MATERIALS FORUM VOLUME 30 - 2006 Edited by R. Wuhrer and M. Cortie Institute of Materials Engineering Australasia Ltd

THE STUDY OF WEAR RESISTANCE OF A HOT FORGING DIE, HARDFACED BY A COBALT-BASE SUPERALLOY

M. Farhani1, A. Amadeh1, H. Kashani1 and A. Saeed-Akbari2*

1Department of Metallurgy and Materials Engineering, Faculty of Engineering, Tehran University.

2 Faculty of Georesources and Materials Engineering, RWTH Aachen University, Germany *Corresponding Author: Alireza Saeed-Akbari, Rudolfstrae 27, 52070, Aachen, Germany. alireza.s.akbari@gmail.com

Tel: (+49241) 9976908 ABSTRACT During hot working processes, due to the simultaneous presence of high temperature and high stress, the relevant dies are under a variety of failure mechanisms. The predominant mechanism depends on the process and its parameters; however various wear mechanisms are known to be of the most important die failure mechanisms. Surface engineering techniques are used to combat wear. In the current study, the hardfacing of a hot forging steel die (H11) by a Cobalt-based super alloy (Stellite 21), was used to study the improvement of wear resistance and the lifetime of the die. Initially some testing blocks of the H11 steel were prepared and then heat treated as of the considered dies. Then the hardfacing process by the TIG method was performed on the testing blocks. Finally, the testing blocks properties, before and after the pin-on-disk wear experiments, was studied using the optical microscopy and hardness testing. Wear tests were performed at three different temperatures: room temperature, 400 and 550 C. After evaluating of the experimental results, a sample die was hardfaced and practiced in service and its dimensions were regularly controlled during service. After a rather long working time, this was brought out of service. The metallographic and hardness testing samples were prepared from the sample die. Comparing the results of the hardfaced and H11 dies and samples, indicated that, the increasing of the high temperature hardness due to the formation of a hard and resistant layer on the surface of hard-faced die, results in the substantial improvement in its wear resistance and lifetime. 1. INTRODUCTION Hot forging is one of the oldest metal-forming processes used in the production of the critical parts for various industrial purposes. As a process, forging can be characterized by good mechanical properties of the workpiece, a short production time, high productivity and optimal material utilization. These advantages are achieved normally for rather large production quantities, because of the high costs of tooling as well as the long set-up times for production line [1]. The dies lifetime is a very important factor determining production cost and rate [2, 3]. Thus, optimizing dies to achieve longer lifetime and cheaper production cost is always desirable in these industries. Hot working tools undergo severe thermal and mechanical shocks during each blow. During the actual hot forging process, the dies surface reaches temperature range of 700-800C [2]. Simultaneous presence of high temperature and high stress results in various die failure mechanisms. Damage of die surface can arise owing to wear, plastic deformation, thermal fatigue and mechanical fatigue [2]. Among these, various wear mechanisms are involved in warm and hot forging processes. It is reported that wear is responsible of approximately 70% of die damage and failure [3-6]. However, the major wear mechanism differs from one situation to another. Figure 1 shows the principal modes of die damages and also indicates the positions in a tool cavity where each type of failure is most likely to occur [1].

Figure 1. Modes of damage and their positions in die

cavity at which each mode is likely to occur [1]. However, there is almost no single material which can encounter all the mentioned wear mechanisms. Even if a material is selected which can withstand more than one of the factors causing wear, making a tool by means of this material is not necessarily economic. Therefore, the preferred strategy is to choose a cheaper material and to cover its critical sections with a material having superior properties. In this regard, various surface engineering techniques are widely utilized. Hard-facing is a weld diffusion process that produces deposits that are metallurgically bonded to substrate. It is now being used increasingly often as an inexpensive means for depositing a hard layer on die surfaces. It

