Cross dies forging : A new method to reduce forging force & price up to 80 % thanks to FEM method

Purpose of this article was to introduce a new method of forging which is called “Cross Die Forging”. In this method, the required force (load) is reduced to the greatest possible degree through elimination of flash channel; however, this would also decrease the positive effect of flash channel, namely filling the gaps and pores within the mold. Cross die forging procedure provides a way for providing a better preform design which ensures that the mold is filled without allowing the material to enter the flash channel. This method has been invented based on the need to decrease the production costs and to use lower tonnage pressing devices for production of heavy parts. This method is an economical method only for parts that: A) Has at least one plane of symmetry and the two ends that are perpendicular to the symmetry plane are flat; B) Has a weight that makes it impossible to be manufactured by rolling or roll forging processes. Examples of such parts are valve’s body, T-junctions, etc.


Introduction
Forging is a manufacturing process in which the part undergoes plastic deformation under compressive load and temperature.This method not only deforms the part, but also improves the mechanical properties due to smaller grain size in this case.[4] Generally, forging process is classified into closed and open die forging.Tools and shape of the die used in open forging process do not match the final shape of the product; while, in closed die forging, shape of the die is almost the same as the final shape of the part; and to give the part its final shape, various types of preform molds are used in a step by step approach.[4] One of the conventional methods used to reduce cost of production is parting-off method in which the preform is turned into a profile whose shape is almost the same as final shape of the part using rolling process.Then, after the cutting process, the resulting part will be used as the preform for the closed die.But, producing the preform for heavy parts by parting-off method requires large force as well as rollers with large diameter which makes it practically impossible to use this method for production of the preform.[3] In cross method, production of preform is performed by a shaped die during a step by step procedure with controlled loading which makes it possible to produce heavy parts.[5], [6]On the other hand, in the closed die forging method, a large force is needed to form (shape) heavy parts which requires application of high tonnage pressing device.[7] Due to elimination of flash channel and providing appropriate preform design, as mentioned above, in cross die method, which is a combination of closed and open die approaches, a pressing device with lower tonnage is needed compared to closed die method, for the same weight.However, the part forged by cross method would be in semiraw state with higher machining costs than closed die method; but, for small production volume, application of this method would be economical.[2] 2 Theoretical Frame Work Current study aims to propose a new method called cross forging method.In this method, the parting line in molds is considered to be the plane of symmetry for the part and molds are assumed to be open-ended, in order to decrease pressure inside the mold.This greatly prevents the materials from entering the flash channel and causes the materials to flow in paths perpendicular to each other.However, this condition mitigates the positive effect of flash channels, i.e. filling the gaps and spaces in the molds.But, this method also greatly reduces the required deformation load.0 In following, percent decrease in the required load is calculated using the available relations: [3] Now, value of S is calculated for cylinders with diameter and length equal to D and L, respectively, positioned horizontally inside the closed and cross dies:  As evident in Figure 1, when the pressure inside the mold decreases to the extent that material flows towards open ends of the mold instead of flowing through the flash channel, the required forging force (load) also decreases.The required forging load increases with increase in weight of the part.Thus, if all the gaps and pores within the mold are filled before material enters the flash channel, then there is no need to increase the pressure inside the mold through designing the flash channel.However, the main problem here is to fill the mold by the preform.
In cross forging method, preform is designed so that it completely fills the mold, and also it prevents materials from entering the flash channel.In this method, all the preforms are manufactures by profile method with two open molds using hammer or pressing device.Then, after being cooled, they are cut and used as the preform.The main goal is to produce a profile that can be used as the preform, when rotated 90 degrees around y axis.This table holds true for weights within range of 1.5 Kg ≤W≤100 Kg; All the measures obtained for the dimensions are related to hot dimensions (simulated with MSC.Superforge software) 1300 tests were performed on steel AIAI 1018, which is considered among the low-carbon steel, according to the following conditions: Mold material was considered to be L6 and temperatures of die and work piece were set at 1100 and 300 degrees Celsius, respectively.Friction coefficient between die and work piece was considered to be 0.3.Also, a hydraulic pressing device has been used with ram speed of 25 mm/s.Now, it is evident that increasing the anvil's width, considering that profile diameter is fixed, decreases E and also increases the required force.Moreover, during the tests, we found out that for each value of E, W and D, if a quadrilateral with dimensions of x1 and y1 fills a profile with diameter D, it would also fill a quadrilateral of dimensions x2 and y2, provided that y2≤y1.Assume that there is a forging part according to Figure 5 (A) with approximate weigh of 1.8 Kg and made of AISI 1018 for which we want to determine the preform to be used in cross forging method.First, hot dimensions of the part are calculated (Figure 5 (B)).Then, dimensions are calculated according to Table III, considering the fact that there must be quadrilateral with dimensions of X, Y for every circular section with cross section areas of A and width of W: If y1≤y2, y1 is chosen as width of the profile and x1 is calculated according to E ratio; and dimensions of the profile is modified in this way.But if y2≤y1, y2 is considered as width of the profile, and x2 is calculated according to E ratio.
By comparing the force obtained from (19) and Figure 9 .it is evident that the required force for producing the part by cross forging method is approximately 70% less than other methods.It must be noted that magnitude of the allowable loading at each pulse is set within 20-30 mm.Otherwise, problem of "overlapping" would occur on the produced profile which would appear as cracks or cavities on surface of the part after being cooled.Now, the resulting preform is tested once on the cross die and once on the closed die as shown in the following Figure 10.Following Figure 9 shows the parts manufactured by the close die and cross die.As expected, due to the decreased pressure inside the mold in the cross die method, material flows towards the open ends of the mold rather than flowing through the flash channel, thus material completely fills the pores and cavities in the mold and no flash is formed.
If we compare the force-time curves obtained for the two methods (Figure 9 (a)), we observe that the force required for manufacturing the part by cross die method is one tenth (1/10) of that required by the closed die method.The approximate force calculated by Table IV (

