Industry News_News Center_Hangzhou Fujing Welding Technology Co., Ltd.

07-29 

What are the technological characteristics of low and high frequency pulse argon tungsten arc welding? Analysis of common use problems of tungsten electrode

What are the technological characteristics of low-frequency pulsed argon tungsten arc welding? Low-frequency pulse tungsten argon arc welding has the following technological characteristics. 1. During the duration of the pulse current, a spot-shaped molten pool is formed on the weldment; during the period of the pulse current stop, the base current can only maintain the stable combustion of the arc, the heat input to the weldment is significantly reduced, and the molten pool metal solidifies to form a welding spot. Therefore, the weld is composed of a series of weld points. 2. The arc is stable and the stiffness is good. When the current is small, general TIG welding is easy to arc, but pulsed TIG welding has good arc stiffness and stability, so this welding method is especially suitable for thin plate welding. 3. Low arc line energy. The pulse arc heats the workpiece centrally and has high thermal efficiency. Therefore, the average current required to penetrate the workpiece with the same thickness is about 20% lower than that of general tungsten arc welding, thereby reducing the heat input, which is beneficial to reduce the heat-affected zone and Reduce welding distortion. 4. Easy to control the weld formation. The welding pool has fast solidification speed and short high temperature residence time, so it can ensure a certain penetration depth, and is not easy to cause overheating, flow or burn-through, which is beneficial to realize single-sided welding without backing and double-sided welding and all-position welding. 5. The weld quality is good. Pulsed tungsten argon arc welding is formed by overlapping welding points, and the thermal cycle of subsequent welding points has a heat treatment effect on the previous welding point. At the same time, because the pulse current has a strong stirring effect on the spot molten pool, the cooling rate of the molten pool is fast, and the high temperature residence time is short, so the weld metal structure is fine and the dendrites are not obvious. These all make the weld performance improved. What are the technological characteristics of high-frequency pulse argon tungsten arc welding? The frequency range of the current is 10~20kHz. The process characteristics of this method are as follows. ①Suitable for high-speed welding The high-frequency pulse arc has a large electromagnetic shrinkage effect and a good arc rigidity. It can avoid the bend or discontinuity of the weld bead caused by the adhesion of the anode spot during high-speed welding; avoid defects such as undercut and poor back forming. Therefore, it is especially suitable for high-speed automatic welding of thin plates. ②Big penetration The arc pressure is large and the energy density is large, so the arc penetration capability is significantly increased. ③Good weld quality The molten pool is subjected to ultrasonic vibration, its fluidity increases, and the physical and metallurgical properties of the welding are improved, which is conducive to the improvement of the quality of the weld. ④Suitable for large groove weld In DC tungsten argon arc welding, if there are too many filler wires, the weld pool and the side of the groove will be poorly fused, and the weld bead will be raised and deflected to one side. When welding the next weld bead, poor melting on both sides of the weld bead easily leads to poor fusion or incomplete penetration. High-frequency pulse tungsten argon arc welding can well overcome this defect. Many characteristics of high frequency argon tungsten arc welding are between general argon tungsten arc welding and plasma arc welding, see table What are the technological characteristics of intermediate frequency pulsed argon tungsten arc welding? The frequency range of the current is 10~500Hz. Its characteristic is that the arc is very stable under small current, and the arc force is not as high as high-frequency argon tungsten arc welding, so it is an ideal method for manual welding of thin plates below 0.5mm. Development trend of argon arc welding tungsten electrode Because of its high melting point and good high temperature performance, tungsten is the best material for electron emission and the best choice for argon arc welding starting electrodes. The earliest used tungsten argon arc welding electrode is a pure tungsten electrode, but the electron work function of the pure tungsten electrode is relatively high, about 4.52eV, and it is not easy to start arcing under the condition of small and medium currents. Therefore, all subsequent researches on electron emission materials are centered on reducing the work function of electrons. The earliest doped tungsten electrode material is thorium tungsten, which is doped with about 1-2% thorium oxide in the tungsten base. (This kind of thorium oxide was not prepared for argon arc welding at the beginning, but a material invented by an American named Pintsch to improve the brittleness of tungsten wires for light bulbs.)

