Note: The following is from a paper presented to the forging committee of the German Iron and Steel Institute March 4, 1982.

Fundamentals of Programmed Forging

Ing. (grad.) Hans Joachim Pahnke, Pahnke Engineering, Düsseldorf

Summary

Increasing requirements for the dimensional accuracy of forgings, the reproducibility and the productivity of the forging process appear to be appropriate reasons for the computer control of the process.
The programming of the open-die forging process can be accomplished with the presses and manipulators available today. For the availability of forging programs, first and foremost the characteristic data of the material must be determined by tests; the deformation formulae are sufficient. The article describes the construction and optimization of forging schedules. They lead to a reproducible process that increases the productivity of the plant and improvement in the quality of the product. Moreover, the computer program enables judgment of the characteristic forging capabilities of the plant and their influence on the productivity

Preamble

The open die forging process as referred to in the following assumptions and formulae is a process where plastic material is compressed in one main axis only and the spread into the two other main axis is not limited.
For the cooperation between press and manipulator it is important to know that a bar being reduced between the two flat dies of the forging press does not only spread or elongate in the forging direction but also towards the manipulator. The manipulator feeding rate, or bite, is an important factor in all calculations, of forging processes and most be kept within close tolerances to allow a correct pre-calculation of the spread of the material.
Tomlinson and Stringer(1) empirically developed formulae describing the relationship between manipulator bite, material shape and reduction for carbon steel. The formulae employed assume that the volume of the forging does not change and the change of length and width of a forging can be described by a spread factor S and the reduction ratio y. Figure 1 shows a typical sketch of a square bar being forged into a rectangular bar and the formulae used to calculate the process. The hatched area is the projection of the die contact surface before the deformation starts. Assuming the material would be completely homogeneous and there would be no friction between the dies, the displaced material would follow the hatch lines and spread and elongation can be described by the spread factor S depending only on the relation of bite and width. The diagram below shows this theoretical curve for S and the experimentally developed curves for carbon steel and chromium steel with 13% chromium. The original formulae from Tomlinson and Stringer show some influence between S and the reduction ratio y. However, 10 years of practical experience with programmed forging at Sandviken, Sweden, by 0la Forslund(2), with almost all grades of alloyed steel, showed that in the forging process: square - rectangular - square the influence of y on the spread is negligible. The influence of other features like the shape of the die edge or the variation of forging temperature also did not show any significant influence on the spread calculation.
The main cause for the difference in behavior of various materials seems to be within the composition of the alloy. Tests carried out with various materials led to five different curves covering the range from low carbon steel to chromium and nickel base alloys, titanium, and aluminum.

Figure 1. Spread behavior of plastic materials

The formula used by Forslund for the spread factor

S = 1 - eA (b/w o ) B

shows A and B as material constants. For low carbon steel the values are

A = -0.7117 B = 0.8296

Computer programs for the forging process. The described formulae have been used to compute the forging process for bars and predict the change of shape as well as other important parameters during the forging process. Figure 2 shows the pass procedure for the forging programs available a) is the process forging from square stock to a square bar. The breakdown procedure is square - rectangular - square where the reduction ratio y can only be varied every second pass. A square forged down to a rectangular section with a certain reduction ratio, turned 90 degrees and forged down again with the same reduction ratio and the same bite ratio, turns automatically into a new square with smaller size. The program has provisions for cornering of vulnerable materials and automatically reduces the reduction ratio of the last two passes if a forging with sharp corners is required. The process square to round as illustrated under b) follows the same breakdown routine square - rectangular - square, then ends up in four passes with an octagon having about 5 to 7 % more area than the final round. The round is finished in a swaging die in one pass. The next two programs c) and d) cover the forging processes round to square and round to round following the same forging principle.
Forging flat sections requires a different technology. The shape factor h/w has to be introduced and leads to a variation of the spread factor as shown in figure 3. Flats with a shape factor exceeding 1: 6 show in the normal forging range practically no spread but only elongation.
To obtain spread on flat forging the bite ratio b/w must be increased to values in the range of 0.75 to 1. This in many cases exceeds the available press force.
The practical use of computer programs to develop forging schedules requires some input of forging know-how. This applies especially to the reduction factor y which has to take care of the shape of the ingot, surface conditions, and brittleness of the material and in most cases must be varied with the dropping forging temperature a, well as the bite ratio b/w which more or less defines the depth of deformation and grain change in the forging.

Figure 2. Pass series for computer program for the forging of steel bars.

Figure 3. Effect of the bar shape on the spread factor.

