P. Ganesh

Laser Cladding of Austenitic Stainless Steel with Hardfacing Alloy Nickel Base

Abstract A nickel base alloy Colmonoy 6 was chosen as a candidate material for hard facing many components in the prototype fast breeder reactor. This alloy offers outstanding wear resistance and high hardness at elevated temperatures. Previous research using the gas tungsten arc welding process revealed that the hardness of the deposit is influenced by the dilution from the base metal up to a deposit thickness as high as 2.5 mm. The present study aims to control the level of dilution in the hard face deposits by carrying out deposition using a low heat input laser cladding process. The samples were characterised by macroscopic and metallographic examination, and microhardness measurements, SEM - EDAX and XRD analysis were used to determine their properties. The study demonstrated that by controlling laser cladding parameters, thin deposits with a low level of dilution can be laid on austenitic stainless steel substrate. Cracking of the deposit was minimised by controlling the heating and cooling rates.

Effect of active flux addition on laser welding of austenitic stainless steel

Abstract The use of active flux in tungsten inert gas (TIG) welding is known to increase its weld depth. The present paper involves study of active flux laser beam welding (ALBW) of austenitic stainless steel sheets with respect to its effect on plasma plume, microstructure and mechanical properties of the resultant weldments. ALBW performed with SiO2 as the flux significantly modified shape of the fusion zone (FZ) to produce narrower and deeper welds. Plasma plume associated with the process was considerably smaller and of lower intensity than that produced during bead on plate laser beam welding (LBW). Flux addition during LBW produced thin and rough weld bead associated with humping. The development of such a weld bead is cause by reversal in the direction of Marangoni flow by oxygen induced inversion of surface tension gradient, widely fluctuating plasma plume and presence of oxides on the weld pool surface preventing free flow of the melt. Active flux laser weldments exhibited lower ductility than that of bead on plate laser weldments.

Comparison of Laser and Gas Tungsten Arc Weldments of Stabilized 17 wt% Cr Ferritic Stainless Steel

Abstract One of the major factors influencing weldability of stabilized 17 wt% Cr ferritic stainless steels is extensive grain coarsening in the fusion zone (FZ) and in the heat affected zone (HAZ). This study aimed at exploiting low heat input characteristics of laser welding to effectively control grain coarsening in the FZ and HAZ of 1.2-mm thick stabilized 17 wt% Cr ferritic stainless steel weldment. The study demonstrated that as compared with gas tungsten arc welding (GTAW), laser welding brought about significant grain refinement in the FZ and HAZ. Laser weldments exhibited superior ductility over their GTA counterparts. An important observation of the study was that, in contrast to GTA weldment, laser weldment was associated with weaker interfaces along columnar grain boundaries in the FZ that served to arrest crack propagation normal to them.

Novel laser surface treatment approach to suppress sensitisation in modified type 316(N) stainless steel weld metal

Abstract 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−1. This paper presents a novel laser surface treatment which suppresses sensitisation in weld metal, even at a slower cooling rate of 65 K h−1. Experiments involving laser surface melting were carried out with 150 W average power pulsed Nd:YAG laser and 10 kW CO2 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 CO2 laser beam. The processed weld metal remained unsensitised after solution annealing followed by slower rate of cooling at 65 K h−1. Numerical simulation study was performed with ANSYS 7·0 software to understand the physical reason behind the difference in sensitisation behaviour of CO2 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 CO2 laser clearly showed evolution of fine grains near the fusion boundary region which enhanced sensitisation resistance.

A Hybrid Laser Surface Treatment for Refurbishment of Stress Corrosion Cracking Damaged 304L Stainless Steel

The paper describes a new hybrid laser surface treatment approach, combining laser surface melting and laser shock peening treatments, for refurbishment stress corrosion cracking damaged type 304L stainless steel specimens. Hybrid laser surface treatment produced crack-free compressively stressed surface. With respect to as-machined specimens, laser-rejuvenated specimens demonstrated significantly reduced susceptibility to stress corrosion cracking in chloride environment with minor increase in mean surface roughness. The results of the study, although particularly applicable to shallow stress corrosion cracking damage, are important for life extension of in-service stainless steel components operating in corrosive chloride environment.

