Technical Paper

The effect of niobium additions on the microstructural morphology in the heat-affected zone of low-carbon steels

Symposium on Fundamentals and Applications of Mo and Nb Alloying in High Performance Steels

With the purpose of evaluating the effect of niobium additions on the microstructures of the HAZ (heat-affected zone) in mild steels, a plain C-Mn steel (without niobium) and three niobium containing steels with 0.01, 0.02 and 0.04 wt%Nb respectively, were investigated through simulated HAZ experiments at heat inputs of 20, 50 and 80 kJ/cm. The microstructures of simulated coarse-grained HAZ’s have been examined by optical metallography and transmission electron microscopy. It was found that the addition of niobium had a significant effect on the transformation in the HAZ. For high energy heat inputs (80 and 50 kJ/cm), the addition of niobium retarded the pearlite formation, even in the case of the 0.01 wt%Nb containing steel. The microstructures of the coarse-grained HAZ of niobium-containing steels consisted mainly of secondary Widmanstätten ferrite, but that of the niobium-free steel contained a large amount of pearlite as well as secondary Widmanstätten ferrite. At the low energy heat input level (20 kJ/cm), the microstructures of the coarse-grained HAZ of the niobium-containing steels were all similar and consisted mainly of interlocking ferrite plates with small amounts of bainite and Widmanstätten ferrite; while that of the niobium-free steel comprised Widmanstätten ferrite and martensite with a small quantity of pearlite. The results from Charpy impact tests indicated that the niobium-containing steels with a simulated HAZ heat input of 20 kJ/cm possessed higher toughness than those treated at heat inputs of 50 and 80 kJ/cm. It is proposed that the interlocking ferrite structure, which forms in the HAZ of the niobium-containing steels after simulation of a 20 kJ/cm heat input level, improves the toughness property. (AU) Copyright © 2014 Companhia Brasileira de Metalurgia e Mineração (CBMM) All rights reserved.
Technical Paper (PDF 2.98 MB)