A post-weld heat treatment process for improving the impact toughness of weld metal from Q690C high-strength steel used in tunnel embedded parts.

Abstract

This invention discloses a post-weld heat treatment process for improving the impact toughness of Q690C high-strength steel weld metal used in tunnel embedded parts, belonging to the field of steel structure welding and heat treatment technology. The process employs a two-stage heat treatment method: first, the weld is heated to 680–700 °C and held for 1.5–2.0 h to decompose residual austenite and coarse tempered martensite; then, it is slowly cooled to 600–620 °C and held for 2.0–2.5 h to promote the precipitation of dispersed carbides and refine the microstructure; finally, it is cooled in the furnace to 300 °C and then air-cooled to avoid secondary embrittlement. After this process, the weld metal microstructure consists of uniform and fine tempered sorbite + dispersed carbides, and the residual stress is significantly reduced. Its mechanical properties are: yield strength ≥680 MPa, tensile strength ≥750 MPa, and impact energy at -40 °C increased to 80–95 J, which is more than 40% higher than that of conventional tempering or normalizing processes.

Description

Technical Field

[0001] This invention belongs to the field of high-strength steel welding and heat treatment technology, and particularly relates to a post-weld heat treatment process for improving the impact toughness of Q690C high-strength steel weld metal used in tunnel embedded parts. Background Technology

[0002] Q690C high-strength steel is widely used in tunnel embedded parts and other high-stress structures due to its high yield strength and good load-bearing capacity. However, its welded joints often suffer from insufficient impact toughness and decreased low-temperature performance, especially in the weld metal zone where coarse martensite and temper embrittlement are prone to form. Existing post-weld heat treatment (PWHT) processes mostly employ high-temperature tempering or conventional normalizing, which can improve the microstructure to some extent, but often lead to significant strength loss or limited toughness improvement, failing to meet stringent service conditions. Therefore, an optimized PWHT process is urgently needed to achieve a coordinated improvement in weld metal strength and toughness.

[0003] Application number CN202411564881.X discloses a heat treatment method for Q690C round steel used in tunnel embedded parts, comprising: 1) placing the hot-rolled round steel in an annealing furnace heated to 500℃ for furnace cooling, wherein the temperature of the hot-rolled round steel when placed in the annealing furnace is ≥450℃; 2) heating the round steel in a quenching furnace to 930-940℃, holding for 50-60 minutes for complete austenitization, and then quenching; and 3) placing the round steel in a tempering furnace heated to 650-660℃, holding for 100-120 minutes, and then air-cooling or water-cooling. The mechanical properties after heat treatment meet the following requirements: yield strength ≥850MPa, tensile strength ≥900MPa, elongation after fracture ≥14%, and KV at 0℃. Summary of the Invention

[0004] The purpose of this invention is to provide a post-weld heat treatment process for improving the impact toughness of weld metal from Q690C high-strength steel used in tunnel embedded parts. Through a two-stage heat treatment plus slow cooling control, the microstructure and properties of the weld metal are significantly improved.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] This invention discloses a post-weld heat treatment process for improving the impact toughness of weld metal from Q690C high-strength steel used in tunnel embedded parts, comprising:

[0007] Q690C high-strength steel is welded using submerged arc welding or gas shielded welding, with the welding heat input controlled at 18-22 kJ / cm.

[0008] Immediately after welding, post-weld heat treatment is performed. The heat treatment adopts a two-stage tempering process. In the first stage, the welded specimen is heated to 680-700 ℃ and held for 1.5-2.0 h. Then, in the second stage, the temperature is lowered to 600-620 ℃ and held for 2.0-2.5 h.

[0009] After heat treatment, the weld metal is cooled in the furnace to below 300 ℃, and then air-cooled to room temperature; thus, the weld metal structure is transformed into fine and uniform tempered sorbite + dispersed carbides, which significantly improves the impact toughness.

