1892 - Indira Gandhi Centre for Atomic Research

More documents

Recommendations

Info

TRANS. INDIAN INST. MET., VOL. 57, NO. 3, JUNE 2004 of the hardface coating, while locked-in residual stresses may increase the risk of micro cracking during thermal cycling in low-ductility coatings. Further, high compressive residual stresses parallel to the surface improves the tribological behaviour of the deposit, 6 and a compressive pre-stress to the deposit beneficially balances the tensile thermal stress during high temperature application. 3 Also, the magnitude and distribution of residual stresses depends on the maximum temperature, holding time, and heating/ cooling rate during thermal cycling. The GP sleeves of PFBR are to be hardfaced at two locations where it comes in contact with the core subassembly (Fig. 1) – one on the top chamfered portion and the other on the inner diameter 470 mm from top. As thinner coatings have better thermal shock resistance due to reduced accumulated residual stresses, the low-dilution PTAW process was chosen. To eliminate the risk of micro cracking and delamination of the deposit, and to minimize the magnitude of residual stress, an optimised PTAW deposition procedure has been qualified. In this work, the effect of thermal cycling during service on the residual stress distribution of hardfaced austenitic SS sleeves has been studied. 2. EXPERIMENTAL PROCEDURE Hardfacing on the inside of the GP sleeve was simulated by hardfacing a 316 SS sleeve (106 mm OD x 80 mm ID x 45 mm height) on its inside surface with nickel base WT-50 hardfacing alloy powder (composition given in Table 1) using the PTAW process (Fig. 2). After hardfacing, the sleeve was cooled slowly in vermiculite powder, machined to the required thickness of 1.5 mm, and examined by liquid penetrant test (LPT). The hardfaced sleeve was cut into two halves along AB (Fig. 2). One half (with location C at the middle) was stress relieved at 1123 K for 35 minutes, using a heating rate of 150 K h –1 and holding time of 2.5 Fig. 1 : Drawing of grid plate sleeve, showing the two hardfacing locations min per mm of thickness, as per the specification. 7 The other half of the sleeve (with location D at the middle) was retained in the as-deposited condition, for comparison. Both the sleeve-halves were then subjected to thermal cycling between 473 and 823 K Table 1 CHEMICAL COMPOSITION OF WT-50 HARDFACING ALLOY POWDER USED Elements C Si Cr Fe B Others Ni Wt.-% 0.46 3.88 10.59 3.00 2.34 < 0.5 Balance 272
DEY, et al., : RESIDUAL STRESS DISTRIBUTION IN HARDFACED AUSTENITIC STAINLESS STEEL SLEEVES for 20 cycles, with holding duration of 1 h at both the temperatures and immediate transfer of samples between furnaces maintained at the two temperatures. The thermal cycling temperatures of 473 and 823 K that were used corresponds to the minimum and maximum temperature of liquid sodium that would be encountered. Residual stress measurements were carried out by X-ray diffraction (XRD) technique on the substrate and across the coating. For the austenitic SS substrate, residual stress measurements were carried out using Cr-K radiation with the X-ray tube operating at 30kV and 7mA target current. The fcc austenite structure gives a diffraction peak (2) at 148.5 o , corresponding to the (311) plane. The effect of peak shift at various tilt-angles () was related to the change in inter-planar (‘d’) spacing of the (311) plane for residual stress measurements. For nickel base hardfacing alloy, residual stress measurement were carried out using Cu-K radiation with the X-ray tube operating at 30kV and 7mA target current. The scan range for the peak detection (2) was between 140 o and 149 o . For both the halfsleeves, in-plane residual stress measurements were carried out in axial direction across the coating at three locations (Fig. 3) in the as-deposited and stressrelieved conditions, and after 5 and 20 thermal cycles. For the austenitic SS substrate, residual stress Fig. 2 : Nickel base WT-50 alloy hardfaced 316 stainless steel sleeve Fig. 3 : One half of the hardfaced sleeve, with the arrow showing the direction of residual stress measurements 273
Page 1: Trans. Indian Inst. Met. Vol.57, No
Page 5 and 6: DEY, et al., : RESIDUAL STRESS DIST

1892 - Indira Gandhi Centre for Atomic Research

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?