Heat resisting steels and iron-containing superalloys
The typical operating life of a modern automobile is 2000 to 3000 hours. That of a civilian aircraft engine is ten times longer, while certain thermal power station components are expected to last more than 300000 hours (1 year = 8760 hours), often with a maximum tolerated strain of only 1%. Materials to be used in such applications must he extremely stable with excellent creep resistance. Although alloy optimisation is based on similar principles to those applied in the case of other steels, special attention must be paid to the possibility of very slow transformations, and it is necessary to reconsider what can he reasonably defined as a state of equilibrium. High temperature strength and stability tends to be expensive. In particular, austenitic structures are intrinsically stronger at high temperatures than ferritic materials, but require large proportions of costly alloying elements. Depending on the operating conditions, a whole series of materials have therefore been developed, ranging from low alloy steels to nickel base superalloys. For this reason, a brief mention is given to alloys that can no longer he considered as steels, but which provide a comparative framework.
KeywordsCast Iron Nickel Base Superalloy Creep Strength Lath Boundary Secondary Carbide
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