Discussion on welding process of galvanized steel barrel vertical seam
Based on the basic principle of steel barrel resistance welding, this paper focuses on the zinc removal treatment process and welding process of galvanized steel drum before welding. The technical measures to ensure the welding quality of galvanized steel drums are put forward.
Keywords: steel drum, zinc removal, seam welding
In the production of barrels, the barrel seam seal is one of the three major sealing points of the steel drum. The requirements for the quality of seam welding are determined by the use of the steel drum itself: sealing, strength and appearance quality. The raw materials for making steel drums are mainly low-carbon steel sheets and galvanized steel sheets. At present, each barrel manufacturer has a good grasp of the quality of the joint welding of the general low-carbon steel plate. Therefore, when it is necessary to package the product in a galvanized barrel, most of the low-carbon steel sheets are subjected to seam welding and then the galvanized layer is removed. And using galvanized steel to directly make steel drums, how to ensure the quality of joint welding, what kind of welding process is used? This is the issue discussed below.
Second, the basic principle of resistance welding
In the production of barrels, we usually use welding methods of resistance lap joints. Resistance welding is a pressure welding method in which an electrode pressed against a weldment is passed through a current, and the current causes the plastic deformation energy provided by the electrode pressure of the weldment joint to form a common grain structure on the two separated surfaces to obtain spot welding. Or seam welding. There are many factors affecting the resistance heat, among which the material of the weldment, electrode pressure, thickness, surface condition and heating temperature are the main factors affecting the resistance heat.
It is known from the electrician principle that when a current passes through a conductor, the conductor will absorb heat and the temperature rises. This is because the conductor resistance converts the absorbed electric heat into thermal energy. This phenomenon is called the thermal effect of current. The resistance heat of the steel plate welding can be calculated according to the Joule law:
Q= I2Rt (Joule)
The above equation shows that the heat generated by the conductor resistance is proportional to the square of the welding current, proportional to the resistance and time, and the current required to complete a certain solder joint or weld is inversely proportional to the square root of time. Therefore, if the time is extremely short, the required current will be large. The combination of too short time and excessive current may cause an undesired temperature distribution in the weld zone, resulting in intense melting of the weldment surface and rapid loss of the electrode, so The welding current is controlled. The transformer secondary circuit of the resistance welding and the workpiece to be welded constitute a series of resistors. The sum of the resistances affects the magnitude of the welding current. In spot weld welding, the total resistance of the joint consists of three major components:
Where: Rw: internal resistance of the welded part;
Rc: contact resistance between the weldments;
Rcw: Contact resistance between the electrode and the plate.
Low-carbon steel has good weldability, mainly in plastic temperature zone width, low welding pressure, moderate welding time and welding current, low sensitivity to thermal cycle, small tendency to shrink and hot crack, and low splash sensitivity. The plasticity of the joint is reduced, and the tendency of the electrode to be damaged is also small. In the point seam welding, it is not necessary to adopt complicated process measures, carbon elements and trace elements are low, and generally no quenching structure or inclusions appear.
Steel drums are made from galvanized steel sheets, which are coated with a layer of zinc (typically 15-20 mm). Therefore, in the barrel joint seam welding, the melting point of the zinc layer on the surface of the steel sheet is low, only 419 Â° C, (the melting point of the steel is 1450 Â° C, the melting point of copper is about 1000 Â° C, in the initial stage of welding, the zinc layer is melted and sticks On the electrode roller, a bad conductor film is produced, the distribution of the wires passing through the surface is uneven, the contact resistance is suddenly increased, the heating pole is uneven or even the surface is burnt. The metal splashes, the surface of the weldment is overheated, and the burnt electrode and the weldment are caused. In contact with the surface, the amount of soldering is reduced. In addition, the zinc layer adhered to the electrode wheel causes the electrode soldering surface to be uneven, and generates a tip discharge when energized, and breaks the contact surface of the soldering member. Thus, the welded seam of the barrel may cause a crack. There are shrinkage holes, cracks, etc., and the mechanical properties are not up to the requirements of the process. Therefore, for the steel drum made of galvanized steel, to ensure the quality of the weld, the weld must be dezinced.