212

also can be used to repair and dimensional restoration of dies [7]. Cobalt-base super alloys are most common hardfacing alloys. Many of them are derived from the Co-Cr-W and Co-Cr-Mo ternaries. Following the success of Cobalt-base tool materials during the World War I, they were then used in the form of weld overlays to protect surfaces from wear since 1922. Low carbon cobalt-base super alloys are employed to combat wear at high temperature services [8]. These alloys have low stacking fault energies and therefore high density of stacking faults and partial dislocations [9]. Solid solution hardening by tungsten and chromium, dislocation-dislocation interactions and impenetrable particle hardening due to metal-carbides are responsible for noticeable hardness in these alloys [10]. Among these alloys, Stellite 21 alloy has been successfully utilized for many years, since 1940s, in the variety of applications, and is still in use, but predominantly as a wear resistant alloy [8]. Carbides observed in this alloy are mostly of the Chromium-rich M23C6 type [10, 11]. These carbides can be observed at above 500C and precipitate in particular on deformation bands and stacking faults [9]. With increasing temperature and deformation, the density of stacking fault, dislocations and deformation as well as volume fraction of carbides increases, thus this alloy exhibits good high temperature hardness [9]. It is also well accepted that cobalt-base super alloys are resistant to deformation at temperature range of 500-900C [10]. 2. EXPERIMENTAL Since the final purpose of this study was the improvement of the wear resistance of a hot forging die made from H11 hot working tool steel, two test block of the same material were prepared. These test blocks were heat treated as for the die and finally a tempered martensitic microstructure achieved. Then, one of them was hardfaced through TIG welding with Stellite 21 rods. The composition of test blocks and the weld rods are shown in table 1. Table 2 shows hardfacing parameters. Then, specimens for metallographic, hardness and wear tests were cut and machined from the experimental blocks. Wear tests were performed using a pin-on-disk method. The disks were made from a T2 high speed steel (table2). These disks were heat treated to achieve a hardness of 64 HRC. Then the surface of the pins and disks were machined to reach a similar condition for all the experiments.

Table 1. Composition of materials used

Alloy Composition (wt %)

H11 C 0.38%, Cr 5%, Mo 1.5%, V .5%, Fe bal.

T2 C 0.9%, Cr 4.5%, W 18%, V 2%, Fe bal.

Stellite 21* C 0.25%, Cr 27%, Ni 2.5% Mo 5.5%, Co bal. * Weld rod; AWS ERCoCr-E, 3.2 mm in diameter

Wear tests were performed at three temperatures; room temperature, 400 and 550 C. Invariable parameters for each wear test involved: sliding speed of 0.4 m/s, normal load of 48 N and total sliding distance of 1000 m. Prior to and after each experiment, pins were ultrasonically cleaned and weighed after drying. Then the weight loss due to wear was measured. Then, the pins were cut along their cylindrical axis and prepared for metallographic and microhardness experiments. In the current study, all of the microhardness tests were performed by means of a knoop indenter and under a 200gr load.

Table 2. TIG hardfacing parameters

Voltage & Current

Pre-heat temp.

Welding velocity

Post--heat

temp.

Heat input

12 V, 100 A 370 C 1.2 m/s 560 C

400 J/mm

According to the results of these experiments, one practical die was hardfaced and put in service. Table 3 shows working conditions of the die.

Table 3. Working conditions of the die

Preheating temperature

Forging temperature Lubricant

250-320 C 1050 C Graphite-Oil Press type Press capacity Workpiece material

Mechanical 620 tons EN3C During service, dimensions of the die were controlled at some stages, like other ordinary dies. After a considerably long period (about 16000 blows) the die was took out of service and specimens were cut from it for metallographic and microhardness experiments. 3. RESULTS AND DISCUSSION 3.1 Test Blocks 3.1.1 Weld overlay microstructure Microstructure of hardfacing weld overlay is shown in figure 2. Dendritic structure and interdendritic carbides can be seen in this micrograph. EDS analysis indicated that interdendritic regions mostly include M23C6 type carbides and a supersaturated content of Cr and Mo. These carbides have formed during solidification of the weld layer. Since solidification speed during welding is very high, the matrix is a supersaturated solid solution of alloying elements (especially Cr and Mo) in Co.

