How to distinguish stainless steel and stainless iron

How to distinguish stainless steel and stainless iron

Stainless iron is a kind of stainless steel, its material is mainly 409, 410, 430, 444, belong to martensitic and ferritic stainless steel, they will be attracted by the magnet. And stainless steel unified refers to the corrosion of atmospheric or acid and other chemical media steel. But stainless steel is not rust, it is just different corrosion behavior in different media.

Common stainless steel materials are divided by metallography, which can be divided into martensitic stainless steel, ferritic stainless steel and austenitic stainless steel three types.

Martensite stainless steel

Common martensitic stainless steel carbon content is 0.1-0.45%, chromium content is 12-14%, is chromium stainless steel, also known as Cr13 stainless steel. Typical material models are 1Cr13, 2Cr13, 3Cr13, 4Cr13 and so on. This kind of stainless steel is usually used to make both can withstand the load and corrosion resistance of a variety of valves, pumps and other parts and some rust tools.

To enhance corrosion resistance, the carbon content of martensitic stainless steel is controlled in a very low range, usually no more than 0.4%. The lower the carbon content, the higher the corrosion resistance of stainless steel, and the higher the carbon content, the higher the carbon content in the matrix, the higher the strength and hardness of stainless steel, but this will form chromium carbide amount is more, so the corrosion resistance of stainless steel becomes worse. Therefore, it can be seen that the strength and hardness indexes of 4Cr13 are better than 1Cr13, but its corrosion resistance is worse than 1Cr13.

1Cr13 and 2Cr13 are resistant to atmospheric, vapor and other medium corrosion, generally used as corrosion resistant structural steel. In order to obtain good comprehensive performance, quenching and high temperature tempering (600~700℃) is often used to obtain tempered soxite, which is used to manufacture turbine blades and boiler tube accessories. And 3Cr13 and 4Cr13 steel, because the carbon content is higher, the corrosion resistance is relatively poor, through quenching and low temperature tempering (200~300℃), tempered martensite, with high strength and hardness (HRC up to 50), so often used as a tool steel, manufacturing medical instruments, cutting tools, hot oil pump shaft.

Ferrite stainless steel

Common ferritic stainless steel carbon content is less than 0.15%, chromium content is 12~30%, is also a chromium stainless steel, typical models are 0Cr13, 1Cr17, 1Cr17Ti, 1Cr28 and so on. Because the carbon content is correspondingly reduced, the chromium content is increased to a certain extent. The microstructure of stainless steel from room temperature to high temperature (960~1100℃) is always single-phase ferrite. Its corrosion resistance, plasticity and weldability are better than martensitic stainless steel. For high chromium ferritic stainless steel, its corrosion resistance of oxidation medium is strong, with the increase of chromium content, corrosion resistance is further improved.

Ferritic stainless steel cannot be strengthened by heat treatment because the phase transition does not occur during heating and cooling. If the grain is coarsened in the heating process, cold plastic deformation and recrystallization can only be used to improve the structure and performance.

This kind of stainless steel if in 450~550℃ stay, will cause the embrittlement of stainless steel, known as "475℃ brittleness". Embrittlement can be eliminated by heating to about 600℃ and then cooling quickly.

Austenitic stainless steel

Austenitic stainless steel is made by adding 8-11% nickel to stainless steel containing 18% chromium. For example, 1Cr18Ni9 is a typical model. The addition of nickel in this kind of stainless steel expands the austenitic region, so that the metastable single-phase austenitic structure can be obtained at room temperature. Because of the high chromium and nickel, and a single-phase austenitic structure, so it has higher chemical stability than chromium stainless steel, corrosion resistance is better, is now used more of a stainless steel.

18-8 type stainless steel in the annealing state shows the structure of austenite plus carbide, the existence of carbide, the corrosion resistance of stainless steel damage is larger, so the general use of solution treatment method, is to heat the stainless steel to 1100℃ after water cooling, so that the carbide dissolved in high temperature obtained austenite, and then through fast cooling, You get monophasic austenitic structures at room temperature.