Third, the weld to zinc treatment process
In order to obtain the weld quality that can be sealed and the strength can be ensured, it is difficult to meet the quality requirements by directly welding on the galvanized sheet. The surface of the weld must be subjected to zinc removal treatment before welding. There are two kinds of processes for removing zinc. One is mechanical dezincification, which uses an edger to remove the zinc layer from the seam weld. The key to the removal of the zinc layer is the selection of the grinding wheel. At present, the grinding wheel used by domestic barrel manufacturers can also edging the general low-carbon steel plate, but this kind of grinding wheel is used for galvanized steel plate edging due to the zinc layer. The melting point is low. When grinding at high speed, the worn zinc layer is stuck on the grinding wheel, blocking the gap of the grinding wheel, reducing the friction coefficient of the grinding wheel, making the surface of the grinding wheel more and more smooth, and the edging to remove zinc can not continue, resulting in Going to zinc is not complete. In this way, it is difficult to weld the seam of the barrel and the welding quality of the weld is not guaranteed.
The other is a chemical zinc removal process that uses hydrochloric acid to react with zinc, namely:
Zn+2HCl= ZnCl2 + H2 â†‘
The zinc layer on the steel surface is completely removed to meet the quality requirements of seam welding.
The following describes the chemical zinc removal process.
Pour a certain concentration of dilute hydrochloric acid solution into a special acid tank. The height of the solution should be controlled to the zinc removal width required by the process. The immersion depth of the galvanized sheet is similar, and is about 1 to 2 mm. This is because the width of the zinc is too large, and the zinc layer is more detached. After the seam welding, the rust prevention ability of the vertical joint is weakened, the rust area of â€‹â€‹the weld is increased, and the amount of the anticorrosive coating that can be applied to the vertical seam is large, resulting in no The necessary waste.
When the zinc-removing solution is prepared, the concentration of hydrochloric acid should also be controlled. If the concentration is too large, a large amount of acid mist will be generated on the liquid surface to corrode the zinc layer on the liquid surface of the galvanized sheet, so that white ash and other defects are generated on the upper surface of the zinc plate. Therefore, choosing a proper acid concentration is critical to the quality of zinc removal. After soaking with hydrochloric acid for a predetermined period of time, the soaked end is immersed in a circulating flow clear water tank (or neutralized in an alkali tank) for cleaning. After cleaning, wipe the soaking cleaning area with cotton gauze, then turn the head to the other end to remove zinc, clean and wipe. When de-zinc immersion, it should be noted that the galvanized sheet should be placed flat in the person's trough, and the zinc height of the whole end face should be kept consistent. This is to ensure the seam size requirement of the seam welding to control the overlap width.
The advantage of chemical dezincification is that the galvanized layer can be completely removed, and the seam welding machine can smoothly carry out the welding without causing the above defects due to the adhesion of the zinc layer to the surface of the roller. However, if spot welding is performed immediately after chemical dezincification, seam welding will cause defects such as splashing and puncture, and the seam will rust quickly after seam welding. This is because a small amount of residual acid is adsorbed on the surface of the zinc-removed end after dezincification, so that there are defects such as impurities and residual acid oxide layer at the weld, which makes the welding quality unstable. In practice, we use zinc removal. The process of grinding the edges. Since the ash is removed, the edging process is easier to carry out. The purpose of edging is to remove residual acid and oxide layer, slow down the rusting speed of the vertical joint, and achieve a uniform thickness of the steel plate. Practice has proved that this process not only improves the welding quality, but also solves the problem of the sealing quality of the T-joint at the seam of the seam weld. The width of the edging should be equal to the width of the de-zinc, so that the above defects can be removed from the surface after de-zinc removal, providing surface quality assurance for the welding process.
Fourth, the welding process
1. Spot welding process We are currently using a manual spot welding process, which is relatively simple to operate and is an auxiliary process for the seam welding process. However, the process control of spot welding cannot be ignored. Inappropriate spot welding processes can cause defects such as disengagement of the solder joints, excessive surface craters, surface cracks, and surface burn-through. These will affect the quality of seam welding. Therefore, controlling the quality of spot welding is a prerequisite for ensuring the vertical seam welding of steel drums.
The quality of spot welding is determined by the appearance, internal shape and nugget size. The so-called nugget refers to the core formed by the solidification of the molten metal in the weld surface of the weldment.
