Process for differential time cooling of a heat-resistant steel tube

Abstract

This invention discloses a differential cooling process for heat-resistant steel pipes, comprising: quenching, heating the steel pipe to A... c3 Cool the furnace at +(120~190)℃, and control the cooling temperature to ≥A. C3 At +30℃, the steel pipe is filled with flowing water at a velocity ≥3m / s for cooling. 内 Cooling is paused when the temperature drops to ≤Ms-150℃, T 内 Return to A C1 -70℃≤T 内 ≤A C1 Refilling with water for cooling begins at -40℃; T 内 Stop water cooling when the temperature is ≤100℃; simultaneously, when the temperature of the outer wall of the steel pipe is T... 外 Down to A C3 -90℃≤T 外 ≤A C3 At -70℃, water mist is sprayed evenly along the circumference of the outer wall of the steel pipe for cooling. 外 Stop spraying water mist for cooling when the temperature is ≤100℃. By utilizing the time difference cooling between the inner and outer walls, the residual stress of the steel pipe can be reduced to ≤35MPa, thus improving the service life of the heat-resistant steel pipe.

Description

Technical Field

[0001] This invention belongs to the field of heat-resistant steel pipe technology, specifically relating to a differential cooling process for heat-resistant steel pipes. Background Technology

[0002] Heat-resistant steel pipes are widely used in key components of thermal power plants, nuclear power plants, and other similar facilities. Their service conditions are typically high temperature and high pressure, subjecting them to continuous temperature fluctuations. Unit start-up and shutdown, load fluctuations, and other factors cause repeated changes in the pipe's operating temperature. Residual stress, inevitably generated during hot or cold working processes, combines with the thermal and mechanical stresses under operating conditions, further exacerbating stress concentration and fatigue damage. This becomes a critical factor affecting the pipe's service safety and lifespan. Residual stress not only affects the dimensional accuracy of the heat-resistant steel pipe after processing but also significantly reduces its fatigue life, internal pressure resistance, and other key mechanical properties. Therefore, effectively reducing or even eliminating residual stress is of great significance for improving the service condition of heat-resistant steel pipes and extending their service life.

[0003] Currently, conventional methods for reducing or eliminating residual stress include high-temperature stress-relief annealing and mechanical and physical methods, but these processes are costly and increase the production process.

[0004] Chinese patent CN201810365440.5 discloses a method for eliminating residual stress in quenched and tempered seamless steel pipes and the bidirectional chain cooling bed used therein. By controlling the straightness of the steel pipe before quenching and tempering after rolling, and the bidirectional chain of the cooling bed after quenching and tempering, residual stress is eliminated, eliminating the need for the quenching and tempering stress-relief annealing process, thus achieving the goal of cost reduction.

[0005] Chinese patent CN201420805596.8 discloses an asymmetric steel pipe straightening roller, which designs a special straightening roller to eliminate residual stress and oxide scale in the steel pipe by controlling the force on the steel pipe during the straightening process.

[0006] Chinese patent CN200910210718.2 discloses a method for controlling the residual stress level of a conveying steel pipe. The patent derives a formula, and by measuring the elasticity of the steel pipe and comparing it with the formula, the residual stress level of the steel pipe can be obtained. This is a method for measuring and characterizing the residual stress level. Summary of the Invention

[0007] The purpose of this invention is to provide a differential cooling process for heat-resistant steel pipes, which solves the technical problems of high residual stress and poor uniformity of microstructure and properties in existing heat-resistant steel pipes. It reduces the residual stress of martensitic heat-resistant steel pipes with an alloy content of ≥10wt% to ≤35MPa and the hardness difference of the entire wall thickness section to ≤10 HBW, thereby achieving low residual stress control and uniform microstructure and properties control of heat-resistant steel pipes and improving their service performance.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows: A differential cooling process for heat-resistant steel pipes includes: Steel pipe quenching involves heating the steel pipe to temperature A. c3 Cool the furnace at +(120~190)℃, and control the cooling temperature to ≥A. C3 The steel pipe is cooled by filling it with flowing water at +30℃; the flow velocity of the flowing water in the steel pipe is ≥3m / s. When the inner wall temperature of the steel pipe is T 内 When the temperature drops to ≤Ms-150℃, water cooling should be suspended. When the inner wall temperature of the steel pipe is T 内 Return to A C1 -70℃≤T 内 ≤A C1 At -40℃, begin water cooling again; When the inner wall temperature of the steel pipe is T 内 Stop water cooling when the temperature is ≤100℃; At the same time, when the temperature of the outer wall of the steel pipe is T 外 Down to A C3 -90℃≤T 外 ≤A C3 At -70℃, water mist is sprayed evenly along the circumference of the outer wall of the steel pipe for cooling. When the outer wall temperature of the steel pipe is T 外 Stop spraying water mist for cooling when the temperature is ≤100℃.