Determination of forging parameters

Reduction ratio. Easy forgeable alloys tolerate relatively large reduction ratios without developing cracks. The normal limit is about y = 1.35. Larger redaction ratios may create overlapping if the dies do not have a taper or a very large radius and it becomes difficult to forge the rectangular section back into a square, because the surface is extremely wavy. The development of diamond shapes may occur. Brittle materials require small reduction ratios to avoid comer cracks or centre overheating. Most materials tolerate higher reductions only in the upper range of the forging temperature and require reduced forging ratios in the lower temperature range. High carbon materials for instance start with ratios of 1.15 and drop to 1.05 at the lower limit of the forging temperature range. If the product to be forged has to come off the press as a finished product with a smooth surface, the reduction in the last planishing passes has to be below 1.1 to avoid bulgy surfaces. The computer program is prepared to calculate this intermediate dimension first before calculating the break down passes.

Manipulator bite. To obtain good center compression and good metallurgical properties the bite ratios should be within the range of 0.4 to 0.6. However, the available press force may limit the possible bite ratio, especially when forging too large ingots on small presses. Bite ratios below 0.25 lead to a change of shape only and not to a change of grain in the forging. They can be tolerated only if the total deformation ratio is in a range where a good center compression is guaranteed in the final passes.

The spread factor. It is relatively simple to determine the spread factor by experiments. As shown in figure 4 the test forging can be marked to allow an exact measurement of the length and the change of length can be used to calculate the spread for a given number of bites with a certain b/w ratio. It is important that the tip of the test bar is partly forged before running the test, in order to eliminate the influence of the die radius.

Figure 4. Test ingot for the establishment of the spread factor.

Hot strength. The change of the strength of the material within the forging temperature range must be incorporated in the program, because programmed forging is only possible if at no time within the forging process the available press capacity is exceeded. Incorporating the shape factors, the forging force for each pass has to be calculated and for safety reasons the manipulator bite has to be reduced whenever 85 % of the available press capacity will be exceeded. The knowledge of the forging force is also necessary to calculate the compression of the hydraulic fluid in the cylinder and its influence on the cycling frequency of the machine. To simplify the calculations the experimentally determined hot strengths curve of a certain material is replaced by a straight line as shown in figure 5, which simplifies eventual modification of the computer program, if the press force estimation, divert from the measurements during practical forging.

Figure 5. Hot strength characteristic curves.

Forging temperature. It is difficult to correctly predict surface and center temperature of a forging for the time of the forging process. The computer program calculates pass by pass the surface radiation as well as heat losses due to die contact hot also calculates the heat input by the forging process and tracks the estimated average temperature of the forging pass by pass. If, during practical forging, it is found that there is too much deviation between calculated temperature and measured temperature the factor for the heat input can be adjusted until both values match.
With the input of press and material parameters the computer produces a pass schedule (click for Forgemaster Software screen shots of main screen and plot screen). The pass schedule has 19 passes. The first four passes deform the round into a square, five to fourteen are square - rectangular - square breakdown passes, fifteen to eighteen are octagon passes and nineteen is the final pass through a swaging die. The values needed to operate the press are dimension (output height), manipulator bite, stroke size (return stroke of the press to allow the manipulation) and rotation angle at the end to the pass or constant rotation during the swage die forging.

Figure 6. Inter-related operation of the press and manipulator.

Requirements of the forging installation

Press and integrated manipulator. As the correct calculation of the forging schedule is only possible with correct and repeatable forging parameters, it goes without saying that programmed forging is only possible on installations with correctly functioning size and stroke control for the press and the manipulator control guaranteeing a certain accuracy of the manipulator bite per press stroke. The cooperation between press and manipulator is illustrated in figure 6. The manipulator peel can only move while the press has lifted and allows the transportation of the forging over the die. The peel is stopped during the penetration period. Two methods of integration are possible: the whole manipulator can operate stop - start or the manipulator string, can travel with the calculated mean speed and only the peel jumps in the carriage. The second method requires less acceleration force and gives a smoother operation.
The program calculation for the forging process must incorporate the technical data of the manipulator to enable articulation of the manipulation time and then adjust the press stroke for manipulation accordingly. Too high return and advance speeds of the press lead to an unnecessary long stroke and to mechanical shocks in the system and do not increase the production. Only the lower part of the curve in the press diagram with penetration, compression and decompression influence the productivity and this section depends more or less only on the installed pump capacity.

Mechanized die change. The integrated press and manipulator should be quipped with a mechanized die changing device. This is especially important for the production of rounds which can economically only be finished in swaging dies. After the octagon has been forged on the flat die a fast switchover from flat die to swaging die is required to finish up in the same heat. A precisely forged octagon requires practically no area reduction in the swaging die. The forging can be finished in one pass.
Installations that are equipped with two inline manipulators are able to hold the forging under tension and keep it straight. They also allow to forge both ends and reach an approximately 15 % higher output.
Other auxiliaries for open die presses depend on the product range. A typical draw-down installation for bar forgings should be equipped with a cross die train only, because this is the fastest method of die changing and the best in respect of maintenance. However, if the production process requires many upsetting operations, or ring and drum forging, the press may need a table moving in the forging direction and the designer then has to compromise on the die changing device.
Pop-up turntables to rotate the forgings or mechanized loading and unloading devices as well as ingot cars may complete the installation to allow a completely mechanized and finally, automated process. A forging installation having all the typical components for a bar forging program is shown in figure 7.

