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As there is no need to add filler metal during the welding process of HFW steel pipe, it has the advantage of high welding speed and high production efficiency, and it is widely used in oil and gas transport, oil pipelines, structures and various mechanical pipe and other fields. But HFW steel pipe can be affected by the quality of raw materials and processes, and many other factors, it is difficult to control the production quality, the yield and the welding process still needs to continuously improve.

The impact of raw materials for welding quality is listed as below:

1. Materials smelting method

Using the traditional method of production, for converter steel, N-containing is high, impurity elements is more, cold brittleness is big and sensitivity increases, which makes the welding quality degraded; for boiling steel, deoxidation incompletely, O-containing is high, S and P and other impurities are unevenly distributed, local content will be exceeded, aging sensitivity and cold crisp sensitivity become larger, hot cracking tendency also increases. So now the general pipe materials are smelted by the blowing alkaline converter or electric furnace and processed by vacuum degassing and Ca treated killed steel, and require non-metallic inclusions in steel should generally be less than 2.0, no ribbon fiber structure and no significant segregation.

2. chemical composition of raw materials

Reducing C, S and P content and increasing the ratio of Mn/Si can improve the weldability the strip materials effectively. High C content is too easy to form segregation and banded structure, and reducing the C content can improve the toughness and resistance to HIC (HIC) performance. S is extremely harmful elements in steel, which can significantly undermine the weldability of steel, causing high temperature cracking, and weld porosity and loose phenomenon. With high content of the sulfide, the S inclusions increase and it easily leads to HIC and SSC (Sulfide Stress cracking). And improve the ratio of Mn\si can reduce weld cracking tendency and improve the quality of welded steel pipe.

3 carbon equivalent (Ceq) of steel

The carbon equivalent of steel refers to convert the the content of alloying elements which will affect the hardness, cold cracking and brittleness into the equivalent content of carbon. By the estimation of carbon equivalent steel and cold cracking sensitivity index, it can palay an important guiding role in judging the welding performance of steel.

Different carbon content and carbon equivalent, can be divided into the weldability of steel is easy to weld area, welding area and difficult to weld

Contact zone 3 areas.鈪�(C) is O. Region of 10% to 0.12% or less is easy to weld zone, "(C) less than 0.12% and an n10% Ceq 0_49% of regional embarrassed pads. when Ceq increases, the tendency hardened steel increases, increased hardness, then steel weld heat affected zone (HAZ) is prone to cold cracking, to obtain good HAz tissue, Ceq should be limited to less than 0.45%.