Welding process of high-strength medium alloy carburizing steel

Abstract

The invention relates to the technical field of metal welding, and discloses a welding process of high-strength medium alloy carburizing steel, which comprises the following steps: preheating the whole weldment to 200-250 DEG C before welding; the overall temperature of a weldment is kept at 200-250 DEG C in the welding process; and after welding is completed, a weldment is integrally heated to 250-300 DEG C, and heat preservation is conducted for preset time. According to the scheme, a new preheating technology in the welding technology is provided for the high-strength medium alloy carburizing steel, overall preheating is adopted before welding, during welding and after welding instead of local preheating, the temperature ranges before welding and during welding are kept consistent, the temperature range after welding is slightly higher than the temperature ranges of the first two stages, it is advantageously kept that the temperature of a mother part in the whole welding stage is stable and uniform, and the welding quality is improved. The temperature gradient in the whole welding process is reduced to the minimum, and through appropriate process steps and welding parameters, it is guaranteed that weld joints do not have welding defects, and meanwhile the mechanical property requirements of products are met.

Description

Technical Field

[0001] The invention relates to the technical field of metal welding methods, and in particular to a welding process for high-strength medium-alloy carburizing steel. Background Art

[0002] 18Cr2Ni4W is a high-strength medium alloy carburizing steel. Due to its good strength, toughness and wear resistance, it is widely used in the manufacture of high-strength parts, such as helicopter journals, gears, turbine shafts and compressor shafts of aircraft engines.

[0003] During the parts manufacturing process, the material needs to be welded and formed. Since it contains a high content of alloying elements (such as chromium, nickel, tungsten, etc.), the main difficulties faced during the welding process include: the carbon equivalent of 18Cr2Ni4W steel is relatively high (usually greater than 0.4%), which means that it has a high hardening tendency. Alloying elements such as chromium, nickel, and tungsten will increase the hardening tendency of the material, making it easy for the weld and heat-affected zone to form a hard and brittle martensitic structure; alloying elements such as chromium and nickel will increase the thermal cracking tendency of the material. Excessive welding heat input will cause the temperature gradient of the weld and heat-affected zone to increase, increasing the risk of thermal cracking; during the welding process, hydrogen will dissolve in the weld and precipitate after cooling, causing hydrogen-induced cracks (cold cracks); high hardening tendency will cause the weld and heat-affected zone to form a hard and brittle martensitic structure, increasing the risk of hydrogen-induced cracks; high alloying elements will increase the thermal expansion coefficient of the material, causing welding deformation, and excessive welding heat input will cause large thermal deformation.

[0004] In the actual welding process, using the existing welding technology, it is very easy to cause softening of the heat affected zone of the weld and the generation of cold cracks. Summary of the invention

[0005] The present invention aims to provide a welding process for high-strength medium-alloy carburizing steel. A new preheating process is proposed for high-strength medium-alloy carburizing steel in the welding process, and the process steps and welding parameters are optimized to ensure that the weld meets the mechanical property requirements of the product without welding defects.

[0006] The basic solution provided by the present invention is: a welding process for high-strength medium-alloy carburizing steel, comprising:

[0007] Carry out welding groove processing, and determine the welding groove processing method and welding method according to the thickness of the workpiece;

[0008] Before welding, preheat the weldment to 200-250℃;

[0009] Gas shielded welding is performed, and the overall temperature of the weldment is maintained at 200-250°C during the welding process;

[0010] After welding, dehydrogenation treatment is performed, and the entire weldment is heated to 250-300℃ and kept warm for a preset time.

