T. Suzuki, J. Inoue, T. Koseki, The University of Tokyo, Tokyo, Japan
In the welding of steel, development of intragranular ferrite (IGF) is one of the keys to improve the toughness of both weld metal and coarse-grained heat affected zone (CG-HAZ). Oxide inclusions which can promote the IGF formation have been extensively searched and the ferrite nucleation mechanism on the inclusions has been discussed for years, but investigations on the effect of volume fractions or number density of the oxide inclusions are limited. In this research, we prepared steel specimens containing different volume fractions of oxide particles by a powder metallurgy technique, and examined the microstructure development after a CG-HAZ simulated thermal cycle. Powder of Ti2O3, initially less than 20mm in diameter, or Al2O3, initially about 1mm in diameter, was mixed with Fe-C and Mn powders by ball milling, and pressed into a disk-shape specimen with a diameter of 12mm and a height of 3mm. The volume fractions of Ti2O3 examined were 0.2, 1.0 and 5.0%, and that of Al2O3 was 1.0%, and the carbon and manganese contents in the steel were kept at 0.14 mass% and 1.4 mass%, respectively. The disk-shaped specimens were sintered at 1473K for 600s, and then subjected to a thermal cycle simulating CG-HAZ with a holding at 1723K for 40s and subsequent cooling at 15K/s in Ar-3%H2. Microstructure of specimen without oxide addition mainly consisted of Widmanstätten ferrite, while in the specimens with 0.2 vol.% Ti2O3, Widmanstätten ferrite was replaced by acicular ferrite. Microstructure was further changed to granular ferrite as the Ti2O3 content was increased to 1.0 and 5.0 vol.%. EBSP analysis showed that the acicular ferrite has Kurdjumov-Sachs orientation relation with austenite. On the other hand, the granular ferrite showed random orientation relations with austenite. In the case of Al2O3 addition, microstructure was almost the same as that of the specimen without oxide addition.
Summary: Oxide particles were dispersed into steel in various volume fractions, and its effect on the formation of intragranular ferrite was investigated. By the dispersion of Ti2O3, Widmanstätten ferrite of steel was replaced with acicular ferrite or granular ferrite, depending on the volume fraction of oxides.
