure and properties. If this type of steel stays at 450-550 ℃, it will cause embrittlement of the steel, known as "475 ℃ brittleness". By heating to about 600 ℃ and then rapidly cooling, embrittlement can be eliminated. It should also be noted that this type of steel, when heated for a long time at 600-800 ℃, will also produce a hard and brittle σ phase, causing the material to exhibit σ phase brittleness.

In addition, during rapid cooling above 925°C, there is a tendency for intergranular corrosion and brittleness caused by significant grain coarsening. These phenomena are serious problems for welding parts. The former can be eliminated by short-term tempering at 650-815°C. This type of steel has obviously lower strength than martensitic stainless steel and is mainly used to manufacture corrosion-resistant parts, widely used in the nitric acid and nitrogen fertilizer industries.

c. Austenitic stainless steel

Adding 8-11% Ni to steel containing 18% Cr results in austenitic stainless steel. 1Cr18Ni9 is the most typical steel grade. This type of steel expands the austenite region due to the addition of nickel, resulting in a metastable single-phase austenite structure at room temperature. Due to its high content of chromium and nickel and single-phase austenitic structure, it has higher chemical stability and better corrosion resistance than chromium stainless steel, making it the most widely used type of stainless steel currently.

18-8 stainless steel exhibits an austenite+carbide structure in the annealed state. The presence of carbides can cause significant damage to the corrosion resistance of the steel. Therefore, a solution treatment method is commonly used, which involves heating the steel to 1100 ℃ and then cooling it with water to dissolve the carbides in the austenite obtained at high temperatures. Through rapid cooling, a single-phase austenite structure is obtained at room temperature.

The commonly known stainless steel refers to ferritic stainless steel and martensitic stainless steel. Used to distinguish it from the most commonly used austenitic stainless steel with good rust prevention performance.

d. Austenitic ferritic duplex stainless steel

It is a stainless steel with austenite and ferrite structures each accounting for about half. In the case of low C content, the Cr content is between 18% and 28%, and the Ni content is between 3% and 10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, N, etc. This type of steel combines the characteristics of austenitic and ferritic stainless steel. Compared with ferritic stainless steel, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance. At the same time, it maintains the 475 ℃ brittleness and high thermal conductivity of ferritic stainless steel, and has characteristics such as superplasticity.

Compared with austenitic stainless steel, it has higher strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Dual phase stainless steel has excellent resistance to pitting corrosion and is also a nickel-saving stainless steel.

How to distinguish between stainless steel and "stainless iron" products?

Identification from markings: Many stainless steel products have steel stamps on their surfaces, such as 13-0, 18-8, etc. The number before the short line indicates the chromium content of the product, and the number after the short line indicates the nickel content of the product. Like 13-0, it only contains chromium and no nickel, commonly known as "stainless iron"; And 18-8 indicates that the product contains both chromium and nickel, which is stainless steel. From voice negotiation: tapping on stainless steel or "stainless iron" products can also be used as a method of judgment. Attract with permanent magnets: True stainless steel is not attracted by magnets, while "stainless iron" can be attracted by magnets. Although there are differences in properties between "stainless iron" and stainless steel, their corrosion resistance is significantly better than that of forged iron and cast iron cookware.