Understanding the Role of Inclusions and Microstructure in Ductile Fracture

Much attention has been devoted to predicting and controlling fracture toughness in ferrous weldments. Extensive studies relat ing toughness with welding parameters and weldment composit ion have been made. All such studies have sought to define the important variables affecting toughness. Unfortunately, such studies often resulted in linear regressions of one variable against toughness, fracture mode was not held constant, and no efforts were made to relate the variable investigated to quantitative features of the weldment microstructure. This work studied the effect of shielding gas on the resultant weldment microstructure and inclusion dist r ibut ion in the GMA process. Quantitative techniques were established for the investigation of these features. The results are discussed in te rms of we ldmen t deox ida t i on , nucleation and growth of inclusions, and the effect of alloy balance on microstructure. D. C. HILL is Group Leader. Linde Research Department and D. E. PASSOJA is Research Scientist, Central Scientific Laboratory, Union Carbide Corporation, Tarrytown, New York. It is concluded that the mechanical properties of weldments fabricated from E70S-3 electrodes by the GMA process are dependent on both microstructure and inclusion size dist r ibut ion. Opt imum strength and toughness resulted when Ar-2% 0 2 or Ar-25% C 0 2 shielding gas was used in preference to C0 2 shielding gas.