Wednesday, November 27, 2013

Precision tube rusting reasons and how to avoid the rusting way

Introducing performance precision tubes, precision tubes often said that it is not easy to rust, and say this is because of the precision tubes containing chromium ingredient, in the air, it combines with oxygen to form a solid surface and dense film is not oxidized state, this film is not oxidized steel surface to prevent oxidation (rust), and may play a role in protecting the surface to prevent corrosion of various corrosion factors. However, if this layer of protective film destroyed, its corrosion occurs, let's look at what causes for the oxide film is destroyed and its corresponding approach.

The main reason for the destruction of the oxide film from its floating iron or harmful atmospheric gas composition affected, because adhesion, accumulation, and sea salt contained in the adhesion caused. These attachments will gradually gather, plus moisture and fixed, so that will destroy the surface of the stainless steel pipe materials are not oxidized film and hinder regeneration of this film, which began rusting stainless steel pipe materials. Rust caused by this situation, mainly precision tube surface rust inside and no rust, so the direct use of artificial means to get rid of surface rust can, once the rust removal, precision tubes can still restore the original shiny, re-use.

For precision tubes are often not in use, before use, when found rusted, they need to be cleaned, in peacetime, we should pay attention to the maintenance of precision tubes, precision tubes not only to avoid corrosion, but also to save a lot of waste of human and financial resources for our business, after all, under the rusty precision tubes if the number of its relatively large number of cases, cleaning up is also a troublesome thing.

Steel pipe weld internal defect repair

No internal defects steel weld penetration refers to a defect between the workpiece and the weld metal or weld layer partially unfused. Incomplete penetration welds weakened section of the work, resulting in severe stress concentration, greatly reduce joint strength, it often becomes the root of the weld cracking. Weld slag sandwiched nonmetallic slag, that is called slag. Slag reduces the weld cross-section of work, causing stress concentration will reduce the weld strength and toughness.

Porosity of the weld metal at high temperatures, excessive absorption of the gas (e.g., H2) or because the bath inside the metallurgical reaction gases (e.g., CO), the bath is cooled and solidified in the time to discharge, and in the inside or the surface of the weld cavities, namely pores. The presence of pores reduces the effective work weld cross section, decreasing the mechanical strength of the joint. If penetrating pores or continuity exists, it will seriously affect the tightness of the weldment. After the welding process, cracks or welding, metal in the welded joint region appears is called a partial fracture cracks. Cracks may occur in the weld may also be generated on both sides of the weld heat affected zone. Sometimes produced in the metal surface, the metal may be generated internally.

Generally in accordance with the mechanism of crack formation can be divided into two types of thermal cracking and cold cracking. Thermal cracking is generated by a liquid to a solid state crystallization process in the weld metal, mostly produced in the weld metal. The main cause of the existence of the low-melting substances (such as FeS, m.p. 1193 ℃) weld which weakens the contact between the grains when subjected to high welding stress, it is easy to cause breakdown between the grains. Weldments and welding rod containing S, Cu impurities such a long time, it is prone to hot cracking. Thermal cracks along the grain boundary characteristic distribution. When the crack through the surface and the outside world, the tendency has obvious hydrogenation. Cold cracks are generated in the cooling process after welding, mostly produced in the base metal or base metal and the weld at the junction of the fusion line. The main reason is due to the generation of weld heat affected zone or formed within the quenched structure, under high stress, caused by the rupture of the grains, when welding high carbon content of the alloying elements more easily or hardened steel, the prone to cold cracking . Excessive weld melt into hydrogen, can cause cold cracking. Crack is the most dangerous kind of defect, which in addition to reducing carry-section outside , but also have serious stress concentration in the use of crack will gradually expand, may eventually lead to the destruction member. So welded structures generally do not allow the existence of such defect , a shovel was found to be necessary to re-welding.

IF tube seam welded pipe line annealing equipment

With weld annealing method, the company can produce high-grade steel pipe. Pipe induction annealing apparatus is an induction heater as a main component of the continuous welded mechanical, welded and quenched to just weld the pipe is heated to above the critical temperature, quenched and annealed steel pipe weld, the weld stress and to minimize the potential crack, resulting fine grain structure can be guaranteed to meet all the requirements of the standard tough steel. As the weld rapid local heating, making steel oxidation and deformation are minimal, accurate and consistent diathermy been fine grain structure also makes the entire weld area and base metal converge.

Center console is the core set of equipment, and set up a manual or automatic conversion, automatic function control system consists of PLC, touch control screen, high-precision thermometer, etc., to achieve a complete high-precision closed-loop temperature control, temperature control accuracy of less than 1%, to ensure the stability of the quality of the steel pipe annealing.

Heating temperature can be set by the man-machine interface (touch screen), you set the parameters according to different specifications and material of the pipe. Internal system can store five kinds of equipment operating data to be stored after adjustment for different pipe size, material, and later have the same specification steel annealing can be directly transferred out of its further ensure the stability of the steel annealing quality, very great convenience to users. Programmable controller for water, electricity and all-round monitoring, alarm and event of a failure, the system immediately stop heating, to ensure the safe operation of the system and displays the fault location can quickly find fault, to resume production.

