Heat treatment method for improving fracture toughness and hydrogen embrittlement resistance sensitivity of steel

Abstract

The invention discloses a heat treatment method for improving fracture toughness and hydrogen embrittlement resistance sensitivity of steel, which comprises the following steps: S1, carrying out preheat treatment on sample steel to obtain the sample steel subjected to preheat treatment; s2, austenitizing treatment is carried out; s3, quenching the sample steel subjected to austenitizing treatment in S2; s4, cooling the sample steel quenched in the step S3 in air; s5, aging treatment is conducted on the sample steel cooled in the step S4; the method is characterized by further comprising the following steps between S1 and S2: carrying out sub-temperature austenitizing treatment on the sample steel subjected to pre-heat treatment; and then oil medium quenching is carried out. After the A-100 ultrahigh-strength steel is subjected to heat treatment, on the premise that the tensile strength and the fracture toughness of the A-100 ultrahigh-strength steel meet design indexes, the hydrogen embrittlement sensitivity resistance of the A-100 ultrahigh-strength steel in a high-embrittlement environment can be greatly improved.

Description

Technical Field

[0001] The invention relates to the field of heat treatment of steel, in particular to a heat treatment method for improving the fracture toughness and hydrogen embrittlement resistance of steel. Background Art

[0002] As an ultra-high strength steel of 2000MPa, A-100 steel has been increasingly used by design departments as the preferred material for the main load-bearing structural parts of aircraft landing gears serving in marine climate conditions due to its excellent adaptability to marine climates. The design department stipulates that the mechanical properties of A-100 steel after final heat treatment are: Rm ≥ 1930MPa, KIC ≥ 110MPam 1 / 2 ;like Figure 1 As shown in the figure, the final heat treatment system specified in the current relevant heat treatment process standards is:

[0003] (Preliminary heat treatment state: normalizing + high temperature tempering) → 885℃ austenitizing (keeping warm for 60min + (0.5~1.0)min / mm condition thickness) → 20℃~60℃ oil cooling or cooling to below 65℃ within 1h~2h → -73℃ ice cooling (keeping warm for 60min + (1~2)min / mm condition thickness, returning to room temperature in air) → 482℃ aging (keeping warm for 5h~8h, cooling to room temperature in air).

[0004] However, due to the characteristics of A-100 steel materials and the many process factors related to the heat treatment quality of the product, after being treated with the current A-100 steel heat treatment technology, on the one hand, the tensile strength and fracture toughness of the material often cannot meet the design indicators at the same time, and it is necessary to take supplementary aging or repeat the heat treatment method as appropriate; but the heat treatment can only be repeated once, which brings certain challenges to on-site operations. On the other hand, since the tensile strength of the material after heat treatment reaches the 2000MPa level, its hydrogen embrittlement sensitivity in a highly brittle environment is also particularly prominent. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a heat treatment method for improving the fracture toughness and hydrogen embrittlement resistance of steel in view of the shortcomings of the prior art, thereby improving the fracture toughness and hydrogen embrittlement resistance of steel.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] A heat treatment method for improving the fracture toughness and hydrogen embrittlement sensitivity of steel, comprising S1, pre-heat treating a sample steel to obtain a pre-heat treated sample steel; S2, further austenitizing; S3, quenching the austenitized sample steel in S2; S4, cooling the quenched sample steel in air in S3; S5, aging the cooled sample steel in S4; characterized in that the following steps are also included between S1 and S2: sub-temperature austenitizing the pre-heat treated sample steel; and then quenching with oil medium.

[0008] The purpose of the present invention is to solve the technical problem that after the steel is treated by the current heat treatment technology, on the one hand, the tensile strength and fracture toughness of the material often cannot meet the design indicators at the same time, and it is necessary to take supplementary aging or repeat the heat treatment method according to the situation; but the number of repeated heat treatments is only allowed once. On the other hand, since the tensile strength of the material after heat treatment reaches the level of 2000MPa, its hydrogen embrittlement sensitivity in a highly embrittled environment is also particularly prominent.