07-29 

Knowing the heat-affected zone can improve the welding quality, and the welding seam quality standards are explained in detail

Welding heat affected zone The welded joint is a welding consisting of three parts: the weld, the fusion zone and the heat-affected zone. Welding heat-affected zone: referred to as HAZ (Heat Affect Zone), under the action of welding heat cycle, the area where the solid base metal on both sides of the weld undergoes significant changes in structure and performance is called the welding heat-affected zone. Microstructure distribution of hardened steel Features: Martensite is not easy to form under welding air cooling conditions. Such as low carbon steel, 16Mn, 15MnV and 15MnTi, etc. According to heating temperature and organizational characteristics, it can be divided into four areas: overheating zone, normalizing zone, partial normalizing zone and recrystallization zone. as the picture shows. Overheating zone (coarse crystal zone) The temperature is between the solidus to 1100°C, and the width is about 1 to 3 mm. During welding, the austenite grains in this area grow up severely. After cooling, a superheated structure with coarse grains is obtained, and the plasticity and toughness are obviously reduced. Phase change recrystallization zone Phase change recrystallization area (normalizing area or fine-grained area) The temperature is between 1100℃～Ac3, and the width is about 1.2～4.0mm. Air cooling after welding makes the metal in this area equivalent to normalizing treatment, so its structure is uniform and fine ferrite and pearlite, and its mechanical properties are better than the base material. Incomplete recrystallization zone Incomplete recrystallization zone (also called partial normalizing zone) The heating temperature is between Ac3 and Ac1. When welding, only part of the structure is transformed into austenite; after cooling, fine ferrite and pearlite are obtained, and the rest is still the original structure, so the grain size is uneven and the mechanical properties are also poor. Recrystallization zone If the base material undergoes cold working deformation before welding, the temperature is between Ac1～450℃, and there is a recrystallization zone. The mechanical properties of the metal in this area have not changed much, but the plasticity has increased. If there is no cold plastic deformation before welding, there is no recrystallization zone in the heat-affected zone. Weld quality standards and the first, second and third level classification of welds 1. Guaranteed items 1. Welding materials should meet the design requirements and relevant standards, and the quality certificate and baking records should be checked. 2. The welder must pass the examination and check the qualification certificate and assessment date of the welder's corresponding welding conditions. 3. Welds of class I and II must be inspected by flaw detection, and should meet the design requirements, construction and acceptance specifications, and check the weld flaw detection report. 4. The surface of the welded seam should not have cracks, weld bead, burn through, arc crater and other defects. Class Ⅱ welds must not have defects such as surface pores, slag inclusions, arc craters, cracks, arc scratches, etc., and class Ⅰ welds must not have defects such as undercuts and insufficient welds. 2. Basic items 1. Appearance of weld seam: uniform weld seam appearance, smooth transition between weld bead and weld bead, weld bead and base metal, and clean welding slag and spatter. 2. Surface pores: Class Ⅰ and Ⅱ welds are not allowed; for class Ⅲ welds, the allowable diameter of each 50mm length of the weld is ≤0.4t; and there are 2 pores ≤3mm; the spacing of pores is ≤6 times the diameter. 3. Undercut: Class I welds are not allowed. Class Ⅱ weld: depth of undercut ≤0.05t, and ≤0.5mm, continuous length ≤100mm, and total length of undercut on both sides ≤10% of weld length. Grade III weld: undercut depth ≤0.lt, and ≤lmm. Note: t is the thinner plate thickness at the joint. 4. The allowable deviation items are shown in Table 5-1. 3. Finished product protection 1. Don't hit the joint after welding, and don't water the steel that has just been welded. Slow cooling measures should be taken at low temperatures. 2. It is not allowed to ignite the arc on the base metal outside the weld at will. 3. Welding can only be performed after the various components are calibrated, and the shim and fixtures should not be moved at will to prevent the size deviation of the components. The welds of concealed parts must go through the concealed acceptance procedures before proceeding to the next concealed process. 4. It is not allowed to clean the slag immediately after low-temperature welding, and it should be done after the welding seam cools down. Fourth, the quality issues that should be paid attention to 1. The size exceeds the allowable deviation: For the deviation of the weld length, width, thickness, center line offset, bending, etc., the relative position and size of the welding part should be strictly controll