[0009] Preferably, after the steel pipe is quenched, it is tempered after the temperature of the steel pipe drops to room temperature. The tempering holding temperature is 720~800℃ and the tempering holding time is 1.5~3.0h.

[0010] Preferably, the steel pipe is a martensitic heat-resistant steel pipe with an alloy content of ≥10wt% excluding Fe and unavoidable impurity elements.

[0011] Using the differential cooling process described in this invention, martensitic heat-resistant steel pipes with an alloy content ≥10wt% excluding Fe and unavoidable impurity elements can be obtained with residual stress ≤35MPa and hardness difference ≤10 HBW across the entire wall thickness.

[0012] In the method described in this invention: Heat-resistant pipe in temperature range A c3After solution treatment at +(120~190)℃, during the high and medium temperature stages of cooling, only thermal expansion and contraction occur without solid-state phase transformation. The resulting stress is mainly thermal stress caused by thermal expansion and contraction, with residual stress distributed as compressive stress on the cooled surface and tensile stress on the uncooled surface. When the cooling temperature enters the martensitic transformation range, the resulting stress is mainly phase transformation stress caused by martensitic phase transformation. The residual stress is distributed as tensile stress in the phase transformation region and compressive stress in the phase transformation region. The final residual stress distribution of the heat-resistant pipe is the superposition level of both thermal stress and phase transformation stress.

[0013] Steel pipe heated to A c3 At +(120~190)℃, through high-temperature solution treatment, carbon and alloying elements are fully dissolved, laying a good foundation for uniform microstructure in the subsequent martensitic phase transformation. The cooling temperature is controlled to be ≥A. C3 At +30℃, the steel pipe is filled with flowing water for cooling, and the flow rate of the flowing water is controlled to be ≥3m / s. The heat on the inner wall is quickly dissipated, forming a temperature gradient in the wall thickness direction and creating lateral compressive stress on the inner wall.

[0014] When the inner wall temperature of the steel pipe is T 内 When the temperature drops to ≤Ms, a martensitic transformation occurs. The martensitic transformation stress is tensile stress, which cancels out the compressive stress formed by the previous thermal shrinkage, thereby reducing the cracking of the inner wall and improving the residual force on the inner wall side.

[0015] This invention controls the inner wall temperature T of the steel pipe 内 Water cooling is suspended when the temperature is ≤Ms-150℃. At this point, the martensitic phase transformation has already occurred on the inner wall of the steel pipe. When the temperature of the inner wall of the steel pipe is T... 内 Return to A C1 -70℃≤T 内 ≤A C1 Refilling with water for cooling at -40℃ causes the surface temperature of the inner wall to rise, resulting in self-tempering of the martensite structure on the inner wall side. This decomposition of martensite and precipitation of carbides release lattice distortion energy, further reducing residual stress. Controlling the flow rate of the flowing water to ≥3m / s allows for stronger cooling, which further refines and homogenizes the martensitic phase transformation lath structure, improving the consistency of performance across the entire wall thickness.

[0016] At the same time, when the outer wall temperature T of the steel pipe drops to A C3 -90℃≤T_outer≤A C3 At -70℃, water mist is uniformly sprayed circumferentially onto the outer wall of the steel pipe for cooling. A cooling time difference of Δt exists between the inner and outer walls at the start of cooling. Utilizing the high hardenability of the heat-resistant steel, the difference in martensitic phase transformation time between the inner and outer walls is achieved. By controlling the superposition of martensitic phase transformation stress and thermal stress, low residual stress and low cracking risk are achieved. Simultaneously, the martensitic lath microstructure across the entire cross-section is refined and homogenized, improving performance consistency; the hardness difference across the entire wall thickness is ≤10 HBW.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: Martensitic heat-resistant steel pipes with an alloy content ≥10wt% (excluding Fe and unavoidable impurity elements) typically exhibit high hardenability and significant martensitic transformation stress. Current technologies employ simultaneous cooling of the inner and outer walls of the steel pipe, or fail to separately control the cooling intensity within the pipe. This results in high stress on the inner wall side after quenching, with residual stress often exceeding 150MPa. This leads to a high risk of quenching cracks and uneven martensitic lath microstructure.

