Classification, characteristics and heat treatment of common stainless steel materials
Stainless steel is a high alloy steel that can resist corrosion in air or chemically corrosive media. According to the organizational state of the steel in the normalized state, it can be divided into ferritic stainless steel, austenitic stainless steel and martensitic stainless steel.
01
ferritic stainless steel
When the chromium content reaches 13%, the iron-chromium alloy will have no γ phase transformation; when the chromium content reaches 12%, it can resist corrosion, so Cr13 ferritic steel becomes ferritic stainless steel.
Features: Ferritic stainless steel has better corrosion resistance and oxidation resistance, especially stress corrosion resistance, but its mechanical properties (higher yield strength than austenitic stainless steel, but lower impact toughness and greater brittleness) and process It has poor performance and is mostly used in acid-resistant structures with little stress and as anti-oxidation steel.
1. Steel types and types of ferritic stainless steel
⑴Cr13 type: such as 0Cr13, 0Cr13Al (Al: expanded F, anti-oxidation), etc., commonly used as heat-resistant steel, anti-oxidation.
⑵Cr16-19 type: such as Cr17, Cr17Ti, Cr17Mo1Nb, etc., which can withstand corrosion in the atmosphere, fresh water, and dilute nitric acid media.
⑶Cr25-28 type: such as Cr25Ti, Cr26Mo1, Cr28, etc., which are acid-resistant steels that are resistant to strong corrosive media.
2. Brittleness of ferritic stainless steel
The disadvantage of high-chromium ferritic steel is that it is brittle. The main reasons are:
⑴Coarse original grains:
① The structure in the cast state is coarse and cannot be refined by phase transformation during heating and cooling, but can only be refined by deformation and recrystallization; ② Ferrite does not produce intergranular corrosion due to fast atomic diffusion (the same principle as F, and Cr diffusion Fast), has low grain coarsening temperature and high grain coarsening rate.
Solution: During production, control the final forging temperature or final rolling temperature at 750°C or lower; add a small amount of titanium to the steel to prevent grain growth with Ti (C, N); increase the content of ferrite in stainless steel. The amount of austenite at high temperature is used to control grain coarsening.
⑵ σ phase: σ phase has high hardness (HRC68 or above) and is often distributed along grain boundaries, so it causes great brittleness and may promote intergranular corrosion. (Quick cooling to reduce precipitation)
⑶475°C brittleness: (After being heated for a long time in the temperature range of 400 to 500°C or when slowly cooling in this temperature range, steel becomes very brittle at room temperature) Reason: When heated at 475°C, the chromium atoms in the ferrite tend to Ordered, many chromium-rich ferrites are formed, which maintain a coherent relationship with the parent phase, causing lattice distortion and internal stress. At this time, the strength of the steel increases, the impact toughness decreases, and the brittleness increases.
⑷ Steel contains C, N, O and other impurities and inclusions
3. Heat treatment of ferritic stainless steel
⑴The balance structure of ferritic stainless steel is ferrite + chromium carbide
⑵Purpose: In order to obtain a ferrite structure with uniform composition, reduce carbide precipitation, eliminate the tendency of intergranular corrosion, and eliminate σ phase precipitation and 475°C brittleness, ferritic stainless steel is often quenched, tempered or annealed after hot rolling. Heat treatment process. (When carbides precipitate, pitting corrosion and intergranular corrosion are prone to occur)
02
Austenitic stainless steel
Austenitic stainless steel is developed with 18% Cr-8% Ni as its typical composition. (Type 18-8 austenitic stainless steel)
Features: High corrosion resistance (higher than M stainless steel, lower than F stainless steel), high plasticity, toughness and low temperature toughness, easy to be processed into steel of various shapes, good welding performance, non-magnetic, etc., that is It has good comprehensive mechanical properties and is the most widely used type of stainless steel.
1. Typical steel types, properties and applications
⑴Cr-Ni series stainless steel: 0Cr18Ni9, 1Cr18Ni9, 1Cr18Ni9Ti, 1Cr18Ni11Nb, 00Cr18Ni10, 00Cr17Ni7Cu2, (adding Ti and Nb reduces grain boundary corrosion; adding Cu reduces stress corrosion and expands the A element)
⑵Cr-Mn-N series, Cr-Mn-Ni-N series stainless steel (adding Mn and N can replace Ni)
Typical steel types: 1Cr17Mn13N, 1Cr18Mn8Ni5N (Analysis: WCr﹪>12﹪ stainless steel; containing Mn, Ni, N is austenitic stainless steel, if it contains Cr, Al, it is F stainless steel)
The solid solution strengthening of N gives steel higher yield strength, plasticity and toughness.
⑶ Metastable austenitic stainless steel: Partial martensite transformation occurs during cold deformation, so that the steel is strengthened by martensite on the basis of cold work hardening.
Supplement: Deformation between Ms and Md induces M phase transformation, and deformation greater than Md stabilizes A mechanically.
2. Balanced structure and heat treatment of austenitic stainless steel
The equilibrium structure of type 18-8 austenitic stainless steel is austenite + ferrite + carbide complex phase structure. The actual single-phase austenite is obtained through solid solution treatment. The purpose is to dissolve both ferrite and carbide into A to obtain single-phase A.
03
martensitic stainless steel
1. Martensitic stainless steel contains 12--18% Cr. Compared with ferritic stainless steel, its composition characteristics are:
⑴The upper limit of chromium content is lower (if too much, it is F)
⑵ It also contains a certain amount of γ phase stabilizing elements such as carbon and nickel. (Not too much nickel)
⑶The corrosion resistance and weldability of this type of steel are worse than austenitic and ferritic stainless steel, and the plasticity is worse than A stainless steel, but because it has a better combination of mechanical properties and corrosion resistance (it has a certain corrosion resistance , bear a certain load)
2. Used to manufacture mechanical parts, medical surgical tools, measuring tools, stainless bearings, springs, etc.
3. Comprehensive comparison of the corrosion resistance and mechanical properties of A (austenite), F (ferrite), and M stainless steel:
M stainless steel has poor corrosion resistance, but can withstand a certain load; A stainless steel has average corrosion resistance, average strength, but good plasticity and toughness; F stainless steel has good corrosion resistance and oxidation resistance, but is brittle.
1. Typical steel types, compositions and applications
⑴① Low carbon 13% Cr steel: such as 1Cr13, 2Cr13; ② Low carbon 17% Cr-2% Ni (Ni: stable A): Cr multi-solid solution strengthening. ① and ② are equivalent to corrosion-resistant tempered steel: the heat treatment is quenching + high-temperature tempering. (The alloy moves the S point to the left, so the effect is similar to modulating steel)
⑵ Medium carbon 13% Cr steel: such as 3Cr13, 4Cr13, equivalent to corrosion-resistant tool steel; quenching + low temperature tempering
⑶ High carbon 18% Cr steel: such as 9Cr18, etc., which is equivalent to corrosion-resistant tool steel. Quenching + low temperature tempering
2. Heat treatment of martensitic stainless steel
⑴Softening treatment: equivalent to preliminary heat treatment
After steel is forged and rolled, martensitic transformation will occur due to air cooling, making the forging hard, causing cracks on the surface of the forging, and making it difficult to machine. ①High temperature tempering ②Complete annealing
⑵Quenching and tempering treatment
⑶Quenching and low temperature tempering
