Ferritic stainless steel

Ferritic stainless steel：

Ferritic stainless steel (400 series) contains 15% to 30% chromium and has a body-centered cubic crystal structure.Chromium is the main alloy element in this group. It may reach amounts higher than 25%, in combination withcarbon content lower than 0.2%. They cannot be hardened through quenching( martensitic transformations).

This type of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements, this type of steel has a large coefficient of thermal conductivity, coefficient of expansion is small, good oxidation resistance, excellent resistance to stress corrosion and other characteristics, mostly used in the manufacture of corrosion-resistant atmospheric, steam, water and oxidizing acid corrosion of the parts.

Theyare more stainless than martensitic steels under oxidizing conditions or in an atmosphenic milieu. Since the grains are not refined through heat treatment, additional care must be taken when reheating them at high temperatures Due to their high content of chromium, these steels tend to weaken when exposed to 500C for a long time, what requires attention when selecting the pieces to be applied.

They have good cold conforming under any conditions because of their ferritic structure(soft They have magnetic sensitivity (ferromagnetism).

Ferritic stainless steel price is not only relatively low and stable, and has many unique features and advantages, has proved that in many applications originally thought to be only austenitic stainless steel (300 series), ferritic stainless steel is an excellent alternative material, ferritic stainless steel is free of nickel, the main elements for chromium (> 10%) and iron, chromium is a stainless steel special corrosion-resistant elements, the price of its relative Stable.

mechanical property：

Ferritic stainless steel can not be strengthened by heat treatment because of the absence of phase change. Generally used after annealing at 700～800℃. Due to the iron and chromium atomic size difference solid solution strengthening effect is small, ferritic stainless steel yield strength, tensile strength is slightly higher than mild steel, ductility is lower than mild steel.

Ordinary ferritic stainless steel is prone to brittleness:

(1) room temperature brittleness. Ordinary ferritic stainless steel notch sensitive, brittle transition temperature in addition to low chromium (such as 405) are above room temperature, the higher the amount of chromium cold embrittlement is greater. This cold embrittlement and steel carbon, nitrogen and other interstitial elements, and ultrapure ferritic steel due to carbon, nitrogen and other interstitial elements containing carbon is very low, you can get good toughness, brittle transition temperature can be reduced to below room temperature.

(2) high temperature embrittlement. Ordinary ferritic stainless steel heated to 927 ℃ above the rapid cooling to room temperature, plasticity and toughness significantly reduced. This high temperature embrittlement and carbon (nitrogen) compounds at 427 ~ 927 ℃ temperature rapidly precipitated at grain boundaries or dislocations. Reducing the carbon and nitrogen content of steel (using ultrapure technology) can greatly improve this embrittlement. In addition, ferritic steel heated to 927 ℃ above the grain capacity coarsening, coarse grains will make the steel plasticity, toughness deterioration.

(3) Formation of σ-phase. According to the iron-chromium phase diagram (see Figure ), in 500 ~ 800 ℃ insulation, chromium-containing 40% ~ 50% alloy will form a single phase σ, chromium-containing alloys containing less than 20% or more than 70%, will form α + σ dual-phase organization. σ-phase formation will significantly reduce the plasticity and toughness of steel. Therefore, such steel should not be used for a long time in 500 ~ 800 ℃.

(4) 475 ℃ brittle. High chromium (> 15%) ferritic steel, insulation at 400 ~ 500 ℃ will be strongly embrittled. This embrittlement takes a shorter time than the σ phase precipitation, for example, 0.080C-0.4Si-16.9Cr steel at 450 ℃ insulation 4h, the room temperature impact toughness drops to almost zero. The degree of embrittlement increases with increasing chromium content, but treatment above 600°C restores toughness. 475°C embrittlement is due to chromium-rich α′ phase precipitation. Heating near 475°C should be avoided for this type of steel.