Lack of fusion is one of the common defects of ERW pipes, which has a great impact on the strength and performance of steel pipes. According to their characteristics, they can be divided into the following three categories:
(1) Completely unfused, that is, the entire wall thickness of the steel pipe weld is not fused.
(2) Partial lack of fusion, that is, partial lack of fusion of the steel pipe weld
(3) Incomplete fusion means poor weld fusion, mainly cold welding.
The macroscopic appearance of completely unmelted and partially unfused areas is generally black or dark gray, and the microscopic morphology is granular oxide. Cold welding is a kind of curved material that is not completely fused. The macroscopic material is gray or dark gray, and the microscopic material is shallow flat dimples with oxides in the dimples. The diameter of the dimples is small, about 2 microns, and the oxides are relatively small. Small and scattered in shallow flat dimples. Cold welding sometimes coexists with unfusion. At this time, the oxides in the unfusion zone are in the form of mesh, and the cold welding zone is tiny dimples accompanied by a large amount of oxides.
When unfusion or cold welding occurs, it is often accompanied by unreasonable metal streamline distribution.
According to X-ray energy spectrometer and electron probe analysis, the unmelted area contains iron, oxygen and other elements, mainly iron oxide; the oxides in the cold welding area are mainly iron oxide, manganese oxide and silicon dioxide, etc., which is formed during the welding process.
Before the steel pipe is welded, the butt welding surfaces on both sides of the strip need to be machined, so the possibility of residual iron oxide can be ruled out. During the welding process, the main media in contact with the butt welding surface are emulsion and air, that is, there is enough water and oxygen in the environment. When the temperature is greater than 570 degrees Celsius, the following reaction will occur:
2Fe+O2=2FeO
Fe+H2O=FeO+H2
Moreover, in the presence of water, iron oxide forms rapidly. Therefore surface iron oxide can easily form during the welding heating process. If the process parameters are improper or the edge shape of the plate is poor, this oxide will remain in the weld.
It should be pointed out that oxides such as silicon dioxide and manganese oxide also exist on the cold welding fracture. This is because steel generally contains a certain amount of silicon, and the following reactions can be found at high temperatures:
2(FeO)+[Si]=(SiO2)+2[Fe]
As a result of the reaction, stable silica is produced.
Similarly, manganese oxide can be generated at high temperature, manganese oxide, and the basic reaction equation is as follows:
(FeO)+[Mn]=(MnO)+[Fe]
In addition, it can also be opened into manganese oxide-silica or Fe(Mn)O-SiO2 composite oxide. Elements such as Al and Cr remaining in steel pipes have a greater tendency to oxidize at high temperatures, and can also produce Al2O3 and Cr2O3 oxides. After the above oxides are formed, if the welding process parameters are improper, they will remain in the weld, causing poor fusion.
Causes of unfusion defects:
1. Influence of welding heat input and strip edge shape
ERW steel pipes are heated and welded using high-frequency current. During welding, due to the skin effect and proximity effect, the metal on both sides of the butt weld is heated to a very high temperature in a short period of time. Under the action of the extrusion force, the metal on both sides is welded together to form a welded joint. In the high-frequency resistance welding process, higher heating temperature is a prerequisite for the metal on both sides to be combined to form a good welded joint. During normal heating, the welding heat input is sufficient, and the edge of the strip is heated to a sufficient temperature. At this time, regardless of the shape of the edge of the strip, the overall temperature is relatively high, and plastic deformation is easy to proceed. Under the action of extrusion force, both sides The metals bond together easily, and the impurities inside the weld have better fluidity at high temperatures and are easy to remove.
2. Influence of strip edge deformation
When inspecting the outer surface of unfused pipes, we can often find the concave shape of the weld and regularly distributed damage, indicating that the edge deformation of the strip is large. After normal welding, the center line of the metal deformation flow at the weld is basically consistent with the center line of the wall thickness, that is, it is basically symmetrical along the center of the plate thickness. However, when the butt edge of the pipe blank is V-shaped or A-shaped and an unfused pipe appears, the metallographic analysis results of the transverse section of the weld show that the center line of the deformation flow of the weld metal is a distance away from the center line of the wall thickness, and the center line of the metal flow is Offset toward the outer surface, with a maximum offset of 2-3mm, and most of the metal deforms and flows toward the inner surface. The formation of this unreasonable metal streamline shows that there must be a bending force moment from the outer surface to the inner surface at the unfusion formation. During the steel pipe production process, the bending deformation of the edge of the formed tube blank is too large and upward. The pressure of the pressure roller is too large, which causes a concave shape on the outer surface of the weld after the steel pipe is welded. Excessive deformation of the edge of the strip strengthens the tendency to form unfusion defects when the welding heat input is insufficient. Similarly, when the edge of the tube is too small or the pressure of the upper pressure roller is insufficient, internal fusion will occur but the outside will not be fused.
3. Influence of strip steel’s moon-shaped bend
Judging from the weld leg indentations on the outer surface of the unfused pipe weld, it is easy to find the phenomenon of weld torsion, that is, one side of the weld leg is at the weld and the other side is at a certain distance from the weld. This is consistent with the moon-shaped bend of the strip. It has a lot to do with it. The torsion of the weld seam causes the weld leg to shift, which is one of the reasons for insufficient welding heat input and greatly promotes the occurrence of unfusion defects in the weld seam.
4. Influence of strip steel edge level corrosion
Rust on the edge of the strip will affect the conductivity of the contact points between the welding feet and the pipe, which can easily lead to insufficient welding heating and various non-fusion defects.
The impact of unfusion on the performance of ERW pipes:
1. Effect on mechanical properties
Complete non-fusion and partial non-fusion will seriously affect the mechanical properties of the weld due to the loss of strength in the defective part. The larger the size of the non-fusion defect, the greater its impact, and vice versa.
2. Impact on performance
When there are unfusion and cold welding defects in the weld, on the one hand, the effective bearing area of the weld is reduced and the strength of the steel pipe weld is reduced. On the other hand, high stress concentrations will occur at the defects, and if such pipes are put into use, there is a risk of pipeline leakage or rupture accidents.
On the issue of brittle fracture. When there are unfusion and cold welding defects in the weld, they are generally accompanied by poor microstructure, coupled with stress concentration at the defect, so brittle fracture may occur. Brittle fracture occurs when the working stress reaches the critical stress for brittle fracture of the material.
Regarding the issue of fatigue fracture, if there are unfusion defects in the pipeline weld, the defects will undergo fatigue expansion under the action of working stress. The pipe medium is more or less corrosive. Therefore, the pipeline actually faces the problem of corrosion fatigue crack growth. The significant difference between corrosion fatigue and ordinary fatigue is that there is no obvious fatigue limit, that is to say, it is more likely to occur than ordinary fatigue.
Conclusion:
In summary, unfused defects should be addressed as the main issue currently affecting the quality of ERW pipe products. We must first strengthen the ultrasonic flaw detection of welds and catch the first and second types of unmelted steel pipes to improve the reliability of the steel pipes. . At the same time, the development and use of welding processes, automatic control of welding temperature and automatic alarm devices for cold welding of welds should be strengthened to ensure that the third type of unfused defective steel pipes are not produced and the quality of welded pipes is improved.
Read more: Analysis of Common Welding Defects of High Frequency Welded Pipe
