Year

1986

Degree Name

Doctor of Philosophy

Department

Department of Metallurgy and Materials Engineering

Abstract

Literature relevant to the weldability of steel by manual metal arc welding has been reviewed. In particular, the process of manual metal arc welding has been examined and current theoretical models that have been developed to predict the structure of the weldment are discussed. Weld defects, particularly hydrogen assisted cold cracking of the heat affected zone (HAZ) are discussed and a review of weldability tests relevent to HAZ cracking has been made. The Japanese developed Tekken Test has been examined in detail and the relevance of Cracking Index measurements has been analysed.

The effects that variations to the extrinsic welding parameters of voltage, current, and speed on Cracking Index as determined by the Tekken Test have been examined. Intrinsic welding variables of microstructure and restraint stress have been examined using optical and electron microscopy and a restraint stress measuring technique. Cracking Index results have been related to the extrinsic welding parameters by variations caused to the intrinsic variables of microstructure and restraint stress.

It was found that increases in welding speed and voltage increased the value of the Cracking Index and increases in welding current decreased the Cracking Index value. The interaction of these effects produced a variation of results for welding at constant heat input.

All three extrinsic parameters were found, independently and collectively to affect the microstructures of both the HAZ and the weldmetal. It was found that increases in welding voltage and current and decreases in welding speed increased the volume fraction of proeutectoid ferrite in the HAZ with an associated decrease in HAZ cracking. Cracking of the weldmetal was found to be related to the ferrite morphology; fine lath (acicular) ferrite was found to be the microstructure least susceptible to cracking. Formation of fine lath ferrite microstrucures was enhanced by low welding speeds. From the various combinations of weldmetal and HAZ microstructures that could be achieved a range of Cracking Index values could be produced.

The influence that root gap of the test piece, the composition (carbon equivalent) of the steel plate, and the classification and manufacturing source of the electrodes had on Cracking Index has also been studied. Increasing root gap and carbon equivalent increased the value of Cracking Index although in the latter case the relationship varied with welding conditions. Cracking Index was also found to vary with electrodes of different classifications and from different manufacturing sources. It is proposed that these Cracking Index variations are attributable to strength incompatibilities and differences in electrode flux compositions respectively.

It is concluded that because of the variation of results, often with considerable scatter, produced by the interaction of all the variables, the Tekken Test is not valid as a general method for determining weldability, but rather provides a useful comparative test of the responses of different steels and electrodes to welding under strictly specified welding conditions.

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