Improved creep strength and creep ductility of type 347 austenitic stainless steel through the self-healing effect of bo
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I. INTRODUCTION
AUSTENITIC stainless steels such as types 316, 321, and 347 are widely used in power generating, petroleum, and chemical plants. However, in order to reduce environmental pollution, there is an urgent need to increase the thermal efficiency of such plants by increasing the working temperatures. Consequently, creep failure limits the life of these materials at these higher working temperatures. To that end, significant efforts have been undertaken to develop new creep resistance steels[1,2,3] as well as to increase the creep strength of existing steels.[4] The increase in creep strength of the existing steels has been accomplished through the optimization of elements present in the steels and also through the addition of minor elements. For example, Ti and Nb are added to austenitic stainless steels to increase their creep strength through the fine intragranular precipitation of Ti, Nb-carbonitride particles. However, the stability of such fine particles as well as the intergranular precipitation of brittle intermetallic phases such as and determine the long-term creep strength of such stabilized steels. Several investigations[2,5] have indicated an optimum level of Ti, Nb, C, and N contents in the austenitic stainless steels for higher long-term creep strength. This has been achieved by adjusting the Ti, Nb, C, and N contents with (Ti, Nb)/(C, N) ratio close to the stoichiometric value of the Ti, Nb-carbonitride precipitates; and also by ensuring that all the prior existing Ti, Nb-carbonitride particles dissolve during the solution annealing treatment of the steel so that they can reprecipitate as fine intragranular particles at dislocations during creep. K. LAHA, Science Officer, is with the Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, Tamil Nadu, India. J. KYONO, Senior Engineer, T. SASAKI, Research Associate, S. KISHIMOTO, Senior Researcher, and N. SHINYA, Research Fellow, are with the Materials Engineering Laboratory, National Institute for Materials Science, Ibaraki 305-0047, Japan. Manuscript submitted December 23, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
In addition, a suggestion[6] also has been put forward to make the steel substoichiometric with respect to the carbonitride particles to increase its long-term creep strength through the “understabilizing” effect. These modifications have increased the long-term creep strength of austenitic stainless steels not only by imparting better stability to a higher density of carbonitride particles but also by delaying the extensive intergranular precipitation of brittle intermetallic phases such as and .[7] More recently, Cu has been added to austenitic stainless steels to increase their creep strengths by the intragranular precipitation of nanosize Cu particles.[4,8] Creep failure proceeds with the nucleation, growth, and coalescence of grain boundary cavities. The evolution of grain boundary cavities during creep is closely connected to the physical properties of the cavity surface a
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