[0009] The preliminary heat treatment, austenitizing treatment, quenching and aging treatment in the present invention can adopt existing processes, such as the austenitizing treatment and quenching process mentioned in the publication number CN114959197B.

[0010] In a preferred embodiment of the present invention, the temperature of the sub-temperature austenitizing treatment is 20°C to 70°C lower than the austenitizing temperature in S2, and preferably, the temperature of the sub-temperature austenitizing treatment is 30°C to 50°C lower than the austenitizing temperature in S2. Only by performing the sub-temperature austenitizing treatment in this temperature range can the austenite grains of the material be refined, and after keeping the temperature for a specified time, quenching with an oil medium can obtain high-density dislocation lath martensite + a small amount of twin martensite + a small amount of residual austenite.

[0011] In a preferred embodiment of the present invention, S5 includes a graded aging treatment at 490-510° C. for 10-30 min, followed by air cooling to room temperature; an aging treatment at 470-490° C. for 5 h-6 h, followed by air cooling to room temperature.

[0012] In a preferred embodiment of the present invention, the graded aging treatment temperature is 500-510°C.

[0013] In a preferred embodiment of the present invention, the preliminary heat treatment of S1 includes normalizing and high temperature tempering.

[0014] In a preferred embodiment of the present invention, the austenitizing temperature in S2 is 871-899° C., and the holding time is 10 to 60 minutes.

[0015] In a preferred embodiment of the present invention, the austenitizing temperature in S2 is 875-895° C., and the holding time is 40-60 min.

[0016] In a preferred embodiment of the present invention, the austenitizing temperature in S2 is 885°C.

[0017] In a preferred embodiment of the present invention, the quenching in S3 is oil medium quenching at 20°C to 60°C.

[0018] In a preferred embodiment of the present invention, the cooling treatment in S4 is a cooling treatment at a temperature of -80°C to -60°C.

[0019] In a preferred embodiment of the present invention, the sample steel in S1 is A-100 sample steel, and the temperature of the sub-temperature austenitizing treatment is 821-869°C.

[0020] The ultimate tensile strength and toughness of metal materials are a pair of "contradictory bodies", and the higher the strength, the higher the sensitivity to hydrogen embrittlement in a hydrogen embrittlement environment under stress! For ultra-high strength steels such as A-100 steel, due to its ultra-high ultimate tensile strength, the contradiction between its toughness and hydrogen embrittlement sensitivity is more prominent.

[0021] The specific technical solutions of the present invention are as follows:

[0022] 1. Based on the preliminary heat treatment state (i.e. normalizing + high temperature tempering state) of the material supply, a sub-temperature austenitization is first performed before the original conventional 885℃ austenitization during the final heat treatment, and then the oil medium quenching is used after the specified heat preservation time;

[0023] 2. According to the current heat treatment technical standards, complete the conventional 885℃ austenitization, oil medium quenching, -73℃ ice cooling treatment;

[0024] 3. Before conventional aging at 482°C, perform graded aging for several minutes, and then air cool to room temperature;

[0025] 4. Finally, according to the current heat treatment technical standards, complete aging at 482℃ for 5h to 6h, and then air cool to room temperature.

[0026] The mechanism of the present invention is as follows:

[0027] A sub-temperature austenitization is performed before the original conventional 885℃ austenitization, which can refine the austenite grains of the material. After keeping warm for a specified time, oil medium quenching is used to obtain high-density dislocation lath martensite + a small amount of twin martensite + a small amount of residual austenite; then the conventional 885℃ austenitization, oil medium quenching, and -73℃ ice cooling treatment are completed to obtain lath martensite with higher dislocation density + a small amount of residual austenite; then, graded aging treatment is performed at a temperature slightly higher than the conventional 482℃ aging temperature for a certain period of time and then air-cooled to room temperature to obtain tempered lath martensite + a small amount of dispersed carbides + an appropriate amount of reversed austenite + a small amount of residual austenite; finally, conventional 482℃ aging is completed for 5h to 6h and then air-cooled to room temperature to obtain tempered lath martensite + fine dispersed coherent carbides + a considerable amount of reversed austenite + a small amount of residual austenite.

