Microstructural zones in the primary solidification structure of weldment of 9Cr-1Mo steel
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I.
INTRODUCTION
A reliable, long-term service performance of components of ferritic steels, especially the 9Cr steels, has been an important consideration in nuclear industry, owing to their applications as core component materials. The main reason for the selection of these ‘‘ferritics’’ for core component applications is their excellent resistance to void swelling. Quite often, the failure of a component is initiated in a localized, microscopic region of the weldment, either during welding or in service.[1,2] This is due to the significant differences in the behavior of various regions of the weldment due to the heterogeneity in their structure.[3–6] This is attributed to the formation of a large number of phases with wide variations in their properties. The major causes of failure of weldments of ferritics have been identified as follows: formation of d-ferrite,[3,7] segregation of impurity elements,[8] overcoarsening of austenite (g ) grains near the weld,[9,10] and formation of intercritical region due to cooling along the duplex (a 1 g ) phase field.[1,2] Apart from these microstructural heterogeneities, the welding process also introduces significant differences in the microchemistry of different regions.[1] These deleterious microstructural and microchemical modifications of the parent metal during welding take place in regions that are as fine as a few angstroms. Hence, the application of analytical electron microscopy becomes essential for the complete understanding and the control of microstructure. The present article describes the results of a detailed analytical electron microscopy study on the microstructural heterogeneities in the weldments of the 9Cr-1Mo steel. The thermal cycle each region undergoes depends on its distance from the source of heat. Accordingly, various ‘‘microstructural zones’’ are found to be present, each zone having experienced a specific type of phase transformation during the thermal cycle. Based on the microstructural studies, the temperature profiles are deduced for the various M. VIJAYALAKSHMI, S. SAROJA, V. THOMAS PAUL and R. MYTHILI, Scientists, Physical Metallurgy Section, Materials Characterisation Group, and V.S. RAGHUNATHAN, Associate Director, Materials Characterisation Group are with IGCAR, Kalpakkam 603 102, India. Manuscript submitted February 19, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
zones. The present article aims to provide a complete description of microstructural and microchemical states of different regions of the weldments of 9Cr-1Mo steel. II.
EXPERIMENTAL DETAILS
The weldments of 9Cr-1Mo steel were fabricated from the normalized and tempered steel supplied by M/s. Creusot Loire & Co., Paris, France. The composition of the steel supplied is as follows: 8.24 wt pct Cr, 0.955 pct Mo, 0.19 pct Si, 0.07 pct C, and balance as Fe. The welding conditions are given in Table I. The geometry of the weld zone during manual metal arc welding in the present study is given in Figure 1(a). The microstructure developed in the weldment as the liquid pool co
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