07-29 

The welding process of manual arc welding, the precautions of manual arc welding

Manual arc welding is a manual welding method that uses the heat generated by the arc to melt the metal to be welded. Because it requires simple equipment and flexible operation, it can easily weld the welds formed by different positions and different joints in space. Therefore, it is still widely used at present. Manual arc welding is shown in the figure: Before welding, connect the welded workpiece and the welding tongs to the two poles of the electric welding machine and clamp the welding rod with the welding tongs. During welding, the welding rod and the workpiece are in instantaneous contact, forming a short circuit, and then they are separated by a certain distance (about 2-4mm), and the arc is ignited. The workpiece under the arc immediately melts to form a semi-oval molten pool. After the electrode coating is melted, part of it becomes a gas that surrounds the arc to isolate it from the air, thereby protecting the liquid metal from oxygen and nitrogen; part of it becomes molten slag, or sprayed into the molten pool alone, or melted with the core The molten droplets of liquid metal are sprayed to the molten pool together. In the arc and the molten pool, the liquid metal, slag and arc gas will undergo certain physical and chemical changes with each other, such as the dissolution of the gas into the liquid metal and the oxidation-reduction reaction. The gas and slag in the molten pool float up due to its light weight. When the arc is removed, the temperature drops and the metal and slag will solidify one after another. In this way, two pieces of metal are joined by melting and crystallizing weld metal. Because the shrinkage of the slag is different from that of the metal, it will slip on the slag shell and the metal boundary, and the slag shell may fall off automatically, or fall off after being knocked, and the metal weld seam with fish scales can be exposed. The main equipment of manual arc welding is electric welding machine. Electric welding machine is a power source that generates welding arc, and there are two kinds of AC and DC. At present, there are many kinds of electric welding machines produced in China, which can be divided into AC electric welding machines and DC electric welding machines according to their structure. There are two different connection methods for DC welding machines. When the electrode is connected to the negative electrode and the workpiece is connected to the positive electrode, it is the positive connection method; the opposite is the reverse connection method. Generally, when welding with alkaline low-hydrogen electrode (such as J507), in order to make the arc burn stably, it is stipulated to use the DC reverse connection method; when using the acid electrode (such as J422) to weld thick steel plates, the forward connection method is used, because the anode part The temperature is higher than the cathode part, and the forward connection method can get a larger penetration depth; when welding thin steel plates and non-ferrous metals, the reverse connection method is used. When welding with alternating current, since the polarity changes alternately, there is no need to choose the polarity connection. The welding material for manual welding is an electric welding rod, which consists of a steel core and a coating on the outside of the steel core. Steel core The role of the steel core (welding core) is mainly to conduct electricity and form a deposited metal with a certain composition at the end of the electrode. The welding core can be made of various steels. The composition of the welding core directly affects the composition and performance of the deposited metal. Therefore, the welding core is required to minimize the content of harmful elements. In addition to limiting S and P, some welding rods have required the welding core to control As, Sb, Sn and other elements. Medicated skin Electrode coating can also be called paint. The main purpose of coating it on the core is to facilitate the welding operation and to ensure that the deposited metal has a certain composition and performance. Electrode coatings can be mixed with hundreds of raw material powders such as oxides, carbonates, silicates, organics, fluorides, ferroalloys, and chemical products in a certain formula ratio. Various raw materials can be divided into the following categories according to their role in the electrode coating: 1. Stabilizer makes the electrode easy to start the arc and can keep the arc burning stably during the welding process. Any substance that is easy to ionize can stabilize the arc. Generally, compounds of alkali metals and alkaline earth metals, such as potassium carbonate, sodium carbonate, marble, etc., are used. 2. Slag-forming agent can form molten slag with certain physical and chemical properties during welding, covering the surface of the molten metal, protecting the welding pool and improving the shape of the weld. 3. Deoxidizer through the metallurgical

