Pearlitic steel rail welding joint production method free of joint heat treatment

Abstract

The invention discloses a pearlite steel rail welding joint production method free of joint heat treatment, and belongs to the technical field of steel rail welding. On the basis of bainite steel rail composition, structure and cooling characteristics, on the premise that the strength, hardness and toughness of a joint are guaranteed, the steel rail welding joint process is optimized by combining the structure transformation characteristics of the bainite steel rail, and compared with a traditional steel rail welding production process, the normalizing heat treatment process step is omitted; the production process is shortened, and the energy consumption and the production cost are reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of rail welding, and in particular relates to a method for producing a pearlite rail welding joint without joint heat treatment. Background Art

[0002] Rail-fixed flash welding, a core technology for seamless railway track connection, presents both process advantages and implementation challenges. This technology achieves metallurgical bonding of rail ends through precise control of the flash welding process. Its significant advantages are reflected in three key areas: First, the resulting weld joint exhibits mechanical properties close to those of the parent metal, with a tensile strength exceeding 980 MPa and a fatigue life meeting the 107 cycle requirement. Second, the automated welding system reduces welding time to under 90 seconds, improving efficiency by over 40% compared to traditional methods. Third, the Vickers hardness deviation in the weld area does not exceed 15 HV, and the low-temperature impact energy at -40°C remains above 27 J, demonstrating excellent adaptability to various operating conditions. However, practical application of this technology faces three technical bottlenecks: the high-precision servo system and closed-loop control module require equipment investment exceeding 10 million yuan; the 12 process parameters that require real-time control during welding are tightly coupled, with flash speed fluctuations required to be controlled within ±0.2 mm / s; and operators require over 800 hours of specialized training to obtain IIW certification.

[0003] The current post-weld treatment process for pearlite rails suffers from significant technical and economic deficiencies. While conventional post-weld heat treatment can restore weld hardness to 90%-95% of the base material through two-stage temperature control (normalizing + tempering), this process suffers from three key issues: 1. Energy consumption is as high as 58 kWh / m, with a treatment cycle of up to 2.5 hours per joint; 2. The heat-affected zone undergoes secondary heating, resulting in a cementite spheroidization rate exceeding 15% and a sharp increase in the hardness gradient to 30 HV / mm; 3. The power factor of mobile induction heating equipment is less than 0.7, and the weight of a single unit exceeds 800 kg, severely restricting construction maneuverability. These technical pain points directly result in the heat treatment link accounting for 42% of the welding cost of heavy-duty railways, and the defect recurrence rate during the service life of the joint is as high as 18%. Therefore, the development of self-regulating in-situ welding technology that can achieve weld microstructure and performance control without subsequent heat treatment has become a technical bottleneck that urgently needs to be overcome in the rail transit sector. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for producing pearlite rail welded joints that does not require joint heat treatment. Based on the composition, structure and cooling characteristics of bainite rails, while ensuring the strength, hardness and toughness of the joints, combined with the structural transformation characteristics of bainite rails, compared with traditional rail welding production processes, the normalizing heat treatment process step is omitted, the production process is shortened, and energy consumption and production costs are reduced.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A production method for a pearlitic rail welded joint with a jointless heat treatment according to the present invention includes the following steps:

[0007] 1) Grind and remove rust from the end faces of two pearlitic rails with the same cross-section. The rust removal depth is ≤ 0.2 mm, and the rail surface in the rust removal area should show metallic luster;

[0008] 2) Use a preheating type fixed flash welding machine for rail welding. The rail welding process is as follows: primary voltage 280 - 420 V, seam closing amount 7 - 9 mm, camber amount 0.4 - 0.5 mm, preheating times 7 - 9 times, secondary preheating current 58 - 62 KA, upsetting amount 10 - 12 mm, upsetting force ≥ 560 KN, total welding time 110 - 120 s, melting end speed 2.3 - 2.42 mm / s;

[0009] 3) After flash welding is completed, the upsetting process should be automatically completed, and the time from upsetting to the completion of upsetting is ≤ 18 s;

[0010] 4) After the joint upsetting is completed, the joint is air-cooled or slowly cooled, and the cooling speed of the joint is not greater than 1.5 °C / s.

[0011] Further, the rail is a 115RE rail.

[0012] Further, it also includes the following steps: 5) Grind the weld bead smoothly, and the height of the weld bead after grinding should not exceed 0.5 mm; aging treatment ≥ 24 h, and then straightening, precision grinding, finished product inspection and flaw detection are carried out.

[0013] Further, the pearlitic rail takes C, Mn, Si, Cr, Ni, Mo as the main alloying elements, and the weight percentages of the rail chemical components are: C: 0.76 - 0.85; Si: 0.47 - 0.57; Mn: 0.9 - 1.18; P ≤ 0.020; S ≤ 0.020; Cr: 0.20 - 0.3; the balance is Fe and unavoidable impurities.

