Laser gas assisted treatment of AISI H12 tool steeland corrosion properties

B.S. Yilbas n, Ihsan-ul-Haq Toor, Jahanzaib Malik, F. PatelDepartment of Mechanical Engineering, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Kingdom of Saudi Arabia

a r t i c l e i n f o

Article history:Received 4 May 2013Received in revised form18 September 2013Accepted 6 October 2013

Keywords:Laser surface treatmentTool steelCorrosionPolarization

a b s t r a c t

Laser gas assisted treatment of AISI H12 tool steel surface is carried out and the electrochemical responseof the laser treated surface is investigated. Morphological and metallurgical changes in the treated layerare examined using a scanning electron microscope, energy dispersive spectroscopy, and X-raydiffraction. Potentiodynamic polarization tests are carried out for untreated and laser treated specimenin 0.2 M NaCl solution at room temperature. It is found that the laser treated AISI H12 workpiece surfacesexhibit higher corrosion resistance as compared to untreated specimen as confirmed by lower corrosionrate, higher pitting potential, and lower passive current density.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

AISI H13 is an air hardening hot work tool steel, which has goodabrasion resistance and toughness at high temperatures. It iswidely used in dies and punches due to its high wear resistance[1]. The surface hardness of H13 tool steel can be further improvedthrough laser gas assisted treatment process, such as laser gasassisted nitriding [2]. In addition, laser treatment is involved withprecision of operation, shallow heat affected zone, and low cost.Although laser gas assisted treatment process provides finemicrostructures and nitride species at the surface, in most ofthe cases, evaporation at the surface modifies the texture of thetreated surface after the process. This, in turn, influences theelectrochemical response of the surface in aqueous environment.Consequently, investigation into electrochemical response of thelaser treated AISI H12 tool steel becomes essential.

Considerable research studies were carried out to examinesurface treatment of tool steel for corrosion protection. Corrosionbehavior of surface treated hot work tool steel in molten alumi-num alloy was investigated by Molinari et al. [3]. The findingsrevealed that plasma nitriding of the surface changed the mechan-ism from a generalized corrosion attack to a localized one whileimproving the corrosion resistance of the treated surface. Thisoccurred because of the reduced wettability and the formation ofthe compound layer at the treated surface. The effects of post-oxidizing treatment on corrosion resistance of gas nitrided AISI

H13 tool steel were studied by Chang et al. [4]. They showed thatincreasing the temperature and soaking time of oxidization wasadvantageous to the corrosion resistance, and a small variation incorrosion current resulted in a significant change in the polariza-tion resistance. The analysis of corrosion behavior of tool steelcoated by Ti–Al–C–N films was carried out by Hsu et al. [5]. Theyindicated that surface coatings improved the corrosion resistanceof the tool steel in 3.5 wt% NaCl solution. The corrosion behavior ofnitrocarburized AISI H13 tool steel was examined by Basso et al.[6]. The nitrocarburizing process considerably improved the corro-sion resistance of the surface in a NaCl environment as comparedto that corresponding to the untreated workpiece surface. Lasersurface modification of tool steels for improved hardness andcorrosion resistance was investigated by Kwok et al. [7]. Theirfindings revealed that laser treatment resulted in improvedcorrosion resistance as compared with the conventionally har-dened specimens. They indicated that the hardness and corrosioncharacteristics of laser surface-melted specimens were stronglydependent on the amount of passivating elements in solid solutionand also on the morphology of the re-precipitated carbides. Lasersurface melting of tool steel and its electrochemical behavior wasstudied by Majumdar et al. [8]. They indicated that improvementin pitting corrosion resistance of the laser treated surface occurredin a 3.56 wt% NaCl solution. The mechanism of laser and fluidcorrosion combining process was examined by Yuan et al. [9]. Theydemonstrated the presence of the chemical reactions taking placeduring laser ablation, which caused the chemical hybrid etching ofthe tool steel surface. The influence of femtosecond laser markingon the corrosion resistance of stainless steels was investigated byValette et al. [10]. The femtosecond laser marking was shown to

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Optics and Lasers in Engineering

0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.optlaseng.2013.10.004

n Corresponding author. Fax: þ966 38602949.E-mail address: bsyilbas@kfupm.edu.sa (B.S. Yilbas).

