study on laser-assisted rejuvenation of inter-granular ... · solution, as prescribed by astm a262...

Study on Laser-Assisted Rejuvenation of Inter-GranularCorrosion Damaged Type 304 Stainless Steel

R. K. Gupta1 & V. K. Bhardwaj1 & D. K. Agrawal1 &

Manoj Kumar1 & B. N. Upadhyay1 &

P. Ram Sankar1 & P. Ganesh1& R. Kaul1 &

S. M. Oak1& L. M. Kukreja1

Accepted: 4 May 2015 /Published online: 30 May 2015# Springer New York 2015

Abstract The paper evaluates laser surface melting treatment for rejuvenation of inter-granular corrosion damaged type 304 stainless steel. Surface melting of inter-granularcorrosion damaged specimens with CW CO2 and pulsed Nd:YAG lasers was quiteeffective in sealing surface damage. However, inter-granular corrosion susceptibility oflaser-rejuvenated surface was strongly influenced by associated thermal exposure. Withrespect to CW CO2 laser, pulsed Nd:YAG laser-rejuvenated surface demonstratedsignificantly suppressed micro-structural damage and IGC-susceptibility. Compactpulsed Nd:YAG laser, with flexible beam transportation, presents an effective tool forin-situ rejuvenation of inter-granular corrosion damaged in-service stainless steelcomponents operating in susceptible environments.

Keywords Stainless steel . Inter-granular corrosion . Laser surface melting .

Rejuvenation

Introduction

Austenitic stainless steels, in spite of their excellent resistance against general corro-sion, are particularly susceptible to localized corrosion, including pitting, crevice, stresscorrosion cracking (SCC) and inter-granular corrosion (IGC). Inter-granular corrosionof austenitic stainless steel (SS) mainly arises as a result of its prolonged exposure tothe susceptible temperature regime of 773–1073 K. This thermal exposure results ininter-granular precipitation of chromium-rich carbides, which is accompanied by

Lasers Manuf. Mater. Process. (2015) 2:135–147DOI 10.1007/s40516-015-0011-6

* R. [email protected]

1 Raja Ramannna Centre for Advanced Technology, P. O.: CAT, Indore 452 013, India

chromium depletion adjacent to grain boundaries to a level below that required to formprotective passive film [1–4]. This micro-structural state is referred as Bsensitization^.In this condition, the resultant chromium-depleted regions become vulnerable tolocalized corrosion in certain acids and aqueous environments with aggressive ions.This kind of corrosion attack along the grain boundaries is known as IGC [5–7].

Inter-granular corrosion accounts for a large number of industrial failures of SScomponents [8–10]. In nuclear fuel reprocessing, waste management industries, and inmany chemical industries, using nitric acid (HNO3) as the process fluid, IGC is themain corrosion problem [11, 12]. In view of long plant down time and high economiccost of failures of key components, there is a strong need for in-situ life extensiontechnique for rejuvenation of in-service austenitic SS components damaged by IGC.Laser, due to its unique ability to process materials with low heat input, low distortionand ease of beam transportation, presents an attractive non-contact tool for in-siturejuvenation of damaged in-service engineering components [13]. There are manyreports on micro-structural repair of sensitized specimens of austenitic SS and alloy600 to enhance their resistance against IGC and inter-granular stress corrosion cracking(IGSCC) through laser surface treatment [14–17]. The usual approach considered forthis application involves dissolution of chromium carbide precipitates (responsible forIGC and IGSCC) through laser surface melting (LSM). To the best of our knowledge,there is no report on repair of IGC-damage through LSM. G Bao et al. [18] used LSMto seal pre-existing stress corrosion cracks in alloy 182 and demonstrated good SCCresistance of as laser-melted surface. In a recently concluded study, performed inauthors’ laboratory, SCC-damaged type 304L SS specimens were effectively rejuve-nated through a new hybrid laser surface treatment, combining LSM and laser shockpeening. Laser-rejuvenated SS specimens demonstrated significantly higher SCCresistance than untreated machined SS specimens [19]. As an extension to thiswork, the present study has been taken up with an objective to evaluate LSMtreatment for rejuvenation of IGC-damaged SS specimens. The results of thisstudy would be important for life extension of austenitic SS structural compo-nents operating in IGC-prone environments.

