Third, the corrosion resistance of steel pipe is poor
Steel pipes that work in corrosive environments are mostly made of stainless steel. Stainless steel is the general term for iron-based alloys containing more than 5% chromium. But from the point of view of real corrosion resistance, the chromium content (w) in the steel must exceed 10%.
In order to improve the machinability of ferritic stainless steel and adjust its grain size, it is necessary to adopt annealing treatment. The annealing temperature is too low, the recrystallization is not complete; If the annealing temperature is too high, the grain will be coarsened significantly, and the grain boundary precipitation will occur during the cooling process, which will reduce the corrosion resistance of the steel.
In order to improve grain boundary corrosion resistance, a small amount of niobium, titanium and other alloy elements that can form stable carbides are often added to steel. The common heat treatment processes of such steel pipes are solution treatment, stabilization treatment and stress relief treatment. The purpose of solution treatment is to fully dissolve chromium-containing carbides into austenite by heating, and then quickly cool to inhibit the precipitation of carbides to obtain single-phase austenite structure, so that the steel has excellent corrosion resistance.
Heat treatment defects of austenitic stainless steel are caused by insufficient heating and cooling speed. Chromium carbides are easy to precipitate along grain boundaries, which leads to the decrease of intergrain corrosion resistance.
In order to improve the intergranular corrosion resistance of austenitic stainless steel containing Ti and Nb, the solution temperature is usually higher than the solution temperature of Cr23C6, (Cr, Fe) 23C6, and lower than the solution temperature of TiC and NbC
3.1 Influence of heat treatment on intergranular corrosion and spot corrosion of steel pipe
Intergranular corrosion of steel pipe is a selective corrosion occurring along grain boundaries, and it is also a type of corrosion most susceptible to thermal treatment. The main reason of intergranular corrosion is chromium deficiency at grain boundaries. When the steel pipe is heat treated at a certain temperature and time parameter or used for a long time at a certain temperature range, the corrosion resistance of the steel pipe will decrease sharply due to the precipitation of chromium carbides along the grain boundary.
Spot corrosion of steel pipe is a typical local corrosion. Under the condition that most of the metal surface remains passive, the worm's eye corrosion caused by the local destruction of passivation film is called spot corrosion. The decline of point corrosion performance of stainless steel is also related to local chromium deficiency.
3.2 Influence of heat treatment on stress corrosion cracking of steel pipe
Stress corrosion cracking of steel pipe is one of the most common corrosion states. The factors affecting the stress corrosion cracking of steel pipe include metallurgy, stress state and environment. It is generally believed that the existence of tensile stress is a necessary condition for stress corrosion cracking. Therefore, if the residual tensile stress on the surface of the steel pipe is not completely eliminated or the residual tensile stress is generated on the surface of the steel pipe due to improper heat treatment, the stress corrosion resistance of the steel pipe will be reduced. Uneven microstructure is easy to produce stress corrosion. The sensitized stainless steel is prone to intergranular stress corrosion cracking.
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