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Influence of laser surface engineering of AISI P20-improved mold steel on wear and corrosion behaviors
Surface & Coatings Technology 377 (2019) 124852
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Influence of laser surface engineering of AISI P20-improved mold steel on wear and corrosion behaviors Changkyoo Parka, Ahjin Simb, Sanghoon Ahna, Heeshin Kanga, Eun-Joon Chunc,
T
⁎
a
Laser and Electron Beam Application Department, Korea Institute of Machinery and Materials, Daejeon 34103, Republic of Korea Laser Industrial Technology Research Group, Korea Institute of Machinery and Materials, Busan 46744, Republic of Korea c Department of Nano Materials Science and Engineering, Kyungnam University, Changwon 51767, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser surface engineering Mold steel AISI P20-improved Fretting Block-on-ring Corrosion
The wear and corrosion characteristics of a mold steel (AISI P20-improved, KP4M) were investigated before and after the laser surface engineering. The laser surface engineering of AISI P20-improved steel was conducted at different laser energy densities by a high-power diode laser. The microstructure of the base metal changed from tempered martensite to lath-type martensite. In addition, the surface hardness was enhanced from 307 HV to 545, 657, and 610 HV when the laser energy density was 220, 315, and 420 J/mm2, respectively. An electron probe micro-analyzer revealed that microstructural homogenization was introduced by the laser surface engineering at 220 and 315 J/mm2, whereas an elemental segregation of chromium and manganese was observed with the laser surface engineering at 420 J/mm2 due to partial melting and solidification process. The laser surface-engineered samples showed an enhancement in the wear resistance in both the fretting and block-on-ring tests as compared to that of the base metal, and the highest wear resistance was detected for the laser surfaceengineered at 315 J/mm2 owing to the largest increment in the hardness. In terms of corrosion resistance, the laser surface-engineered AISI P20-improved steel at 315 J/mm2 showed a marginal improvement in the corrosion resistance as compared to that of the base metal because of the microstructural homogenization. Contrastingly, a marginal deterioration of the corrosion resistance was examined for the laser surface-engineered AISI P20-improved steel at 420 J/mm2 because of the elemental segregation.
1. Introduction The use of plastics has been increasing in various applications such as automobiles, shipbuilding, airplanes, and home appliances owing to their low density, low cost, and ease of manufacturing [1], and more than 30% of plastic components have been manufactured by the injection molding process [2]. In the injection molding of plastics, the mold is a critical element in terms of the productivity and quality of the final products. Damages in a mold deteriorate productivity and lead to poor-quality final products. Thus, such a mold should be discarded [3,4]. During the injection molding process, the molds are subjected to wear conditions due to vibration and misalignment over time, and this can cause surface damages in the form of corrosion, abrasion, and fatigue cracking [5–8]. AISI P20-improved steel is a chromium- and molybdenum-containing low carbon steel with high hardenability, which is commonly utilized for general mass production of molds. In their lives, molds are expected to produce several millions of plastic components. Although
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the above- mentioned steel shows reasonable mechanical properties, the lifetime of the mold can be extended by surface treatments. Various surface modification processes for steels have been introduced, such as surface coating and alloying [9–15], shock peening [16], and surface heat treatment [17–20]. Only limited studies have reported the effect of laser-assisted surface modification on the wear and corrosion resistances of steels. Telasang et al. reported the influence of laser surface hardening and melting on the wear and corrosion behavior of AISI H13 tool steel [20]. Lesyk et al. introduced consecutive processes of diode laser heat treatment and ultrasonic impact treatment to enhance the surface hardness and wear resistance of AISI D2 tool steel [21]. However, only a few studies have investigated the effect of laser-assisted surface treatment on the wear and corrosion behaviors of mold steels. In this study, AISI P20-improved, mold steel, was subjected to diode laser surface engineering (LSE) at different laser energy densities to induce surface hardening and microstructural homogenization. Fretting and block-on-ring tribotests were conducted to investigate the correlation between the LSE and wear resistance. Moreover, the
Corresponding author. E-mail address: [email protected] (E.-J. Chun).
https://doi.org/10.1016/j.surfcoat.2019.08.006 Received 17 May 2019; Received in revised form 31 July 2019; Accepted 2 August 2019 Available online 28 August 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 377 (2019) 124852
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Fig. 1. Experimental set-up for the laser surface engineering of AISI P20-improved steel.
electrochemical behavior was examined to investigate the effect of the LSE on the corrosion behavior.
Laser energy denisty (J / mm2) =
Laser Power (W ) Beam depth (mm) × Speed (mm / s ) (1)
In the case of the LSE at 1000 and 1200 °C, the surface temperature of the specimen was measured by a two-color pyrometer (LASCON), which was coaxially mounted on the laser head. However, in the case of the LSE at 1500 °C, a thermocouple (B-type) was used to measure the surface temperature because the target surface temperature exceeded the pyrometer limit. The laser head was tilted 5° to the normal direction of the sample to prevent damage of the laser head by the reflection of the laser beam.
2. Materials and methods 2.1. Experimental set-up for laser surface engineering Fig. 1 shows the experimental set-up for the LSE process. A commercial AISI P20-improved steel with a dimension of 100 (W) × 150 (D) × 15 (H) mm was used for this study, and its chemical composition is provided in Table 1. A 4-kW diode laser (TeraBlade Laser, TeraDiode Inc., USA) with a continuous wave mode and wavelength of 970 nm was utilized for the laser surface engineering. The laser beam had a flat top energy distribution with a dimension of 24 (W) × 1 (D) mm. The scan speed of the laser beam was 5.0 mm/s, and the focal length was 310 mm. The laser energy densities for the LSE process of AISI P20improved steel were set as 220, 315, and 420 J/mm2, corresponding to the specimen surface temperature of 1000, 1200, and 1500 °C, respectively. The laser energy density was calculated as
2.2. Macrostructure and microstructure The macrostructures and microstructures of the base metal and laser surface-engineered AISI P20-improved steels were observed via optical microscopy (OM; BX51M, Olympus) and scanning electron microscopy (SEM; SU5000, Hitachi). The qualitative analysis of the base metal and laser surface-engineered AISI P20-improved steels was conducted by Xray diffraction (XRD, SmartLab, Rigaku) with Cu-Kα radiation. The
Table 1 Chemical composition of the AISI P20-improved steel (mass %).
AISI P20-improved
C
Ni
Si
Mn
P
S
Cr
Mo
Cu
Al
Fe
0.38
0.12
0.26
1.02
0.0092
0.022
1.72
0.46
0.061
–
Bal.
2
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Fig. 2. Experimental set-up for the (a) fretting test and (b) block-on-ring test. An alumina ball and a steel ring were used as the deformation tool for the fretting and block-on-ring tests, respectively.
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LSE at 420 J/mm2, the average laser surface-engineered region has a depth of 754 ± 44 μm, whereas those for the LSE at 220 and 315 J/ mm2 are 490 ± 35 and 631 ± 25 μm, respectively. The depth of the laser surface-engineered region was measured ten times and averaged. Two different zones are observed at the laser surface-engineered region (labeled as zone 1 and zone 2) for the LSE at 420 J/mm2. Fig. 3 (b) shows the top-sectional SEM images of the base metal and laser surfaceengineered AISI P20-improved steels. For the base metal, typical tempered martensite is observed, whereas lath-type martensite is found for the laser surface-engineered AISI P20-improved steels [22]. Fig. 4 demonstrates the X-ray diffraction profiles of the surface of the base metal and laser surface-engineered AISI P20-improved steels. All the peaks indicate the martensite phase for both the base metal and laser surface-engineered AISI P20-improved steels. The XRD peaks are marginally shifted to lower 2θ value with increasing laser energy densities. Moreover, the marginal broaden XRD peaks are detected for the laser surface-engineered AISI P20-improved steels in comparison to those of the base metal. The peak broadening is caused by the presence of the residual stress and the decrease of crystallite size in the microstructure after the laser surface engineering [23]. The EPMA analysis was conducted to examine the elemental distribution before and after the LSE. Fig. 5 shows the observation points, back-scattered electron (BSE) images, and EPMA results for the base metal and LSE at 315 and 420 J/mm2. For the base metal, the elemental distribution was analyzed near the surface, as shown in Fig. 5 (a). In the elemental distribution maps of carbon, iron, and chromium, nonhomogeneous elemental distributions are detected. On the other hand, for the LSE at 315 J/mm2 (Fig. 5 (b)), the homogenous elemental distributions of carbon, iron, chromium, and manganese are clearly observed in the EPMA results, and similar results are obtained for the LSE at 220 J/mm2. Fig. 5 (c) shows the EPMA results for zone 1 of the LSE at 420 J/mm2. The cell structure with coarse grains is clearly detected from the BSE image, and this indicates that the specimen is partially melted and solidified by the LSE. The elemental distribution maps of manganese and chromium reveal that these elements are highly segregated at the grain boundary regions. This result is consistent with that of G. Telasang et al., who studied the correlation between microstructure and mechanical properties of AISI H13 tool steel before and after laser surface treatment [23]. Moreover, a relatively severe nonhomogeneity of carbon is detected from the EMPA result. Fig. 5 (d) shows the EMPA results for zone 2 of the LSE at 420 J/mm2, and no cell structure is detected from the BSE image. This suggests that the sample was heat-treated by the LSE without melting. The EMPA results reveal that iron, chromium, and manganese are homogeneously distributed, whereas non-homogeneity of carbon is locally detected.
