For manual arc welding "single-sided welding double-sided forming" process according to the method of operation, can be divided into continuous welding method and intermittent arc extinguishing welding method.
In low carbon steel and low alloy steel welding the first layer almost always use intermittent arc welding. This welding method can use a larger current, with greater penetration, and can control the pool temperature and switch, can do the root weld through, while continuous welding method, that is, continuous welding without interrupting the arc must use a smaller welding current, the temperature is low at the start of welding, but after welding a section of the workpiece temperature rises, it is not easy to control the pool temperature and pool size, so it is difficult to ensure that the root weld through and no tumor, so the first layer is rarely used. Therefore, the first layer is rarely used, but for the second layer after the welding.
Intermittent arc extinguishing method is mainly through the control of arc burning and arc extinguishing time, the use of reasonable transport bar action to control the melt pool temperature, melt pool time, melt pool switch and the thickness of the liquid metal layer, in order to obtain a good reverse formation and internal quality, but regardless of which welding method, the arc on the bevel melting degree, and is divided into penetration fill the gap between the butt. From the surface, it is root forming but in essence the root of the bevel does not really melt through and cannot pass the reverse bending test, so it is no longer used. Generally are used to achieve single-sided welding double-sided forming by penetrating the root of the welding method.
Single-sided welding double-sided forming method of operation, regardless of carbon steel, low-alloy steel or stainless steel welding, as well as the use of DC power or AC power, despite the significant differences in welding performance, but the operating principles are the same, mainly to control the following three aspects.
1, the root gap assembly gap should be appropriate
In the case of a reasonable bevel angle, there must be a proper root gap to ensure that the welding rod is delivered to the root, to ensure that the arc through the northern part, melting through the root. In order to easily achieve uniform penetration, the general root gap size deviation should be about 1 mm.
Root gap size should be equivalent to the diameter of the welding rod used or greater than the diameter of 0.5 ~ 1.0 mm or so is appropriate.
The size of the root gap is related to many factors and should be selected with comprehensive consideration.
① thickness of the workpiece, such as the weld is thin, heat dissipation is slow, the weld heat is not easy to dissipate, the root gap can be smaller, thicker weld should be appropriate larger, in order to facilitate the root penetration.
② process parameters, welding current is small, the root gap should be slightly larger, when the welder is used to using a larger current operation, the root gap should be reduced accordingly.
③Welding position, the root gap of the flat seam and the horizontal seam can be smaller, while the back seam and the vertical seam need to be slightly larger.
④Blunt edge size: If the blunt edge is large, the root gap should also be larger.
⑤ welding order, the root gap should be welded first, the root gap should be welded after the large, in addition to consider the thermal expansion and other factors.
2, the bevel angle should be appropriate, and to have a certain size of the blunt edge.
Bevel angle must be in accordance with the "rules" and the technical conditions of the design of the bevel angle directly affects the quality of the joint and the size of the weld, must choose a reasonable angle, generally "v" bevel 60 ° ~ 70 °.
Blunt edge is along the thickness of the welded part of the direction of the end of the bevel is not open. According to the thickness of the workpiece is generally left with 0.5 ~ 2.0 mm size of the blunt edge. Such as wall thickness of 3 mm, the blunt edge should be 0.5 mm, such as wall thickness of 12 mm or more, generally should be 1.5 mm, the maximum does not exceed 2 mm is appropriate, the blunt edge is too thick easy to root not weld through. Too thin easy to be pierced, a large melt hole.
With a blunt edge, the arc after the arc to preheat the workpiece can be longer, the preheating range can be larger, thereby improving the welding process conditions, increasing the mobility of liquid metal, easy to ensure that the weld through.
With a blunt edge, it can withstand larger welding currents and will not penetrate the root as soon as the arc is induced. With a blunt edge it is easy to control the size of the melt pool, which is conducive to root penetration. Especially in the supine welding position, a slightly higher current must be selected to operate, otherwise it is not only impossible to form but also difficult to overcome process defects such as porosity and slag. Therefore there is a certain size of the blunt edge is very necessary.
3, the use of penetration welding method.
Breakthrough welding method, that is, in the welding process, the penetrating force of the leading arc, melting penetration of the root to ensure that the root weld through the formation of a welding method.
The specific method of operation is: after the arc is triggered, the arc is stretched to preheat (flat welding preheat time is short, not very obvious, the position of the weld is very obvious), when the state of semi-melting (i.e., in the welding goggles to see the bevel edge of the preheated "sweat beads" when about 3 to 4 seconds), the arc is depressed, melting penetration blunt edge, so that the appearance of A slightly larger than the gap between the buttress "melt hole", thus ensuring that part of the deposited metal transition to the root and back of the weld and molten base material together to form a molten pool.
As the electrode continues to melt, the penetration of the melt hole is welded on, at this time to take the appropriate arc extinguishing techniques, so that it cools to form the weld. Then hit again, melting blunt edge, and then form a molten hole, and then weld on this repeatedly to achieve the back of the weld formation.
The formation of molten hole means that the root has been welded through. The size of the melt hole size, that is, the size of the mark back weld. Generally control the diameter of the melt hole for the butt gap of about 1.1 to 1.5 times. Specific size according to the workpiece thickness, welding position, specification parameters and root clearance, steel grade and other factors to adjust. Generally, the process test is carried out first, and the law is figured out before welding to ensure the quality of welding.
After the second layer of welding using continuous welding method, pay attention to reduce process defects, welding current to moderate, for carbon steel and low-alloy steel weldments, after welding to control the slow cooling, to obtain good organizational properties of the joint and create conditions for gas escape, for austenitic stainless steel weldments, it is required to choose a smaller welding process specifications, after welding natural cooling or make it fast cooling, to prevent the tendency to produce intergranular corrosion due to overheating .
Cover layer (strengthen the welding layer) welding, should first be "filler", so that the weld meat height consistent, and not more than the bevel surface, retain the bevel profile, adjust the welding current, a cover, to achieve beautiful appearance.
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