07-29 

Alternative welding method of arc welding spot welding, the essentials of argon arc welding wire feeding

The automotive industry has always been the first to innovate and adopt automation more quickly and completely than most other industries. Driven by factors such as fuel efficiency, safety standards, and unique design specifications, industrial automation has promoted greater use of next-generation metals and new materials. For the automotive industry and the general manufacturing industry, this has stimulated the creation and improvement of alternative connection methods. Laser welding Robotic laser welding can produce strong and repeatable welds at relatively high speeds with extremely high accuracy, thereby increasing productivity in the workshop and enabling manufacturers to weld materials that were once considered unweldable. This process is very suitable for mass production. The process uses a focused laser beam to provide precise heat input and align the weld. For metals of various thicknesses, this is a good choice. Laser welding can be performed by thermal conduction (ie remote laser welding), deep penetration welding (ie laser seam stepping) or hybrid laser arc welding. Variants of traditional laser welding have become a suitable substitute for spot welding. The precise and clean weld provided by the laser spot welder or laser seam stepper is very suitable for Class A surface treatment. Like the laser used in laser ablation, the laser can also help remove material to prepare the surface to be coated. In addition to laser welding, manufacturers have discovered that there are several other popular mixed metal welding methods. Especially in terms of cost saving and cycle time reduction, these welding methods are beneficial. Flow drilling A very clean single-sided process that uses rotating screws to generate heat and friction during drilling, melt the base metal, and then use the screws as filler metal. This method is very suitable for connecting dissimilar sheet metal, such as sheet metal used in electric vehicle (EV) battery trays or motor brackets. Friction stir welding The solid-state connection process (the metal does not melt) requires a rotating planer to apply pressure and friction to melt the metal. This method excels in continuous welds, requires very few consumables, and does not require filler metal. It is also ideal for joining alloys or dissimilar metals with a thickness of 0.5 to 65 mm. Brazing Using a traditional process similar to GMAW or GTWA, the process usually uses silicone bronze filler brazing material to form a metallurgical bond between two dissimilar metals (that is, stolen materials of dissimilar steel or aluminum). The low melting point filler metal flows to the base part or joint without deformation. This method is usually used to fill gaps or glue or reinforce parts. It is recommended for parts with larger gaps. Friction element connection Connecting lightweight materials such as aluminum with high-strength steel, this method uses steel friction elements to penetrate the upper layer before being welded to the substrate. The friction force generated by proper process control and mechanical force directly acts on the friction element. The generated frictional heat acts on the element and the substrate without reaching the melting temperature, and only the adhesive is generated between the friction element and the substrate. This process is beneficial for the connection process of strict automotive structural parts. Self-piercing riveting A welding method that can automatically riveting and punching holes in harder materials by using self-piercing rivets, thereby eliminating the cost of pre-drilling and pre-drilling. This process is very suitable for metal-to-metal fastening applications, and is very suitable for joining mild steel, brass, aluminum and stainless steel. These welding process libraries enable manufacturers to adapt to changing customer needs. Although the best solution for a given task will depend on compatibility, cost, and cycle time, effective use of any of these robotic processes has the potential to increase throughput and product quality. Beginners argon arc welding spot welding skills The length of time to hold the switch is related. If the material is thick, press a little longer and send the welding wire in. When it is burned, you can easily burn through if you don’t send the welding wire. If the material is thin, it will take a short time to press it lightly. These are all related to the welding machine. First of all, you have to adapt to that welding machine. In addition, it is possible to weld very thin plates without adding welding wire; hold the tungsten needle with your hand and hold it flat against the place to be welded and lightly tap it. It’s easy to burn through the weldment, so reduce the current so that you don’t think it will burn through the weldment. After the current is low, the speed of melting the welding wire will slow down. Don’t worry, weld little by little. For the ultimate goal, just solder the place to

07-29 

How to understand the relationship between welding defects and non-destructive testing, it is recommended to collect!

