Chromium on the corrosion resistance of stainless steel decisive role in the definition of stainless steel, ω (Cr) ≥ 10.5%, which is the main element of stainless steel, corrosion resistance, the higher the chromium, corrosion resistance higher possible. This is because the steel in the oxidation of the medium can be formed to Cr2O3 as a stable substrate surface protection film (about 10μm), that generate passive phenomenon, in which enrichment of the chromium layer membrane.
Meanwhile, the chromium effectively improve solid solution (ferrite, martensite or austenite) of the electrode potential, so that the original iron (low carbon steel) of the electrode potential from negative to positive, so that the steel from corrosion. Adding chromium stainless steel, the electrode potential follows the n / 8 occur on a volume change. When the chromium content of each atom to 1 / 8, 2 / 8, 3 / 8, … … n / 8, or 12.5%, 25%, 37.5% … … the mole fraction, the electrode potential there is a jump, corrosion weakened. Atomic concentration of chromium and 1 / 8 (or 12.5%, molar fraction), if compared to 11.7% by mass, so the chromium content of chromium stainless steel is generally 12% (by mass) above. When the chromium atom content reaches 25%, there will be second mutation, this time the corrosion resistance of chromium steel to further improve.
In addition, the chromium on the mechanical properties of stainless steel and technological properties have very good role. Chromium can increase the hardenability of stainless steel, in low-alloy structure has been widely used. Such as reduced chromium austenite to ferrite and carbide transformation rate, so that the isothermal transformation diagram of austenite was shifted to the right, thus reducing the critical cooling rate of quenching of stainless steel, resulting in increased hardenability of steel Some martensitic stainless steel air quenching martensite are available.
Chromium can improve the oxidation resistance of stainless steel, with the increase of chromium content of steel significantly improved oxidation resistance. In the martensitic chromium stainless steel, oxidation resistance higher than ordinary stainless steel, 4 to 9 times, martensitic chromium stainless steel can not afford the skin temperature is about 700 ~ 850 ℃.
Chromium is passivated the steel and stainless steel to give good corrosion resistance of industrial use of the only elements of value. With the increase of chromium content, can increase resistance to atmospheric corrosion. In the oxidizing medium (such as dilute nitric acid), with the chromium content increases, the corrosion resistance of stainless steel increased; but in reducing media, with the chromium content increases, the corrosion resistance of stainless steel decreased.
Chromium can affect the physical properties of steel were as follows: chromium can increase the steel lattice constant than the increase in volume with chromium content increases linearly, and significantly lower iron-chromium alloy thermal conductivity, but also increase the resistance of steel. Resistance of martensitic chromium stainless steel is common 4 to 6 times. In the quenching conditions, due to the increase in the stability of chromium ferrite increased, thereby reducing thehardness and tensile strength of steel. In the annealed condition, the low carbon iron – chromium alloys with chromium percentage increased, strength and hardness increase, while elongation decreased slightly.
Chromium in pure γ-Fe in the maximum solubility is about 12.0%; when ω (C) ≈ 0.5%, the maximum solubility in austenite is about 20%. In pure α-Fe solubility in the infinite. In Cr-Mn-N steel can increase the N solubility. The formation of chromium carbides in the steel tendency than manganese, and less than tungsten. Chromium steel, the temperature can increase strength and wear resistance of high carbon steel.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)