Plastic deformation analysis of wear on insert component and die service life in hot forging process

Indian Journal of Engineering & Materials Sciences Vol. 22, December 2015, pp. 686-692 Plastic deformation analysis of wear on insert component and die service life in hot forging process R Rajiev a * & P Sadagopan b a Department of Mechanical Engineering, Bannari Amman Institute of Technology, Sathyamangalam 638 401, India b Department of Production Engineering, PSG College of Technology, Coimbatore 641 004, India Received 18 July 2014; accepted 29 June 2015 One of the main critical problems in the hot forging process is the temperature rise in the die cavity and huge stresses generated during forging operation which leads to die failure due to wear, deformation of die, cracks etc. In this study, wear analysis is carried out in a die in local industry. The simulation of the forging process on the die and the work-piece is carried out by using commercially available software (DEFORM). The flow of the material in the die, die filling, contact pressure distribution, sliding velocities and temperature distribution of the die have been investigated. The depth of wear on the die surface is evaluated using the finite element simulation and then the total wear depth was determined. By comparing the numerical results with the measurement taken from the worn die, the wear coefficient is evaluated at different locations of the die surface and finally an average value of wear coefficient is suggested. Keywords: Hot forging, Finite Element analysis, Wear model, Wear coefficient. Forging is defined as the process in which a metal billet or blank is shaped by tools or dies with application of temperature and pressure. Castro et al. 1 presented that the forging dies are metal blocks having cavities so shaped as to impart the desired shape to a metal work-piece when they are brought together. The dies have to be made by modern manufacturing methods from appropriate die materials in order to provide acceptable die life at a reasonable cost. Often the economic success of a forming process depends on die life and die costs per piece produced. Die wear is predominantly due to material removal from the die surface by pressure and sliding of the deforming material. Wear resistance of the die material, die surface temperature, relative sliding speed at the die material interface and the nature of the interface layer are the most significant factors influencing abrasive die wear. Dai et al. 2 indicated that proper selection of the die material and of the die manufacturing technique determines, to a large extent, the useful life of forming dies. Dies may have to be replaced for a number of reasons, such as changes in dimensions due to wear or plastic deformation, deterioration of the surface finish, breakdown of lubrication, and cracking or breakage. The surface hardness of a die decreases owing to the *Corresponding author (E-mail: cheryuvaraj@gmail.com) thermal softening of hot forging dies. This thermal softening effect accelerates die failures. The limiting factors of die service life can occur simultaneously or separately during hot forging process. Dehghani and Jafari 3 described about wear as a loss of dimension between two sliding surfaces and it is related to interactions between surfaces and more specifically the removal and deformation of material on a surface as a result of mechanical action of the opposite surface. Plastic deformation such as yield stress is excluded from the wear definition if it doesn't incorporate a relative sliding motion and contact against another surface. Brucelle and Bernhart 4 and Persson et al. 5 discussed about the high work-piece temperatures and high contact pressures during the forging process that lead to large mechanical, thermal softening, wear and plastic deformation of dies. Kim et al. 6 indicated that when the initial die temperature is high, the temperature difference between die and work-piece becomes small, and this small temperature difference assists the metal flow. Long contact time between dies and deforming materials at high temperature induce thermal softening of hot forging tools. This thermal softening decreases resistance to wear or plastic deformation. Tercelj et al. 7 investigated that the initial temperature of die, heat transfer between the die and the material under deformation and the material and

