Influence of long-term aging and superimposed creep stress on the microstructure of 2.25cr-1Mo steel
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TRODUCTION
A 2.25Cr-lMo steel has been used f o r hightemperature service applications in p o w e r generation and chemical industries for over five decades. Apart from possessing adequate creep strength, the corrosion resistance o f this grade o f steel is superior to that o f 0 . 5 M o , 1Cr-0.50Mo, and 1.25Cr-0.50Mo steels. M i c r o structural changes due to long-term tempering and the creep-deformed structures were investigated a few decades ago using carbon extraction replicas.~l-4] Recently, the microstructure of the Cr-Mo and Cr-Mo-V steels exposed to 20 years o f service life has been studied to understand the microstructural degradation35-9] Some investigators used accelerated aging at 630 °C to produce microstructures that simulate 25,000 to 1,000,000 hours o f service exposure at 540 °C.t~°~ Though these studies established various structural changes due to prolonged service exposure, the influence o f exposure time and temperature vis-d-vis that o f superimposed creep stress in bringing about the observed microstructural changes is still not clear. Considering the recent interest in predicting the remaining life o f the components exposed to the designed life,m-~2] a simultaneous study o f the two factors becomes important. W e have recently reported the results o f our investigations on the role o f hightemperature exposure time and that o f the superimposed creep stress on the microstructures o f 1.25Cr-0.5Mo, 1Cr0 . 5 M o , and 0.5Cr-0" 5Mo-0.25V s t e e l s . [13'14'151 Since creep-resistant steels are presently imported by India, Tata Steel embarked on a p r o g r a m o f producing 2.25Cr-1Mo grade steel to international specifications. N. GOPE, Manager, AMIT CHATTERJEE, Deputy General Manager, Research and Development, and T . MUKHERJEE, Additional General Manager, Iron and Steel, are with T h e T a t a Iron & Steel Company Ltd., Jamshedpur 831 0 0 1 , India. D.S. SARMA, Professor and Department Head, is with the Department of Metallurgical Engineering, Benaras Hindu University, Varanasi 221 0 0 5 , India. Manuscript submitted November 2 6 , 1990. METALLURGICAL TRANSACTIONS A
Stress-rupture tests have been carried out at various temperatures, and the results o f these tests have been reported elsewhere, t16-18~ In the present investigation, the results o f a detailed transmission electron microscopy study carried out on both the shoulder and gage portions o f the stress-rupture tested specimens o f a 2.25Cr-lMo steel are presented in o r d e r to arrive at an understanding o f the effects o f high-temperature exposure and those o f superimposed creep stress. II.
EXPERIMENTAL
A 2.25Cr-lMo steel, with the chemistry shown in Table I, was made in a 6-ton electric arc furnace and teemed into a mold fitted with a hot top. The ingot was subsequently rolled to 250 × 250 mm square blooms, and these blooms were machined down to 210 × 210 mm square gothic bars and subsequently rolled to seamless tubes o f 215-mm outside diameter and 4 0 - m m wall thickness through a piercing elongator and pilgering proce
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