Slag-metal reactions during welding: Part I. Evaluation and reassessment of existing theories

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I.

INTRODUCTION

DURING submerged arc welding (SAW), as in other flux-shielded processes, chemical reactions take place between the slag and the metal. This interaction results in compositional changes affecting the structure and properties of the weldment and hence is of considerable practical importance. In order to control the mechanical properties of the weld metal and to match them with those of the workpiece, it is desirable to estimate the extent of interaction between the slag and the metal. Although several researchers have studied this interaction over the last two decades, thus far no general method for determining weld metal composition is available. Empirical equations developed from laborious experiments have been used to predict weld chemistry for specific welding consumables under narrow ranges of welding conditions, and more scientific approaches have given, at best, only a qualitative explanation of the changes occurring in weld chemistry. The present work aims at establishing a quantitative relationship between the composition of welding consumables used, the welding parameters employed, and the resulting weld metal chemistry. The work differs from many investigations of the past in that it incorporates fundamental principles of chemical metallurgy and solidification processing while at the same time incorporating many of the more practical features of the SAW process. The work is presented in three parts. In Part I, the work done by previous investigators is reviewed, and the strengths and shortcomings of prevailing theories are described along with preliminary experiments which prove or disprove certain hypotheses. This critical review, although somewhat lengthy, is necessary to set the stage for the theory and experiments which follow. In Part II, t35] an entirely new theory is presented to explain the changes in weld chemistry, and a kinetic model is formulated to predict weld metal composition. Finally, Part 11II341presents the verification of the theory through several different experiments. In addition, the theory has been U. MITRA, Project Leader and Senior Member, Research Staff, is with the Thin Film Materials Department, Philips Laboratories, North American Philips Corporation, Briarcliff Manor, NY 10510. T.W. EAGAR, Richard P. Simmons Professor of Metallurgy, Leaders for Manufacturing Professor of Materials Engineering, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted September 18, 1989. METALLURGICAL TRANSACTIONS B

successfully tested by applying it to numerous data published by previous investigators. II. ANALYSES BASED ON METALLURGICAL THERMODYNAMICS

Most previous researchers of slag-metal reactions during welding have used only thermodynamic considerations in their analyses t1-~51 and have considered reactions such as xM + y O = ( M x O y )

[1]

xM + y(FeO) = (MxOy) + yFe

[21

and

The equilibrium constants for the above two reactions are g i -~

(aMxOy)

[3]

[aM]~[ao] y

K2 =

(aM,O

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