Classification of stainless steel pipes
The so-called “stainless” in stainless steel pipes has only a relative meaning. Stainless steel pipe refers to a series of steel grades with high chemical stability in air, water, salt and other aqueous solutions, acids and other corrosive media. Existing stainless steel pipes are all high chromium steels in terms of chemical composition; because in the atmosphere, when the chromium content in the steel exceeds about 12%, it will basically not rust, so it is customary to set the chromium content to exceed this content. The steel grades are collectively referred to as stainless steel pipes.
According to the different alloying element content and microstructure, stainless steel pipes can be divided into:
(1) 200, 300 series austenitic stainless steel pipes-namely chromium-nickel and chromium-nickel-manganese grades. It can only be hardened by cold working: heat treatment can only soften it. It is non-magnetic in the annealed state, and sometimes becomes slightly magnetic after cold working.
(2) 400 series ferritic stainless steel pipe-the grade of stainless steel pipe containing chromium but not nickel. Cold working can harden it to a certain degree, but heat treatment cannot. This type of steel is always magnetic.
(3) 400 series martensitic stainless steel pipe-this type of stainless steel pipe contains chromium, except for two cases, the rest do not contain nickel. The difference with the above two types is that it can be hardened by heat treatment.
(4) Duplex stainless steel pipes-more corrosion resistant than austenitic stainless steel pipes, and toughness better than pure ferritic stainless steel pipes;
(5) Precipitation hardening stainless steel pipe-is obtained by adding the content of ferrite forming elements to the pure austenite structure.
In order to tap the potential of stainless steel pipes, save energy, and find the best cooperation between various elements, researchers are also constantly deepening the research on stainless steel pipe materials. Future research will mainly start from the following aspects:
(1) Research and purification of fine or ultra-fine crystal
In the metallurgical industry, the research on the purification of materials has been deepened, and the concepts of clean steel and zero-inclusion steel have been put forward. Combining the production of stainless steel pipe materials with smelting technology, the research on pure and ultra-pure stainless steel pipes will surely get a good development. At the same time, the material grains can be refined, the microscopic characteristics and structure of the material can be changed, so that the material can obtain special properties that it does not have under the normal state, and the use space of the material can be expanded.
(2) Nickel stainless steel pipe
With the increase in the output of stainless steel pipes, the demand for nickel continues to expand, and there will be a shortage of nickel supply in the near future. Without reducing its original performance, reducing the proportion of nickel in stainless steel pipes or replacing nickel with other elements is a practical and feasible method, which will still be valued and developed in the future. For example, the study of adding nitrogen to high-manganese and low-nickel 200 series stainless steel pipes is one.
(3) High-strength stainless steel pipe
With the development of certain large-scale components and the requirements for lightweight components, the requirements for material strength are also increasing. Combining alloying, especially microalloying with high purity, high uniformity refining, casting technology and ultra-fine structure control technology, to develop a new material that combines good strength, toughness, corrosion resistance and manufacturability is a high-strength stainless steel pipe in the future The main direction of development.
(4) High corrosion resistance stainless steel pipe
The transportation and storage of chemical raw materials are difficult, mainly because chemical raw materials are highly corrosive, and general materials do not have high corrosion resistance. It is urgent to study stainless steel pipe materials with good corrosion resistance that meet the requirements of various special industries (seawater purification equipment, chemical plants, coastal sodium chloride corroded buildings, etc.).
(5) Heat-resistant and anti-oxidation stainless steel pipe
The continuous deepening of the exploration of the universe has promoted the development of the aerospace and aviation industry. Due to the special requirements of the aviation and aerospace industries, materials that can withstand high temperatures and heat are needed. The research on heat resistance and oxidation resistance of stainless steel pipe materials is an important direction, which can expand the application of materials and promote the development of materials.