Effect of Ferrite and Martensite Hardening Variation on Mechanism of Void Formation in Low-Alloy Dual-Phase Steel

The effect of ferrite and martensite hardening variation on the mechanism of void formation in a low-carbon low-alloy steel with different ferrite-martensite dual-phase (DP) microstructures has been investigated. Accordingly, several ferrite-martensite DP microstructures containing a wide range of martensite volume fractions (V-m) from 23% to 87% were developed by application of intercritical annealing heat treatment processes. Optical and field-emission scanning electron microscopy were supplemented by hardness measurements and tensile testing to follow the microstructural changes and correlate them with the mechanism of void formation. The experimental results indicate that the mechanism of void formation changes from ferrite/martensite (F/M) interface decohesion to martensite cracking as V-m increases in the microstructures. For DP specimens containing low V-m, F/M interface decohesion is the dominant mechanism of void formation, while in DP samples with moderate level of V-m, both mechanisms of F/M interface decohesion and martensite cracking are involved in the void formation. In DP samples containing high V-m, void nucleation and growth due to martensite cracking is the dominant damage mechanism. This change in the void formation mechanism with V-m can be rationalized based on the change in the plastic behavior of ferrite and martensite microphases developed as a consequence of ferrite and martensite hardening variation. The martensite hardening response decreases monotonically with increasing V-m from 23% to 87%, while that of ferrite shows an increasing trend up to 73% then decreases with further increase up to 87%.