Friday, October 24, 2014

The cause of the pipeline peeling

Line pipe having a high compressive strength is required, they also require a higher low temperature toughness and excellent weldability. However, in the production and use of the Central Standing Committee found that certain finished steel pipe welds on both sides and there are a lot of varying lengths from the skin defect, seriously affecting the material properties.

Under the inspection and analysis, from the skin defect causes are the following:
(1) oxide inclusions. Slab of large inclusions mainly from powder entrapment. In analyzing the composition of slab surface cracks found near the crack contains a lot of O, Ca, Al and Si and other elements, brittle inclusions composed of high melting point such elements can not easily be adsorbed on the protection of molten slag, it is easy into the liquid steel billet and thus remain in the interior. In the casting process, the large inclusions (such as Al2O3 inclusions) will first be peeled and shell matrix form nascent microcracks, this crack propagation along the strand matrix shell spherical inclusions, and then in the slab surface cracks. Such cracks can lead to lamellar tearing, serious damage to the welding properties of the material, thick steel plate to reduce ductility.

(2) the impact of Cu. Cu case of low melting point elements, easy to segregation at the austenite grain boundaries, and precipitates between the iron oxide layer and the substrate layer, resulting in the formation of low-melting slab surface enrichment-rich phase. Meanwhile, Cu segregation reduces the austenite grain boundary energy to reduce the grain boundary strength.

(3) N elements effect. Steel [N] content and plays a significant layer defects correspondence. The parameters in the mold and cooling the same water flux performance is substantially the same conditions, with the steel [N] content is reduced, significantly reduced from the skin defect. Tests showed that the steel of the [N] is the mass fraction of less than 60 × 10-6 from the skin when the defect rate of 5.8%, [N] is equal to the mass fraction of from greater than 60 × 10-6 at a substantial increase in the defect rate of the skin, can be as high as 76.2 %. For aluminum-containing steel, since Al forms AlN precipitation can reduce the low temperature ductility of the steel, at the same time, AlN tend along the austenite grain boundaries under stress in the vicinity of precipitates will form micro-cracks, leading to crystallization boundary embrittlement. This can be found from the detection of high aluminum slab cracks containing proven.

(4) mold cooling water, pouring temperature and casting speed impact. Appropriate to reduce the amount of water can reduce mold slab surface microcracks, improve the slab quality, reducing hot-rolled sheet rolled skin defects. Reduce the amount of water mold, in essence, with the use of high crystallization rate fluxes similar effect is to reduce the heat transfer rate, slow to realize the slab in the mold of cold, promote uniform growth of shell to improve slab quality. The ability to reduce the pouring temperature and casting speed will help to improve the slab quality, which is a slab mold shell thickness related to high pouring temperature and casting speed will increase the heat flux density, shell thickness thinning, solidified shell to withstand external forces lower, the larger the probability of cracks.

Passivation process of line pipe

Where the line pipe parts, such as electroless plating or other coating requirements, are generally required in pretreatment (including pickling go black, polished, etc.) through the passivation treatment to use when finished or assembled parts. Passivation can improve the thermodynamic stability of line pipe in environmental media, localized corrosion prevention line pipe, line pipe surface so that cleanliness has sufficient and can also eliminate thermal processing line pipe surface oxide.

Passivation process line pipe can be divided into two types of wet and dry. Concrete can be divided into many types. Passivation effect depends both on line pipe passivation process, but also on line pipe material itself, the specific factors are contained elements line pipe, line pipe metallurgical structure, line pipe processing status. In the composition of the elements, chromium, nickel and strong elements belonging passivating iron followed by passivation, and therefore, chromium and nickel content, the higher, the stronger the passivation of the line pipe. Austenitic, ferritic steel pipeline with a more uniform structure, can be passivated better, martensitic steel pipeline through heat treatment to strengthen its multi-phase microstructure of the organization, thus passivation is not strong. Line pipe by passivating the surface finish after machining is preferably, casting, sand blasting, etc. The resulting surface roughness is not conducive to passivation.

