Several Commonly Used Stainless Steels and Their Heat Treatment Properties

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According to the structural composition of the steel after normalizing heat treatment, it can be divided into martensitic stainless steel, ferritic stainless steel, austenitic stainless steel and transition stainless steel (semi-austenitic stainless steel, that is, both austenitic and The advantages of martensitic stainless steel). The composition and characteristics of various stainless steels are described below.

1. Martensitic stainless steel

The chromium content in the steel is 12% to 18%, and the carbon content is 0.10% to 0.4%. After quenching, a martensitic structure can be obtained. This kind of stainless steel is called martensitic stainless steel.

Martensitic stainless steel has good strength, plasticity, toughness and corrosion resistance after quenching and tempering. Both martensitic or semi-martensitic stainless steels are magnetic.
The structure of martensitic stainless steel after normalizing or quenching is martensite. The structure of half martensitic stainless steel after normalization is martensite + ferrite. The structure after quenching depends on its composition and quenching temperature. It can be martensite or martensite + austenite. They can all be strengthened by heat treatment.
Commonly used martensitic stainless steels are 2Cr13, 3Cr13, 4Cr13. And 1Cr13 belongs to half martensitic stainless steel.

After quenching and high temperature tempering, 1Cr13 and 2Cr13 steels have good comprehensive mechanical properties. It is suitable to manufacture parts that require good plasticity and are subjected to impact loads, and are mostly used for structural parts that require certain strength and toughness. Such as the blades of steam turbines, hydraulic valves, parts of thermal cracking equipment, screws, nuts and daily life utensils under high temperature. After quenching and low-temperature tempering, 3Cr13 and 4Cr13 steels have high hardness and wear resistance, and are suitable for high -hardness wear-resistant parts, corrosion-resistant parts and devices. Such as hot oil pump shafts, plungers, springs, rolling bearing accessories, medical devices, tools, instrument shafts, etc.

※ Note:

Cr13 type stainless steel has poor thermal conductivity. Preheating should be carried out during quenching heating. The preheating temperature is 700～900℃ and the time is 1～3h to avoid cracking.

When heating 3Cr13 and 4Cr13 steel, attention should be paid to prevent surface decarburization, because decarburization will reduce the hardness after quenching.

After quenching, it should be tempered in time to avoid cracking, and the interval should not exceed 8h.

2. Ferritic stainless steel

Typical ferritic stainless steels include Cr17, Cr17T1, Cr25, Cr25T1, Cr28, etc. They have good corrosion resistance in strong oxidizing media. This type of steel can resist the corrosion of strong corrosive solutions such as nitric acid and hot phosphoric acid. So it is one of the superior materials used in chemical equipment. At the same time, because they generally have good high temperature oxidation resistance, they are also used as high temperature resistant materials.
This type of steel has a very low carbon content (≤0.15%) and a high chromium content (>17%). The structure does not change during heating and cooling, so it cannot be strengthened by heat treatment.

The heat treatment of ferritic stainless steel is relatively simple, and it is generally used in an annealed state. The purpose of annealing is to eliminate processing stress and obtain a single ferrite structure. Generally, it is heated to 750-850°C for annealing treatment. Excessive heating temperature will cause the grains to grow and make the steel brittle; and the grown grains cannot be refined by heat treatment, so the heating temperature should be strictly controlled. The annealing holding time is 1～2h, and it can also be calculated according to the material thickness (1.5min/mm). Air cooling or water cooling after annealing.

※ Note:

Ferritic stainless steel staying in the temperature range of 400～425℃ will cause the embrittlement of the steel, which is usually called “475℃ brittleness”. For the resulting brittleness, it can be heated to a temperature above 475℃ and then quickly cooled . eliminate.

The disadvantages of ferritic stainless steel are low toughness and high brittleness. There are three reasons for brittleness.

The first is the coarse grains, ferritic stainless steel does not undergo phase change during heating and cooling, and the coarse as-cast structure cannot be changed by heat treatment.

The second is the brittleness of 475℃ and the high chromium ferritic stainless steel with Cr content greater than 15%. When staying in the temperature range of 400～500℃ for a long time or slowly cooling in this temperature range, it will cause room temperature embrittlement and increase in strength. High, plasticity and toughness are close to zero. Because the embrittlement is the most serious around 475°C, it is called brittleness at 475°C.

The third is the brittleness of the sigma phase. The high-chromium ferritic stainless steel with a Cr content of more than 15% is called the sigma phase, which is the intermetallic compound FeCr precipitated from the δ-ferrite when heated for a long time in the temperature range of 520 to 820℃. The σ phase is often distributed along the grain boundary, and it is accompanied by volume changes during precipitation, which increases the brittleness, and also causes intergranular corrosion of steel and reduces oxidation resistance.

3. Chromium-nickel stainless steel (18-8 type chromium-nickel stainless steel)

The 18-8 type chromium-nickel stainless steel is a single-phase austenitic stainless steel with a carbon content of less than 0.20%, while the chromium content is generally greater than 18%, and the nickel content is greater than 9%. Its strength and hardness are lower, and its plasticity and toughness are better. Used to manufacture parts that are resistant to acid and alkali solutions, such as chemical containers, acid-resistant pump parts, etc.

18-8 type stainless steel includes 1Cr18Ni9 Ti, 1Cr18Ni9, 2Cr18N19 and so on. Adding alloy element titanium (Ti) can effectively prevent intergranular corrosion. It belongs to chromium-nickel stainless steel, which has good corrosion resistance and oxidation resistance, and has high plasticity and weldability, but its machinability is poor. It is widely used in aviation, shipbuilding, automobile, chemical, medical equipment and other industries. And because of the magnetism of this kind of steel, it is also widely used in the instrumentation industry .

Editing by Baccata

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