Spiral seam welded pipe hydrostatic burst test

Spiral seam welded pipe hydrostatic burst test is a destructive test, tube sheet through the entire board surface ultrasonic flaw inspection, pipe weld inspection through the X-ray inspection, hydrostatic testing, ultrasonic flaw detection, thickness and tensile after passing performance test to pass as a pipe burst test tube. For the important oil and gas pipelines with spiral welded pipe, in the book require technical standard pipe manufacturers do hydrostatic burst test, and provide a burst test report.

Blasting field locations should be selected in a safe place in the absence of personnel and motor vehicles through; If set in the underground or semi-underground, but also set an alert when blasting test mark, to prevent personnel and motor vehicles. The device consists of blasting proving ground burst test units, feed pumps, high-pressure injection pump, electric control box, computer control systems, pressure sensors, temperature sensors and other components. 4m from the bursting pipe installed at both ends of the process of electronic cameras, surveillance blast site, can be seen bursting through the monitor screen.

By blasting tests to verify product performance and process rationality. Design units and provide reference data for the bursting pressure; pipe manufacturers to find areas of improvement through test analysis helps to improve product quality. Spiral seam welded pipe burst test device. Spiral seam welded pipe manufacturers should have burst Proving Ground, generally divided into temporary and permanent blasting blasting Proving Ground test site.

Drawback of magnetic particle testing or penetration detection

Magnetic particle testing or penetration detection can be useful to find the appearance of cracks, folds, double skin, hairline, pinhole appearance shortcomings. About ferromagnetic materials, magnetic particle testing method should be preferred, because of its high detection sensitivity; on non-ferromagnetic materials, such as stainless steel seamless pipe, then use soaked detection.

When the two remaining reserved ADX less, because the layout reasons detecting apparatus, two heads are sometimes not useful in detecting, and the end is capable of cracks or other defects site. If there is a potential end cracking tendency of the weld heat affected apparatus also allows potential expansion of cracks. Thus, we should also pay attention to the detection zone must seamless steel welding, expanding pipe ends to discover defects. On -line use of austenitic seamless insulation is damaged or when to have rain infiltrated the site, should be soaked focus detection to detect stress corrosion cracking or pitting and other shortcomings. However, detection of magnetic or soaked steel outer appearance can only be detected, internal appearance of the shortcomings of the powerless. Detection of internal appearance, particularly the detection of cracks class disadvantage, it is necessary to carry out after ultrasonic detector.

Thursday, November 21, 2013

Prevent intergranular stress corrosion cracking method

As intergranular stress corrosion cracking is probably caused because of the chromium depleted zone, preventing intergranular stress corrosion cracking method may include post-weld heat treatment to restore the diffusion of chromium, the carbon content to very low levels and adding titanium to inhibit precipitation of chromium carbide.

To determine the impact of post-weld heat treatment, the carbon material of 100ppm two passes through a heating cycle sensitization, followed by the different conditions in the third pass of the heating cycle. Similar to the above using U -bend stress-corrosion cracking test to evaluate the samples prepared. The results showed that the range of 550 ~ 700 ℃ heated few minutes, sensitized specimens crack. This effect may be due to the heat treatment to increase the diffusion of chromium, thus narrowing the chromium depleted zone. PWHT using short (a few minutes), you can prevent intergranular stress corrosion cracking, which is the actual laying of pipeline efficiency without hinder.

To determine the lower carbon content, and adding titanium influence on different carbon content and the titanium content of the material was evaluated. The samples were 450 ℃, 1000s a heating cycle of treatment, this condition is likely to cause sensitization, a similar U-shaped pattern bending stress corrosion cracking tests. With the change of test conditions, the U-shaped bend in the sample by generating a gap. The results show that addition of titanium to reduce carbon content and inhibit the generation of cracks. This is probably because the soldering is suppressed and the dissolution of carbon into titanium carbide and chromium depleted suppressed cause precipitation of chromium carbides. Therefore, reducing the carbon content and the addition of titanium to improve the material resistance to intergranular stress corrosion cracking an effective manner.

Tank ultrasonic flaw detection precision tubes Analysis

Sink is the use of ultrasonic detection precision tubes spin forward type, ultrasound probe stationary. Chongqing Precision Pipe Factory technicians mentioned were seized through gutters and precision tubes full of water at the bottom of the coupling characteristics, to ensure the thickness of the coupling constant. But because ultrasonic detection of internal defects on the surface of the main and sub- surface defects exist blind, preventing detection, coupled with spiral forward type, precision tubes for 12m long 30m venue needs such as space shortage has affected the precision tube detection method selection and promotion.

Therefore, the domestic large-caliber precision tubes for the detection, commonly used magnetic flux leakage method or hydrostatic test. In China, good performance is still not suitable for large -caliber precision tubes produced magnetic flux leakage testing equipment, the use of which is imported. Imported magnetic flux leakage detection equipment is expensive, for the majority of domestic enterprises is difficult to accept; while the hydraulic test inefficient, labor-intensive, especially when the operator is not high sense of responsibility, the water pressure test useless. Visible to achieve large-diameter seamless precision tubes metallurgical testing has become the steel industry problems to be solved.

Large diameter pipe is characterized by precision large diameter, wall thickness is relatively thick, so take advantage of this feature under ultrasound and eddy current testing internal surface and subsurface features combine to achieve "blind" testing. Through the use of "precision tubes in situ rotation detection probe forward combinations," not only solve the detection problem, but also solve the narrow space occupied space.