The welding quality defects of ERW welded pipes include cracks, lap welds, leaks, scratches, etc. Only the two main defects of cracks and lap welding are analyzed below:
Cracks are the main defects of welded pipes, and their manifestations can include ordinary cracks, local periodic cracks, and irregular intermittent cracks. There are also steel pipes with no cracks on the surface after welding, but cracks appear after flattening, straightening or hydraulic testing. When the cracks are severe, water leaks. There are many reasons for cracks. Eliminating cracks is one of the most difficult problems in welding adjustment operations.
The following analyzes are carried out from the aspects of raw materials, forming and welding pass patterns and process parameter selection.
1. Raw materials
(1) Steel type, that is, the chemical composition of the steel has a significant impact on the welding performance. The chemical elements contained in the steel affect the welding performance to a greater or lesser extent, for better or worse. Due to high welding temperature and large extrusion force, high-frequency welding has a wider chemical range than low-frequency welding, and can weld carbon steel, low-alloy steel, etc. Carbon steel mainly contains five elements: carbon, silicon, manganese, phosphorus and sulfur. Low alloy steel can also contain various elements such as manganese, titanium, vanadium, aluminum, and nickel.
The effects of various elements on welding performance are described below.
1) Carbon As the carbon content increases, the welding performance decreases, the hardness increases, and it is prone to brittleness. Mild steel is easy to weld.
2) Silicon Silicon reduces the weldability of steel, mainly because it easily generates SiO2 inclusions with low melting points; it increases the amount of slag and molten gold.
The fluidity of the genus causes serious splashing, thus affecting the quality.
3) Manganese Manganese increases the strength and hardness of steel, reduces welding performance, and easily causes brittle cracking.
4) Phosphorus Phosphorus is detrimental to the weldability of steel. Phosphorus is the primary cause of blue brittle.
5) Copper When the copper content is less than 0.75%, it does not affect the weldability of steel. When the content is higher, the fluidity of the steel will increase, which is not conducive to welding.
6) Nickel Nickel has no significant adverse effect on the weldability of steel.
7) Chromium Chromium reduces the welding performance of steel, high melting point oxides
Difficult to exclude from welds.
8) Titanium Titanium can refine the grains, increase the welding performance of steel, and make the fluidity of steel worse and the viscosity higher.
9) Sulfur Sulfur causes hot cracking of welds. During the welding process, sulfur is easily oxidized and the generated gas escapes, resulting in many pores and looseness in the weld. Sulfur is not conducive to welding and reduces the mechanical properties of steel. Sulfur in steel is usually limited to specified trace amounts.
10) Vanadium Vanadium can significantly improve the welding performance of ordinary low alloy steel. Vanadium can refine the grains, prevent the grains in the heat-affected zone from growing and coarsening, and can fix part of the carbon in the steel and reduce the hardenability of the steel.
The influence of aluminum on the welding performance of steel depends on the aluminum content in the steel. Generally speaking, the aluminum remaining in the steel after deoxidation has little effect on the welding performance. If the amount added as an alloying element is large, When, it has a similar effect to silicon, reducing the welding performance of steel.
12) Oxygen Oxygen is regarded as a harmful element in steel. Higher oxygen content will form more FeO remaining at the weld during welding, thus reducing the welding performance.
13) Hydrogen Hydrogen is the cause of cracking.
14) Niobium: Adding 0.005~0.05% niobium to steel can increase the yield strength and impact toughness and improve the welding performance.
15) Zirconium Zirconium can improve the density of welding metal.
16) Lead Lead has no significant effect on the welding performance of steel.
The influence of various elements in a certain steel on the comprehensive welding performance of the steel is measured by carbon equivalent. The upper limit of carbon equivalent is 0.65~0.70%. If the upper limit is exceeded, the weld will be easily brittle, the hardness will increase, the welding quality will be poor, and the flying saw will cut off and cut off.
(2) The components of strip steel are segregated, especially the heads of B2F and B3F strip coils, which are usually the heads of boiling steel ingots. The composition segregation is serious, containing more impurities and oxides, and the welding performance is poor. Therefore, the same coil of strip steel is prone to cracks at the head.
(3) There are interlayers in the strip, especially edge interlayers, which are one of the main causes of cracks.
(4) The strip width is not enough. For strips exceeding the negative deviation, the extrusion and rolling holes cannot be filled, and the welding pressure is insufficient or even no pressure, causing cracks.
(5) The thickness of the strip changes greatly, and the welding current suddenly increases and decreases, causing cracks.
