Welding Quality Control Measures of LSAW Steel Pipe

Large diameter LSAW steel pipe with its big wall thickness, good material, stable processing technology, becomes the preferred pipe by largest domestic and foreign oil and gas transportation project. In the large diameter LSAW steel pipe welded joints, weld and heat affected zone are the most prone to produce a variety of defects, and welding undercut, pores, slag, not fused, not penetration, welding cracks are the main form of welding defects, and also are the origins those often make LSAW steel pipe produce quality accident. The quality control measures will be listed below, if you you are interested in the production process of LSAW steel pipe, you can click the hyper-link.

1.The quality control before welding:

1) First of all, pipe manufacturer should take control of the quality of raw materials, after passing the inspection, raw materials can formally enter the construction site. And pipe manufacturer resolutely put an end to the use of substandard steel.
2) The next is the management of the welding material. Checking whether the welding material is a qualified product, whether storage and baking system is implemented, whether the surface of the welding material is clean and no rust, and whether the electrode coating is intact, with mildew or not.
3) The next is the welding area cleaning management. Check the cleaning quality of the welding area, there shall not be water, oil, rust and oxide film and other dirt, which plays an important role to prevent the occurrence of external defects in the weld.
4) Finally, choosing the appropriate welding method, it should be implemented before the test welding, after welding principle.

2. The quality control during welding:

1) According to the welding process to review the specifications of the wire and flux is correct or isn't, to prevent the wrong welding wire and flux caused by welding quality accidents.
2) Monitor the welding environment, when the welding environment is not good (temperature below 0 ℃, relative humidity greater than 90%) pipe manufacturer should take appropriate measures after welding.
3) Checkout the groove size, including the gap, blunt edge, angle and wrong, etc. meet the process requirements or not before pre-welding.
4) Checkout the selected welding current, welding voltage, welding speed and other process parameters are correct or aren't in the submerged arc internal and external welding process.
5) Supervise the welding personnel in the submerged arc internal and external welding full use of the length of the pipe end of the arc plate to strengthen the internal and external welding arc plate use efficiency, which helps to improve the quality of pipe end welding.
6) Supervise the welding staff in the welding is the first clean up the slag, whether it has been completely handle joints, groove whether there is oil, rust, slag, water, paint and other dirt.

Differences between LSAW Steel Pipe and Spiral Welded Pipe

Welded carbon steel pipe includes LSAW (longitudinal submerged arc welded) steel pipe and spiral welded pipe. Some people those do not very understand this industry may not know differences between the former and the latter very clearly. Don’t worry, today we will talk about this topic.

Spiral welded pipe is made by using low carbon construction steel strip or low alloy structural steel strip to roll into a tube according to a certain helix angle (called the forming angle), and then welding the seam together. It can use the narrow band steel to produce steel pipe with large diameter. Spiral welded pipe is mainly used for oil and natural gas pipeline, its specifications are expressed by outer diameter * wall thickness. Spiral welded pipe with also includes single-sided welding and double-sided welding.

LSAW steel pipe is made by using strip steel as raw materials and doing longitudinal welded in the high-frequency welding equipment(hence the name due to the weld forms a straight line). In accordance with the use of different, there are different following production process (can be divided into scaffolding tube, fluid tube, wire casing, bracket tube, fence tube, etc.).

From the production process, the production process of straight seam welded pipe is simple, the production efficiency is high, the cost is low, and it develops rapidly. But the strength of the spiral welded pipe is generally higher than that of the LSAW steel pipe. Spiral welded pipe with larger diameter can be produced with the narrow billet, and the pipe with different diameter can also be produced with the same width. However, compared with the same length of the LSAW steel pipe, the weld length of spiral welded pipe increased by 30 to 100%, and the production rate of it is low.

From the welding process, the welding method of spiral welded pipe and straight seam welded pipe is consistent. But the straight seam welded pipe will inevitably have a lot of T-shaped welds, so the probability of appearing welding defects are greatly improved. And the welding residual stress at TIG welding is large, the weld metal is often in three-dimensional stress condition, which increases the possibility of producing crack. Moreover, according to the submerged arc welding process, each weld should have starting arc place and arc blowout place, but each LSAW steel pipe can not achieve the conditions in the welding ring, which may cause there are more welding defects in the arc blowout place.

