Heat treatment method of 34CrMo4 steel for high-pressure gas cylinder

Abstract

The invention discloses a heat treatment method of 34CrMo4 steel for a high-pressure gas cylinder, which comprises the following steps: putting 34CrMo4 steel in a hot rolling state into a quenching furnace, setting the temperature of the quenching furnace at 940-1020 DEG C, preserving heat for 100-130 minutes after the 34CrMo4 steel reaches the set temperature, taking out the 34CrMo4 steel after heat preservation, quenching the 34CrMo4 steel in medium oil, and cooling the 34CrMo4 steel to normal temperature for later use after quenching; and the tempering temperature of the tempering furnace is set to be 600-620 DEG C, after the temperature of the tempering furnace reaches the set temperature, the quenched 34CrMo4 steel is put into the tempering furnace to be tempered for 180 min, the quenched 34CrMo4 steel is taken out and naturally cooled to the normal temperature after tempering, and heat treatment of the 34CrMo4 steel for the high-pressure gas cylinder is completed. The 34CrMo4 steel is subjected to heat treatment through ultrahigh-temperature quenching and tempering, the yield ratio of the high-pressure gas cylinder prepared after treatment is reduced on the premise that it is guaranteed that the mechanical property meets the requirement, the safety of the high-pressure gas cylinder can be effectively improved, and the service life of the high-pressure gas cylinder can be effectively prolonged.

Description

Technical Field

[0001] The invention belongs to the technical field of steel material processing, and in particular relates to a heat treatment method for 34CrMo4 steel for high-pressure gas cylinders. Background Art

[0002] High-pressure gas cylinders are pressure vessels that hold gas. Some medium-carbon steel cylinders typically undergo quenching and tempering before being put into production. This treatment typically results in higher yield strength and tensile strength, along with a higher yield-to-strength ratio. Currently, high-pressure gas cylinders primarily use high-strength steels such as 34CrMo4. These steels have higher yield strength and tensile strength, requiring less quenching and tempering than medium-carbon steel, but they also suffer from a higher yield-to-strength ratio.

[0003] To improve the safety and extend the service life of high-pressure gas cylinders, it is necessary to reduce the yield strength ratio while ensuring that the mechanical properties meet the requirements. The current treatment process for 34CrMo4 steel involves a sub-tempering at 780°C in dielectric oil, followed by a tempering treatment at 550°C, followed by natural cooling. This treatment also meets the requirements of TSG21-2016, "Regulations on Safety Technical Supervision of Stationary Pressure Vessels," for Rm ≤ 1060 MPa, a yield strength ratio ReL / Rm ≤ 0.90, and A ≥ 16%. However, due to compositional variations among different batches of 34CrMo4 steel, their AC-3 temperatures vary. AC-3 is the critical temperature for austenitization of hypoeutectoid steel, and only above this temperature can austenite fully form. This makes selecting the optimal sub-tempering temperature difficult. If the optimal temperature is not chosen correctly, the resulting 34CrMo4 steel will contain excessive undissolved ferrite, which will affect the yield strength and yield ratio, compromising the quality of the high-pressure gas cylinders. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to address the deficiencies of the above-mentioned existing technologies and provide a heat treatment method for 34CrMo4 steel for high-pressure gas cylinders. The method adopts an ultra-high temperature quenching and tempering heat treatment process to reduce the yield strength ratio while ensuring that the mechanical properties of the high-pressure gas cylinders meet the requirements, thereby improving the safety of high-pressure gas cylinders and avoiding the uncertainty of sub-temperature quenching.

[0005] The technical solution adopted by the present invention is: a heat treatment method for 34CrMo4 steel for high-pressure gas cylinders, the heat treatment method comprising the following steps:

[0006] Step 1: Place the hot-rolled 34CrMo4 steel into a quenching furnace. Set the quenching furnace temperature to 940°C-1020°C. When the quenching furnace temperature reaches the set temperature, keep it warm for 100-130 minutes. After keeping warm, take it out and quench it in medium oil. After quenching, cool it to room temperature in the medium oil and take it out for use.

[0007] Step 2: Set the tempering temperature of the tempering furnace to 600℃-620℃. After the tempering furnace temperature reaches the set temperature, place the quenched 34CrMo4 steel into the tempering furnace and temper it for 180 minutes. After tempering, take it out and naturally cool it to room temperature. The heat treatment of 34CrMo4 steel for high-pressure gas cylinders is completed.

[0008] Preferably, the quenching furnace temperature is 940° C., the holding time is 120 min, the tempering temperature is 600° C., and the tempering time is 180 min.