213

Figure 2. Weld overlay microstructure before the wear

test. Microstructures of the wear test pins after the experiment are shown in figure 3. As can be seen, there is not considerable difference between these microstructures and that of weld layer before the tests; it includes dendrites and primary carbides. It seems that even at 550C the test duration (about 41 minutes) was not enough for considerable precipitation of carbides. 3.1.2 Hardness The macrohardness of H11 test block after the heat treatment was 530 HV. Hardness profile in the weld overlay of the hardfaced test block (before wear tests) is shown in figure 4. The hardness of the weld overlay was at the same level at every depth from the surface. The hardness profiles after the wear tests are shown in figure 5. The hardness of the weld overlay increases near the surface. By increasing the wear test temperature, the hardness increment increases. H11 (non-hardfaced) pin shows no considerable change in hardness after room temperature wear test, but after wear test at 550C, the macrohardness of the pin decreased to 460 HV.

(a)

(b)

Figure 3. Microstructure of hardfaced pins after the

wear test at: a) room temperature and b) 550C.

300

350

400

450

500

550

600

650

0 1 2 3 4 5

Distance from Surface (mm)

Mic

roha

rdne

ss (H

V)

Figure 4. Microhardness profile in weld overlay before the wear tests.

300

350

400

450

500

550

600

650

0 0.1 0.2 0.3 0.4 0.5 0.6

Distance from Surface (mm)

Mic

roha

rdne

ss (H

V)

(a)

300

350

400

450

500

550

600

650

0 0.1 0.2 0.3 0.4 0.5 0.6

Distance from Surface (mm)

Mic

roha

rdne

ss (H

V)

(b) Figure 5. Microhardness profiles in hardfaced pins after the wear test at: a) room temperature and b)

550C. 214

3.1.3 Wear Test Wear test results are shown in figure 6. At room temperature, H11 pin shows a better resistance (lower weight loss) to wear than hardfaced pin. At 400C wear resistance of the H11 pin decreases considerably while the wear resistance of hardfaced pin had no considerable change. At 550C, the wear resistance of both pins increases compared to 400C and the hardfaced pin shows a better resistance. Comparing room temperature and 550C results, the wear resistance of the H11 pin shows a rise by increasing temperature, while that of the hardfaced one is more satisfying at higher temperature.

0

5

10

15

20

25

25 400 550Wear Test Temperature (C)

Wei

ght L

oss

(mg)

H11 pinsHardfaced pins

Figure 6. Wear rate (weight loss) results.

3.2 Discussion on the Results of the Test Blocks In case of H11 pins, at 400C, the decrease in the wear resistance relative to room temperature test can be due to the decrease in hardness and strength at higher temperatures. Transformation of surface layers to a more tempered structure causes a considerable decrease in the hardness and wear resistance. Moreover, formation of localized metallic oxides on the surface and their removal during the test, result in a more weight loss and lower wear resistance in H11 pin. It should be noted that localized and scattered oxide spots act with respect to a mechanism called oxidation-scarpe-reoxidation and cause a decrease in the wear resistance. On the other hand, continuous oxide layers can act as a ceramic coating on the surface and can protect it against wear, providing that the sublayers have enough strength. The localized oxide spots form in the hot spots of the surface due to friction. At low ambient temperatures these oxides are discontinuous and scattered, but at some higher ambient temperatures these oxides can coalescence and form a continuous coating. After the formation of this layer, the wear reaches a steady state before which the wear resistance is relatively low. The formation of this continuous oxide layer on the surface after a while, leads into the increase of the wear resistance at 550C in comparison with 400C. In case of hardfaced pins, no considerable variation in hardness is observed at 400C in comparison with

room temperature. Furthermore, due to the oxidation resistant nature of the cobalt-base superalloys, no oxidation can occur. Thus, no considerable variation occurred in wear resistance. At 550C a surface layer with a high hardness of about 600 HV is formed. Deformation of the surface layer and work hardening are responsible for this increase in hardness. According to these results, it seems that higher deformation on the surface and higher temperature, can lead into a better wear resistance in hardfaced specimens. Therefore, a single die was hardfaced and put in service.

