Stainless steel is widely used in various industries and our life. But do you think you know enough about stainless steel? Even if you are some engineer in the steel industry. Here I’d like to write an article of detailed introduction of stainless steel.
This article focuses on the types and characteristics of common stainless steels, as well as the main purpose.
1. Martensitic stainless steel
Typical martensitic stainless steel has 1Cr13-4Cr13 and 9Cr18 and so on
1Cr13 steel processing technology performance is good. Deep drawing, bending, curling and welding without preheating. 2Crl3 cold deformation does not require preheating, but before welding to be preheated, 1Crl3, 2Cr13 is mainly used to make corrosion-resistant structural parts such as turbine blades, and 3Cr13, 4Cr13 is mainly used to make medical equipment surgical scalpel and wear parts; 9Crl8 can do corrosion bearing and cutting tools.
2. Ferritic stainless steel
Ferritic stainless steel containing Cr is generally 13% -30% carbon content of less than 0.25%. Sometimes other alloying elements are added. Metallographic organization is mainly ferrite, heating and cooling process without α <=> γ transition, can not be used to strengthen the heat treatment. Strong antioxidant. At the same time, it also has a good hot workability and a certain cold workability. Ferritic stainless steel is mainly used to produce high corrosion resistance and low strength requirements of the components, widely used in the manufacture of nitric acid, nitrogen and other equipment and chemical use of stainless steel pipe lines.
Chromium-containing stainless steel with a chromium content greater than 14%, chromium-free stainless steel with a chromium content of 27%, and molybdenum, titanium, niobium, silicon, aluminum, tungsten, vanadium, etc., on the basis of the above ingredients Elements of stainless steel, the chemical composition of the formation of ferrite elements in absolute dominance, the matrix structure is ferritin. This type of steel in the quenching (solid solution) of the organization for the ferrite, annealing and aging state of the organization can be seen in a small amount of carbide and intermetallic compounds.
Of these categories are Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28 and so on. Ferritic stainless steel because of high chromium content, corrosion resistance and oxidation resistance are better, but poor mechanical properties and process performance, and more for the use of acid-resistant structure and the use of anti-oxidation steel.
Typical ferritic stainless steels are Crl7, Cr25 and Cr28.
3. Austenitic stainless steel
Austenitic stainless steel is to overcome the lack of resistance to the stainless steel and stainless steel is too large and developed. The basic composition of Crl8%, Ni8% referred to as 18-8 steel. It is characterized by the carbon content of less than 0.1%, using Cr, Ni with the single-phase austenite.
This kind of steel contains more elements to expand y and stabilize austenite, which are y phase at high temperature, and have austenite structure at room temperature due to Ms point below room temperature during cooling. 18-8, 18-12, 25-20, 20-25Mo and other chromium nickel stainless steel, manganese instead of part of the nickel and nitrogen low nickel stainless steel such as Cr18Mnl0Ni5, Cr13Ni4Mn9, Cr17Ni4Mn9N, Cr14Ni3Mnl4Ti steel and so belong to this category.
Austenitic stainless steels have many advantages already mentioned before, although the mechanical properties are relatively low, and ferritic stainless steel – samples can not be heat treated to strengthen, but can be cold working deformation method, the use of work hardening to improve their strength. The disadvantage of this type of steel is sensitive to intergranular corrosion and stress corrosion, through the appropriate alloy additives and process measures to eliminate.
Austrian for stainless steel is generally used in the manufacture of nitric acid, sulfuric acid and other chemical equipment components, refrigeration industry, low-temperature equipment components and deformation can be used as stainless steel spring and clockwork and so on.
Austenitic stainless steel has a good resistance to uniform corrosion performance, but in the local anti-corrosion, there are still the following problems:
(1) Intergranular corrosion of austenitic stainless steel
Austrian for stainless steel in the 450-850 ℃ insulation or slow cooling, there will be crystal corrosion. The higher the carbon content, the greater the eutectic tendency. In addition, intergranular corrosion can occur in the heat affected zone of the weldment. This is due to precipitation of Cr-rich Cr23C6 in the grain boundary. So that the surrounding matrix produced by the poor chromium area, resulting in the formation of corrosion of the original battery. This intergranular corrosion is also present in the previously mentioned ferritic stainless steels.
Engineering often use the following methods to prevent intergranular corrosion:
1) Reduce the amount of carbon in the steel, so that the amount of carbon in the steel below the equilibrium state in the austenite saturation solubility, that is fundamentally solve the chromium carbide (Cr23C6) in the grain boundary precipitation problems. Usually in the steel to reduce the amount of carbon to 0.03% or less to meet the requirements of resistance to intergranular corrosion.
2) Addition of Ti, Nb, etc. can form stable carbide (TiC or NbC) elements, to avoid precipitation in the grain boundary Cr23C6, can be anti-austenitic stainless steel intergranular corrosion.
