1. What are the characteristics of heat treatment of martensitic stainless steel?
Answer: Martensitic stainless steel includes Cr13 type (low carbon and medium carbon), Cr17Ni type (low carbon), and Cr18 type (high carbon). Its heat treatment process includes annealing, quenching + tempering, stress relief, etc.
(1) Cr13 type stainless steel: Alloys with Cr≥12% have the properties of stainless steel and can be quenched and strengthened. Ferrite structure exists in the austenitized structure and is retained after quenching. If the Cr content is too high, single-phase ferrite will be formed when heated, and quenching and strengthening treatment cannot be performed.
1) Annealing: Soft annealing can meet the requirements for cutting processing. Heat treatment after forging can prevent cracking of forgings. For extrusion deformation, complete annealing is required. There is a large amount of chromium carbide in the annealed workpiece, and the chromium content in the solid solution is reduced. At the same time, these chromium carbide particles and the matrix form many micro-batteries, which accelerate the corrosion of steel parts. The relationship between the quenching hardness of 1Cr13 and the carbon content is relatively large. When the carbon content is ≥0.13% and the chromium content is ≥12.75%, the hardness is greater than 46. Carbon has a greater impact on the carbon content, while chromium has a smaller impact. 5HRC. The quenching hardness of 2Cr13 is about 50HRC. The hardness of 3Crl3 and 4Crl3 steels after quenching is 51~56HRC.
2) Tempering after quenching: The common quenching temperature of 1Cr13 is 1000~1050℃, and 2Cr13 generally uses 980~1000℃. The tempering temperature of 1Cr13 and 2Cr13 is generally 600~700℃. For the design technical requirements of 1Cr13, try not to choose the hardness range of 40-43HRC (401HB). In this hardness range, the tempering temperature and time after quenching are difficult to control, as shown in Figure 8-2. If it must be selected in this hardness range, the heat treatment quenching temperature should be reduced accordingly, but the control difficulty in actual production is still relatively large.
Second, reducing the austenite heating temperature will affect the solid solution of carbides, and the impact performance is also relatively low.
Third, when the tempering temperature is 500-600℃, carbides with high dispersion will precipitate in the organization, which not only has low corrosion resistance but also low impact toughness. In actual production, the quenching temperature of 3Cr13 and 4Cr13 is 1000-1020℃, the low temperature is 200-300℃, the corrosion resistance is good, and the tempering hardness of 3Cr13 ≥48HRC, 4Cr13 ≥50HRC. High-temperature tempering is generally 600-750℃. After quenching, Cr13 steel needs to be tempered within 8 hours to prevent cracking.
(2) Cr17Ni stainless steel: This type of steel is developed by adding 2% Ni to 1Cr17 ferritic stainless steel. When quenched at high temperature, it is austenite + ferrite. Quenching is generally done by oil cooling. The quenched structure is: martensite + ferrite + a small amount of participating austenite. This type of forging is prone to white spots, and it is necessary to remove the white spots after forging. In the heat treatment technical requirements, there is often a requirement that the ferrite content is ≤15%. However, due to the slight fluctuation of the carbon content, the ferrite content will exceed the standard. Generally, the carbon content is required to be ≥0.15%, Cr is 16%~17.5%, and Ni is 2%~2.5%. The quenching heating temperature is generally 980~1020℃, the tempering temperature is 275~350℃ and 550~700℃, and air cooling is used after tempering. The tempering number is sometimes required to be twice, in order to eliminate the participating austenite structure. The corrosion resistance and impact toughness of tempering temperatures in the range of 350-550℃ are relatively low, so it is not recommended to use it.
(3) It is recommended to use a protective atmosphere furnace during heat treatment to prevent oxidation and decarburization.

1. What is the heat treatment process of commonly used martensitic steel?
Answer: The heat treatment process of commonly used martensitic steel includes annealing, stress relief, quenching and tempering, and the heat treatment heating insulation coefficient.

2. What is the heat treatment process of austenitic stainless steel?
Answer: 1Cr18Ni9Ti is a typical (18-8 type) chromium-nickel austenitic stainless steel. Since 18-8, stainless steel basically maintains a single austenitic structure in the solid state, there is no a→γ allotropic transformation during heating and cooling. Therefore, except for precipitation hardening austenitic stainless steel, it is impossible to use heat treatment methods to strengthen the steel. General 18-8 austenitic steel can only be strengthened by cold deformation.