Figure 7. Micrograph from H11 die surface after service.

3.3 Dies 3.3.1 H11 die 3.3.1.1 Metallography Microstructure of a H11 die after its service is shown in figure 7. White layer on the surface (left) is a mixture of martensite and retained austenite. The dark layer beneath, is a mixture of ferrite and carbides. Figure 8 shows a micrograph of this layer at higher magnification. As can be seen, it includes fine spherical carbides in ferrite matrix. The final sublayer (at right) is the original tempered martensite. According to this micrograph, it is evident that the die surface reaches to a high temperature enough for the austenitization of the surface layer. This austenite has been quenched by the lubricant and has formed the mentioned martensite. This transformation should be repeated at every blow. The heat diffusion to the next layer was not enough for austenitization. Nevertheless, during the total time of service, it was enough for annealing this layer even to spherical carbides. Thus, a very hard surface layer (probably fully martensitic at the surface) and a very low-hardness layer just beneath it, has been formed in H11 die. Moreover, the formation of a brittle oxide layer on the surface is possible.

215

Figure 8. Microstructure of the sublayer beneath the

white surface layer (figure 7).

The die surface had too many cracks which have shown partly in figure 9. These cracks are formed because of the thermal and mechanical shocks as well as stresses due to the transformation. Propagation of these cracks in soft (ductile) sublayers could be the result of either thermal or mechanical fatigue. These cracks join each other at the sublayers or propagate parallel to the surface, and lead into the removal of large particles from the surface.

(a)

(b)

(c)

Figure 9. Surface cracks in H11 die after service: a) propagation; b) coalescence and c) propagation parallel

to the surface. 3.3.1.2 Hardness The hardness profile from the surface to the depth of an H11 die is shown in figure 10. As can be seen, a very hard layer has been formed at the surface, and just beneath of this layer, the hardness falls into a very low level. Retained austenite and annealed structure of the sublayers are responsible for this low hardness. At more depths from the surface, the hardness rises to its primal level.

Figure 10. Microhardness profile in the H11 die after

service

(a)

216

(b)

Figure 11. Microstructure of the hard-faced die after service: a) far from the surface and b) near the surface.

3.3.2 Hardfaced die 3.3.2.1 Metallography The microstructure of two different regions of the weld overlay is shown in figure 11. As can be seen, far from the surface, the structure contains dendrites, primary carbides, some precipitated carbides on the grain boundaries. But close to the die surface, a recrystallized structure including the precipitated carbides in regular lines inside the grains and grain boundaries, and primary carbides could be distinguished. This indicates that the deformation at a sufficiently high temperature for recrystallization has occurred. Regular lines are the stacking faults or other planar defects in the crystal structure which are decorated by the carbides precipitation. The arrow on the micrograph shows a thermal twin. Determining the defects type is beyond our discussions in the current study. Surface cracks were observed in the hardfaced die, although with a lower frequency as of the H11 die. Figure 12 shows a crack propagating along interdendritic regions.

Figure 12- A surface crack in the hardfaced die after the service.

3.3.2.2 Hardness The hardness profile for the hardfaced die after the service is shown in figure 13. The hardness of the surface layer is high because of the mentioned recrystallized structure and carbides precipitation on the grains internal defects. The main difference with that of the H11 die is the absence of the soft sublayer beneath the hard surface layer. 3.3.3 Comparison of the dies performance Dimensions of both dies were controlled during the service. Results of these controls are shown in figure 14. The hardfaced die has lost only about 0.25 mm of its dimensions after about 16000 blows, while the H11 die has lost about 2.25 mm of its dimensions after about 4000 blows.

100

200

300

400

500

600

0 2 4 6

Distance from Surface (mm)

Mic

roha

rdne

ss (H

V)

8

Figure 13. Microhardness profile in the hardfaced die after the service.

0

0.5

1

1.5

2

2.5

0 3000 6000 9000 12000 15000 18000

Number of blows

Dim

ensi

on L

oss

(mm

)

Hardfaced Die

H11 Die

Figure 14. The dies loss of dimension during the

service.