Practical Examples
Usually in the forging industry segments are designed to build them is not possible with existing presses.This repetition can be seen especially in the military industry, but here we are comparing two ways to produce Gate Valve 7 1/6 "3000 PSI economically: In order to compare costs associated with cross die method, with that of other conventional methods, a comparison was made on a profile related to a valve's body (7 1/6" 3000 Psi) between traditional and cross die methods.First, a 6.5 tons ingot was preheated and converted into a quadrilateral using open die forging.Then, the part was shaped into the desired profile using a shaped open mold and a hydraulic pressing device during several pulses.After being cooled down, the ingot was cut and the resulting profile was machined.I.

Total Cost Related to Manufacturing the Valve by Conventional Method (A)
Preparing the casted ingot; squeezing and converting the ingot into a quadrilateral; subjecting the ingot to tension to produce the profile; machining (rough-end mill); final machining.
Describing profile process Based on Figure14(a): Similar to previous method, in this method, the resulting profile, whose dimensions were calculated according to table (3), cools down; then, it is cut to a particular length to be used as a preform for cross dies.3K valve's bodies are manufactured from a dodecahedral ingot of 6.5 tons; while using the same ingot, 5 valves can be produced by cross die method.2-Weight of the part to be forged decreases from 1247 kg to 900 kg, when using cross die method.3-Time of machining the part decreases from 130 hours to 50 hours, in case of cross die method.4-Better mechanical properties can be achieved by cross method due to higher squeezing ratio in this case.Also, it is possible to manufacture the product under PSL4 condition.
5-With regard to the oil & drilling industry's annual need to 1000 Qty.Gate Valves 7-1/16" 3000 & 5000 Psi, that much of it is imported.In case of using available presses in Iran, the amount will be about 124,858,080,000Rials (4,161,936 $) can be saved by cross die method.

Results
1-In this method, forging force can be reduce up to 80 percentage 2-This method can be used for some have forging parts which are not possible to produce theoretically 3-Cross die method is applicable only for parts that have at least one plane of symmetry.4-Using cross die method is economical for forging parts with heavy weights and intermediate production volume.5-In cross die method, the material flow through the flash channel is prevented by decreasing the pressure inside the mold and providing an appropriate preform design; thus, the required forging force in cross die method is 1/10 (one tenth of) closed dies.6-In cross die method, the forged part must be in its semi-raw state and also requires machining after the final production.