Experimental Details

The experimental study was performed on a 5 mm thick sheet of high carbon type 304SS in mill annealed condition. Chemical composition (in wt.%) of SS plate is presentedin Table 1. The methodology adopted to introduce IGC damage involved (i) heattreatment at 923 K for 30 min to generate Bditched^ micro-structure (as referred inASTM A262 [20]), followed by (ii) 15 h exposure to boiling Cu-CuSO4-16 % H2SO4

solution, as prescribed by ASTM A262 practice E [20]. The resultant IGC-damagedspecimens (dimensions: 80×10×5 mm) were used as substrates for LSM treatment. Itmay be noted that high carbon variety of type 304 SS was selected as the substrate tointroduce severe IGC-damage in the specimen so that the effectiveness of LSMtreatment for its rejuvenation can be critically determined. Moreover, heat treatmentparameters were also selected in such a way (on the basis of a separate set ofexperiments) to obtain fully Bditch^ microstructure while producing severe IGC dam-age in ASTM A262 practice E test.

136 Lasers Manuf. Mater. Process. (2015) 2:135–147

Initial LSM experiments were performed with an in-house developed 600 Wcontinuous wave (CW) CO2 laser. Laser processing set-up comprised of laser systemintegrated with a beam delivery system and a computer controlled 2-axis workstation.The laser beam was focused with a zinc selenide focusing lens of 100 mm focal length,housed in a copper nozzle. During the course of LSM treatment, argon gas was flownthrough the nozzle (flow rate = 3 L/min.), which served the dual purpose of protectingfocusing lens from possible spatter from the substrate as well as partly shielding thesubstrate from atmospheric contamination. LSM involved scanning surface of thespecimen with a defocused laser beam. For complete surface coverage, LSM wasstarted from one edge of the specimen and progressed towards the terminating edgethrough overlapping laser passes with about 50 % overlap between successive laserpasses. Towards later part of the study, surface treatment with an in-house developed250 W average power pulsed Nd:YAG laser was also considered for rejuvenation ofIGC-damaged SS specimens. In surface melting experiments with Nd:YAG laser, thelaser beam was transported to the focusing nozzle through a 600 μm core diametersilica-silica optical fiber with numerical aperture of 0.22. The output from optical fiberwas focused by using a 35 mm collimating and a 125 mm focusing lenses to a achieve afocal spot diameter of 2.2 mm. Table 2 summarizes experimental parameters of LSM.

Laser surface melted specimens were characterized with respect to surface micro-structure, IGC susceptibility (as per ASTM A262 practices A and E) and degree ofsensitization (DOS) through double loop electro-chemical potentio-kinetic reactivation(DL-EPR) test as per ASTM G108 [21]. ASTM A262 practice A is a rapid screeningtest for identifying austenitic SS specimens that are certain to be free from susceptibilityto IGC. The test involves electro-chemical etching of the polished surface of thespecimen in 10 % oxalic acid (H2C2O4.2H2O) solution at a current density of 1.0 A/cm2 for 90 s. The test classifies the microstructure of wrought austenitic SS as Bstep^,Bdual^ and Bditch^. BStep^ microstructure displays only steps between grains (with nochromium carbide precipitates or chromium depletion) while Bditch^ microstructure is

Table 1 Chemical composition (in wt %) of 304 stainless steel sheet used for the study