Figute 7. Integrated steel bar forge.

1) Deposit grid for ingots
2) Walking beam furnace.
3) Roller conveyor.
4) Transfer equipment
5) Forging manipulators
6) Forging Press
7) Control cabin
8) Transfer equipment
9) Cooling bed
10) Billet connection trough

Special equipment. A new swaging die developed by Professor Strandell(3) of the Technische Hochschule, Stockholm, Sweden, presents a solution to eliminate the octagon passes. The die shown in figure 8 converts a 1: 2 rectangular section into a round with 27% area reduction without rotation. On the next pass back the same die is used to smooth the surface and finish the round. It requires a computer program to calculate the correct manipulator advance per press stroke for this operation with a step by step calculation of the spread, because a too high feeding speed creates a flash and a too low feeding speed leaves flat spots on the rounds. A similar die has been tested to make square bars in less passes(4).
Programs for complicated open die forgings are already in development and the new electronic press controls combined with the measuring instruments on press and manipulator allow copying and repetition of forging processes developed by the press operator with manual input. The programs me stored on disc they can be modified off-line, and allow repetition of the process in automatic operation.

Figure 8. Forging die for round rods according to Prof. Strandell

Rigidity of the press frame. Accuracy and repeatability of the programmed operation depend in many ways of the design of the press frame. Only a rigid frame with minimized deflection and a good guide system can give an excellent forging tolerance. The standard open die forging press with four round columns can not be considered as the best machines for precision programmed operation. However, if a press is revamped and equipped with computer control for programmed operation, the four column press should at least be equipped with two diagonally arranged stroke sensors to compensate for the inclination of the moving cross head. To improve the guide system, revamping of such a press should, wherever possible, include an elongation of the supports for the guide bushings.

Using program calculations to compute production factors

The computer program for forging schedules includes the calculation of the forging time. The base of this is an exact calculation of the pressing cycle with penetration, compression, decompression and manipulation time. A reference program therefore offers excellent possibilities to check which parameters influence the productivity of a forging installation and how much. It could also be used to mathematically compare similar installations. The reference program should calculate the elongation of a bloom of a length of 100 with a size averaging the planned press production. The elongation of the bloom in % divided by the required forging time in minutes can then be used as production factor PQ to compare the productivity. Figures 9 to 12 illustrate which factors influence the productivity of a press installation

Figure 9. Reference ingot for the establishment of the production factor.

Manipulator bite or the ratio b/w (figure 10). The diagram shows that the productivity increases up to a b/w of about 0.5 and finally decreases when the spread is more than the elongation. A large manipulator bite, however, requires a large press force. The dimensions and strength of the reference block is correctly chosen if for b/w = 0.6 the limitation of the press force is reached.

Figure 10. Production factor as a function of the advance ratio b/w.

Figure 11. Production factor as a function of press speed.

Pressing speed. As mentioned before, the idling speeds have no significant influence on the production of the press, because they cover the manipulation time. An increase of production can therefore only be reached with an increase of penetration speed. Most direct drive systems installed in recent years operate with penetration speed of 50 to 120 mm/ sec. However, figure 11 shows that the penetration speed could be much higher than presently used in forging installations, which would result in higher productivity.

Figure 12. Production factor as a function of the advancement power of the manipulator.

Manipulator acceleration. A high manipulator acceleration increases the cycling frequency of the press and therefore the productivity. However, figure 12 shows that here are limitations too. An increase of the acceleration is proportional to an increase of the manipulator drive power. The given example shows that an increase of the drive power from 50 to 100 kW, which would result in a peel acceleration from 2.5 to 5 m/sec only leads to an increase in productivity of 9%. As on the other hand, this increase in acceleration will also increase the maintenance problems, the design engineer will have to compromise for the best reliability of the installation. Figure 12 also shows a comparison of a manipulator traveling stop - start or a manipulator travelling with constant carriage drive and jumping peel. With the same drive capacity the constantly traveling manipulator reaches a higher production.

References

(*1) Tomlinson, A., and Stringer, J.D., J. Iron Steel Inst. 193 (1959) No. 2,p 157/62

(*2) Forslund, O.: Stahl and Eisen 100 (1980) No. 4, p. 168/73

(*3) Swedish Patent Application 324, 345, April 11, 1969.

(*4) Kallstrom, R.: Scand. J. Mettalurg. 12 (1983) No. 1, p. 29/33

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