Classification Characteristics and Heat Treatment of Common Stainless Steel Materials

Stainless steel is a type of steel. Steel refers to steel containing less than 2% carbon (C), and more than 2% is iron. During the smelting process, alloy elements such as chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), and molybdenum (Mo) are added to improve the performance of steel and make it corrosion-resistant (i.e., rust-proof). Stainless steel is a high-alloy steel that can resist corrosion in air or chemical corrosive media. According to the organizational state of 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 good corrosion resistance and oxidation resistance, especially good stress corrosion resistance, but poor mechanical properties (yield strength is higher than austenitic stainless steel, but impact toughness is low and brittle) and process performance. It is mostly used for acid-resistant structures with low stress and as oxidation-resistant steel.

1. Types and types of ferritic stainless steel

⑴ Cr13 type: such as 0Cr13, 0Cr13Al (Al: expanded F, anti-oxidation), etc., often used as heat-resistant steel, anti-oxidation.

⑵ Cr16-19 type: such as Cr17, Cr17Ti, Cr17Mo1Nb, etc., can resist corrosion in the atmosphere, fresh water, and dilute nitric acid media.

⑶ Cr25-28 type: such as Cr25Ti, Cr26Mo1, Cr28, etc., are acid-resistant steels resistant to strong corrosive media.

2.Brittleness of ferritic stainless steel    

The disadvantage of high chromium ferritic steel is its high brittleness, and the main reasons are:

⑴ Coarse original grains:

① The structure is coarse in the cast state and cannot be refined by phase transformation during heating and cooling, but can only be refined by deformation recrystallization;

② Ferrite has a low grain coarsening temperature and a high grain coarsening rate due to the fast atomic diffusion (the principle is the same as F, no intergranular corrosion occurs, and Cr diffuses quickly). Solution: In production, the final forging temperature or final rolling temperature is controlled at 750℃ or lower; a small amount of titanium is added to the steel to prevent grain growth with Ti (C, N); the amount of austenite at high temperature in ferritic stainless steel is increased to control grain coarsening.

⑵σ phase: σ phase has high hardness (HRC68 or above) and is often distributed along the grain boundary, so it causes great brittleness and may promote intergranular corrosion. (Quick cooling to reduce its precipitation)   

⑶475℃ brittleness: (After long-term heating in the temperature range of 400-500℃ or slow cooling in this temperature range, the steel becomes very brittle at room temperature)

Reason: When heated at 475℃, the chromium atoms in the ferrite tend to be ordered to form many chromium-rich ferrites, 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

⑷The steel contains impurities and inclusions such as C, N, and O

 3.Heat treatment of ferritic stainless steel

⑴The equilibrium structure of ferritic stainless steel is ferrite + chromium carbide

⑵Purpose: In order to obtain a ferrite structure with uniform composition, reduce carbide precipitation, eliminate intergranular corrosion tendency, and eliminate σ phase precipitation and 475℃ brittleness, ferritic stainless steel is often treated with quenching, tempering or annealing heat treatment after hot rolling. (Pitting corrosion and intergranular corrosion are likely to occur when carbides precipitate)

02 Austenitic stainless steel

Austenitic stainless steel is developed with 18% Cr-8% Ni as the typical composition. (18-8 type 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 process into various shapes of steel, good welding performance, non-magnetic, etc., that is, good comprehensive mechanical properties, is the largest type of stainless steel.

1、Typical steel grades, properties and applications

⑴Cr-Ni stainless steel: 0Cr18Ni9, 1Cr18Ni9, 1Cr18Ni9Ti, 1Cr18Ni11Nb, 00Cr18Ni10, 00Cr17Ni7Cu2, (adding Ti and Nb to reduce intergranular corrosion; adding Cu to reduce stress corrosion and expand A element)

⑵Cr-Mn-N, Cr-Mn-Ni-N stainless steel (adding Mn and N can replace Ni) Typical steel grades: 1Cr17Mn13N, 1Cr18Mn8Ni5N (Analysis: WCr﹪>12﹪ stainless steel; containing Mn, Ni, N is austenitic stainless steel, such as containing Cr and Al is F stainless steel) N solid solution strengthening makes the steel have higher yield strength, plasticity and toughness.

⑶Metastable austenitic stainless steel: partial martensitic 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 makes A mechanically stabilized.

2、Equilibrium structure and heat treatment of austenitic stainless steel

The equilibrium structure of 18-8 type austenitic stainless steel is austenite + ferrite + carbide complex phase structure, and the actual single-phase austenite is obtained by 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:

⑴ The upper limit of chromium content is lower (if too much, it is F)

⑵ It also contains a certain amount of carbon and nickel and other γ-phase stabilizing elements. (Nickel cannot be too much)

⑶ This type of steel has worse corrosion resistance and weldability than austenitic and ferritic stainless steels, and worse plasticity than A stainless steel, but because it has a good combination of mechanical properties and corrosion resistance (has certain corrosion resistance and can withstand certain loads)

2、Used to manufacture mechanical parts, medical surgical tools, measuring tools, stainless bearings, springs, etc.

3、Comprehensively compare the corrosion resistance and mechanical properties of A (austenitic), F (ferritic), and M stainless steels: M stainless steel has poor corrosion resistance, but can withstand a certain load; A stainless steel has moderate corrosion resistance, average strength, but good plasticity and toughness; F stainless steel has good corrosion resistance and oxidation resistance, but is brittle.

4、Typical steel grades, composition and applications

⑴① Low-carbon 13% Cr steel: such as 1Cr13, 2Cr13;

② Low-carbon 17% Cr-2% Ni (Ni: stabilizes A): Cr is mostly solid solution strengthened. ① and ② are equivalent to corrosion-resistant tempered steel: heat treatment is quenching + high temperature tempering. (The alloy makes the S point shift to the left, so the effect is similar to tempered steel)

⑵ Medium carbon 13% Cr steel: such as 3Cr13, 4Cr13 is equivalent to corrosion-resistant tool steel; quenching + low temperature tempering

⑶ High carbon 18% Cr steel: such as 9Cr18, etc. is equivalent to corrosion-resistant tool steel. Quenching + low temperature tempering

2、Heat treatment of martensitic stainless steel

3、⑴ Softening treatment: equivalent to preliminary heat treatment After forging and rolling, the steel will produce martensitic transformation due to air cooling, which will harden the forging and produce cracks on the surface of the forging, and it is also not easy to cut.

① High temperature tempering

② Complete annealing

⑵ Quenching and tempering treatment

⑶ Quenching and low temperature tempering