Laser-assisted removal of weld heat tints from stainless steel surface

Materials Science and Engineering: A, 2000

The objective of the present work was to estimate the influence of pulsed laser irradiation on the removal of the oxide layer, which is developed on the surface of stainless steels during their exposure to high temperature oxidation. In general, this layer is a protective one, mainly against corrosion. However, in many manufacturing applications or maintenance work, the removal of the oxides is necessary; for example, the metallic surfaces should be cleaned before welding, otherwise the presence of oxides increases the tendency to brittle behaviour of the joint. In this study, a pulsed Nd:YAG laser (u= 1.064 mm, 10 ns) was used for the surface cleaning of three stainless steels with different chemical compositions and/or surface treatments. The influence of the laser irradiation on the material, as well as on the mechanisms involved, was investigated by in-situ mass measurements and post-situ microscopic observations (Scanning Electron Microscopy/SEM, and Energy Dispersion Spectroscopy/EDS). For energy density applied (1.0-2.0 J cm − 2 ), irrespective of the composition and the thickness of the surface layer, the laser irradiation resulted in the expulsion of the oxide layer, while no material removal of the underlying metal occurred.

Autogenous welded specimens of austenitic (S30400 and S31603), duplex (S31803) and super duplex (S32760) stainless steels were fabricated by laser penetration welding (LPW) with a CW Nd:YAG laser in an argon atmosphere. The microstructure and the phases present in the resolidified zone of the laser-welded specimens were analyzed by optical microscopy and X-ray diffractometry, respectively. The pitting and galvanic corrosion behavior of the stainless steels in the laser-welded and unwelded conditions in 3.5% NaCl solution at 23 • C was studied by means of electrochemical measurements. X-ray diffraction analysis showed that the phases present in the weld metal depended on the composition of the base metal. While the laser weld for S31603 retained the original austenitic structure, the laser weld of S30400 contained austenite as the major phase and ␦-ferrite as the minor phase. On the other hand, a slight change of ␦-ferrite to austenite ratio was found in both the laser welds of S31803 and S32760, with austenite present at the ␦-ferrite grain boundaries. The welds exhibited passivity but their pitting corrosion resistance was in general deteriorated as evidenced by a lower pitting potential and a higher corrosion current density compared with those of the unwelded specimens. The decrease in pitting corrosion resistance of the welds was attributed to microsegregation in the weld zone of S31603, and to the presence of ␦-ferrite in S30400. For the duplex grades S31803 and S32760, the disturbance of the ferrite/austenite phase balance in the weld zone could be the cause of the decrease in corrosion resistance. The initial free corrosion potentials of the unwelded specimens were considerably higher than those of the corresponding laser welds, indicating that the welds were more active and were expected to act as anodes in the weldment. The ranking of galvanic current densities (I G ) of the couples formed between the laser-welds (LW) and the as-received (AR) specimens with area ratio 1:1, in ascending order, is: AR S31603/LW S31603 < AR S31803/LW S31803 < AR S32760/LW S32760 < AR S30400/LW S30400. The recorded I G in all couples was low (in the range of nA/cm 2 ). welding (LPW) can produce low-distortion and precise weldments with minimal heat-affected zones . In LPW of stainless steels, phase transformation is common. The mechanical properties and corrosion resistance of laser-welded stainless steels may be deteriorated due to microsegregation, unfavorable phase content, presence of porosities, solidification cracking, micro-fissures and loss of materials by vaporization. Galvanic cell may also be set up between different parts of the weldment. Galvanic corrosion in weldments should not be overlooked because it can lead to accelerated deterioration of the anodic region especially in hostile environments. Pitting corrosion and galvanic corrosion have been investigated in the couples between dissimilar alloys such as 316L, Ti, Nb and [3], CoCr and REX 734 [4], annealed and cold-worked 316L [5], and GTAW welded and unwelded N08031 [6]. The pitting corrosion resistance of several austenitic stainless steels welded by a CO 2 laser has been investigated by Vilpas [7]. However, reports on the galvanic 0924-0136/$ -see front matter

Laser ablation is the method for material removing from a surface using pulsed lasers. It is used for surface structuring, laser drilling or laser marking. A change of the corrosion behavior of a base material caused by thermal effects during laser processing is an important problem of this technology. Possibilities of surface modification of stainless steel using laser ablation are presented in this contribution. The most important processes that take place during laser treatment of material surface are described. The influence of laser processing parameters on resulting material surface state and a possible connection of laser parameters with corrosion tests results are discussed.

Welded components are subjected to solution annealing heat treatment for achieving full stress relief and restoration of mechanical properties and corrosion resistance. During such heat treatments, optimum cooling rate has to be selected because very slow cooling rate will result in sensitisation and susceptibility to intergranular corrosion whereas fast cooling will result in reintroduction of residual stress. For 316 LN stainless steel which is welded using modified E316-15 electrodes (0?045-0?055%C), critical cooling rate above which there is no risk of sensitisation is 75 K h 21 . This paper presents a novel laser surface treatment which suppresses sensitisation in weld metal, even at a slower cooling rate of 65 K h 21 . Experiments involving laser surface melting were carried out with 150 W average power pulsed Nd:YAG laser and 10 kW CO 2 laser, in both continuous wave and pulse modulated (100 Hz) modes. Best results were obtained when surface melting was performed with high frequency pulse modulated CO 2 laser beam. The processed weld metal remained unsensitised after solution annealing followed by slower rate of cooling at 65 K h 21 . Numerical simulation study was performed with ANSYS 7?0 software to understand the physical reason behind the difference in sensitisation behaviour of CO 2 laser melted specimens under continuous wave and high frequency pulse modulated conditions and the predictions were validated using results of electron backscattered diffraction studies. Weld metal specimens treated with high frequency pulse modulated CO 2 laser clearly showed evolution of fine grains near the fusion boundary region which enhanced sensitisation resistance.

