Abstract
Four slabs cut out from a forging ring were normalized at 870°C, 885 °C, 900°C and 915°C for three hours. Cooling from normalizing to room temperature proceeded under forced air. Test specimens were extracted for Charpy impact testing at −46°C and metallographic examination. Furthermore, open-source image analysis software ImageJ was used on SEM micrographs to evaluate the size distribution of pearlite nodules.
The four different normalizing temperatures yielded impact test values above the minimum threshold of SA – 350 for LF2 Cl1 classification. However, the slab normalized at 870 °C showed the best performance, being significantly higher (122 Joules average – 53 Joules minimum) than slab normalized at 915°C (86 Joules average – 33 Joules minimum). Normalizing temperatures of 885°C and 900°C behaved similarly (104 J and 113 J average, respectively).
After the metallographic characterization, it was concluded that higher normalizing temperatures increase the austenite grain size and promote finer pearlites made of single-colony nodules. Lower normalizing temperatures restrict the austenitic grain growth, leading to coarser pearlites made of smaller nodules with multiple colonies. These smaller nodules of coarse pearlite oppose more effectively to brittle crack propagation since colony boundaries function as an additional barrier.
