LASER SURFACE MELTING OF 17-4 PH PRECIPITATION-HARDENABLE STAINLESS STEEL Paper 1203

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1 LASER SURFACE MELTING OF 7- PH PRECIPITATION-HARDENABLE STAINLESS STEEL Paper 0 Zhichao Cheng, Chi Tat Kwok, Kin Ho Lo, Department of Electromechanical Engineering, University of Macau, Taipa, Macau Abstract Preciptation-hardenable (PH) stainless steels are widely employed in industry for their high mechanical strength, reasonable toughness and moderate corrosion resistance. In the present study, laser surface melting of 7- PH precipitation-hardenable stainless steel (Fe-7%Cr-%Ni- %Cu-0.%Nb) was attempted using a.-kw continuous wave Nd:YAG laser for enhancing its corrosion resistance and hardness. The pitting corrosion behavior of laser surface-melted samples processed under different processing conditions in.% NaCl solution at C was studied by open circuit potential measurement and potentiodynamic polarization technique. Compared with the annealed and aged 7- PH, the corrosion resistance of the laser surface-melted samples was significantly improved, as evidenced by a noble shift in open circuit potential, a higher pitting potential, a wider passive range and a lower passive current density. The enhanced corrosion resistance was attributed to the refinement of precipitation of copper particles in the ferrite matrix. In addition, the hardness of the laser surface-melted 7- PH was found to be higher than that of the aged and annealed ones by % and 7% respectively. Keywords: Laser surface melting, Nd:YAG laser, preciptation-hardenable stainless steel, pitting corrosion, hardness Introduction 7- PH (AISI 0) is a precipitation-hardening martensitic stainless steel which contains about wt% Cu for strengthening by precipitation of nearly pure ε-copper in the martensitic matrix []. It possesses high strength, reasonable toughness and moderate corrosion resistance. Its corrosion resistance is comparable to that of AISI 0 in most media. This steel could be aged in various conditions and therefore often used as the structural materials for chemical and power plants [, ]. Generally, it consists of a mixture of martensite (α ) as the matrix and mutable content of δ-ferrite and ε-copper precipitation depending on the ageing conditions. In addition, the typical service temperatures in power plant applications are below 00 o C for prolong period of time. The increase in hardness and tensile strength accompanied by embrittlement for 7- PH were reported at temperatures ranging from 00 to 00 o C by long-term aging. Moreover, this steel is susceptible to pitting corrosion in the chloride containing environment because of the inhomogeneous compositions of the various phases. Lasers have been widely applied for surface treatment of PH stainless steels due to their intense energy density and sharp thermal gradient [,]. Laser surface melting (LSM) of metallic materials is a rapid solidification technique and depends on the processing parameters which will achieve a high cooling rate at the surface and produce a refined grain or dendritic structure. Highly supersaturated structures can be obtained after surface melting and rapid solidification through increasing driving force for nucleation and growth, may increase the kinetics of ageing in precipitation hardening systems []. LSM can be applied to improperly heated 7- PH for eliminating the embrittlement, enhancing corrosion resistance, and even removing surface cracks. The microstructural characterization and the kinetic of aging of - PH stainless steel after LSM has been reported [] but the corrosion study on the PH stainless steels is scarcely found in the literature. In the present study, LSM of 7- PH stainless steel was attempted using a.-kw continuous wave Nd:YAG laser with different processing conditions in order to enhance corrosion resistance and surface hardness.. Laser surface melting Experimental details 7- PH stainless steel (Fe-7%Cr-%Ni-%Cu-0.%Nb) in form of plate with thickness of -mm was used in the present study. The as-received 7- PH stainless steel was aged at 80 o C for hours. It was also solution-annealed at 00 o C for one hour for comparison. LSM of 7- PH was carried out by the continuous wave Nd:YAG laser with power of 0. to kw, beam diameters of Φ and Φ mm, and a scanning speed of mm/s. Argon flowing at 0 l/min was used as the shielding gas. The surface was achieved by overlapping the melt tracks with degree of overlapping of 0%. The details of laser processing parameters are depicted in Table.. Microstructural and hardness analysis The microstructure of the laser-surface melted specimens was studied by the scanning electron microscope (SEM) and the X-ray diffractometry (XRD). Hardness test was carried out by a micro-hardness tester at a load of 00 g and a loading time of s.

