Heat treatment of stainless steel
Stainless steel has been invented from the early 20th century to less than a century, but the momentum of its development and application is exceptionally rapid. Especially since the late 1960s, the world’s production of stainless steel basically maintained an average annual growth rate of 4%, the application of stainless steel has gradually expanded to all areas of the national economy. The reason why stainless steel can be so rapid development, an important factor is its corrosion resistance, heat resistance. The advantages and disadvantages of stainless steel heat treatment process on the corrosion and heat resistance of stainless steel has a great impact, and the processing performance of stainless steel plays a decisive role. Therefore, the heat treatment process of stainless steel has been in a very important position in the production process of stainless steel.
Characteristics of stainless steel heat treatment
Heat treatment of stainless steel is to change its physical properties, mechanical properties, residual stress and restore the corrosion resistance due to pre-processing and heating is seriously affected, in order to get the best performance of stainless steel or stainless steel can be further cold and hot processing. The so-called heat treatment is for different organizations, different types of stainless steel for the corresponding annealing, quenching and tempering, normalizing and other treatments.
Stainless steel is a special type of steel, steel with high nickel and chromium content, due to the presence of nickel, chromium and other alloying elements, its heat treatment with ordinary steel heat treatment does not have the following characteristics.
The heating temperature is higher, and the heating time is relatively long.
Stainless steel has low thermal conductivity and poor temperature uniformity at low temperatures.
Austenitic stainless steel high temperature expansion is more serious.
Furnace atmosphere control is important to prevent carburization, nitriding and decarburization and peroxidation.
The surface luster of stainless steel has a decisive impact on the use and price of the product, and the iron oxide produced during heat treatment will seriously affect the surface luster.
To ensure that the surface of stainless steel to avoid scratches and protection: heat treatment produces deformation.
Classification of stainless steel
Stainless steel according to its organization can be divided into austenite, martensite and ferritic three categories (in addition to precipitation hardening type, ferritic austenite type, etc.), these three types of stainless steel heat treatment either processing methods or purposes are not the same.
This paper discusses the heat treatment methods and characteristics of the three types of stainless steel, which are widely used.
Austenitic stainless steel
This type of stainless steel is the most widely used, and the largest amount of use. It is characterized by austenite organization at room temperature, no phase change, can not be hardened by heat treatment, but can be hardened by cold working. Commonly used heat treatment method is solid solution treatment.
The representative steel grade of austenitic stainless steel is 18-8 steel (304), because it is austenitic organization, so it has no magnetic and no hardening characteristics. Since this steel grade does not undergo phase transformation, its heat treatment is to heat to high temperature (generally above 1000 ℃), austenite recrystallization at the same time, so that the carbide produced in the processing and. Phase decomposition solid solution to the austenite, and then rapidly cooled, so that the carbon is solid solution state of austenite to maintain to room temperature, which – the treatment process is solid solution treatment. Table 1 is the reference annealing temperature of austenitic stainless steel.
Table 1: Austenitic stainless steel reference annealing temperature
|GB4237||AISI ASTM||JIS-G 4303||Annealing temperature/℃|
|0Cr1 7Ni12Mo2||36||SUS 316||1010-1120|
|00Cr1 7Ni12Mo2||316L||SUS 316L||1010-1150|
Austenitic stainless steel heating temperature is mainly based on the solidification rate of carbide and determined, there is information like 304 steel carbide solid solution at 1065 ℃ need 3 minutes, at 1176 ℃ need 1.5 minutes, at 1000 ℃ need up to 10 minutes. From this point of view, the higher the heating temperature the better, but the heating temperature is high at the same time may cause excessive grain growth, iron oxide thickening and other defects. Because austenitic stainless steel can not refine the grain through phase transformation, if the grain is too large, the material’s tensile strength will be significantly reduced.
As far as the heating time is concerned, the thermal conductivity of stainless steel is low (especially at low temperatures), and only after rising to high temperatures (700-800 ℃) thermal conductivity has improved. Therefore, for the section of large austenitic stainless steel are required to preheat to 700-800 ℃, and then quickly ramp up, for the section of small austenitic stainless steel (such as strip steel) if the heating rate is too slow, carbide will be fully precipitated, it will lead to too long solid solution time, the United States Amco has developed a classic heating schedule (see Table 2, Table 3). As the chromium in stainless steel formed chromium-based oxides in pickling is more difficult to remove, so the formation of chromium-based oxides should be controlled during heat treatment, for stainless steel with special requirements, bright annealing form of heat treatment can be used.
Table 2: Austenitic stainless steel solution treatment holding time
Table 3: Austenitic stainless steel solution treatment of heating time
|Working diameter or thickness/mm||Heating time to solid solution||Holding time at solid solution temperature|
In order to prevent the solid solution carbide precipitation, the cooling rate is also important, especially at 600-700 ℃, carbide precipitation more and sensitization, so must be fast cooling. Due to the low thermal conductivity of austenitic stainless steel, for the larger section of the material, no matter how fast cooling, the central part of the cooling degree is still very slow, often due to carbide precipitation and more sensitization. So in actual production, the section of the larger material is generally considered to use the addition of Ti, Nb and other elements of stabilized austenitic stainless steel, because Ti, Nb and other elements have a greater affinity for carbon, such stabilized austenitic stainless steel (such as 321, 347, etc.) can be fast cooling without water quenching or other rapid cooling measures.
This type of stainless steel is generally no ν-α transition, at high temperature and room temperature are ferrite organization, no phase change. But when the steel contains a certain amount of carbon, nitrogen and other austenite forming elements, at high temperatures can also form austenite organization, such steel can not be strengthened by heat treatment, can only be annealed to eliminate internal stress and facilitate further processing.
