An evaluation of two cold pilger die designs
Making tool design recommendations based on tube transitional quality
A number of elements contribute to the success and failure of pilgering seamless tube. An analysis of the finished product and consideration of these processes can help a shop choose the right tooling for the process.
The cold pilger process is a common method of finishing seamless stainless steel hollows. Many aspects of the pilgering process influence machine productivity and finished tube quality.
This article discusses an analysis done on two different tool die designs and their effects on tube quality and machine productivity.
The evaluation examined several elements, such as material elongation, wall reduction, load forces, product size, and surface finish. Analyzing these conditions allows you to make tool design recommendations to maximize machine productivity and improve the final tube product's quality.
When performing an evaluation of any part of the pilger process, you must consider the other variables that may affect the evaluation, including condition of wear plates and bearings, operator training, tube hollow quality (the dimensional aspects), machine speed, feed rates, lubricants, tube material alloy, and the many forces generated from the material reduction. However, this article does not attempt to discuss these factors.
Any producer's goal is to optimize tube quality and productivity. Therefore, it is important to do a comparative analysis of tool designs.
Both tool design concepts analyzed by the author used the same size tube hollow to produce the same finished tube size. To minimize any machine variability, testing of both tool designs was consecutive. The die diameter and other basic tooling dimensional characteristics were the same on both designs, with the exception of the transition area.
The experiment incorporated two AISI grades of stainless steel (304 and 316) for each tool design. The criteria and equipment regarding all tube hollows and pilgered finish tubes remained constant throughout the experiment.
The tooling evaluation analyzed a number of related elements, including area, outside diameter (O.D.), wall reductions, elongation, separating forces, and finish tube quality.
Area Reduction. Theoretically, calculations for the tube area reduction of both tool designs are approximately the same. However, as shown in Figure 1, tool Design A resulted in a slightly lower total area reduction in the beginning stages of the transition. OD reduction rates of both Design A and B indicated no significant difference in performance.
Wall Reduction. The theoretical calculations for wall reductions of both designs indicated that Design A maintained a heavier wall in earlier stages of the transition (see Figure 2). The data coincides with the aforementioned area reduction rate indicated in Figure 1.
Elongation.the theoretical elongation of the two designs shows only a slight difference in the latter stages of the tube transition—Design A indicated slightly larger elongation near the finish tube section.
Roll Separation Forces. Some of the real differences between the tool designs became apparent when the theoretical roll separating forces were reviewed (see Figure 3). While the forces of Design A were higher in the earlier stages of the transition, they fell below Design B's levels in the latter stages.
Roll Gap. The actual roll gap measurements reflected more design differences (see Figure 4). Design A showed a smaller roll gap, which implies that less total work force was needed for production. The direction of the roll stand was entry to exit. Roll gap measurements were collected during actual production.
Analyzing the Finish Product
The finish tube quality was analyzed in a number of areas.
Hardness. All finish tube samples achieved nearly the same hardness. In reviewing the strain-hardening rate on several transitions, you will see the rates-of-hardness increase for Design A indicated a lower hardening rate (see Figure 5). The data appeared to correspond with the smaller roll gap reported in Figure 4.
Ovality. Design A samples validated an improvement in ovality of more than 50 percent. Measuring a number of tube transitions from both tool designs reveals Design A provided a more rounded transition (see Figure 6). The final ovality improvement was caused by improved control of the transition ovality and indicated significant differences between Designs A and B.
Surface Finish. The surface finishes of all tube samples from both tool designs did not demonstrate significant differences in the OD or the inside diameter (ID). Surface readings of the OD ranged from 12 to 17 microns (RMS). The ID surfaces ranged from 20 to 30 microns (RMS), as measured using a profilometer.
Horsepower. The percentage of horsepower used during both tool design experiments was recorded and analyzed. Though both tool designs used nearly the same average percentage of horsepower, there were distinct differences in applied horsepower during early transition stages. The working range (peak to peak) for Design A used 6 percent less horsepower than Design B. The smaller range used by Design A implies less severity in peak loads.
Evaluating the Results
The analysis revealed differences in conditions that are beneficial in a comparative evaluation. When the experiment was set up, elements of the process were selected that were reasonably attainable for measurement (i.e., ovality, rate of hardness, roll gap, separating forces, elongation, wall reduction, area reduction).
