Structure–property relationships for normalised flanges for low temperature service

Abstract

The influence of grain boundary carbides on the impact behaviour of normalised flanges made to the ASTM A350 LF2 specification has been determined. Previous work has derived the structure–property relationship for normalised and normalised and tempered forged flanges from very simple and easy to measure microstructural measurements, i.e. the volume fraction of pearlite and ferrite grain size. These measurements enabled the prediction of the Charpy V notch impact energy at −46°C to a high degree of accuracy from metallographic examination of small slivers taken off the sides of commercially produced flanges. In the present work, measurement of the grain boundary carbide thickness and the coverage of the carbides at the boundaries has been made to establish whether inclusion of these variables in the analysis would enhance the accuracy of prediction. The forged flanges had compositions in the range 0·17–0·33%C, 0·4–1·24%Mn with and without Al additions and all had ferrite–pearlite structures. An increase in the thickness of the grain boundary carbides resulted in a reduction in the impact energy. A further improvement in predictability can therefore be achieved by incorporating a grain boundary carbide thickness term in the analysis. No benefit to predictability was found from inclusion of a term for the degree of coverage of the boundaries by the carbides, probably because of the strong interrelationship between this term and the grain size. In addition to this work, the Charpy impact energy at 0°C was also obtained for flanges so that the technique could be extended to predict the impact energies for normalised flanges requiring compliance with specifications requiring a minimum impact energy at 0°C.