C Cr Ni Mn Si Mo S P Fe

0.1 17.8 8.4 1.3 0.51 0.23 – – Bal

Table 2 Experimental parameters for laser surface melting

Laser used: 600 W CW CO2

Laser power Spot dia Scan rate Track-to-trackshift

Incident laser energy per mm2

600 W 1.5 mm 3.2 mm/s 0.75 mm 160 J

Laser used: 250 W average power pulsed Nd:YAG

Pulse energy Pulseduration

Repetition rate Spot dia Scan rate Track-to-track shift

Incident laserenergy per mm2

34 J 20 ms 1 Hz 2.2 mm 0.5 mm/s 1 mm 8.94 J

Lasers Manuf. Mater. Process. (2015) 2:135–147 137

characterized by one or more grains completely surrounded by chromium-depletedregions. On the other hand, Bdual^ microstructure implies partial coverage of grainboundary of a grain by chromium-depleted regions. A Bstep^ structure is not consideredprone to IGC, while a Bditch^ structure may be prone to IGC in a corrosive environ-ment. The test is used for the acceptance of material against IGC. IGC susceptibility ofBditch^ micro-structure needs to be confirmed by another appropriate practice ofASTM A262.

The specimens used for DL-EPR tests were mounted in cold-setting resin insuch a way that only the desired surface is exposed. Before mounting of thespecimen, an electrical connection was provided by joining a metallic wire to theback of the specimen through a suitable electrically conducting adhesive tape.The specimen’s surface was subsequently ground and polished. All the edges ofthe polished specimens were masked with a lacquer to avoid any crevice attackduring the test. DL-EPR tests were conducted in a de-aerated solution of 0.5 Msulphuric acid (H2SO4) and 0.01 M potassium thiocyanate (KSCN). A platinumelectrode was used as counter electrode while a saturated Ag/AgCl electrode wasused as a reference electrode. DL-EPR test involved sweeping electrode potentialfrom open circuit potential in the active region to +300 mV (with respect tosaturated Ag/AgCl electrode) in the passive region at a rate of 6 V/h followed byreverse scan back to the open circuit potential. The basic principle involved inEPR testing involves first passivating specimen’s surface, which is then followedby subjecting it to an active scan. In the reverse scan of DL-EPR, the resultantcurrent arises mainly from incompletely passivated chromium-depleted zones.Degree of sensitization, as determined from DL-EPR test, is expressed as%DOS = (Ir/Ia) × 100, where Ir is the maximum reactivation current in reversescan and Ia is the maximum activation current in forward scan.

Results and Discussion

Introduction of IGC-Damage in SS Specimen

Mill annealed SS sheet used for the study displayed a Bstep^ micro-structure as perASTM A262 practice A, indicating that the material was not susceptible to IGC. U-bending of two specimens, extracted from the plate, did not generate any cracking onthe convex surface of the bent specimens, thereby confirming good ductility of thematerial in the initial mill annealed condition. Figure 1 presents micro-structure andphotograph of the U-bent SS specimen in the initial mill annealed condition. Heattreatment of SS specimens at 923 K for 30 min. resulted in the development ofcompletely Bditch^ micro-structure, as shown in Fig. 2a. The heat treated specimendisplayed high value of %DOS (40) in the DL-EPR test. Fifteen-hour exposure of heattreated SS specimens to boiling Cu-CuSO4-16%H2SO4 solution generated severe IGCdamage in the 300–600 μm thick surface layer, as shown in Fig. 2b. The resultantspecimens were used as substrates for subsequent experiments on laser-assisted reju-venation. In the subsequent part of paper, these specimens are referred as BIGC-damaged^ specimens. U-bending of two such specimens as per ASTM A262 practiceE produced complete surface delamination, thereby demonstrating extensive IGC


damage in these specimens. Figure 3 shows top and side views of one of these twoBIGC-damaged^ specimens after U-bending.

