Effect of Aluminum Additions on Microstructures and Mechanical Properties of Ti Micro-Alloyed Steels Containing Ni Subjected to Simulating HAZ Thermal Cycle

This work reports the results of an investigation of micro-alloyed steel plates subjected to high heat input (4.5 kJ/mm), simulating the thermal cycle of heat affected zone (HAZ) conducted by induction heating in a computer controlled weld thermal cycle simulator. Three samples of steels having Carbon and Nickel content of 0.08 and 0.8 wt. %, respectively, as well as different Aluminum contents of 0.004, 0.026, and 0.057 wt. %, were investigated. The variation in microstructure, hardness and crack tip opening displacement (CTOD) fracture toughness properties with Al contents were evaluated, and compared with another set of the three samples of steel having the same chemical compositions but with neglected amount of Ni content. The fracture toughness tests revealed the decrease in their CTOD values due to increasing Al contents. In contrast, the hardness measuring revealed the increase of the hardness due to increasing Al wt. %. On the other hand, it was found that the presence of 0.8 wt. % Ni promotes the formation of finer microstructure at a high heat input of 4.5kJ/mm, which leads to higher fracture toughness CTOD value(δ =1.05), which is two times the value of those which have traces of Ni content(δ =0.52). However, the hardness test results showed similar values in each case regardless the amount of Ni contents. Meanwhile, the aspect ratio of the nucleated acicular ferrite was found to be increasing together with the refinement of its grain size due to Ni addition. Hence, it can be concluded that such development and the specific additions of Aluminum positively affects the microstructures and mechanical properties of the investigated steels and their used as marine structural steel. Copyright © 2016 VBRI Press.