Study on forming technology of hot push bend

The forming principle and characteristics of hot push bend are briefly described. The process, parameter selection, process control and detection of hot push bend are briefly described.

The principle and characteristics of the hot push bend process

20220226093003 17614 - Study on forming technology of hot push bend

Hot pushing is the use of steel pipe heated to reduce the strength, plasticity increases the characteristics of the bending power is greatly reduced after the steel pipe bending system. Hot push pipe bend selected equipment for the pushing machine, its structure is divided into four parts: heating and cooling device, mainly induction loop and cooling water system; transmission structure, consisting of electric motor, gearbox, worm gear transmission mechanism, etc.; bending mechanism, consisting of guide wheel frame, top wheel frame, pipe gripper and longitudinal and transverse top pipe mechanism and other components; manipulation system, consisting of electrical control system, manipulator, angle controller and other components Composition. Its working principle is to be bent part of the steel pipe set on the induction loop, in the use of heating coils will be heated at the same time, the forward propulsion device rocker traction steel pipe rotation, so that the workpiece is bent under the conditions of local heating, when the steel pipe temperature rises to the plastic state, the rear end of the steel pipe with mechanical thrust propulsion, bending, bending out of the steel pipe part of the rapid cooling with coolant, cooling device using water-cooled method to reduce the The temperature of the steel pipe, so that the side heating, pushing, bending, cooling, and constantly bend the bend out, complete the bend forming.

Hot push bend has the following characteristics:

  • ① Induction circle for heating, uniform heating, and does not produce carbon, sulfur and other pollutants harmful to steel;
  • ② Bend forming quality is good, orifice roundness, overall flatness and bending angle is good, bending section inside smooth, no wavy folds;
  • ③ Bending after the residual stress is small, bending angle range, adaptability, high production efficiency.
  • ④ The process can avoid the traditional bending process forming, the convex side of the pipe wall by pulling thinning, the concave side of the pipe wall by pressure thickening caused by the uneven wall thickness of the bent pipe.
  • ⑤ If the various process parameters are reasonably designed, the process can ensure that the bend forming process in all parts of the wall thickness always remains the same, that is, has been equal to the wall thickness of the straight pipe billet before deformation.
  • ⑥ The process can be used in the same bullhorn mandrel pushing out the same caliber 45 °, 90 °, 180 ° and other bending angles of the bend, high production efficiency.

The assessment of the bend process

In order to ensure that the quality of the bend to achieve optimal, formal product production before the need for different pipe diameters, different wall thickness of the steel pipe process assessment, refer to the ASME specification NB-4212 forming and bending process requirements for implementation.

  • 1. Impact test evaluation. In accordance with the requirements of NB-4213 on the impact performance assessment. The use of specimens for process evaluation test, test materials should be used with the parts of the material with the same specifications, grade and heat treatment system, and has a similar impact performance. These specimens should be subjected to the same forming or bending process and heat treatment system with the component materials, and the corresponding test to determine the impact performance after forming still meet the specification requirements.
  • 2. Evaluation of the wall thickness of the pipe. According to NB-4223.1 wall thickness requirements for assessment. Bending before and after the measurement of the wall, the calculated thinning rate shall not be less than the specification or technical specifications required. In the bending radius of less than 10 times the nominal pipe size of the nominal pipe in all standard wall thickness of the pipe and special (non-standard wall thickness) pipe bending, bending its wall thickness should be measured by ultrasonic mode at each tangent point and the middle of the bending arc, complete a set of three thickness measurements. For more than 45 degrees of bending, should be in the middle of the arc and the midpoint between the points of tangency for additional groups of measurements, should be carried out on the outer arc of the bend or the root of a group of three measured values: one at the plane of the bend, the other two at about 60 degrees from the center of the pipe, each distributed above and below the plane of the bend. The minimum value of the measured thickness should be greater than the minimum wall thickness after the bend specified in the technical specifications.
  • 3. Other physical and chemical test performance assessment. Specimen bending and forming before and after the chemical composition of the material, grain size, intergranular corrosion sensitivity, tensile strength and other mechanical properties should also be assessed according to the corresponding material specification requirements, to ensure that the bending process does not have an impact on its chemical composition and mechanical properties.
  • 4. Assessment of forming dimensions. In accordance with the PFI pipe prefabricated standards, the bend after forming the size: bending angle, ellipticity, waviness, bending radius assessment, should meet the PFIES-24 tolerance requirements.