Figure 1 .
Figure 1.The required forging load for parts with different weights forged by closed dies (FT) and cross (F0)

Figure 2 .Figure 3 .
Figure 2. The percent decrease in the required forging load (%S) for parts with different weights forged by cross dies (F0)

Figure 4 .
Figure 4. Schematic diagram related to testing the cross molds

Figure 6 .Figure 7 .
Figure 6.(a) Dimensions of raw preform and 6(b) final preform, considering slopes and R values for corners and edges Now, according to standard DIN 7523, R related to edges and corners and values of slopes are calculated for the raw preform (Figure 6 (A)) to produce the part as shown in Figure 6 (B).Now, since the preform must be manufactured from a section with a shaped open mold by a hammer or a pressing

Figure 8 .
Figure 8. (A) schematics of the cross dies, right diagram,9(B) schematics of the closed die.as shown, the die is assumed to be open ended in this case.(l=lower die, u: upper die).

Figure 9 .
Figure 9.The part manufactured by cross die method

Figure 9 .Figure 10 .
Figure 9. (a) Comparison of the force required for manufacturing the part (Figure 8 (A), 8 (B)) using closed die and cross die methods (simulated with MSC.Superforge software)

Figure 12 Figure 13 .
Figure 12 Cross (a) and closed (b) die forging

Figure 15 .
Figure 15.The part forged with cross die method-7 1/16" 3000 Psi valve's body (Simulated with MSC.Superforge Software) II.Total Cost of Producing a Valve by Cross Die Method (B) Preparing the casted ingot; squeezing and converting the ingot into a quadrilateral; subjecting the ingot to tension to produce the profile; cutting and cross die forging; machining (rough-end mill); final machining.Describing cross die forging process Based on Figure14(b): 1-The average original cost for producing one 4130L dodecahedral ingot with net weight of 5320: 268,925,200 Rials 2, 3 and 4-The cost of transforming a dodecahedral ingot to a quadrilateral ingot= 27,487,360Rials In this method, 5 valves can be produced from one ingot: 296,412,560/5=59,282,510 Rials 5-There is 20 minutes overtime in this stage: 1/3(2,695,810+12,786,000+4,509,000)=6,663,600 Rials Rials 65,946,110=59,282,510+6,663,600 6-Cost of machining for each valve: 50 hours*1,200,000=60,000,000 7-Original cost of producing a valve: 60,000,000+65,946,110=125,946,110Rials~ 4200 $III.Benefits of Using Cross Die Methods for Production of Valve's Body (7 1/16" 3000Psi valve) inComparison to Traditional Method: 1-By using current conventional methods, three 7 1/16" 3K valve's bodies are manufactured from a dodecahedral ingot of 6.5 tons; while using the same ingot, 5 valves can be produced by cross die method.2-Weight of the part to be forged decreases from 1247 kg to 900 kg, when using cross die method.3-Time of machining the part decreases from 130 hours to 50 hours, in case of cross die method.4-Better mechanical properties can be achieved by cross method due to higher squeezing ratio in this case.Also, it is possible to manufacture the product under PSL4 condition.

Figure 16 .Figure 17 .FFAA
Figure 16.Image of a cross die and the mechanism by which it is fixed on the work table of 315 tons mechanical press

Table 1 .
Values of S, F0 and FT for Cylinders with Different Values of D and L (L=1.8×D)

Table 2 .
Dimensions of quadrilateral sections used to fill the shaped molds with internal diameter of 150 mm and width of 100 mm Em=1.4.For each Em and D and different anvil width, W, quadrilaterals with different sizes were tested by considering the size limitations mentioned in left side of the table.
m : Escape confection D n : Diameter of Round Bar (mm) A: Area of Square bar (mm 2 ) A 0: Area of Round Bar (mm 2 )

Table 3 .
Gide Table for Designing Prefome, By Knowing Values of W and D or A0, Dimensions of A Quadrilateral That Could Fully Fill the Shaped Open Die with Diameter of D (without Formation Of Flash) And Minimum Required Force Is Shown In The Below Table.(A=A0×E, K=Y/X, F=Fz(Forging Force))