Metals, 2021

Welding parameters can greatly affect the final product. In this study, there was a variation given on the pulse energy, i.e., heat input parameters. The microstructure was analyzed and presented in relation to the efficiency of corrosion. The microstructural study showed the changes of the fusion zone (FZ) and the heat-affected zone (HAZ) with an increase in pulse energy. The development of a prominent austenite process on the weld material had a prolonged effect on its corrosion resistance property. Electrochemical impedance spectroscopy (EIS) and potentiodynamic measurements were used to test the electrochemical activity of laser-weld 2205 duplex stainless steel in an aqueous 3.5% NaCl solution. Finally, the findings of the EIS analysis were supported by Raman spectroscopy. Based on the obtained results, the 2205 duplex stainless steel (DSS) weld obtained at a higher pulse energy showed higher corrosion resistance than the welded sample obtained at a low pulse energy. The impedan...

ISIJ International, 2006

Microstructure and high temperature oxidation behaviour of laser weldments of 316L and 316LN steels have been found to influence by the variation in welding speed. Ferrite content and ferrite morphology change for both type 316L and 316LN laser weld with welding speed. Laser weldments consisting mainly of weld metal and base metal region of two austenitic stainless steels (ASS) were oxidized in dry air at 973 K for 240 h. Steel weldment was found to have a higher oxidation rate when joined with lower welding speed of 11.66 mm/s as compared to 25 mm/s speed. On the other hand, 316LN steel weldments have indicated much superior oxidation resistance property under similar condition. Oxidation behaviour of two ASS weldments has been correlated with microstructure and oxide scales formed over the different regions have been characterized by scanning electron microscopy (SEM/EDXA).

Optics & Laser Technology, 2011

Surface modification of AISI316 stainless steel by laser melting was investigated experimentally using 2 and 4 kW laser power emitted from a continuous wave CO 2 laser at different specimen scanning speeds ranged from 300 to 1500 mm/min. Also, an investigation is reported of the introduction of carbon into the same material by means of laser surface alloying, which involves pre-coating the specimen surfaces with graphite powder followed by laser melting. The aim of these treatments is to enhance corrosion resistance by the rapid solidification associated with laser melting and also to increase surface hardness without affecting the bulk properties by increasing the carbon concentration near the surface. Different metallurgical techniques such as optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the microstructure of the treated zone. The microstructures of the laser melted zones exhibited a dendritic morphology with a very fine scale with a slight increase in hardness from 200 to 230 Hv. However, the laser alloyed samples with carbon showed microstructure consisting of g dendrite surrounded by a network of eutectic structures (g +carbide). A significant increase in hardness from 200 to 500 Hv is obtained. Corrosion resistance was improved after laser melting, especially in the samples processed at high laser power (4 kW). There was shift in I corr and E corr toward more noble values and a lower passive current density than that of the untreated materials. These improvements in corrosion resistance were attributed to the fine and homogeneous dendritic structure, which was found throughout the melted zones. The corrosion resistance of the carburized sample was lower than the laser melted sample.

Electrochimica Acta, 2007

Martensitic high nitrogen stainless steels offer a combination of wear-, corrosion-and fatigue properties. But for some applications a higher surface hardness is required. A laser hardening with rapid heating (without smelting) and cooling (quenching) rates can improve the surface hardness with compressive residual stresses in the near surface layer. Yet, some cases of pitting corrosion in chloride media are reported.

Crystals

Small differences in the contents of surface active elements can change flow direction and thus heat transfer, even for different batches of a given alloy. This study aims to determine the effects of sulfur on weld bead morphology in the laser process. The paper presents the results related to the weld bead shape of two thin AISI 304 industrial stainless steel casts. One cast contains 80 ppm (0.008%) of sulfur, considered as a high sulfur content, and the other one contains 30 ppm (0.003%) sulfur, which can be considered low sulfur. The welds were executed using a CO2 laser. The effects of laser power (3.75, 3.67, 6 kW), welding speed (1.25, 2.40, 2.45, 3.6 m/min), focus point position (2, 7, 12 mm), and shield gas (Helium, mixed 40% helium + 60% argon and mixed 70% helium + 30% argon) with a flow rate of 10 L/min on the depth of the weld (D) and the aspect ratio (R = D/W) were investigated using RSM (response surface methodology). The experimental results show that the transfer of ...

Materials & Design, 2000

Laser surface melting (LSM) were carried out in three types of stainless steels — austenitic, martensitic and ferritic — in order to improve pitting corrosion resistance. After the LSM, most of the steels present a shift in the pitting potential to more noble values, a longer passivity stage and lower passive current density than the substrate. These improvements are achieved by means of a modified surface layer, more homogeneous with a fine cellular dendritic structure free of large precipitates. Nevertheless, the corrosion resistance depends critically on the laser processing parameters, particularly for the cases of ferritic and martensitic stainless steels. Therefore, care must be taken in the choice of the laser-processing parameters that leads to optimal properties in each material.