[0009] Preferably, the quenching furnace temperature is 980° C., the holding time is 120 min, the tempering temperature is 600° C., and the tempering time is 180 min.

[0010] Preferably, the quenching furnace temperature is 1020° C., the holding time is 120 min, the tempering temperature is 600° C., and the tempering time is 180 min.

[0011] Preferably, the quenching furnace temperature is 980° C., the holding time is 120 min, the tempering temperature is 620° C., and the tempering time is 180 min.

[0012] Preferably, the quenching furnace temperature is 1020° C., the holding time is 120 min, the tempering temperature is 620° C., and the tempering time is 180 min.

[0013] The beneficial effects of the present invention are as follows: by heat treating 34CrMo4 steel through ultra-high temperature quenching and tempering, the high-pressure gas cylinder prepared after the treatment can meet the requirements of Rm≤1060MPa, yield strength ratio ReL / Rm≤0.90, and A≥16% stipulated in TSG21-2016 "Regulations on Safety Technical Supervision of Stationary Pressure Vessels". At the same time, the reduction of the yield strength ratio can effectively improve the safety and service life of the high-pressure gas cylinder. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a metallographic diagram of Example 1 of the present invention;

[0015] Figure 2 This is a metallographic diagram of Example 2 of the present invention;

[0016] Figure 3 This is a metallographic diagram of Example 3 of the present invention;

[0017] Figure 4 This is a metallographic diagram of Example 4 of the present invention;

[0018] Figure 5 This is a metallographic diagram of Example 5 of the present invention;

[0019] Figure 6 This is the metallographic diagram of comparative example 1 of the present invention. DETAILED DESCRIPTION

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] Example 1

[0022] The heat treatment method for 34CrMo4 steel for high-pressure gas cylinders provided in this embodiment includes the following steps:

[0023] Step 1: Place the hot-rolled 34CrMo4 steel into a quenching furnace. Set the quenching furnace temperature to 940°C. When the quenching furnace temperature reaches 940°C, keep it warm for 120 minutes and then take it out. Then place it in medium oil for quenching. After quenching, cool it to room temperature in the medium oil and take it out for use.

[0024] Step 2: Set the tempering temperature of the tempering furnace to 600°C. After the tempering furnace temperature reaches 600°C, place the quenched 34CrMo4 steel into the tempering furnace and temper it for 180 minutes. After tempering, take it out and cool it naturally to room temperature, that is, use air cooling to room temperature. After cooling, the heat treatment of 34CrMo4 steel for high-pressure gas cylinders is completed.

[0025] Example 2

[0026] This embodiment is basically the same as embodiment 1, except that the temperature of the quenching furnace is changed to 980°C and the tempering temperature is 600°C.

[0027] Example 3

[0028] This embodiment is basically the same as embodiment 1, except that the temperature of the quenching furnace is changed to 1020°C and the tempering temperature is 600°C.

[0029] Example 4

[0030] This embodiment is basically the same as embodiment 2, except that the tempering temperature is changed. The quenching furnace temperature is 980°C and the tempering temperature is 620°C.

[0031] Example 5

[0032] This embodiment is basically the same as embodiment 4, except that the temperature of the quenching furnace is changed. The quenching furnace temperature is 1020°C and the tempering temperature is 620°C.

[0033] Examples 1 to 5 were processed into standard specimens with a diameter of 8 mm and subjected to tensile tests according to GB / T228.1-2021 "Tensile Tests of Metallic Materials Part 1: Room Temperature Test Methods". The test results are shown in Table 1.

[0034] Table 1:

[0035]

[0036] The ends of the tensile specimens of Examples 1 to 5 were sampled, polished, and etched, and metallographic analysis and observation were performed using a DMM-480C inverted optical microscope. Figure 1-Figure 5 , the analysis results are shown in Table 2,

[0037] Table 2

[0038]

[0039]

[0040] From the tensile tests and metallographic analyses of Examples 1 to 5, it was concluded that the mechanical test results of Examples 1 to 5 all met the requirements of TSG21-2016, and that the microstructures of Examples 1 to 5 after the heat treatment process were all tempered bainite; the elongation and reduction in area of the Example 5 process were the highest, and the elongation and reduction in area of the Example 1 process were relatively the lowest; the strength of the Example 2 process was the highest, and the strength of the Example 3 process was the lowest; the yield strength ratio of the Example 2 process was the highest, and the yield strength ratio of the Example 3 process was the lowest; the grains of the Example 1 process were the finest, the grains of the Example 3 process were the coarsest, and the mixed crystals existed in the Example 2 process.