Technical Requirement for Welded Steel Pipe

According to the API/ASTM stadnards, there are many technical requirements for producing welded steel pipe. 

The nominal pipe diammeter ranges from 6-150mm, nominal wall thickness ranges from 2.0-

6.0mm and the length of the pipe is usually from 4 to 10. The pipe surface shoule be smooth, no collapse, no cracks, no delamination, and no lap welding defects exist. The wall thickness is not allowed to over the deviation, no scratches, no weld dislocation, no burns and scarring and other minor defects. 

Welded steel pipe should perform mechanical test, flattening test and flaring test to achieve the standard requirements. 2.5 Mpa internal pressure test is allowed to keep over 1 minue without leakage. It allows use eddy current testing method instead of the hydrostatic test. After the eddy current testing, welded steel pipe will be cut to a predetermined length by flying saw and then flip-off from the assembly line. Then both ends of the pipe should be flat head chamfering, painting, marking and hexagonal packing before delivery.

High frequency straight seam welded steel pipe

High frequency straight seam welded steel pipe also know as straight seam resistance welding pipe. The outside diameter is small, usually is under DN 600. For the outside diameter is larger than DN 600, the production process will use double sided submerged arc welding process. The high frequency straight seam welded steel pipe is mainly used in water engineering, petrochemical industry, chemical industry, power industry, agriculture irrigation and urban construction. For the transportation of liquids, the pipe can be applied for water supply, drainage. For gas transportation, the pipe can be used for transmission gas, steam, liquefied petroleum gas. For structural purposes, the pipe can be used as piling pipe fro bridges, piers, roads, buildings and other structures. 

The characteristics for high frequency straight seam welded steel pipe is below:
1.  the welding speed is large.
2. the welding area is small.
3. clean up is not necessary after welding
4. the welding process can produce thin walled tube, and also metal pipe.

The high frequency straight seam welded steel pipe process is below:
Slitting---Uncoiling---Strips Leveling---Head and Tail Cutting---Strips Welding---Looper Accumulator---Forming---Welding---Burr Removing---Sizing---Testing---Cutting---Inspection---Straightening---Processing---Hydrostatic Test---Flow Detection---Coating and Marking---Finished.

Heat Treatment Process of LSAW Steel Pipe

First, reasonable selection. Straight pipe joints should choose a good micro deformable material straight seam steel pipe steel, for carbide segregation serious straight seam steel casting steel should be reasonable and transfer of heat treatment, larger and unable to cast a straight seam steel pipe steel may be solution bis refined heat treatment. Reasonable choice of heating temperature control heating rate, on a straight seam steel pipe can be heated slowly, preheat and other ways to reduce heating balanced straight seam steel pipe heat deformation.

Second, the correct heat treatment process operation and reasonable heat treatment process is also a useful way to reduce the distortion of straight pipe joints. Straight seam steel pipe deformation reason is often messy, but we only need to master the rules of the deformation analysis of the onset of the reason, the choice of avoiding abnormal deformation method of straight pipe joints can be reduced, and also can be controlled.

Third, straight seam steel pipe to be pre-heat treatment to eliminate the machining process of the remaining episodes of stress. For straight seam steel pipe, in the case of the conditions allow, try to use cryogenic vacuum heat treatment and quenching after quenching. In ensuring straight seam steel hardness of the premise, try to use pre-cooling, grading cooling quenching or austempering process.

Fourth, straight seam steel pipe planning portrayed to be reasonable, not too poor thickness, shape to symmetrical, straight pipe joints on large deformation deformed to master the rules, to reserve allowance, on a large, straight seam steel pipe can be used in combination plan. For some precision messy straight seam steel pipe can be used pre-heat treatment, aging heat treatment, quenching and nitriding heat treatment to control the accuracy of straight pipe joints. When the patch straight seam steel trachoma, porosity, wear and other defects, the choice of cold welding and other small heat-affected patch repair equipment, to prevent the onset of deformation process.

