According to the manufacturing method, steel pipes can be divided into two categories: seamless steel pipes and welded steel pipes. Of these, ERW pipes are the main variety of welded steel pipes. Today, we are discussing two types of steel pipe used as raw material for oil casing: seamless casing and ERW casing.
Seamless steel tubes are tubes manufactured by four methods: hot-rolled, cold-rolled, hot-drawn and cold-drawn. The tube body itself has no welded seams.
ERW (electro welded wire) tubes are straight seam welded tubes manufactured by the high frequency resistance welding process.The raw material for ERW welded tubes, steel plates (coils), are made from low carbon micro alloy steel rolled by the TMCP (thermomechanically controlled process).
1. Outer diameter tolerances
Seamless steel pipes: the sizing is done at around 8000°C using a hot rolling forming process. The raw material composition of the roll, cooling conditions and cooling state has a great impact on its outer diameter, so the outer diameter control is difficult to precise and fluctuate in a wide range.
ERW steel pipes: cold-forming and sizing with 0.6 % reduction. Process temperature is basically constant at room temperature, so the outside diameter control is accurate and fluctuates in a small range, which is conducive to the elimination of black skin buckles.
2. Wall thickness tolerance
Seamless steel pipe: produced by piercing round steel with large wall thickness deviations. Subsequent hot rolling can partially eliminate wall thickness unevenness, but the most advanced equipment can only be controlled within ± 5 ~ 10% t.
ERW steel pipe: hot-rolled coil as raw material, modern hot-rolled thickness tolerances can be controlled within 0.05mm.
The hot rolling process cannot eliminate the outer surface defects of the billets used in seamless steel tubes, and can only be wiped off after the finished product is made. The spiral path left behind after perforation can only be partially eliminated in the wall reduction process.
ERW steel tubes are made from hot-rolled coils. The surface quality of the coil is the surface quality of the ERW steel pipe. The surface quality of hot rolled coils is easily controlled and of high quality.
Seamless steel pipe: hot roll forming process is used. The raw material composition of the tube, the cooling conditions of the roll and the cooling state have a great impact on its outside diameter, so it is difficult to accurately control the outside diameter and fluctuate in a wide range.
ERW steel pipe: cold-formed, so the outside diameter is accurately controlled and the fluctuation range is small.
5. Tensile test
The tensile properties of seamless and ERW steel pipes are in accordance with API standards, but the strength of seamless steel pipes is generally at the upper limit and plasticity at the lower limit. In contrast, the strength of ERW steel pipe indicators in the best state, plasticity indicators than the standard 33.3% higher. The reason for this is the performance of the hot rolled coils of ERW steel raw material through micro-alloy smelting, off-furnace refining, controlled cold rolling and controlled rolling.