Seamless steel pipe for structure Seamless steel pipe

Chromium-molybdenum steel, also known as medium temperature hydrogen resistant steel, is an alloy of chromium (Cr), molybdenum (Mo), iron (Fe), carbon (C). Because of its special high quality performance, it is often used to manufacture some high temperature and high pressure resistant valves and pressure vessels, such as chromium-molybdenum steel safety valve, chromium-molybdenum steel gate valve, screwdriver head, bicycle and so on
Heat resistance
The addition of chromium, molybdenum, rocky and other elements effectively improves the high-temperature oxidation resistance and high-temperature strength of steel, and its mechanism is as follows: Chromium mainly exists in the cementite (Fe3C), dissolved in the cementite of chromium, improve the decomposition temperature of carbide, prevent the occurrence of graphitization phenomenon, and improve the heat resistance of steel. Molybdenum can strengthen ferrite by solution and improve the stability of carbides, so it is beneficial to improve the high temperature strength of steel. The addition of vanadium can keep the fine grain structure of steel at higher temperature and improve the thermal stability and strength of steel.
Hydrogen corrosion resistance
Chromium, molybdenum and other elements improve the stability of carbides, prevent the decomposition of carbides, reduce the chance of carbides and precipitated carbon and hydrogen reaction to form methane. The addition of vanadium can keep the fine grain structure of steel at higher temperature, which greatly increases the stability of steel to hydrogen at high temperature and high pressure.
Temper brittleness
The temper brittleness of chromium-molybdenum steel refers to the phenomenon that the impact toughness will decrease when the steel is operated in the temperature range of 370℃ to 595℃ for a long time. Our usual hydrogen devices operate in this temperature range for a long time. According to the experimental results, the cr-Mo steel with Cr content of 2%-3% has the most serious tendency of embrittlement. P, Sb, Sn, As, Si, Mn and other elements have a great influence on the temper brittleness. Embrittlement is reversible and severely embrittlement materials can be deemable by proper heat treatment.
Brittle hard tendency is large, easy to produce delayed crack
Due to the addition of chromium, molybdenum, vanadium and other alloying elements, the critical cooling rate of steel is reduced and the stability of undercooled austenite is improved. If the welding cooling rate is faster, it is not easy to change from austenite to pearlite in the heat affected zone, but to change to martensite and form hardened structure at lower temperature. Under the combined action of complex residual stress and diffused hydrogen, hydrogen-induced delayed cracks can easily occur in the hardened structure of weld zone and heat-affected zone.