Laser assisted deposition of graded overlay of Stellite 6 on austenitic stainless steel

Hardfacing by Stellite is extensively employed in many engineering applications for enhancing high temperature wear, oxidation and corrosion resistance of austenitic stainless steel components. These overlaid components, because of large difference in thermo-physical and mechanical properties between clad and substrate, are largely prone to cracking. Cracking resistance of Stellite 6 clad austenitic stainless steel components can be greatly enhanced by providing smooth composition gradient across substrate-clad interface. This article presents development of graded overlay of Stellite 6 on AISI 304 stainless steel substrate using 2.5 kW continuous wave CO 2 laser and their comparison with directly Stellite 6 clad AISI 304 stainless steel specimens. Graded overlaying of Stellite 6 effected significant reduction in microhardness gradient across substrate-clad interface with respect to that of directly Stellite 6 clad specimens. Thermal cycling tests, performed on laser-cladded specimens, demonstrated superi...

Emerging Laser Materials Processing Techniques for Future Industrial Applications

Lasers are not only the proven and indispensable tools for some of the contemporary manufacturing technologies but have the potential for providing solutions to some of the upcoming intricate problems of industrial materials processing. The ongoing research is spearheading in the direction to develop novel fabrication techniques for improving qualities of the products, possibilities to engineer integrated multi-materials and multi-functional components and enhancing economic or procedural benefits. To explore the possibilities of achieving some of these objectives, we have carried out studies on the laser rapid manufacturing of structures of different metals with control over porosity, bimetallic integration, and other technologically important mechanical characteristics, laser melting based surface processing , laser shock peening , hybrid welding , and laser profile cutting of metal sheets. The results of these studies with comprehensiveness are presented and discussed in this chapter. A brief review of their scope for the industrial acceptability and adaptability has also been presented to assess the real potential of these research areas.

Microstructural characterisation of a dissimilar weld of alloy D9 and AISI 316M stainless steel produced using a 2.5 kW CW CO 2 laser

The dissimilar weld, a fuel clad tube (D9 alloy) and an end plug made of AISI 316M forms a part of the fuel pin in the 500 MW Indian Prototype Fast Breeder Reactor (PFBR). The present study aimed at the development of a crack resistant microstructure of the dissimilar weld by optimisation of 2.5 kW CO 2 laser welding parameters. The D9 alloy, because of its low Cr eq /Ni eq ratio (close to 1), solidified first as primary austenite which makes its welds particularly susceptible to solidification cracking. In this study, the primary mode of solidification in the fusion zone was modulated by controlling the relative degree of fusion of the fuel clad tube and the end plug. A defect free weld with predominantly a primary ferrite mode of solidification was obtained by optimising laser parameters and displacing the focused laser beam towards the end plug side to achieve a greater contribution of end plug material to the fusion zone. The formation of a crater and associated defects at the termination site of lase...

Enhancement of intergranular corrosion resistance of type 304 stainless steel through laser shock peening

ABSTRACT The present experimental study evaluates laser peening for suppressing intergranular corrosion (IGC) susceptibility of sensitised 304 stainless steel. Under the given experimental conditions, an increase in the number of laser peening treatments of sensitised 304 stainless steel specimen brought about progressive reduction in its susceptibility to IGC. Triple laser peening of sensitised stainless steel specimen introduced remarkable reduction in its IGC susceptibility. The results of the study suggest that breakage of inter-granular network of chromium carbides/chromium-depleted regions through plastic deformation, imposed by multiple laser peening treatment, was responsible for causing significant suppression in IGC susceptibility of sensitised 304 stainless steel specimens.

Laser welding of 3 mm thick laser-cut AISI 304 stainless steel sheet

The objective of the present work was to study the laser weldability of laser-cut 3 mm thick AISI 304 austenitic stainless steel sheet (using oxygen as an assist gas). For minimizing heat input during laser cutting, which is an important factor influencing the thickness of the oxide layer on the cut surface, laser cutting was performed in pulsed mode. The results of the study demonstrated that although the laser welding of laser-cut specimens did not result in the formation of weld defects, the resultant laser weldments exhibited reduced ductility with respect to base metal and bead-on-plate laser weldments. Laser-cut and laser-welded specimens also displayed higher notch sensitivity than the base metal. However, laser-cut and laser-welded specimens still possessed enough ductility to pass guided bend tests.