[0011] The working principle and advantages of the present invention are:

[0012] The carbon equivalent of a material is an important indicator for evaluating the weldability of the material. When the carbon equivalent is less than 0.4%, the weldability of the steel is good. When the carbon equivalent is between 0.4% and 0.6%, the weldability is slightly poor. When the carbon equivalent is greater than 0.6%, the weldability is poor. The carbon equivalent of 18Cr2Ni4W is about 0.72%-0.92%, which is a difficult-to-weld material. Compared with other types of steel, this type of steel has an increased sensitivity to cold cracks, and the risk of welding cracks increases accordingly. At the same time, due to the high content of alloy elements, the hardening tendency of the steel is increased, the plasticity is reduced, and the risk of welding cracks is further increased. Therefore, how to determine a specific process for high-strength medium-alloy carburizing steels such as 18Cr2Ni4W to ensure welding quality is technically difficult.

[0013] In conventional welding, preheating and post-heating can be used to avoid the problem of workpiece temperature gradient to prevent cracks. Preheating before welding can slow down the cooling rate after welding, and post-heating is conducive to the escape of diffused hydrogen in the weld metal, avoiding the formation of hydrogen pores and hydrogen cracks, and at the same time reducing the hardening degree of the weld and heat-affected zone. However, when considering preheating before welding, the preheating range of general products is three times the wall thickness of the welding area (test plate thickness), and not less than 150-200mm can meet the general product to prevent welding cracks. This scheme found that in the actual application of 18Cr2Ni4W steel, most of them are medium and thick plates (plate thickness below 4.5mm is thin plate, 4.5-25mm is medium and thick plate, 25-100mm is thick plate). After repeated tests, it was found that the use of local preheating during welding will cause uneven surface and preheating temperature field of the parent material, forming a temperature gradient. Compared with other parameters, this is an important factor that causes high-strength medium-alloy carburizing steel to be very sensitive to welding cracks. The reason why this solution was able to discover this is that it is based on the ability to complete in-depth testing of 18Cr2Ni4W steel in actual application scenarios of special and important ships. However, for ordinary enterprises and ordinary products, in-depth testing and analysis in such application scenarios are technically difficult.

[0014] Based on this, this scheme changes the heat treatment method in the welding process, and adopts overall preheating rather than local preheating before, during and after welding to ensure the uniformity of the overall temperature field of the base material in each stage, reduce the generation of temperature gradient of the base material, help reduce the generation of welding stress, avoid the formation of stress concentration points, and avoid the generation of welding cracks; at the same time, the temperature range for preheating and holding before and during welding is kept consistent, so that the overall temperature transition of the weldment is uniform, reducing the generation of temperature gradient of the base material between the two welding stages, further reducing the accumulation of welding stress, and reducing the incidence of welding cracks; setting the insulation range of the post-weld stage higher than that of the first two stages can not only balance the temperature gradient of the base material that may be generated in the first two stages, but also accelerate the escape of hydrogen from the weld and heat-affected zone. By analyzing the relationship between the temperature at each stage and the welding process status of the material and the ambient temperature and other factors and conducting repeated tests, it was determined that the preheating before welding should be 200-250℃, the temperature during welding should be maintained at 200-250℃, and the temperature after welding should be heated to 250-300℃. The temperature range is set reasonably, and the workpiece is always maintained in a good and reasonable dynamic change range of the temperature gradient, providing a good underlying temperature environment adapted to each stage of the entire welding process, reducing the temperature gradient during the entire welding process, avoiding excessive temperatures in the weld and heat-affected zone, reducing the probability of thermal cracks, ensuring uniform distribution of weld temperature, reducing uneven stress distribution caused by uneven temperature during welding, and improving the stability of the welded structure. At the same time, good control of each welding parameter is achieved to ensure that the weld has no welding defects and its mechanical properties meet product requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A schematic flow chart of a welding process for high-strength medium-alloy carburizing steel provided in an embodiment of the present invention;

[0016] Figure 2 This is a weld bead arrangement diagram for a plate with a thickness of 20 mm provided in an embodiment of the present invention;

[0017] Figure 3 This is a weld bead arrangement diagram for a plate with a thickness of 45 mm provided in an embodiment of the present invention;

[0018] Figure 4 A macroscopic metallographic image of a weld provided by an embodiment of the present invention;

[0019] Figure 5 This is a microscopic metallographic diagram of the weld provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0020] The following is a further detailed description through specific implementation methods:

[0021] As attached Figure 1 As shown: A welding process for high-strength medium alloy carburizing steel, comprising the following steps:

[0022] (1) Perform welding groove processing and determine the welding groove processing method and welding method according to the thickness of the workpiece.

