How to solve these problems encountered in welding

1.Cracks

Hot crack, in the process of weld cooling, because the sulfur and phosphorus impurities in the steel or welding material form a variety of brittle and hard eutectic with low melting point, in the process of weld cooling, the solidified eutectic with low melting point is in a tensile state, which is easy to crack.

Cold crack, produced in the range of 200 ℃ to room temperature, has delay characteristics, and appears in a few minutes to a few days after welding, so it is also called delay crack. It is closely related to the selection of steel, structural design, storage and application of welding materials and welding process.

Preventive measures: for cold cracks, the welding rod with good crack resistance, low hydrogen or ultra-low hydrogen and low strength should be selected, and the preheating temperature and linear energy should be controlled to reduce the tendency of cold cracks. For hot cracks, the steel with high nickel content should be selected, and the refining method should be adopted to increase the purity of steel, reduce the impurity content, and control the concavity of weld less than 1 mm to reduce the linear energy, In order to reduce the tendency of hot cracking.

2.Lack of fusion and penetration

The main reasons are improper process parameters, measures and groove size, insufficient cleanliness of groove and weld surface or impurities such as oxide skin and welding slag, and poor welder process.

Preventive measures: before welding, ensure that there are no impurities on both sides of the groove edge, and determine the groove shape and assembly clearance. During welding, reasonably select the welding current, welding rod angle and moving speed. If the weldment has fast heat conduction and large heat dissipation area, it can be preheated before welding or heated with flame at the same time of welding. Long arc preheating can be used at the beginning and joint of the weld before welding, In order to achieve full penetration of the weld, one-sided welding and two-sided forming process should be used as far as possible. In addition, in order to avoid the phenomenon of magnetic bias blowing, the arc should not deviate to one side to ensure uniform heating everywhere.

3.Lamellar tearing

When the welding temperature is below 400 ℃, due to the large thickness of the plate and the high impurity content, especially the high sulfur content, the low alloy high strength steel with strong segregation along the rolling direction of the plate will produce "step" layered cracking along the rolling direction when it is subjected to the force perpendicular to the thickness direction in the welding process.

Preventive measures: reduce the size of welding corner to reduce the volume of weld metal, correctly select Z-direction steel structure, use low hydrogen welding method and welding material with low strength combination, adopt appropriate small heat transfer to reduce heat effect and shrinkage strain, but at this time, cold crack must be prevented.

It is beneficial to control the weld size strictly, adopt small pass and multi pass welding, avoid too large weld leg, preheat properly, but it is necessary to prevent the increase of shrinkage strain, and take intermediate annealing to eliminate stress.

4.Stomata

The main difference between hydrogen pores and nitrogen pores is that hydrogen pores are mainly single pores, while nitrogen pores are mostly dense pores; The main reason of weld porosity is related to the selection, preservation and use of welding materials, the selection of welding process parameters, the cleanliness of groove base metal and the protection degree of molten pool.

Preventive measures: before welding, the oil and rust on the surface of base metal and welding material shall be strictly cleaned, the welding material shall be dried, the welding material shall be correctly selected, the gas source shall be strictly controlled, the protection of welding area shall be strengthened, and the dissolved gas in the molten pool shall be eliminated. Appropriate welding process parameters shall be adopted to optimize the welding process.

Basic knowledge about welded pipe