Thermal expansion smls pipe technology
Thermal expansion smls pipe is what we often call thermal expansion pipe. Steel pipes (seamless steel pipes) with relatively low density but strong shrinkage can be called thermally expanded pipes. Thick tube finish rolling process that enlarges the diameter of the tube by cross rolling or drawing. Thickening steel pipes in a short time, producing non-standard and special-shaped smls pipes with low production costs and high production efficiency is the current development trend in the field of international pipe rolling.
The warping and diameter expansion of thermally expanded seamless steel pipes is a pressure processing process that applies force from the inner wall of the steel pipe through hydraulic or mechanical means to expand the steel pipe radially outward. Mechanical methods are simpler and more efficient than hydraulic methods. The process is:
Mechanical diameter expansion is to use the segments at the end of the diameter expander to expand the diameter in the radial direction, so that the pipe blank gradually undergoes plastic deformation along the length direction, and realizes the plastic deformation process of the entire length of the pipe. profiles to produce thermally expanded seamless steel pipes. There are 5 stages:
1. Pre-cycle stage. Open the sector blocks until all sectors touch the inner wall of the steel pipe. At this time, the radii of each point within the step range of the inner circular tube of the steel pipe are almost the same, and the steel pipe obtains a preliminary full circle.
2. Nominal inner diameter grade. The sector block starts from the front position and reduces its moving speed until it reaches the required position, that is, the inner circumferential position of the finished tube with quality requirements.
3. Springback compensation stage. The sector block begins to decelerate at the second stage position until it reaches the required position, that is, the position of the inner circumference of the steel pipe before rebound required by the process design.
4. Stable pressure maintaining stage. The sector block remains stationary on the inner circumference of the steel pipe for a period of time, and then rebounds. This is the pressure-maintaining and stable stage required by the equipment and diameter expansion process.
5. Uninstall the regression phase. The sector block quickly retracts from the inner circumference of the steel pipe before springing back until it reaches the initial expansion position, which is the minimum contraction diameter required for the sector block expansion process.
In practical applications, when the process is simplified, steps 2 and 3 can be combined and simplified, which will have no impact on the quality of steel pipe diameter expansion.
Prevent heat treatment deformation of smls pipes
SMLS pipe heat treatment refers to a seamless steel pipe thermal processing process that changes the chemical composition and structure of the surface or interior of seamless steel pipes through heating, insulation, cooling and other means to obtain the required performance.
The reasons for the deformation of seamless steel pipes are often complicated, but they can be reduced and controlled as long as the deformation rules are mastered, the causes are analyzed, and different methods are used to prevent the deformation of seamless steel pipes. Generally speaking, the following methods can be used to prevent heat treatment deformation of seamless steel pipes.
(1) Reasonable selection of materials. For seamless steel pipes, micro-deformation mold steel (such as air-hardened steel) with good material should be used. Mold steel with severe carbide segregation should be reasonably forged and tempered. For larger, non-forgeable mold steels, solution double refinement heat treatment can be performed.
(2) The structural design of seamless steel pipes should be reasonable, the wall thickness should not be too different, and the shape should be symmetrical. For molds with large deformation, the deformation rules should be mastered and machining allowance should be reserved. For large, precise and complex molds, a combined structure can be used.
(3) Seamless steel pipes should be preheated to eliminate residual stress generated during mechanical processing.
(4) Reasonably select the heating temperature and control the heating speed. For seamless steel pipes, balanced heating methods such as slow heating and preheating can be used to reduce heat treatment deformation of seamless steel pipes.
(5) On the premise of ensuring the hardness of seamless steel pipes, pre-cooling, graded cooling quenching or warm quenching processes should be used as much as possible.
(6) For seamless steel pipes, vacuum heating quenching and cryogenic treatment after quenching should be used as much as possible when possible.
(7) For some sophisticated and complex molds, preheating treatment, aging heat treatment, and quenching and tempering nitriding heat treatment can be used to control the accuracy of seamless steel pipes.
(8) When repairing defects such as trachoma, pores, and wear of seamless steel pipes, repair equipment with small thermal impact such as cold welding machines should be used to avoid deformation during the repair process.
In addition, correct heat treatment process operations (such as hole plugging, hole binding, mechanical fixing, appropriate heating methods, correct selection of the cooling direction and movement direction of seamless steel pipes in the cooling medium, etc.) and reasonable tempering heat treatment processes are also important in reducing unnecessary waste. Effective measures to prevent deformation of seamed steel pipes.
Tensile test and hardness test of smls pipes
Tensile testing is to make a smls pipe into a specimen, pull the specimen on a tensile testing machine until it breaks, and then measure one or more of the mechanical properties. Usually only tensile strength, yield strength, elongation after fracture and reduction of area are measured. Tensile testing is the most basic mechanical property testing method for metal materials. Almost all metal materials require tensile testing as long as they have requirements for mechanical properties. Especially for materials whose shapes are inconvenient for hardness testing, tensile testing has become the only means of testing mechanical properties.
SMLS pipe tensile test is divided into four stages
(1) Flexible stage. At this stage, the deformation of the sample is completely elastic. After all payloads have been written, the samples are restored to their original length. At this stage, the elastic modulus E of the material can be determined.
(2) The condescending stage. The elongation of the specimen increases sharply, while the load reading on the universal testing machine fluctuates within a small range (jagged line in the figure). If small fluctuations in load readings are ignored, this stage can be represented by a horizontal line segment on the tensile graph. If the sample is polished, you will see stripes on the surface of the sample that are approximately 45° from the plot line, called slip lines
(3) Strengthening stage. After the sample reaches the yield stage, if the smls pipe continues to be stretched, the resistance in the sample will not increase because the material is not strengthened during the deformation process.
(4) Narrowing stage and breaking stage. When the sample is stretched to a certain extent, the load reading gradually decreases. Seamless steel pipes that pass the tensile test can be put into construction
Since tensile testing is inconvenient and it is convenient to convert hardness to strength, more and more people only test the hardness of materials and rarely test their strength. Especially due to the continuous improvement and innovation of hardness tester manufacturing technology, the hardness of some materials that could not be directly tested before, such as seamless steel pipes, stainless steel plates and stainless steel strips, can now be directly tested for hardness. . Therefore, there is a trend that hardness testing gradually replaces tensile testing.
The hardness test is to slowly press a hard indenter into the surface of the sample under specified conditions, and then test the depth or size of the indentation to determine the hardness of the material. Hardness testing is the simplest, fastest and most convenient method for testing the mechanical properties of materials. Hardness testing is non-destructive and there is an approximate conversion relationship between material hardness values and tensile strength values. The hardness value of the material can be converted into a tensile strength value, which is of great practical significance.
For annealed seamless steel pipes with an inner diameter greater than 6.0 mm and a wall thickness less than 13 mm, the W-B75 Webster hardness tester can be used. It is very fast and easy to test, suitable for fast, non-destructive qualification inspection of seamless steel pipes. For seamless steel pipes with an inner diameter greater than 30mm and a wall thickness greater than 1.2mm, use a Rockwell hardness tester to test the HRB and HRC hardness. For seamless steel pipes with an inner diameter greater than 30mm and a wall thickness less than 1.2mm, use a surface Rockwell hardness tester to test the HRT or HRN hardness. For seamless steel pipes with an inner diameter less than 0mm and greater than 4.8mm, the pipe-specific Rockwell hardness tester is used to test the HR15T hardness. When the inner diameter of the seamless steel pipe is greater than 26mm, the hardness of the inner wall of the pipe can also be tested with a Rockwell or surface Rockwell hardness tester.