Welding is the construction of large-scale installation of a key, safe operation and manufacturing schedule its quality, efficiency will directly affect the project. Due to the different levels of skilled workers, the welding process varies greatly, there are many shortcomings easily; now finishing the types and causes of defects, to reduce or prevent welding defects, improving the quality of project completion.

First, the weld size does not comply, mainly for the solder wave rough, uneven shape, weld height is too low or too high strengthen, varying the width of the solder wave and fillet unilateral or excessive subsidence etc. The reason is : 1. weldment bevel angle or improper assembly uneven gaps; 2. the welding current is too large or too small, improper welding specification selection; 3. uneven transport speeds, improper welding (or welding the) angle.

Second, crack, mainly for the crack tip shape sharp, severe stress concentration, to withstand alternating and shock loads, static tension greater impact, it is the most dangerous weld defects. Press the causes can be divided into cold cracking, thermal cracking and reheat crack. (Cold crack) means a crack at below 200 ℃ produced, it has a close relationship with the hydrogen, its main causes are: 1. Selection of big thick piece measures preheat temperature and slow cooling after welding is inappropriate; 2. Welding inappropriate material used; 3. welded joints rigid, process is unreasonable; 4. produce brittle welds and nearby tissue; 5. Welding inappropriate choice. (Thermal cracking) refers to crack above 300 ℃ produced (mainly solidification cracking), its main causes are: 1. Effect of ingredients. Welding pure austenitic steels, prone to high nickel alloy and when certain non-ferrous metals. 2. Weld contains more elemental sulfur and other harmful impurities. 3. The welding conditions and joint forms of inappropriate choice. (Reheat cracking) that is stress relief annealing cracks. Refers to the high intensity of the welding zone, due to the post weld heat treatment or high temperature, the intergranular cracks in the heat affected zone generated by its main causes are: 1. Eliminate undue stress annealing heat treatment conditions. 2. Effect of alloy composition. Such as chromium, molybdenum and vanadium boron and other elements have a tendency to increase reheat cracking. 3. welding consumables, welding specification poor choice. The structural design of large stress concentration caused by the irrational.

Third, the pores; the welding process, the hole was too late because of the gas to escape in time and in the weld metal formed by the interior or surface, its causes are: 1. welding, flux drying is not enough; 2. the welding process is not stable enough arc voltage is high, the arc is too long, excessive speed and welding current is too small; 3. filler metal and the base metal surface oil, rust, etc. are not cleaned; 4. point arc melting method did not use backward; 5. Preheat temperature is too low; 6. Not to arc and arc position shifted; 7. protection pads bad, the pool area is too large; 8. AC prone pores, DC reverse the tendency of the smallest pores.

Fourth, the weld; the welding process, the molten metal to flow on the outer weld unmelted metal base material formed by the tumor, it changes the cross-sectional area of the weld, for dynamic load negative. The causes are: 1. Arc too long, the bottom welding current is too large. 2. When the vertical welding current is too large, improper transport strip swing. 3. Weld assembly gap is too big.

Fifth, the crater; weld ending at the obvious lack of meat and depression. The causes are: 1. When welding Crater improper operation, extinction time is too short. 2. send automatic welding wire and the power supply cut off, without first stopping off wire again.

Sixth, the undercut; the base material after arc weld edges melted not add weld metal and leaving a gap. Undercut weakened cross section of the joint force, so reducing the joint strength, causing stress concentration, so that might lead to a breach in the biting edges. The causes are: 1. The current is too large, improper arc is too long, the transport speeds, the arc heat is too high. 2. submerged arc voltage is too low, too high welding speed. 3. The electrode, the inclination angle of the wire is not correct.

Seven slag; the presence of non-metallic inclusions in the weld metal or fusion line inside the site. Slag influence on the mechanical properties, the impact and the number of inclusions and shape. The causes are: 1. When each layer of the multilayer welding slag is not removed. 2. weldment left thick rust. 3. improper physical properties of electrode coating. 4. Defective solder layer shape, bevel angle poorly designed. Too small than 5. molten weld width and penetration, the undercut is too deep. 6. The current is too small, the welding speed is too fast, too late floating slag.

Eight, lack of penetration; the existence of local phenomena not fusion or base metal and weld metal between the base metal. It is generally found in the weld root side welding, very sensitive to stress concentration, fatigue strength and other properties a greater impact. The causes are: 1. poor design bevel angle is small, blunt edge large and small gap. 2. welding rod, wire angle is incorrect. 3. The current is too small, low voltage, welding speed is too fast, the arc is too long, there are magnetic blow like. 4. There are thick rust weldment not clean. 5. Partial submerged arc welding when.