Creep deformation of induction pressure welded 2·25Cr-1Mo steel

Int. J. Pres. Ves. & Piping 57 (1994) 327-330

Creep deformation of induction pressure welded 2.25Cr-lMo steel S. Ahila, S. Ramakrishna lyer & V. M. Radhakrishnan Indian Institute o f Technology, Madras 600 036, India

(Received 13 February 1993; accepted 1 April 1993) In this paper, experimental results involving the effect of stress and temperature on creep behaviour of induction pressure welded (IPW) 2.25Cr-lMo steel are presented. Creep rupture tests were conducted at 550-700°C in steps of 50°C over a stress range of 112-5-180MPa. Above 650°C failure of the specimen was enhanced due to the microstructural instability. Failure in the specimens occurred invariably in the heat affected zones (HAZ), and the fracture surfaces indicated ductile failure. recent years. 3'4 Normalised and tempered 2 . 2 5 C r - l M o steel boiler tubes have b e e n welded by induction pressure welding. The t e m p e r a t u r e , pressure, current and upset during a typical weld cycle is shown in Fig. 1. F r o m these w e l d e d tubes, creep specimens were made with the weldment at the centre of the gauge length.

1 INTRODUCTION

Superheaters and reheaters in fossil fired steam plants are frequently made of 2 - 2 5 C r - l M o steel. Failures along main steam pipes are primarily due to creep deformation. In p a r e n t / w e l d metal composite structures, where similar metals are joined, creep life is affected due to local changes in structure and mechanical properties. 1 C r e e p cracking occurs at w e l d e d joints, in particular, close to weldments. There are several reasons for the presence of failures a r o u n d welded joints: 2

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1. Variation of creep properties across a weld leading to stress concentration. 2. Z o n e s with low creep strength (fine grained region of heat affected zone). 3. Z o n e s with low creep ductility (coarse grained microstructure close to fusion boundary).

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The purpose of the present study is to evaluate the creep rupture b e h a v i o u r of induction pressure welded ( I P W ) 2 - 2 5 C r - l M o steel.

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2 EXPERIMENTAL

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I P W is one of the fastest welding p r o c e d u r e s used in boiler plants and has gained popularity in

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Int. J. Pres. Ves. & Piping 0308-0161/94/$07.00 ~) 1994 Elsevier Science Limited.

Fig. 1. Induction pressure welding--process details. 327

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S. Ahila, S. Rarnakrishna Iyer, V. M. Radhakrishnan 12 IPW

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These specimens were subjected to creep tests at stress levels of l l 2 . 5 - 1 8 0 M P a in a temperature range of 550-700°C. Percentage elongation with time, rupture life and total elongation were measured. The fractured samples were polished and etched with 2% nital and studied under the optical microscope. The fracture surface was studied using a scanning electron microscope.

3 RESULTS A N D D I S C U S S I O N Figures 2(a), 2(c) and 2(d) show the creep curves at various temperatures and stress levels. As expected, an increase in temperature or stress, increased the creep rate and decreased the rupture life. At 650 ° and 700°C, the samples failed at very early stages. Figure 3 shows a logarithmic plot of creep rate,

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The failed specimens indicated a microstructural instability at 650°C and above. The extensive precipitation of carbides at grain boundaries and in the grain interior resulted in a reduction of solid solution strengthening. A study of the fracture surfaces of the specimens revealed the presence of voids, mostly on the prior austenite grain boundaries (Fig. 5). Cracks were developed by the link up of these voids which might have originated at M 2 3 C 6 and M6C precipitates formed in the ferritic steel at a distance typically a few millimetres from the fusion boundary. 5 Cracks formed near the surface were oxidised as in Fig. 6. This led to a reduction in the available cross-section of the sample, thereby increasing the stress and leading to final failure. Figure 7 clearly indicates the formation of oxides along the grain boundaries and oxide islands in the interior of grains. This oxide formation also contributed to the weakening of the material on creep testing.

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330

S. Ahila, S. Ramakrishna lyer, V. M. Radhakrishnan

4 CONCLUSION From the results obtained from creep testing of induction pressure welded joints of 2 . 2 5 C r - l M o steel at different temperatures and stress levels, the following conclusions are drawn:

Fig. 7. Oxide islands along grain boundaries and inside the grains. All of the failures occurred in the H A Z and not in the weld metal or parent metal. The higher strength of the weld portion is closely related to the dissolution of carbides during welding. 6 Microstructural instability involves dissolution of M23C 6 carbides at regions well away from the fusion line and their precipitation initially as M3C along the prior austenite grain boundaries very close to the fusion line. These carbides subsequently transform to chromium-rich M23C6 and molybdenum-rich M6C on prolonged exposure during creep testing. This results in weakening of the H A Z during creep testing. The type and form of carbides p r o d u c e d by the normalising and tempering operations have been d o c u m e n t e d by B a k e r and Nutting. 7 Creep testing at temperatures favourable for carbide formation can be considered as a tempering process; at the test temperatures, used in this work the final microstructures correspond to the thermodynamic equilibrium structure. The continuous evolution causes the appearance of minimum creep rate, rather than a stationary creep rate. s

1. The material 2 - 2 5 C r - l M o obeys p o w e r law creep and the M o n k m a n Grant relationship. 2. The failure is of the ductile type with the fracture surface showing ductile dimples. 3. A b o v e 650°C, the microstructure was unstable, resulting in early failure, compared to that at lower temperatures. 4. The heat affected zones, with a microstructure containing both fine and coarse grained material was more prone to failure under creep conditions rather than the parent metal or weld metal.

REFERENCES 1. Ivarson, B. & Sandstorm, R., Met. Technol., 7 (1980) 440. 2. Hertzman, S., Sandstorm, R. & Wale, J., High. Temp. Technol., 15 (1987) 33. 3. Milner, D. R. & Rowe, G. W., Met. Rev., 7 (28) (1962) 433. 4. Dharmar, R., In Effect of weld voltage and weld upset on IPW of 2.25Cr-lMo reheater tubes. ME Thesis, Regional Engineering College, Trichy, India, 1986. 5. Li, C. C., In Proc. Conf. Joining Dissimilar Metals, Pittsburg, PA, 1982, AWS/EPRI, p. 107. 6. Nakazawa, T., Hosoi, Y., Shimada, H., Komatsu, H., Fujii, T. & Nogata, H., In Proc. Int. Conf. on Welding Technology for Energy Applications. Oak Ridge National Laboratory, TN, 1982, p. 217. 7. Baker, R. G. & Nutting, J., J. Iron & Steel Inst., 192 (1959) 257. 8. De Witte, M. & Steen, M., In Third Int. Conf on Creep and Fracture of Engineering Materials and Structures,

Pineridge Press, Swansea, 1987, p. 773.