[0014] Further, the rail welding process is as follows: primary voltage 400 V, seam closing amount 8 mm, camber amount 0.45 mm, preheating times 8 times, secondary preheating current 60 KA, upsetting amount 11 mm, upsetting force ≥ 560 KN, total welding time 114 s, melting end speed 2.38 mm / s.

[0015] Further, the rail is produced by smelting, continuous casting, slow cooling of the steel billet, rolling, and heat treatment according to the composition requirements; the rolling reduction ratio of the rail should not be less than 9:1, and the final rolling temperature should not be higher than 950 °C, so as to ensure the grain size of the original austenite of the rail.

[0016] Furthermore, the obtained joint satisfies the following: tensile strength ≥ 1190 MPa, yield strength ≥ 1010 MPa, joint elongation ≥ 8.9%, joint reduction of area ≥ 37%, joint impact ≥ 34 J, the ratio of the average hardness of the joint to the average hardness of the base metal Hj / Hp ≥ 0.98, the ratio of the average hardness of the soft spots of the joint to the average hardness of the base metal Hj1 / Hp ≥ 0.85, the width of the joint softening zone W ≤ 10 mm, and all mechanical properties of the joint meet the standard requirements.

[0017] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0018] Based on the composition, structure, and cooling characteristics of pearlitic steel rails, through the optimization of the welding process and the simplification of the production process flow, while ensuring the strength, hardness, and toughness of the joint, compared with the traditional steel rail welding production process, the normalizing heat treatment and tempering heat treatment process steps are omitted, the production process is simplified, the production cost is reduced, and the production efficiency is improved.

[0019] Based on the above invention content, the obtained joint has a tensile strength ≥ 1190 MPa, a yield strength ≥ 1010 MPa, a joint elongation ≥ 8.9%, a joint reduction of area ≥ 37%, a joint impact ≥ 34 J, the ratio of the average hardness of the joint to the average hardness of the base metal Hj / Hp ≥ 0.98, the ratio of the average hardness of the soft spots of the joint to the average hardness of the base metal Hj1 / Hp ≥ 0.85, the width of the joint softening zone W ≤ 10 mm, and all mechanical properties of the joint meet the standard requirements. The present invention combines the characteristics of the phase transformation of the air-cooled pearlitic steel rail structure, so that the joint has high strength, hardness, plasticity, and toughness and other characteristics. Brief Description of the Drawings

[0020] The present invention will be further described below in conjunction with the drawings.

[0021] Figure 1 It is a test curve graph of the softening zone width of 0.9Hp. Detailed Embodiments

[0022] A production method for a pearlitic steel rail welding joint without joint heat treatment includes:

[0023] 1) The pearlitic steel rails welded in the present invention use C, Mn, Si, Cr, Ni, and Mo as the main alloying elements, and the weight percentages of the chemical components of the steel rail are C: 0.76 - 0.85; Si: 0.47 - 0.57; Mn: 0.9 - 1.18; P ≤ 0.020; S ≤ 0.020; Cr: 0.20 - 0.3; the balance is Fe. The above components are used for the production of steel rails through smelting, continuous casting, slow cooling of steel billets, rolling, heat treatment, etc.

[0024] 2) The production of rails through smelting, continuous casting, slow cooling of steel billets, rolling, heat treatment, etc. The rolling reduction ratio of the rails should not be less than 9:1, and the final rolling temperature should not be higher than 950 °C to ensure the grain size of the original austenite of the rails; the rolled section of the rails is 115RE.

[0025] 3) The welding production process flow of the pearlitic rails of the present invention is: rust removal → welding → rough milling → aging treatment → straightening → precision grinding → finished product inspection → flaw detection

[0026] 4) Grind and remove rust from the end faces of two pearlitic rails with the same section. The rust removal depth ≤ 0.2 mm, and the metal luster should be visible on the rail surface in the rust removal area

[0027] 5) Use a preheating type fixed flash welding machine for rail welding. The welding process for 115RE rails is: primary voltage 400V, seam closing amount 8mm, camber amount 0.45mm, preheating times 8 times, secondary preheating current 60KA, upsetting amount 11mm, upsetting force ≥ 560KN, total welding time 114s, melting end speed 2.38mm / s.

[0028] 7) After flash welding is completed, the upsetting process should be automatically completed, and the time from upsetting to upsetting completion ≤ 20s.

[0029] 8) After the joint upsetting is completed, the joint can be air-cooled or slowly cooled, and the cooling rate of the joint should not be greater than 1.5 °C / s.

[0030] 9) Grind the weld bead smoothly. After grinding, the height of the weld bead should not exceed 0.5 mm; the aging treatment ≥ 24h, and then straightening, precision grinding, finished product inspection and flaw detection are carried out.

[0031] During the implementation process, mechanical property tests are carried out on the joints of the present invention. The test results of the rail head (test line 1) on the longitudinal section of the joint are shown in Table 1.