Ferritic stainless steel is chromium as the main alloying element, the content of 12%-30% Cr. Such steel is a single-phase organization, no phase change, with strong magnetic properties. Its typical representative steel grade is 430. the United States in the 1960s to reduce the cost of stainless steel as the goal of the development of cheap stainless steel 409, widely used in cars, motorcycles, silencers and sewer pipes, etc., is also part of the ferritic Diao; stainless steel this category.
The steel without hardenability, the same as austenitic steel can not be hardened by heat treatment, and due to heating caused by grain growth than austenitic steel both fast and grain size and large. Therefore, in order to avoid grain growth and austenite phase transformation during heat treatment, the heating temperature should not be too high, – generally annealing the maximum heating temperature does not exceed 850 ℃. Table 4 is the reference annealing temperature of ferritic stainless steel.
Table 4: Reference annealing temperature of ferritic stainless steel
|GB 4237||AISI ASTM||JIS-G 4303||Annealing temperature/℃|
Ferritic stainless steel in annealing treatment, must be shortened in the temperature range of 370-550 ℃ dwell time, especially for high chromium ferritic stainless steel. Material if the residence time in this temperature range is too long, it is easy to occur 475 ℃ embrittlement phenomenon, that is, the hardness increases, elongation decreases significantly, or even zero, while the material corrosion resistance is also reduced. Some experiments show that: 27Cr steel in 475 ℃ after 100 hours of heating, the material at room temperature tensile strength increased by 50%, yield strength increased by l50%, while the elongation is zero. In addition, the poor welding performance of the steel (weld heat affected zone grain coarse and brittle).
Martensitic stainless steel
This type of stainless steel has obvious phase change points, at high temperatures for austenite organization, cooling can occur when the martensite phase change, transformed into martensite organization and hardening. Because of its high chromium content, good hardenability, you can use a variety of heat treatment methods such as quenching and tempering.
Martensitic stainless steel with the first two stainless steel characteristics are clearly different, as the name suggests, from the high temperature austenitic state fast cooling (quenching) into a martensitic organization. This type of stainless steel has obvious phase change points, can be hardened by quenching. And because of its high chromium content, good hardenability, tempering can be adjusted in a wide range of its strength and toughness, therefore, martensitic stainless steel can be used for both structural steel, but also for tool steel.
Martensitic stainless steel used as tool steel, in a quenched state. In order to quench, must be heated up to above the critical point, so that the carbide solid solution to the austenite. In the heating so that the carbide solid solution, because the carbon diffusion rate is slow, in order to get uniform austenite organization, force mouth heat temperature is generally higher than the critical point temperature of more than 50 ℃, but also must have a certain holding time, in order to make the carbide fully and uniformly dissolved. Of course, the heating time is too long, the heating temperature is too high will cause martensite organization is not uniform, residual austenite organization increased, so that the material internal internal stress due to the expansion difference.
Martensitic steel is hot crack-sensitive steel, the steel at low temperature, low thermal conductivity, rapid heating is very easy to produce cracks, so when dealing with large cross-sectional materials, should be preheated, and then rapid temperature rise. Table 5 shows the reference annealing temperature of martensitic stainless steel.
Table 5: reference annealing temperature of martensitic stainless steel
|GB 4237||AISI ASTM||JIS-G 4303||Annealing temperature/℃|
|1Cr12||403||SUS 403||750~ 900|
|2Cr13||420J1||SUS 420 J1||750-900|
When used as structural steel, should be tempered on the basis of quenching (tempered state). Martensitic stainless steel has tempering brittleness, tempering temperature should generally not be lower than 580 ℃. Cooling from the tempering temperature, in order to avoid tempering brittleness is generally used oil cooling; sometimes in order to get a higher yield limit, can also be used air cooling, but then an important mechanical index of structural steel impact value will decline. It should be noted that martensitic stainless steel in the quenching, should be tempered as soon as possible, such as can not be tempered as soon as possible, the material is prone to cracking.
Precipitation hardening stainless steel
Austenitic, ferritic, martensitic three types of stainless steel, although the application is wide, but as a structural steel use, there are also some difficult to overcome the defects. Austenitic stainless steel yield strength is low, only about 200N/mm2, not suitable for use as structural steel; and martensitic stainless steel, although it can be quenched, tempered and other forms of heat treatment to obtain a higher yield strength, but its corrosion resistance is poor. For those applications requiring the best corrosion resistance and maximum strength, a new type of Cr-Ni stainless steel was developed – precipitation-hardening stainless steel (also known as PH stainless steel).
The heat treatment of this new stainless steel includes homogenization, complete annealing, solution heat treatment, aging treatment and transformation cooling. Its characteristics are.
Fully annealed state is softer and easier to rework.
The required mechanical properties can be obtained through appropriate aging treatment.
Has the same corrosion resistance as similar stainless steel, improved resistance to stress corrosion fracture performance.
Transformation cooling is suitable for cooling below a certain temperature. The commonly used precipitation-hardening stainless steel is martensitic, and its representative steel grade is 631 (0Crl7Ni7A1). The heat treatment of this steel grade is first solution treatment, in the material heated to 1000-1100 ℃ after fast cooling, and then according to the different mechanical properties required for aging treatment at different temperatures, such as 621 ℃, 565 ℃, 510 ℃ aging.
From the above analysis, it can be seen that the heat treatment of stainless steel is relatively complex, according to the characteristics of different steel grades, to take different heat treatment to meet the different requirements of users.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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