Welding-Induced Brittleness Susceptibility In Mo-V-Ti-N Microalloyed Steel And Metallurgical Methods For Restraint

In this work, to investigate the brittleness susceptibility and metallurgical methods to depress it in the welding-induced coarse grain heat-affected zone (CGHAZ) of a low carbon Mo-V-Ti-N microalloyed structural steel, three experimental steels representing typical addition of N (120ppm), single addition of B (12ppm) and dual addition of N and B (210ppm+17ppm), respectively were prepared, by which the relevant CGHAZ samples simulated under different t8/5(6-100s, referring to the time interval for cooling from 800oC to 500oC ) were made using a Gleeble simulator. Their microstructures were examined quantitatively and the substructure of martensite-austenite constituents (M-A) as a second brittle phase was emphasized by transmission electron microscope (TEM) observation. The role of micromechanical aspects of M-A and its neighboring matrix phase in controlling the brittleness was also evaluated by the nanoindentation hardness measurements. The Charpy-V-Notch (CVN) impacts were additionally performed on each simulated CGHAZ at -20℃, and the fracture mode was revealed by scanning electron microscope (SEM) observations of the morphology of fractured surfaces and secondary cracking in sections perpendicular to them.It is shown that the M-A contained multi-phases microstructure formed in all the CGHAZs but in different morphologies. With the increasing t8/5, the amount of proeutectoid ferrite (PF) increased significantly at an expense of bainitic ferrite (BF) and acicular ferrite (AF). As a result, the densely distributed flaky M-A was gradually replaced by the more isolated massive M-A, which is accompanied with an increase in size and a change in substructure of M-A from the lath to the twin. The difference of nanohardness between the M-A constituents and the neighboring matrix was also evidenced to increase markedly with the increasing t8/5. Moreover, an increase in t8/5 causes the CVN impact energy of simuated CGHAZ to decrease and the fracture mode to change from dimple plastic fracture to quasi-cleavage brittle rupture, On the other hand, it is indicated that a trace addition of boron into the Mo-V-Ti-N steel resulted in a decreased fraction and size as well as a denser distribution of M-A by suppressing the transformation of PF in the CGHAZ obtained at each t8/5. The CVN toughness of CGHAZ was therefore significantly improved. With all the above results considered, the brittleness susceptibility in the CGHAZs of the Mo-V-Ti-N steel can be attributed to their microstructural and micromechanical features and as a metallurgical method to depress it, a relatively low heat input for welding process or a trace addition of boron in the steel is recommended.