1. Common welding defects can be divided into four categories: (1) The size of the weld seam does not meet the requirements: such as the weld seam is super high, super wide, too narrow, the height difference is too large, and the weld transition to the base metal is not smooth. (2) Welding surface defects: such as undercut, weld bump, indentation, overflow, incomplete penetration, surface pores, surface cracks, etc. (3) Weld internal defects: such as pores, slag inclusion, cracks, lack of fusion, tungsten inclusion, and incomplete penetration of double-sided welding. (4) The performance of the welded joint does not meet the requirements: the mechanical properties and corrosion resistance of the welded joint are reduced due to overheating, overburning and other reasons. 2. The hazards of welding defects to welded components mainly include the following aspects: (1) Cause stress concentration The distribution of stress in welded joints is very complicated. Where there is a sudden change in the cross section of the structure, the stress distribution is particularly uneven, and the stress value at some points may be many times larger than the average stress value. This phenomenon is called stress concentration. There are many reasons for stress concentration, and the existence of process defects in the weld is one of the most important factors. The cracks, incomplete penetration and other defects with sharp notches in the weld make the weld cross section discontinuous and produce abrupt positions, which will cause great stress concentration under the action of external force. When the stress exceeds the breaking strength of the metal material at the front end of the defect, the material will crack and fail. (2) Shorten the service life For components subjected to low-cycle fatigue loads, if the size of the defect in the weld exceeds a certain limit, the defect will continue to expand and grow after a certain number of cycles, until the component breaks. (3) Causes embrittlement and endangers safety Brittle fracture is a kind of low-stress fracture, which is a rapid and sudden fracture of structural parts without plastic deformation, which is very harmful. Welding quality has a great influence on the brittle fracture of the product. 3. Welding defects (1) Welding deformation Generally, the workpiece will be deformed after welding. If the deformation exceeds the allowable value, it will affect the use. Several examples of welding distortion are shown in Figure 2-19. The main reason is the uneven local heating and cooling of the weldment. Because during welding, the weldment is only heated to high temperature in a local area. The closer it is to the weld, the higher the temperature and the greater the expansion. However, the metal in the heating area cannot expand freely because it is blocked by the metal with lower surrounding temperature; and it cannot shrink freely due to the restraint of the surrounding metal during cooling. As a result, this part of the heated metal has tensile stress, while the other part of the metal has a balanced compressive stress. When these stresses exceed the yield limit of the metal, welding deformation will occur; when the strength limit of the metal is exceeded, cracks will appear. (2) External defects of the weld 1. Welding seam enhancement is too high As shown in Figure 2-20, this phenomenon will occur when the angle of the welding groove is too small or the welding current is too small. The dangerous plane of the weldment weld has transitioned from the M-M plane to the N-N plane of the fusion zone. Due to stress concentration, it is prone to damage. Therefore, in order to improve the fatigue life of the pressure vessel, it is required to shovel the reinforcement height of the weld. 2. The weld is too concave As shown in Figure 2-21, the strength of the joint is reduced due to the reduction of the working section of the weld. 3. Weld undercut The depression formed on the workpiece along the edge of the weld is called undercut, as shown in Figure 2-22. It not only reduces the working section of the joint, but also causes serious stress concentration at the undercut. 4. Welding The molten metal flows to the unmelted workpiece at the edge of the molten pool, and it accumulates to form a weld bump, which is not fused with the workpiece, as shown in Figure 2-23. The weld bead has no effect on the static load strength, but it will cause stress concentration and reduce the dynamic load strength. 5. Burn through As shown in Figure 2-24. Burn through means that part of the molten metal leaks from the opposite side of the weld, or even burns through into a hole, which reduces the strength of the joint. The above five kinds of defects exist on the surface of the weld, which can be found by naked eyes and can be repaired in time. If the operation is skilled, it can generally be avoided. (3) Internal defects of the weld 1. Incomplete penetrat