[0010] The chemical composition of the Q690C high-strength steel by mass fraction is: C 0.12~0.16, Si 0.20~0.40, Mn 1.20~1.60, Nb 0.02~0.05, V 0.03~0.08, Ti ≤0.025, with the balance being Fe and unavoidable impurities.

[0011] Furthermore, the heating rate in the first stage is controlled at 8–12 °C / min to prevent coarsening of the weld microstructure caused by rapid heating.

[0012] Furthermore, during the second stage of heat preservation, the furnace temperature fluctuation shall not exceed ±5℃ to ensure the uniformity of carbide precipitation.

[0013] Furthermore, the furnace is cooled to 300 ℃ and then air-cooled to effectively prevent secondary tempering and embrittlement.

[0014] Furthermore, the welding wire used is a low-hydrogen type, model AWS A5.28 ER110S-G, to ensure the low-hydrogen performance of the weld metal.

[0015] Furthermore, when using submerged arc welding, the welding current is controlled at 500–600 A, the arc voltage at 28–32 V, and the welding speed at 30–40 cm / min.

[0016] Furthermore, when the welding method is gas shielded welding, the shielding gas is a mixture of 80% Ar + 20% CO2, and the flow rate is controlled at 18-22 L / min.

[0017] Furthermore, the weld metal microstructure after this process is fine and uniform tempered sorbite with dispersed carbides, and the grain size reaches level 8 to 9.

[0018] Furthermore, the mechanical properties after this process meet the following requirements: yield strength ≥ 680 MPa, tensile strength ≥ 750 MPa, elongation ≥ 16%, and impact energy at -40℃ ≥ 80 J.

[0019] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0020] (1) At -40℃, the impact energy of the weld is increased from 40-55J in conventional process to 80-95J, an increase of more than 40%; (2) The yield strength of the weld metal is stable at ≥680 MPa, the tensile strength is ≥750 MPa, the strength and toughness are well matched, and there is no significant decrease in strength; (3) The process is simple, no complicated equipment is required, and it can be implemented with only conventional heat treatment furnace, which has excellent economy and operability; (4) This invention is especially suitable for tunnel embedded parts, bridge high-strength connectors and wind power high-strength structural parts, and has significant application and promotion value.

[0021] This invention optimizes the post-weld heat treatment process to achieve homogenization and refinement of the weld metal microstructure (by controlling the microstructure, the weld metal obtains a fine and uniform tempered sorbite + dispersed carbide microstructure, which significantly improves low-temperature toughness), effectively solving the technical problem of "high strength but low toughness" in existing Q690C high-strength steel weld joints, and has important engineering significance and industrialization prospects. Detailed Implementation

[0022] The following is a detailed description of a post-weld heat treatment process for improving the impact toughness of weld metal made of Q690C high-strength steel for tunnel embedded parts according to the present invention.

[0023] A post-weld heat treatment process for improving the impact toughness of weld metal from Q690C high-strength steel used in tunnel embedded parts is disclosed. This process is applicable to improving the performance of welded joints of Q690C and similar high-strength steels in structural components such as tunnel embedded parts. The core of this method is a two-stage tempering process combined with optimized cooling, which can significantly improve low-temperature impact toughness while ensuring weld strength. The implementation method is as follows:

[0024] Material preparation: The base material is Q690C high-strength steel, with a typical chemical composition range (mass fraction, %) as follows: C 0.12~0.16, Si 0.20~0.40, Mn 1.20~1.60, Nb 0.02~0.05, V 0.03~0.08, Ti ≤0.025, with the balance being Fe and unavoidable impurities. Low-hydrogen welding wire (such as AWS A5.28 ER110S-G) is selected as the welding material to ensure the low-hydrogen performance and crack resistance of the joint.

[0025] Welding process: Submerged arc welding or gas shielded welding is adopted, and the welding heat input is controlled at 18-22 kJ / cm to reduce the overheating zone and coarse grain formation of the weld joint, and ensure that the initial microstructure of the weld metal is in an adjustable state.