RAJIEV & SADAGOPAN: HOT FORGING PROCESS 687 environment temperature have definite influence on the magnitude and temperature distribution in dies. Wei et al. 8 mentioned that even though H11 steel was earlier introduced to industrial segments, it was much less studied because of popularity of H13 steel. H13 steel is well known having flexible heat treatments and tribological properties. Stupkiewicz and Mroz 9 and Sailesh Babu 10 made a detailed study and found that the wear and friction are important phenomena occurring at tool-work-piece contact interface in metal forming processes. Cui et al. 11 had renewed interest in analyzing forging process by different simulation techniques. Doddamani and Uday 12 significantly used simulation software in the forging industry which determines and display a selection of useful parameters such as, the effective plastic strain, effective strain rate, effective stress, material flow, temperature, force-time relationship. Yiping 13 found that there was a discrepancy in the simulated profile and the worn die measured profile and they indicated the need for modification of wear coefficient. The objective of this research work is to simulate and analyze the closed die forging process. The results obtained from the computer simulation were compared with the worn die measurement taken from the forging industry and the wear coefficients have been evaluated. Finite Element Analysis and Wear Analysis Abachi et al. 14 used the wear model, which is based on Archard s wear model. From the model, it is observed that the depth of wear is a function of sliding length, hardness, and normal stress and wear coefficient. P L d = k H (1) The relation given in Eq. (1) indicates that d is depth of wear (mm) at time increment t (s), k is non-dimensional wear coefficient, P is contact pressure (MPa), L is sliding distance (mm) at time increment t(s) and H is hardness of die (Pa). The sliding distance is replaced in terms of sliding velocity as shown in Eq. (2) and wear coefficient k is substituted for k/h as shown in Eq.(3) L= θ t (2) where sliding velocity (mm/s) at time increment t. The Eq. (1) can be re-written using Eq. (2) which is shown in Eq. (3) d = k. (P.θ. t) (3) Eq. (3) is used to estimate wear depth during one forging cycle/forging of one component. In order to obtain total wear depth in a die for any batch quantity, the equation can be rewritten as d fin = n k. P 1 i. θ i. t i (4) where d fin is the final wear depth, n is the total number of increments/total forging cycle in forging process simulation. Methodology Wear in a die depends on properties like surface hardness, surface finish, friction coefficient, lubrication, temperature etc. Forging wear is a complex phenomenon and takes place during forging as well as during ejection of the component from the die. In this work, the wear analysis were carried out on the hot forging die which is used for manufacturing insert component as shown in Fig. 1. The upper die and lower die cavity dimensions are shown in Figs 2(a) and 2(b), respectively. Before starting the forging process, the dies were preheated to 150 C initially to prevent die failure due to thermal stress. Billet dimensions are shown in Fig. 3. Billets as shown in Fig. 4(a) were heated to temperature of 1100 C. Using screw forging press of 100 Ton capacity, the insert was forged as shown in Fig. 4(b). Modeling and meshing the dies In order to establish the factors that contribute for die failure, modeling of the components were done to find the component stress distribution, temperature Fig. 1 Insert component dimensions

688 INDIAN J. ENG. MATER. SCI., DECEMBER 2015 distribution and velocity of metal flow. Venkatesan et al. 15 in their study used the commercially available software for finding out the die wear. In a similar manner, in this study commercially available software is used to establish die wear. The parameters like work-piece and die materials, forging machine, forging temperature are kept the same in all the simulations using friction wear model. Work-piece and dies were modeled in CATIA V5 and meshed in the software. Tetrahedral mesh type was used in this simulation; this mesh type was assigned automatically by the software using automatic mesh generator. In the software, the objects were positioned manually by using positioning option. Drag option was used to drag the dies and workpiece to position. The alignment of the dies with the work-piece and initial position of them is shown in Fig. 5. During simulation die filling at different stages of insert is shown in Fig. 6. Evaluation of wear depth by using finite element simulation The flow of the material in the die, die filling, contact pressure distribution, sliding velocities and the temperature distribution were evaluated for 700 iterations/cycles in numerical simulation of the die. The input data, which were used in the simulation for the forged part and the forging press, are shown in Tables 1 and 2, respectively. Fig. 3 Billet dimensions Fig. 2 (a) Upper die dimensions and (b) Lower die dimensions Fig. 5 Position of die and work piece in initial contact Fig. 4 (a) Billet before forging and (b) forged component