3) By adjusting the ratio of the austenite forming elements to the ferrite forming elements in the steel, it has austenite + ferrite biphase structure, in which ferrite is 5% to 12%. This biphasic structure is less prone to intergranular corrosion.
4) the use of appropriate heat treatment process, can prevent intergranular corrosion, to obtain the best corrosion resistance.
(2) stress corrosion of austenitic stainless steel
The cracking caused by the combined effect of stress (mainly tensile stress) and corrosion is called stress corrosion cracking, referred to as SCC (Stress Crack Corrosion). Austenitic stainless steel is prone to stress corrosion in corrosive media containing chloride ions. When the Ni content of 8% to 10%, the austenitic stainless steel stress corrosion tendency of the largest, continue to increase the amount of Ni to 45-50% stress corrosion tendency gradually reduced until disappear.
The main way to prevent stress corrosion of austenitic stainless steel is to add Si2-4% and to control N content below 0.04% from smelting. In addition, should also minimize the P, Sb, Bi, As and other impurities. In addition, A-F duplex steel is used, which is insensitive to stress corrosion in Cl- and OH- media. When the initial microcracks encounter ferrite phase no longer continue to expand, ferrite content should be about 6%.
(3) Deformation strengthening of austenitic stainless steel
Single-phase austenitic stainless steel has good cold deformation properties, can be cold drawn into very fine steel wire, cold rolled into a thin strip or stainless steel seamless pipes.
After a large number of deformation, the strength of steel to improve, especially in the zero temperature rolling, the effect is more significant. Tensile strength of up to 2000 MPa or more. This is because in addition to cold hardening effect, but also superimposed on the deformation induced M transformation.
Austenitic stainless steel can be used to create stainless steel spring, spring and clock, spring and clockwork, aviation structure, such as wire rope. Deformation after the need to weld, you can only use the spot welding process, deformation to increase the stress corrosion tendency. And due to partial γ -> M transformation and produce ferromagnetism, in use (such as instrument parts) should be considered.
The recrystallization temperature changes with the deformation of the deformation, when the deformation of 60%, the recrystallization temperature is reduced to 650 ℃ cold deformation of austenitic stainless steel recrystallization annealing temperature of 850-1050 ℃, 850 ℃ is required to heat 3h, 1050 ℃ When you can burn, and then water-cooled.
(4) Heat treatment of austenitic stainless steel
Austenitic stainless steel commonly used heat treatment processes are: solution treatment, stabilization and stress treatment.
1) Solution treatment. The steel is heated to 1050-1150 ° C after the water quenching, the main purpose is to dissolve the carbide in austenite, and this state to keep room temperature, so that the corrosion resistance of steel will be greatly improved. As described above, in order to prevent crystal corrosion, the solution treatment is usually carried out so that Cr23C6 is dissolved in austenite and then rapidly cooled. Normally the products are water-cooled, but in some applications, for the thin-walled pieces can be used for air-cooled, such as the cooling method of solution treatment of stainless steel bright annealed tubes, it is the typical air cooled solution treatment.
2) Stabilization treatment. Usually after the solution treatment, commonly used in Ti, Nb 18-8 steel, solid treatment, the steel heated to 850-880 ℃ after the air-cooled insulation, Cr carbide at this time completely dissolved, and titanium The carbides are not completely dissolved and are sufficiently precipitated during the cooling process so that the carbon can no longer form chromium carbides, thus effectively eliminating intergranular corrosion.
3) To stress treatment. The stress treatment is to eliminate the residual heat stress of the steel after cold working or welding. The heat treatment process is generally heated to 300-350 ° C for tempering. For non-stabilized elements Ti, Nb steel, the heating temperature does not exceed 450 ℃, so as not to precipitate chromium carbide caused by intergranular corrosion. For ultra-low carbon and Ti, Nb stainless steel cold work and welding parts, need to 500-950 ℃, heating, and then slow cooling, to eliminate stress (to eliminate the upper limit of welding stress), can reduce the tendency to intergranular corrosion and improve the steel Stress corrosion resistance.
4, Austenitic – ferrite duplex stainless steel
On the basis of Austenitic stainless steel, the Cr content is increased and the Ni content is reduced. In combination with the refolding process, a biphasic structure with austenite and ferrite (containing 40-60% δ-ferrite ) Of stainless steel, the typical steel number 0Cr21Ni5Ti, 1Cr21Ni5Ti, OCr21Ni6Mo2Ti and so on. Duplex stainless steel has good weldability, no heat treatment after welding, and its intergranular corrosion, stress corrosion tendency is also smaller. However, due to high Cr content, easy to form σ phase, should be used to pay attention.
This type of steel due to the expansion of the y area and the stability of the role of austenitic elements, not enough to make the steel at room temperature or high temperature with pure austenite, and therefore austenite-ferrite complex state, The amount of ferrite varies depending on the composition and the heating temperature and can be varied in a large range.
