Practical knowledge of stainless steel

2-1. Ferritic steel

Low-carbon chromium stainless steel containing more than 14% chromium, any chromium stainless steel containing 27% chromium, and molybdenum, titanium, niobium, silicon, aluminum, tungsten, vanadium, etc. added to the above composition Elemental stainless steel, the element that forms ferrite in the chemical composition is absolutely dominant, and the matrix structure is ferrite. The structure of this type of steel in the quenched (solid solution) state is ferrite, and a small amount of carbides and intermetallic compounds can be seen in the structure of the annealing and aging state.

Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. belong to this category. Because of its high chromium content, ferritic stainless steel has good corrosion resistance and oxidation resistance, but its mechanical properties and process properties are poor. It is mostly used for acid-resistant structures with little stress and as anti-oxidation steel.

2-2. Ferritic-Martensitic Steel

This type of steel is in a ya (or δ) two-phase state at high temperatures, and yM transformation occurs during rapid cooling, and ferrite is still retained. The normal temperature structure is martensite and ferrite. Due to the difference in composition and heating temperature, the structure The amount of ferrite in can vary from a few percent to several tens. 0Crl3 steel, lCrl3 steel, 2Cr13 steel with the upper limit of chromium and the lower limit of carbon, Cr17Ni2 steel, Cr17wn4 steel, and many modified 12% chromium hot-strength steels developed on the basis of ICrl3 steel (this type of steel is also called heat-resistant stainless steel Many steel grades in ), such as Cr11MoV, Cr12WMoV, Crl2W4MoV, 18Crl2WMoVNb, etc. belong to this category.

Ferritic-martensitic steel can be partially quenched and strengthened, so higher mechanical properties can be obtained. However, their mechanical properties and process properties are largely affected by the content and distribution of ferrite in the structure. This kind of steel belongs to two series of 12″14% and 15″18% according to the chromium content in the composition. The former has the ability to resist the atmosphere and weakly corrosive media, and has good shock absorption and a small linear expansion coefficient; the latter’s corrosion resistance is equivalent to the ferritic acid-resistant steel with the same chromium content, but to a certain extent Some of the shortcomings of high-chromium ferritic steel are also retained.

2-3. Martensitic steel

This type of steel is in the y-phase zone at the normal quenching temperature, but their y-phase is only stable at high temperatures, and the M point is generally around 300°C, so it transforms into martensite during cooling.

This type of steel includes 2Cr13, 2Cr13Ni2, 3Cr13 and some modified 12% chromium hot-strength steels, such as 13Cr14NiWVBA, Cr11Ni2MoWVB steel and so on. The mechanical properties, corrosion resistance, process performance and physical properties of martensitic stainless steel are similar to those of ferrite-martensitic stainless steel containing 12″14% chromium. Because there is no free ferrite in the structure, the mechanical properties are more The above-mentioned steels are higher, but have lower overheating sensitivity during heat treatment.

2-4. Martensite-Carbide Steel

The carbon content of the co-precipitation point of Fe-C alloy is 0.83%. In stainless steel, the S point is shifted to the left due to chromium. Steels containing 12% chromium and more than 0.4% carbon (Figure 11-3), and 18% chromium and Steels with more than 0.3% carbon (Fig. 3) are all hypereutectoid steels. This type of steel is heated at the normal quenching temperature, and the secondary carbides cannot be completely dissolved in austenite, so the quenched structure is composed of martensite and carbides.

There are not many grades of stainless steel belonging to this category, but some stainless steels with relatively high carbon content, such as 4Crl3, 9Cr18, 9Crl8MoV, 9Crl7MoVCo steel, etc., 3Crl3 steel with upper carbon content may also be quenched at a lower temperature Such an organization. Due to the high carbon content, although the above three steel grades such as 9Cr18 contain more chromium, their corrosion resistance is only equivalent to that of stainless steel containing 12″14% germanium. The main purpose of this type of steel is to require high hardness and resistance Grinding parts, such as cutting tools, bearings, springs and medical equipment, etc.