RAJIEV & SADAGOPAN: HOT FORGING PROCESS 689 Based on the literature work of Abachi et al. 14, an initial value of wear coefficient k=1 10-12 Pa -1 was assumed and the wear at different locations were obtained by simulation using Eq. (5) for one and 700 forging cycles. Figures 7 and 8 show the wear depth values obtained during simulation at different locations of the die for one and 700 forging cycles, respectively. Problem type Table 1 Input data for finite element simulation Hot forging-closed die with flash Die material H13 Work-piece material EN 19 Die initial (preheating) temperature ºC. 150 Work-piece temperature ºC. 1100 Die Hardness 45 HRC Heat transfer coefficient to ambient 50 W/m.K Friction coefficient 0.3 No. of output steps 100 Initial contact distance 88.8 mm Flash thickness 2.8 mm Ultimate tensile strength of die (MPa) at 1990 room temperature Yield tensile strength of die (MPa) at room 1650 temperature Ultimate tensile strength of workpiece(mpa) 930 Yield tensile strength of work-piece(mpa) 770 n d fin Pi θ i. t i ) = 12 1 (. (5) 10 Coordinate measuring machine measurement on worn die Yohng et al. 16 used coordinate measuring machines for worn die measurements. In the present work worn die measurements were carried at various points using coordinate measuring machine as shown Fig. 9 and the wear depth values are marked in mm. The depth of wear obtained from this measurement is compared with that obtained from the simulation results. Evaluation of wear coefficient By comparing the wear profile obtained by measurement, it is found that there are some Parameter Table 2 Specification of forging screw press Value Type of press Mechanical screw press Capacity (tons) 100 Diameter of screw (mm) 132 Stroke of ram (mm) 250 Number of stroke/min 25 Power (hp) 10 Fig. 6 Die filling at different stages of insert during forging simulation Fig. 7 Wear depth (mm) for k =1x10-12 Pa -1 in one cycle of forging Fig. 8 Wear depth (mm) in seven hundred cycles of forging

690 INDIAN J. ENG. MATER. SCI., DECEMBER 2015 discrepancies between these two which is in agreement with the work done by Yiping 13. From the measured values of the depth of wear at different locations of the die, new values of wear coefficient were calculated using Eq. (6). k = d true n ( P. θ. ). 1 i i ti (6) Figure 10 shows the dimensional wear coefficient values evaluated at different points in the upper die cavity using Eq. (6) and it varies from 0.98 10-13 Pa -1 Fig. 9 CMM probe and the die wear measurement to 27.35 10-13 Pa -1 and the average value of wear coefficient is found to be 9.39 10-13 Pa -1. Wear analysis was done using the said evaluated average value of k, and new worn out profile was obtained. Figure 11 shows the comparison between worn die profile obtained by simulation using the average value of wear coefficient k=9.39 10-13 Pa -1 and worn die profile measured by CMM. It is noted that the theoretical estimation is in good agreement with actual measurement and justifies in using the average evaluated wear coefficient of 9.39 10-13 Pa -1 for die wear analysis in a similar fashion as reported by Rodrigues and Martin 17. Result and Discussion From the Fig. 7, it is understood that the wear depth is larger on the central projected portion in the upper die cavity. The maximum wear depth is around 2.142 10-3 mm which is found to be at the edge of the central projected zone where sliding is more. The effective stress distribution with respect to time at die pre-heating temperature of 150 o C is shown in Fig. 12. It is observed that the stress value fluctuates between 120 MPa to 1450 MPa, approximately. Fig. 10 Dimensional wear coefficients evaluated at different points in the upper die cavity Fig. 12 Effective stresses versus time at 150ºC die preheating temperature Fig. 11 Comparison between worn die profile for k=9.39 x 10-13 Pa -1 and worn die profile Measured by CMM