The selection of process parameters

Process assessment to obtain the best bending effect at the same time, the process parameters are also selected, these parameters include the bending heating temperature, propulsion speed and cooling rate.
Heating temperature: the choice of heating temperature should be higher than the sensitization temperature of steel, choose the solid solution temperature for heating the best, this temperature interval heating can effectively avoid the tendency of steel intergranular corrosion. From the point of view of strength and toughness balance, the heating temperature is not easy to too high, too high may lead to too much grain growth, affecting the mechanical properties of the material, the heating temperature with the increase in the content of alloying elements can be reduced. Heating should be fast, the speed is generally greater than 20 ℃ / S.
Propulsion speed: determine the speed of feeding not only to consider the production efficiency, but also to pay attention to the difference between the inner and outer heating temperature of the raw steel pipe, the cooling rate and heating rate, etc.
Cooling speed: cooling speed is an important parameter of the bending process, cooling speed depends on the wall thickness of the raw steel pipe, bending radius and feeding speed, etc. As the cooling process also has to go through the sensitization temperature interval, the residence time in this temperature interval should be minimized. In addition, the choice of cooling and heating speed, should also take into account the internal stress generated by the bending process.
The above three parameters with reasonable, in order to achieve good bending results.

The process control

Process control mainly includes: parameter control, anti-deformation.
Process parameters control: process evaluation, has been a reasonable bending process parameters, the implementation of the bend should be strictly in accordance with the selected parameters requirements.
Anti-deformation control: pipe bend is heated to high temperatures in the steel pipe is completed, for large diameter, large wall thickness of the bend, should be used to support the completed bending parts such as brackets, so as not to form parts by gravity sagging, resulting in poor bend flatness, especially continuous bending of the bend should pay more attention to set up support brackets.

The quality of the pipe bend check

After the end of bending, the pipe bend should be checked for appearance and size. Bend appearance should be no surface cracks, crazing, folds or other quality-damaging phenomena. Bend size inspection mainly includes: wall thickness, bending angle, bending radius, waviness, flatness, ellipticity, etc.
Bend wall thickness inspection, should refer to the requirements of the process evaluation.
By the material organization and uneven wall thickness and other factors, the actual bending, bending parts prone to wavy phenomenon, the measurement of waviness can be referred to PFI standard ES-24 requirements: the average value of the two largest adjacent wave diameter of the pipe after bending and the minimum difference in the diameter of the valley of the pipe between the wave should not exceed 3% of the nominal outside diameter of the pipe. When the waviness exceeds the tolerance requirements should be trimmed in an appropriate manner, and should ultimately meet the standard requirements of the tolerance range.
For the measurement of bending radius, bending angle and other dimensions can refer to the tolerance requirements recommended in PFI standard ES-24. The data should be recorded after the dimensional measurement is completed.
The influence of temperature and its distribution on the geometry of the pushed bend is key. The heating temperature is an important process parameter and is directly controlled by the power regulation of the medium frequency power supply. For carbon and alloy steels, the heating temperature is determined by the principle that the material austenitization temperature is above, and the main compressive stress on the inner wall of the pipe bend is less than the yield limit of the material at this temperature when pushing. The higher the material austenitization temperature, the higher the heating temperature; the higher the material high temperature yield limit, the higher the heating temperature. Medium frequency induction heating, the highest temperature of WB36 steel is 850-900 ℃, A335 P22 steel is 900-950 ℃, the highest point of heating temperature of A335 P91 material is 900-1000 ℃. The temperature measurement method is a combination of fixed far-infrared pyrometer and manual far-infrared pyrometer.
The temperature distribution is an important process parameter, which is directly controlled by the shape of the induction ring and the relative position of the induction ring and the mandrel head. The shape of the inductor ring is the main factor and the relative position of the inductor ring to the mandrel head is a secondary factor. Temperature along the mandrel head axial distribution law for low, medium, high, medium, temperature along the mandrel head radial distribution law for low, medium, high. Heating temperature is high, the wall thickness of the pushed bend increases.
Propulsion speed on the impact of pushing the bend geometry propulsion speed as an important process parameter is directly controlled by the hydraulic system flow adjustment.

The principle of determining the propulsion speed is the bend wall compressive stress is less than the yield limit of the material at this temperature, the bend wall elongation is less than the maximum elongation of the material at this temperature. Material heat transmission coefficient, magnetic permeability and medium frequency power, fast propulsion speed. Fast propulsion speed, productivity increases, but the wall thickness thinning rate of the pushed bend increases.

Source: China Pipe Bends Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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