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Common storage tank welding

    Increasing product range of solar water heater, the tank material changes more and more, in addition to the traditional stainless steel, carbon steel enamel but also the emergence of nano-PP, slush and other new materials, new processes. However, generally speaking, solar water heater storage tank as a result of high temperature, corrosion resistant requirements, so a lot of stainless steel and carbon steel tank or enamel in two ways, the following two forms of the tank will be welding methods and processes for details.

    A stainless steel tank

    Stainless steel water tank is generally the material SUS304, are austenitic stainless steel type, for stainless steel solar storage tank, commonly used in welding is spot welding, seam welding, and welding.

    1 spot

    Spot welding joints are welded parts in the individual points on the contact surface to be welded together VSR device. Spot welding of metal required to have a good plasticity. Shown in Figure 1, for the most simple example of spot welding applications.
 

    VSR welding device, first clean the surface welding, then lap welded sheet metal assembled, pressed between two cylindrical copper electrodes, clamping force applied P, shown in Figure 2. When a large enough current through the contact at the plate produce large amounts of resistance heat, the hottest area of ??the metal center quickly heated to high plasticity, or molten state, forming a lens-shaped liquid bath. Continue to maintain pressure P, off current, the metal cools, forming a solder joint. Shown in Figure 3, is a spot welding machine diagram of VSR.

    Spot because there is a certain spacing between the solder joints, so there is no sealing required only for structural and sheet metal mesh overlap, cross-weld steel structural parts of the VSR device. If the cylindrical electrode into the disc-shaped electrode, and rotating electrodes pressed weldments, weldment between the disc-shaped electrode is only continuous feed, coupled with the pulsed power. We can form a continuous and overlapping joints, the formation of the weld, which is seam welding. It is mainly used for sealing requirements or require a higher joint strength structural parts of the sheet lap welding VSR device, such as fuel tanks, water tanks.

    Spot welding process

    (1) pre-press stage – down to the current electrode connected to stage, to ensure that the electrode clamping the workpiece, the workpiece with an appropriate pressure.

    (2) welding time – welding current through the workpiece, heat the formation of nugget.

    (3) maintain the time – cut off the welding current, electrode force to maintain enough strength nugget solidification.

    (4) stop time – start to bring the electrode to the electrode began to decline again to start the next welding cycle.

    In order to improve the performance of welded joints, and sometimes need to be following one or more of the increase in the basic cycle:

    (1) increase the pressure to remove the thick pre-clearance between the workpiece, so tightly fit.

    (2) warm-up pulses to improve the plasticity of metal to make parts easy to fit tightly to prevent splashing; bumping when doing so, more bump before soldering in power even with the flat contact points to ensure consistent heating.

    Spot welding joint design

    Spot usually folding joints and lap joints, the joint can be two or more ranges such as the thickness of the workpiece thickness or composition. Spot in the design of the structure must be considered up to the electrode, that electrode must be able to easily reach parts of the welding site. Should also be considered, such as margins, overlap volume, pitch, assembly space and joint strength of various factors.

    Margins of the minimum depends on the type of welded metal, thickness, and welding conditions. For high yield strength of metal, thin pieces or less desirable conditions using a strong value. Amount of overlap is twice the margin, the minimum recommended amount of overlap in Table 1.

    Pitch that is adjacent to two of the center distance, the minimum and the thickness of the welded metal, conductivity, surface cleanliness, and the diameter of the nugget. Table 2 shows the recommended minimum pitch.

    Welding conditions

    As the high resistance of stainless steel, thermal conductivity is poor, so compared with the low-carbon steel, the welding current can be smaller and shorter welding time. Such materials have high temperature strength, the electrode must be high pressure to prevent shrinkage, cracks and other defects. Stainless steel thermal sensitivity, usually shorter welding time, strong internal and external water cooling, and to accurately control the heating time, welding time and welding current to prevent the heat-affected zone grain growth and the emergence of intergranular corrosion phenomenon.

    Stainless steel welding electrodes recommended for Class 2 or Class 3 electrode alloy, in order to meet the needs of high-pressure electrode. The following table is stainless steel spot welding conditions:

    For stainless steel tanks, often just for positioning spot welding, spot welding seam after welding but also for the general.

    2 seam welding

    Welding seam is rolled plate with a pair of cylindrical electrodes instead of spot welding electrodes, and the workpiece relative motion, resulting in one take each stack nugget sealed seam welding method.

