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In the postweld heat-treated (PWHT) fusion welded modified 9Cr-1Mo steel joint, a soft zone was identified at the outer edge of the heat-affected zone (HAZ) of the base metal adjacent to the deposited weld metal. Hardness and tensile tests were performed on the base metal subjected to soaking for 5 minutes at temperatures below Ac 1 to above Ac 3 and tempering at the PWHT condition. These tests indicated that the soft zone in the weld joint corresponds to the intercritical region of HAZ. Creep tests were conducted on the base metal and cross weld joint. At relatively lower stresses and higher test temperatures, the weld joint possessed lower creep rupture life than the base metal, and the difference in creep rupture life increased with the decrease in stress and increase in temperature. Preferential accumulation of creep deformation coupled with extensive creep cavitation in the intercritical region of HAZ led to the premature failure of the weld joint in the intercritical region of the HAZ, commonly known as type IV cracking. The microstructures across the HAZ of the weld joint have been characterized to understand the role of microstructure in promoting type IV cracking. Strength reduction in the intercritical HAZ of the joint resulted from the combined effects of coarsening of dislocation substructures and precipitates. Constrained deformation of the soft intercritical HAZ sandwich between relatively stronger constitutes of the joint induced creep cavitation in the soft zone resulting in premature failure.

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ture range of application of alloy 625 by the present authors. This may have a large impact on the mechanical properties and life of the cracker tubes.The various investigations of serrated flow suggest that the measurement of c , the critical strain for the onset/termination of serrations, [16] and its dependence on strain rate and temperature (T) is essential to understand the underlying mechanisms. This dependence is generally expressed as [1] where m and ␤ are the respective exponents in the relations for the variation of vacancy concentration (C v ) and mobile dislocation density ( m ) with plastic strain (C v ϰ m and m ϰ ␤ , respectively), K is a constant, Q is the activation energy, k is the Boltzman constant, and T is the absolute temperature. [17,18] One can obtain the exponent (m ϩ ␤) as the slope in the plot of ln vs ln c at a constant temperature. There are several methods for evaluating Q associated with serrated flow, which are described as follows.(1) Following Eq. [1], the slope of a plot of ln c vs 1/T at constant can be used to evaluate Q as Q ϭ slope ϫ (m ϩ ␤) ϫ k. However, as pointed out by Qian and Reed-Hill, [19] this method involves the use of an average value of (m ϩ ␤) obtained over a range of and T values.(2) From the ln vs ln c plots, one can obtain intercepts on the ln axis corresponding to different levels of critical strain at different temperatures. A replot of these intercept values of ln vs 1/T corresponding to different critical strain levels will yield a set of parallel lines, whose average slope is related to Q as Q ϭ slope ϫ k. [19,20] It may be noted that this intercept method does not involve the use of (m ϩ ␤).(3) The quasistatic aging model [17] relates the concentration dependence of c asAlloy 625 ammonia cracker tubes were service exposed for 60,000 hours at 873 K. These were then subjected to a solution-annealing treatment at 1473 K for 0.5 hours. The effects of temperature and strain rate on the tensile properties of the solution-annealed alloy were examined in the temperature range of 300 to 1023 K, employing the strain rates in the range of 3 ϫ 10 Ϫ5 s Ϫ1 to 3 ϫ 10 Ϫ3 s Ϫ1 . At intermediate temperatures (523 to 923 K), various manifestations of dynamic strain aging (DSA) such as serrated flow, peaks, and plateaus in the variations of yield strength (YS) and ultimate tensile strength (UTS) and work-hardening rate with temperature were observed. The activation energy for serrated flow (Q) was determined by employing various methodologies for T Ͻ 823 K, where a normal Portevien-Le Chatelier effect (PLE) was observed. The value of Q was found to be independent of the method employed. The average Q value of 98 kJ/mol was found to be in agreement with that for Mo migration in a Ni matrix. At elevated temperatures (T Ն 823 K), type-C serrations and an inverse PLE was noticed. The decrease in uniform elongation beyond 873 K for 3 ϫ 10 Ϫ5 s Ϫ1 and 3 ϫ 10 Ϫ3 s Ϫ1 and beyond 923 K for 3 ϫ 10 Ϫ4 s Ϫ1 strain rates seen in this alloy has been ascribed to reduction in ductility due...