Studies On The Influence Of Rare Earth Ce On Microstructure And Properties Of Huge Heat Input SAW Seam Metal

Due to submerged arc welding's(SAW)high efficiency and economic characteristics it becomes one of the most widely used welding methods in shipping manufacture. Practice showed that, when welding heat input achieved 50kJ/cm above, HSLA welding joints stayed in the high temperature environment for more time, It would cause grains grow, notch toughness of the weld seam metal get lower. Therefore, the research of how to control weld seam grain growth and improve weld seam metal notch toughness in SAW are very meaningful.This paper developed a kind of suitable for huge heat input SAW with rare earth (CeO2)alkaline sintering flux, And comprehensively research on Ce to refine grains, improve weld metal toughness .Xabi impact test indicated that adding Ce weld metal'AKV was significantly higher than which not adding Ce weld metal's. Among them, adding rare earth Ce 2% in weld metal's AKV was the highest. It had a 57% increase AKV compared with that not adding Ce. Adding Ce 4% in the weld metal' AKV reduced 26% to adding Ce 2% Results showed that the addition of rare earth had a "range". Addition amount for 2% was the best.Weld seam metal metallographic experiments showed that the rare earth Ce had significant influence on the microstructure. The weld seam was mainly consisted of primary ferrite(PF),acicular ferrite(AF) and pearlite(FC). Weld seam without adding rare earth Ce microstructure was mainly consisted of large size PF, and a little AF,PC. Adding rare earth Ce 2% weld seam had lots of AF and the grain was the finest. It showed that rare earth Ce can restrain the growth of PF, increase the amount of AF, and refine the grains. But with the improvement of content of Ce , the PF content increased and content of AF was cut down. And when content of Ce at 8%, ferrite side plates(FS) appeared. When the content of Ce increased, grain -phase was polluted, So we can't get fine grains.The inclusions in weld seam metal were analyzed by environmental scanning electron microscopy(ESEM) and Transmission electron microscopy(TEM).The test results showed that rare earth had obvious effect on the inclusions'shape and size. Rare earth Ce made inclusions spheroidizing and refined. Inclusions in the weld seam were mostly round shape or directed a ball. There were only a few inclusions'size more than 1μm , most inclusions' size were 0.2μm~1μm in a beneficial inclusions range. These beneficial inclusions can be the nucleating-centers of AF. Meanwhile, a new acicular ferrite grows on some AF, We call it secondary acicular ferrite. Secondary acicular ferrite nucleation, growth, refining the original grain, further improve the weld metal toughness.