Many of these grades of duplex stainless steel, such as low-carbon 18-8 chrome-nickel steel, titanium, niobium, molybdenum 18-8 chromium nickel steel, especially in the cast steel can be seen in the organization of ferrite, in addition to Chromium is more than 14 to 15% and less than 0.2% chromium chromium stainless steel (such as Cr17Mnll), and the current research and has been applied to most of the chromium manganese nitrogen stainless steel. Compared with pure austenitic stainless steel, the advantages of such steel, such as high yield strength, high resistance to intergranular corrosion, stress corrosion sensitivity is low, the tendency to produce hot cracks when welding is small, good casting flow and many more. The disadvantage is poor pressure processing performance, point corrosion tendency is larger, easy to produce c-phase brittleness, in the strong magnetic field under the performance of weak magnetic and so on. All of these advantages and disadvantages are derived from the organization of ferrite.
Duplex stainless steel has the advantages of austenitic stainless steel and ferritic stainless steel. Although not such excellent as austenitic stainless steel, the cold deformation performance of duplex stainless steel is good, so can be produced to duplex stainless steel pipes and tubes, duplex steel flanges, duplex steel mechanical parts and so on.
5. Ferritic – martensitic steel
This kind of steel at high temperature for y + a (or δ) two-phase state, fast cooling occurs yM transition, ferrite is still retained, the normal temperature structure of martensite and ferrite, due to the composition and heating temperature difference , The amount of ferrite in the organization can be in the range of several to several percent change. 0Cr13 steel, Cr17Ni2 steel, Cr17wn4 steel, and in the ICrl3 steel developed on the basis of many modified 12% chromium hot steel (this type of steel is also called heat-resistant stainless steel ) In many steel, such as Cr11MoV, Cr12WMoV, Crl2W4MoV, 18Crl2WMoVNb are all dry this category.
The ferrite-martensitic steel can be partially quenched to obtain higher mechanical properties. But their mechanical properties and process performance to a large extent by the organization of ferrite content and distribution of the impact. This type of steel according to the composition of the chromium content of 12 to 14% and 15 to 18% of the two series. The former has the ability to resist atmospheric and corrosive media, and has a good shock absorption and a small linear expansion coefficient; the latter corrosion resistance with the same chromium content of ferritic acid-resistant steel rather, but to a certain extent Also retains some of the shortcomings of high chromium ferrite.
6. Martensite-Carbide Steel
The iron content of the Fe-C alloy is 0.83%, and the S is left in the stainless steel due to chromium, with 12% chromium and more than 0.4% carbon (Figure 11-3), and 18% chromium and More than 0.3% carbon steel (Figure Bu) 3) are all eutectoid steel. This type of steel in the normal quenching temperature heating, secondary carbides can not be completely dissolved in austenite, so after quenching the organization of martensite and carbide composition.
Belong to this category of stainless steel grades are few, but it is some high carbon content of stainless steel, such as 4Crl3, 9Cr18, 9Crl8MoV, 9Crl7MoVCo steel, carbon content of the upper limit of 3Crl3 steel at a lower temperature quenching, may also occur Such an organization. Due to the high carbon content, the above three steel grades such as 9Cr18 contain more chromium, but its corrosion resistance is only equivalent to 12 to 14% germanium stainless steel. The main purpose of this type of steel is to require high hardness and wear parts, such as cutting tools, bearings, springs and medical equipment.
7. Austenitic-martensitic steel
The Ms point of this type of steel is lower than room temperature, and after solution treatment it is austenitic and easy to form and weld. There are usually two process methods to make martensitic transformation. First, after the solution treatment by 700 ~ 800 degrees of heating, austenite due to precipitation of chromium carbide and into a stable state, Ms point increased to above room temperature, cooling into martensite; Second, after the solution solution directly Cooling to Ms and Mf points, so that the austenite into martensite. The latter method can obtain high corrosion resistance, but after the solution treatment to the cryogenic interval time should not be too long, otherwise it will be due to the stability of austenite aging effect of the deepening of the strengthening effect. After the above treatment, the steel is further aged from 400 to 500 degrees to strengthen the precipitated intermetallic compound. The typical steel grades of this type are 17Cr-7Ni-A1, 15Cr-9Ni-A1, 17Cr-5Ni-Mo, 15Cr-8Ni-Mo-A1 and so on. This type of steel is also known as austenitic-martensitic aging stainless steel, and because in fact the organization of these steel in addition to austenite and martensite, there are different amounts of ferrite, it is also known as half Stretch hardening stainless steel.
This type of steel is a new type of stainless steel developed and applied in the late 1950s. They are characterized by high strength (C up to 100-150) and good thermal strength, but due to lower chromium content and chromium carbide deposition during heat treatment , So the corrosion resistance than the standard austenitic stainless steel is lower. It can also be said that the high strength of this type of steel is at the expense of a part of the corrosion resistance and other properties (such as non-magnetic) to obtain the case, the current steel is mainly used for aviation industry and rocket missile production, general machinery manufacturing applications Is not yet universal, and there is also a series that classifies them into ultra-high strength steels.