Answer: Martensitic stainless steel pipes include Cr13 type (low carbon and medium carbon), Cr17Ni type (low carbon), and Cr18 type (high carbon). The heat treatment process includes annealing, quenching + tempering, stress relief, etc.
(1) Cr13 stainless steel pipe: Alloys with Cr ≥ 12% have the properties of stainless steel pipes and can be quenched and strengthened. There is a ferrite structure in the austenitized structure, which remains 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 accelerates 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 >46. Carbon has a large influence on the carbon content and chromium has a small influence. 5HRC. The quenching hardness of 2Cr13 is about 50HRC. The hardness of 3Crl3 and 4Crl3 steel after quenching is 51~56HRC.
2) Tempering after quenching: The common quenching temperature for 1Cr13 is 1000~1050℃, 980~1000℃ is generally used for 2Cr13, and the tempering temperature of 1Cr13 and 2Cr13 is generally 600~700℃.
① 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. See Figure 8-2. If you must choose In this hardness range, the heat treatment and quenching temperature should be reduced accordingly, but the control difficulty in actual production is still relatively large.
② Lowering the austenite heating temperature will affect the solid solution of carbides, and the impact performance will also be relatively low.
③ When the tempering temperature is between 500 and 600°C, carbides with high dispersion will precipitate in the structure, 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℃, they have good corrosion resistance, and the tempering hardness is 3Cr13≥48HRC, 4Cr13≥50HRC. High-temperature tempering is generally between 600 and 750°C. Cr13 steel needs to be tempered within 8 hours after quenching to prevent cracking.
(2) Cr17Ni stainless steel pipe: This steel type is developed based on 1Cr17 ferritic stainless steel pipe by adding 2% Ni. During high-temperature quenching and heating, it is austenite + ferrite. Oil cooling is generally used for quenching. The quenching structure is martensite + ferrite + a small amount of austenite. It is easy to produce white spots after this kind of forging, and it needs to be treated to remove the white spots after forging. In the heat treatment technical requirements, there is often an index requirement for ferrite content ≤15%. However, due to small fluctuations in carbon content, the ferrite content will exceed the standard. Generally, the carbon content is required to be ≥0.15%, Cr between 16% and 17.5%, and Ni between 2% and 2.5%. The quenching heating temperature is generally 980~1020℃, and the tempering temperature is 275~350℃ and 550~700℃. Air cooling is used after tempering. The tempering times sometimes need to be twice to eliminate the participation of the austenite structure. The corrosion resistance and impact toughness in the tempering temperature range of 350 to 550°C are relatively low and are not recommended.
(3) It is recommended to use a protective atmosphere furnace during heat treatment to prevent oxidation and decarburization.
1. What is the commonly used heat treatment process for martensitic steel?
Answer: Commonly used heat treatment processes for martensitic steel include annealing, stress relief, quenching, and tempering. See the heat treatment heating and insulation coefficient.
2. What is the heat treatment process for austenitic stainless steel pipes?
Answer: 1Cr18Ni9Ti is a typical (18-8 type) chromium-nickel austenitic stainless steel pipe. Since the 18-8 stainless steel pipe maintains a single austenite structure in the solid state, there is no a→ during the heating and cooling process. Allotropic transformation of γ, so except for precipitation hardening austenitic stainless steel pipes, steel cannot be strengthened by heat treatment. General 18-8 austenitic steel can only be strengthened through cold deformation. Commonly used heat treatments for 18-8 steel include stress relief treatment, solid solution treatment, sensitization treatment, stabilization treatment, and heat treatment to eliminate σ phase.
1) Stress relief annealing: To eliminate cold working stress, it can be heated to 300~350℃, kept for 1~2h, and air-cooled. When eliminating welding stress, heating at 850 to 950°C, heat preservation for 1 to 3 hours, and air cooling or water cooling are generally used.
2) Solid solution treatment: The solid solution treatment process is similar to the quenching process, except that no phase change occurs in the steel, so 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 pipes have excellent corrosion resistance. The commonly used heating temperature for solution treatment is 1050~1100℃. The upper limit temperature is taken when the carbon content is high, and the upper limit temperature is taken when the carbon content is low. It should be cooled quickly after treatment. 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 the heating equipment and an ammonia decomposition atmosphere is used as the heating medium. Because chloride salts will corrode steel, it is not advisable to use a salt bath for heating. To ensure the heating quality, the surface of the parts must be cleaned before processing.
3) Sensitization treatment: Heating in the temperature range of 400 to 800°C to test the intergranular corrosion resistance of steel is called sensitization treatment. This temperature range is called the sensitization temperature. Except for special circumstances, heating of steel in the sensitization temperature range should be avoided as much as possible. Solid solution treatment is to resolute the precipitated chromium carbide in austenite. The solid solution treatment process can be used to eliminate the influence of sensitization treatment.
