Nonequilibrium solute segregation to austenite grain boundaries in low alloy ferritic and austenitic steels
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I.
INTRODUCTION
THEimportance of chemical composition within the grain boundary region in ferritic and austenitic steels has been widely recognized, 1'2 and its influence on the creep deformation, weldment reheat cracking, and stress corrosion cracking susceptibility of electrical power plant components is well known. 3'4'5 In ferritic steels changes in composition can result from either equilibrium or nonequilibrium segregations of alloying or impurity elements to the prior austenite grain boundaries. Nonequilibrium segregations have been observed in a range of alloy systems following quenching or cooling below a critical rate.6-1~ The original evidence for these was inferred by Westbrook ~who measured marked hardness increases at grain boundaries. In general, nonequilibrium segregations developed by quenching increase in extent with increasing initial austenitizing temperature, and it is considered that segregation of solute arises from the diffusion and decomposition of vacancy-solute complexes. 2'6-~~Changes in equilibrium vacancy concentration with temperature provide the driving force for diffusion to grain boundaries which act as sinks during the period of cooling. The extent of the solute segregation therefore depends upon the excess vacancy concentration and the strength of the vacancy-solute atom bond. Scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectrometry (EDS) enables X-ray microanalyses to be undertaken on thin foil specimens to a spatial resolution that is sufficient to measure such solute segregations to grain boundaries. 11'12 This technique has the advantage that solute distribution can be related to the detailed microstructure. In this contribution segregations of substitutional chromium, a major alloying element, and tin, an impurity element, which occur during quenching low alloy 2.25 pct Cr 1 pct Mo, Type 316 stainless and 2.25 pct Cr 1 pct Mo 0.08 pct Sn steels are measured in thin foils using the STEM-EDS X-ray microanalysis technique. The results are compared with the predictions of an analysis based upon a model for segregation during quenching involving migration of vacancy-solute atom pairs.
P. DOIG, Acting Section Head, and P. E.J. Flewitt, Acting Branch Manager, are with the Scientific Services Department, Central Electricity Generating Board, South Eastern Region, Gravesend, Kent, United Kingdom. Manuscript submitted November 18, 1985. METALLURGICALTRANSACTIONSA
II.
SEGREGATION ANALYSIS
An analysis based upon conjugate vacancy-solute migration to a grain boundary sink has been described previously. 7 This can be applied to examine the magnitude and spatial extent of quench-induced solute segregations to austenite grain boundaries in steels. In addition to the vacancy-solute diffusion to the boundary, the analysis includes concomitant reverse diffusion of solute atoms as a consequence of the thermodynamic driving force that results from the nonequilibrium concentration of solute atoms developing about the grain boundary. These atoms hav
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