elemental distribution was analyzed by electron probe X-ray microanalyzer (EPMA, JXA-8530F, JEOL) before and after the laser surface engineering. Moreover, the chemical composition of wear debris was analyzed by energy dispersive X-ray (EDX) detector and scanning electron microscopy (SEM; SU5000, Hitachi). 2.3. Mechanical properties The residual stress at the surface of the base metal and laser surfaceengineered AISI P20-improved steels was determined by X-ray diffraction (XRD, D/MAX-2500/PC, Rigaku) using a two-angle sin2Ψ technique at {211} plane, employing the Cu-Kα radiation. Vickers hardness testing (MMT-X7, Matsuzawa) was conducted five times with a load of 0.5 kgf and dwell time of 10 s, and the obtained values were averaged. 2.4. Tribological test A reciprocating friction instrument (RFW160, NeoPlus; Fig. 2 (a)) with a 10 mm diameter alumina ball was utilized for the fretting test. The dimension of the samples was 40 (W) × 40 (D) × 10 (H) mm, and the normal loads of 10, 20, and 30 N were applied to the samples for 30,000 cycles. The frequency was 4 Hz, and the displacement amplitude was 300 μm. The samples were polished by SiC papers before the fretting tests, and the average surface roughness (Ra) was less than 0.3 μm. All the fretting tests were conducted three times for each condition and averaged the obtained wear losses. A dry sliding wear test was performed by a block-on-ring tribotest (BRW-150, NeoPlus; Fig. 2 (b)) with a S45C steel ring. The outer diameter of the ring was 45 mm, whereas the dimension of the block specimen was 16 (W) × 5 (D) × 10 (H) mm. The sliding speed and distance were 0.236 m/s and 1300 m, respectively. The normal loads were set as 170, 255, and 340 N. During sliding, the surface temperature of the steel ring was measured using a thermocouple (K-type). To prevent abnormal wear behavior in the initial stages of the tribotests, a run-in process was performed using the SiC papers (220 grit) attached on the ring surface. All the block-on-ring tribotests were performed three times for each condition and averaged the obtained wear losses. The tribological tests were performed at 25 °C with a relative humidity of 40–60%. After the tribological tests, the total wear loss and depth profiles of the wear scars were examined via laser microscopy (VK-8710, Keyence). The surface morphologies of the produced wear scars were observed via SEM. 2.5. Corrosion test The corrosion tests were performed using a computerized potentiostat/galvanostat (VMP3, BioLogic Science Instruments). The corrosion circuit consisted of three electrodes: a platinum sheet as the counter electrode, an AISI P20-improved steel as the working electrode, and a saturated calomel electrode (SCE) as the reference electrode. A 3.5% NaCl solution was used as the electrolyte. The potentiostatic/ galvanostatic study was performed from −2 to +1 V at a fixed scan rate of 3.0 mV/s. After the corrosion tests, the EC-Lab® V11.21 software was used to build a Tafel plot and analyze the corrosion potential and corrosion rate. Moreover, the surface morphologies of the corroded samples were examined via SEM to evaluate the corrosion degradation.
3.2. Mechanical properties Fig. 6 shows the residual stress at the surface of AISI P20-improved steels before and after the laser surface engineering process. The similar level of residual stress is detected for the LSE at 220 J/mm2 with the base metal, while a significant compressive residual stress (−1195 MPa) is measured for the LSE at 315 J/mm2. The introduced compressive stress at the surface may increase the wear resistance and fatigue strength [24,25]. On the other hand, in the case of the LSE at 420 J/mm2, the tensile residual stress (870 MPa) is detected at the surface due to rapid quenching from partially melted state to solid state. These state changes may deteriorate the fatigue strength, resulting in the reduction of the prevention of surface cracks propagation [26]. Fig. 7 shows Vickers hardness of the base metal and laser surfaceengineered AISI P20-improved steels as a function of the depth from the surface. The hardness was measured five times at every 100 μm of depth, and the values were averaged. The error bars in Fig. 7 indicate the range of measured microhardness. The surface hardness of the base metal is measured as 307 HV, whereas it is found to be 545, 610, and 657 HV for the LSE at 220, 420, and 315 J/mm2, respectively.
3. Results and discussion 3.1. Macrostructure and microstructure Fig. 3 (a) shows the cross-sectional OM images of the base metal and laser surface-engineered AISI P20-improved steels at 220, 315, and 420 J/mm2, where the white dashed rectangle line indicates the laser surface-engineered region. A deep laser surface-engineered region was observed when a high laser energy density is applied to the sample. The 4
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Fig. 3. (a) Cross-sectional and (b) top-sectional macrostructures of the base metal and laser surface-engineered AISI P20-improved steels at 220, 315, and 420 J/mm2. Two different zones are detected for the cross-sectional images of the laser surface engineering at 420 J/mm2.
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Fig. 10 shows the cumulative wear loss as a function of the cycles at a normal load of 30 N for the base metal and laser surface-engineered AISI P20-improved steels. Every fretting test was conducted three times, and the obtained wear loss was averaged. The error bars in Fig. 10 indicate the range of measured wear loss. The base metal shows the highest cumulative wear loss. At 10,000 cycles, the cumulative wear loss is 5.28 × 10−3 mm3, and it increases to 13.91 × 10−3 mm3 at 30,000 cycles. On the other hand, the lowest cumulative wear loss is observed for the LSE at 315 J/mm2. The cumulative wear loss is 3.76 × 10−3 mm3 at 10,000 cycles and 7.71 × 10−3 mm3 at 30,000 cycles. The cumulative wear loss for the LSE at 315 J/mm2 at 30,000 cycles is reduced by 44.6% in comparison to that of the base metal. This result indicates that the increase in the surface hardness by the LSE causes the increase in the wear resistance and decrease in the wear loss. Fig. 11 shows the depth profiles of the wear scars developed after 30,000 cycles of the fretting test at a normal load of 30 N. The highest depth of the wear scars is detected from the base metal, which shows the largest cumulative wear loss, whereas the lowest depth of the wear scars is observed for the LSE at 315 J/mm2, which shows the smallest cumulative wear loss. In addition, in the case of the base metal, numerous peaks are detected in the depth profile, and this indicates that the surface roughness of the wear scar is relatively high due to the debris accumulation and generated abrasion grooves during the fretting test. Contrastingly, in the case of the laser surface-engineered AISI P20improved steels, relatively smoother depth of profiles are observed compared to that of the base metal because of the low debris accumulation, and fewer generated abrasion grooves at the wear scars. The total wear losses of the base metal and laser surface-engineered AISI P20-improved steels at 10, 20, and 30 N of normal loads and 30,000 cycles are summarized in Table 2. It is evident that the LSE of the AISI P20-improved steel increases the wear resistance, resulting in a smaller amount of wear loss for the laser surface-engineered AISI P20improved steels than that of the base metal. Fig. 12 shows the SEM images of the produced wear scars for the base metal and laser surface-engineered AISI P20-improved steels at a normal load of 30 N after 30,000 cycles of the fretting test. For the base metal, a large amount of wear debris is accumulated over the wear scar. Moreover, relatively deep and thick abrasion grooves are observed at the wear scar. The average surface roughness (Ra) of the wear scar is found to be 6.33 μm. On the other hand, a relatively small amount of wear debris is detected at the wear scar for the laser surface-engineered AISI P20-improved steel. The wear debris accumulation is dispersed at the wear scar for the base metal due to a large amount of wear debris generation during the fretting test. However, for the laser surface-engineered AISI P20-improved steel, the wear debris accumulation is detected mostly at the edge of the wear scar. Also, relatively thin and sallow abrasion grooves are detected at the wear scar for the laser surface-engineered AISI P20-improved steel. The average surface roughness (Ra) is 4.22 μm for the LSE at 220 J/mm2, 3.61 μm for the LSE at 315 J/mm2, and 3.75 μm for the LSE at 420 J/mm2. These results suggest that the LSE improves the wear resistance of the AISI P20-improved steel, resulting in the smaller amount of wear debris and thinner abrasion grooves compared to that of the base metal. A comparable width and length of the wear scars are detected for the base metal and laser surface-engineered AISI P20-improved steels, and this is because the number of fretting test cycles is sufficient to develop matured wear scars. The produced wear debris was collected after the fretting test of laser surface-engineered AISI P20-improved steels at 315 J/mm2, and the chemical composition was characterized by EDX analysis. The fretting test was conducted at the normal load of 20 N with 30,000 cycles. Fig. 13 shows the SEM image, EDX spectrum, and chemical composition of the collected wear debris. The EDX area analysis is performed at the region of SEM images. Oxygen is detected from the result of chemical composition, and this confirms the oxidation of wear debris during the fretting test. The hard wear debris may act as an
Fig. 4. XRD diffraction pattern of the base metal and laser surface-engineered AISI P20-improved steels at 220, 315, and 420 J/mm2.