[0041] In the heat treatment process of ultra-high temperature quenching and tempering, when the tempering temperature is the same, as the quenching temperature increases, the grains become coarser and the yield strength ratio decreases. It can be seen that appropriate grain coarsening is beneficial to reducing the yield strength ratio.

[0042] In summary, Example 1 is the optimal example. The mechanical test results after treatment in Example 1 have the best comprehensive performance, and the performance meets the requirements of TSG21-2016. The structure is tempered troostite, and the grain size is 9.0. Combined with the material's yield strength, tensile strength, yield strength ratio, elongation after fracture, and cross-sectional shrinkage, it is considered that this process has the best comprehensive physical and chemical indicators and is relatively safer to use.

[0043] Comparative Example 1

[0044] This embodiment is basically the same as embodiment 1, except that the quenching temperature and the tempering temperature are changed, that is, a sub-temperature quenching heat treatment process is adopted, the quenching temperature is 790°C, and the tempering temperature is 580°C.

[0045] Comparative Example 1 was processed into a standard specimen of φ8 mm and subjected to a tensile test in accordance with GB / T228.1-2021 "Tensile Tests of Metallic Materials Part 1: Room Temperature Test Methods". The results are shown in Table 3.

[0046] Table 3

[0047]

[0048] Then, the ends of the tensile specimens of Comparative Example 1 were sampled, polished, and etched, and then metallographically analyzed and observed using a DMM-480C inverted optical microscope. Figure 6 The analysis results are shown in Table 4.

[0049] Table 4

[0050]

[0051] From the tensile test, the mechanical test results after the sub-temperature heat treatment process meet the requirements of TSG21-2016; however, the microstructure after the sub-temperature heat treatment process is tempered bainite and a small amount of undissolved ferrite. The presence of undissolved ferrite will affect the yield strength and yield strength ratio of the heat-treated material. If the sub-temperature quenching process is used, it is necessary to test the composition of the hot-rolled steel during quenching, so as to select a sub-temperature quenching temperature exceeding AC-3 to avoid the production of excessive undissolved ferrite, thereby ensuring that the structure of the treated steel is tempered bainite. Therefore, the ultra-high temperature quenching process of the present invention can completely avoid the uncertainty of using sub-temperature quenching, thereby reducing process steps, improving process efficiency, and ensuring product quality.

[0052] The above description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any modification and replacement based on the technical solution and inventive concept provided by the present invention should be covered by the protection scope of the present invention.

Claims

1. A heat treatment method for 34CrMo4 steel for high-pressure gas cylinders, characterized by: The heat treatment method comprises the following steps: Step 1: Place the hot-rolled 34CrMo4 steel into a quenching furnace. Set the quenching furnace temperature to 940°C-1020°C. When the quenching furnace temperature reaches the set temperature, keep it warm for 100-130 minutes. After keeping warm, take it out and quench it in medium oil. After quenching, cool it to room temperature in the medium oil and take it out for use. Step 2: Set the tempering temperature of the tempering furnace to 600℃-620℃. After the tempering furnace temperature reaches the set temperature, place the quenched 34CrMo4 steel into the tempering furnace and temper it for 180 minutes. After tempering, take it out and naturally cool it to room temperature. The heat treatment of 34CrMo4 steel for high-pressure gas cylinders is completed.

2. The heat treatment method of 34CrMo4 steel for high-pressure gas cylinders according to claim 1, characterized in that: The quenching furnace temperature is 940° C., the holding time is 120 min, the tempering temperature is 600° C., and the tempering time is 180 min.

3. The heat treatment method of 34CrMo4 steel for high-pressure gas cylinders according to claim 1, characterized in that: The quenching furnace temperature is 980° C., the holding time is 120 min, the tempering temperature is 600° C., and the tempering time is 180 min.

4. The heat treatment method of 34CrMo4 steel for high-pressure gas cylinders according to claim 1, characterized in that: The quenching furnace temperature is 1020° C., the holding time is 120 min, the tempering temperature is 600° C., and the tempering time is 180 min.

5. The heat treatment method of 34CrMo4 steel for high-pressure gas cylinders according to claim 1, characterized in that: The quenching furnace temperature is 980° C., the holding time is 120 min, the tempering temperature is 620° C., and the tempering time is 180 min.

6. The heat treatment method of 34CrMo4 steel for high-pressure gas cylinders according to claim 1, characterized in that: The quenching furnace temperature is 1020° C., the holding time is 120 min, the tempering temperature is 620° C., and the tempering time is 180 min.

Images