[0023] Specifically, in this embodiment, the steel ingot is forged to obtain a 18Cr2Ni4W workpiece; after forging, the steel ingot is heat treated, normalized and tempered, to increase the grain size of the 18Cr2Ni4W base material and improve the performance of the 18Cr2Ni4W base material; the 18Cr2Ni4W workpiece is heat treated in a natural furnace or a resistance furnace, and when the furnace temperature reaches 500°C, the 18Cr2Ni4W workpiece is charged into the furnace, and the temperature is raised to 930±30°C within 5-10 hours, and then the temperature is kept, and the temperature is kept for 6-9 hours before the workpiece is taken out of the furnace and cooled to room temperature in the air; then the temperature is raised to 700±20°C within 5-7 hours, and then the temperature is kept, and the temperature is kept for 5-12 hours before the workpiece is taken out of the furnace and cooled to room temperature in the air to obtain the treated 18Cr2Ni4W base material workpiece; the 18Cr2Ni4W base material workpiece is processed to the required size by turning to form a part; according to GB / T6402 Level I requires that parts be inspected by ultrasonic flaw detection; after passing the inspection, the corresponding workpiece is obtained and then enters welding.

[0024] Since the nozzle diameter of the gas shielded welding gun is generally 23.8mm, if a single-sided groove is still selected when the plate thickness data is too large, the welding gun cannot reach the bottom of the groove and cannot play a protective role. Welding defects are likely to occur during the welding process, such as: porosity, slag inclusion, incomplete penetration, and lack of fusion. Therefore, determining the welding groove processing method and welding method according to the thickness of the workpiece can ensure the welding quality.

[0025] When the workpiece thickness is ≤25mm, that is, thin plate or medium and thick plate, usually medium and thick plate, single-sided groove is opened and single-sided welding is carried out; when the workpiece thickness is greater than 25mm, that is, thick plate, double-sided groove is opened and double-sided welding is carried out.

[0026] like Figure 2 As shown in (a) and (b) in the figure, an example of a weld bead layout diagram for a plate thickness of 20 mm is given, where t1 = 20 mm, t2 = 20 mm, b = 0-1 mm, c = 1-2 mm, and 1 / 4 in (b) is the base layer, 7 is the cover layer, and 2 / 3 / 5 / 6 are the middle layer.

[0027] like Figure 3 As shown in (a) and (b), a weld layout diagram for a plate thickness of 45 mm is given, where t1 = 45 mm, t2 = 45 mm, t3 = 25 mm, b = 0-2 mm, c = 2-3 mm, and in (b), 1 and 4 are base layers, 7 / 8 / 9 / 10 are cover layers, and 2 / 3 / 5 / 6 are intermediate layers.

[0028] The groove angle is 25-60°. When the plate thickness is less than 25mm, a V-shaped groove can be opened, preferably with a groove angle of 60°. When the plate thickness is above 25mm, an X-shaped groove can be opened, preferably with a groove angle of 50°. Under the condition of the same plate thickness, the larger the groove, the more welding layers there are, the higher the temperature between the weld layers, and the more likely it is to cause coarse weld grains, thereby affecting the performance of the weld joint. The groove can be moderate without affecting the operation of the welding gun. At the same time, the filling amount of welding material also depends on the size of the groove angle. The larger the groove, the more filling, and the greater the material consumption.

