Causes of Low Temperature Toughness in ASTM A350 LF2 Flanges

Recent destructive analysis of six ASTM A350 LF2 Grade 2 flanges has revealed vastly different low temperature (-50F) Charpy impact toughness from 3 ft-lbs to greater than 220 ft-lbs.  These relatively low strength flanges, minimum 36 ksi yield and 70-95 ksi tensile strength, had nominally the same yield and UTS despite the difference in toughness.  Detailed chemical and microstructural analysis was undertaken to elucidate the cause of the toughness range.  The majority of the flanges had aluminum additions and a fine grain size with the toughness differences mostly explained by the cooling rate after normalizing with the still air cool showing the lowest toughness and the fastest air cooled sample the highest.  For flanges of this strength level a quench and temper operation is not required to obtain good low temperature toughness but forced air cooling after normalizing is a minimum cooling rate to ensure good mechanical properties.