The process flow of ultrasonic testing for submerged arc welded spiral steel pipes and straight seam welded pipes.
2014-07-04
Welded steel pipes, also known as welded pipes, are steel pipes made by welding steel plates or strips that have been rolled into shape. The production process of welded steel pipes is simple, with high production efficiency, a wide variety of specifications, and low equipment investment, but their general strength is lower than that of seamless steel pipes. Since the 1930s, with the rapid development of high-quality strip steel continuous rolling production and advancements in welding and inspection technology, the quality of weld seams has continuously improved. The variety and specifications of welded steel pipes have increased significantly and have replaced seamless steel pipes in more and more fields. Welded steel pipes are divided into straight seam welded pipes and spiral welded pipes based on the form of the weld seam.
The production process of straight seam welded pipes is simple, with high production efficiency and low cost, leading to rapid development. Spiral welded pipes generally have higher strength than straight seam welded pipes; they can produce larger diameter welded pipes from narrower raw materials and can also produce different diameter welded pipes from raw materials of the same width. However, compared to straight seam pipes of the same length, the length of the weld seam increases by 30-100%, and the production speed is lower. Therefore, smaller diameter welded pipes mostly use straight seam welding while larger diameter ones mostly use spiral welding. Although the quality produced by a production line for welded steel pipe is guaranteed, some unavoidable minor issues may arise; thus, non-destructive testing before leaving the factory is essential and can be said to be crucial.
Common defects in general steel pipe weld seams include: porosity, slag inclusion, lack of fusion, lack of penetration, and cracks. So far there has not been a mature method for accurately assessing the nature of these defects; instead, comprehensive assessments are made based on the shape of defect waves obtained on a fluorescent screen combined with changes in reflection wave height along with defect location and welding process. The assessment of internal defect nature as well as causes for defects and preventive measures can be summarized as follows:
Porosity
In spiral steel pipe welds, individual porosity echo heights are low with a single wave pattern that is relatively stable. When detected from various directions, reflection waves are generally similar but disappear upon slight movement of the probe; dense porosity may show clusters of reflection waves whose heights vary with pore size. When rotating at a fixed point with the probe, an alternating phenomenon occurs. The main causes for this type of defect include: welding materials not dried at specified temperatures; deterioration or shedding of electrode coating; rusting of core wire; unclean cleaning before manual welding; excessive current during manual welding or too long an arc; excessive voltage or significant fluctuations in network voltage during submerged arc welding; low purity protective gas during gas shielded welding etc. If porosity exists in spiral steel pipe welds it not only damages metal density but also reduces effective cross-sectional area leading to decreased mechanical properties—especially when chain-like pores exist which significantly reduce bending and impact toughness.
Preventive measures against such defects include: not using electrodes that have cracked coatings or deteriorated due to rusting; rusty wires must be derusted before use. Welding materials should be dried at specified temperatures; clean both sides around joints thoroughly while selecting appropriate welding current, arc voltage and speed.
Slag Inclusion
The echo signal from point slag inclusion in steel pipe resembles that from point porosity while strip slag inclusion echoes often show serrated waveforms with lower amplitudes resembling branched shapes where small peaks appear beside main peaks—amplitude varies when moving probes across different directions during detection. Causes for this type include: insufficient welding current or excessive speed preventing slag from floating up properly due to unclean edges on base metal or layers being joined together along with inappropriate chemical compositions containing high sulfur or phosphorus levels etc. Preventive measures involve: correctly selecting appropriate welding currents ensuring joint angles aren’t too small cleaning joints thoroughly before starting multi-layer welds ensuring slag removal between layers while rationally choosing travel angles speeds etc.
Lack Of Penetration
High reflectivity along with higher amplitude stability occurs when moving probes across both sides yielding roughly similar reflection amplitudes during detection at either side around weld seams. This type not only reduces mechanical performance at joints but also creates stress concentration points at gaps where lack-of-penetration occurs leading often to cracks under load making it a dangerous defect.|Common causes include: excessively small gaps between joint edges insufficiently low currents too fast travel speeds small joint angles incorrect travel angles along with arc blow etc Preventive measures involve: rational selection regarding joint types assembly gaps employing correct processes etc.
Lack Of Fusion
When moving probes reflectivity remains stable yet amplitudes differ between sides sometimes detectable only from one side due to reasons like unclean grooves overly fast speeds insufficiently high/low currents incorrect electrode angles arc blow etc Preventive measures involve: correctly selecting groove types currents ensuring cleanliness around grooves proper operation preventing misalignment etc.
Cracks
Echo heights are relatively large exhibiting wide amplitudes showing multiple peaks reflecting continuous variations upon probe movement while peaks shift up/down when rotating probes indicating presence thereof Cracks represent one among most dangerous defects reducing strength at joints additionally creating stress concentrations due sharp notch-like ends after loading resulting structural failures Cracks can be categorized into three types: hot cracks cold cracks reheat cracks Hot crack causes stem from rapid cooling rates within molten pools causing segregation uneven heating generating tensile stresses.
Thus detecting flaws within spiral steel piping proves critical however it’s paramount to implement preventive measures beforehand minimizing defective products emerging thereby reducing unnecessary losses Preventive strategies entail limiting segregation-prone elements harmful impurities within base/welding materials primarily restricting sulfur content increasing manganese levels enhancing alkalinity within rods/fillers lowering impurity levels improving segregation degrees modifying structural forms adopting rational sequences boosting freedom during shrinkage.
Causes of cold cracks: The welded material has a high hardenability, making it prone to cracking under the tensile forces during cooling; during welding, if the cooling rate is very fast, hydrogen cannot escape in time and remains in the weld seam. Hydrogen atoms combine to form hydrogen molecules, which enter the microscopic pores of the metal in a gaseous state and create significant pressure, causing local metal to experience high pressure and form cold cracks; cold cracks are likely to form when welding stress tensile stress coincides with hydrogen precipitation and quenching embrittlement.
Preventive measures: Preheat before welding and cool slowly after welding, allowing the decomposition of austenite in the heat-affected zone to occur within a sufficient temperature range, avoiding the formation of hardened structures while also reducing welding stress; perform low-temperature annealing and dehydrogenation treatment promptly after welding to eliminate stresses generated during welding and allow hydrogen to diffuse outwards in time; use low-hydrogen type electrodes and basic flux or austenitic stainless steel welding rods/wires, dry the welding materials as specified, and clean the groove thoroughly; enhance protection during welding and clean the surface of the welded area to prevent hydrogen intrusion; select reasonable welding specifications and adopt a rational assembly sequence to improve the stress state of steel pipes.
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Shenyang Tengfei Iron Corporation
Shenyang Tengfei Steel Pipe Co., Ltd.
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