Keywords: Hot-work steel; Vacuum heat treatment; Hard chromium plating; Thermal fatigue.

Size: px

Start display at page:

Download "Keywords: Hot-work steel; Vacuum heat treatment; Hard chromium plating; Thermal fatigue."

Jewel Wood
5 years ago
Views:

1 AISI H13 Hot-Work Steel with Hard Chromium Plating Thermal Fatigue Behavior Evaluation Shu-Hung Yeh 1,a, Liu-Ho Chiu 1,b, Shou-Chi Lin 2,c and Yeong-Tsuen Pan 2,d 1 40, Chung-Shan North Road, 3rd Sec., Chung-Shan District, Taipei City 10451, Taiwan, R. O. C. 2 1 Chung Kang Road, Siaogang District, Kaosiung 81233, Taiwan, R.O.C. a shooter0630@hotmail.com, b lhchiu@ttu.edu.tw, c @mail.csc.com.tw, d t112@mail.csc.com.tw Keywords: Hot-work steel; Vacuum heat treatment; Hard chromium plating; Thermal fatigue. Abstract. A hard-coating on hot work tool steel can be used to obtain higher corrosion resistance, as well as better wear resistance. This study investigates the thermal fatigue performance of AISI H13 hot work tool steel with and without hard chromium plating. Treated specimens were characterized using microstructural analysis, X-ray diffraction analysis and microhardness measurement. The thermal fatigue test is based on cyclic induction heating and water cooling. The specimen was heated to the maximum surface temperature of 670 o C followed with water injection to bring the specimen down to a minimum temperature of 25 o C. The thermal fatigue testing in this study was conducted using 500 cycles. A vacuum heat treated specimen with a hardness of 47 HRC was used as the reference material. The hard chromium plated layer with a thickness 35 μm had a hardness of 930 HV 0.1. The damage factor, defined as crack depth crack width, of quenched and tempered H13 specimens and hard chromium plated specimens were 800 and 1760, respectively. The damage factor evaluation verified the vacuum heat treated specimen thermal fatigue resistance is superior to that of the hard chromium plated specimen. Introduction AISI H13 steel has good hardening ability, high toughness and excellent red hardness with less structural distortion, maintaining good dimensional stability throughout the hardening process [1, 2]. During the die casting process mold steel is heated using molten metals such as aluminum or magnesium, followed by cooling with water which causes mechanical stress on the hot work die. Such cyclic thermal stress will lead to plastic deformation, causing further heat crack initiation [3-5]. Thermal fatigue cracking is the most common failure behavior in all hot work die damage mechanisms [5]. Hence, a higher level of thermal fatigue resistance, thermal shock resistance and excellent high temperature strength are required in hot work die steel. Chromium electrodeposition is a well-established technique for the production of engineering component functional coatings that take advantage of high hardness levels, resistance to corrosion and wear, or low coefficient of friction. The deposition process causes extensive micro-cracking in these coatings, presenting a crystallographic texture [6-8]. Hard chromium plating is almost always applied to items that are made of steel, usually hardened steel. Hard chromium plating has been applied as a fairly heavy coating for wear resistance, lubricity, oil retention and other wear purposes. Some examples would be hydraulic cylinder rods, rollers, piston rings mold surfaces, thread guides and gun bores [8]. In a previous study the thermal fatigue effect on the hardness of hard chromium electroplating was investigated by Hadavi et al [9]. They reported that the hardness of all coatings, i.e. crack-free low contraction, cracked and high contraction, drastically reduced after applying 50 thermal cycles. This could be due to stress release. Birol [10] concluded that the Cr content in Stellite 6 alloy facilitated the formation of stable Cr-rich oxides which sustained the thermal stresses generated at the surface without spalling owing to their plasticity and thereby retarded crack initiation. Hard chromium plating treatment for wear properties improvement in tool steels is known [8], but only a few reports investigated thermal fatigue behavior applications to hot work mold steels [9]. Hence,

