Welding crack, deformation and stress control

1) In order to prevent welding cracks, the following technological measures are mainly taken: (1) Preheating and preheating are effective measures to prevent cracking. The preheating temperature is determined mainly by considering the weldability of the steel. The following tests should be carried out before welding. a) Calculate the carbon equivalent based on the chemical composition of the steel.
b) Determination of crack sensitivity of steel by the rigid restraint test method.
c) Determine the mechanical properties (mainly impact toughness) of the welded joint under different heat input conditions, and analyze the metallographic structure and hardness of the heat affected zone.
The preheating temperature and welding process parameters of the welding are determined by the above comprehensive test results. The reference values ​​of the preheating temperature of the medium carbon alloy structural steel are shown in Table 39. Table 39 Preheating temperature of carbon alloy structural steel Recommended value Steel minimum preheating temperature thickness <20mm Thickness=20~50mm Thickness>50mm 20CrNiMoH 90 150 200 40CrNiMoH 280 320 370 42CrMnMoH 280 320 370 30CrMoH 150 180 200 20CrNi2MoH 150 180 200 40CrMnMoH 260 320 370
(2) Welding method selection According to the structure type, size and specific conditions of production, the welding method is selected according to Table 40. Table 40 Welding method Comparison of welding methods Advantages and disadvantages Manual arc welding operation is flexible, and it is applied to small gear welding and welding joints, welds Continued unevenness, poor working conditions and low efficiency. Submerged arc self-welding welding seam is beautiful in appearance and high in efficiency. It is suitable for large gear welding with poor visibility and low heat affected zone performance. O2 gas shielded welding has deep penetration, high efficiency, low price, easy to realize mechanization, automatic spatter, weld Forming poor cored wire CO2 gas shielded welding with high efficiency, beautiful weld bead formation, joint plastic, high toughness, high smoke, Ar80%, ten CO220%, gas shielded weld, beautiful shape, joint plasticity, high toughness, increased ratio, argon price High (3) welding material selection There are two kinds of main ring joints for welding tooth blanks, which are the joint welds (butt or corner joint) of the rim and the web. The weld is made of carbon alloy structural steel (rim) and The low alloy steel [web] is welded; the other ring weld is a joint welded joint between the web and the rim. The weld is welded by low alloy steel (16Mn) and 35 or ZG35. The above two kinds of welds can be welded with structural steel with strength of 16Mn, which not only ensures that the welded joint has no crack, but also obtains a weld with good plastic toughness, and ensures that the weld has good bearing capacity under dynamic load conditions. In addition, when selecting welding materials (electrodes, fluxes, etc.), it is preferred to use alkaline electrodes and strictly control the carbon, sulfur, phosphorus content and diffusible hydrogen content of the deposited metal to ensure excellent toughness and resistance of the weld metal. Split piece,
(4) Welding line energy and weld bead arrangement use a small line energy to prevent carbon in the base material from melting into the weld to prevent cracking of the weld bead and prevent high temperature tempering in the heat affected zone. The strength and hardness caused by softening of the zone are reduced.
Multi-layer multi-pass welding can improve the microstructure and performance of the heat-affected zone. This method is especially suitable for gas shielded welding.
(5) After the heat welding, 250 to 300 ° C / (1 ~ 2h) hydrogenation treatment, and then slowly cooled to room temperature;
(6) Post-weld stress relief heat treatment 2) Process measures to reduce welding deformation (1) The three major parts of rim, web and wheel are added: 1: Accuracy and tolerance are standardized, and countries around the world have their own standards, such as The excavator's low-speed heavy-duty welding gear assembly in the United States requires the outer diameter of the web to be 1.6mm smaller than the inner diameter of the rim, and the tolerance is 0.8mm, that is, the rim and the web are assembled before the welding. 0.8 to 16 mm.
The inner diameter of the web is equal to the outer diameter of the wheel plus 0.8 to 1.0 mm.
It can be seen that the assembly clearance of the gear is much smaller than the assembly clearance of the general welded part, which is a key measure to prevent deformation.