Selection of elements was based on their relevance and cost effectiveness. All of these elements relate directly to the transition, which is a result of the die and mandrel design. The relationship of these conditions to each other is very important and provides indications for design change.
The experiment utilized two types of data, theoretical and actual. It is not purposeful, at this time, to attempt to confirm theoretical data with actual data. All theoretical calculations were derived from unpublished, proprietary sources.
Any discussion of the tool designs is solely to indicate which specific elements were used for evaluation. The actual die and mandrel designs are proprietary and, therefore, not specified. In addition, tool life and wear patterns played an important role in design evaluation. Because of time and expenses, the experiment did not address all these variables.
The theoretical data from both designs in area reduction, wall reduction, and elongation resulted in the same outcome. Design A indicated differences that imply less total force was needed, and work distribution by the pass design altered the tube quality. The theoretical data on separating forces for Design A did not appear to confirm other theoretical data; thus, it required further investigation.
The actual roll gap measurements were smaller in Design A and indicated a lower strain-hardening rate throughout the transition stages. The transition ovality of Design A maintained a more rounded tube, resulting in improved transition tube quality. Though the surface quality did not change appreciably, the improvement in transition ovality appears to have contributed to the reduction of horsepower used by Design A. The actual data differences imply that Design A used less work force and resulted in improved tube quality through better work distribution via the pass design.
Without quantifying design differences, we can say Design A theoretically provided tighter side relief for transition control. Additionally, Design A pass design enhanced the development of a more rounded tube transition. By using less work force, Design A could be altered to provide a larger tube hollow, thereby increasing the overall area reduction. Conceptually, this would provide better tube quality and higher productivity.
Better tube ovality and higher productivity can be achieved without increasing horsepower by changing the tool design.
As mentioned, the theoretical data from Design A and B were inconclusive in determining any real design improvement. Confirmation of theoretical data would assist in conclusive evidence. The actual data collected confirmed that Design A improved tube ovality. As indicated from the transition ovality, the redistribution of work reflected improved differences in pass designs.
Author's Note: Wear effects on Design A are unknown, and further examination of the design will continue. Changes in hardening rate and work distribution are expected to occur.
ASM News, Vol. 22, No. 7 (July 1992).
B. Avitzur, Handbook of Metal-Forming Processes(New York: Wiley-Interscience Publication, 1983).
P. Dunstan and R. Johnson, "Factors Influencing I.D. Crack Propagation In Tube Pilgering," in proceedings from conference sponsored by ASM, 1988.
W. Dahl, "The Theory of the Cold Reduction of Tubes," in proceedings from Cold Pilger Symposium, Moenchengladbach, Germany, 1969.
S. Randall and H. Prieur, "Tubular Production in the Cold Pilger Machine," Iron and Steel Engineer (August 1967).
S. Semiatin and J. Jonas, Formability and Workability of Metals(Metals Park, Ohio: ASM Publications, 1984).
H. Stinnertz, "Cold Pilger Tool Design: Prevention of Product Defects," in proceedings from conference sponsored by ASM, 1988.
Unpublished proprietary designs and calculations.
Written by Glen Stapleton
Stapleton Engineering Consultants
2822 Forest View Way
Carlsbad California 92008
Glen Stapleton has a degree in mechanical and tool engineering. His 27 years of experience includes work as a consultant, engineering and maintenance manager, senior project engineer, senior process development engineer, plant engineer, tooling engineer, methods engineer, production supervisor, maintenance supervisor, and safety director.
Stapleton has presented extrusion and tube reducing papers for numerous educational conferences hosted by several different trade organizations. He also is the author of an industry handbook, "Cold Pilger Technology," and has published articles on extrusion and tube reducing in TPJ - The Tube & Pipe Journal® and The FABRICATOR®.
Stapleton's company, Stapleton Engineering Consultants, Inc., offers a consulting engineer to provide technical assistance for extrusion and tube reducing (pilger mill) equipment and processes. His services include troubleshooting equipment, diagnosing malfunction, recommending types of modifications or repairs, and developing operational and training manuals.