Local radial mold plastic forming of seamless steel pipes is an important development direction of deep processing of pipes, and plastic processing of pipes has a very important position in modern industrial production. In the actual production process of partial radial mold plastic processing and forming of seamless steel pipe, the problem of uncontrolled local plastic deformation of seamless steel pipe often occurs, and its local deformation exceeds the strength limit of the material leading to the generation of cracks, and due to the influence of the shape of the material and mold contact tool, the material generates flow leading to the generation of indentation, and the product of the required shape size and performance cannot be obtained.
Seamless steel pipe in the mold of the local radial plastic forming mechanism is very complex, the various parts of the billet in the forming process have different functions, the state of force and deformation properties are different, the role and influence of each other is also very complex. The study of its forming laws and control methods, the seamless steel pipe in the mold in the local radial plastic forming and control will be of great significance.
Seamless steel pipe local radial mold plastic forming process
The former seamless steel pipe local radial die plastic forming process is to place the steel pipe on the lower die, with the top of the convex die not open circular groove radially loaded on the steel pipe, the convex die to do radial displacement will be pressed into the desired shape, the forming process and the typical defects.
Physical simulation of partial radial die plastic forming of seamless steel pipe
1. Physical simulation of mold improvement design
Through the analysis of plastic forming process and mechanical analysis of partial radial die for seamless steel pipe, it is known that changing the concentrated load to surface load can reduce the uneven compression of seamless steel pipe by convex die and reduce the stress concentration, which is an effective method to prevent the instability and cracking of partial radial die for seamless steel pipe during plastic forming.
In order to prevent plastic destabilization and cracking of the seamless steel pipe during forming, the design and manufacture of the convex die were improved during the physical experiments in this paper, and the top of the convex die was opened with a radius of 29 mm (old die) and 34 mm (new die) respectively, with a circular groove of 2 mm depth (the radius of the circular groove is the same as the outer diameter of the steel pipe being processed), thus ensuring that the contact between the convex die and the seamless steel pipe is a surface contact.
2. Experimental process
The surface of the seamless steel pipe is evenly coated with graphite and put into the mold before the experiment starts, and then it is installed on the movable platform of the 30-ton microcomputer-controlled electronic universal material testing machine for this experiment, and the temperature of the test environment is room temperature, and the experiment is loaded slowly by displacement loading, which is visible as a quasi-static process due to the slow loading speed. After turning on the tester, the movable platform of the tester drives the mold to rise slowly together to ballast on the steel pipe, this test observation recorded the forming process of each test piece, and recorded the load value at the end of ballasting.
In the pre-doing experiments found that the forming quality of the steel pipe and its wall thickness t, the amount of pressure Δh between the close relationship, so this paper the ratio of the wall thickness of the steel pipe to its external diameter and the amount of pressure to its external diameter ratio Therefore, in this paper, the ratio of the wall thickness to the outside diameter and the ratio of the pressed volume to the outside diameter are called the thickness to diameter ratio and the relative radial pressed volume of the steel pipe, respectively, and are considered to be the main parameters affecting the forming quality of the steel pipe when the die parameters are certain. Therefore, the experimental process of this paper also focuses on the following phenomena were observed and measured.
1. the deformation of the seamless steel pipe when forming in the mold.
2. the collapse and cracking of the middle of the seamless steel pipe when the thickness to diameter ratio and the relative radial compression of the steel pipe are different.
3. the indentation of the pressed surface of seamless steel pipe when the relative radial indentation of the die groove is different and the radial indentation of the die groove is different.
4. The load value at the end of ballast; measurement of specimen dimensions after deformation, cross-sectional collapse and cracking in the deformed area.
Experimental phenomenon observation and experimental results and analysis
1. Experimental phenomenon
When the steel pipe thickness to diameter ratio is the same and the tool shape parameters are certain, 8 specimens of seamless steel pipe Φ48mm×5mm×200mm (4 pieces each of new and old molds) are tested with good repeatability.
At the initial ballast stage, the free spreading part of the steel pipe firstly undergoes elastic bending deformation, and the contact surface between the lower part of the steel pipe and the concave die increases continuously. The steel pipe is deformed to both sides by the tension of the free spreading part, and the contact surface of the steel pipe and the groove also begins to change, the steel pipe has a more obvious plastic deformation, with the expansion of the plastic deformation zone, the steel pipe material strain hardening phenomenon, while the steel pipe plastic flattening, when the steel pipe flattening to a certain extent, the thickness of the steel pipe and the contact surface of the convex die increases at both ends, the middle part of the steel pipe is basically formed, at this time also reached the required Ballast volume.
The test process of the four test pieces of the new mold, the test phenomenon and the old mold is basically the same, as can be seen by its force displacement curve, the new mold elastic modulus is significantly smaller than the elastic modulus of the old mold test phenomenon, the same does not appear after the parts are formed cracking phenomenon, and the surface of the parts also did not appear obvious indentation.
Conclusion
According to the characteristics of local radial plastic forming of seamless steel pipe in the mold and the defects that are easy to appear in the forming process, a brief mechanical analysis was conducted, and based on this analysis, the mold was improved and designed, and the physical simulation of local radial plastic forming of seamless steel pipe in the mold was carried out with the improved mold. Through the analysis of the physical simulation results of the local radial mold plastic forming of seamless steel pipe, some conclusions can be obtained as follows.
1. opening the top of the convex die with a circular groove can solve the cracking problem of plastic forming in the local radial mold of seamless steel pipe.
2. widening the radius of the recess of the convex die can reduce or eliminate the surface indentation of the partial radial die plastic forming of seamless steel pipe.
3. the concept of thickness to diameter ratio and relative radial indentation of seamless steel pipe is proposed; they are closely related to the forming quality under the condition of certain tool shape.
4. when the thickness to diameter ratio is the same, in the case of small relative radial indentation, the section shape is poor.
5. different thickness to diameter ratio, the compression of the steel pipe collapse amount is different, the smaller the thickness to diameter ratio, the greater the collapse of the steel pipe.
6. load value increases with the increase in thickness to diameter ratio.