Rejuvenation of IGC-Damaged SS Specimens Through Laser Surface Melting

Initial set of experiments involved surface melting of top surface (80×10 mm) of IGC-damaged SS specimens with a 600 W CW CO2 laser. Laser surface treatment wasperformed with incident laser power density of 3.4×104 W/cm2 with beam interactiontime of ~470 ms. It is to be noted that in the initial part of these experiments, laserscanning was performed in both the directions. Laser melted surface displayedcomplete sealing of IGC damage. Cross-sectional metallographic examination of theselaser surface melted specimens exhibited about 250–600 μm thick melted and re-solidified layer associated with typical cast micro-structure, as shown in Fig. 4a.Electrolytic etching of the cross-section of laser surface melted specimen as per ASTMA262 practice A exhibited ditches along the grain boundaries. Figure 4b presentsmagnified view of the micro-structure of laser melted and re-solidified zone, asproduced by ASTM A262 practice A. DL-EPR testing of laser melted surface exhibiteda drop in %DOS from 40 (in heat treated condition) to 22 (in CO2 laser meltedcondition). In order to determine IGC susceptibility of rejuvenated surface of IGC-damaged specimens, laser surface melted specimens were subjected to ASTM A262

Fig. 1 (a) Microstructure of 304 stainless steel sheet in initial mill annealed condition and (b) photograph ofthe stainless steel specimen after U-bending

Fig. 2 (a) Micro-structure of heat treated (923 K/30 min.) stainless steel specimen and (b) cross-section ofthis specimen after undergoing IGC test as per ASM A262 practice E


practice E. Important features, as observed on IGC-tested laser-rejuvenated specimens,were: (i) extensive IGC damage on untreated side surfaces of the specimen which werenot subjected to laser melting and (ii) on laser melted surface, initial part of the lasermelted surface remained free of cracks while numerous cracks were seen on rest of thesurface (i.e. in the regions which were exposed later to LSM), as shown in Fig. 5a. It isto be noted that during IGC testing of laser-rejuvenated specimens, IGC-damagepresent on untreated side surfaces provided numerous open channels for ingress of testsolution into the specimen which would produce erroneous test results. In order toavoid this problem in the next set of experiments, LSM treatment was extended to foursurfaces (two numbers of 80×10 mm and two numbers of 80×5 mm) of IGC-damagedSS specimens. The above approach was found to be successful in suppressing IGCdamage on side surfaces of laser-rejuvenated specimens in ASTM A262 practice E test.However, these IGC-tested specimens did display preferential cracking in the later partof laser surface treated surface, as shown in Fig. 5b. Cross-sectional metallographicexamination of transverse cross-section of specimens shown in Fig. 5 revealed anincrease in inter-dendritic ditching with progress in LSM. Figure 6 compares micro-structures of laser melted surface layer in the initial and final stages of LSM treatment.It may be noted that surface melting of small SS specimens with CW CO2 laser isassociated with progressive heat build-up which results in constant increase in overalltemperature of the specimen with associated reduction in cooling rate. These twoeffects are responsible for greater micro-structural degradation (seen as pronouncedditching in ASTM 262 practice A test) and enhanced IGC susceptibility (reflected inthe form of cracking in ASTM 262 practice E test) seen on later part of laser melted

Fig. 3 (a) Top and (b) side views of heat treated (923 K/30 min.) + IGC tested (as per ASTM A262 practiceE) stainless steel specimen showing delamination of exposed surface layer

Fig. 4 (a) Cross section of CO2 laser surface melted BIGC-damaged^ stainless steel specimen and (b)magnified view of melted and resolidified region showing dark ditches at the grain boundaries


surface. In order to suppress above effect in the next set of experiments, followingmeasures were considered:

(i) post-LSM solution annealing treatment of laser surface melted specimens at 1323Kfor 30 min. to dissolve chromium carbide precipitates and heal associatedchromium depletion at the grain boundaries generated during LSM,

(ii) introduction of cooling periods between successive laser tracks by performinglaser scanning in one direction while keeping the back stroke idle to preventexcessive heat buildup in the specimen.