2 . Corrosion study To investigate the electrochemical corrosion behavior, the samples were embedded in cold-curing epoxy resin, exposing a surface area of cm. Open-circuit potential (OCP) measurement and potentiodynamic polarization test in.% NaCl solution, open to air at ± oc, were performed using a PAR VersastatII potentiostat according to ASTM Standard G-9 []. All potentials were measured with respect to a saturated calomel electrode (SCE, 0.V versus SHE at oc) as the reference electrode. Two parallel graphite rods served as the counter electrode for current measurement. After the OCP measurement for hours, the potential was then increased at a rate of mv s-, starting from 00 mv below the OCP. From the polarization curve, corrosion current density (Icorr) was evaluated by Tafel extrapolation method by software (PowerCORR, V..). Table Processing parameters for various samples Power Powder Scanning Sample Beam density Pd speed v number size Φ P (mm) (kw) (kw/cm) (mm/s) Aged at 80 oc for h, air cooled and is similar to that of the laser surface-melted - PH stainless steel []. On the other hand, the solution-annealed 7- PH contains largely lath martensite with a minor fraction of ferrite [] but no precipitation of copper is observed indicate that the martensite is supersaturated with Cu and Cr. The microstructure of sample remained unchange because no melting occured at low power density. (a) Melt zone Substrate (b).9 o Annealed at 00 C for h, air cooled Results and discussion. Microstructural analysis The SEM micrographs of overall view of transverse crosssection and microstructure at high magnification of the laser surface-melted samples are shown in Figs. to. No pore and crack is observed. The melt depth of sample and sample are about 0. mm and 0. mm respectively. At higher magnification, it is obvious that a finer microstructure in the melt zone was obtained by LSM as shown in Figs. (b) and (b). The etched vermicular phase is primary ferrite. Owing to microsegregation at the solidification temperature, the cores of the cells are compositionally stablished ferrite, and appears in a vermicular morphology. The microstructure of the substrate and the aged 7- PH resemble a low-carbon lath martensite. Sometimes some copper preciptates (ε) occasionally were found at the boundaries of primary ferrite []. Comparing with the aged 7- PH (the substrate) [Fig. (c)], the melted zone shows an refined structure of interdendritic ferrite pointing along the thermal gradient with some pure ε-copper particles at the grain boundaries (c) Figure SEM micrographs of sample (Φ mm, P.0 kw): (a) overall view, (b) melt zone and (c) substrate.

3 surface melted samples were removed by grinding prior to the corrosion tests. The phases present in the various samples are summarized in Table. (a) Melt zone Substrate (b) Figure XRD spectra for of aged and laser surface-melted -7 PH. Table Phase present and hardness of various samples Sample Phase Max. hardness Condition present (HV) number Aged α, 0 Figure SEM micrographs of sample (Φ mm, P 0.8 kw): (a) overall view and (b) melt zone. Φ 0. kw 80 Φ 0.8 kw 7 Φ.0 kw 99 Φ 0. kw α, 0 Annealed α, δ α = lath martensite, δ = ferrite, ε = copper precipitate. Figure SEM micrograph of sample (Φ mm, P 0. kw). Fig. shows the typical XRD spectra of the laser surfacemelted -7 PH (sample ) and the aged -7 PH (sample ). The highest relative intensity of both samples is at θ =.. It indicates that the major phase is martensite. Besides, a small proportion of ε-copper precipitate and iron oxide are also detected in the laser-surface melted -7 PH due to thermal oxidation. The oxide layer on the laser- Hardness profile Fig. shows the micro-hardness profiles of various samples along the melt depth. The average hardness of the aged 7- PH (the substrate) is 0 HV. Compared with the aged 7- PH, increment of hardness was observed in the melt zone of the laser surface-melted samples, whose depth with the maximum hardness depends on the processing parameter, i.e. the power density. The maximum hardness increases with the increase in power density (Table ). While the hardness of the solution-annealed sample is the lowest ( HV). Among the laser surface-melted samples, the hardness of sample is the highest. The hardness of sample is increased by % and 7% when compared with aged 7- PH and solution-annealed 7- PH respectively Hardness profiles of the melt zones indicated that the amount of εcopper removed from primary ferrite during solidification due to microsegregation. Highly supersaturated structures can be obtained after LSM and rapid solidification through

