In order to understand the effect of niobium additions on the microstructures of the HAZ (heat-affected zone) in mild steels, a plain C-Mn steel (without niobium content) and three niobium containing steels with 0.01, 0.02 and 0.04 mass%Nb, respectively were investigated through simulated HAZ experiments with the heat inputs of 2×106, 5×106 and 8×106 J/m. The microstructures of simulated coarse-grained HAZ 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. For high energy heat inputs (8×106 and 5×106 J/m), the additions of niobium evidently retarded the pearlite formation even in the case of 0.01 mass%Nb containing steel; the microstructures of coarse-grained HAZ of niobiumcontaining steels consisted mainly of secondary Widmanstätten ferrite, but that of the niobium-free steel comprised a large amount of pearlite besides secondary Widmanstätten ferrite. In the condition of low energy heat input (2×106 J/m), the microstructures of coarse-grained HAZ of 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 test indicated that the niobium-containing steels treated with the heat input 2×106 J/m could possess higher toughness than those treated with the heat inputs of 5×106 and 8×106 J/m. It is proposed that the interlocking ferrite structure, which forms in niobium-containing steels under 2×106 J/m heat input, improves the toughness property.
(Received September 1, 1998; In Final Form December 11, 1998)
Keywords: niobium containing steels, coarse-grained heat-affected zone, thermal simulation, Widmanstätten ferrite, bainite, interlocking ferrite structure, transmission electron microscopy
† Graduate student, National Taiwan University, Present address: Chang Mien Industries Co., LTD., Kaohsiung, Taiwan.
†† Graduate student, National Taiwan University, Present address: Department of Mechanical Engineering, Yuan-Ze University, Chung-Li, Taoyuan, Taiwan.