Compared to the base metal, the surface hardness for the LSE at 315 J/ mm2 is increased by 114.1% caused by the microstructural change from the tempered martensite to martensite [22]. For the LSE at 420 J/mm2, a slightly lower hardness is measured than that for the LSE at 315 J/ mm2, and this may be attributed to the elemental segregation, as shown in the EMPA results. 3.3. Wear behavior The effect of the LSE on the wear behavior of the AISI P20-improved steel was investigated by the fretting and block-on-ring tests. 3.3.1. Fretting test Fretting tests were conducted for up to 30,000 cycles at different normal loads of 10, 20, and 30 N for the base metal and laser surfaceengineered AISI P20-improved steels. Fig. 8 shows the average coefficient of friction (COF) as a function of the normal load. The average coefficients of friction are obtained under a steady state, and the error bars represent the range of coefficient of friction. The base metal has the highest average COF at each normal load. An intermediate average COF is obtained for the LSE at 220 J/mm2, and the lowest average COF is obtained for the LSE at 315 and 420 J/mm2. Moreover, a high average COF is achieved when a high normal load is applied to the samples. Fig. 9 displays COF as a function of cycles for the base metal and laser surface-engineered AISI P20-improved steels at a normal load of 30 N. For each sample, high peaks of COF are observed in the initial stage of the fretting test, then COF reaches the steady state. The initial increase in COF is caused by a strong adhesion at the surface asperities of the sample and counter material (e.g., alumina ball), hindering the reciprocating motion. Therefore, the friction force and COF increase. At some point, the tangential force applied at the contacting area overcomes the adhesive bond of the interface, and then fracture occurs, resulting in a decrease of the friction force and COF. The wear mechanism changes from a two-body to three-body condition with continuing wear debris generation and accumulation. As a result, COF reaches a steady state [27–29]. Compared to the base metal, the laser surface-engineered AISI P20-improved steels show a relatively smaller variation in COF. For the base metal, the average COF is 0.583, and the COF values vary from 0.562 to 0.609. Contrastingly, for the LSE at 315 J/mm2, the average COF is 0.36, and the COF values fluctuate from 0.341 to 0.379. The significant variation in COF during the fretting test can be influenced by the strong interaction between the interface and wear debris. 6
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Fig. 5. EPMA analysis for the (a) base metal, (b) laser surface-engineered at 315 J/mm2, (c) zone 1 (partially melted and solidified) of the laser surface-engineered at 420 J/mm2, and (d) zone 2 (heat-treated) of the laser surface-engineered at 420 J/mm2.
abrasive, resulting in surface damage of the mold [25].
surface-engineered AISI P20-improved steels. The sliding distance was 1300 m, and the normal loads were set as 170, 255, and 340 N. Fig. 14 shows the average COF for the base metal and LSE at 220, 315, and 420 J/mm2. The average coefficients of friction are obtained under a
3.3.2. Block-on-ring test Block-on-ring tests were performed for the base metal and laser 7
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Fig. 5. (continued)
measured surface temperature of the steel ring as a function of the sliding distance during the block-on-ring tribotest for the base metal (Fig. 15 (a)), LSE at 220 J/mm2 (Fig. 15 (b)), LSE at 315 J/mm2 (Fig. 15 (c)), and LSE at 420 J/mm2 (Fig. 15 (d)). The normal load was set as 340 N. In the initial stage of the tribotest, a considerable variation in
steady state, and the error bars indicate the range of coefficient of friction. At each normal load, the highest average COF is obtained for the base metal and followed by the LSE in the order of 220, 420, and 315 J/mm2. Moreover, when a high normal load is applied to the samples, a high average COF is derived. Fig. 15 shows COF and the 8
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Fig. 6. Residual stress at the surface of the base metal and laser surface-engineered AISI P20-improved steels at 220, 315, and 420 J/mm2. Fig. 9. Coefficient of friction for the base metal and laser surface-engineered AISI P20-improved steels during 30,000 cycles of the fretting test. The normal load was set as 30 N.
Fig. 7. Vickers hardness of the base metal and laser surface-engineered AISI P20-improved steels as a function of the depth from the surface. Fig. 10. Cumulative wear loss for the base metal and laser surface-engineered AISI P20-improved steels as a function of cycles at a normal load of 30 N.
Fig. 8. Average coefficient of friction of the base metal and laser surface-engineered AISI P20-improved steels as a function of the normal load for the fretting test. Fig. 11. Depth profiles of the wear scars after 30,000 cycles of the fretting test at a normal load of 30 N for the base metal and laser surface-engineered AISI P20-improved steels. 9
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during the tribotest. The increase in the surface temperature of the steel ring is 51.1 °C for the base metal, whereas it is 29.6 °C for the LSE at 315 J/mm2. The large increase in the surface temperature suggests that severe wear environments present at the interface of the AISI P20-improved steels and steel ring, resulting in significant and non-uniform COF values. Fig. 16 shows the cumulative wear loss as a function of the sliding distance at a normal load of 340 N. The block-on-ring tests were performed three times for every condition, and the measured wear loss was averaged. The error bars indicate the range of measured wear loss. The base metal shows the largest cumulative wear loss, followed by the LSE at 220, 420, and 315 J/mm2. A linear increase in the wear loss is detected for both the base metal and laser surface-engineered AISI P20improved steels, and this indicates that there is no change in the wear mechanism during the tribotest. The cumulative wear loss is 58.85 × 10−2 mm3 at 1300 m of sliding distance for the base metal, and this value decreases to 23.30 × 10−2 mm3 for the LSE at 315 J/ mm2. Fig. 17 shows the depth profile of the base metal and LSE at 220, 315, and 420 J/mm2 after 1300 m of sliding distance at a normal load of 340 N. The deepest and largest wear scar is observed for the base metal. The depth of the deepest wear scar is found to be 275.4 μm, and its width is 7.04 mm. Contrastingly, the shallowest and smallest wear scar is detected for the LSE at 315 J/mm2. The deepest depth and width of this wear scar is 119.2 μm and 4.71 mm, respectively. The total wear loss for the base metal and laser surface-engineered AISI P20-improved steels after the sliding distance of 1300 m at the normal load of 170, 255, and 340 N is summarized in Table 3. A relatively small-sized wear scar and a small amount of wear loss are observed for the laser surfaceengineered AISI P20-improved steels than those for the base metal, and
Table 2 Summary of the wear loss from the fretting tests of the base metal and laser surface-engineered AISI P20-improved steels at the laser energy densities of 220, 315, and 420 J/mm2. The fretting test was performed at the normal loads of 10, 20, and 30 N with 30,000 cycles. Wear loss (×10−3 mm3) for 10 N Base metal 220 J/mm2 315 J/mm2 420 J/mm2
7.06 4.38 3.98 4.06
± ± ± ±
0.64 0.55 0.31 0.20
Wear loss (×10−3 mm3) for 20 N
Wear loss (×10−3 mm3) for 30 N
10.77 ± 0.88 6.76 ± 0.41 5.86 ± 0.33 5.79 ± 0.41
13.91 ± 1.09 8.99 ± 0.77 7.71 ± 0.62 7.91 ± 0.58
COF is observed both for the base metal and laser surface-engineered AISI P20-improved steels, and this may be caused by the oxide layer removal and surface leveling. After this significant variation in COF, relatively stable and uniform COF values are obtained for the laser surface-engineered AISI P20-improved steels. However, non-uniform COF values are obtained for the base metal during the tribotest. The numerous peaks and large fluctuation in the COF values are observed for the base metal owing to the relatively low surface hardness [11]. This result indicates that a relatively uniform wear behavior is maintained during the tribological test for the laser surface-engineered AISI P20-improved steels compared to that for the base metal, and the surface temperature profiles of the steel ring support this result. The initial surface temperature is 33.6 °C, and it increases to 84.7 °C after 1300 m of sliding distance for the base metal. Contrastingly, for the LSE at 315 J/mm2, the surface temperature increases from 34 to 63.6 °C
Fig. 12. Surface morphologies of the developed wear scars after 30,000 cycles of the fretting test at a normal load of 30 N for the base metal and laser surfaceengineered AISI P20-improved steels. 10
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Fig. 13. SEM image, EDX spectrum, and chemical composition of produced wear debris during the fretting test for the laser surface-engineered AISI P20-improved steels at 315 J/mm2.
Fig. 14. Average coefficient of friction of the base metal and laser surface-engineered AISI P20-improved steels from the block-on-ring test at different normal loads.
these results suggest that the LSE increases the surface hardness, resulting in an improvement in the wear resistance. Fig. 18 shows the SEM image of the surface morphology of the produced wear scar by the block-on-ring tribotest. The sliding distance was 1300 m, and the normal load was set as 340 N. In the case of the base metal, thick abrasion grooves, spalling, and delamination are observed at the wear scar, confirming that the dominant wear mechanism is a mix of adhesive and abrasive mechanisms. Spalling and delamination may cause an unstable wear behavior, resulting in a large fluctuation in the COF, as shown in Fig. 15 (a). Contrastingly, in the case of the LSE at 220 J/mm2, a relatively less spalling is detected; however, thick abrasion grooves are still observed at the wear scar. In the case of the LSE at 315 and 420 J/mm2, thin and sallow abrasion grooves and smooth surface morphologies are detected, confirming a stable wear behavior, as shown in Fig. 15 (c) and (d).
