1892 - Indira Gandhi Centre for Atomic Research

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TRANS. INDIAN INST. MET., VOL. 57, NO. 3, JUNE 2004 measurements were carried out in the as-deposited condition only. Optical metallographic examination of the as deposited WT-50 hardface coating was carried out to study the microstructure of the deposit and the deposit/substrate interface. Microhardness measurements were also carried out across deposit/ substrate interface at intervals of 0.2 mm at a load of 200 g. 3. RESULTS AND DISCUSSION The residual stress values measured in the radial direction on the top surface of the austenitic SS substrate in the as-deposited condition, corresponding to the locations A, B, C and D in Fig.2, are given in Table 2. The results show that the stress values are very low as the deposit is significantly (~7 mm) below the top surface. The residual stresses measured across the hardface deposit in axial directions on both half-sleeves (Fig. 3) in the as deposited and stress relieved conditions, and after thermal cycling are given in Figs.4 and 5. The results show very high values of compressive residual stress in the as deposited condition at all the three locations across the deposit width. The compressive residual stresses across the deposit are caused by the low coefficient of thermal expansion of the WT-50 deposit material (14–15 m m –1 K –1 ) as compared to 316 SS substrate material (17– 18 m m –1 K –1 ). During cooling after deposition, the austenitic SS substrate will shrink more due to its higher coefficient of thermal expansion. There<strong>for</strong>e, tensile residual stresses develop in the substrate and balancing compressive residual stresses develop in the deposit. The residual stress at the centre of the deposit (location 2 in Fig. 3) is higher than those at the periphery of the deposit (locations 1 and 3 in Fig. 3), because the total restraint of the substrate and deposit is higher at the centre than at the periphery. Fig. 4 : Residual stress profile across the coating in halfsleeve ‘C’ in as deposited, stress relieved and thermal cycled conditions. Fig. 5 : Residual stress profile across the coating in the halfsleeves ‘D’ in as deposited, and thermal cycled conditions Stress relieving heat treatment at 1123 K (half-sleeve C) is found to significantly the compressive residual stresses (110–200 MPa) reduce across the hardface deposit at all locations (Fig. 4), as the tensile thermal stresses generated during the stress-relieving heat treatment offsets the compressive stresses present in the as-deposited condition. Table 2 RESIDUAL STRESS ON THE SURFACE OF THE AUSTENITIC SS SUBSTRATE MEASURED IN RADIAL DIRECTION CORRESPONDING TO THE LOCATIONS SHOWN IN FIG.2. Locations A B C D Residual stress (MPa) –12 ± 23 15 ± 18 18 ± 26 –22 ± 20 274
DEY, et al., : RESIDUAL STRESS DISTRIBUTION IN HARDFACED AUSTENITIC STAINLESS STEEL SLEEVES Thermal cycling reduces both the peak compressive residual stress and the residual stress gradient across the deposit (Figs. 4 and 5). Repeated expansion and contraction during thermal cycling results in local yielding thereby relaxing the prior residual stresses and smoothening the residual stress distribution. It is observed that after thermal cycling, there is an increase in compressive residual stresses at peripheral locations in the stress-relieved deposit. Differential shrinkage between the coating and the substrate, which depends on the cooling rate and the difference in thermal expansion coefficients, increases the compressive residual stress. On the other hand, local yielding decreases the compressive residual stress. The combined effect of these two factors results in the observed changes in the residual stress in the peripheral locations. However, these changes in the residual stress distribution on thermal cycling did not have any adverse effect on the integrity of the deposit, as LPT, ultrasonic testing and radiography of the hardfaced sleeves did not show any cracking either in the deposit or at the deposit/substrate interface. Fig. 7 : Micrograph of the WT- 50 hardface coating The microstructure across the deposit/substrate interface (Fig. 6) reveals a narrow dilution zone and a planar interface. It also shows the presence of dispersed precipitates of borides and carbides, 8 which are responsible <strong>for</strong> the high hardness and wear resistance of this hardfacing alloy. No appreciable variation in the microstructural features is observed in the deposit from the interfacial region to the interior of the deposit. The microstructure of the hardface deposit at higher magnification (Fig. 7) clearly reveals Fig. 8 : Microhardness profile across 316 SS substrate/WT- 50 deposit interface in the as-deposited condition. the dendritic structure with dispersed precipitates in the inter-dendritic regions. The microhardness profile across the substrate/deposit interface (Fig. 8) shows that the hardness increases appreciably from ~290 VHN at the interface to ~410 VHN in the deposit across a distance of 0.2 mm. The short distance over which the hardness increases across the interface is indicative of the narrow dilution zone in this PTAW deposit. 4. CONCLUSIONS Fig. 6 : Micrograph across WT- 50/316 SS interface in asdeposited condition (1) Thermal cycling reduces the peak value and gradient in compressive residual stress across the hardface deposit in both the as-deposited and stress-relieved conditions. 275
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1892 - Indira Gandhi Centre for Atomic Research

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