2-5. Austenitic steel

This type of steel contains more elements that expand the y zone and stabilize austenite. They are all in the y phase at high temperatures. During cooling, the Ms point is below room temperature, so it has an austenite structure at room temperature. 18-8, 18-12, 25-20, 20-25Mo and other chromium-nickel stainless steels, and low-nickel stainless steels with manganese instead of part of nickel and nitrogen added, such as Cr18Mnl0Ni5, Cr13Ni4Mn9, Cr17Ni4Mn9N, Cr14Ni3Mnl4Ti steel, etc. belong to this category.

Austenitic stainless steel has many advantages mentioned above. Although its mechanical properties are relatively low and cannot be strengthened by heat treatment like ferritic stainless steel, its strength can be improved by cold working deformation method using work hardening. The disadvantage of this type of steel is that it is more sensitive to intergranular corrosion and stress corrosion, which needs to be eliminated by appropriate alloy additives and technological measures.

2-6. Austenitic-Ferritic Steel

This type of steel is not enough to make the steel have a pure austenite structure at room temperature or very high temperature due to the expansion of the y zone and the effect of stabilizing austenite elements, so it is in the austenite-ferrite complex state. The amount of ferrite can also vary within a relatively wide range due to the composition and heating temperature.

There are many stainless steels belonging to this category, such as low-carbon 18-8 chromium-nickel steel, 18-8 chromium-nickel steel with titanium, niobium, and molybdenum, especially ferrite can be seen in the structure of cast steel. Chromium-manganese stainless steels with chromium greater than 14″15% and carbon lower than 0.2% (such as Cr17Mnll), as well as most of the chromium-manganese-nitrogen stainless steels currently researched and applied. Compared with pure austenitic stainless steels, this type of steel There are many advantages, such as high yield strength, high resistance to intergranular corrosion, low sensitivity to stress corrosion, low tendency of hot cracking during welding, good casting fluidity, etc. The disadvantage is that the pressure processing performance is poor. Corrosion tends to be large, easy to produce c-phase brittleness, showing weak magnetism under the action of a strong magnetic field, etc. All these advantages and disadvantages are derived from the ferrite in the structure.

2-7. Austenitic Bowl-Martensitic Steel

The Ms point of this type of steel is lower than room temperature, and the austenitic structure after solution treatment is easy to form and weld. There are usually two processes used to make martensitic transformation. One is that after the solution treatment is heated at 700″800 degrees, the austenite is transformed into a metastable state due to the precipitation of chromium carbide, the Ms point rises above room temperature, and it changes to martensite when cooled; the second is directly after the solution treatment Cooling to between the Ms and Mf points will transform austenite into martensite. The latter method can obtain higher corrosion resistance, but the interval between solution treatment and deep cooling should not be too long, otherwise it will be caused by austenite. The aging and stabilization effect of the aging body reduces the strengthening effect of cryogenic. After the above treatment, the steel is aged at 400″500 degrees to further strengthen the precipitated intermetallic compounds. Typical steel grades of this type of steel are 17Cr-7Ni-A1, 15Cr-9Ni-A1, 17Cr-5Ni-Mo, 15Cr-8Ni-Mo-A1 and so on. This type of steel is also called austenitic-maraging stainless steel, and because in fact these steels have different amounts of ferrite in addition to austenite and martensite, they are also called semi-austenite A precipitation hardening stainless steel.

This type of steel is a new type of stainless steel developed and applied in the late 1950s. Their general characteristics are high strength (C up to 100 to 150) and good thermal strength, but due to the low chromium content and chromium carbide precipitation during heat treatment , So the corrosion resistance is lower than that of standard austenitic stainless steel. It can also be said that the high strength of this type of steel is obtained at the expense of some corrosion resistance and other properties (such as non-magnetic). At present, this type of steel is mainly used in the aviation industry and the production of rockets and missiles, and is used in general machinery manufacturing. They are not yet universal, and there is also a series that classifies them as ultra-high-strength steels.