Fig. 15. Coefficient of friction and surface temperature of the steel ring from the block-on-ring test as a function of the sliding distance for the (a) base metal and laser surface-engineered at (b) 220 J/mm2, (c) 315 J/mm2, and (d) 420 J/ mm2 at a normal load of 340 N.
3.4. Corrosion behavior Fig. 19 displays the polarization curve for the base metal and LSE at 315 and 420 J/mm2. For the base metal, the corrosion potential is found to be −0.967 V/SCE. There is a marginal shift in the corrosion potential to the noble direction (−0.937 V/SCE) for the LSE at 315 J/ mm2. However, in the case of the LSE at 420 J/mm2, the corrosion potential is shifted to −0.985 V/SCE. The polarization curve, as shown
in Fig. 19, was used to calculate the corrosion rate [30], which is summarized in Table 4. The corrosion rate of the base metal is 0.147 mm/year, and this value shifts to 0.051 and 0.215 mm/year for the LSE at 315 and 420 J/mm2, respectively. Fig. 20 presents the SEM images of the corroded morphology after the electrochemical behavior 11
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Fig. 17. Depth profiles of the wear scars after 1300 m of sliding distance from the block-on ring tests at a normal load of 340 N. Table 3 Summary of the wear loss from the block-on-ring tests of the base metal and laser surface-engineered AISI P20-improved steels at the laser energy densities of 220, 315, and 420 J/mm2. The normal loads were set as 170, 255, and 340 N, and the sliding distance was 1300 m. Wear loss (×10−2 mm3) for 170 N Base metal 220 J/mm2 315 J/mm2 420 J/mm2
37.45 22.45 15.59 16.77
± ± ± ±
2.21 1.95 1.62 1.51
Wear loss (×10−2 mm3) for 20 N 47.55 ± 2.95 28.95 ± 2.25 19.25 ± 1.35 20.53 ± 2.2
Wear loss (×10−2 mm3) for 30 N 58.85 36.59 23.30 24.50
± ± ± ±
3.57 2.53 2.56 2.10
of general corrosion is detected at the corroded surface, as shown in Fig. 20 (c). The LSE at 315 J/mm2 shows the best corrosion resistance, and this is attributed to the microstructural homogenization by the LSE. The martensite structure with the homogenous distribution of chromium (Fig. 5 (b)) enhances the corrosion resistance [31]. Opposingly, the LSE at 420 J/mm2 shows a reduction in the corrosion resistance compared to that of the base metal. The occurrence of chromium segregation at the grain boundary region (as shown in the EMPA result, Fig. 5 (c)) by the partial melting and solidifying process decreases the chromium content in the remainder of the matrix, causing a Galvanic attack and deterioration in the corrosion resistance. This study focuses on the influence of the laser surface engineering on wear and corrosion behaviors of AISI P20-improved mold steel. However, the mechanical properties such as yield strength and fatigue strength are also essential factors in terms of the lifetime of the mold. The correlation between laser-induced microstructure and mechanical properties will be investigated for future research.
Fig. 15. (continued)
4. Conclusion In this study, the laser surface engineering of AISI P20-improved mold steel was conducted at different laser energy densities, and wear and corrosion behaviors was investigated before and after the laser surface engineering. The details of the investigation are summarized as follows:
Fig. 16. Cumulative wear loss as a function of the sliding distance from the block-on-ring tests at a normal load of 340 N.
tests. In the case of the base metal and LSE at 420 J/mm2, general and uniform corrosion can be observed over the corroded surface. By contrast, the LSE at 315 J/mm2 shows a relatively higher corrosion resistance in general corrosion in comparison to that of the base metal and LSE at 420 J/mm2. Pitting corrosion with a relatively small degree
1. The laser surface engineering was performed at the laser energy densities of 220, 315, and 420 J/mm2. For the LSE at 220 and 315 J/ mm2, the samples were heat-treated without melting. Whereas, for the LSE at 420 J/mm2, the sample was partially melted and solidified at the surface, and a cell structure was built. The 12
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Fig. 18. Worn surface morphologies from the block-on-ring tribotests after 1300 m of sliding distance at a normal load of 340 N.
2.
3.
4.
Fig. 19. Polarization curves of the base metal and laser surface-engineered at 315 and 420 J/mm2 in 3.5% NaCl solution. Table 4 Summary of the corrosion properties of the base metal and laser surface-engineered AISI P20-improved steels at different laser energy densities in 3.5% NaCl solution.
Base metal 315 J/mm2 420 J/mm2
Corrosion potential (V/SCE)
Corrosion rate (mm/year)
−0.967 −0.937 −0.985
0.147 0.051 0.215
5.
13
microstructural change from tempered martensite to lath-type martensite was detected after the laser surface engineering. The EPMA results revealed that the LSE at 315 J/mm2 improved the microstructural homogenization compared to that in the base metal. However, in the case of the LSE at 420 J/mm2, elemental segregation of chromium and manganese was detected at the grain boundary region, resulting in the reduction of microstructural homogenization. The measured residual stress at the surface of the base metal was 556 MPa (tensile), and it decreased to 305 MPa (tensile) and − 1195 MPa (compressive) for the laser surface engineering at 220 and 315 J/mm2, respectively. On the other hand, larger tensile residual stress (870 MPa) was detected for the laser surface engineering at 420 J/mm2 in comparison to that of the base metal. The surface hardness of the base metal was 307 HV, and it increased to 545, 657, and 610 HV for the LSE at 220, 315, and 420 J/mm2, respectively. A microstructural change caused the increase in the hardness. The wear behaviors of the base metal and laser surface-engineered AISI P20-improved steels were investigated by the fretting and block-on-ring tests. Both the tribotests showed a reduction in the coefficient of friction and wear volume for the laser surface-engineered AISI P20-improved steels in comparison to the values for the base metal. The base metal showed the highest coefficient of friction and wear loss, and followed by the LSE at 220, 420, and 315 J/mm2. The trend in the improvement of the wear resistance coincided with the increase in the surface hardness caused by the LSE at different laser energy densities. In the surface morphology images of the wear scars, a large amount of wear debris accumulation, spalling, and thick abrasion grooves were detected for the base metal. By contrast, a relatively smoother
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Fig. 20. Surface morphologies of the corroded (a) base metal, (b) laser surface-engineered at 420 J/mm2, and (c) laser surface-engineered at 315 J/mm2 from the electrochemical behavior tests in 3.5% NaCl solution.
References
surface morphology with a small amount of wear debris accumulation and thin abrasion grooves was observed for the laser surfaceengineered AISI P20-improved steels. 6. The laser surface-engineered AISI P20-improved steel at 315 J/mm2 showed a marginal shift in the corrosion rate and corrosion potential in the noble direction compared to those of the base metal. The improvement in the corrosion resistance was due to a rise in the microstructural homogeneity provided by the LSE. Contrastingly, the laser surface-engineered AISI P20-improved steel at 420 J/mm2 showed a reduction in the corrosion resistance compared to that of the base metal. This was attributed to the microstructural inhomogeneity introduced by the elemental segregation of chromium during the melting and solidifying process.
[1] F.J. Shiou, C.H. Cheng, Ultra-precision surface finish of NAK80 mould tool steel using sequential ball burnishing and ball polishing processes, J. Mater. Process. Technol. 201 (2018) 554–559. [2] G. Singh, A. Verma, A brief review on injection moulding manufacturing process, Materials Today: Proceedings 4 ( (2017) 1423–1433. [3] I. Martínez-Mateo, F. Carrión-Vilches, J. Sanes, M. Bermúdez, Surface damage of mold steel and its influence on surface roughness of injection molded plastic parts, Wear 271 (2011) 2512–2516. [4] K.G. Budinski, Guide to Friction, Wear and Erosion Testing, ASTM International West Conshohocken, PA, 2007. [5] U. Bryggman, S. Söderberg, Contact conditions in fretting, Wear 110 (1986) 1–17. [6] J. Warburton, The fretting of mild steel in air, Wear 131 (1989) 365–386. [7] R. Magaziner, V. Jain, S. Mall, Wear characterization of Ti–6Al–4V under fretting–reciprocating sliding conditions, Wear 264 (2008) 1002–1014. [8] K.K. Liu, M.R. Hill, The effects of laser peening and shot peening on fretting fatigue in Ti–6Al–4V coupons, Tribol. Int. 42 (2009) 1250–1262. [9] Z. Zhang, T. Yu, R. Kovacevic, Erosion and corrosion resistance of laser cladded AISI 420 stainless steel reinforced with VC, Appl. Surf. Sci. 410 (2017) 225–240. [10] X. Tong, M. Dai, Z. Zhang, Thermal fatigue resistance of H13 steel treated by selective laser surface melting and CrNi alloying, Appl. Surf. Sci. 271 (2013) 373–380. [11] D.-C. Wen, Plasma nitriding of plastic mold steel to increase wear-and corrosion properties, Surf. Coat. Technol. 204 (2009) 511–519. [12] R. Ibrahim, M. Rahmat, R. Oskouei, R.S. Raman, Monolayer TiAlN and multilayer TiAlN/CrN PVD coatings as surface modifiers to mitigate fretting fatigue of AISI P20 steel, Eng. Fract. Mech. 137 (2015) 64–78. [13] F. Silva, R. Martinho, M. Andrade, A. Baptista, R. Alexandre, Improving the wear resistance of moulds for the injection of glass fibre–reinforced plastics using PVD
Acknowledgments This research was conducted as part of the project NK217C, funded by the National Research Council of Science and Technology, Republic of Korea.
