◆ 특집 ◆ 최신 정밀 설계재료 기술 Ⅱ
SCM440H 금형강의 표면 처리에 따른 피로 특성 연구
Effect of Surface Treatment on Fatigue Strength of SCM440H
염현호1, 이문구1, 이춘만2, 전용호1,
Hyunho Yeom1, Moon Gu Lee1, Choon Man Lee2, and Yongho Jeon1,
1 아주대학교 기계공학과 (Department of Mechanical Engineering, Ajou Univ.) 2 창원대학교 기계설계과 (Department of Mechanical Design & Manufacturing, Changwon National Univ.)
Corresponding author: [email protected], Tel: +82-31-219-3652 Manuscript received: 2013.6.2 / Revised: 2013.6.27 / Accepted: 2013.6.27
Increased efficiency and improved performance associated with light-weight materials were investigated in this study. Numerous studies have investigated surface treatments to improve the fatigue strength of metals. Laser heat treatment is a promising method because the power and spot size can be easily controlled, allowing a small heat affected zone (HAZ). However, changes in the material properties can result; in particular, the material can become more brittle. In this study, a combination of laser heat treatment and vibration peening was proposed to increase fatigue strength without changing the material characteristics. SCM440H was investigated experimentally, and specimens were tested using a giga-cycle ultrasonic fatigue tester. The results show that the combination of these two processes significantly increased the fatigue strength and, furthermore, different fracture types were observed after a small and large number of cycles.
Key Words: Laser Heat Treatment (레이저 열처리), Regulable Vibration Peening (주파수 변조 진동 피닝), Fatigue Life (피로수명), Ultrasonic Fatigue Test (초음파 피로 시험)
1. Introduction
In order to design and manufacture a high efficient machine the terms such as the high performance, light weight, high efficient and long life have been focused on various industries. Especially, the long life, i.e. fatigue failure resistance, is very important because it is directly connected to the cost and the users’ safety. Recently, the high speed train, airplane, space ship or satellite, and some of automobile parts are required giga cycle fatigue strength because of their harsh working environment. To enable this requirement, the development of base material
has been actively studied but large investments, time and efforts for researching, and validating its mechanical properties are necessary. For this reason, various studies have been made to offer a superior mechanical property by the treatment of the existing materials.
The surface treatment is a process which uses external energies to improve the material properties. For example, the peening, laser treatment, and the coating or etching processes are commonly used in serial production.
They use a mechanical, thermal, and chemical energy, respectively.
The laser heat treatment uses a laser heat source
material transforms their phase, the grains expand when it heats and shrink quickly when it quenched. For this reason, the surface will have compressive residual stress and this enables the improvement of fatigue strength.
The peening process uses mechanical energy which transfers into the surface layer of the material. The energy makes the grains small and provides the compressive residual stress to improve the mechanical properties. The shot peening is a well-known process but it is difficult to make uniform treatment and have a shallow treated surface. For this reason, several studies have been made by using the ultrasonic vibration to provide the external mechanical energy. Yasuoka2 et al. examined that the ultrasonic vibration peening can make the small surface grains by the plastic deformation and increase the fatigue strength because of the hardened surface layer.
After laser heat treatment, the surface of material changes to harder and brittle. In other words, the material will lose its ductility. Some of application may not allow losing the ductility on the material and this is the main reason why the process combination of laser heat treatment and peening is proposed.
In this work, the combination of two processes, laser heat treatment first and vibration peening later, will be studied. The experiments will be made with untreated, peening only, and different heat treatment temperatures which are under or over the transformation temperature, about 750 oC in steel and peening cases. The fatigue strength and fracture pattern of SCM440H will be examined and compared by ultrasonic fatigue tester and fractograph in each experimental case.
2. Experiments
The SCM440H is chosen because it is commonly used in the die or mold industry and structural parts of engine or transmission in automotive industry. The test specimens are prepared with combination of two processes, laser heat treatment and vibration peening.
Both processes selected because they can control the process conditions easily. To investigate the effect of the two process combination, ultrasonic fatigue test were chosen because it can expedite the fatigue experiments.
Fig. 1 shows the specimen which used in this study.
The specimen is a circular tapered hourglass-shaped with 4mm of minimum diameter, 14.5mm of notch radius, and 76.53mm of total length. The specimen is specially designed to make a resonance in the ultrasonic fatigue tester.
Fig. 2 shows the laser heat treatment system. The laser is 1kW diode laser (Laserline, LDM-1000-100).
Recently, the diode laser generally uses for the heat treatment because it can offer various beam shapes and relatively good absorptivity in steels. The laser power is controlled with a pyrometer and this enable the constant temperature of heat treatment on the specimen. The laser spot is 3mm diameter at 138mm focal length but 5mm laser spot is used to reduce the heat flux and prevent local melting. The laser heats ±5mm from the center of the specimen with controlled 600 and 800 oC of heating zone temperature.