[0026] Post-weld heat treatment (PWHT) regime: Immediately after welding, the specimen is placed in a heat treatment furnace and subjected to a two-stage tempering process: First stage: rapid heating to 680–700 ℃ and holding for 1.5–2.0 h to decompose coarse martensite and transform residual austenite into fine structure, initially releasing welding residual stress; Second stage: cooling to 600–620 ℃ and holding for 2.0–2.5 h to promote the precipitation and stable distribution of dispersed carbides, refine grains, and improve toughness; Cooling regime: after heat treatment, the specimen is slowly cooled in the furnace to 300 ℃, and then air-cooled to room temperature to effectively avoid secondary tempering embrittlement and eliminate most of the residual stress.

[0027] Microstructure and Performance Results: After processing using the process of this invention, the weld metal microstructure consists of uniform and fine tempered sorbite with dispersed carbides, achieving a grain size of 8-9. The microstructure is homogeneous, and residual stress is significantly reduced. Its mechanical properties are as follows: yield strength ≥ 680 MPa, tensile strength ≥ 750 MPa, elongation ≥ 16%, and impact energy at -40 ℃ stable at 80-95 J, which is more than 40% higher than that of traditional tempering or normalizing processes.

[0028] Process applicability: The post-weld heat treatment process of the present invention does not rely on complex equipment and can be implemented with only conventional resistance furnaces or box furnaces. It is easy to operate and the cost increases by less than 5%, making it suitable for large-scale industrial application.

[0029] Example 1:

[0030] The base metal was Q690C steel with a chemical composition of C 0.13%, Si 0.32%, Mn 1.42%, Nb 0.035%, V 0.045%, and Ti 0.018%. AWS A5.28 ER110S-G welding wire was used. Submerged arc welding was employed, with a welding heat input of approximately 20 kJ / cm. Immediately after welding, the weld was placed in a resistance furnace for heat treatment. The heat treatment regime was as follows: heating to 690℃ at a rate of 8℃ / min and holding for 1.5 h; then cooling to 610℃ and holding for 2.0 h; finally, furnace cooling to 300℃ and air cooling to room temperature. The results showed that the weld metal microstructure transformed into uniform and fine tempered sorbite, accompanied by dispersed carbide precipitation, and residual stress was significantly released. Performance test results: yield strength 682 MPa, tensile strength 756 MPa, elongation 16.5%, impact energy at -40℃ 85J, which is about 55% higher than the untreated state.

[0031] Example 2:

[0032] The base material was Q690C steel with the following composition: C 0.15%, Si 0.28%, Mn 1.50%, Nb 0.040%, V 0.050%, and Ti 0.020%. ER110S-G welding wire was also used. The welding method was gas-shielded welding, with heat input controlled at 19 kJ / cm. Post-weld heat treatment was performed immediately. The heat treatment regime was: heating to 700℃ and holding for 2.0 h; slowly decreasing to 600℃ and holding for 2.5 h; furnace cooling to 320℃ followed by air cooling. Results showed that the weld zone microstructure consisted of fine tempered sorbite with a small amount of bainite, and carbides were dispersed, resulting in significantly improved toughness. Performance test results showed a yield strength of 690 MPa, tensile strength of 761 MPa, elongation of 17%, and impact energy at -40℃ of 93 J, representing an improvement of approximately 70% compared to the control group, with optimal strength-toughness matching.

[0033] Comparative Example 1 (Single Tempering):

[0034] The welding materials and welding process were the same as in Example 1. Only one tempering was performed after welding: 650 ℃ × 2 h, followed by furnace cooling to room temperature. Results showed that the weld microstructure still contained a significant amount of tempered martensite, with uneven precipitation of some coarse carbides and insufficient residual stress release. Performance test results: yield strength 695 MPa, tensile strength 765 MPa, but impact energy at -40 ℃ was only 52 J, indicating severely insufficient toughness.