[0028] Compared with the prior art, the beneficial effect of the present invention is that the present invention can achieve the premise of ensuring that the tensile strength and fracture toughness of A-100 steel meet the design indicators after the final heat treatment, while significantly improving its own ability to resist hydrogen embrittlement sensitivity in a highly brittle environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is the current standard roadmap for heat treatment technology of A-100 steel.

[0030] Figure 2 It is a standard roadmap of heat treatment technology in one embodiment of the present invention.

[0031] Figure 3 This is the metallographic structure diagram of the A-100 steel prepared in Example 2 of the present invention.

[0032] Figure 4 1 is the metallographic structure diagram of the A-100 steel prepared in Example 1 of the present invention.

[0033] Figure 5 This is a photograph of a sustained loading test (SLT) to test hydrogen embrittlement sensitivity in an embodiment of the present invention. DETAILED DESCRIPTION

[0034] The technical path adopted by the present invention is the current standard route for heat treatment of A-100 steel Figure 1 Based on the design of the invention, it is a process reengineering of the current A-100 steel heat treatment standard technology path. Figure 1 and Figure 2 It can be seen that the technical method adopted by the present invention is to austenitize at 885°C (see Figure 1 Before step 02), austenitization at a lower temperature is first performed and kept warm for a specified time (see Figure 2After step 2) in the process, quench with 20℃~60℃ oil medium (see Figure 2 Then complete the conventional 885℃ austenitization (see Figure 1 Step 02 and Figure 2 Step 4), 20℃~60℃ oil medium quenching (see Figure 1 Step 03 and Figure 2 Step 5), -73℃ ice-cold treatment (see Figure 1 Step 04 and Figure 2 Step 6) to obtain lath martensite with higher dislocation density + a small amount of twin martensite + a very small amount of retained austenite; then conventional aging at 482℃ for 5h to 8h (see Figure 1 Before step 05), first perform graded aging treatment at slightly above 482℃ for several minutes, then air cool to room temperature (see Figure 2 Step 7 in the process) to obtain tempered lath martensite + a small amount of dispersed carbides + an appropriate amount of reversed austenite + a small amount of retained austenite; finally, complete the conventional aging at 482℃ for 5h~6h, and then air cool to room temperature (see Figure 2 Step 8) in the process is used to obtain tempered lath martensite + fine dispersed coherent carbides + a considerable amount of reversed austenite + a small amount of retained austenite.

[0035] Embodiment 1:

[0036] 1) Compared with the current traditional heat treatment process, the process method of this embodiment is to first perform a sub-temperature austenitizing at a temperature of about 30°C to 50°C lower than the conventional austenitizing temperature of 885°C, and then perform oil medium quenching at 20°C to 60°C based on the material supply state (900-920°C normalizing + 680-720°C tempering);

[0037] 2) Then complete the conventional 885℃ austenitizing and 20℃~60℃ oil medium quenching;

[0038] 3) After being cooled at -73°C, the mixture was allowed to warm to room temperature in air;

[0039] 4) After graded aging treatment at 500°C for 20 minutes, air-cool to room temperature;

[0040] 5) After conventional aging treatment at 482°C for 5 to 6 hours, air cool to room temperature.

[0041] Embodiment 2:

[0042] 1) Compared with the current traditional heat treatment process, the new process is to first perform a sub-temperature austenitizing at a temperature of about 30℃ to 50℃ lower than the conventional austenitizing temperature of 885℃, and then perform oil medium quenching at 20℃ to 60℃ based on the material supply state (900-920℃ normalizing + 680-720℃ tempering);

[0043] 2) Then complete the conventional 885℃ austenitizing and 20℃~60℃ oil medium quenching;

[0044] 3) After being cooled at -73°C, the mixture was allowed to warm to room temperature in air;

[0045] 4) After graded aging treatment at 490°C for 40 min, air-cool to room temperature;

[0046] 5) After conventional aging treatment at 482°C for 5 to 6 hours, air cool to room temperature.