4 increasing driving force for nucleation and growth, may increase the kinetics of ageing in precipitation hardening systems. The increase in hardness of sample may attributed to the presence of more ε-copper precipitates in the primary ferrite with high dislocation density. Fig. 8 shows the potentiodynamic polarization curves for various samples the.% NaCl solution and the key corrosion parameters such as I corr and pitting potential (E pit) are summarized in Table. All samples show passivation in.% NaCl solution. Compared with the aged and annealed -7 PH, the corrosion resistance of the laser surfacemelted samples was significantly improved, as evidenced by a noble shift in OCP, a higher pitting potential, a wider passive range and a lower passive current density. The laser surface-melted sample possesses the lowest passive current density (0 µa/cm ) whereas sample possesses the highest E pit (+0 mv) as well as the widest passive range. The enhanced corrosion resistance was attributed to the refinement of precipitation of copper particles in the ferrite matrix. Figure Hardness profiles for various samples.. Open circuit potential measurement Fig. 7 shows the plots of OCP vs elapsed time for various samples in.% NaCl solution (open to air) at o C. The OCP of the aged and annealed -7 PH become almost stable at -7 mv and - mv respectively. The OCP of sample is nobler than that of aged and annealed -7 PH and it indicates that LSM with higher power density can homogenize the microstructure resulting in higher thermodynamic stability. Figure 8 Polarization curves for various samples. Table Corrosion parameters of various samples. Sample OCP I Condition corr E pit number (mv) (na/cm ) (mv) Aged -7 8 Φ 0.kW Φ 0.8kW Φ.0kW Φ 0.kW -7 7 Annealed Conclusions Figure 7 Plot of OCP vs time for various samples.. Potentidynamic polarization test. LSM of 7- PH stainless steel was successfully achieved by a CW Nd:YAG laser for enhancing its corrosion resistance and hardness. No crack or pore was observed in the laser surface-melted samples.. Compared with the annealed and aged 7- PH, the corrosion resistance of the laser surface-melted samples was significantly improved, as evidenced by a noble

5 shift in open circuit potential, a higher pitting potential, a wider passive range and a lower passive current density.. The enhanced corrosion resistance was attributed to the refinement of precipitation of ε-copper particles in the ferrite matrix.. The hardness of the laser surface-melted 7- PH was also found to be higher than that of the aged and annealed ones by % and 7% respectively. Acknowledgments The work described in this paper was fully supported by research grant from the Science and Technology Development Fund (FDCT) of Macau SAR (Grant no. 09/009/A). References [] Ozbaysal, K., Inal, O.T. (990) Thermodynamics and structure of solidification in the fusion zone of CO laser welds of - stainless steel, Mater. Sci. Eng., A0, 0-7. [] Liu, R.L., Yan, M.F. (009) Improvement of wear and corrosion resistances of 7-PH stainless steel by plasma nitrocarburizing, Materials and Design, doi: 0.0/j.matdes [] Rack, H.J. Kalish, D. (97) Metall. Mater. Trans. A, 9. [] Ozbaysal, K., Inal, O.T. (99) Age-hardening kinetics and microstructure of PH - stainless steel after laser melting and solution treating, Journal of Material Science, 9, [] Murayama, M., Katayama, Y., Hono, K. (999) Microstructural Evolution in a 7- PH Stainless Steel after Aging at 00 o C, Metallurgical and Materials Transactions, 0A, -. [] ASTM Standard G-9: Standard Referemce Test Method for Making Potentiodynamic Anodic Polarization Measurement, ASTM Standards, ASTM, Philadelphia, USA.