2 this study evaluates the hard chromium plating effects on the mechanical and thermal fatigue behavior of AISI H13 hot work steel. Experimental procedures Materials and Characterization. The chemical composition of AISI H13 hot-work steel is listed in Table 1. The specimens were sectioned from a round steel bar and machined to the dimensions shown in Fig. 1. All of the specimens were heated to 1025 C austenitizing temperature for 20 minutes, followed by 2 kg/cm 2 N 2 gas quenching and tempered at 610 C. The vacuum hardened specimens were used as reference materials, with some of the vacuum hardened specimens subjected to hard chromium plating treatment. Before the hard chromium plated treatment specimens were ground using 1200-grit abrasive paper and polished to a surface roughness (Ra) of about 0.03 μm, obtained using a Mitutoyo Surftest SV-400 surface profile. The hard chromium plating was formed in a Sargent bath to obtain 35 μm thickness. Table 1 Chemical compositions of JIS SKD61 (wt%). Wt% C Si Mn Cr Mo V Ni Fe JIS SKD Bal. Fig. 1 Dimension of the thermal fatigue specimen. The specimen cross-section microstructure and thermal fatigue pattern were revealed using a 95ml alcohol + 5ml HNO 3 chemical solution and observed using an optical microscope (Nikon OPTIPHOT-100). The prior austenite grain size given by austenitizing treatment was about ASTM No. 9. The phases in the hard chromium plated specimen case layer were characterized using an X-ray diffractometer using Cu-Kα radiation source over a 2θ range from 30 o to 90 o. The dominant peaks of the vacuum heat treated specimen in the XRD pattern were those of the tempered martensite phase. The substrate and case layer hardness were measured using a Vickers micro-hardness tester with an applied load of 0.98 N. Thermal Fatigue Testing. The thermal fatigue test equipment was based on cyclic induction heating using 150 khz frequency and 12 kw power to induce fast heating on the specimen surface. The specimen was heated to the maximum surface temperature of 670 ± 20 o C during 1.5 s to rapidly simulate the die casting temperature condition. Water injection brought the specimen to a minimum temperature of 25 o C during 2.0 s. The cycling time was 3.5 s. The temperature was recorded during the thermal cycles at the surface as illustrated in Fig. 2. The specimen temperature was measured during thermal cycling using K-type thermocouples fixed at the steel surface. Medium carbon steel S45C was employed to double check the temperature measurement. There was no evident martensite transformation at the surface for S45C after 500 cycles of thermal fatigue testing. It was confirmed that the surface temperatures were not much different with respect to the thermal fatigue tests performed with hot work mold steel.

Fig. 2 Temperature recorded during thermal cycles. The thermal fatigue testing was conducted using 100~1000 cycles. After testing the surface oxides were removed by soaking the specimen in a 15 vol.

The thermal fatigue properties including the damage factor (crack width crack depth) and crack density (number of cracks per unit length) were revealed from a polished cross-section investigated

3 Fig. 2 Temperature recorded during thermal cycles. The thermal fatigue testing was conducted using 100~1000 cycles. After testing the surface oxides were removed by soaking the specimen in a 15 vol. % HCl solution for 15 min. The surface thermal crack morphology and propagation behavior was investigated using an optical microscope (OM, Nikon OPTIPHOT-100). The thermal fatigue properties including the damage factor (crack width crack depth) and crack density (number of cracks per unit length) were revealed from a polished cross-section investigated using an optical microscope. Microhardness profiles after thermal cycling for all treated and tested specimens were measured using Vickers indentation on the polished cross-section with an applied load of 0.98 N. Results and Discussion Hard Chromium Plated Layer Characteristics. Figure 3 shows the surface and cross-sectional micrographs of hard chromium plated H13. The surface roughness with a Ra value of about 2~3 μm obtained in the case of hard chromium plated specimens is consistent with the optical microscopy observations. A severe bumpy morphology was detected on the surface, as shown in Fig. 3 (a). A hard Cr plated layer with a thickness of 35 ± 2 μm can be observed at the surface, soundly bonded with the martensite phase underneath. Micro-cracks were also observed in the plated layer, as indicated in Fig. 3 (b). The dominant peaks of the hard chromium plated specimen in the X-ray diffraction pattern are chromium metal. The microhardness value of the hard chromium plated layer and substrate are 930 ± 11 HV 0.1 and 470 ± 7 HV 0.1, respectively. Fig. 3 (a) surface and (b) cross-sectional micrographs of hard chromium plated specimen. Surface Cracking. The thermal stresses generated inside the steel can be estimated from, σ surface = α (T) E(T) (ΔT) where α and E are the thermal expansion coefficient and the Young s modulus of the steel, respectively and ΔT is the temperature gap between the maximum and minimum specimen temperatures. The thermal stresses acting on the surface are estimated to be 700 MPa. The thermal stress is safely below the room temperature yield strength of hot work steel with a hardness of 47 HRC. However, the yield strength could gradually be degraded, leading to steel failure when cyclic thermal stress is applied [10]. The surface cracking morphologies of hard chromium plated and vacuum hardened specimens with a hardness of 47 HRC (1500 MPa) are shown in Fig. 4. After 100 thermal fatigue test cycles, shorter and thinner cracks were observed at the center of the heated area. Several slight cracks

gradually stretched and broadened as the thermal cycles increased to 300 cycles.