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coatings: A comparative study, Coatings 7 (2017) 28. [14] G. Telasang, J.D. Majumdar, G. Padmanabham, M. Tak, I. Manna, Effect of laser parameters on microstructure and hardness of laser clad and tempered AISI H13 tool steel, Surf. Coat. Technol. 258 (2014) 1108–1118. [15] G. Telasang, J.D. Majumdar, N. Wasekar, G. Padmanabham, I.J.M. Manna, M.T. A, Microstructure and mechanical properties of laser clad and post-cladding tempered AISI H13 tool steel, 46 (2015) 2309–2321. [16] J.S. Hoppius, L.M. Kukreja, M. Knyazeva, F. Pöhl, F. Walther, A. Ostendorf, E.L. Gurevich, On femtosecond laser shock peening of stainless steel AISI 316, Appl. Surf. Sci. 435 (2018) 1120–1124. [17] P. Sun, S. Li, G. Yu, X. He, C. Zheng, W. Ning, Laser surface hardening of 42CrMo cast steel for obtaining a wide and uniform hardened layer by shaped beams, Int. J. Adv. Manuf. Technol. 70 (2014) 787–796. [18] C. Wang, H. Zhou, N. Liang, C. Wang, D. Cong, C. Meng, L. Ren, Mechanical properties of several laser remelting processed steels with different unit spacings, Appl. Surf. Sci. 313 (2014) 333–340. [19] K.-H. Lee, S.-W. Choi, T.-J. Yoon, C.-Y. Kang, Microstructure and hardness of surface melting hardened zone of mold steel, SM45C using Yb: YAG disk laser, Journal of Welding and Joining 34 (2016) 75–81. [20] G. Telasang, J.D. Majumdar, G. Padmanabham, I. Manna, Wear and corrosion behavior of laser surface engineered AISI H13 hot working tool steel, Surf. Coat. Technol. 261 (2015) 69–78. [21] D. Lesyk, S. Martinez, B. Mordyuk, V. Dzhemelinskyi, А. Lamikiz, G. Prokopenko, Y.V. Milman, K. Grinkevych, Microstructure related enhancement in wear resistance of tool steel AISI D2 by applying laser heat treatment followed by ultrasonic impact treatment, Surface Coatings and Technology 328 (2017) 344–354.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Influence of laser surface engineering of AISI P20-improved mold steel on wear and corrosion behaviors Changkyoo Parka, Ahjin Simb, Sanghoon Ahna, Heeshin Kanga, Eun-Joon Chunc,
T
⁎
a
Laser and Electron Beam Application Department, Korea Institute of Machinery and Materials, Daejeon 34103, Republic of Korea Laser Industrial Technology Research Group, Korea Institute of Machinery and Materials, Busan 46744, Republic of Korea c Department of Nano Materials Science and Engineering, Kyungnam University, Changwon 51767, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser surface engineering Mold steel AISI P20-improved Fretting Block-on-ring Corrosion
The wear and corrosion characteristics of a mold steel (AISI P20-improved, KP4M) were investigated before and after the laser surface engineering. The laser surface engineering of AISI P20-improved steel was conducted at different laser energy densities by a high-power diode laser. The microstructure of the base metal changed from tempered martensite to lath-type martensite. In addition, the surface hardness was enhanced from 307 HV to 545, 657, and 610 HV when the laser energy density was 220, 315, and 420 J/mm2, respectively. An electron probe micro-analyzer revealed that microstructural homogenization was introduced by the laser surface engineering at 220 and 315 J/mm2, whereas an elemental segregation of chromium and manganese was observed with the laser surface engineering at 420 J/mm2 due to partial melting and solidification process. The laser surface-engineered samples showed an enhancement in the wear resistance in both the fretting and block-on-ring tests as compared to that of the base metal, and the highest wear resistance was detected for the laser surfaceengineered at 315 J/mm2 owing to the largest increment in the hardness. In terms of corrosion resistance, the laser surface-engineered AISI P20-improved steel at 315 J/mm2 showed a marginal improvement in the corrosion resistance as compared to that of the base metal because of the microstructural homogenization. Contrastingly, a marginal deterioration of the corrosion resistance was examined for the laser surface-engineered AISI P20-improved steel at 420 J/mm2 because of the elemental segregation.
1. Introduction The use of plastics has been increasing in various applications such as automobiles, shipbuilding, airplanes, and home appliances owing to their low density, low cost, and ease of manufacturing [1], and more than 30% of plastic components have been manufactured by the injection molding process [2]. In the injection molding of plastics, the mold is a critical element in terms of the productivity and quality of the final products. Damages in a mold deteriorate productivity and lead to poor-quality final products. Thus, such a mold should be discarded [3,4]. During the injection molding process, the molds are subjected to wear conditions due to vibration and misalignment over time, and this can cause surface damages in the form of corrosion, abrasion, and fatigue cracking [5–8]. AISI P20-improved steel is a chromium- and molybdenum-containing low carbon steel with high hardenability, which is commonly utilized for general mass production of molds. In their lives, molds are expected to produce several millions of plastic components. Although
⁎
the above- mentioned steel shows reasonable mechanical properties, the lifetime of the mold can be extended by surface treatments. Various surface modification processes for steels have been introduced, such as surface coating and alloying [9–15], shock peening [16], and surface heat treatment [17–20]. Only limited studies have reported the effect of laser-assisted surface modification on the wear and corrosion resistances of steels. Telasang et al. reported the influence of laser surface hardening and melting on the wear and corrosion behavior of AISI H13 tool steel [20]. Lesyk et al. introduced consecutive processes of diode laser heat treatment and ultrasonic impact treatment to enhance the surface hardness and wear resistance of AISI D2 tool steel [21]. However, only a few studies have investigated the effect of laser-assisted surface treatment on the wear and corrosion behaviors of mold steels. In this study, AISI P20-improved, mold steel, was subjected to diode laser surface engineering (LSE) at different laser energy densities to induce surface hardening and microstructural homogenization. Fretting and block-on-ring tribotests were conducted to investigate the correlation between the LSE and wear resistance. Moreover, the
Corresponding author. E-mail address: [email protected] (E.-J. Chun).
https://doi.org/10.1016/j.surfcoat.2019.08.006 Received 17 May 2019; Received in revised form 31 July 2019; Accepted 2 August 2019 Available online 28 August 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Experimental set-up for the laser surface engineering of AISI P20-improved steel.
electrochemical behavior was examined to investigate the effect of the LSE on the corrosion behavior.
Laser energy denisty (J / mm2) =
Laser Power (W ) Beam depth (mm) × Speed (mm / s ) (1)
In the case of the LSE at 1000 and 1200 °C, the surface temperature of the specimen was measured by a two-color pyrometer (LASCON), which was coaxially mounted on the laser head. However, in the case of the LSE at 1500 °C, a thermocouple (B-type) was used to measure the surface temperature because the target surface temperature exceeded the pyrometer limit. The laser head was tilted 5° to the normal direction of the sample to prevent damage of the laser head by the reflection of the laser beam.
2. Materials and methods 2.1. Experimental set-up for laser surface engineering Fig. 1 shows the experimental set-up for the LSE process. A commercial AISI P20-improved steel with a dimension of 100 (W) × 150 (D) × 15 (H) mm was used for this study, and its chemical composition is provided in Table 1. A 4-kW diode laser (TeraBlade Laser, TeraDiode Inc., USA) with a continuous wave mode and wavelength of 970 nm was utilized for the laser surface engineering. The laser beam had a flat top energy distribution with a dimension of 24 (W) × 1 (D) mm. The scan speed of the laser beam was 5.0 mm/s, and the focal length was 310 mm. The laser energy densities for the LSE process of AISI P20improved steel were set as 220, 315, and 420 J/mm2, corresponding to the specimen surface temperature of 1000, 1200, and 1500 °C, respectively. The laser energy density was calculated as
2.2. Macrostructure and microstructure The macrostructures and microstructures of the base metal and laser surface-engineered AISI P20-improved steels were observed via optical microscopy (OM; BX51M, Olympus) and scanning electron microscopy (SEM; SU5000, Hitachi). The qualitative analysis of the base metal and laser surface-engineered AISI P20-improved steels was conducted by Xray diffraction (XRD, SmartLab, Rigaku) with Cu-Kα radiation. The
Table 1 Chemical composition of the AISI P20-improved steel (mass %).
AISI P20-improved
C
Ni
Si
Mn
P
S
Cr
Mo
Cu
Al
Fe
0.38
0.12
0.26
1.02
0.0092
0.022
1.72
0.46
0.061
–
Bal.
2
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Fig. 2. Experimental set-up for the (a) fretting test and (b) block-on-ring test. An alumina ball and a steel ring were used as the deformation tool for the fretting and block-on-ring tests, respectively.
