What will affect the impact toughness of erw pipe?

What will affect the impact toughness of erw pipe?

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

1. The influence of pipeline steel raw materials on the impact toughness of welds

1.1 Chemical composition

The main components of pipeline steel are Fe, C and alloy elements, and also contain impurities such as S and P, among which C and S have the greatest impact on impact toughness. With the increase of C content, the content of pearlite in steel also increases accordingly, because pearlite contains a large amount of brittle flaky cementite, so the impact toughness is poor. Therefore, the impact toughness of low carbon steel is better than that of high carbon steel. Some often exist in the steel in the form of composite FeS, and distribute with Fe at the grain boundary to form a low melting point (985°C) FeS+S binary eutectic. Therefore, when the steel is hot-worked at 1000-1200°C, due to the eutectic melting of the low melting point on the grain boundary, the steel cracks along the grain boundary. Part of P dissolves into ferrite in steel, which increases the strength and hardness of ferrite, reduces plasticity and toughness, and partially forms brittle compounds, which makes the plasticity and toughness of steel drop sharply at room temperature. Si is mainly dissolved in ferrite to strengthen it, thereby increasing the strength, hardness and elastic limit of steel, and correspondingly reducing plasticity and toughness. Mn dissolved in ferrite reduces brittleness, and when the mass fraction of Mn is 1% to 1.5%, the impact toughness of steel can be improved.

1.2 Inclusions

Inclusions in steel reduce the impact toughness of steel. Oxide inclusions have a great influence on the fatigue strength of steel. When the melting point of the weld metal is lower than that of the oxide inclusions, the oxides are difficult to discharge and remain in the weld.

2. The influence of welding process on the impact toughness of weld

The influence of the welding process on the welding quality is reflected in the adjustment of heat input energy, welding extrusion pressure, welding speed, opening angle, edge shape of the tube blank, placement of electrodes or inductors and impedance devices, etc. Reflect the overall situation of the 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°C) at the same time during welding, and the C element on the surface is burned or the C-rich liquid phase is extruded. It is bright white after being etched by 5% picric acid solution. The shape, state and width of the fusion line are closely related to parameters such as the energy of the welding line, the size of the forming extrusion pressure, and the welding speed, and are important indicators to measure the welding quality.

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 weld energy, but also an important basis for determining whether other welding process parameters are appropriate. The higher the weld energy, the greater the heat influence of the base metal, and the wider the heat-affected zone of the "waist drum", and vice versa. When the welding wire energy is constant, the slower the welding speed, the slower the heat diffusion, and the wider the "waist drum", and vice versa. When the welding wire energy and welding speed are relatively reasonable and stable, if the extrusion pressure is too small, a large amount of molten metal cannot be squeezed out, and the "waist drum" will also become wider, otherwise it will become smaller.

2.3 Metal streamline shape

Metal flow lines are one of the most important display features in all forms of welded parts. It is a crystal structure of a special shape formed by partial melting or semi-melting of metal under certain conditions under pressure, and it is also a comprehensive reflection of the size and direction of welding extrusion pressure, line energy and welding speed.

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