[0029] After welding groove processing, grind to remove oil stains on the groove surface. Use a grinder to remove oil, rust, water, carbon gouging slag and other dirt within 20mm of the groove and its surrounding area, and the original color of the parent metal is exposed on the groove surface.

[0030] (2) Before welding, preheat the entire weldment to 200-250℃.

[0031] Specifically, the workpiece is placed in a natural gas furnace for overall preheating. First, the furnace temperature is set to 200°C. When the furnace temperature reaches the set temperature, the workpiece is placed in the furnace for 2-3 hours to the preheating target temperature. Factors such as the heating rate are not considered during the process.

[0032] (3) Perform gas shielded welding, and keep the overall temperature of the weldment at 200-250℃ during the welding process.

[0033] Specifically, the protective gas composition is 80% Ar+20% CO2, and the gas flow rate is 20L / min-30L / min.

[0034] Gas shielded welding includes welding the base layer, the middle layer and the cover layer in sequence at the weld joint; the base layer welding current is 220-250A, the voltage is 20-25V, the welding speed is 20-25cm / min, and the heat input is 2-3KJ / cm; the middle layer and the cover layer welding current is 260-300A, the voltage is 25-30V, the welding speed is 30-40cm / min, and the heat input is 3-4KJ / cm. This data range is based on the data range with the best weld quality obtained through repeated tests after the temperature of each stage is determined.

[0035] like Figure 2 (b) and Figure 3 (b) The base layer, cover layer, and middle layer shown in numbers are all general terms. The thicker the thickness, the more middle layers there are. The current and voltage of the base layer are smaller than those of the middle layer and cover layer.

[0036] During welding, the workpiece is heated by natural gas. When the overall temperature of the workpiece is detected to be as low as 200°C, natural gas is used for direct heating. A hole is opened in the natural gas pipeline to ensure that the entire weldment is within the natural gas heating range to ensure the uniformity of the overall temperature of the product. The temperature of the weldment is measured with an infrared thermometer.

[0037] (4) After welding, heat the weldment to 250-300°C and keep it warm for a preset time to remove hydrogen.

[0038] Post-weld dehydrogenation treatment refers to low-temperature heat treatment when the weld has not cooled below 100°C. It is generally heated to 250-300°C and kept warm for 2-6 hours. Its main function is to accelerate the escape of hydrogen from the weld and heat-affected zone to prevent the occurrence of welding cracks.

[0039] Specifically, the workpiece is placed in a trolley natural furnace or a trolley resistance furnace, and the temperature is raised to 250°C with the furnace, the heating rate is 60-80°C / h, and the temperature is kept for 2 hours. The method of the present application is described in detail below using a specific embodiment.

[0040] Use 45mm thick steel plate, open 50° X-shaped groove, preheat 200-250℃ before welding, keep warm 200-250℃, heat 250-300℃ after welding, corresponding Figure 3 , the welding parameters are shown in Table 1 (the parameters in the welding process are a fluctuating value, not a specific fixed value):

[0041] Table 1 Welding parameters

[0042]

[0043] The above method can obtain a smooth welded joint without defects such as cracks and pores (such as Figure 4 ), the grain size of the base material is basically the same as that of the weld (such as Figure 5 ). Good mechanical properties, requiring the yield strength of the weld metal at room temperature to be ≥835MPa, the tensile strength to be ≥1050MPa, the elongation at fracture to be ≥10%, and the impact energy of the U-notch impact test to be ≥78J. The actual measured values ​​are: the yield strength of the weld metal is 968MPa, the tensile strength is 1064MPa, the elongation at fracture is 11%, and the impact energy (according to the standard requirements, the impact energy of each group of three specimens) is 83J, 86J, and 80J respectively. The bending test joint is qualified without cracks.