(iii) surface melting with pulsed Nd:YAG laser. High peak power provided by pulsedlaser provides necessary energy for surface melting, while its low average powerwould serve to suppress unwanted micro-structural degradation. Surface meltingwith pulsed laser is characterized by high cooling rate besides providing inter-spersed long cooling periods between successive short laser pulses.

Post-LSM Solution Annealing of CO2 Laser Surface Melted IGC-Damaged Specimens

Solution annealing (at 1323 K for 30 min.) of CO2 laser surface melted specimensbrought about complete breakdown of cast micro-structure of laser melted surface.

Fig. 5 CO2 laser surface melted IGC-damaged stainless steel specimens after undergoing ASTM A262Practice E test - (a) only top surface was laser melted and (b) top and side surfaces were laser melted. Onconcerned surfaces, laser melting was started from edge BS^ and terminated at BT .̂ Note preferential crackingin the Blater part^ of laser melted surface

Fig. 6 Transverse cross-section of specimen presented in Fig. 5a showing micro-structures of laser meltedand re-solidified layer in (a) initial and (b) final stages of laser surface melting treatment


During solution annealing, laser surface melted and resolidified regions underwentsignificant grain coarsening and displayed typical Bstep structure, indicating no sus-ceptibility to IGC. Figure 7a presents coarse grained micro-structure of laser meltedregion of IGC-damaged SS specimen after undergoing solution annealing treatment.Dinda et al. [22] have also reported evolution of similar micro-structural changes inlaser-aided direct metal deposited structure of Inconel 625 after its exposure to 1273–1473 K. IGC testing of CO2 laser surface melted and solution annealed specimens asper ASTM A262 practice E did not generate any cracking, although the bent surfacedisplayed characteristic Borange peel^ effect which is attributed to coarse grainedsurface micro-structure. Figure 7b presents a CO2 laser surface melted and solutionannealed IGC-damaged specimen after its exposure to ASTM A262 practice E test. Theresults demonstrate that micro-structural degradation introduced by CO2 LSM has beencompletely erased by subsequent solution treatment and the resultant specimen was notsusceptible to IGC.

CO2 LSM of IGC-Damaged Specimens with Idle Back Stroke

CO2 LSM of IGC-damaged SS specimens, performed with idle back stroke, broughtabout noticeable control on surface temperature. Laser melted surface of IGC-damagedSS displayed %DOS of 12, a drop from 22 recorded by CO2 laser surface meltedspecimen involving laser scanning in both the directions. It may be noted that thereported value of %DOS is also contributed by additional thermal exposure (in 773–1073 K regime) introduced by LSM of other surfaces. The resultant specimenssuccessfully passed ASTM A262 practice E test demonstrating that the specimen wasnot susceptible to IGC. Figure 8 presents photograph of a CO2 laser surface melted(with idle back stroke) IGC-damaged specimen after undergoing ASTM A262 practiceE test. The results confirm that deleterious thermal effects and associated micro-structural degradation introduced by CO2 LSM could be effectively reduced byproviding cooling periods between successive laser passes and the same is responsiblefor specimen’s reduced susceptibility to IGC.

Fig. 7 (a) Cross-section of a CO2 laser melted IGC-damaged stainless steel specimen after undergoingsolution annealing treatment - note significant grain coarsening (b) photograph of same specimen after itsexposure to ASTM A262 practice E test


Pulsed Nd:YAG LSM of IGC-Damaged SS Specimens

Based on the input received from CO2 LSM, surface melting with pulsed Nd:YAG laserwas also performed on all side surfaces. Incident peak laser power density of 4.4×104