(6) The edges of the steel strip are irregular, jagged, missing flesh, or torn, and irregular intermittent small cracks appear after welding.
(7) Due to the excessive disc knife gap or serious blade wear during slitting, the strip edge burrs are too large, and cracks are easily caused when the burrs are outward during forming. In order to prevent cracks from occurring, tilt the strip upward in the opposite direction when uncoiling, so that the burrs face inward during forming.
2. Forming and welding pass patterns
1) The guide ring of the molded closed hole is damaged, causing edge gnawing and cracks.
2) The bearing of the squeeze roller is damaged, resulting in insufficient welding pressure and prone to cracks.
3) The extrusion roller pass design is unreasonable, or the wrong pass shape is used to form the strip into a peach shape. The welding pressure is large on the inside and small on the outside, and cracks are prone to occur.
4) The processing of the extrusion roller is unqualified, and the upper and lower bearing tables, the intermediate shaft hole and the outer circular hole are not concentric. When the non-concentricity is relatively large (0.10~0.20 mm), the extrusion roller rotates eccentrically, causing unstable extrusion force and prone to cracks.
5) The hole pattern of the extrusion roller is seriously worn, causing the upper pressure to decrease and form a peach shape, and cracks appear after welding.
3. Process parameter selection
1) The welding speed is too high, resulting in low temperature and cracks.
2) The welding speed is too low, causing the temperature to be too high, which is prone to overburning and easy to crack after being flattened. In severe cases, pores may even appear in the weld.
3) The pressure is low, and inclusions with low melting points are not easy to extrude. Moreover, the bonding force between molten metals is small due to low pressure, and cracking is likely to occur when the weld is stressed.
4) If the welding temperature is low and the welding fails, the speed should be reduced.
5) The welding temperature is too high and the current is too large, which may easily cause overburning or even pores and oxides. The speed should be increased.
6) The position of the electrode or sensor is inappropriate and too far from the center line of the extrusion roller, resulting in large heat consumption, low temperature, and poor welding quality.
7) The opening angle is too small, resulting in unstable welding current, small blasts at the lintel to form blisters, and welding cracks.
B) Lap welding
Lap welding is when the two edges of the tube blank are misaligned during welding. Even if the burrs are removed, traces of misalignment can still be seen. The reasons are:
1) The molding quality is not good. When the two edges of the formed tube blank are uneven, there will be common overlap welds. When the edges of the formed tube are wavy and bulging, there will be local small overlap welds.
2) When producing thin-walled pipes, improper extrusion force can easily cause premature failure. Installing a small supporting roller inside the central tube body of the extrusion roller can effectively overcome the lap welding of thin-walled tubes.
3) The extrusion roller is installed unevenly, one is higher and the other is lower, causing lap welding.
4) The edge of the strip is poorly slit, the edge is curled or the edge burrs are too large, resulting in lap welding.
5) The rolling center lines of the forming vertical rollers, horizontal rollers and extrusion rollers are not in a straight line, or the strip steel has a sickle curve, resulting in unstable forming, twisted welds, and lap welding.
6) When the front end of the resistor is exactly at the center line of the extrusion roller, the expansion strength and flattening strength values exceed the center line of the extrusion roller. When it extends to the side of the sizing machine, the expansion strength and flattening strength are at their best. The flattening strength decreased significantly. When it is not on the center line but on one side of the molding machine, the welding strength is also reduced.
The influence of edge steel shape on welding quality:
1. Non-metallic inclusions
Non-metallic inclusions formed due to segregation during ingot casting are exposed to the edge of the strip after hot rolling and slitting. During form welding, they affect the welding performance and reduce the welding quality.
2. Edge sandwich
The shrinkage cavities and loose structures formed during ingot casting will remain at the edge of the strip after hot rolling and slitting, and interlayers can be clearly seen or inconspicuous interlayers, which will affect the welding quality and cause cracks during welding.
3. Residual oxides
A large amount of oxide remains on the edge, which cannot be completely removed from the weld during welding and remains inside the weld, affecting the welding quality.
Tips: ERW steel pipe is formed by rolling strip and welding the seam, with tighter dimensional tolerances and less weight. The weld seam is heat treated after welding that no untempered martensite remains, and the weld flash can be removed from both inner and outer surfaces. ASTM A53 ERW steel pipe is a typical carbon steel pipe. It is largely used to convey fluids at low / medium pressures such as oil, gas, steam, water, air and also for mechanical applications.