After the laser heat treatment, the vibration peening is applied. Fig. 3 shows the vibration peening system. To apply the vibration, a Terfenol transducer (CU18A, Etrema Product, Inc) is used because it can provide larger
Fig. 1 SCM440H test specimen
Fig. 2 Laser heat treatment system
displacement and higher force to compare the PZT. A 1.2mm tungsten carbide tip hits the laser treated surface with 9.3 kHz and 8 ㎛ displacement. Current peening tip is designed to resonate at 9.3 kHz and 8 ㎛ displacement is chosen to run the stable experiments. The rotation speed and feed was 30 rpm and 0.1 mm/min, respectively.
This experimental condition referred from previous Ultrasonic Nano-Crystal Surface Modification (UNSM) study which uses 20 kHz vibration by Suh3 et al. to make the similar peening result. The conditions were modified because of the system differences.
To investigate the effect of those combinations, the fatigue tests were performed with ultrasonic fatigue tester (UFT) at a frequency of 20kHz, stress ratio of R=-1 and displacement range is ~20㎛ at room temperature. The benefit for UFT is expediting the experiments. Fig. 4 shows the UFT and the system uses a 20 kHz PZT transducer and a horn for transferring and amplifying the axial vibration. The specimen vibrates in ultrasonic resonance at one of its longitudinal modes. The
maximum displacement amplitude and stress is made at the end and middle of specimen, respectively. The compressed air is applied in the middle of specimen and put some cooling period while the testing to reduce heat generation in the specimen.
3. Results
Before examine the effect of combination of laser heat treatment and peening process on fatigue strength, a preliminary study is performed to find out whether the process combination is beneficial. Fig. 5 shows that the results for the comparison among the processes. Different process condition is given for each of specimen. No treatment, 600 oC laser heat treatment only, peening only, and 600 oC laser heat treatment and peening specimens were made and examined the fatigue strength. All the specimens were fractured at ~305MPa but the number of cycle was completely different. The samples which applied single process can stand about mega cycles but the combined process sample stands 4.69 x 108 cycles.
This result can suggest that the process combination is not changing its material characteristics but beneficial on fatigue strength. This means that the process combination can be applicable to the case which is not allowed losing its ductility. Further study will be made to investigate why this happened by examine the microstructures.
The preliminary study indicate that the process combination is beneficial but it still requires to examine how much it can improve on the fatigue strength by comparing different cases. The S-N fatigue behaviors of untreated, peening only, and 600 and 800 oC laser heat Fig. 3 Vibration peening system
Fig. 4 Ultrasonic Fatigue Tester (UFT)
Fig. 5 Number of cycles for the different conditions
treatments and peening SCM440H specimen with using UFT are shown in Fig. 6. The results show that the fatigue strength of untreated specimen can stand 6.66 x 107 cycles at 232.59 MPa. The peening only specimen can stand 7.53 x 107 cycles at 276.64 MPa. Both untreated and peening only specimens fracture at 105 cycle range at ~300MPa. The S-N curve shows that the peening only case has smaller slope then the untreated case. The results shows that the peening conditions which used in the experiments are effectively reduce the surface notches and the grain size and this can improve the fracture strength. The lower stress and higher cycles seem more effective because the peening depth is not enough to stand the high applied stress.
The effect of laser treatment shows that the fatigue strength is increased as the laser temperature increased.
In the high stress and low cycle case, the untreated specimen fractured 2.09 x 105 cycles at 305.6 MPa where the peening only case fractured but the 600 and 800 oC laser heat treated and peening cases fractured 5.07 x 105 cycles at 334.85 MPa and 2.06 x 105 cycles at 381.2 MPa, respectively. This means 600 and 800 oC laser heat treated and peening can increase the fatigue strength
~10 % and ~25%, respectively.
Fig. 7 is a fractograph of the untreated specimen which fractured 3.27 x 106 cycles at 268 MPa and 2.08 x 105 cycles at 305 MPa, respectively. Fig. 7(a) shows that the fracture initiates a single point on the surface but multiple points were observed in Fig. 7(b). Cho4 et al summarized the fracture types with respect to the number of cycles and this is a typical fracture mechanism of
megacycle and low cycle (~104 cycles) fracture.
Fig. 8 shows the fracture surface of the peening only specimen which fractured at 1.79 x 107 cycles, 303.68 MPa. Although the fracture type is observed similarly in Figs. 7 and 8, the fracture initiation was different. Fig. 9 compares the fracture initiation. The fracture initiated from the surface and ~30㎛ below the surface in the case of untreated and peening only, respectively.