[0018] This invention controls the cooling of the inner and outer walls of a steel pipe at different starting temperatures, creating a temperature gradient between them. By utilizing the high hardenability of heat-resistant steel, the timing difference in the martensitic transformation between the inner and outer walls is achieved, thus regulating the superposition of martensitic transformation stress and thermal stress and reducing residual stress in the steel pipe. Simultaneously, the flow rate of the water flowing through the inner wall is controlled to ≥3 m / s for forced cooling, maintaining the inner wall temperature T... 内 When the temperature is ≤Ms-150℃, water cooling is suspended. The tensile stress from the martensitic phase transformation counteracts the compressive stress formed by the earlier thermal shrinkage, reducing the residual stress on the inner wall of the steel pipe. This is achieved when the inner wall temperature T... 内 Return to A C1 -70℃≤T 内 ≤A C1 When the temperature reaches -40℃, water cooling is initiated again, and the martensitic structure undergoes self-tempering. The martensite decomposes, carbides precipitate, lattice distortion energy is released, and residual stress is further reduced.

[0019] Using the cooling process described in this invention, the residual stress of martensitic heat-resistant steel pipes with an alloy content of ≥10wt% can be reduced to ≤35MPa, the hardness difference of the entire wall thickness section can be ≤10 HBW, a uniform tempered martensitic structure can be obtained throughout the wall thickness, and the serviceability of the heat-resistant steel pipe can be improved. Detailed Implementation

[0020] The present invention will be further described below with reference to the embodiments.

[0021] The steel pipe used in the embodiments and comparative examples of this invention has a size of φ559mm×100mm (outer diameter×wall thickness). The specific process parameters of the embodiments and comparative examples of this invention are shown in Table 1.

[0022] The performance results of the embodiments and comparative examples of the present invention are shown in Table 2.

[0023] As can be seen from Tables 1 and 2, the residual stress of the heat-resistant tubes obtained in the embodiments of the present invention is ≤35MPa, which is significantly lower than the residual stress of the heat-resistant tubes prepared by the comparative process. The hardness difference of the entire wall thickness section of the heat-resistant tubes obtained by the method of the present invention is ≤10HBW, and the uniformity distribution is better than that of the steel tubes prepared by the comparative process.

[0024] In Comparative Example 1, the steel pipe was cooled by air cooling throughout, and the residual stress of the steel pipe reached 205 MPa, which is significantly higher than that in the embodiment of the present invention.

[0025] In Comparative Example 2, water mist was sprayed only on the outer wall of the steel pipe for cooling, and the residual stress of the steel pipe reached 292 MPa, which is significantly higher than that in the embodiment of the present invention.

[0026] In Comparative Example 3, only the inner wall of the steel pipe was cooled with water, and the temperature reversal of the inner wall was controlled, while the outer wall was air-cooled. The residual stress of the steel pipe reached 151 MPa, which is significantly higher than that in the embodiment of the present invention. This proves that simple internal cooling control is insufficient to significantly reduce stress; it must be combined with external cooling to produce a synergistic effect.

[0027]

[0028]

Claims

1. A differential cooling process for heat-resistant steel pipes, characterized in that, include: Steel pipe quenching involves heating the steel pipe to temperature A. c3 Cool the furnace at +(120~190)℃, and control the cooling temperature to ≥A. C3 The steel pipe is cooled by filling it with flowing water at +30℃; the flow velocity of the flowing water in the steel pipe is ≥3m / s. When the inner wall temperature of the steel pipe is T 内 When the temperature drops to ≤Ms-150℃, water cooling should be suspended. When the inner wall temperature of the steel pipe is T 内 Return to A C1 -70℃≤T 内 ≤A C1 At -40℃, begin water cooling again; When the inner wall temperature of the steel pipe is T 内 Stop water cooling when the temperature is ≤100℃; At the same time, when the temperature of the outer wall of the steel pipe is T 外 Down to A C3 -90℃≤T 外 ≤A C3 At -70℃, water mist is sprayed evenly along the circumference of the outer wall of the steel pipe for cooling. When the outer wall temperature of the steel pipe is T 外 Stop spraying water mist for cooling when the temperature is ≤100℃.

2. The differential cooling process for heat-resistant steel pipes as described in claim 1, characterized in that, After the steel pipe is quenched, it is tempered after the temperature drops to room temperature. The tempering holding temperature is 720~800℃ and the tempering holding time is 1.5~3.0h.

3. The differential cooling process for heat-resistant steel pipes as described in claim 1, characterized in that, The steel pipe is a martensitic heat-resistant steel pipe with an alloy content of ≥10wt% excluding Fe and unavoidable impurity elements.

4. A martensitic heat-resistant steel pipe with an alloy content ≥10wt% processed using the differential cooling process described in any one of claims 1 to 3, characterized in that, The residual stress of the martensitic heat-resistant steel pipe with an alloy content of ≥10wt% is ≤35MPa, and the hardness difference of the entire wall thickness section is ≤10 HBW.

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