A Study of Notch Toughness and Weld Softening in Submerged Arc Weld Heat-Affected Zone for Q & T High Tensile Steel (Report 1)

Characteristics of commercial high tensile steels

Abstract

Preparatory study was conducted in a series of researches on the characteristics of weld heat-affected zone, especially on the embrittlement in grain coarsened region adjacent to weld and softening in spheroidized zone welded with submerged arc process for quenched & tempered high tesile steel, based on the fact that decrease of notch toughness in weld heat-affected zone for Q & T steel was great when welded with large heat input such as in submerged arc welding.
In this study submerged arc single bead weld was deposited on an unsymmetrical single Vee groove of 20 mm plate, and a 2 mm V Charpy and small tensile specimen was taken out of HAZ.
Results show that decrease of notch touhgness caused by large heat input welding is greatest in bonded region HAZ and the slower the rate of cooling, the lower the notch toughness.
Ni is effective for improving bonded region HAZ notch toughness; however, other elements such as Mn, Cr, Mo and V, all show the tendency to decrease it.
Carbon equivalent which was described in Suzuki et al papers does not always appear to be suitable for predicting the notch toughness in large heat input weld HAZ and further study is needed.
By contrast, joint strength is hardly affected by Ni, but other elements increase this strength. Carbon equivalent which represents heat-affected zone hardnability is more useful to predict joint strength than notch toughness.
Microstructures in bonded zone are mostly bainitic except the case of 60 kg/mm² grade high tensile steel, i.e., steel A; and grains grow up to extreme size and their boundaries are intermittent, which is presumed to be due to the burning effect.
In higher strength steel, maximum hardness in HAZ is detected at 1 mm inside of HAZ from bond, which seems to be caused by diffusion of alloying element into weld and burning in grain boundaries.
Inspection by electron microprobe analyser indicates the diffusion region around bond approximately 120 μ for submerged arc welding and shows that it is located in HAZ.