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LSE at 420 J/mm2, the average laser surface-engineered region has a depth of 754 ± 44 μm, whereas those for the LSE at 220 and 315 J/ mm2 are 490 ± 35 and 631 ± 25 μm, respectively. The depth of the laser surface-engineered region was measured ten times and averaged. Two different zones are observed at the laser surface-engineered region (labeled as zone 1 and zone 2) for the LSE at 420 J/mm2. Fig. 3 (b) shows the top-sectional SEM images of the base metal and laser surfaceengineered AISI P20-improved steels. For the base metal, typical tempered martensite is observed, whereas lath-type martensite is found for the laser surface-engineered AISI P20-improved steels [22]. Fig. 4 demonstrates the X-ray diffraction profiles of the surface of the base metal and laser surface-engineered AISI P20-improved steels. All the peaks indicate the martensite phase for both the base metal and laser surface-engineered AISI P20-improved steels. The XRD peaks are marginally shifted to lower 2θ value with increasing laser energy densities. Moreover, the marginal broaden XRD peaks are detected for the laser surface-engineered AISI P20-improved steels in comparison to those of the base metal. The peak broadening is caused by the presence of the residual stress and the decrease of crystallite size in the microstructure after the laser surface engineering [23]. The EPMA analysis was conducted to examine the elemental distribution before and after the LSE. Fig. 5 shows the observation points, back-scattered electron (BSE) images, and EPMA results for the base metal and LSE at 315 and 420 J/mm2. For the base metal, the elemental distribution was analyzed near the surface, as shown in Fig. 5 (a). In the elemental distribution maps of carbon, iron, and chromium, nonhomogeneous elemental distributions are detected. On the other hand, for the LSE at 315 J/mm2 (Fig. 5 (b)), the homogenous elemental distributions of carbon, iron, chromium, and manganese are clearly observed in the EPMA results, and similar results are obtained for the LSE at 220 J/mm2. Fig. 5 (c) shows the EPMA results for zone 1 of the LSE at 420 J/mm2. The cell structure with coarse grains is clearly detected from the BSE image, and this indicates that the specimen is partially melted and solidified by the LSE. The elemental distribution maps of manganese and chromium reveal that these elements are highly segregated at the grain boundary regions. This result is consistent with that of G. Telasang et al., who studied the correlation between microstructure and mechanical properties of AISI H13 tool steel before and after laser surface treatment [23]. Moreover, a relatively severe nonhomogeneity of carbon is detected from the EMPA result. Fig. 5 (d) shows the EMPA results for zone 2 of the LSE at 420 J/mm2, and no cell structure is detected from the BSE image. This suggests that the sample was heat-treated by the LSE without melting. The EMPA results reveal that iron, chromium, and manganese are homogeneously distributed, whereas non-homogeneity of carbon is locally detected.
elemental distribution was analyzed by electron probe X-ray microanalyzer (EPMA, JXA-8530F, JEOL) before and after the laser surface engineering. Moreover, the chemical composition of wear debris was analyzed by energy dispersive X-ray (EDX) detector and scanning electron microscopy (SEM; SU5000, Hitachi). 2.3. Mechanical properties The residual stress at the surface of the base metal and laser surfaceengineered AISI P20-improved steels was determined by X-ray diffraction (XRD, D/MAX-2500/PC, Rigaku) using a two-angle sin2Ψ technique at {211} plane, employing the Cu-Kα radiation. Vickers hardness testing (MMT-X7, Matsuzawa) was conducted five times with a load of 0.5 kgf and dwell time of 10 s, and the obtained values were averaged. 2.4. Tribological test A reciprocating friction instrument (RFW160, NeoPlus; Fig. 2 (a)) with a 10 mm diameter alumina ball was utilized for the fretting test. The dimension of the samples was 40 (W) × 40 (D) × 10 (H) mm, and the normal loads of 10, 20, and 30 N were applied to the samples for 30,000 cycles. The frequency was 4 Hz, and the displacement amplitude was 300 μm. The samples were polished by SiC papers before the fretting tests, and the average surface roughness (Ra) was less than 0.3 μm. All the fretting tests were conducted three times for each condition and averaged the obtained wear losses. A dry sliding wear test was performed by a block-on-ring tribotest (BRW-150, NeoPlus; Fig. 2 (b)) with a S45C steel ring. The outer diameter of the ring was 45 mm, whereas the dimension of the block specimen was 16 (W) × 5 (D) × 10 (H) mm. The sliding speed and distance were 0.236 m/s and 1300 m, respectively. The normal loads were set as 170, 255, and 340 N. During sliding, the surface temperature of the steel ring was measured using a thermocouple (K-type). To prevent abnormal wear behavior in the initial stages of the tribotests, a run-in process was performed using the SiC papers (220 grit) attached on the ring surface. All the block-on-ring tribotests were performed three times for each condition and averaged the obtained wear losses. The tribological tests were performed at 25 °C with a relative humidity of 40–60%. After the tribological tests, the total wear loss and depth profiles of the wear scars were examined via laser microscopy (VK-8710, Keyence). The surface morphologies of the produced wear scars were observed via SEM. 2.5. Corrosion test The corrosion tests were performed using a computerized potentiostat/galvanostat (VMP3, BioLogic Science Instruments). The corrosion circuit consisted of three electrodes: a platinum sheet as the counter electrode, an AISI P20-improved steel as the working electrode, and a saturated calomel electrode (SCE) as the reference electrode. A 3.5% NaCl solution was used as the electrolyte. The potentiostatic/ galvanostatic study was performed from −2 to +1 V at a fixed scan rate of 3.0 mV/s. After the corrosion tests, the EC-Lab® V11.21 software was used to build a Tafel plot and analyze the corrosion potential and corrosion rate. Moreover, the surface morphologies of the corroded samples were examined via SEM to evaluate the corrosion degradation.
3.2. Mechanical properties Fig. 6 shows the residual stress at the surface of AISI P20-improved steels before and after the laser surface engineering process. The similar level of residual stress is detected for the LSE at 220 J/mm2 with the base metal, while a significant compressive residual stress (−1195 MPa) is measured for the LSE at 315 J/mm2. The introduced compressive stress at the surface may increase the wear resistance and fatigue strength [24,25]. On the other hand, in the case of the LSE at 420 J/mm2, the tensile residual stress (870 MPa) is detected at the surface due to rapid quenching from partially melted state to solid state. These state changes may deteriorate the fatigue strength, resulting in the reduction of the prevention of surface cracks propagation [26]. Fig. 7 shows Vickers hardness of the base metal and laser surfaceengineered AISI P20-improved steels as a function of the depth from the surface. The hardness was measured five times at every 100 μm of depth, and the values were averaged. The error bars in Fig. 7 indicate the range of measured microhardness. The surface hardness of the base metal is measured as 307 HV, whereas it is found to be 545, 610, and 657 HV for the LSE at 220, 420, and 315 J/mm2, respectively.
3. Results and discussion 3.1. Macrostructure and microstructure Fig. 3 (a) shows the cross-sectional OM images of the base metal and laser surface-engineered AISI P20-improved steels at 220, 315, and 420 J/mm2, where the white dashed rectangle line indicates the laser surface-engineered region. A deep laser surface-engineered region was observed when a high laser energy density is applied to the sample. The 4
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Fig. 3. (a) Cross-sectional and (b) top-sectional macrostructures of the base metal and laser surface-engineered AISI P20-improved steels at 220, 315, and 420 J/mm2. Two different zones are detected for the cross-sectional images of the laser surface engineering at 420 J/mm2.
