Water quenching heat treatment process of seamless steel pipe

The mature heat treatment process of 42CrMo seamless steel pipe uses oil or quenching liquid for quenching, but the cost is high, and quenching crack is a serious quality problem, which will lead to the scrap of steel pipe parts. In this paper, the heat treatment process of 42CrMo seamless steel pipe is proposed by using water as quenching cooling medium.

Using water as quenching medium

Firstly, the small batch heat treatment test of 42CrMo seamless steel pipe was carried out φ 98.43mm (outer diameter) × 93mm (wall thickness), tensile strength > 655mpa, yield strength 552 ~ 665mpa, elongation > 20%. The heat treatment quantity of small batch is 12 pieces, the quenching temperature is 920 ℃, the heat preservation time is 50 min, the quenching cooling medium is water, the quenching adopts the method of immersion and internal spraying, the tube temperature after quenching is below 40 ℃, the tempering temperature is 735 ℃, and the heat preservation time is 105 min. After quenching and tempering, the mechanical properties of the steel pipe were qualified, but a large number of cracks were found on the inner surface of 8 pipes, which led to the scrapping of the steel pipe.

The photos of cracks are shown in Fig. 1. In order to analyze the characteristics of cracks, infer the stages and causes of cracks, put forward targeted prevention measures, and carry out process improvement and effect verification.

20210608210640 12515 - Water quenching heat treatment process of seamless steel pipe

Fig. 1 crack of steel pipe after initial process heat treatment
(1) The microstructure was observed by Zeiss Axio imager A2M metallographic microscope.
(2) According to ASTM A370 standard, the longitudinal tensile properties of samples were tested, and the test equipment model was sht560560t. According to ASTM E23 standard, longitudinal impact test at 0 ℃ was carried out on the sample, and the inspection equipment model was ni300f.

Characteristics and causes of cracks

Macroscopic characteristics of cracks

The cracks are straight and strong on the inner surface, with a depth of 2 ~ 5mm and a length of 20 ~ 200 mm. The cracks are sharp, indicating that the stress during cracking is large, which is a typical quenching crack. Figure 2 is a microscopic photograph of the crack.

20210608210710 29004 - Water quenching heat treatment process of seamless steel pipe

Fig. 2 characteristics of cracks under microscope (50 ×)

Microstructure of crack area

The structure of the crack zone is normal tempered sorbite, and there is no obvious oxidation and decarburization at the crack, which indicates that the crack is produced in the process of quenching, which is consistent with the characteristics of quenching crack.

Crack cause analysis

The steel pipe of this specification has small diameter, thin wall thickness, small heat energy stored after austenitizing, strong hardenability of 42CrMo and fast water cooling speed, so it is easy to completely harden. The morphology of martensite can be divided into lath martensite and sheet martensite. Lath martensite has high strength, high hardness, and high plasticity and toughness, while sheet martensite has high strength, but poor toughness and brittleness. When WC ≤ 0.2%, it is lath martensite, when WC = 0.2% ~ 1%, it is lath martensite and sheet martensite mixed structure, and after 42CrMo quenching, it is lath martensite and sheet martensite mixed structure. There are temperature stress and microstructure stress in the cooling process of steel pipe. It is not long after undercooled austenite transforms into martensite that cracks begin to appear. The temperature stress can lead to the deformation of the steel pipe. The plasticity of austenite is good at this stage, and the stress is released by deformation. The density of austenite is less than that of martensite. After transformation into martensite, volume expansion will occur, resulting in structural stress. The more sufficient the degree of transformation into martensite is, the greater the stress will be, and the greater the stress on the surface of steel pipe will be, resulting in cracks.

Optimization and application effect of heat treatment process

According to the cause analysis of quenching cracks, the following optimized quenching process measures are put forward

  • 1) The quenching temperature of steel pipe is reduced to 880 ℃ to reduce the temperature stress.
  • 2) In order to reduce the stress of the structure, the pressure of the pump and the flow velocity of the water should be reduced.
  • 3) Reduce the quenching depth, so that 1 / 4 of the steel pipe in the circumferential direction is on the water surface, and 3 / 4 of the steel pipe is under the water, so as to reduce the structural stress.
  • 4) Reduce the cooling time of steel pipe in water and increase the outlet water temperature of steel pipe to 120 ~ 160 ℃.
  • 5) The residual temperature self tempering is used to reduce the stress concentration.
  • 6) The tempering process is still maintained at 735 ℃ for 105 min.

The optimized heat treatment process was used for the heat treatment production of 60 steel pipes. After ultrasonic testing and surface quality inspection, it was found that the quality of steel pipes was good, and the qualified rate was 100%. Fig. 3 is a picture of the microstructure and grain size of the steel pipe produced by the improved heat treatment process.

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Figure 3 microstructure and grain size (500) of 42CrMo after heat treatment process optimization ×)
The microstructure of the steel pipe produced by the improved heat treatment process is tempered sorbite with good hardenability, no strip or block ferrite, uniform distribution of carbides, compact structure and average grain size of 20 μ m. The average grain size is 8.0, which provides the basis for obtaining good mechanical properties. The test results of mechanical properties are shown in Table 1.

Table 1 performance results after optimization of heat treatment process

Process type Tensile strength / MPa Yield strength / MPa Elongation (%)  shock absorption energy / J
1-1 718 593 27 178
1-2 726 608 28 185
Technical agreement ≥655 552-655 ≥20 ≥48

It can be seen from table 1 that the strength and toughness are within the requirements of the technical agreement. The improved heat treatment process does not reduce the strength and toughness of the material. The use of residual temperature self tempering can effectively reduce the stress concentration in the quenching process and ensure the quality of the steel pipe.

Concluding remarks

Using water as quenching cooling medium, the production cost of heat treatment of 42CrMo steel pipe is reduced. The biggest risk of water quenching is quenching crack. Aiming at quenching crack, an improved quenching process is proposed, which provides guidance data for 42CrMo steel pipe heat treatment with water as quenching cooling medium.

Authors: Wang Jinyong, Qi Xilun, Fei Zhiwei, ye Bingyi

Source: China Seamless Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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