[0044] This scheme provides a welding process for high-strength medium-alloy carburizing steel, which changes the heat treatment method in the welding process. It adopts overall preheating rather than local preheating before, during and after welding to ensure the uniformity of the overall temperature field of the parent material in each stage, reduce the generation of temperature gradient of the parent material, help reduce the generation of welding stress, avoid the formation of stress concentration points, and avoid the generation of welding cracks; at the same time, the temperature range for preheating and holding in the two stages before and during welding is consistent, so that the overall temperature transition of the weldment is uniform, the generation of temperature gradient of the parent material between the two welding stages is reduced, and the accumulation of welding stress is further reduced, and the occurrence rate of welding cracks is reduced; the insulation range of the post-welding stage is set higher than that of the first two stages, which can not only balance the temperature gradient of the parent material that may be generated in the first two stages, but also accelerate the escape of hydrogen from the weld and heat-affected zone. Through this process, from the welding preheating process, the temperature gradient of the whole welding process is minimized, and a product with beautiful weld shape, no obvious defects such as welding cracks, and a small difference between the weld grain size grade and the parent material can be obtained, ensuring that the weld has no welding defects and its mechanical properties meet the product requirements.

[0045] The above is only an embodiment of the present invention. The common sense such as the known specific structure and characteristics in the scheme is not described in detail here. The ordinary technicians in the relevant field are aware of all the common technical knowledge in the technical field of the invention before the application date or priority date, can obtain all the existing technologies in the field, and have the ability to apply the conventional experimental means before that date. The ordinary technicians in the relevant field can improve and implement the scheme in combination with their own abilities under the enlightenment given by this application. Some typical known structures or known methods should not become obstacles for the ordinary technicians in the relevant field to implement this application. It should be pointed out that for the technicians in this field, without departing from the structure of the present invention, several deformations and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent.

Claims

1. A welding process for high-strength medium-alloy carburizing steel, characterized in that: include: Carry out welding groove processing, and determine the welding groove processing method and welding method according to the thickness of the workpiece; Before welding, preheat the weldment to 200-250℃; Gas shielded welding is performed, and the overall temperature of the weldment is maintained at 200-250°C during the welding process; After welding, keep warm to remove hydrogen, heat the entire weldment to 250-300℃ and keep warm for a preset time.

2. The welding process of a high-strength medium alloy carburizing steel according to claim 1, characterized in that: The welding groove processing method and welding method are determined according to the thickness of the workpiece. When the workpiece thickness is ≤25mm, a single-sided groove is opened and single-sided welding is performed; when the workpiece thickness is greater than 25mm, a double-sided groove is opened and double-sided welding is performed.

3. The welding process of a high-strength medium alloy carburizing steel according to claim 1, characterized in that: The groove angle is 25-60°.

4. The welding process of a high-strength medium alloy carburizing steel according to claim 1, characterized in that: After welding groove processing, grind and remove oil stains on the groove surface.

5. The welding process of high strength medium alloy carburizing steel according to claim 1, characterized in that: The gas shielded welding comprises a shielding gas composition of 80% Ar+20% CO2, and a gas flow rate of 20L / min-30L / min.

6. A welding process for high-strength medium alloy carburizing steel according to claim 5, characterized in that: The gas shielded welding comprises welding of a base layer, an intermediate layer and a cover layer in sequence at the weld; wherein the welding current of the base layer is 220-250A, the voltage is 20-25V, the welding speed is 20-25cm / min, and the heat input is 2-3KJ / cm; the welding current of the intermediate layer and the cover layer is 260-300A, the voltage is 25-30V, the welding speed is 30-40cm / min, and the heat input is 3-4KJ / cm.

7. The welding process of high strength medium alloy carburizing steel according to claim 1, characterized in that: The preset time is 2-6 hours.

8. The welding process of high strength medium alloy carburizing steel according to claim 1, characterized in that: During the welding process, a manual drying gun is used to heat the entire weldment.

9. The welding process of high-strength medium alloy carburizing steel according to claim 1, characterized in that: The high-strength medium-alloy carburizing steel is 18Cr2Ni4W.

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