    Seam welding electrodes

    Seam welding electrode is circular with a rolling tray, rolling disk diameter is generally 50 ~ 600mm, commonly used diameter of 180 ~ 250mm. Roll plate thickness of 10 ~ 20mm. Contact surfaces are cylindrical and spherical shape are two kinds of individual cases where the conical surface. In addition to cylindrical roller disk in the form of bilateral chamfer, the chamfer can also be made in the form of a single measure to meet the hem seam welding joints. Contact with the workpiece surface, as the thickness of different width for the ω 3 ~ 10mm, spherical radius R is 25 ~ 200mm. Cylindrical roller plate is widely used in welding of steels and superalloys, spherical roller tray for easy cooling, the transition indentation uniform, light alloy commonly used in welding.

    Roll plate usually external cooling. Welding non-ferrous metals and stainless steel, with clean tap water can, welding general steel to prevent rust, commonly used in aqueous solution containing 5% borax cooling. Sometimes using rolling disk inner loop water cooling, especially welding aluminum welding, but its structure is much more complex.
 

        Seam welding method

    Rolling disk by way of rotation and feed points, seam welding can be divided into continuous seam welding, seam welding and intermittent step seam welding.

    Continuous seam welding, the roll plate continuous rotation, the current constantly through the workpiece. This method is easy to overheat the surface, the electrode wear serious, and therefore rarely used. However, in high-speed seam welding (4 ~ 15m/min) 50Hz AC every week to form a half joints, AC zero crossing is equivalent to resting time, which in turn is similar to the following intermittent seam welding, and thus in the tank system, cooper industries was applied.

    Intermittent seam welding, the roll plate continuous rotation, intermittent current through the workpiece, take each other by the formation of the weld nugget Diego\\ s composition. As the current is interrupted by the resting time, roll to the cooling plate and the workpiece, which can improve the rolling disk life, reduce the heat-affected zone width and deformation of the workpiece, get on better welding quality. This approach has been widely used in a variety of the following 1.5mm steel, high temperature alloys and titanium seam welding. Intermittent seam welding, due to the continued roll-out plate welding zone, the pressure decreases nugget in the case of crystallization from hot surfaces, so it is easy to produce, shrinkage and cracking (as in welding high temperature alloy). Although the solder joints take the stack length of more than 50% nugget, a little after the molten metal can be a little before filling shrinkage, but the last point is difficult to avoid shrinkage. However, currently developed computer control box, can reduce weld finishing part of the welding current point by point, to solve this problem.

    Seam welding step, the roll plate intermittent rotation, the workpiece does not move when the current through the workpiece, the metal melting and crystallization were not moving when rolling plate, heat and pressure to improve the solid condition, and thus can more effectively improve the welding quality, extend the life of rolling plate. This approach than aluminum, magnesium alloy seam welding. For seam welding high temperature alloys, but also can improve the quality of welding, but this type of domestic AC welder small, and therefore has not been applied. When welding aluminum hard. And a thickness of 4 +4 mm above the metal must be used for seam welding step, to form a solder joint each time the same can be applied as spot welding forging force, or while using warm cold pulse. But the latter is rarely used.

    By connector type points, seam welding can be divided into lap seam welding, flatten seam welding, pad foil welding of seams, seam welding electrodes such as copper.

    Spot welding with the same lap joint, lap joint can be used one pair of roll plate or a plate and a mandrel roll seam welding electrodes. The minimum amount of overlap joints with the same spot.
    Lap seam welding seam welding in addition to commonly used double-sided, there is a single-sided seam welding seam, single-sided double-slit seam welding and seam welding and other small-diameter circle. Small-diameter circular seam welding can be used:

    (1) deviated from the axis of the roll pressure plate electrode;

    (2) attach a horizontal seam welder positioning device;

    (3) the ring electrode, the electrode surface tapered cone tip must fall on the circumference of small diameter weld center electrode to the workpiece in order to eliminate slippage.

    Pressing the lap seam welding seam welding than the general much smaller, about 1 to 1.5 times the thickness of the welding joints flattened at the same time, the thickness of the joints after welding thickness of 1.2 to 1.5 times. Usually cylindrical surface of the roll plate, the width of the lap joints should be completely covered part. Welding welding pressure to use a larger and continuous current. In order to obtain a stable weld quality must be precisely controlled amount of overlap. Usually clamped to the workpiece firmly fixed in advance or by tack welding. This method can get a good weld appearance, commonly used in low-carbon steel and stainless steel food containers and frozen products such as welding machines bush.