as the thermal cycles increased to 1000 cycles, see Fig. 4 (d).

different from the quenched and tempered H13 specimen.

Fig. 4 General views of the surface crack morphology on vacuum hardened and hard chromium

The cross section microstructures of vacuum hardened and hard chromium plated specimens

One can see no obvious microcracks in the surface of the hardened H13 specimen less than 100

The thermal cracks were easily and clearly observed at the surface after 300 thermal cycles

4 gradually stretched and broadened as the thermal cycles increased to 300 cycles. After 500 cycles of thermal fatigue testing the cracks gradually cross-linked and the crack width increased to about 8~10 μm. The cracks propagated continuously and then cross-linked together with a dendritic structure as the thermal cycles increased to 1000 cycles, see Fig. 4 (d). However, the crack morphologies of the hard chromium plated H13 specimen were totally different from the quenched and tempered H13 specimen. The broadened cracks were not obvious on the hard chromium plated H13 specimen surface after 500 thermal cycles. Only a slight bumpy morphology could be observed at the surface, as indicated in Fig. 4 (e). Fig. 4 General views of the surface crack morphology on vacuum hardened and hard chromium plated H13 specimens in the course of thermal cycling. Crack Depth and Crack Density. The cross section microstructures of vacuum hardened and hard chromium plated specimens during thermal cycling are shown in Fig. 5. One can see no obvious microcracks in the surface of the hardened H13 specimen less than 100 thermal cycles. The thermal cracks were easily and clearly observed at the surface after 300 thermal cycles and the crack depth increased with increasing thermal cycles. The crack depth was about 250 μm when the thermal cycles were increased to The cracks were obviously formed from the defects existing in the plated layer, as shown in Fig. 5 (e). 100 cycles 300 cycles 500 cycles 1000 cycles 500 cycles Fig. 5 General views of the cross-section crack morphology on (a) ~ (d) vacuum hardened and (e) hard chromium plated specimens in the course of thermal cycling. The polished cross-section micrograph also analyzed crack features such as damage factor and crack density. Two crack features in the surface of vacuum hardened and hard chromium plated

specimens are shown in Fig. 6. The hardened specimens with a series of 100, 300, 500 and 1000 thermal cycles had damage factors of 30, 240, 800 and 2020 μm 2, respectively.

5 specimens are shown in Fig. 6. The hardened specimens with a series of 100, 300, 500 and 1000 thermal cycles had damage factors of 30, 240, 800 and 2020 μm 2, respectively. As the thermal cycles were increased from 100 to 300, the damage factor rapidly increased 8 times to 240 μm 2, and further increased about 67 times to 2020 μm 2 as the thermal cycles were increased to This indicates that the damage factor has a clear dependence on the number of cycles, as shown in Fig. 6 (a). However, the crack density was not strongly dependent on the number of cycles (Fig. 6 (b)). Some cracks propagated into the material due to the stress concentration. The damage factor of quenched and tempered H13 specimens and hard Cr plated specimens were 800 and 1760, respectively. Compared to the hardened specimen after 500 thermal cycles, the hard Cr plated specimen damage factor increased 2.2 times to 1760 μm 2. This reveals that a cracked plated layer at the hot work steel surface causes negative influence on the thermal fatigue property. The stress might be concentrated rapidly at the original plated specimen micro cracks during cyclic induction heating, causing substrate cracks to form beneath the plated layer micro cracks and propagating more easily [11]. Fig. 6 (a) damage factor and (b) crack density of vacuum hardened and hard chromium plated specimens. Hardness after Thermal Fatigue. The microhardness profile was measured using a Vickers microhardness tester with an applied load of 0.98 N to investigate the hardness variation corresponding to thermal fatigue. Figure 7 shows the microhardness profiles of vacuum hardened specimens with different thermal fatigue test cycles and hard chromium plated specimens after 500 thermal fatigue test cycles, respectively. One can see that thermal cycling to 670 C causes softening in the surface area for all hardened specimens tempered at 610 C. After 100 ~ 1000 cycles at 670 C, the initial surface hardness for the hardened specimens was reduced approximately in the 25~32% range. The hardness increased with increasing depth from the surface down to the substrate and the heat effect zone depth was about 700 μm, see Fig. 7 (a). On the other hand, after 500 cycles the hardness of the hard Cr plated layer was reduced 63 % from 930 to 345 HV. The heat effect zone depth for the hard Cr plated H13 specimen was about 1100 μm, as shown in Fig. 7 (b). Persson [12] mentioned that surface engineering processes negatively influence the tool mechanical properties and further decreased the resistance against surface cracking. The broader and deeper cracks in the hard Cr plated specimen facilitate stress relief in the surface layer [4]. Fig. 7 Microhardness profiles of (a) vacuum hardened specimens with different thermal fatigue test cycles and (b) hard chromium plated specimens after 500 thermal fatigue test cycles.

