Relationship between Microstructure and Fatigue Crack in Mild Steel Weld

The authors performed a study on fatigue in the weld metal and heat-affected zone of mild steel using V-notched specimens subjected to plane bending and found that the fatigue cracks in the weld metal and heat-affected zone propagated along the grain boundary more remarkably than those in the base mtal. The study was carried out to clarify the relationships between the respective microstructures of the welded part and the mechanisms of the propagation of fatigue cracks. The metallographic observations of fatigue cracks on the notched specimens taken from the base metal, the weld metal and the heat-affected zone of SM41 welded were performed under stress level a little higher than fatigue limit of each specimen and the following results were obtained. 1. More precipitates, which were identified as cementite by electron diffraction, were found at the grain boundaries in the weld metal and heat-affected zone than in the base metal. By transmission electron microscopy, no precipitate could be found in the grains of the fine grained zone, the spheroidized zone and the base metal, while carbons seem to hava precipiated on the dislocations in the weld metal and the embrittled zone. 2. It was found that the fatigue cracks in the heat affected zone propagated along the grain boundaries more frequently with an increase in the amounts of carbides at the grain boundaries.