THERMAL TREATMENTS AND FATIGUE LIFE BEHAVIOUR IN DUAL-PHASE STEELS

I. ALVAREZ; M. BALBI

Conferencia; XVIII International Colloquium MECHANICAL FATIGUE OF METALS; 2016

Dual-phase (DP) steels consist of martensite islands embedded in a ferrite matrix exhibited impressive mechanical properties. One of the most practical approaches to produce dual phase steels involves intercritical annealing. The grain refinement in DP steels have revealed more homogeneous distribution and more spherical shape of martensite islands, formation of martensite cracks an cleavage fracture in ferrite would be suppressed, thereby encouraging ductile fracture mechanisms. A plain low carbon steel (AISI 8620) was treated by different intercritical annealing processes to produce DP steels with different martensite content and grain size (Table 1).DesignationApplied heat treatment cycleM001Austenitization at 870°C for 20 min, followed by intercritical annealing at 780°C for 60 min and water quenchingM002Austenitization at 870°C for 20 min, water quenched. Intercritical annealing at 780°C for 60 min and water quenchingM003Austenitization at 1000°C for 2 hs, water quenched. Austenitization at 870°C for 15 min, water quenched. Intercritical annealing at 780°C for 30 min and water quenchingMicrostructures of the different obtained DP were studied by optical and scanning electron microscopy. The volume fraction of martensite in ferrite in all the samples 50:50. The samples M002 and M003, with fully martensitic structure (after austenitization and water quenched), have fine grain size and produces better mechanical properties than M001 (initial austenitic structure after austenitization). Low cycle fatigue test showed two stages, firstly a hardening during the first few cycles and then a continuous softening up to fracture. The results have shown that the DP designed M002 perform mechanical properties similar to a commercial DP and the fatigue life is close to that of the as-received material (ferritic-perlitic structure). In order to analyze fatigue damage evolution and failure mechanism, the microstructure has been evaluated by transmission electron microscopy. A High-Strength Low Alloyed (HSLA) steel (0.1%wt C) will be studied and compare with the AISI 8620.