Common heat treatments for 18-8 steel include stress relief treatment, solution treatment, sensitization treatment, stabilization treatment, and heat treatment to eliminate σ phase.
1) Stress relief annealing: In order to eliminate cold working stress, it can be heated to 300-350℃, kept warm for 1-2h, and air-cooled. When eliminating welding stress, it is generally heated at 850-950℃, kept warm for 1-3h, and air-cooled or water-cooled.
2) Solution treatment: The solution treatment process is similar to the quenching process, but no phase change occurs in the steel. Therefore, the room temperature structure after treatment is a supersaturated γ-Fe solid solution instead of a supersaturated α-Fe solid solution. The main purpose of solution treatment is to make austenitic stainless steel have excellent corrosion resistance.
The heating temperature of solution treatment is usually 1050-1100℃. The upper limit temperature is taken when the carbon content is high, and the lower limit temperature is taken when the carbon content is low. Heat treatment heating insulation coefficient in an air furnace. After treatment, it should be cooled quickly. Generally, water cooling is used, and air cooling can be used for thin-walled parts. This type of steel should be heated in a neutral or weakly oxidizing atmosphere. For this purpose, an air furnace is often used as a heating device, and an ammonia decomposition atmosphere is used as a heating medium. Because chloride salts can corrode steel, it is not suitable to use salt bath heating. To ensure the quality of heating, the surface of the parts must be cleaned before treatment.
3) Sensitization treatment: Heating in the temperature range of 400-800℃ to test the steel’s resistance to intergranular corrosion is called sensitization treatment. This temperature range is called the sensitization temperature. Except for special circumstances, heating the steel in the sensitization temperature range should be avoided as much as possible. The solution treatment is to dissolve the precipitated chromium carbide in austenite again. The effect of sensitization treatment can be eliminated by the solution treatment process.
4) Stabilization treatment: There are many forms of metal corrosion. One type of corrosion is along the grain boundaries on the metal surface, which is called intergranular corrosion. Alloy elements such as titanium and niobium are added to austenitic stainless steel to prevent intergranular corrosion. Stabilization treatment is only used for chromium-nickel austenitic stainless steel containing titanium or niobium. After solution treatment, the intergranular corrosion tendency of steel increases due to the precipitation of chromium carbide along the grain boundaries. Therefore, a stabilization treatment should be carried out again after solution treatment to transfer the carbon atoms in chromium carbide to titanium carbide or niobium carbide, thereby improving the steel’s ability to resist intergranular corrosion. The process of stabilization treatment is: heating to 850-900℃, keeping warm for 2-6h, air cooling, or water cooling.
5) Heat treatment to eliminate σ phase: σ phase is a hard and brittle FeCr intermetallic compound. Its presence reduces the toughness, corrosion resistance, and oxidation resistance of steel. σ phase is most likely to appear in high-chromium ferrite. It may also appear in austenitic-ferritic steel and austenitic steel. σ phase can be dissolved in austenite at high temperatures, and the temperature at which it can exist in steel is 820℃. Heat treatment to eliminate the σ phase is to heat at a temperature higher than its upper limit of existence. For 1Crl8Ni9Nb, the σ phase disappears after heating at 850℃. The upper limit temperature of the σ phase varies with the steel composition, so the specific heating temperature should be determined through experiments.

3. What is the heat treatment process of ferrite-austenite stainless steel?
Answer: Commonly used ferrite-austenite stainless steel heat treatments are: 0Cr21Ni5Ti, 1Cr21Ni5Ti, 1Cr18Mn10Ni5Mo3N, 0Cr17Mn13Mo2N, 1Cr18Ni11Si4AlTi, 00Cr18Ni5Mo3Si, 00Cr25Ni5Mo2.
The heat treatment process of 0Cr17Mn13Mo2N is heating at 1050～1080℃, water cooling, and the structure is austenite + 20～30% δ ferrite.
The heat treatment process of 1Cr18Mn10Ni5Mo3N is heating at 1100～1150℃, water cooling.
0Cr21Ni5Ti, 1Cr21Ni5Ti heat treatment process 950 ~ 1050 ℃ heating, water cooling or air cooling.
1Cr18Ni11Si4AlTi heat treatment process 950 ~ 1050 ℃ heating, water cooling.
00Cr18Ni5Mo3Si, 00Cr25Ni5Mo2 heat treatment process 950 ~ 1000 ℃ heating, water cooling.

4. What is the reason why 18-8 austenitic stainless steel products are prone to corrosion? What are the ways to prevent corrosion?
Answer: The types of metal corrosion are: continuous corrosion, intergranular corrosion, pitting corrosion, stress corrosion, etc. The defects that need to be prevented during the heat treatment of 18-8 austenitic stainless steel products are continuous corrosion, pitting corrosion, and intergranular corrosion.

1) Causes of corrosion: The corrosion of stainless steel is caused by the presence or precipitation of carbides and intergranular chromium depletion. The theory of chromium depletion in the grain boundary area holds that the intergranular corrosion of austenitic stainless steel is caused by the precipitation of Cr23C6 along the grain boundary during the aging process, which causes the austenite near the grain boundary to be depleted in Cr, causing the chromium content in the solid solution to drop below the limit required for passivation.
2) Preventive measures:
(1) Use solid solution treatment to prevent or reduce the precipitation of carbides in the matrix;
(2) After the solid solution treatment, use sensitization treatment, heating temperature 400 ~ 800 ℃, and treatment time from tens of hours to more than 1000 hours, the purpose of extending the sensitization treatment time is to allow the chromium-depleted area caused by the precipitation of carbides to obtain chromium compensation from the chromium-rich area by diffusion, and restore corrosion resistance.
(3) For 18-8 type Ti or Nb-containing austenitic stainless steel, add stabilization treatment after solid solution treatment. Stabilization process: 850-900℃, keep warm for 2-4h, air cooling.

5. How to eliminate the σ phase of 18-8 austenitic stainless steel?
Answer: σ phase sometimes appears in 18-8 chromium-nickel austenitic stainless steel. When 18-8 steel contains ferrite-forming elements such as titanium, niobium, molybdenum, and silicon, because the ferrite is rich in chromium, the σ phase is formed from the chromium-rich ferrite. The formation of σ phase must have a certain temperature and time.

No σ phase is found in 18-8 steel with pure austenite structure, while σ phase can often be found in cast 18-8 titanium steel, which may be related to the component segregation of the casting. That is the local σ phase, which may be related to the component segregation of the casting. More chromium is enriched in the local ferrite, making it easy for the σ phase to nucleate and grow. The formation of σ phase leads to the brittleness of steel and also reduces the solubility of σ phase in ferrite at high temperatures. In Fe-Cr alloy, this upper limit temperature is about 820℃, so the brittleness caused by σ phase can be eliminated by heating or solution treatment above 820℃.

Due to the different compositions of steel, the upper limit dissolution temperature of the σ phase also varies, so the specific temperature can be determined by experiment. For example, for 1Crl8Ni11Nb steel, σ phase begins to dissolve at 800℃, and σ phase disappears when heated at 850℃.