Influence factors on impact toughness of erw welded pipe

Influence factors on impact toughness of erw welded pipe 

With the rapid development of oil and gas industry, the world oil and gas pipeline construction has entered a period of rapid development. With the continuous improvement of oil and gas pipeline pressure, higher requirements are put forward for welding quality of oil and gas pipeline steel pipe. The following factors affect the impact toughness of ERW welded pipe weld:

1. Influence of pipeline steel raw materials on weld impact toughness

1.1 Chemical Composition

The main components of pipeline steel are Fe, C and alloying elements, and there are impurities such as S and P, among which C and S have the greatest influence on impact toughness. With the increase of C content, the content of pearlite in steel increases correspondingly, because pearlite contains a lot of brittle lamellar cementite, so the impact toughness is poor. Therefore, low carbon steel is better than high carbon steel impact toughness. S often exists in the form of compound FeS in steel, and forms a low melting point (985℃) FeS+S binary co-crystal with Fe distributed at the grain boundary. Therefore, when the steel is hot-worked at 1000 ~ 1 200℃, the steel cracks along the grain boundary due to the melting of the low melting point co-crystals on the grain boundary. Part of P is dissolved into ferrite in steel, which increases the strength and hardness of ferrite, decreases the plasticity and toughness, and partly forms compounds with great brittleness, so that the plasticity and toughness of steel at room temperature decrease sharply. Si is mostly dissolved in ferrite to strengthen it, thus improving the strength, hardness and elastic limit of steel, and correspondingly reducing the plasticity and toughness. Mn dissolved in ferrite reduces the brittleness of steel, and the impact toughness of steel can be improved when the mass fraction of Mn is 1% to 1.5%.

1.2 inclusion

Inclusions in steel reduce the impact toughness of steel. The oxide inclusion has a great influence on the fatigue strength of steel. When the melting point of the weld metal is lower than that of the oxide inclusion, the oxide is difficult to discharge and remains in the weld.

2. Influence of welding process on impact toughness of weld

The influence of welding process on welding quality is shown in adjusting heat input energy, welding extrusion pressure, welding speed, opening Angle, shape of tube blank edge, placement of electrode or inductor and impedance device, etc. High frequency welding metal streamlines can directly reflect the overall situation of welding process.

2.1 fusion line

The fusion line is a bright line formed when the two edge metals are heated to a high temperature (≥1400 ℃) at the same time during welding, and the surface C element is burned or the C-rich liquid phase is extruded. After hot corrosion of 5% picric acid solution, the color was bright white. The shape, state and width of fusion line are closely related to the level of welding line energy, the size of forming extrusion pressure, welding speed and other parameters, which are important indicators to measure the quality of welding.

2.2 "Waist drum" heat-affected zone

The shape and width of the heat affected zone of the weld are not only the reference basis for determining the energy of the welding line, but also the important basis for determining whether other welding process parameters are suitable. The higher the welding line energy, the greater the heat influence of the base metal, and the "waist drum" heat influence zone becomes wider, and vice versa. When the welding line energy is fixed, the slower the welding speed is, the slower the heat diffusion is, and the "waist drum shape" will also become wider, and vice versa. When the welding line energy and welding speed are relatively reasonable and stable, if the extrusion pressure is too small, a large number of molten metal is not extruded, the "waist drum" will also become wider, on the contrary, it will be smaller.

2.3 Metal streamline shape

Metal flow line is one of the most important display characteristics in various forms of weld parts. It is a kind of special shape crystalline structure formed by partial melting or semi-melting metal under certain conditions under the action of pressure, and it is also a comprehensive embodiment of the magnitude and direction of welding extrusion pressure, line energy and welding speed.

2.4 Center distance is too large

Center distance refers to the distance between the center of the metal streamline and the center of the roll wall thickness. In addition to the unsteadiness of the extrusion force, the parallelism of the plate edge also affects it.

(1) When the extrusion pressure is not adjusted properly, the center of the metal streamline is either up or down, and it is difficult to match the thickness center of the plate wall. The greater the center distance, the more uneven the seam gap, the easier it is to produce "wrong edge" at the weld, and the "oxide" is difficult to extrude, and the worse the welding quality. When the center distance is close to zero, the welding point can be close to the center of the extrusion roller, the extrusion pressure distribution is uniform, and the welding quality can reach the best state.

(2) When the plate edge is not parallel, it is also easy to cause the center distance is too large, it is difficult to do parallel joints, local solder joint thermal effect may be reduced, low-temperature welding state, weld "oxide" is difficult to exclude, the tendency to form "gray spot" increases.

3. Influence of heat treatment process on impact toughness of weld

Some research data show that heating temperature is more important than heating time in the influencing factors of heat treatment on weld impact toughness.
The impact toughness will decrease when the temperature is too high. The heating time of 650 -- 950℃ should be varied in the range of 12 -- 37s. When the maximum heating temperature is 950℃, the heating rate from 650 to 950℃ is not sensitive to the temperature rise. Although the decrease of heating rate can reduce the temperature difference between inside and outside the tube wall and fully homogenize the microstructure, too low heating rate may cause grain growth and reduce the weld toughness.

4 conclusion

(1) Improving the purity of raw materials is the prerequisite for ensuring the impact toughness of ERW welded pipe welds.

(2) Reasonable and correct welding process is an important method to improve the impact toughness of ERW welded pipe welds.

(3) Heat treatment is also one of the reasons affecting the impact toughness of ERW welded pipe weld.