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Fig. 10 shows the cumulative wear loss as a function of the cycles at a normal load of 30 N for the base metal and laser surface-engineered AISI P20-improved steels. Every fretting test was conducted three times, and the obtained wear loss was averaged. The error bars in Fig. 10 indicate the range of measured wear loss. The base metal shows the highest cumulative wear loss. At 10,000 cycles, the cumulative wear loss is 5.28 × 10−3 mm3, and it increases to 13.91 × 10−3 mm3 at 30,000 cycles. On the other hand, the lowest cumulative wear loss is observed for the LSE at 315 J/mm2. The cumulative wear loss is 3.76 × 10−3 mm3 at 10,000 cycles and 7.71 × 10−3 mm3 at 30,000 cycles. The cumulative wear loss for the LSE at 315 J/mm2 at 30,000 cycles is reduced by 44.6% in comparison to that of the base metal. This result indicates that the increase in the surface hardness by the LSE causes the increase in the wear resistance and decrease in the wear loss. Fig. 11 shows the depth profiles of the wear scars developed after 30,000 cycles of the fretting test at a normal load of 30 N. The highest depth of the wear scars is detected from the base metal, which shows the largest cumulative wear loss, whereas the lowest depth of the wear scars is observed for the LSE at 315 J/mm2, which shows the smallest cumulative wear loss. In addition, in the case of the base metal, numerous peaks are detected in the depth profile, and this indicates that the surface roughness of the wear scar is relatively high due to the debris accumulation and generated abrasion grooves during the fretting test. Contrastingly, in the case of the laser surface-engineered AISI P20improved steels, relatively smoother depth of profiles are observed compared to that of the base metal because of the low debris accumulation, and fewer generated abrasion grooves at the wear scars. The total wear losses of the base metal and laser surface-engineered AISI P20-improved steels at 10, 20, and 30 N of normal loads and 30,000 cycles are summarized in Table 2. It is evident that the LSE of the AISI P20-improved steel increases the wear resistance, resulting in a smaller amount of wear loss for the laser surface-engineered AISI P20improved steels than that of the base metal. Fig. 12 shows the SEM images of the produced wear scars for the base metal and laser surface-engineered AISI P20-improved steels at a normal load of 30 N after 30,000 cycles of the fretting test. For the base metal, a large amount of wear debris is accumulated over the wear scar. Moreover, relatively deep and thick abrasion grooves are observed at the wear scar. The average surface roughness (Ra) of the wear scar is found to be 6.33 μm. On the other hand, a relatively small amount of wear debris is detected at the wear scar for the laser surface-engineered AISI P20-improved steel. The wear debris accumulation is dispersed at the wear scar for the base metal due to a large amount of wear debris generation during the fretting test. However, for the laser surface-engineered AISI P20-improved steel, the wear debris accumulation is detected mostly at the edge of the wear scar. Also, relatively thin and sallow abrasion grooves are detected at the wear scar for the laser surface-engineered AISI P20-improved steel. The average surface roughness (Ra) is 4.22 μm for the LSE at 220 J/mm2, 3.61 μm for the LSE at 315 J/mm2, and 3.75 μm for the LSE at 420 J/mm2. These results suggest that the LSE improves the wear resistance of the AISI P20-improved steel, resulting in the smaller amount of wear debris and thinner abrasion grooves compared to that of the base metal. A comparable width and length of the wear scars are detected for the base metal and laser surface-engineered AISI P20-improved steels, and this is because the number of fretting test cycles is sufficient to develop matured wear scars. The produced wear debris was collected after the fretting test of laser surface-engineered AISI P20-improved steels at 315 J/mm2, and the chemical composition was characterized by EDX analysis. The fretting test was conducted at the normal load of 20 N with 30,000 cycles. Fig. 13 shows the SEM image, EDX spectrum, and chemical composition of the collected wear debris. The EDX area analysis is performed at the region of SEM images. Oxygen is detected from the result of chemical composition, and this confirms the oxidation of wear debris during the fretting test. The hard wear debris may act as an
Fig. 4. XRD diffraction pattern of the base metal and laser surface-engineered AISI P20-improved steels at 220, 315, and 420 J/mm2.
Compared to the base metal, the surface hardness for the LSE at 315 J/ mm2 is increased by 114.1% caused by the microstructural change from the tempered martensite to martensite [22]. For the LSE at 420 J/mm2, a slightly lower hardness is measured than that for the LSE at 315 J/ mm2, and this may be attributed to the elemental segregation, as shown in the EMPA results. 3.3. Wear behavior The effect of the LSE on the wear behavior of the AISI P20-improved steel was investigated by the fretting and block-on-ring tests. 3.3.1. Fretting test Fretting tests were conducted for up to 30,000 cycles at different normal loads of 10, 20, and 30 N for the base metal and laser surfaceengineered AISI P20-improved steels. Fig. 8 shows the average coefficient of friction (COF) as a function of the normal load. The average coefficients of friction are obtained under a steady state, and the error bars represent the range of coefficient of friction. The base metal has the highest average COF at each normal load. An intermediate average COF is obtained for the LSE at 220 J/mm2, and the lowest average COF is obtained for the LSE at 315 and 420 J/mm2. Moreover, a high average COF is achieved when a high normal load is applied to the samples. Fig. 9 displays COF as a function of cycles for the base metal and laser surface-engineered AISI P20-improved steels at a normal load of 30 N. For each sample, high peaks of COF are observed in the initial stage of the fretting test, then COF reaches the steady state. The initial increase in COF is caused by a strong adhesion at the surface asperities of the sample and counter material (e.g., alumina ball), hindering the reciprocating motion. Therefore, the friction force and COF increase. At some point, the tangential force applied at the contacting area overcomes the adhesive bond of the interface, and then fracture occurs, resulting in a decrease of the friction force and COF. The wear mechanism changes from a two-body to three-body condition with continuing wear debris generation and accumulation. As a result, COF reaches a steady state [27–29]. Compared to the base metal, the laser surface-engineered AISI P20-improved steels show a relatively smaller variation in COF. For the base metal, the average COF is 0.583, and the COF values vary from 0.562 to 0.609. Contrastingly, for the LSE at 315 J/mm2, the average COF is 0.36, and the COF values fluctuate from 0.341 to 0.379. The significant variation in COF during the fretting test can be influenced by the strong interaction between the interface and wear debris. 6
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Fig. 5. EPMA analysis for the (a) base metal, (b) laser surface-engineered at 315 J/mm2, (c) zone 1 (partially melted and solidified) of the laser surface-engineered at 420 J/mm2, and (d) zone 2 (heat-treated) of the laser surface-engineered at 420 J/mm2.
abrasive, resulting in surface damage of the mold [25].
surface-engineered AISI P20-improved steels. The sliding distance was 1300 m, and the normal loads were set as 170, 255, and 340 N. Fig. 14 shows the average COF for the base metal and LSE at 220, 315, and 420 J/mm2. The average coefficients of friction are obtained under a
3.3.2. Block-on-ring test Block-on-ring tests were performed for the base metal and laser 7
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Fig. 5. (continued)
measured surface temperature of the steel ring as a function of the sliding distance during the block-on-ring tribotest for the base metal (Fig. 15 (a)), LSE at 220 J/mm2 (Fig. 15 (b)), LSE at 315 J/mm2 (Fig. 15 (c)), and LSE at 420 J/mm2 (Fig. 15 (d)). The normal load was set as 340 N. In the initial stage of the tribotest, a considerable variation in
steady state, and the error bars indicate the range of coefficient of friction. At each normal load, the highest average COF is obtained for the base metal and followed by the LSE in the order of 220, 420, and 315 J/mm2. Moreover, when a high normal load is applied to the samples, a high average COF is derived. Fig. 15 shows COF and the 8
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Fig. 6. Residual stress at the surface of the base metal and laser surface-engineered AISI P20-improved steels at 220, 315, and 420 J/mm2. Fig. 9. Coefficient of friction for the base metal and laser surface-engineered AISI P20-improved steels during 30,000 cycles of the fretting test. The normal load was set as 30 N.
Fig. 7. Vickers hardness of the base metal and laser surface-engineered AISI P20-improved steels as a function of the depth from the surface. Fig. 10. Cumulative wear loss for the base metal and laser surface-engineered AISI P20-improved steels as a function of cycles at a normal load of 30 N.
Fig. 8. Average coefficient of friction of the base metal and laser surface-engineered AISI P20-improved steels as a function of the normal load for the fretting test. Fig. 11. Depth profiles of the wear scars after 30,000 cycles of the fretting test at a normal load of 30 N for the base metal and laser surface-engineered AISI P20-improved steels. 9
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during the tribotest. The increase in the surface temperature of the steel ring is 51.1 °C for the base metal, whereas it is 29.6 °C for the LSE at 315 J/mm2. The large increase in the surface temperature suggests that severe wear environments present at the interface of the AISI P20-improved steels and steel ring, resulting in significant and non-uniform COF values. Fig. 16 shows the cumulative wear loss as a function of the sliding distance at a normal load of 340 N. The block-on-ring tests were performed three times for every condition, and the measured wear loss was averaged. The error bars indicate the range of measured wear loss. The base metal shows the largest cumulative wear loss, followed by the LSE at 220, 420, and 315 J/mm2. A linear increase in the wear loss is detected for both the base metal and laser surface-engineered AISI P20improved steels, and this indicates that there is no change in the wear mechanism during the tribotest. The cumulative wear loss is 58.85 × 10−2 mm3 at 1300 m of sliding distance for the base metal, and this value decreases to 23.30 × 10−2 mm3 for the LSE at 315 J/ mm2. Fig. 17 shows the depth profile of the base metal and LSE at 220, 315, and 420 J/mm2 after 1300 m of sliding distance at a normal load of 340 N. The deepest and largest wear scar is observed for the base metal. The depth of the deepest wear scar is found to be 275.4 μm, and its width is 7.04 mm. Contrastingly, the shallowest and smallest wear scar is detected for the LSE at 315 J/mm2. The deepest depth and width of this wear scar is 119.2 μm and 4.71 mm, respectively. The total wear loss for the base metal and laser surface-engineered AISI P20-improved steels after the sliding distance of 1300 m at the normal load of 170, 255, and 340 N is summarized in Table 3. A relatively small-sized wear scar and a small amount of wear loss are observed for the laser surfaceengineered AISI P20-improved steels than those for the base metal, and
Table 2 Summary of the wear loss from the fretting tests of the base metal and laser surface-engineered AISI P20-improved steels at the laser energy densities of 220, 315, and 420 J/mm2. The fretting test was performed at the normal loads of 10, 20, and 30 N with 30,000 cycles. Wear loss (×10−3 mm3) for 10 N Base metal 220 J/mm2 315 J/mm2 420 J/mm2
7.06 4.38 3.98 4.06
± ± ± ±
0.64 0.55 0.31 0.20
Wear loss (×10−3 mm3) for 20 N
Wear loss (×10−3 mm3) for 30 N
10.77 ± 0.88 6.76 ± 0.41 5.86 ± 0.33 5.79 ± 0.41
13.91 ± 1.09 8.99 ± 0.77 7.71 ± 0.62 7.91 ± 0.58
COF is observed both for the base metal and laser surface-engineered AISI P20-improved steels, and this may be caused by the oxide layer removal and surface leveling. After this significant variation in COF, relatively stable and uniform COF values are obtained for the laser surface-engineered AISI P20-improved steels. However, non-uniform COF values are obtained for the base metal during the tribotest. The numerous peaks and large fluctuation in the COF values are observed for the base metal owing to the relatively low surface hardness [11]. This result indicates that a relatively uniform wear behavior is maintained during the tribological test for the laser surface-engineered AISI P20-improved steels compared to that for the base metal, and the surface temperature profiles of the steel ring support this result. The initial surface temperature is 33.6 °C, and it increases to 84.7 °C after 1300 m of sliding distance for the base metal. Contrastingly, for the LSE at 315 J/mm2, the surface temperature increases from 34 to 63.6 °C
Fig. 12. Surface morphologies of the developed wear scars after 30,000 cycles of the fretting test at a normal load of 30 N for the base metal and laser surfaceengineered AISI P20-improved steels. 10
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Fig. 13. SEM image, EDX spectrum, and chemical composition of produced wear debris during the fretting test for the laser surface-engineered AISI P20-improved steels at 315 J/mm2.