07-29 

Attention should be paid to the selection and groove design of welded joints

The welded structure is made up of many parts, components, and parts connected by welding, so the performance and quality of the welded joint is directly related to the performance, safety and reliability of the welded structure. Over the years, the welding engineering community has conducted extensive experimental research on welded joints, which has played a great role in improving the performance and reliability of welded structures and expanding the application range of welded structures. 1. Welded joints 1) Basic types of welded joints The main welding methods such as welding, pressure welding and brazing can be used to make welded structures. These welding methods are used to connect metal structures to form non-detachable connection joints—welding joints to form welded joints, pressure welded joints and brazing, respectively. Joints to form a welded structure. But the most widely used is fusion welding, here is the focus on fusion welding joints. 1) Fusion welded joint: The welded joint is composed of weld metal, fusion line, heat-affected zone and base metal. The weld metal is a cast structure formed by solidification of the filler material and part of the base material after melting. The structure of each part of the fusion welded joint is not uniform, and there are also differences in performance. This is because the chemical composition and metallographic structure of the above four regions are different, and the original cross-section and shape of the component are often changed at the joints, discontinuities, even defects, and different degrees of stress concentration, as well as welding residual stress and deformation. , Large rigidity, etc. all have an impact on the performance of the joint. As a result, the joint not only has uneven mechanical properties, but also has differences in physical and chemical properties. In order to ensure the reliable work of the welded structure, it is hoped that the welded joint has the same mechanical properties as the base metal. In some cases, it is also hoped to obtain the same physical and chemical properties, such as electrical conductivity, magnetic permeability, corrosion resistance, and the same gloss and color. As far as weld metal is concerned, columnar crystal casting structure is often formed, which is generally stronger and harder than the base metal, but the toughness is reduced. For high-strength steel, using appropriate technological measures, such as preheating, slow cooling, or using appropriate heat transfer, can also obtain weld metal with required properties. Generally speaking, the strength of the weld metal may be higher or lower than the strength of the base metal. The former is called high matching and the latter is called low matching. The heat-affected zone with small width, due to the large welding temperature field gradient, the thermal cycle of each point is very different, resulting in the difference in structure and performance. This difference is related to the structure and composition of the welded metal and the heat loss of welding. In particular, it should be pointed out that the "dynamic strain aging" (thermal strain aging) that occurs after the welding thermal cycle will deteriorate the joint performance. After pre-straining steel, aluminum, etc., the "ageing" phenomenon of brittleness will occur. This kind of pre-straining and aging occurs at low temperature (room temperature), and is usually called "static strain aging". The welding heat-affected zone will produce thermal strain after welding thermal cycle, and the high temperature of welding accelerates the aging embrittlement, so "dynamic strain aging" greatly reduces the performance of the joint, and attention should be paid to prevent it. Fusion welds mainly include butt welds and fillet welds. Welded joints composed of these two types of welds include butt joints, fillet joints, T-shaped (cross) joints, lap joints and plug welded joints. According to GB/T 985-1988 "Basic forms and dimensions of gas welding, electrode arc welding and gas shielded weld grooves" and GB/T 986-1988 "Basic forms and dimensions of submerged arc welding weld grooves" commonly used The basic form of the weld groove and the above-mentioned joint form are shown in Figure 5-1. Figure 5-1 shows butt joints (see Figure 5-1 a~n), corner joints (see Figure 5 -1o~u), T-shaped and cross joints (see Figure 5 -1 v~Y and z) , A') and lap joints (see Figure 5-1 b', c') of the groove form, size, and weld metal formed by melting (indicated by thin solid lines in the figure). The relevant dimensions represented by the symbol letters are shown in Table 5-6. Table 5-6 is listed with reference to GB/T 985-1988 and GB/T 986-1988 standards. In addition to the above two standards, the choice of which groove form can also be determined by the thickness of the weldment according to industry and enterprise standards, and there is a suitable interval. For example, for the butt connection of plates with a t

07-29 

There are many types of cracks in welded joints, and the measures and methods are explained in detail