[0047] The metallographic structure of the steel after heat treatment in Example 1-2 is as follows: Figure 3-4 The tensile test and fracture test were carried out (the tensile test standard method is GB / T 228.1, and the fracture toughness test standard method is GB / T 4161), and the results are shown in Table 1.

[0048] Table 1 A-100(23Co 14 Ni 12 Cr3MoE) steel after heat treatment according to Example 1-2 Test results table

[0049]

[0050] Note: 1) Same furnace batch A-100 (23Co 14 Ni 12 After the Cr3MoE) steel material is treated by the traditional heat treatment process, its tensile strength is ob = 1933 ~ 1950MPa, and the fracture toughness KIC = 110 ~ 117MPam 1 / 2 .

[0051] 2) According to the traditional heat treatment process (see Figure 1 ) after treatment of A-100(23Co 14 Ni 12 Cr3MoE) steel, after hydrogen embrittlement sensitivity test (the standard method for hydrogen embrittlement sensitivity test is ASTM F519 or HB 5067.1), such as Figure 5 As shown in the figure, the A-100 (23Co 14 Ni 12 Cr3MoE) steel, according to the hydrogen embrittlement sensitivity test, can last at least tens of hours to 200 hours.

Claims

1. A heat treatment method for improving the fracture toughness and hydrogen embrittlement resistance of steel, comprising S1, pre-heat treating a sample steel to obtain a pre-heat treated sample steel; S2, performing austenitizing treatment; S3, quenching the austenitized sample steel in S2; S4, cooling the quenched sample steel in S3 in air; S5, performing aging treatment on the cooled sample steel in S4; characterized in that, The steps between S1 and S2 also include: performing sub-temperature austenitization treatment on the sample steel after the preliminary heat treatment; and then performing oil medium quenching.

2. The heat treatment method for improving the fracture toughness and hydrogen embrittlement resistance of steel according to claim 1, characterized in that: The temperature of the sub-temperature austenitizing treatment is 20°C to 70°C lower than the austenitizing temperature in S2. Preferably, the temperature of the sub-temperature austenitizing treatment is 30°C to 50°C lower than the austenitizing temperature in S2.

3. The heat treatment method for improving the fracture toughness and hydrogen embrittlement resistance of steel according to claim 1, characterized in that: S5 includes a graded aging treatment at 490-510°C for 10-30 minutes, followed by air cooling to room temperature; an aging treatment at 470-490°C for 5h-6h, and then air cooling to room temperature.

4. The heat treatment method for improving the fracture toughness and hydrogen embrittlement resistance of steel according to claim 3, characterized in that: The graded aging treatment temperature is 500-510°C.

5. The heat treatment method for improving the fracture toughness and hydrogen embrittlement resistance of steel according to any one of claims 1 to 4, characterized in that: The preliminary heat treatment of S1 includes normalizing and high temperature tempering.

6. The heat treatment method for improving the fracture toughness and hydrogen embrittlement resistance of steel according to any one of claims 1 to 4, characterized in that: The austenitizing temperature in S2 is 871-899°C, and the holding time is 10 to 60 minutes.

7. The heat treatment method for improving the fracture toughness and hydrogen embrittlement resistance of steel according to claim 6, characterized in that: The austenitizing temperature in S2 is 875-895°C, and the holding time is 40-60 min; preferably, the austenitizing temperature in S2 is 885°C.

8. The heat treatment method for improving the fracture toughness and hydrogen embrittlement resistance of steel according to any one of claims 1 to 3, characterized in that: The quenching in S3 is oil medium quenching at 20℃~60℃.

9. The heat treatment method for improving the fracture toughness and hydrogen embrittlement resistance of steel according to any one of claims 1 to 3, characterized in that: The cooling treatment in S4 is a cooling treatment at a temperature of -80°C to -60°C.

10. The heat treatment method for improving the fracture toughness and hydrogen embrittlement resistance of steel according to any one of claims 1 to 3, characterized in that: The sample steel described in S1 is A-100 sample steel, and the temperature of the sub-temperature austenitizing treatment of the A-100 sample steel is 821-869°C.

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