Cause Analysis of LSAW Steel Pipe Surface Crack

Surface microcracks of lsaw steel pipe impact on product quality greatly, ranging from commuted, while in retirement, especially niobium, vanadium, titanium Longitudinal most prominent, has seriously affected the improvement of product quality and finished product rate. This not only gives the enterprise has brought huge economic losses, but also restricts the development of varieties of plate production line. Longitudinal surface microcracks mainly has edge crack, longitudinal, star cracks, its causes both improper rolling process parameter control, there are abnormal production of continuous casting process, the slab cracks genetic, residual element enrichment.

Longitudinal surface microcracks severely affect product quality. Longitudinal surface micro cracks main reason is the continuous casting mold copper coating off. This aspect will lead to copper penetration into the solidified shell austenite grain boundaries, grain boundary cracks; on the other hand will be uneven due to thermal stress cracks and crack sensitivity containing titanium steel strong performance is particularly evident . Control measures: To improve the quality of mold copper, copper to prevent dew; strengthen inspection before production of slab quality and production after Longitudinal; Longitudinal protection through good casting and other measures to improve Longitudinal steel cleanliness.

Pre-welding process for LSAW Steel Pipe

LSAW steel pipes (UOE, JCOE) manufacturing processes, welding processes are set to continuous advance. Its role is to be finalized after the tube forming, shaping and positioning within the weld seam on the back cover, in order to ensure the pipe dimensions and is conducive to tube welded pipe to improve quality and production efficiency.

Pre-welding process
After forming the tube by the next roller conveyor transported to a pre-welder's torch, rising rotating roll, rotate to adjust an open position to weld at the top of both sides of the pressure on the pressure roller and the pressure roller so that the tube seam, drop gun and adjust the tracking of the weld seam, open the protective gas and cooling water, start welding, welding condition monitoring, welded to the tube end welding arc stops (automatic delay stop gas), lift the gun and the top pressure rollers, pipe welder pre sent , pipe ends and welding defects.

Pre-welding process characteristics
Pre-welding using Ar + CO2 mixed gas shielded arc welding, continuous welding seam on the back of the tube to move at a faster speed. Since the bevel angle, tube shape, the tube along the longitudinal direction of the opening and both sides of the plate edge distortion factors of inequality, forming the seam after seam quality is not stable, larger changes. Need to change the tube seam welding, the arc of a steady state, the weld position deviated from the case and other real-time tracking and adjusted accordingly. Thus presoldered have welding speed, impact seam quality factors, changes and adjustments randomness and other characteristics.

Welding Technology of LSAW Steel Pipe

Molding method for manufacturing large diameter lsaw steel pipe are UOE molding method, a row of roll forming method (CFE), CE molding. For the vast majority of the final steps of the molding process is to conduct a post-weld tube expanding full-length, in order to improve the quality of welded steel pipe shape. Expanding become large-caliber Longitudinal production to ensure the quality of the finished tube an important process.
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Expanding is a use of hydraulic or mechanical means from urging the steel pipe along the pipe wall bulged radially outward pressure molding process. Mechanical than hydraulic mode, the device is simple and more efficient, the world's most advanced large-caliber Longitudinal several enlarged diameter pipeline system processes are adopted, the process is as follows: Mechanical expanding the use of the split-side portion of the enlarged diameter of the segments radially expand, so the tube along the longitudinal direction in a stepwise manner to achieve the full length of the tube segments of plastic deformation process. It is divided into five stages

1. Initial stage full circle. Segments open until all the segments are exposed to the pipe wall, this time within the range of steps at various points within the steel tube radius are almost identical, steel preliminary full circle.

2. The nominal inside diameter of the stage. Segments from the front position began to decrease velocity, until you reach the desired position, this position is within the quality requirements of the finished pipe circumferential position.

3. Springback compensation stage. Segments in position to start the second stage further low speed until you reach the desired position, this position is within the missile complex process design requirements of previous pipe circumferential position.