A good solder joint appearance requires shallow, smooth, uniform transitions with no visible shoulders or partially extruded surfaces. It is not allowed to have annular or radial cracks on the outer surface, and there must be no copper alloy that melts or adheres on the surface. The internal shape requires uniform uniformity of the surrounding nugget, and there is no penetrating or exceeding the specified value inside the core. The joint line extends and shrinks the hole. Within the specified range, there are no serious overheated tissues and impermissible defects around the core.
To meet the above quality requirements, it is important to choose the right spot welding process parameters. First, after the spot welding, the size of the edge of the spot weld should be taken well. Generally, the width of the edge should be less than 30-40% of the size of the nugget (that is, the size of the end face of the electrode tip). The end faces of the electrodes should be ground and flat, and no defects such as bumps and protrusions are allowed, which is beneficial to the spot welding quality. In spot welding, attention should also be paid to the adjustment and control of the main parameters such as welding current, electrode pressure, and welding time. Changes in the welding current change the size of the solder joint (weld spot diameter and penetration rate), which in turn affects the performance of the solder joint. If it is excessively increased, causing defects such as metal 1 splashing, deep cratering, excessive shrinkage, etc., the performance of the joint is reduced. Therefore, in the process, we generally use the welding current at the time of starting the sputtering as the maximum current to be used. The effect of welding time on the core size is similar to the effect of changing the welding current. The electrode pressure determines the degree of deformation of the indirect contact of the welding piece. The electrode pressure is too large, the penetration rate is greatly reduced, and the welding strength is insufficient. The electrode pressure is too small, the heating is too fast, and splashing is easy. The electrode pressure is too small, the heating is too fast, and splashing is easy. The electrode pressure should be determined within the range of -, when the pressure is too large, when the welding current or welding time is properly increased, the assembly gap between the weldments can be eliminated, the stiffness is uneven, and the gas pressure is fluctuated. The influence of the heating stability can improve the stability of the welding quality. Therefore, when selecting parameters in the process, these parameters must be matched to each other in order to meet the quality requirements of spot welding.
2, seam welding process
Seam welding has more parameter adjustment points than spot welding. Because of the influence of the welding thin current, the roller electrode pressure and the welding time, the solder joint pitch, the welding speed, the welding cycle, the roller electrode end face size, the surface quality of the material, etc., all affect the seam welding quality. Therefore, when adjusting the parameters of the seam welder, the above factors should be taken into consideration, especially the galvanized barrel vertical seam welding. After removing the zinc and grinding the edge, the surface thickness of the welding material should be uniform. In order to eliminate the adhesion of residual zinc on the surface and the trimming of the electrode, the roller should be knurled, and the roller electrode should be turned and trimmed to ensure uniform seam welding and weld surface quality and electrode life.
Since the seam welding process has a larger current shunt than the spot welding process, the welding current should be increased by 15 to 40% compared to spot welding, but excessive current may cause excessive pressure or burn through in the weld. Therefore, the specific value should depend on the electrical conductivity, thickness or dot pitch (or amount of overlap) of the material. The welding speed determines the contact area between the roller electrode and the plate and the contact time between the two. The current distribution, the size of the split, the conduction and heat dissipation of the preheating and slow cooling are all related to the speed, and the influence on the quality of the seam welding is obvious. If the welding current is increased at a faster welding speed to maintain the necessary joint strength, the surface of the weldment will be overheated and the electrode is quickly damaged when the heat in the weld zone is raised. Therefore, it is difficult to increase the welding speed by increasing the current. . Good weld quality is generally obtained at low seam welding speeds.
We are currently using a semi-automatic seam welder, which operates as a critical and complex operation in barrel production. The main reason is to properly adjust the parameters of the device. In the specific operation, the feeding speed and the time of the electrode wheel pressing are all controlled by the operator. Therefore, the operating skill required by the operator is required to produce the weld quality in accordance with the process technology.
In the galvanized steel barrel vertical seam welding process, the choice of welding and zinc removal treatment before seam welding directly affects the quality of galvanized steel barrel joint. The chemical weaving process after zinc removal can obtain good weld quality. When welding, through the correct matching of the process parameters of spot welding and seam welding, the operator can master the operation skills skillfully. It can produce steel drums that meet the sealing, strength and appearance quality requirements.
Author unit: Huzhou Chemical Factory
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