How to distinguish between stainless steel and stainless iron
Stainless iron is a kind of stainless steel. Its materials are mainly 409, 410, 430, 444, etc., which belong to martensitic and ferritic stainless steels, and they will be attracted by magnets. The stainless steel uniform refers to the steel that can resist the corrosion of chemical media such as atmosphere or acid. But stainless steel is not rust-free, it just has different corrosion behavior in different media.

Common stainless steel materials can be divided into three types: martensitic stainless steel, ferritic stainless steel and austenitic stainless steel by metallographic structure.

Martensitic stainless steel

Common martensitic stainless steels have a carbon content of 0.1-0.45% and a chromium content of 12-14%. They are chromium stainless steels, generally called Cr13 stainless steels. Typical material models are 1Cr13, 2Cr13, 3Cr13, 4Cr13, etc. This type of stainless steel is usually used to make various valves, pumps and other parts and some stainless tools that can bear both load and corrosion resistance.

In order to enhance corrosion resistance, the carbon content of martensitic stainless steels is controlled to a very low content range, usually not more than 0.4%. The lower the carbon content, the higher the corrosion resistance of the stainless steel, and the higher the carbon content, the higher the carbon content in the matrix, the higher the strength and hardness of the stainless steel, but this will form the amount of chromium carbides. The more, the worse the corrosion resistance of such stainless steel becomes. Therefore, it can be seen that the strength and hardness of 4Cr13 are higher than that of 1Cr13, but its corrosion resistance is worse than that of 1Cr13.

Both 1Cr13 and 2Cr13 are resistant to atmospheric, steam and other media corrosion, and are generally used as corrosion-resistant structural steel. In order to obtain good comprehensive performance, quenching and high temperature tempering (600~700℃) are often used to obtain tempered sorbite to manufacture steam turbine blades, boiler tube accessories, etc. However, 3Cr13 and 4Cr13 steels have relatively poor corrosion resistance due to higher carbon content. Through quenching and 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.

Ferritic stainless steel

The carbon content of common ferritic stainless steel is less than 0.15%, and the chromium content is 12~30%. It is also a kind of chromium stainless steel. Because the carbon content decreases accordingly, the chromium content increases to a certain extent. The microstructure of stainless steel is always single-phase ferrite when heated from room temperature to high temperature (960~1100℃). Its corrosion resistance, plasticity and weldability are better than martensitic stainless steel. For high-chromium ferritic stainless steel, its ability to resist oxidative medium corrosion is strong, and with the increase of chromium content, the corrosion resistance is further improved.

Ferritic stainless steels cannot be strengthened by heat treatment because they do not undergo phase transformation when heated and cooled. If the grains are coarsened during the heating process, only cold plastic deformation and recrystallization can be used to improve the structure and improve the properties.

If this kind of stainless steel stays at 450~550℃, it will cause embrittlement of stainless steel, which is called “475℃ brittleness”. Embrittlement can be eliminated by heating to about 600°C followed by rapid cooling.

Austenitic stainless steel

Add 8-11% nickel to stainless steel containing 18% chromium, and it becomes austenitic stainless steel. Such as 1Cr18Ni9 is a typical model. This type of stainless steel expands the austenite region due to the addition of nickel, so that a metastable single-phase austenite structure can be obtained at room temperature. Because it contains high chromium and nickel, and has a single-phase austenite structure, it has higher chemical stability and better corrosion resistance than chromium stainless steel. It is a kind of stainless steel that is widely used now.

18-8 type stainless steel shows the structure of austenite and carbides in the annealed state. The existence of carbides does great damage to the corrosion resistance of stainless steel. Therefore, the solution treatment method is generally used, which is to heat the stainless steel 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.