Cross-Weld Creep Performance in Grade 91 Steel: Macro-Based Assessment

Abstract

The martensitic 9% Cr creep strength enhanced ferritic (CSEF) steels Grades 91 and 92 (Table 1) are carefully alloyed and processed to develop a distribution of carbonitrides such as M 23 C 6 and MX-type precipitates that pin the martensitic matrix and dislocation substructure. Upon application of a welding thermal cycle, this microstructure is significantly altered and results in local re-gions [e.g., the heat-affected zone (HAZ)] that possess inferior creep properties compared to that of the unaffected base material. In-service performance of fabricated 9% Cr CSEF steel structures will, in many cases, be limited by the multiaxial stress-state response of the HAZ. It is thus important for well-planned and executed cross-weld creep test programs to carefully evaluate and quantify the extent of damage in the HAZ.The complexity of the martensitic microstructure and the evolution of damage necessitates careful evaluation using well-controlled procedures and for multiple length scales as the microstructural features span several orders of magnitude in length scale using the techniques listed in Table 2. The general length scale for the important features in martensitic CSEF steels are summarized below:Prior austenite grain size ~ 10 to 100s m Packet boundary size ~ 1 to 10s m BN and inclusions ~ 1 m AlN, Laves phase, and M 23 C 6~ 100s nm MX ~ 10s nm Dislocations ~ <1 nm Present damage may be manifested as voids (~1 m), microcracks (~ 100s m), or macrocracks (≥ 1 mm) Significant advances have recently been made for welldeveloped technologies regarding the ability to obtain large datasets such as hardness testing (e.g., automated hardness mapping); conventional microscopy (e.g., development of confocal laser microscopy and light-emitting diode (LED) microscopy); and the ease at which advanced electron microscopy can be applied to obtain large, statistically relevant datasets. It is also true that ever more advanced techniques are becoming more commonplace and are now available to researchers to elucidate the specifics regarding the evolution of damage in base material and cross-weld samples. This was recently highlighted in a special feature in Materials Science and Technology, Ref. 1). The complexity of martensitic CSEF steels necessitates the application of these methods to WELDING RESEARCH ABSTRACTMeaningful characterization of the microstructure in metallurgically complex steels is complicated by the diversity of thermal cycles experienced by multipass fusion welds. To overcome the problems of relevant documentation, it is necessary to balance information from macro-, micro-, and nano-evaluation with appropriate analysis. This paper presents details regarding recommended approaches that optimize this characterization.Initially, specific procedures relevant to macroanalysis, including hardness mapping and calculation of the peak temperature through the width of the heat-affected zone (HAZ), are described. Then, assessment of the distribution of creep damage in feature-type, cross-weld creep tests using laser m...