First, the effect of nitrogen on the microstructure and heat treatment of steel pipes
①Nitrogen and carbon can be solidly dissolved in iron, forming interstitial solid solutions.The effect of nitrogen on the microstructure and heat treatment of steel pipes
② Nitrogen expands the austenitic phase region of steel, which is a strong forming and stabilizing austenitic element, and its effectiveness is about 20 times that of nickel, which can replace some nickel in steel within a certain limit.
(3) Nitrogen and chromium, aluminum, vanadium, titanium and other elements that penetrate the surface of the steel can generate extremely stable nitrides, which become surface hardening and strengthening elements.
④ Nitrogen makes the structure of high chromium and high chromium nickel steel dense and solid.
(5) Excessive residual nitrogen content in steel can lead to loose macro organization or porosity.
Second, the influence of nitrogen on the mechanical properties of steel pipes
① Nitrogen has a solid solution strengthening effect, which can improve the hardenability of steel.
② In nitrogen-containing ferritic steel, when tempering after fast cooling or staying at room temperature for a long time, precipitation hardening can occur due to the precipitation of ultra-microscopic nitrides, and nitrogen also causes strain aging phenomenon of low carbon steel. While the strength and hardness increase, the toughness of the steel decreases and the notch sensitivity increases. The brittleness of steel caused by nitrogen is similar to that of phosphorus, and its effect is much greater than that of phosphorus. Nitrogen is also the main cause of blue brittleness in steel.
③ Improve the strength and impact toughness of high chromium steel and high chromium nickel steel, while the plasticity is not reduced.
④ Improve the creep strength and high temperature endurance strength of steel.
Third, the impact of nitrogen on the physical, chemical and technological properties of steel pipes
(1) Nitrogen has no significant effect on corrosion resistance of stainless steel pipes.
②When the mass fraction of nitrogen is greater than 0.16%, the oxidation resistance will deteriorate.
③ The cold working deformation hardening rate of nitrogen-containing steel is higher.
(4) Nitrogen can reduce the grain growth tendency of high chromium ferrite steel, thereby improving its weldability.
The application of nitrogen in steel
(1) Nitrogen as an alloying element, the content in steel is generally less than 0.3%(mass fraction), and can be as high as 0.6% in special cases.
② Mainly used in nitriding tempered steel, ordinary low alloy steel, stainless acid-resistant steel and heat-resistant non-peeling steel, of which heat-resistant non-peeling steel can be manufactured turbine components.
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