(2) Assembling and welding the tire, assembling on the tire with higher precision and realizing automatic welding can prevent the deformation of the weldment and improve the production efficiency and quality.
3) Process measures to reduce welding stress 1. The main cause of stress in welding stress welding and temperature gradient, stress is related to sheet thickness and structural rigidity. The thicker the sheet, the greater the structural rigidity, the more clamping or fastening force High, the greater the internal stress.
The presence of stress tends to cause cracks in the weld zone and affect the machining accuracy and dimensional stability, which is unfavorable for gear meshing quality and service life. Therefore, it is important to reduce the welding stress for gear members with high precision requirements. The welding stress should be eliminated by high temperature tempering at 600 °C, and the gears are welded after the rim is etched. The post-weld tempering temperature can only be lower than the quenching and tempering temperature of 30 to 50 °C. Otherwise, it will affect the performance of the material and will not meet the strength and hardness requirements. Therefore, in addition to post-weld heat treatment, it is important to reduce the stress to the lowest value from the welding process measures. The process measures are as follows:
(1) Welding under preheating and heat preservation can prevent the welding stress caused by the temperature difference, and the slow cooling after welding can also achieve the reduction of stress effect.
(2) The clamping device of the fixed gear should be moderately rigid to reduce the welding stress.
(3) Submerged arc welding and gas shielded welding are used to fix the weldment and the welding head through the tire. The weldment is rotated by the positioner, at a given position of the wire, welding current, arc voltage and Under the condition of welding speed, the arcing and stopping time of the arc is reduced and the weld defects at the joint may be caused. The entire ring welding is uniform and uniform, and the shrinkage stress caused by the uneven weld is reduced.
(4) Every time a ring seam is welded, it is necessary to strike the slag and hammer the weld in time to reduce the welding stress.
(5) Preventing the existence of grooves between the undercut and the lap weld, avoiding local stress concentration. 2. The weld rim between the stress rim and the web and the web and the hub caused by the dissimilar steel phase welding and the closed weld. The weld is closed for the thick plate, and the chemical composition, mechanical properties and coefficient of linear expansion of the material to be welded are different. Welding under the condition of rigid restraint is easy to generate stress, which leads to cracking of the weld zone. Common methods used in production are:
(1) Preheating of metal (medium carbon alloy steel) with a small coefficient of linear expansion before welding.
(2) Measures for adding intermediate metal transition layer. Therefore, in the gear welding production, the welding of the rim and the web selects the following two different joints according to the carbon equivalent, weldability, mechanical properties and product structure of the rim material. Type:
a) surfacing the transition layer on the rim. When the rim material has a high carbon content and poor weldability, a layer of deposited metal close to the web material should be piled on the rim. Welding of dissimilar steels with large differences in performance was changed to two materials with similar properties to weld, thereby eliminating welding stress.
The specific method is to preheat the rim of the surfacing. The preheating temperature is determined according to Table 39 and the results of the weldability test. The surfacing materials are J507, H 08M r2SiA, H 08MnA, etc., and the surfacing method is preferred to use gas shielded welding. And submerged arc welding.
After the completion of the surfacing, the magnetic particle inspection and the intermediate heat treatment are performed (the temperature is 30 ° C to 50 ° C lower than the rim tempering temperature, and after the intermediate heat treatment, whether the inner diameter of the weld overlay has a machining allowance by the position of 8 points on the circumference is confirmed. After the error, the surfacing surface is machined, the welding edge and the welding bead are polished by the grinding wheel, and the ultrasonic welding and magnetic particle inspection are carried out after the surface of the surfacing surface is machined. After the inspection, the rim can be assembled and welded with the web.
b) No surfacing is carried out on the rim. When the rim material has a low carbon content and good weldability, the surfacing layer may be omitted and assembled directly with the web.
Since the welding of the web and the hub is small due to its difference in performance, it is not necessary to provide a weld overlay layer and directly weld the joint.

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