Monday, October 20, 2014

The superiority of spiral pipe in uses

According to the nature of doing HDPE plastic spiral coil, with a light texture, high strength, good toughness, easy to lay, water resistance, low cost, corrosion resistance, etc., its performance and economic benefits far exceed the reinforced concrete pipe and general specifications iron pipe, engineering pipe replacement products. In metallurgy, petrochemical and other industries exchanger design, there are various forms of heat, but the presence of a conventional heat exchanger ineffective, difficult to clean, fouling resistance and large more drawbacks, can not meet production of customer needs. According to the spiral knot spiral hose heat exchanger to do its good effect of heat transfer, easy to clean, low fouling resistance, compact structure and so on.

Plastic spiral steel pipe (also called hollow wall winding plastic tube) is a wall structure is very reasonable plastic tube. The overall use spiral structure. In for urban drainage projects, improve drainage capacity facility (or narrow diameter) and engineering service life, easy construction, shorten the construction period, when the water table is high, the depth of excavation and restoration of large or require high road, the project cost significantly reduced. Spiral has many superior heat transfer characteristics and structural features straight tube. Heat transfer coefficient is higher than the straight tubes, more heat transfer area can be obtained in the same space, the longer the pipe is arranged to reduce the seam to improve the safety. So spiral heat exchangers and steam generators in the field of power engineering, chemical, aerospace, and nuclear energy has been widely applied.

Large plastic spiral steel pipe is formed with a "T" shaped ribs with a sheet of polyvinyl chloride (UPVC) mixing the carbon black or high density polyethylene (HDPE) using a hot press method. Then on the volume of the machine roll cut into spiral. By quickly locking mechanical connection, the application of an adhesive or melted PVC reinforced molded plastic spiral. It has a large diameter, high corrosion resistance, good anti-leakage effects, quick construction and other characteristics.

Friday, October 17, 2014

Galvanized surface pretreatment

In order to get a good layer of zinc, steel surface must be very clean, rust, iron oxide, and this will reduce the adhesion properties of the coating, prevent binding of iron and zinc. Therefore, before plating steel surface preparation must be carried out. The process includes a general degreasing, washing, pickling, and then washing with water. Also included in the dry storage of galvanized, painted solvent.

The purpose is to remove grease clarinet skim the surface, dirt, steel pickling plant so as not to affect the results, degreasing can be achieved by burning or caustic, often caustic. Alkaline cleaning solution is an aqueous solution containing about 10-12% NaOH, the temperature is 70-80 ℃. Cleaning time is 8-10 minutes.

The purpose is to remove pickling steel surface oxide skin, the acid is generally used sulfuric acid and hydrochloric acid, sulfuric acid pickling effect depends on the temperature, the effect of hydrochloric acid depends on the concentration of H ions. The pickling solution is an aqueous solution of sulfuric acid containing 10-15% sulfuric acid at a temperature of 60-80 ℃, hydrochloric acid pickling solution containing 10-15% aqueous hydrochloric acid, at a temperature below 40 ℃, the pickling time is usually 30 minutes. The advantage of hydrochloric acid is that it can operate at room temperature, removing the oxide scale capability, less erosion of the base metal by absorption of the hydrogen gas and small in the possibility of contamination of the residue of carbon black, ferric oxide generated acid matter, dark angle, easy to remove, no effect on zinc from galvanized perspective, hydrochloric acid is more appropriate. However, the low sulfuric acid prices, easy regeneration, therefore, countries choose pickling solution under different circumstances, the United States and more with sulfuric acid, hydrochloric acid and West Germany, multi-purpose. Adding an appropriate amount of inhibitor in the acid, the acid can etch the surface of the pure steel decreases, so that the contact is reduced with hydrogen in the steel pipe. In addition, also reduce acid consumption and mist generated, the amount inhibitor is typically 0.1-0.5%. Usual amount inhibitor is generally 0.1-0.5%, commonly used inhibitor of water such as plastic.

Seamless steel tube for pipeline engineering welding techniques

Steel pipe transported to the site, firstly according to equipment and fittings position pipes, and in the field will be specific length of each pipe of measurement is good, mark the specific size and arrangement of the position, so that when the interface condemnation, the use of lower crane tube, down tube process be used in nylon lifting sling to pipe insulation protection.