W/cm2 (for pulse energy of 34 J and pulse duration of 20 ms) was selected to achievesatisfactory surface melting with low repetition rate of 1 Hz to control average laserpower input while providing long cooling periods (980 ms) between successive laserpulses. Laser surface melting of IGC-damaged SS specimens with pulsed Nd:YAGlaser brought about complete sealing of IGC damaged surface, as shown in Fig. 9a.Cross-sectional examination of the specimen exhibited that the depth of melted and re-solidified layer ranged from 200 to 500 μm, which fell short of the depth of IGC-damaged region, particularly in the overlap region. Figure 9b presents cross-section of aNd:YAG laser surface melted IGC-damaged SS specimen. Although LSM was suc-cessful in sealing IGCdamageon the surface, adeeper part of IGC-damaged regionhadremained unsealed.Metallographic examination of the cross-section ofNd:YAG lasersurface melted specimen, as per ASTMA262 practice A, displayed fine solidificationstructure with no signatures of inter-dendritic ditches, as shown in Fig. 10. DL-EPRtesting of Nd:YAG laser surface melted IGC-damaged SS specimen displayed a verylow value of 0.3 % DOS. Figure 11 compares %DOS of heat treated (used forintroducing IGC-damage) and rejuvenated IGC-damaged SS specimens through CO2

and Nd:YAG LSM. Significantly low value of % DOS of Nd:YAG laser melted IGC-damaged SS surface is attributed to its significantly lower average energy input (68 J/mm2 with respect to 244 J/mm2 for CO2 LSM), higher cooling rate (~106 K/s [23] with

Fig. 8 Photograph of CO2 laser surface melted (with idle back stroke) IGC-damaged stainless steel specimenafter undergoing ASTM A262 practice E test

Fig. 9 (a) Surface and (b) cross-section of pulsed Nd:YAG laser surface melted IGC-damaged stainless steelspecimen


respect to lower cooling rate of 103 K/s for CW LSM [24, 25]) and long cooling periods(980 ms) between successive short laser pulses (20 ms) which effectively suppressedchromium carbide precipitation in laser melted surface layer.

Inter-granular corrosion testing of two Nd:YAG laser surface melted IGC-damagedSS specimens was carried out as per ASTM A 262 practice E. Out of these twospecimens, no IGC damage was noticed on one of the IGC-tested specimens, as shownin Fig. 12a. Small edge cracks seen near the edge of the U-bent specimen shown in theabove figure need to be ignored as per ASTM A262 practice E. On the other hand, thesecond IGC-tested Nd:YAG laser surface melted IGC-damaged SS specimen displayeda few linear defects across the bending direction, as shown in Fig. 12b. Longitudinalcross-sectional examination of the second IGC-tested specimen (showing cross-sectionof defects seen in Fig. 12b) showed that the defects had originated from the unfusedIGC-damaged regions below the laser melted surface layer, as shown in Fig. 13.Gradual reduction in defect opening towards the surface implies that the direction ofdefect propagation was from the underlying defect to the surface, unlike IGC whichinitiates from the surface and extends into the material. Hence, it is evident that theconcerned defect was indeed a crack which had originated from an unfused IGC-

Fig. 10 Microstructure of pulsed Nd:YAG laser surface melted and resolidified layer

Fig. 11 Comparison of % DOS of heat treated and laser-rejuvenated surfaces of BIGC damaged^ stainlesssteel specimens, as obtained from DL-EPR curves


damaged region below the laser-melted surface layer (acted as a stress concentrationsite) and propagated towards the surface under the influence of tensile stress imposedby bending of the specimen. Detailed examination of a number of IGC-tested Nd:YAGlaser melted IGC-damaged SS specimens did not show any evidence of IGC on lasermelted surface.

On synthesizing above results, it is evident that LSM is quite effective in healingsurface damage on IGC-damaged SS specimens. During heating part of LSM treatmentCr-rich carbide precipitates present on the surface of IGC-damaged specimen arecompletely dissolved but subsequent cooling period determines the extent to whichthese Cr-rich carbides (with associated chromium depletion) are re-precipitated. Hence,IGC susceptibility of rejuvenated surface is strongly controlled by associated heat inputas well as the manner in which it is imparted. High heat input associated withcontinuous wave CO2 LSM treatment results in greater micro-structural degradation(in the form of Cr-rich carbide precipitation and associated Cr-depletion which isreflected as high %DOS) leading to IGC susceptibility of rejuvenated SS surface.The two measures adopted to overcome the above limitation, associated with CO2