6 Conclusions OM observations showed the hard Cr plated H13 specimen with a coating layer 35 ± 2 μm in thickness presented a severely bumpy morphology with a surface roughness (Ra value) of about 2~3 μm. The highest surface microhardness value, 930 ± 11 HV 0.1, was obtained for the hard chromium plated layer. From the transverse crack morphology observation, the hardened specimen damage factors under thermal testing for 100, 300, 500 and 1000 cycles were 30, 240, 800 and 2020 μm 2, respectively. The damage factor has a clear dependence on the number of cycles. The cracks were formed obviously from the defects existing in the plated layer. The damage factor of quenched and tempered H13 specimens and hard Cr plated specimens were 800 and 1760, respectively. The damage factor for the hard Cr plated specimen increased 2.2 times to 1760 μm 2 compared with the vacuum hardened specimen. The cracked layer at the hot work steel surface causes negative influence on the thermal fatigue property. Acknowledgment Financial support of this research by China Steel Corporation, under the grant RE is gratefully acknowledged. References [1] H.K. Rafi, G.D.J. Ram, G. Phanikumar and K.P. Rao: Mater. and Des. Vol. 32 (2011), p. 82. [2] D.S. Ma, J. Zhou, Z.Z. Chen, Z.K. Zhang, Q.A. Chen and D.H. Li: J. Iron Steel Res. Vol. 16(5) (2009), p. 56. [3] M. Koneshlou, K.M. Asl and F. Khomamizadeh: Cryogenics Vol. 51 (2011), p. 55. [4] C. Wang, H. Zhou, P.Y. Lin, N. Sun, Q. Guo and P. Zhang: Mat. Desig. Vol. 31 (2010), p [5] P. Ans, J. Dillea and M. Degrez: Surf. Coat. Technol. Vol. 205 (2011), p [6] M.P. Nascimento, R.C. Souza, I.M. Miguel, W.L. Pigatin and H.J.C. Voorwald: Surf. Coat. Technol. Vol. 138 (2001), p [7] J. Pina, A. Dias, M. Francois and J.L. Lebrun: Surf. Coat. Technol. Vol. 96 (1997), p [8] T. Sahraoui, S. Guessasma, N.E. Fenineche, G. Montavon and C. Coddet: Mat. Lett. Vol. 58 (2004), p [9] S.M.M. Hadavi, A.A. Zadeh and M.S. Jamshidi: J. of Mat. Proc. Technol. Vol. 147 (2004), p [10] Y. Birol: Mater. Sci. Eng. A Vol. 527 (2010), p [11] S.H. Yeh, L.H. Chiu, W.C. Lo and C.L. Huang: Mat. Trans. Vol. 54 (2013), p [12] A. Persson, S. Hogmark and J. Bergstrom: Surf. Coat. Technol. Vol. 191 (2005), p. 216.

Available online at ScienceDirect. Procedia Engineering 114 (2015 )

Available online at ScienceDirect. Procedia Engineering 114 (2015 ) Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 114 (2015 ) 606 612 1st International Conference on Structural Integrity Fatigue Performance Of AISI 4340 Steel Ni-Cr-B-Si-Fe