Low carbon steel weld metal microstructures: The role of oxygen and manganese
A review of relevant welding literature as well as ongoing research at the University of Illinois at Chicago indicate the important influences of manganese and oxygen on the weld metal microstructure of low carbon steel welds. These microstructures are composed of several distinct ferrite morphologies, bainite, and other microconstituents. Both manganese and oxygen affect the transformation behavior of the weld metal as it cools from the austenitic range. A high oxygen content, in the form of oxide inclusions, shifts the transformation curve to the left by increasing the number of effective nucleation sites for high temperature transformation products (proeutectoid and side-plate ferrite). Manganese, on the other hand, shifts the transformation curve to the right, thereby increasing the weld metal hardenability and promoting the formation of lower temperature transformation products (acicular ferrite, bainite). Welds with a high proportion of acicular ferrite possess superior fracture toughness because the short, interlocking needles resist crack propagation.
The submerged-arc and gas metal-arc welding processes were both used to make high and low heat input welds with two different levels of manganese (1.0 and 1.6%). Photomicrographs taken with the light microscope along with microhardness measurements were used to distinguish the ferrite morphologies and other constituents present in the weld metal. Quantitative metallography performed on all samples confirmed an increased proportion of acicular ferrite at the higher manganese level. Grain size measurements of the prior austenite also indicated that manganese refines the secondary microstructure.
Submerged-arc welds with oxygen contents from 170 to 1400 ppm were also made. Area fraction measurements show an increase in acicular ferrite as the oxygen level is reduced from 1400 to 400 ppm. However, below an oxygen level of 250 ppm, further reductions in the oxygen content reduce the amount of acicular ferrite as bainite formation increases.
KeywordsWelding Ferrite Bainite Weld Metal Heat Input
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