[0035] Comparative Example 2 (normalizing treatment):

[0036] The base material was the same as in Example 2. Post-weld normalizing was performed: heating to 850°C, holding for 1.5 h, and air cooling to room temperature. Microstructural observation showed that the weld metal microstructure consisted of coarse ferrite and pearlite, with significantly enlarged grains. Performance results: yield strength 662 MPa, tensile strength 738 MPa, elongation 15%, impact energy at -40°C 60 J. Although the toughness was improved compared to Comparative Example 1, the strength decreased significantly, failing to meet the design requirements of Q690C.

[0037] Comparative Example 3 (single low-temperature tempering):

[0038] The base material was the same as in Example 1. A single low-temperature tempering was performed after welding: heating to 600°C, holding for 2.0 h, and furnace cooling. The microstructure was coarse tempered sorbite with insufficient carbide precipitation and localized embrittlement. Performance test results: yield strength 688 MPa, tensile strength 750 MPa, impact energy at -40°C 55 J. Although the strength remained high, the improvement in toughness was limited.

[0039] Table 1. Process Comparison between Examples and Comparative Examples

[0040] serial number parent material Welding method Heat input (kJ / cm) PWHT process regime Cooling method Remark Example 1 Q690C Submerged arc welding 20 690 ℃×1.5 h → 610 ℃×2.0 h The furnace was cooled to 300°C and then air-cooled. Two-stage tempering Example 2 Q690C Gas shielded welding 19 700 ℃×2.0 h → 600 ℃×2.5 h The furnace was cooled to 320°C and then air-cooled. Two-stage tempering Comparative Example 1 Same as Example 1 Submerged arc welding 20 650 ℃×2.0 h The furnace cooled to room temperature Single tempering Comparative Example 2 Same as Example 2 Gas shielded welding 19 850 ℃×1.5 h air cooling Zheng Huo Comparative Example 3 Same as Example 1 Submerged arc welding 20 600 ℃×2.0 h Furnace cooling Single-stage low-temperature tempering

[0041] Table 2 Performance Comparison Table

[0042] serial number Weld microstructure characteristics Yield strength (MPa) Tensile strength (MPa) Elongation (%) Impact energy at -40℃ (J) in conclusion Example 1 Fine tempered sorbite + dispersed carbides 682 756 16.5 85 Good strength and toughness matching Example 2 Tempered sorbite with a small amount of bainite 690 761 17.0 93 Optimal improvement in resilience Comparative Example 1 Coarse tempered martensite + inhomogeneous carbides 695 765 15.5 52 High strength, but insufficient toughness Comparative Example 2 Coarse ferrite + pearlite 662 738 15.0 60 Improved toughness, but insufficient strength Comparative Example 3 Coarse tempered sorbite with insufficient carbide precipitation 688 750 15.8 55 Strength remains the same, but toughness improvement is limited.

[0043] As can be seen from the embodiments and comparative examples, the present invention has the following advantages:

[0044] (1) Significantly improved low-temperature toughness: The impact energy of the example at -40℃ reached 85-93J, which is 40%-70% higher than that of the comparative example of 52-60J, effectively solving the problem of brittleness of Q690C high-strength steel weld metal in low-temperature environment.

[0045] (2) The embodiment maintains a yield strength ≥680 MPa and a tensile strength ≥750 MPa while significantly improving the impact toughness, while the comparative embodiment either has insufficient toughness (such as single tempering) or a significant decrease in strength (such as normalizing).

[0046] (3) Refinement and homogenization of the microstructure: The two-stage tempering process enables the weld metal to obtain a fine and uniform tempered sorbite + dispersed carbide microstructure, which is superior to the coarse tempered martensite or ferrite + pearlite microstructure in the comparative example.