Dual control heating pipes, under normal circumstances can reduce 25% of project cost (using FRP do protective layer) and 10% (high-density polyethylene made ​​protective layer) around. Pipe shall have a cross weld anywhere. When tack welding, weld all electrode positioning number (or grade) should be formally welding the same, but some fine electrode diameter optional. Positioning the welding current is selected from the weld becomes larger than the time of welding, is usually 10% -15% larger, in order to ensure penetration. For pipe welding, the weld should be smooth, uniform width, root penetration, no obvious defects and undercut bump phenomenon, should be above the pipe welds to strengthen the surface area of ​​about 2 mm, welding the groove edge 2-3 mm. When the pipe and flange welding, pipe flanges should be inserted thirds, flange and pipe should be perpendicular to the axis of the two coincide.

a, horizontal pipe slope of not less than 0.3%.
b, should be checked before welding welding environment, welding installation equipment, welding materials, and dry cleaning, must comply with the specifications and requirements of welding operations.
c, welding pieces of double-sided welding, weld root should be cleaned and checked back, after cleaning up the defect before welding seam on the back. The provisions of gouging welds should be visually inspected after gouging before welding.
d, the wall of the pipe must fight more than 4mm groove before welding, bevel surface may not have dissection, cracks, damage processing, burrs and flame cutting slag and other defects. After coating should knock welding clean, weld should be rendered uniform scaly, without pores, slag, cracks, welding tumor and other phenomena, biting flesh length should be within specifications.

Thursday, October 16, 2014

The purpose of precision seamless steel tube heat treatment

Precision seamless steel tube as a product must have a certain performance to meet the needs of using conditions.There are two ways to improve the performance of the steel, it is adjusting the chemical composition of steel, which uses alloying method;The other is the combination of heat treatment and heat treatment and plastic deformation.In the field of modern industrial technology, heat treatment in the aspect of improving the performance of the steel is still dominant.

There are many types of precision seamless steel pipe, and formulate the corresponding standards, the use of steel seeds range, sometimes users according to their own conditions attached to increase demand.In the standard, as one of the terms of delivery performance indicators must meet specified requirements.The performance of precision seamless steel tube mainly refers to the mechanical properties, physical properties, performance and process capability, etc.And the heat treatment by heat, heat preservation and cooling to obtain certain steel microstructure and properties of the corresponding, in order to meet the product standard and the user's requirements.

Precision seamless steel tube heat treatment embodied in the following aspects: the purpose of homogenization for precision seamless steel tube group By heat treatment can eliminate the original precision seamless steel tube production of uneven performance of organization, to achieve the required uniform organization and physical properties;To eliminate the defects in production Such as cold drawn precision seamless steel tube processing production after hardening of 1.3 1.6 times higher strength, elongation rate was reduced by 30% - 35%, corresponding to cold working can pull off, so you need to intermediate annealing to eliminate internal stress;When hot rolled seamless precision seamless steel tube in the residual stress is bigger, affect the corrosion resistance, the fight against performance also need to eliminate the residual stress in heat treatment.

Spiral steel pipe heating process

In spiral steel pipe production process, the main processing was conducted under the thermal state, so the heating operation is very important to determine product quality process.

Heating operation lies in: the temperature of billet uniformly heated to suitable for processing due to punch a great influence on the quality, that is to say, the temperature of the perforation process is one of the important conditions affect the quality, so often to the perforation process of billet temperature control.Improper heating will be inside the tube billet surface or outer surface crack, folding and eccentric waste products.Blank hole temperature varies according to different materials.But mostly at about 1200 ℃, carbon and other alloying elements is large temperature is somewhat low.Heating operation the second key is to minimize the number of scale: especially in hot extrusion elbow, from the tool life and surface quality of extruded tube, more strict.Such as hot stove, according to their functions, divided into two kinds of furnace and furnace again;The former is used to heat the blank from room temperature to the processing temperature;The latter will be used in the processing process, then heated to the necessary processing temperature.There are many kinds of heating furnace, but the main use of annular furnace.This kind of furnace has annular furnace bottom, it can turn slowly, the diameter of the blank from the entrance along the bottom direction loading, turning a can be used at the exit of the heating and soaking to the specified temperature of a stove.Spiral steel pipe production process more slowly, but the factory use the spiral steel pipes are all conform to the specifications of the products.