Optics and Lasers in Engineering 54 (2014) 8–13

provide an electrochemical ennoblementa and the chemical com-position at the treated surface was not changed considerably sothat the passive character of stainless steel surface was maintainedwhile providing the susceptibility to localized corrosion. Lasertreatment of tool steel surface for improved hardening andcorrosion resistance was investigated by Pyzalla et al. [11]. Theyshowed that, due to the laser heat treatment, there was dissolu-tion of precipitates during heating and re-precipitation duringcooling cycles, which affected the mechanical and the corrosioncharacteristics of the treated surfaces. Laser surface modificationof steel surfaces and the corrosion resistance of the treated surfacewere studied by Steyer et al. [12]. They indicated that, in oxidativeregime, laser treatment drastically affected both microstructureand chemical composition at the surface; in which case, chromiumdepletion took place in subsurface while leading to a loss ofpassive ability. Laser surface treatment of A286 alloy and corrosionresistance of the treated surface was carried out by Yilbas et al.[13]. They showed that the laser treatment improved the corrosionresistance of the alloy surface through forming a passive layer atthe treated surface.

In the present study, laser gas assisted treatment of AISI H12tool steel surface is carried out and the corrosion resistance of thetreated surface is investigated in the aqueous solution. Lasertreatment conditions are set to obtain minimum surface asperities

including cavitations, micro-cracks, and voids. Nitrogen at highpressure is used as an assisting gas in the experiments. Theelectrochemical tests are conducted in 0.2 M NaCl solution atroom temperature. The morphological changes prior and afterthe electrochemical tests are examined using a scanning electronmicroscope and energy dispersive spectroscopy.

2. Experimental

CO2 laser (LC-ALPHAIII) was used to irradiate the workpiecesurface. The nominal focal length of the focusing lens was kept at127 mm, which resulted in �0.7 mm diameter of the focusedbeam at the irradiated workpiece surface. Nitrogen assisting gasemerging from the conical nozzle and co-axially with the laserbeam was used. Laser treatment tests were carried out for variouslevels of the laser parameters including laser scanning speed andthe laser output power. The laser parameters resulting in defectfree surfaces were selected for the treatment of the workpiecesprior to electrochemical tests. It was observed that increasing laseroutput power while lowering laser scanning speed gave rise toevaporation at the surface while enhancing the surface asperitiesand roughness. However, increasing laser scanning speed andlowering laser output power resulted in shallow depth of lasertreated layer. Therefore, the laser parameters resulting in thelowest asperities at the surface and the treated layer depth of onthe order of 40 mm were selected. The laser treatment parametersare given in Table 1.

AISI H12 plates with 2.5 mm thickness were used as work-pieces, which was prepared at dimensions 20�20�2.5 (length�width� thickness) mm3. Material characterization of the lasertreated surfaces was carried out using SEM, EDS and XRD. Jeol6460 electron microscopy was used for SEM examinations and

Table 1Laser treatment conditions.

Scanningspeed (cm/s)(mm/min)

Power(W)

Frequency(Hz)

Nozzlegap(mm)

Nozzlediameter(mm)

Focussetting(mm)

N2

pressure(kPa)

10 95 1000 1.5 1.5 127 650

Laser Scanned Tracks

Overlapping of Laser Spots

0.5 mm

Fig. 1. Optical photograph and SEM micrographs of laser treated surface: (a) optical photograph showing laser scanned tracks, (b) overlapping of laser spots, and (c) finestructures formed at the surface.

B.S. Yilbas et al. / Optics and Lasers in Engineering 54 (2014) 8–13 9

Bruker D8 Advanced having CuKα (λ¼1.5406 Å) radiationwas usedfor XRD analysis. A typical setting of XRD was 40 kV and 30 mAand scanning angle (2θ) was ranged 201–1301.

Corrosion tests (Potentiodynamic polarization and Tafel beha-vior) were carried out in a three electrode cell, which composed ofa specimen as a working electrode, a platinum (Pt) wire as acounter electrode, and a saturated calomel reference electrode

(SCE). The specimens were degreased in benzene, cleaned ultra-sonically, and subsequently washed with distilled water prior toelectrochemical tests. The investigations were carried out with anexposed working electrode area of 0.07 cm2 in 0.2 M NaCl solutionat room temperature in PCI4/750 Gamry potentiostat and repeatedseveral times to ensure the reproducibility of the data. DC105corrosion software was used to analyze the Tafel region, whilePotentiodynamic polarization experiments were performed at ascan rate of 0.5 mV/s.

3. Results and discussion

Laser gas assisted nitriding of AISI H12 tool steel and thecorrosion resistance of the treated surface is examined. Micro-structural and morphological changes at the treated surface areinvestigated using an electron scanning microscope (SEM) andenergy dispersive spectroscopy (EDS).

