Underwater Wet Welding Of E40Steel Using Flux-cored Wire

According to the underwater wet welding environment and the problem of poreand crack flaws produced by the welding process, a new Ni-based flux-cored wirewhich can be used in the underwater wet welding is developed. The seam formability,arc stability, slag spreadability and detachability are evaluated in detail. Meanwhile,microstructure, mechanical properties and fractures of the E40steel butt jointsobtained by different heat inputs are particularly analyzed.Because of the effects of cooling of vapour and hydraulic pressure, the weldseam of underwater welding has smaller fusion width and larger reinforcement thanthat obtained in the air while employing the same welding parameter. In theunderwater welding, the influence of welding voltage on the fusion width isunconspicuous when the welding current is less than120A, the fusion width remainsat7-8mm when the welding voltage increases from25V to33V. However, the fusionwidth scale turns to7-12mm when the welding current is over160A.The arc stability of the newly developed flux-cored wire turns out to be moderate.Short circuiting transfer frequency is not uniform, and the arc burning period is a littlebit long(0.15s), with the characteristic of large droplet short circuiting transfer. Theaverage arc interruption time is3.9per second, and the restarting ability is good,therefore, favorable weld seam can be got when low welding speed is employed. Thespreadability of slag is very good, with over95%seam covered by the slag. Thethickness of cross section of slag is homogeneous, the weld seam presents metallicluster after removing the slag. The average slag detachability of underwater claddingis86.1%, while that of butt welding is only30%.The microstructure of the butt joint is mainly composed of Ni-based γ-solidsolution and dispersed γ′[Ni3(Al, Fe)]. When the heat input increases from22.5kJ/cmto25.5kJ/cm, the region of the columnar crystals decreases, equiaxed grains increase,fine grain size at top of weld seam increases from30μm to40μm, and the distributionof γ′becomes inhomogeneous. When the heat input is22.5kJ/cm, the microstructurein CGHAZ is mainly composed of tempered martensite, and fine carbide precipitatesalong the original austenite boundary. When the heat input becomes25.5kJ/cm, themicrostructure in CGHAZ consists of proeutectoid ferrite, sorbite and granular bainite.The tensile strength of joints are414MPa and381MPa, respectively, when the heatinput is22.5kJ/cm and25.5kJ/cm. The fractured weld seams turn out to be brittleintergranular fracture. The oxide inclusions in the fracture increase with the heat inputincreases, and the hardness in the weld seam increases from180HV to190HV, thehardness in the CGHAZ decreases from331HV to280HV.