Comparative Analysis Of Seamless Casing And ERW Casing

Steel pipes can be divided into two categories according to the manufacturing method: seamless steel pipes and welded steel pipes. Among them, ERW steel pipes are the main type of welded steel pipes. 

The main discussion is about two types of steel pipes used as raw materials for oil casing: seamless casing and ERW casing.

Seamless casing - casing made of seamless steel pipe as raw material.

Seamless (Seamless) pipe refers to steel pipes made by four methods: hot rolling, cold rolling, hot drawing, and cold drawing. The pipe body itself does not have any welds.

ERW casing - casing made of electric welded pipe as raw material.

ERW (Electric Resistant Weld) pipe refers to a straight seam welded pipe made by the high-frequency resistance welding process. The raw steel plate (coil) of ERW welded pipe is low-carbon micro-alloy steel rolled by TMCP (Thermo-Mechanical Control Process).

1. Outer diameter tolerance: 

Seamless steel pipe: A hot rolling forming process is adopted, and its sizing is completed at about 8000C. The raw material composition, cooling conditions, and cooling state of the rolls of the steel pipe have a great influence on its outer diameter, so it is difficult to accurately control the outer diameter and the fluctuation range is large.

ERW steel pipe: cold bending forming is adopted, and the sizing is completed by 0.6% diameter reduction. The process temperature is constant at room temperature, so the outer diameter control is accurate and the fluctuation range is small, which is conducive to eliminating black buckles;

2. Wall thickness tolerance:

Seamless steel pipe: round steel perforation is used for production, and the wall thickness deviation is large. The subsequent hot rolling can partially eliminate the uneven wall thickness, but the most advanced unit can only control it within ±5~10%t.

ERW steel pipe: A hot-rolled coil is used as raw material, and the thickness tolerance of modern hot rolling can be controlled within 0.05mm.

3. Appearance:

The outer surface defects of the billet used in seamless steel pipes cannot be eliminated by the hot rolling process, and can only be polished off after the finished product is completed; for the spiral track left after perforation, only part of it can be eliminated during the wall reduction process.

ERW steel pipe uses hot-rolled coil as raw material. The surface quality of the coil is the surface quality of the ERW steel pipe, and the surface quality of the hot-rolled coil is easy to control and has high quality, so the surface quality of the ERW steel pipe is much better than that of the seamless steel pipe.

4. Ovality:

Seamless steel pipe: the hot rolling forming process is adopted. The raw material composition, cooling conditions, and cooling state of the roll of the steel pipe have a great influence on its outer diameter, so the outer diameter control is difficult to be accurate, and the fluctuation range is large.

ERW steel pipe: cold bending is adopted, so the outer diameter control is accurate and the fluctuation range is small.

5. Tensile test:

The tensile performance indexes of seamless steel pipes and ERW steel pipes meet the API standard, but the strength of seamless steel pipes is generally at the upper limit and the plasticity is at the lower limit. In comparison, the strength index of ERW steel pipes is in the best state, and the plasticity index is 33.3% higher than the standard. The reason is that the raw material of ERW steel pipes-hot-rolled strip coils is guaranteed by micro-alloying smelting, refining outside the furnace, and controlled cooling and rolling; seamless steel pipes mainly rely on increasing carbon content, which is difficult to ensure reasonable matching of strength and plasticity.

6. Hardness:

The raw material of ERW steel pipes-hot-rolled strip coils has extremely high precision in controlled cooling and rolling during the rolling process, which can ensure the uniform performance of each part of the strip coil.

7. Grain size:

The raw material of ERW steel pipes-hot-rolled strip coils use wide and thick continuous casting billets, with thick fine-grained surface solidification layers, no columnar crystal areas, shrinkage cavities and looseness, small composition deviation, and dense structure; in the subsequent rolling process, the application of controlled cooling and rolling technology further guarantees the grain size of the raw materials.

8. Anti-collapse test:

ERW steel pipe has the characteristics of raw materials and pipe-making process. Its wall thickness uniformity and ovality are far better than seamless steel pipes, which is the main reason why its anti-collapse performance is higher than that of seamless steel pipes.

9. Impact test:

Since the impact toughness of the ERW steel pipe parent material is several times that of seamless steel pipes, the impact toughness at the weld is the key to ERW steel pipes. By controlling the impurity content of raw materials, the height and direction of longitudinal shear burrs, the shape of the forming edge, the welding angle, the welding speed, the heating power and frequency, the welding extrusion amount, the medium frequency annealing temperature and depth, the length of the air cooling section and other process parameters, the impact work of the weld is guaranteed to reach more than 60% of the parent material. If further optimized, the impact work of the weld can be close to the parent material, thereby achieving seamless performance.

10. Burst test:

The burst test performance of ERW steel pipe is far higher than the standard requirements, mainly due to the high wall thickness uniformity and uniform outer diameter size of ERW steel pipes.

11. Straightness:

Seamless steel pipes are formed in a plastic state, and the straightness of the pipe ends is relatively difficult to control with a single length (3-4 lengths for continuous rolling);

ERW steel pipes are cold-processed and have online straightening in the state of diameter reduction, they have infinite lengths, so the straightness is better.

12. Steel consumption of 10,000-meter-long casing: 

The wall thickness of ERW steel pipes is uniform, and its wall thickness tolerance can be ignored, while the control accuracy limit of the wall thickness difference of seamless steel pipes is ±5%t, which is generally controlled at ±5-10%t. To ensure that its minimum wall thickness can meet the standard requirements and performance, the only solution is to increase the wall thickness appropriately. Therefore, for casings of the same specifications and weight, ERW steel pipes are 5-10% longer than seamless steel pipes, or even more, which reduces the steel consumption of 10,000-meter-long casing by 5-10%. Even at the same price, ERW casings can save users 5-10% of procurement costs.