In recent years, the application of low-alloy high-strength materials on special equipment has become more and more common, which is related to the high temperature and high pressure working conditions of boilers and pressure vessels. However, in the manufacturing process of special equipment, cracks are often found in the welds after heat treatment, especially for 2.25Cr- Materials such as 1Mo and 13MoNiMoR have attracted the attention of manufacturers. 01There are many types of cracks in welded joints Crystal cracks: when the weld pool solidifies and crystallizes, in the temperature range where the liquid and solid phases coexist, due to the effects of crystal segregation and shrinkage stress and strain, the weld metal is cracked along the primary crystal grain boundary. Such cracks only occur in welds (including arc craters). Liquefaction crack: during the welding process, under the action of the peak temperature of the welding thermal cycle, in the interlayer metal of the multi-layer weld and the metal near the base metal, due to the intergranular metal/heated remelting, under a certain shrinkage stress , The phenomenon of cracking along the austenite grain boundary is called "hot tearing" in some literature. High temperature and low plasticity cracking: After the liquid phase crystallization is completed, the welded joint metal begins to cool from the plastic recovery temperature of the material. For some special materials, when it is cooled to a certain temperature range, due to the mutual strain rate and certain metallurgical factors The effect causes a decrease in plasticity and causes the weld joint metal to crack along the grain boundary. Generally, it occurs in the heat-affected zone farther from the fusion line than the liquefaction crack. Reheating cracks: after welding, the cracks that develop along the austenite grain boundaries under certain conditions during the process of service at a certain temperature after the residual stress heat treatment or without any heat treatment of the weldment. In fact, reheat cracking is one of the main problems to be solved in the weldability of low-alloy high-strength steels, especially some low-alloy high-strength steels and In the welding seam of hot-strength steel thick plates, reheat cracks are often generated during the post-weld stress relief heat treatment process. The treatment of these defects is labor-intensive and time-consuming, which has a great impact on production. The following is a brief analysis of the formation mechanism of reheat cracks and the preventive measures and inspection methods in the manufacturing process. 02 Mechanism of reheat cracking The formation of reheat cracks, in simple terms, is due to the high strengthening strength in the grain and the weaker grain boundary strength. During the post-weld heat treatment, the deformation during stress relaxation is concentrated on the grain boundary. Once the grain boundary strain exceeds the grain boundary At the limit of the strength of the boundary, it will cause cracks along the grain boundary. (1) The internal cause of reheat crack formation during welding, the heat-affected zone near the fusion line is heated to about 1200°C, especially after the thick plate is heated many times, the grains are coarse, and the precipitation of strong carbides is slower during cooling In the same submerged arc welding, due to the large heat input, the grains in the middle of the weld are also coarser. During the subsequent SR treatment (480～680℃), carbides (V4C3, NbC, MoC, etc.) are in the crystals. The internal dispersion precipitates, thereby strengthening the intragranular (good intragranular thermal strength), so that during heat treatment, the strain during stress relaxation is concentrated on the grain boundaries; the coarse grains make the number of grain boundaries that bear strain drop sharply, and the same strain unit The grain boundary strain is greatly increased; in addition, during the post-weld SR treatment, low melting point impurities and trace elements such as B, Sb, Sn, As and so on segregate in the grain boundary, which weakens the plasticity of the grain boundary, and the strain exceeds the plastic limit of the grain boundary. Form cracking. (2) External causes of reheating cracks The internal causes of reheating cracks are briefly described above. However, to produce reheating cracks, external causes are required. The generation of external causes should be considered from the welding residual stress and expansion stress. In the post-weld stress relief heat treatment, the welding residual stress is reduced through relaxation creep deformation. When the deformation of the material is difficult to meet the deformation requirements, cracks will occur. In the welding zone, the presence of low melting point compounds, segregation and coarse grain embrittlement zone, due to insufficient grain boundary strength and toughness, can not resist creep expansion and deformat

07-29 

What are the taboos of the argon tungsten arc welding process? Process characteristics analysis

The heat generated by the anode during DC tungsten arc welding is much greater than that of the cathode, so when welding with DC positive connection (workpiece is connected to the positive), the tungsten electrode is not easy to overheat due to the small heat generation, and the tungsten electrode of the same diameter can use a larger current. At this time, the workpiece generates a large amount of heat, has a large penetration depth, and has a high productivity. The tungsten electrode thermionic emission capability is stronger than that of the workpiece, which makes the arc stable and concentrated. Therefore, most metals (except aluminum, magnesium and their alloys) should be welded by direct current welding. In DC reverse welding, the situation is opposite to the above, and it is generally not recommended.