The advanced ferritic and ferritic-martensitic steels of current interest have evolved5 from their prede­cessors, the creep-resistant ferritic steels, over nearly a century. The first of the series was the carbon and C-Mn steels with a limited application to about 523 K. Subsequent developments through different levels of chromium, molybdenum have increased the high temperature limit to 873, leading to the current ferritic and ferritic-martensitic steels, that is, the 9-12% Cr-Mo steels. In addition to being economi­cally attractive, easy control of microstructure using simple heat treatments is possible in this family of steels, resulting in desired mechanical properties.

The propensity to retain different forms of bcc ferrite, that is, ferrite or martensite or a mixture at room temperature in Cr-Mo steels, depends crucially on the alloying elements. Extent of the phase field traversed by an alloy on heating also depends on the amount of chromium, silicon, molybdenum, vana­dium, and carbon in the steel. The combined effect of all the elements can be represented by the net chromium equivalent, based on the effect of the aus­tenite and ferrite stabilizing elements. A typical pseu­dobinary phase diagram6 is shown in Figure 1(a). Increase in chromium equivalent by addition offerrite stabilizers or V or Nb would shift the Fe-9Cr alloy into the duplex phase field at the normalizing temperature. The phase field at the normalizing tem­perature and the decomposition mode7-9 of high temperature austenite (Figure 1(b)) dictate the result­ing microstructure at room temperature and hence, the type of steel. Accordingly, the 9CrMo family of steels can either be martensitic (9Cr-1Mo (EM10) or stabi­lized 9Cr-1MoVNb (T91)), ferritic (12Cr-1MoVW (HT9)) or ferritic-martensitic (9Cr-2Mo-V-Nb (EM12)) steel. The stabilized variety of 9-12 CrMo steels could result10 in improved strength and delayed grain coarsening due to the uniform distribution of fine niobium or vanadium carbides or carbonitrides.

The transformation temperatures and the kinetics of phase transformations depend strongly on the composition of the steels. Sixteen different 9Cr steels have been studied11,12 and the results, which provide the required thermodynamic database are shown in Figure 2, with respect to the dependence of melting point, Ms temperature and the continuous heating transformation diagrams. The constitution and the kinetics of transformations dictate microstructure and the properties.

In the early stages, the oxidation resistance and creep strength were of prime importance, since the Cr-Mo steels were developed4 for thermal power stations. In addition to the major constituent phases discussed above, the minor carbides which form at temperatures less than 1100 K, dictate the long term industrial performance of the steels. Evaluation of tensile and creep properties of Cr-Mo steels exposed

to elevated temperature for prolonged durations have been extensively studied.5,13,14 The following trends

were established: The optimized initial alloy compo­sition considered was 9Cr, W-2Mo = 3, Si = 0.5, with C, B, V, Nb, and Ta in small amounts. Higher chro­mium content has two effects: it increases the hard — enability leading to the formation of martensite and also promotes the formation of 8-ferrite thereby reducing the toughness. A reduction in the chromium

content lowers the oxidation resistance. If W + Mo concentration is kept <3%, creep strength will reduce, while higher amount promotes the formation of 8-ferrite and brittle Fe2Mo Laves phase. The addi­tion or partial replacement of molybdenum with tungsten and boron increased the stability of M23C6, and slowed down the kinetics of recovery. Lower nickel introduced 8-ferrite, while its increase reduces creep strength. When Si is less than 0.3%, oxidation resistance gets lowered, while higher silicon content led to agglomeration of carbides with an increased amount of 8-ferrite. On similar lines, the composition of all other elements could also be optimized, based on structure-property correlation studies.

The components of the steam generators are often subjected to repeated thermal stresses as a result of temperature gradients that occur on heating and cool­ing during start-ups and shutdowns or during
variations in operating conditions. Steady state operation in between start-up and shutdown or transients would produce creep effects. Therefore the low cycle fatigue (LCF) and creep-fatigue inter­action assume15 importance in the safe life design approach of steam generator components. The alloy exhibited a decrease in fatigue life with increasing temperature, thus limiting its upper limit oftempera — ture up to about 773 K.

The joining technologies of Cr-Mo steels have been well investigated.16,17 One of the major pro­blems during welding of ferritic steels has been the formation of 8-ferrite, if the amount of ferrite stabi­lizers is high. The partial substitution of Mo with W enables austenite stabilization and hence reduces the tendency to form 8-ferrite. In fact, there needs to be an intricate balance between austenite and ferrite stabilizing elements in 9-12Cr-Mo steels.

This would ensure a satisfactory solidification process with a fully austenitic structure. Additionally, this enables easier hot workability during primary proces­sing and tubemaking, without losing high tempera­ture creep resistance. The formation of 8-ferrite reduces toughness due to the notch sensitivity, pro­motes solidification cracking and embrittlement due to sigma-phase precipitation and reduces the creep ductility at elevated temperatures of operation. Other problems relate to solidification cracking, hydrogen cracking, and reheat cracking, which have been exten­sively studied.18 The Type IV cracking in ferritic steel weldments and the brittle layer formation in the dis­similar welds are discussed in detail later.