RAJIEV & SADAGOPAN: HOT FORGING PROCESS 691 Fig. 13 Die temperatures with respect to component contact time with 150ºC die preheating Fig. 15 Variation of wear depth versus temperature at die locations 1-12 Fig. 14 Locations 1 to 24 on the die where depth of wear is evaluated Figure 13 shows the die temperature with respect to component contact time. It is seen that as the time of contact of component with die increases, the die temperature also increases to a maximum value of 678ºC. Figure 14 shows the various locations marked as 1 to 24 in the upper die cavity, where wear depths were evaluated at those locations at die preheating temperatures varying from 0 C to 300 C and plotted in two segments with locations from 1 to 12 and 13 to 24 as shown in Figs 15 and 16, respectively. From the Figs 15 and 16, it is observed that the wear depth is at minimum level at the die temperature of 150 C. Also, it is found that as the die temperature increases above 150 C, die wear also increases. However, it is also found that the wear increases as the temperature of die decreases below 150 C. Increase in wear of the die above 150 C die temperature may be due to softening effect of the die at higher temperatures. For die temperature below 150 C, the upper layer of the surface of the die starts Fig. 16 Variation of wear depth versus temperature at die locations 13-24 wearing out possibly due to thermal shocks and this phenomenon is repeated subsequently if the forging is continued at low temperatures. It is also observed from Fig. 15 that the die wear is slightly higher in the region between 11 and 12 marked, whereas, it is lower in the region between 1 to 10. This may be due to high velocity of material flow on the die surface in the region 11 and 12 compared to the region 1 to 10. Similarly die wear is found little higher at locations 20, 21 and 22 compared to that at other locations as shown in Fig. 15. This phenomenon is attributed to higher velocity of material flow where higher wear has taken place. Conclusions From this study of H13 die and EN19 work-piece insert made by forging process, the following conclusions can be drawn: (i) The wear depth is minimum at the die preheating temperature of 150 C. Wear is found higher when the preheating die temperature is above 150 C or below 150 C.

692 INDIAN J. ENG. MATER. SCI., DECEMBER 2015 (ii) It is also observed that during forging simulation the die service life is affected by high effective stresses, high sliding velocity and high contact pressure. (iii) Further, it is found that by assuming constant initial value of dimensional wear coefficient of k=1 10-12 Pa -1 as reported in literature, the die wear simulation result do not show in good agreement with the worn die measurement. (iv) By comparing the wear at different locations of the die measured with CMM, wear coefficient at those locations were established and the average wear coefficient is obtained as 9.39 10-13 Pa -1. This value may be useful for die wear estimation and hence the life of die during design stage for components similar in nature to the insert discussed. References 1 Castro G, Fernandez V & Cidb J, Wear, 23 (2007) 1375-1385. 2 Dai G, Zhang Z, Wu S, Dong L & Liu L, Mater Sci Eng, 13 (2007) 434-438, 3 Dehghani K & Jafari A, Mater Sci-Poland, 28 (2010) 139-152. 4 Brucelle O & Bernhart G, J Mater Process Technol, 87 (1999) 237-246. 5 Persson A, Hogmark S & Bergstrom J, Surf Coat Technol, 191 (2005) 216-227. 6 Kim D H, Lee H C, Kim B M & Ki K H, J Mater Process Technol, 166 (2005) 372-380. 7 Tercelj M, Turk R & Knap M, Appl Therm Eng, 23 (2003) 113-125. 8 Wei M X, Wang S Q, Wang L, Cui X H & Chen K M, Tribol Int, 44 (2011) 898-905. 9 Stupkiewic S & Mroz Z, Wear, 231 (1999) 124-138. 10 Sailesh Babu M S, A material based approach to creating wear resistant surfaces for hot Forging, Ph.D Thesis, Ohio State University, 2004, 1-150. 11 Cui Junjia, Lei Chengxi, Xing Zhongwen & Li Chunfeng, Mater Sci Eng, 535 (2012) 241-251. 12 Doddamani M R & Uday M, Int J Eng Innovat Technol, 1 (2012) 2277-3754. 13 Yiping Y Zhao, J Forg Technol, 6 (2000) 43-47. 14 Abachi Siamak, Akkok Metin & Gokler Mustafa Ilhan, Tribol Int, 43 (2010) 467-473. 15 Venkatesan K, Subramanian C & Summerville E, Wear, 203 (1997) 129-138. 16 Yohng J O, Kim H & Chang, Int J Precis Eng Manuf, 10 (2008) 105-113. 17 Rodrigues J M C & Martin P A F, Finite Elements Anal Des, 38 (2002) 295-305.