4) Stabilization treatment: There are many forms of metal corrosion. There is a type of corrosion that proceeds along the grain boundaries on the metal surface, called intergranular corrosion. Titanium, niobium, and other alloying elements are added to austenitic stainless steel pipes to prevent intergranular corrosion. Stabilization is only used for chromium-nickel austenitic stainless steel tubes 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 performed after solid 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 stabilization treatment process is: heating to 850~900℃, holding for 2~6 hours, and air cooling or water cooling.
5) Heat treatment to eliminate σ phase: σ phase is a hard and brittle FeCr intermetallic compound. Its existence reduces the toughness, corrosion resistance, and oxidation resistance of steel. The σ phase is most likely to appear in high-chromium ferrite. It may also appear in austenitic-ferritic steel and austenitic steel. The σ phase can be dissolved in austenite at high temperatures, and the temperature at which it exists in steel is 820°C. The heat treatment to eliminate the σ phase is to heat at a temperature higher than the upper limit of its existence. For 1Crl8Ni9Nb, the σ phase will disappear after heating at 850°C. With different steel compositions, the upper limit temperature for the existence of the σ phase is also different, so the specific heating temperature should be determined through experiments.
3. What is the heat treatment process for ferritic-austenitic stainless steel pipes?
Answer: Commonly used ferritic-austenitic stainless steel pipes are: 0Cr21Ni5Ti, 1Cr21Ni5Ti, 1Cr18Mn10Ni5Mo3N, 0Cr17Mn13Mo2N, 1Cr18Ni11Si4AlTi, 00Cr18Ni5Mo3Si, 00Cr25Ni5Mo2. The 0Cr17Mn13Mo2N heat treatment process is 1050~1080℃ heating and water cooling, The structure is austenite+20～ 30% delta ferrite. 1Cr18Mn10Ni5Mo3N heat treatment process: heating at 1100~1150℃ and water cooling. 0Cr21Ni5Ti, 1Cr21Ni5Ti heat treatment process: heating at 950~1050℃, water cooling or air cooling. 1Cr18Ni11Si4AlTi heat treatment process is heated at 950~1050℃ and water-cooled. 00Cr18Ni5Mo3Si and 00Cr25Ni5Mo2 heat treatment process is heated at 950~1000℃ and water-cooled.
4. What is the reason why 18-8 type austenitic stainless steel pipe products are prone to corrosion? What are the ways to prevent corrosion?
Answer: The corrosion types of metal include: continuous corrosion, intergranular corrosion, pitting corrosion, stress corrosion, etc. The defects that need to be prevented during the heat treatment of 18-8 type austenitic stainless steel pipe products are continuous corrosion, pitting corrosion, and intergranular corrosion.
1) Causes of corrosion: The corrosion of stainless steel pipes is caused by the presence or precipitation of carbides and intergranular chromium depletion. The theory of chromium depletion in the grain boundary area believes that the intergranular corrosion of austenitic stainless steel pipes is due to the aging process. , Cr23C6 precipitates along the grain boundaries, causing the austenite near the grain boundaries to be Cr-poor, causing the chromium content in the solid solution to fall below the limit required for passivation.
2) Preventive measures:
(1) Adopt solid solution treatment to prevent the carbide in the matrix from precipitating or less;
(2) After solid solution treatment, sensitization treatment is used, with a heating temperature of 400 to 800°C and a treatment time from dozens of hours to more than 1,000 hours. The purpose of extending the sensitization treatment time is to diffuse the chromium-deficient area caused by the precipitation of carbides. It functions to obtain chromium compensation from the chromium-rich area and restore corrosion resistance.
(3) Type 18-8 Ti- or Nb-containing austenitic stainless steel pipes are stabilized after solid solution treatment. Stabilization process: 850~900℃, heat preservation for 2~4 hours, air cooling.
5. How to eliminate the σ phase of 18-8 type austenitic stainless steel pipe?
Answer: σ phase sometimes appears in 18-8 type chromium-nickel austenitic stainless steel. When 18-8 steel contains titanium, niobium, molybdenum, silicon, etc. to form ferrite elements, because the ferrite is rich in chromium, the σ phase is formed from the chromium-rich ferrite. The formation of the σ phase requires a certain temperature and time. No σ phase is found in 18-8 steel with pure austenitic structure, while σ phase can often be found in as-cast 18-8 titanium steel, which may be related to the composition segregation of castings. That is the local σ phase, which may be related to the composition segregation of the casting. More chromium is enriched in the ferrite, making it easy for the σ phase to nucleate and grow. The formation of the σ phase causes the brittleness of steel and also reduces the σ phase’s ability to dissolve in ferrite at high temperatures. In iron-chromium alloys, the upper limit temperature is about 820°C, so the brittleness caused by the σ phase can be eliminated by temperatures above 820°C. Eliminate by heating or solution treatment. Due to different compositions of steel, the upper limit dissolution temperature of the σ phase also changes, so the specific temperature can be determined by experiments. For example, in 1Crl8Ni11Nb steel, at 800°C, the σ phase has begun to dissolve, and the σ phase will disappear when heated to 850°C.
Post time: May-28-2024