Behavior of Graphite in Electron Beam Welding of Nodular Cast Iron

Abstract

Concerning electron beam welding of nodular cast iron, effects of factors as pretreatment of base metal, heat input and number of passes on behavior of graphite in welds by bead welding and insert type welding (insert metal; SUS304, 0.5 mm thickness) have been investigated by microscopic observation, electron probe microanalysis, thermal cycle measurement, tensile test, etc. The results obtained are as follows:
(1) Microstructures of fusion zone by bead welding are almost eutectic structures. Carbon contents of fusion zone by insert type welding (4.5 kJ/cm) are in the range of 0.51 to 1.63% according to penetration ratio and microstructure of fusion zone (as cast, 1 pass) by low penetration ratio shows almost austenitic structures and microstructures of fusion zone by other conditions (as cast; 2 passes, annealed; 1, 2 passes) show austenitic plus martensitic structures.
(2) Carbon contents of melted zone surrounding graphte in bond are in the range of 4.3 to 5.0% (hypereutectic structures). Carbon contents of acicular martensite outside of melted zone are in the range of 1.2 to 1.8%. Carbon contents of these zones and diffusion distance of carbon from graphite show tendency to increase as heat input and number of passes increase. As cast material has tendency to show higher carbon contents of these zones than annealed material.
(3) In tensile tests of welded joints by insert type welding, fractures have tendency to initiate from hyper eutectic zones in bond and propagate through these brittle zones. Lowering of joint efficiency are influenced by these brittle zones.