In order to enhance the performance of the stainless steel pipe, usually use some cold processing technology, such as cold drawing, cold rolling, cold rolling, cold, cold, cold expansion distortion, but finally achieve effect, also will inevitably suffer some damage, these cold working process and welding processing, will damage the performance of the stainless steel pipe, especially corrosion or heat resistance.
There are five main reasons for the damage of stainless steel pipes. First of all, it will increase the micro defects and surface roughness of the lattice dislocation of the pipe material, and also induce martensite transformation and precipitation of carbides. For example, austenitic stainless steel presents a magnetic increasing phenomenon after cold working.
Secondly, the lattice dislocation or phase transition of stainless steel pipe will occur on the surface, and it will also become the starting position of pitting corrosion. This phenomenon will produce a direct adverse effect when the deformation degree reaches 20% reduction section rate.
The residual stress in the stainless steel pipe will be left after the cold working, and the residual stress is very unfavorable to the stress corrosion cracking of the pipe. Any degree of cold processing will lead to a great increase in the sensitivity of the material to stress corrosion cracking.
The cold working degree also has an adverse effect on the high temperature endurance strength of austenitic stainless steel. The higher the general working temperature or the higher the fracture life requirement, the lower the cold working degree can be.
For stainless steel pipes subjected to alternating loads, cold processing will cause cracking and expansion rates to increase due to the decrease of elongation and residual elongation.
In order to solve the above problems, there are several solutions. First of all, the stainless steel standards in most countries, especially the European unified stainless steel pipe standard, stipulate that all seamless stainless steel pipes must be supplied in solid solution or annealed state, so as to remove the damage caused by cold processing and welding.
Secondly, three key parameters should be paid attention to in solution treatment, i. e. heating temperature, rapid cooling method and high temperature residence time. Excessive solution treatment temperature or residence time will be detrimental to the corrosion resistance of stainless steel. Determine whether the solution is in place, hardness determination, expansion, curling, flattening, tensile data can be determined, in which hardness is the most simple.
The solid solution or by high temperature heating annealing and pickling treatment, it significantly increased the manufacturing cost and production cycle, with acid mist and other waste gas and waste water emissions plus, some enterprises omit this process, the use of this product, easy to cause the production and personal accident after use.
Finally, for some stainless steel pipes which may be difficult to implement solid solution or annealing, it is necessary to control the degree of cold processing and local annealing at low temperature, which can effectively reduce the harmful effects.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)