Fig. 14. Average coefficient of friction of the base metal and laser surface-engineered AISI P20-improved steels from the block-on-ring test at different normal loads.
these results suggest that the LSE increases the surface hardness, resulting in an improvement in the wear resistance. Fig. 18 shows the SEM image of the surface morphology of the produced wear scar by the block-on-ring tribotest. The sliding distance was 1300 m, and the normal load was set as 340 N. In the case of the base metal, thick abrasion grooves, spalling, and delamination are observed at the wear scar, confirming that the dominant wear mechanism is a mix of adhesive and abrasive mechanisms. Spalling and delamination may cause an unstable wear behavior, resulting in a large fluctuation in the COF, as shown in Fig. 15 (a). Contrastingly, in the case of the LSE at 220 J/mm2, a relatively less spalling is detected; however, thick abrasion grooves are still observed at the wear scar. In the case of the LSE at 315 and 420 J/mm2, thin and sallow abrasion grooves and smooth surface morphologies are detected, confirming a stable wear behavior, as shown in Fig. 15 (c) and (d).
Fig. 15. Coefficient of friction and surface temperature of the steel ring from the block-on-ring test as a function of the sliding distance for the (a) base metal and laser surface-engineered at (b) 220 J/mm2, (c) 315 J/mm2, and (d) 420 J/ mm2 at a normal load of 340 N.
3.4. Corrosion behavior Fig. 19 displays the polarization curve for the base metal and LSE at 315 and 420 J/mm2. For the base metal, the corrosion potential is found to be −0.967 V/SCE. There is a marginal shift in the corrosion potential to the noble direction (−0.937 V/SCE) for the LSE at 315 J/ mm2. However, in the case of the LSE at 420 J/mm2, the corrosion potential is shifted to −0.985 V/SCE. The polarization curve, as shown
in Fig. 19, was used to calculate the corrosion rate [30], which is summarized in Table 4. The corrosion rate of the base metal is 0.147 mm/year, and this value shifts to 0.051 and 0.215 mm/year for the LSE at 315 and 420 J/mm2, respectively. Fig. 20 presents the SEM images of the corroded morphology after the electrochemical behavior 11
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Fig. 17. Depth profiles of the wear scars after 1300 m of sliding distance from the block-on ring tests at a normal load of 340 N. Table 3 Summary of the wear loss from the block-on-ring tests of the base metal and laser surface-engineered AISI P20-improved steels at the laser energy densities of 220, 315, and 420 J/mm2. The normal loads were set as 170, 255, and 340 N, and the sliding distance was 1300 m. Wear loss (×10−2 mm3) for 170 N Base metal 220 J/mm2 315 J/mm2 420 J/mm2
37.45 22.45 15.59 16.77
± ± ± ±
2.21 1.95 1.62 1.51
Wear loss (×10−2 mm3) for 20 N 47.55 ± 2.95 28.95 ± 2.25 19.25 ± 1.35 20.53 ± 2.2
Wear loss (×10−2 mm3) for 30 N 58.85 36.59 23.30 24.50
± ± ± ±
3.57 2.53 2.56 2.10
of general corrosion is detected at the corroded surface, as shown in Fig. 20 (c). The LSE at 315 J/mm2 shows the best corrosion resistance, and this is attributed to the microstructural homogenization by the LSE. The martensite structure with the homogenous distribution of chromium (Fig. 5 (b)) enhances the corrosion resistance [31]. Opposingly, the LSE at 420 J/mm2 shows a reduction in the corrosion resistance compared to that of the base metal. The occurrence of chromium segregation at the grain boundary region (as shown in the EMPA result, Fig. 5 (c)) by the partial melting and solidifying process decreases the chromium content in the remainder of the matrix, causing a Galvanic attack and deterioration in the corrosion resistance. This study focuses on the influence of the laser surface engineering on wear and corrosion behaviors of AISI P20-improved mold steel. However, the mechanical properties such as yield strength and fatigue strength are also essential factors in terms of the lifetime of the mold. The correlation between laser-induced microstructure and mechanical properties will be investigated for future research.
Fig. 15. (continued)
4. Conclusion In this study, the laser surface engineering of AISI P20-improved mold steel was conducted at different laser energy densities, and wear and corrosion behaviors was investigated before and after the laser surface engineering. The details of the investigation are summarized as follows:
Fig. 16. Cumulative wear loss as a function of the sliding distance from the block-on-ring tests at a normal load of 340 N.
tests. In the case of the base metal and LSE at 420 J/mm2, general and uniform corrosion can be observed over the corroded surface. By contrast, the LSE at 315 J/mm2 shows a relatively higher corrosion resistance in general corrosion in comparison to that of the base metal and LSE at 420 J/mm2. Pitting corrosion with a relatively small degree
1. The laser surface engineering was performed at the laser energy densities of 220, 315, and 420 J/mm2. For the LSE at 220 and 315 J/ mm2, the samples were heat-treated without melting. Whereas, for the LSE at 420 J/mm2, the sample was partially melted and solidified at the surface, and a cell structure was built. The 12
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Fig. 18. Worn surface morphologies from the block-on-ring tribotests after 1300 m of sliding distance at a normal load of 340 N.
2.
3.
4.
Fig. 19. Polarization curves of the base metal and laser surface-engineered at 315 and 420 J/mm2 in 3.5% NaCl solution. Table 4 Summary of the corrosion properties of the base metal and laser surface-engineered AISI P20-improved steels at different laser energy densities in 3.5% NaCl solution.
Base metal 315 J/mm2 420 J/mm2
Corrosion potential (V/SCE)
Corrosion rate (mm/year)
−0.967 −0.937 −0.985
0.147 0.051 0.215
5.
13
microstructural change from tempered martensite to lath-type martensite was detected after the laser surface engineering. The EPMA results revealed that the LSE at 315 J/mm2 improved the microstructural homogenization compared to that in the base metal. However, in the case of the LSE at 420 J/mm2, elemental segregation of chromium and manganese was detected at the grain boundary region, resulting in the reduction of microstructural homogenization. The measured residual stress at the surface of the base metal was 556 MPa (tensile), and it decreased to 305 MPa (tensile) and − 1195 MPa (compressive) for the laser surface engineering at 220 and 315 J/mm2, respectively. On the other hand, larger tensile residual stress (870 MPa) was detected for the laser surface engineering at 420 J/mm2 in comparison to that of the base metal. The surface hardness of the base metal was 307 HV, and it increased to 545, 657, and 610 HV for the LSE at 220, 315, and 420 J/mm2, respectively. A microstructural change caused the increase in the hardness. The wear behaviors of the base metal and laser surface-engineered AISI P20-improved steels were investigated by the fretting and block-on-ring tests. Both the tribotests showed a reduction in the coefficient of friction and wear volume for the laser surface-engineered AISI P20-improved steels in comparison to the values for the base metal. The base metal showed the highest coefficient of friction and wear loss, and followed by the LSE at 220, 420, and 315 J/mm2. The trend in the improvement of the wear resistance coincided with the increase in the surface hardness caused by the LSE at different laser energy densities. In the surface morphology images of the wear scars, a large amount of wear debris accumulation, spalling, and thick abrasion grooves were detected for the base metal. By contrast, a relatively smoother
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Fig. 20. Surface morphologies of the corroded (a) base metal, (b) laser surface-engineered at 420 J/mm2, and (c) laser surface-engineered at 315 J/mm2 from the electrochemical behavior tests in 3.5% NaCl solution.
References
surface morphology with a small amount of wear debris accumulation and thin abrasion grooves was observed for the laser surfaceengineered AISI P20-improved steels. 6. The laser surface-engineered AISI P20-improved steel at 315 J/mm2 showed a marginal shift in the corrosion rate and corrosion potential in the noble direction compared to those of the base metal. The improvement in the corrosion resistance was due to a rise in the microstructural homogeneity provided by the LSE. Contrastingly, the laser surface-engineered AISI P20-improved steel at 420 J/mm2 showed a reduction in the corrosion resistance compared to that of the base metal. This was attributed to the microstructural inhomogeneity introduced by the elemental segregation of chromium during the melting and solidifying process.
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