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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. [DOI: 10.1115/1.1830490] Keywords: Void, Central Burst, Multi-Pass Drawing, Finite Element Analysis, Die Angle, Allocation of Reduction

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.

A new structure design was presented to prolong the tool life for hot metal working. The tool surface deteriorates quickly when attacked under continuous severe sliding conditions by hot metal at a temperature above 1473 K. Observations were made on guide shoes used in seamless tube rolling to reach the conclusion that enlargement of the size and change in the structure of chromium carbide prolong the tool life. This is contrary to the structure design of high-speed steel for which small carbide is recommended. The validity of the new tool was verified both in laboratory tests and in the production line.

A cone-type piercing mill was developed by the authors for materials with poor hot workability. This piercing mill is called 'the super piercer' in Europe. It has a pair of cone-type main rolls supported at both ends with their roll axes inclined and crossed so as to enable piercing at high feed and cross angles. In order to ensure the best performance of the rotary piercing, disc rolls are adopted instead of plate guide shoes. The super piercer was put into practice at the small-diameter seamless tube plant operated in 1983. The recent progress in our research and development on the super piercer has resulted in the concept of 'the new super piercer,' which allows expansion piercing. Namely, the development of the skewing technology for disc roll axes and its application to the cone-type piercing technology has realized remarkable increase in the expansion ratio. The new super piercer was adopted as the core technology of the new medium-diameter seamless tube plant operated in 1997. In this paper, studied in detail were the influences of the expansion ratio, feed and cross angles on the rotary forging effects, redundant shear deformations and power consumption.

A new design philosophy was tried out on a carbide-reinforced tool for hot working of stainless steel. The carbide particle in the microstructure of the new tool was selected so as to meet the relatively large optimum size. Attention has been focused, in the conventional tool, on the enlargement of the area fraction of fine carbide particles on the surface. The tool was made by a plasma transferred arc process, and it was found in the laboratory that an optimum mixture ratio of NbC and AISI316L powder realizes the best performance against wear and galling. It was also found that coating of the new tool material on the mother metal of moderate strength, toughness, and ductility ensures the longevity of the tool. The new tool was subjected to hot rolling of a stainless-steel seamless tube by a plug mill to verify the high performance.

Investigations were carried out on the direct applicability of a set of grooves of multi-roll wire mills for different materials having different stress-strain curves. Special evaluation was made on the stability of the geometry among different materials when rolled under the same mill setting conditions. Influence of inter-stand tension generated by the change in roll force due to the difference in stress-strain curve was studied both numerically and experimentally. A group of three-roll and four-roll-type mills is superior to two-roll-type mill in getting rid of the time for mill setting. [DOI: 10.1115/1.1360695]

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. [ABSTRACT FROM PUBLISHER]