[0032] Table 1 Test results of the rail head (test line 1) on the longitudinal section

[0033]

[0034] Using the Rockwell hardness test data of each measuring point on the rail head (test line 1) of the longitudinal section of the welded joint, a hardness curve is plotted on the coordinate graph. On both sides of the weld, the width with a hardness value lower than 0.9Hp is used as the softening zone width, denoted as W. The standard requires W ≤ 20 mm, and the test results are shown in the softening zone width test curve Figure 1 .

[0035] From Table 1, Figure 1It can be seen that for the joint of the present invention, the ratio Hj / Hp of the average hardness of the joint to the average hardness of the base material is ≥ 0.98, the ratio Hj1 / Hp of the average hardness of the soft spots of the joint to the average hardness of the base material is ≥ 0.85, and the width W of the softening zone of the joint is ≤ 10 mm. At the same time, the tensile properties and impact properties of the joint are inspected, and the obtained tensile strength of the joint is ≥ 1065 MPa, the yield strength is ≥ 723 MPa, the elongation of the joint is ≥ 10%, the reduction of area of the joint is ≥ 35%, and all mechanical properties of the joint meet the standard requirements. The present invention combines the characteristics of the phase transformation of the air-cooled pearlitic steel rail structure, so that the joint has the characteristics of high strength, hardness, plasticity and toughness, etc.

[0036] The embodiments described above are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A production method for a pearlitic rail welded joint with a joint-free heat treatment, characterized in that: It includes the following steps: 1) Grind and remove rust from the end faces of two pearlitic steel rails with the same cross-section. The rust removal depth is ≤0.2 mm, and the steel rail surface in the rust removal area should show metallic luster; 2) Use a preheating type fixed flash welding machine for steel rail welding. The steel rail welding process is as follows: primary voltage 280 - 420 V, seam closing amount 7 - 9 mm, camber amount 0.4 - 0.5 mm, preheating times 7 - 9 times, secondary preheating current 58 - 62 KA, upsetting amount 10 - 12 mm, upsetting force ≥560 KN, total welding time 110 - 120 s, melting end speed 2.3 - 2.42 mm / s; 3) After flash welding is completed, the upsetting process should be automatically completed. The time from upsetting to the completion of upsetting is ≤18 s; 4) After the upsetting of the joint is completed, the joint is air-cooled or slowly cooled, and the cooling rate of the joint is not greater than 1.5 °C / s.

2. The production method of the pearlitic steel rail welded joint with joint-free heat treatment according to claim 1, characterized in that: The steel rail is 115RE steel rail.

3. The production method of the pearlitic rail welded joint with heat treatment without joints according to claim 1, characterized in that: It also includes the following steps: 5) Grind the weld bead smoothly. After grinding, the height of the weld bead should not exceed 0.5 mm; aging treatment ≥24 h, and then straightening, precision grinding, finished product inspection and flaw detection are carried out.

4. The production method of a pearlitic rail welded joint with heat treatment without joints according to claim 1 or 2, characterized in that: The pearlitic steel rail takes C, Mn, Si, Cr, Ni, Mo as the main alloying elements. The weight percentages of the chemical components of the steel rail are: C: 0.76 - 0.85; Si: 0.47 - 0.57; Mn: 0.9 - 1.18; P≤0.020; S≤0.020; Cr: 0.20 - 0.3; the balance is Fe and inevitable impurities.

5. The production method of a pearlitic rail welded joint with heat treatment without joints according to claim 4, characterized in that: The steel rail welding process is as follows: primary voltage 400 V, seam closing amount 8 mm, camber amount 0.45 mm, preheating times 8 times, secondary preheating current 60 KA, upsetting amount 11 mm, upsetting force ≥560 KN, total welding time 114 s, melting end speed 2.38 mm / s.

6. The production method of a pearlitic rail welded joint with heat treatment without joints according to claim 4, characterized in that: The steel rail is produced by smelting, continuous casting, slow cooling of the steel billet, rolling, and heat treatment according to the composition requirements; the rolling reduction ratio of the steel rail should not be less than 9:1, and the final rolling temperature should not be higher than 950 °C to ensure the grain size of the original austenite of the steel rail.

7. The production method of a pearlitic rail welded joint with joint-free heat treatment according to claim 4, characterized in that: The obtained joint meets the following requirements: tensile strength ≥1190 MPa, yield strength ≥1010 MPa, joint elongation ≥8.9%, joint area reduction ≥37%, joint impact ≥34 J, the ratio of the average hardness of the joint surface to the average hardness of the base metal Hj / Hp≥0.98, the ratio of the average hardness of the soft spot of the joint to the average hardness of the base metal Hj1 / Hp≥0.85, the width of the joint softening zone W≤10 mm, and all mechanical properties of the joint meet the standard requirements.

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