Fig. 1 shows optical photograph and SEM micrographs of thetop surface of the laser treated workpiece. The treated surfacecomposes of equally spaced laser scanned tracks. The overlappingof the laser irradiated spots along the laser tracks is visible, sincethe laser repetition rate is kept at 1000 Hz during the scanning.

Treated Layer Thickness

Overlapping Region

Fig. 3. Cross-section of the laser treated layer: (a) cross-section, (b) dense layer at the surface, (c) small feathery like structure in the surface vicinity, (d) elongated needlelike and feather structures in the region next to the surface vicinity, and (e) overlapping region of laser scanning tracks.

Fig. 2. Roughness of laser treated and untreated surfaces.

B.S. Yilbas et al. / Optics and Lasers in Engineering 54 (2014) 8–1310

This provides �70% of the overlapping ratio of the irradiated spotsat the surface. Moreover, the overlapping between the scanningtracks is about 0.130 mm, which correspond to 1/e points of thelaser intensity distribution at the surface. The surface is free fromasperities such as micro-cracks, large voids, and deep cavities. Theroughness of the treated surface is also shown in Fig. 2. It can beobserved that the surface roughness is on the order of 2.2 mm,which is considerably low for the laser treated surface. Thisindicates that the laser pulse intensity used in the experiment islow enough to avoid large scale surface ablation. At the same time,the laser pulse intensity is sufficiently high enough to generateacceptable depth of the treated layer, which is on the order of40 mm (Fig. 3). The closed examination of the surface reveals thatthe fine structures are formed at the treated surface due to thehigh cooling rates at the surface. The gas assisted processingincreases the cooling rates at the surface due to the convectioneffect while generating high stress levels in the surface vicinity.Although the temperature gradients remain high at the surfacedue to high cooling rates, no micro-crack is observed along thelaser scanning tracks. This is attributed to the self annealing effectof the previously formed laser tracks, which acts as a heat sourcefor the newly formed tracks while modifying the cooling rates atthe surface.

Fig. 3 shows SEM micrographs of the cross-section of the lasertreated layer. The treated layer extends uniformly below thesurface at a depth of on the order of 40 mm. The dense layercomposed of fine structures occurs in the surface vicinity of thetreated layer. This is attributed to the high cooling rates takingplace at the surface. In addition, formation of nitride speciescontributes to the formation of dense layer through volumeshrinkage in the surface vicinity. The overlapping region of thelaser scanning tracks is also shown in Fig. 3. Although the meltdepth reduces slightly in the overlapping region of the laserscanning tracks, microstructure does not alter in the surfaceregion. The presence of nitride species are also evident from theXRD diffractogram, which is shown in Fig. 4. The presence ofε-Fe2N and ε-Fe3N peaks indicate the nitride rich species, whichare formed at the treated surface. In addition, the formation CrNcompound at the surface is evident from the diffractogram. As thedepth below the surface increases, grain size remains small, whichis attributed to: (i) nitrogen diffusion, which replaces Fe2�3N withγ'-Fe4N, and (ii) the cooling rate, which attains high values, sincethis region is close to the surface vicinity. The nitride rich grainsform feathery like structures, which can be also observed fromFig. 3. It should be noted that γ'-Fe4N peaks indicate the presenceof nitride precipitates below the surface and nitrogen diffusiontakes place from grain boundaries to grains. As the depth below

the surface increases further, the feathery like structures replacewith fine elongated grains because of the relatively lower coolingrates in this region as compared to that at the surface.

Fig. 5 shows the results of potentiodynamic polarizationresponse of the laser treated and untreated AISI H12 samples in0.2 M NaCl solution at room temperature. It is clear from Fig. 5 thatlaser treated surface exhibited better corrosion resistance thanuntreated surface in terms of pitting potential (Epit) and passivecurrent density (ip). Corrosion potential (Ecorr) is found to be, i.e.�480 mV4�700 mV for laser treated and untreated surfaces,respectively. Former one (laser treated surface) shows a pittingpotential of 170 mVSCE as compared to �280 mVSCE for untreatedsurface. Passive current density (ip) as well as corrosion current(icorr) density of the laser treated surface is much less than that ofuntreated surface. Table 2 summarized the results of potentiody-namic polarization and Tafel analysis.

The findings suggest that the formation of stable and moreprotective film at the laser treated surface takes place while

Fig. 4. XRD diffractogram of the laser treated workpiece.

Fig. 5. Potentiodynamic polarization response of laser treated H12-1 and H12-2specimens in 0.2 M NaCl solution.

Table 3EDS data for the laser treated surface (wt%).