3.4 Discussion on the results of the dies According to the results for the H11 die, the hard surface layer formed during its work, breaks out, as a result of a weak support of the very soft sublayer beneath it, and is then removed from the surface. The formation repeating cycle of this hard layer and its break-out and removal cause a severe mass removal and dimension loss in the H11 die. Additionally, the initiation and propagation of the cracks due to the thermal and mechanical shocks as well as the

217

transformation stresses result in the removal of relatively large particles from the die surface. Furthermore, the oxidation of the surface can occur and leads into a more mass removal; however, the ultrasonic cleaning of the specimens in the current study made the detection of the oxide particles almost impossible. In case of the hardfaced die, oxidation resistant nature of the weld overlay prevents the oxidation based wear mechanisms to be occurred. As a result of deformation, work hardening, recrystallization of surface layer, and precipitation of carbides on defects inside grains, a hard surface layer forms. This layer has the strong support of a tough sublayer and does not break out easily. Thus, this hard layer can act as a protective coating against wear. The frequency the surface cracks in the hardfaced die was very low in comparison with the H11 die. This could be the result of superior fatigue properties of the Stellite 21 in comparison with the H11 steel. 4. CONCLUSIONS The results of this study can be summarized as follow:

Wear is one of the most important failure mechanisms in the H11 hot forging dies. The formation cycle of a hard surface layer which has a weak support of a soft sublayer, and its break out and removal, leads into a severe wear and dimension loss in some of the hot forging dies.

Another wear mechanism for these dies could be the cracks propagation and their coalescence under the surface and thus, removal of rather large particles.

In case of hardfacing with Stellite 21 superalloy, a hard surface layer forms on the surface, as a result of deformation, recrystallization and carbides precipitation on crystal defects inside the grains. This hard surface layer has the good support of a tough sublayer, and creates a protective coating against wear on the die surface.

Due to absence of cyclic thermal or mechanical shocks in pin-on-disk wear experiments, there is a difference between the

wear mechanisms during the pin-on-disk experiment and the real industrial usage of the hot forging dies. Nevertheless, the wear resistance trends in pin-on-disk wear tests demonstrate an acceptable consistency with the industrial experiments.

Overall, the lifetime of the H11 hot forging dies could be substantially increased via hardfacing by a Stellite 21cobalt-base superalloy.

Acknowledgements The authors wish to thank MOHAM Industries for their kind cooperation and supports. References 1. J. Kohopo, H. Hakonen and S. Kivivuori, Wear,

1989, Vol. 130, pp 103-112. 2. K. Venkatesen and C. Subramanian, Wear, 1997,

Vol. 203-204, pp 129-138. 3. K. Venkatesen and C. Subramanian, Materials &

Design, 1995, Vol. 16, pp 289-294. 4. R. S. Lee, J. L. Jou, J. Mat. Proc. Tech., 2003, Vol.

140, pp 43-48. 5. J. H. Kang, I. W. Park, J. S. Jae and S. S. Kang, J.

Mat. Proc. Tech., 1999, Vol. 94, pp 183-188. 6. C. Bournicon, Trait. Therm. (France), 1991,

Vol. 246, pp 70-77. 7. Surface Engineering of tool and die steels, ASM

Specialty Handbook: "Tool Materials", J. R. Davies, Ed., ASM international, 1995, pp 383-389.

8. Cobalt-base Alloys, ASM Specialty Handbook: "Nickel, Cobalt and their alloys", J. R. Davies, Ed., ASM International, 2000, pp 362-370.

9. P. Revel, M. Clavel, G. Berager and P. Pilvin, Mat. Sci. Eng., 1993, Vol. A169, pp 85-92.

10. J. L. de Mol van Otterloo and J. Th. M. De Hosson, Scrip. Mat., 1997, Vol. 36, No. 2, pp 239-245.

11. Cobalt and Cobalt-base Alloys, ASM Alloying : "Understanding the basics", J. R. Davies, Ed., ASM International, 2001, pp 540-549.

218