Carbon extraction replica analysis is employed to measure the quantities and size distributions of
particles in Ti-Nb
microalloyed steel and its simulated
CGHAZ (coarse grain heat affected zone) to reveal the response behavior of second phase
particle to the welding
thermal cycle. The micro-structure and impact
toughness of CGHAZs in Ti-Nb microalloyed steel and a kind of high strength low alloyed steel with similar chemical content and manufacture process are investigated with OM, TEM and series charpy impact test. Experiment results and analysis show that the particles in the steel are irregular in shape, and exhibited a slight size-dependence in that the relative content of Nb decrease with the increase of particle size. The Nb contents of particles in parent metal fell in a wide range from 25%-82%. After weld thermal cycle, the shape of remaining particles in simulated CGHAZ changes from irregular shape to cubicle one, the particle number per unit area decrease, and the mean particle size increases. During the weld thermal cycling, the particles prohibit the original austenite grain from coarsening, prevent the formation of coarse upper bainite, accelerate the formation of acicular ferrite, and promote the decomposition of M-A constituent into ferrite and cemen-tite, thus improve the toughness of CGHAZ in high strength low alloyed steel remarkably. With time t (cooling time form 800 ℃ to 500 ℃) increasing from 16 s to 120 s, the toughness of CGHAZ in Ti microalloyed steel increases because that secondary microstructure of CGHAZ change from upper bainite to acicular ferrite and the austenite grain size coarsening is not serious.
More abstracts about the EFFECT OF SECOND PHASE PARTICLES ON MICROSTRUCTURE AND TOUGHNESS OF CGHAZ IN Ti-Nb MICROALLOYED STEE