Conditions for WJP are simulated using FEM method and it is found that high pressure increases the surface roughness because of increased erosion rate. Optimizing SOD is the basic criteria for achieving more compressive residual stress. It has been observed that compressive residual stress increase as large as 60% of yield strength of the material.
Ramulu et al. 7 compared the strength of the material after water jet peening and abrasive water jet cutting. The degree of plastic deformation and state of material surface were found to be strongly dependent on the peening condition applied. Water jet peening introduces compressive residual stress equivalent to those introduced by shot peening and other operations. Anand Rao et al. 8 studied the effect of different filler material and current in the TIG welded joints of 310 SS. The filler material 309L having same Cr content as base metal shows better tensile strength compared to 316L & 347 fillers.Sakthivel et al. 9 studied the creep deformation and rupture behaviour of 316L(N) Austenitic stainless steel processed by activated TIG and multipass TIG welding. Activated TIG welding increased the creep rupture life of steel weld joints over MP TIG welding.
Kuang Hung Tsung et al. 10 studied the effect of oxide fluxes MnO, TiO2, SiO2, Al2O3 used in Activated TIG welding process. The use of activated TIG welding increase the joint penetration and weld depth to width ratio, thereby reducing angular distortion of weld metal. Akbari Mousavi et al. 11 studied The effect of groove in the residual stress distribution of TIG welded 304L SS. U groove has minimum residual stressed in both longitudinal and transverse direction compared to V groove. 500 V groove has minimum residual stress compared to 100, 200, 300, 400 V grooves.Srivastava et al 12 studied the effect of various parameters such as pressure, SOD, traverse speed, current, voltage and no. of passes in water jet peening of welded joints. The main variables that influence are pressure and SOD. Increasing pressure reduces the tensile stresses in the welded joints. Maintaining optimum SOD in WJP is more important.
TIG – Tungsten inert gas welding, technically called gas tungsten arc welding is the most commonly used welding process that uses non consumable electrode that delivers the current to the welding arc. The tungsten and weld puddle are protected and cooled with an inert gas, typically argon and helium. 316L SS plates of 150*125*6 mm are welded using following TIG welding parameters as shown in the table 1.
TIG welded samples are then subjected to WJP operation using high pressure water droplets, to create local plastic deformation that induces residual compressive stress on the welded surface. By varying pressure & SOD, 6 different conditions (pressure – 200MPa, 250MPa, 300MPa & SOD- 50mm, 75mm). for each pressure conditions, two SOD were used for analysis.
Hardness of TIG welded and WJP samples are then analysed using micro Vickers hardness. XRD were taken for measuring residual stress. Microstructural analysis were done to visualize the microstructure of different zones of welded an peened metals. Surface roughness and SCC rate were determined.
The microstructures of as welded and peened samples observed in optical microscope are shown in the figure. The lines observed in base metal is due to multipass rolling. Discontinuous structure of columnar grains are found across the weldment along the welding direction. Fine grains are observed in the peened region. This might be due to the impact of high pressure water jet that broke the columnar grains into fine grains.
Micro Vickers hardness is used for measuring hardness variation from peened surface to distance in depth, since it is only way to measure minute variation in hardness that arises as a result of peening of TIG welded specimens.
Base metal hardness – 185 HV. A load of 0.5kg and dwell time of 12s were used to measure hardness. Hardness were measured at a level of 0.02mm from the peened surface as shown in the table 2.
From the values obtained for six different condition of peening, it is observed that high hardness of 237 HV is obtained at 250Mpa-50mm but in this, depth of penetration is limited to peened region as shown in the graph. In
300Mpa-50mm& 300Mpa-75mm penetration depth extend from peened surface (origin in the graph) to 60 m with hardness of around 215HV.
It is observed that pressure and SOD plays vital role in increasing the hardness. High pressure and small SOD helps to achieve high hardness with increased penetration depth in cross section. High SOD results in lowering the hardness value as seen in the 6 different conditions of WJP.
XRD had been used for measuring residual stress which were induced during welding & peening process.
The results of XRD shows that TIG welded samples have residual tensile stress of 97.5Mpa. WJP process transforms residual tensile stress to compressive with use of high pressure water droplets. It is observed that for peened samples under 250Mpa-50mm condition, high residual compressive stress of 118.8Mpa is obtained which reflects in the high hardness of 237HV. As SOD increases, the effect of pressure in inducing compressive stresses decreased. XRD results justifies the hardness values yet high compressive stresses leads to high hardness.At 50mm, the water droplets has sufficient energy with continuous flow for creating higher stresses which is needed to strengthen the welded region while in 75mm, the path of water flow diversifies, covering larger for peening but not with much high potential for inducing compressive stresses.
Surface roughness is the combination of short wavelength deviation of the surface from the nominal surface.