- Molten metal falls due to gravity, and the shape and size of the molten pool should not be controlled.
- It is difficult to transport the strip, and the surface of the weldment should not be welded flat.
- Defects such as slag inclusion, incomplete penetration, weld beading and poor weld formation are easy to occur.
- The molten weld metal splashes and diffuses, which is easy to cause scald accidents.
- Overhead welding is less efficient than other welding positions.
Overhead welding tips:
- For butt weld overhead welding, when the thickness of weldment ≤ 4mm, type I groove shall be adopted, φ3.2mm electrode shall be selected, and the welding current shall be moderate; When the welding thickness ≥ 5mm, multi-layer and multi pass welding shall be adopted.
- For overhead welding of T-joint welds, single-layer welding shall be adopted when the weld leg is less than 8mm, and multi-layer and multi pass welding shall be adopted when the weld leg is greater than 8mm.
- According to the specific situation, adopt the correct strip transportation method:
(1) When the size of welding leg is small, linear or linear reciprocating strip transportation shall be adopted, and single-layer welding shall be completed;
When the welding leg size is large, multi-layer welding or multi-layer and multi-pass welding strip transportation can be adopted.
The first layer shall adopt linear strip transportation, and the other layers can adopt inclined triangular or inclined ring strip transportation.
(2) No matter which method of strip transportation is adopted, the weld metal excessive to the molten pool should not be too much each time.
- The fusion welding metal mainly depends on its own weight to transition to the molten pool.
- The shape and metal of molten pool are easy to maintain and control.
- When welding metal with the same plate thickness, the welding current at the flat welding position is greater than that at other welding positions, and the production efficiency is high.
- The slag and molten pool are prone to mixing, especially when welding flat fillet welds, the slag is easy to advance and form slag inclusion.
It is difficult to distinguish the slag and molten pool of acid electrode; Alkaline electrode is clear; HG20581 standard clearly states that acid electrode cannot be used for class II and III vessels.
- Improper welding parameters and operation are easy to form defects such as weld beading, undercut and welding deformation.
- When the back is free formed by single side welding, the first weld is easy to produce images such as uneven penetration procedure and poor back forming.
Flat welding tips:
- According to the plate thickness, the welding rod with larger diameter and larger welding current can be selected for welding.
- During welding, the electrode and weldment shall form an included angle of 60 ~ 80 °, and the separation of slag and liquid metal shall be controlled to prevent slag from leading.
- When the plate thickness is ≤ 6mm, I-type groove is generally opened for butt flat welding, and the front weld should be adopted φ3.2 ~ 4 electrode short arc welding, and the penetration can reach 2 / 3 of the plate thickness; Before back sealing, the root may not be cleaned (except for important structures), but the slag shall be cleaned, and the current can be larger.
- In case of unclear mixing of slag and molten pool metal in butt flat welding, lengthen the arc, tilt the welding rod forward, and push the slag behind the molten pool to prevent slag inclusion.
- When welding horizontal and inclined welds, uphill welding should be adopted to prevent slag inclusion and molten pool from moving forward and slag inclusion.
- When multi-layer and multi pass welding is adopted, pay attention to the number of welding passes and welding sequence, and each layer shall not exceed 4 ~ 5mm.
- For T-type, fillet and lap flat angle welded joints, if the thickness of the two plates is different, the electrode angle shall be adjusted to deflect the arc to one side of the thick plate to make the two plates heat evenly.
- Correctly select the strip transportation method
(1) When the welding thickness is less than or equal to 6mm, i-groove butt and flat welding. When double-sided welding is adopted, straight-line strip transportation is adopted for the front weld, which is slightly slower; The back weld also adopts linear strip transportation, with slightly larger welding current and faster speed.
(2) When the plate thickness is ≤ 6mm, multi-layer welding or multi-layer multi pass welding can be used when other forms of groove are opened.
The first layer of backing welding should adopt low current electrode, low standard current, linear or serrated electrode welding.
When welding the filler layer, the electrode with larger diameter and short arc welding with larger welding current can be selected.
(3) When the leg size of flat fillet welding of T-joint is less than 6mm, single-layer welding can be selected, and linear, oblique ring or sawtooth strip transportation method can be used;
When the welding leg size is large, multi-layer welding or multi-layer multi pass welding should be adopted.
The linear strip transportation method is adopted for backing welding, and the inclined sawtooth and inclined ring strip transportation can be selected for filling layer.
(4) Multi layer and multi pass welding should generally adopt linear strip welding method.
- Molten pool metal and slag fall due to their own weight and are easy to separate.
- When the molten pool temperature is too high, the molten pool metal is easy to flow down, forming defects such as weld beading, undercut and slag inclusion, and the weld is uneven.
- Incomplete penetration is easy to form at the root of T-joint weld.
- The degree of penetration is easy to master.
- The welding productivity is lower than that of flat welding.
Vertical welding tips:
- Keep correct electrode angle;
- Vertical welding up is commonly used in production, and special welding rod shall be used for vertical welding down to ensure the weld quality. The welding current of vertical welding upward shall be 10 ~ 15% less than that of flat welding, and the smaller electrode diameter (< 0.05) shall be selected φ4mm）
- Short arc welding is adopted to shorten the distance from droplet transition to molten pool.
- Adopt correct strip transportation method.
(1) When the T-groove butt joint (commonly used for thin plate) is vertically welded upward, the linear, serrated and crescent strip transportation method is commonly used for welding, and the maximum arc length is no more than 6mm.
(2) In other forms of groove butt vertical welding, the first layer of weld often adopts broken welding, crescent type with small swing and triangular strip welding.
The subsequent layers can be transported in crescent or sawtooth shape.
(3) During vertical welding of T-joint, the electrode shall have appropriate residence time on both sides and top corners of the weld, the swing amplitude of the electrode shall not be greater than the weld width, and the operation of electrode transportation is similar to that of vertical welding of other groove forms.
(4) When welding the cover layer, the shape of the weld surface depends on the strip transportation method.
Crescent shaped strip can be used if the weld surface requirements are slightly higher; Sawtooth strip transportation can be used for flat surface (the middle concave shape is related to the pause time).
- The molten metal is easy to fall on the groove due to its own weight, resulting in undercut defects on the upper side and tear drop weld beading or incomplete penetration defects on the lower side.
- Molten metal and slag are easy to separate, slightly like vertical welding.
Transverse welding tips:
- The groove of butt horizontal welding is generally V-type or K-type, and the butt joint with plate thickness of 3 ~ 4mm can be welded with type I groove on both sides.
- Select small diameter electrode, the welding current is smaller than that during flat welding, and the short arc operation can better control the flow of molten metal.
- When welding thick plates, in addition to backing welds, multi-layer and multi pass welding should be adopted.
- During multi-layer and multi pass welding, special attention shall be paid to controlling the overlapping distance between welding passes.
For each overlap welding, welding shall be started at 1 / 3 of the previous weld to prevent unevenness.
- According to the specific situation, maintain an appropriate electrode angle, and the welding speed shall be slightly block and uniform.
- Adopt correct strip transportation method.
(1) For type I butt horizontal welding, it is better to use reciprocating linear strip transportation method for front weld;
For thicker parts, linear or small inclined annular strip should be used, and linear strip should be used on the back. The welding current can be increased appropriately.
(2) Other groove butt horizontal welding is adopted, and the gap is small, and straight-line strip transportation can be used for backing welding;
When the gap is large, the reciprocating linear strip transportation shall be adopted for the backing layer, the inclined annular strip transportation can be adopted for multi-layer welding of other layers, and the linear strip transportation shall be adopted for multi-layer multi pass welding