Masayoshi Akiyama

Effect of die geometry on residual stress level present after bar drawing

Abstract A new drawing die designed to reduce the residual stresses in bar after drawing is presented. The die design consists of two consecutive straight taper portions, and the outlet portion has a very small die angle. The marked beneficial effect of adding the outlet portion was predicted by finite element modelling (FEM), and subsequently verified by measuring the residual stresses in drawn bars. The optimum range of the outlet die angle was determined using FEM. The new design requires a slightly longer die length than the conventional one, but it is short enough to be adopted in current production lines.

Influence of back-tension on effect of skin pass in minimising residual stress levels after cold rod drawing

Abstract Experimental and numerical investigations have been carried out on the influence of back-tension on residual stress levels after cold bar drawing. In particular, the influence on the skin pass effect was studied. Experimental and numerical results show that back-tension has little influence on the reduction of residual stress obtained following skin pass drawing. It is concluded that a skin pass can be incorporated into the final stage of drawing regardless of the draft schedule of the overall drawing process.

Design Method of Die Geometry and Pass Schedule by Void Index in Multi-Pass Drawing

The concept for die-geometry design is shown to suppress void growth in multi-pass drawing. First, the void index to evaluate the void fraction in multi-pass drawing was newly proposed based on the well-known equation to predict the fracture limit in cold working. Using finite element analysis, the influence of die geometries on the void index was investigated and it was clarified that the extra-low-angle die is effective whatever reduction in area is adopted. Moreover, dies were designed to have the effect of suppressing void with the minimum die length. A series of experiments was then carried out that successfully verified the numerical results.

Influence of carbon content and carbide morphology of carbon steels on stress strain curve in vicinity of yield point

Abstract Experimental analysis was carried out to examine the influence of carbon content on the stress–strain curve in the vicinity of yield point. Numerical analysis was then carried out to investigate the mechanism in the microstructure through which the influence occurred. Attention was focused upon the morphology of the carbide, and the homogenisation method and the elastic–plastic finite element method were used for numerical evaluation. Despite very primitive assumptions that the macroscopic stress–strain curve of extra low carbon steel applies to the ferrite in the microstructure and the carbide deforms only elastically, the numerical results have explained well the stress–strain curves obtained experimentally. Finally, TEM observations were carried out to check the validity of the result predicted by the homogenisation method. The TEM images show a concentration of dislocations in the ferrite existing between adjacent carbides lying parallel with the macroscopic loading direction. This result explains qualitatively the localisation of strain in the ferrite predicted by the homogenisation method. By using the numerical analysis inversely, it would be possible to design the morphology of the carbide so that the macroscopic mechanical behaviour of a carbon steel meets the required value in the vicinity of yield point.

Die design for reducing tube outside diameter by cold pressing and mechanism of thinning and bending phenomenon

Abstract Numerical investigation by axisymmetric finite element analysis (FEA) and experimental investigation were carried out on the excessive decrease in outside diameter occurring on a tube and a bar cold worked through a die by press. The mechanism through which excessive decrease in outside diameter appears was determined by FEA, and the cause of discrepancy between the die diameter and the outside diameter of cold worked tube was discussed. Throughout the course of this investigation, a peculiar phenomenon of bending was observed on the cold worked tubes, and further investigation was carried out on the dominating parameter of this bending phenomenon. Eccentricity, i.e.non-uniformity of wall thickness in the cross-section of tube or failure in concentricity between the circles defining outside and inside surfaces of parent tube, is the parameter that most influences the occurrence of bending phenomenon. Discussion and estimation were given on the mechanism through which this peculiar bending occurred.

Trigger for the occurrence of grain coarsening phenomenon of BS304S31 austenite stainless steel under small plastic strain at high temperature

Observation by optical microscopy and EBSP have made it clear that the trigger for the grain coarsening phenomenon of austenite stainless steel BS304S31 may be the stacking faults concentrating selectively in a thin layer lying just beneath the grain boundary. When macroscopic plastic strain reached 6 percent, selective concentration of stacking faults was observed. When it reached 20 percent, the distribution of stacking faults became uniform in each grain. After these specimens were heated, concentration of stacking faults disappeared, and grain coarsening occurred at the point with 6 percent strain, but no grain coarsening occurred at the point with 20 percent strain. In order to investigate this concentration of stacking faults, an attempt was made to analyze the deformation in each crystal by using image‐based FEM. The result suggested that there is a possibility that plastic strain concentrates in the vicinity of the grain boundary when the macroscopic plastic strain is small.

Applications for plasma transferred arc welded tools during hot steel tube making

Abstract This paper describes applications of a tool reinforcement technology, using plasma transferred arc (PTA) welding, in hot metal rolling. The coating of large NbC particles shows a remarkable performance in the suppression of galling. As the rolling temperature increases, a matrix material with high temperature resistance to deformation is recommended. The tool life is slightly longer when a thick scale layer exists on the hot metal surface, but when the scale is eliminated the new tool demonstrates excellent capability. The new tool also shows a higher level of performance when rolling hot titanium and hot stainless steel. The application philosophy of PTA welded tools is demonstrated.

Experimentally Based Study on the Effect of Initial Grain Size for Characterization of the Grain Coarsening Phenomenon During Hot Rolling of a Medium Carbon Steel

Laboratory experiments using a hot deformation simulator were carried out to clarify the effect of initial grain size on the grain coarsening phenomenon in hot metal working with light reduction. As an example of medium carbon steel, AISI1045 was adopted and specimens of various grain sizes were prepared by changing the thermo-mechanical histories. They were subjected to a hot deformation testing by the simulator. The threshold grain size to cause the grain coarsening phenomenon was then determined and a criterion was presented to ensure a fine grain after hot rolling. The validity of the new criterion was checked through hot rolling of bars by a prototype mill in a laboratory.

Influential factor to dimensional precision of cold-drawn tubes:

Elastic–plastic finite element analyses were carried out to sort out the influential factors on the dimensional precision of cold-drawn stainless steel seamless tube for energy industry. Good agreement was obtained between the predicted dimensions and the measured dimensions of tube after drawing experiment for validity check. Geometrical parameters of tools and geometry of parent tube have little influence on the geometrical precision of finished tube. Most influential factor on the geometrical precision of drawn tube is the temperature of tools. Observation and measurement on a production line showed that the temperature of tools increases according to the increase in the number of drawn tubes. Increase in the temperature of tool leads to the change in the geometry of drawn tube. A methodology for evaluating the thermal expansion of tools is shown to take it into consideration the designing procedure of cold-drawing process by using finite element analysis.

Influence of die geometry on undershooting phenomenon and bending phenomenon in cold extrusion of steel tube

Abstract Numerical and experimental investigations were carried out on the excessive decrease in outside diameter occurring on a tube cold extruded through two conventional types of die, a taper die and an R die. Wide ranges of the parameters of die profile were adopted in order to precisely examine the influence of die geometry on the undershooting phenomenon, and appropriate die geometries for the suppression of the undershooting phenomenon were shown. The R die is more practical in the manufacturing operation compared with the taper die. Bending was also investigated, and it was shown that the bending intensity becomes lower as the undershooting intensity decreases. However, it is not necessary to suppress bending sufficiently, even if the undershooting is suppressed to zero in the case of adopting the ordinary types of die – there seemed to be another governing factor for the occurrence of bending and this is a remaining problem.