Evaluation of Simulated Heat Affected Zone of Molybdenum- and Niobium Alloyed Ultra-High-Strength Steels

Gleeble 3800-thermomechanical simulator was used to simulate the heat affected zone of quenched and tempered 0.16 wt.% C steels with variation of molybdenum-and niobium contents. The purpose of the study was to evaluate the effect of alloying content on the properties of the coarse-grained zone of HAZ region (CGHAZ) and partially re-austenitised inter-critical zone (ICHAZ) with two different t 8/5 times (5 s and 15 s). Results showed that Mo and Nb decreased the amount of softening in the HAZ-region, especially with longer t 8/5 -time (15 s). 0Mo steel had mixed microstructure of bainite and martensite in the CGHAZ region with t 8/5 time of 15 s, which led to higher degree of softening compared to other steels. Shorter t 8/5 time of 5 s produced martensitic microstructure in CGHAZ region in all cases leading to higher hardness values. Impact energy values at-40 °C were at least 34 J/cm 2 in all cases. Highest hardness values in the ICHAZ-region were achieved in the case of 0.5Mo steel. Also, at-40 °C impact energy values of ICHAZ were at least 34 J/cm 2 in all cases, however Mo-free steel achieved clearly higher impact energies in ICHAZ region, which is result from softer microstructure with relatively low hardness compared to other steels. Overall, it can be concluded that longer t 8/5 time can be used, which corresponds to higher heat input in welding, if Mo and/or Nb alloying is used.