Local Structure Investigation of Cu Precipitates in Modified 18CrNiMo7-6 Steels by Synchrotron X-ray Absorption Spectroscopy

This paper studied the copper precipitation in an 18CrNiMo7-6 martensitic steel (0.19 mass% C) with copper addition and its resulting improved mechanical behavior. The development of nano-precipitates in two modified alloys with 1.0 and 1.5 mass% copper addition was investigated by means of synchrotron X-ray absorption spectroscopy. The first-principles calculation has enabled the modeling of the unavailable copper standards: solid solution, B2, BCC, 2H, 9R and 3R, for calculating the XAS spectra and successfully identified the unknown phases after aging for the first time in this steel group. The samples alloyed with 1.5 mass% copper yielded the semi-coherent 9R structure when aged at 500 degrees C between 166 to 360 minutes. The ones containing 1 mass% copper formed the B2 ordered structure after aging at 480 degrees C for 50 minutes and revealed the co-existence of the 9R after 240 minutes. The analysis reveals the precipitation kinetics of copper in low carbon martensitic steel and helps determine the optimum tempering parameters to adjust peak strength.