Fig. 6 Stress and number of cycle (S-N) curve
(a) (b) Fig. 7 (a) SEM image, No treatment; 268MPa, 3.72x106
cycle (b) SEM image, No treatment; 305MPa, 2.08x105 cycle
Fig. 8 SEM image, peening; 303.68MPa, 1.79x107 cycle
(a) (b) Fig. 9 (a) SEM image, No treatment; 268MPa, 3.72x106
cycle (b) SEM image, peening; 303.68MPa, 1.79x107 cycle
Additionally, the grains of the near surface region are much smaller. This proves that the peening process can offer denser grains which affect positively to the fatigue strength.
Figs. 10 and 11 show the fracture surface on 600 and 800 oC laser heat treated and peened specimen. Both show that the fracture initiates from the one inner point but the initiation points are 300㎛ and 1000 ㎛ from the surface in each conditions.
4. Conclusion
This work has shown that laser heat treatment and vibration peening the SCM440H can increase the fatigue strength. The S/N curve shows that this process combination can achieve significantly higher fatigue strength in certain conditions. Additionally, the fracture
type validation of the SCM440H has been made with different cycles. The main finding about this study is that the process combination can increase the fatigue strength without changing the material’s own mechanical properties. The specific conclusions include the following:
1. The process combination, laser heat treatment and vibration peening, can drastically increase the service life to compare with the untreated, laser heat treated, and peened cases.
2. The process combination can achieve significant high fracture strength even the laser heat treatment temperature is not reaching the transformation temperature. In other words, this process combination can increase the fatigue life without losing its ductility.
3. The S/N curve slope is decreased when the peening process is applied. This happens because the surface notches and grains get smaller by the external mechanical energy. Additionally, current peening conditions are more effective on the lower stress condition.
4. The fracture strength increased ~10 and ~25% by the process combination. The untreated specimen fractured 2.09 x 105 cycles at 305.6 MPa. 600 oC and the 800 oC laser heat treated and peening case fractured 5.07 x 105 cycles at 334.85 MPa and 2.06 x 105 cycles at 381.2 MPa, respectively.
5. The fractograph shows that the fracture initiates from the surface in the untreated case but the initiation point is getting deeper from the surface as the laser heating temperature increased. The initiation points were on the surface, 300㎛ and 1000 ㎛ from the surface at untreated, 600 oC and 800 oC laser heat treated cases, respectively.
The process combination enables increased fracture strength and service life. This will be beneficial on the safety, environment, and cost. However, Na5 et al.
claimed that the surface melting by laser reduces the hardness which directly affect to the service life and Yasuoka2 et al. proposed that the over peening can increase the surface micro cracks and reduce the fatigue life. For this reason further studies will be required regarding the reasons why process combination is beneficial, the process conditions optimization, fracture mechanism on the giga cycle, and effect on Fig. 10 SEM image, 600 oC laser heat treatment and
peened; 307.23MPa, 4.69x108 cycle
Fig. 11 SEM image, 800 oC laser heat treatment and peened; 308.5MPa, 6.59x108 cycle
university research fund and by the Industrial Strategic technology development program, 10039982, development of next generation multi-functional machining systems for Eco/Bio components funded by the Ministry of Knowledge Economy (MKE, Korea).
REFERENCES
1. Kennedy, E., Byrne, G., and Collins, D. N., “A review of the Use of High Power Diode Lasers in Surface Hardening,” Journal of Materials Processing Technology, Vol. 155-156, pp. 1855-1860, 2004.
2. Yasuoka, M., Wang, P., Zhang, K., Qiu, Z., Kusaka, K., Pyoun, Y. S., and Murakami, R., “Improvement of the Fatigue Strength of SUS304 Austenite Stainless Steel Using Ultrasonic Nanocrystal Surface Modification,” Surface & Coatings Technology, Vol.
218, pp. 93-98, 2013.
3. Suh, C. M., Pyoun, Y. S., Cho, I. H., and Baek, E. B.,
“UNSM Surface Technology for Manufacturing and Remanufacturing Torsion Bars for Crawler Vehicles,” Journal of Ocean Engineering and Technology, Vol. 25, No. 6, pp. 80-85, 2011.
4. Cho, I. S., Shin, C. S., Kim, J. Y., and Jeon, Y. H.,
“Accelerated Ultrasonic Fatigue Testing Applications and Research Trends,” Trans. Korean Soc. Mech.
Eng. A., Vol. 36, No. 6, pp. 707-712, 2012.
5. Na, G. D., Yoo, Y. T., Shin, H. J., and Shin, B. H.,
“Characteristics Induction and Laser Surface hardening of SM45C Steel,” J. Korean Soc. Precis.
Eng., Vol. 23, No. 7, pp. 39-50, 2006.