Spectrum N C Cr Si V Mo Fe

Spectrum 1 2.7 0.1 5.1 1.3 0.5 1.5 BalanceSpectrum 2 4.0 0.2 5.3 1.6 0.7 1.6 BalanceSpectrum 3 9.2 0.1 5.1 1.4 0.6 1.5 BalanceSpectrum 4 7.8 0.2 4.9 1.3 0.8 1.6 BalanceSpectrum 5 6.2 0.1 5.0 1.2 0.7 1.4 Balance

Table 2Results of electrochemical tests for laser treated H12-1 and H12-2 specimens in0.2 M NaCl solution.

Source ofvariables

Ecorr(mVSCE)

icorr ip Epit(mVSCE)

Corrosionrate

Untreated �0.70 7.37E�9 4.0E�6 �0.28 3.3 mpyLaser treated �0.48 1.4E�9 2.6E�6 0.17 0.175 mpy

B.S. Yilbas et al. / Optics and Lasers in Engineering 54 (2014) 8–13 11

indicating the laser treatment has a positive effect on the corro-sion properties of the alloy surface. The improved corrosionresistance of H12 tool steel is mainly due to dissolution of chromecarbides, which are present at the untreated surface. Thesecarbides are usually formed during the initial fabrication of theworkpieces. During the laser treatment process, such carbidesdissolved and form CrN species at the surface during the treatmentprocess (Fig. 4). It should be noted that the carbon content of H12steel is within the range of 0.3–0.4% (wt) and after laser treatmentno carbide compound is observed, such as chromium carbide(CrC), while indicating dissolution of carbides. The reduction ofcarbon content in the laser treated region can also be observedfrom Table 3. Nevertheless, both CrN and CrC have high resistanceto corrosion [15,16]. This ultimately increases the corrosion resis-tance of the treated surface. Cr enrichment at the treated surface isalso evident from EDS data, which are given in Table 3. Conse-quently, formation of nitride species and fine grains at the surfaceact as a self protective layer at the surface while improving thecorrosion resistance of the laser treated surface, which is in linewith the previous study [14].

Fig. 6 shows SEM micrographs of laser treated and untreatedsurfaces after the electrochemical tests. The presence of pit sites isevident at the treated and untreated surfaces. However, pit sitesize is small and pits are locally scattered at the treated surface. Ingeneral, pits are shallow and no secondary pitting is observed atthe surfaces of laser treated and untreated workpieces. Thelocalized pitting results in low pitting potential during the elec-trochemical tests as seen from Fig. 5. The nitride species and denselayer formed at the surface act like a protective layer whilelowering the pit size at the treated surface. In addition, the selfannealing affect of the initially formed laser tracks causes thereverse transformation of some δ-ferrite into γ-austenite in thesurface region and the presence of high δ-ferrite and γ-austenitecontent improve the localized corrosion resistance of the lasertreated surface.

4. Conclusion

Laser gas assisted treatment of AISI H12 tool steel surface iscarried out and the corrosion resistance of the treated anduntreated surfaces are tested in aqueous 0.2 M NaCl electrolyticsolution. Laser treated surfaces are characterized using scanningelectron microscopy and energy dispersive spectroscopy. Thepotentiodynamic polarization and Tafel analysis are carried outprior and after the laser treatment process. The laser treatmentparamaters are selected based on the minimum surface asperitiesand the depth of treated layer of 40 mm. It is found that lasertreated surfaces are free from large scale asperities such as deepcavities, large size pores, and microcracks. The treated surfacecomposes of regular laser tracks, which acts as the self annealingsites for the newly formed laser tracks while modifying the coolingrates and lowering the thermal stress levels in the surface region.The dense layer consisting of fine grains are formed at the surfaceregion of the laser treated layer. The formation of nitride speciescontributes the formation of dense layer through volume shrink-age in then surface region. The presence of nitride species resultsin the formation of feathery like structure below the surfacevicinity. The laser treated AISI H12 specimen exhibited improvedcorrosion resistance as was confirmed by higher pitting potentialand lower passive current density. The laser treatment dissolvedthe chrome carbides and, thus, improved the chrome concentra-tion at the surface, which ultimately improved the corrosionresistance properties. In addition, formation of nitride species,particularly CrN, enhances the corrosion resistance of the lasertreated surface.

Acknowledgments

The authors acknowledge the support of King Fahd Universityof Petroleum and Minerals, Dhahran, Saudi Arabia for this work.

Fig. 6. SEMmicrographs of electrochemically tested untreated and treated surfaces: (a) untreated surface, (b) laser treated surface, (c) untreated surface, and (d) laser treatedsurface.

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