At the work site, it is often heard that some people refer to the rusted stainless steel pipes or guardrails and say that this must be a stainless steel material. But, some people use magnets to judge whether the material is stainless steel. It is not stainless steel that is considered magnetic, but stainless iron. Stainless steel is not magnetic. But be aware that even austenitic stainless steels are somewhat magnetic in some cases. Although the magnetism is relatively weak. So, what is stainless steel? what is stainless iron? what are the main differences between stainless steel and stainless iron? The following is some of the information I found and a summary.

Stainless iron

Stainless iron is a kind of stainless steel. Its materials are mainly 409, 410, 430, 444, etc. It belongs to martensitic and ferritic stainless steel. They will be attracted by magnets.

The main models of stainless iron are 409, 410, 430, 444

409 Stainless Steel: Inexpensive. It is a ferritic stainless steel commonly used in machinery, construction and other projects. Has good mechanical properties, excellent strength and hardness. Often used in exhaust pipes.

410 stainless steel: It belongs to martensitic stainless steel. After quenching and tempering, the strength, plasticity and toughness are well matched. Has good corrosion resistance and machinability. Commonly used in valves and blades.

430 stainless steel: has good corrosion resistance. But there is a tendency for intergranular corrosion. Mainly used in architectural decoration, fuel burner parts.

444 stainless steel: It is a high-alloy ferritic stainless steel with good resistance to pitting corrosion, crevice corrosion and stress corrosion. Suitable for heat exchangers, water heaters, water storage tanks, solar cell phone panels, etc.

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 behaves differently in different media. Common stainless steel materials are divided into three types. Martensitic stainless steel. Ferritic stainless steel. Austenitic stainless steel.

Martensitic stainless steel

The carbon content of common martensitic stainless steel is 0.1-0.45%, and the chromium content is 12-14%. It is a chromium stainless steel, generally called Cr13 stainless steel. Typical material models are 1Cr13, 2Cr13, 3Cr13, 4Cr13, etc. These stainless steel are usually used to make various valves, pumps and some stainless tools. These products are both load-resistant and resistant to corrosion.

To enhance corrosion resistance, the carbon content of martensitic stainless steel is controlled to a very low content range. Usually no more than 0.4%. The lower the carbon content, the better the corrosion resistance of the stainless steel. The higher the carbon content, the higher the carbon content in the matrix. Then the strength and hardness of stainless steel are higher. But this also increases the amount of carbides that form chromium. In this way, the corrosion resistance of stainless steel becomes worse. So, it can be seen that the strength and hardness of 4Cr13 are higher than those of 1Cr13. But its corrosion resistance is worse than 1Cr13.

Both 1Cr13 and 2Cr13 are resistant to atmospheric, steam and other medium corrosion. Generally used as corrosion-resistant structural steel. In order to get good comprehensive properties, quenching and high-temperature tempering (600~700℃) are often used. Tempered sorbate is obtained. To manufacture steam turbine blades, and boiler tube accessories. And 3Cr13 and 4Cr13 steel, because the carbon content is higher. Corrosion resistance is relatively poor. Tempered martensite is obtained by quenching and low-temperature tempering (200~300℃). Has high strength and hardness (HRC up to 50). So, it is often used as tool steel. Manufacture of medical equipment, cutting tools, hot oil pump shafts, etc.

Ferritic stainless steel

Common ferritic stainless steels have a carbon content of less than 0.15% and a chromium content of 12-30%. It is chrome stainless steel. Typical models are 0Cr13, 1Cr17, 1Cr17Ti, 1Cr28. Since the carbon content is correspondingly reduced, the chromium content is increased to a certain extent. Stainless steel is heated from room temperature to high temperature (960~1100℃). Its microstructure has always been single-phase ferrite. 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. With the increase of chromium content, the corrosion resistance is further improved.

Ferritic stainless steel, because no phase transformation occurs when heating and cooling. So, stainless steel cannot be strengthened by heat treatment. If the grains are coarsened during heating. Only cold plastic deformation and recrystallization can be used to improve the structure and properties.

If this kind of stainless steel stays at 450~550℃, it will cause the embrittlement of the stainless steel. It 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. Thus, 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. So, it has higher chemical stability than chromium stainless steel. Corrosion resistance is also better. It is the most widely used stainless steel.

18-8 type stainless steel shows the structure of austenite and carbides in the annealed state and the existence of carbides. The corrosion resistance of stainless steel is greatly damaged. So, the solution treatment method is generally used. It is to heat the stainless steel to 1100 ℃ and then water-cool it. The carbides are dissolved in the austenite obtained at a high temperature. Through rapid cooling, a single-phase austenite structure is obtained at room temperature.