This section begins with the optimization of chemis­try and initial microstructure to develop swelling and

creep-resistant ferritic steels. The microstructural instability during service exposure is briefly pre­sented. The superior swelling performance of ferritic steels is understood based on mechanisms of void swelling suppression. Following this, the irradiation — induced/-enhanced segregation/precipitation causing irradiation hardening is discussed. The irradiation creep and embrittlement, their mechanisms and meth­ods to combat the problems are highlighted. The R&D efforts of today to reduce the severity of embrittle­ment in ferritic steels, using modeling methods, are outlined. Finally, typical problems in the weldments

of ferritic steels, when used for out of core applica­tions, are presented, emphasizing the advantage of modeling in predicting the materials’ behavior.

4.03.4.1 Influence of Composition and Microstructure on Properties of Ferritic Steels

Rapid strides have been made the world over, in the design and development of advanced creep-resistant ferritic or ferritic-martensitic steels. The low alloy steels can be used as either 100% ferrite-martensite
or a mixture of both. It is possible to choose the required structure by the appropriate choice of either the chemistry or the heat treatment. For example, a completely ferrite matrix, yielding high toughness, can be obtained in steels with chromium content higher than 12%, with carbon reduced to less than

0. 03%. The same steel can be used to provide higher strength by choosing the 100% martensite structure, if carbon content is increased to about ~0.1%. The 9Cr steels have always been used in the 100% mar­tensite state. Extensive studies have been carried out on phase stabilities of these steels, with changes in chemistry and heat treatment.

The creep resistance of the plain Cr-Mo steels has, further, been increased by the addition of carbide stabilizers like Ti or V or Nb, leading to the modified variety of 9-12Cr-Mo steels. These

elements led24 to copious, uniform precipitation of Monte Carlo (MC) type of monocarbides, which are very fine and semicoherent. Such precipitates are very efficient in pinning the mobile dislocations, lead­ing to improved creep behavior at higher tem­peratures. These carbides are stable at temperatures higher than even 1273 K and hence, do not cause deterioration of long-term mechanical properties during service exposure.

The development of high creep-rupture strength 9-12% steels with various combinations of N, Mo, W, V, Nb, Co, Cu, and Ta is based on optimizing the constitution (Table 2.) and 8-ferrite content, increasing the stability of the martensite, dislocation structure and maximizing the solid solution and pre­cipitation hardening. The concentration of each ele­ment in ferritic steels has been optimized based on an in-depth understanding of the influence of the specific element on the behavior of the steel. The extensive studies related to optimization of chemistry are summarized in Table 3. Based on the strong scientific insights, large number of commercial steels have been developed (Table 4) in the later half of the last century.

Most of this family of ferritic-martensitic steels is used in the normalized and tempered condition or fully annealed condition to achieve the desirable phase. The type of structure that is deliberately favored in a given steel depends on the end application.

The microstructure of the steels in normalized and tempered conditions consists24 (Figure 5) of (a) martensite laths containing dislocations with a Burgers vector 1/2a0<111> with a density of approximately 1 x 1014m~2 (b) coarse M23C6 particles located at

prior austenite and ferrite grain boundaries with finer precipitates within the laths and at martensite lath and subgrain boundaries. M2X precipitates rich in Cr are isomorphous with (CrMoWV)2CN.

The initial microstructure of the normalized and tempered steels described above does not remain stable during service in a nuclear reactor. Pro­longed exposure at high temperature causes changes in the initial microstructure, which has been studied extensively. The M2X precipitates in the normal­ized and tempered stabilized 9Cr-1Mo steels are gradually replaced (Figure 6) by MX, intermetallic, and Laves phases during prolonged aging at high temperature.

The high temperature and the irradiation over prolonged time of exposure introduce microstructural instabilities. These instabilities are caused mainly by the point defects caused by irradiation and complex coupling of these defects with atoms in the host lattice, their diffusion or segregation and finally the precipitation. There is a recovery of the defect structure since the irradiation-induced vacancies alter the dislocation dynamics. There are three types of processes with respect to evolution of secondary phases: irradiation-induced precipitation, irradiation — enhanced transformations, and the irradiation modi­fied phases. It is seen that the evolution of these phases depends on the composition and structure of the steel and the irradiation parameters like the temperature, dose rate, and the dose. Evolution of irradiation — induced phases and their influence on hardening and embrittlement is discussed later.