[0047] (4) Effective release of residual stress: The combination process of "high temperature tempering + secondary stable tempering + slow cooling" adopted in the example can eliminate welding residual stress to the maximum extent and avoid crack initiation. In contrast, the residual stress release is insufficient.

[0048] (5) Simple process and outstanding economy: This invention can be completed using a conventional resistance furnace or box furnace. The process is easy to control and the cost increases by less than 5%. Compared with high temperature normalizing or tempering process, it is more suitable for large-scale industrial promotion.

[0049] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A post-weld heat treatment process for improving the impact toughness of weld metal from Q690C high-strength steel used in tunnel embedded parts, characterized in that, include: Q690C high-strength steel is welded using submerged arc welding or gas shielded welding, with the welding heat input controlled at 18–22 kJ / cm. Immediately after welding, post-weld heat treatment is performed. The heat treatment adopts a two-stage tempering process. In the first stage, the welded specimen is heated to 680-700 ℃ and held for 1.5-2.0 h. Then, in the second stage, the temperature is lowered to 600-620 ℃ and held for 2.0-2.5 h. After heat treatment, the weld metal is cooled in the furnace to below 300 ℃, and then air-cooled to room temperature; thus, the weld metal structure is transformed into fine and uniform tempered sorbite + dispersed carbides, which significantly improves the impact toughness. The chemical composition of the Q690C high-strength steel by mass fraction is: C 0.12~0.16, Si 0.20~0.40, Mn 1.20~1.60, Nb 0.02~0.05, V 0.03~0.08, Ti ≤0.025, with the balance being Fe and unavoidable impurities.

2. The post-weld heat treatment process for improving the impact toughness of Q690C high-strength steel weld metal used in tunnel embedded parts according to claim 1, characterized in that, The heating rate in the first stage is controlled at 8–12 °C / min to prevent coarsening of the weld microstructure caused by rapid heating.

3. The post-weld heat treatment process for improving the impact toughness of Q690C high-strength steel weld metal used in tunnel embedded parts according to claim 1, characterized in that, During the second stage of heat preservation, the furnace temperature fluctuation should not exceed ±5℃ to ensure the uniformity of carbide precipitation.

4. The post-weld heat treatment process for improving the impact toughness of Q690C high-strength steel weld metal used in tunnel embedded parts according to claim 1, characterized in that, Cooling the furnace to 300°C and then air cooling effectively prevents secondary tempering and embrittlement.

5. The post-weld heat treatment process for improving the impact toughness of Q690C high-strength steel weld metal used in tunnel embedded parts according to claim 1, characterized in that, The welding wire used is a low-hydrogen type, model AWS A5.28 ER110S-G, to ensure the low-hydrogen performance of the weld metal.

6. The post-weld heat treatment process for improving the impact toughness of Q690C high-strength steel weld metal used in tunnel embedded parts according to claim 5, characterized in that, When using submerged arc welding, the welding current should be controlled at 500–600 A, the arc voltage at 28–32 V, and the welding speed at 30–40 cm / min.

7. The post-weld heat treatment process for improving the impact toughness of Q690C high-strength steel weld metal used in tunnel embedded parts according to claim 6, characterized in that, When the welding method is gas shielded welding, the shielding gas is a mixture of 80% Ar + 20% CO2, and the flow rate is controlled at 18-22 L / min.

8. The post-weld heat treatment process for improving the impact toughness of Q690C high-strength steel weld metal used in tunnel embedded parts according to claim 1, characterized in that, The weld metal microstructure after this process is fine and uniform tempered sorbite with dispersed carbides, and the grain size reaches level 8 to 9.

9. The post-weld heat treatment process for improving the impact toughness of Q690C high-strength steel weld metal used in tunnel embedded parts according to claim 1, characterized in that, The mechanical properties after this process meet the following requirements: yield strength ≥ 680 MPa, tensile strength ≥ 750 MPa, elongation ≥ 16%, and impact energy at -40℃ ≥ 80 J.