LSM, yielded noticeable reduction in IGC susceptibility of rejuvenated SS surface.Post-LSM solution annealing, although effective in erasing chromium carbide precip-itation formed during LSM, produces undesirable gain coarsening (adversely affectsductility), besides introducing undue distortion in the rejuvenated component. More-over, solution annealing treatment may not be a practical solution, particularly for large

Fig. 12 Nd:YAG laser surface melted IGC-damaged stainless steel specimens after undergoing ASTM A 262practice E test. Edge defects seen in BA^ need to be ignored as per standard, while three linear defects are seenin the specimen shown in BB^

Fig. 13 Cross-section of one of the defects seen in Fig. 12b showing crack propagation from unfused IGC-damaged region below laser melted surface layer under the imposed bending stress


in-service components. With respect to the above, the second approach of providingcooling period between successive CO2 laser passes (by using idle back stroke) appearsto be a more practical option to obtain rejuvenated SS surfaces of lower IGC suscep-tibility. Although, the results of the study provide qualitative input, application of theprocess would require a close loop real time control of surface temperature to suppressundue micro-structural damage (in the form of inter-granular chromium carbide pre-cipitation and associated chromium depletion). In contrast to CO2 LSM, inherently lowheat input of pulsed Nd:YAG LSM, along with significantly higher cooling rate andlong cooling periods between successive laser pulses, brought about drastically reduced%DOS of rejuvenated SS surface, which was found to be Bnot susceptible to IGC^. Inalmost all practical situations, it is only one surface of the component (e.g. pipes,cylindrical vessels etc.) which is exposed to the media and suffers from IGC damageand therefore, LSM treatment needs to be confined to the concerned surface only. Sincelaser-rejuvenated SS components are not likely to undergo any subsequent stressing/forming operation, sub-surface unfused IGC-damaged regions should not be damagingenough to result in crack formation. However, during Nd:YAG LSM good pulse-to-pulse and track-to-track overlap (~70 % or more) should be provided to obtain uniformthickness of laser melted surface layer. With respect to CO2 laser, Nd:YAG LSM due tocompactness of the laser system [in the present case, volume of Nd:YAG laser (0.43 m3)was about ten times smaller than that of CO2 laser (4.3 m3- excluding volume oflaser gas cylinders)], lower heat input, ease of laser beam delivery through optical fiberand not very stringent requirement of real-time process control, appears to be a betterchoice for rejuvenation of IGC-damaged SS components.

Conclusions

The results of present investigation have demonstrated the following:

1. IGC-damaged type 304 stainless steel can be effectively rejuvenated through asuitable laser surface melting treatment.

2. Inter-granular corrosion susceptibility of laser rejuvenated surface is stronglyinfluenced by thermal exposure associated with laser surface melting treatmentand hence, its control is essential to obtain low IGC susceptibility of rejuvenatedIGC-damaged specimen.

3. With respect to CW CO2 laser, pulsed Nd:YAG laser surface melting, due toassociated lower heat input, higher cooling rate and long cooling periods betweensuccessive laser pulses, effectively suppresses micro-structural damage during lasersurface melting.

4. Pulsed Nd:YAG laser surface melting, due to its relaxed requirement of real-timeprocess control and ease of beam delivery, provides a promising non-contact toolfor rejuvenation of in-service IGC-damaged stainless steel components operatingin susceptible environments.

Acknowledgments Authors are thankful to Mr. D. C. Nagpure for his contribution in conducting inter-granular corrosion tests. They wish to thank to Mr. Sabir Ali and Mr. B. K. Saini for their assistance during


surface melting experiments with Nd:YAG laser. Technical assistance of Mr. Ram Nihal Ram in metallo-graphic specimen preparation is also thankfully acknowledged.

Compliance with ethical standards

Conflict of interest We the authors hereby declare that there is no conflict of interest.

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