What is the difference between stainless steel and stainless iron? How to tell?

What is the difference between stainless steel and stainless iron? How to tell?
 

Stainless iron is a type of stainless steel. The models are: 409 410 430 444. It belongs to martensitic and ferritic stainless steel. It will be magnetic when using a magnet. Austenitic stainless steel includes 201 202 304 321 316L, etc.

Stainless steel (also known as stainless acid-resistant steel) refers to steel that can resist corrosion by chemical media such as the atmosphere or acid. Stainless steel is not rust-free, but its corrosion behavior in different media is different. Commonly used stainless steels Commonly used stainless steels can be divided into three types: martensitic stainless steel, ferritic stainless steel and austenitic stainless steel according to their organizational characteristics.

a. Martensitic stainless steel

Commonly used martensitic stainless steel has a carbon content of 0.1~0.45% and a chromium content of 12~14%. It is a chromium stainless steel, usually referred to as Cr13 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 as well as some stainless tools that can withstand loads and require corrosion resistance.

In order to improve corrosion resistance, the carbon content of martensitic stainless steel is controlled in a very low range, generally no more than 0.4%. The lower the carbon content, the better the corrosion resistance of the steel, and the higher the carbon content, the higher the carbon content in the matrix, the higher the strength and hardness of the steel; the higher the carbon content, the more likely it is to form chromium. The more carbides there are, the worse the corrosion resistance becomes. It is not difficult to see from this that the strength and hardness indicators of 4Cr13 are worse than 1Cr13, but its corrosion resistance is not as good as 1Cr13.

1Cr13 and 2Cr13 have the ability to resist corrosion from atmosphere, steam and other media, and are often used as corrosion-resistant structural steel. In order to obtain good comprehensive performance, quenching + high-temperature tempering (600~700°C) is often used to obtain tempered sorbite to manufacture steam turbine blades, boiler tube accessories, etc. As for 3Cr13 and 4Cr13 steel, due to their higher carbon content, their corrosion resistance is relatively poor. Through quenching + low temperature tempering (200~300℃), tempered martensite is obtained, which has higher strength and hardness (HRC Up to 50), so it is often used as tool steel to manufacture medical equipment, cutting tools, hot oil pump shafts, etc.


b. Ferritic stainless steel

Commonly used ferritic stainless steel has a carbon content of less than 0.15% and a chromium content of 12 to 30%. It is also a chromium stainless steel. Typical steel grades include 0Cr13, 1Cr17, 1Cr17Ti, 1Cr28, etc. As the carbon content decreases and the chromium content increases accordingly, when steel is heated from room temperature to high temperature (960~1100°C), its microstructure is always a single-phase ferrite structure. Its corrosion resistance, plasticity and weldability are better than martensitic stainless steel. For high-chromium ferritic stainless steel, its ability to resist corrosion in oxidizing media is strong. As the chromium content increases, the corrosion resistance further improves.

Adding titanium to steel can refine the grains, stabilize carbon and nitrogen, and improve the toughness and weldability of the steel. Ferritic stainless steel does not undergo phase change when heated and cooled, so the steel cannot be strengthened by heat treatment. If the grains are coarsened during the heating process, cold plastic deformation and recrystallization can only be used to improve the structure and performance. If this type of steel stays at 450~550℃, it will cause embrittlement of the steel, which is called "475℃ brittleness". Embrittlement can be eliminated by heating to about 600°C and then cooling quickly. It should also be noted that long-term heating of this type of steel at 600~800°C will produce a hard and brittle σ phase, causing the material to become σ phase brittle. In addition, when quenched above 9250C, intergranular corrosion tendencies and brittleness caused by significant grain coarsening will occur. These phenomena are serious problems for welding parts. The former can be eliminated by short-term tempering at 650~815℃. This type of steel is obviously lower in strength than martensitic stainless steel and is mainly used to make corrosion-resistant parts and is widely used in the nitric acid and nitrogen fertilizer industries.

c. Austenitic stainless steel

Adding 8~11% Ni to steel containing 18% Cr is the best austenitic stainless steel. For example, 1Cr18Ni9 is the most typical steel grade. Due to the addition of nickel, this type of steel expands the austenite area, so that a metastable single-phase austenite structure can be obtained at room temperature. Due to its high content of chromium and nickel and its single-phase austenite structure, it has higher chemical stability and better corrosion resistance than chromium stainless steel. It is currently the most widely used type of stainless steel.

Type 18-8 stainless steel exhibits an austenite + carbide structure in the annealed state. The presence of carbides will greatly damage the corrosion resistance of the steel. Therefore, solution treatment is usually used, that is, the steel is heated to 1100°C. After water cooling, the carbides are dissolved in the austenite obtained at high temperature, and then through rapid cooling, a single-phase austenite structure is obtained at room temperature.

Commonly known as stainless steel refers to ferritic stainless steel and martensitic stainless steel. It is used to distinguish it from austenitic stainless steel, which has good anti-rust properties and is the most commonly used.