Impact of Quenching and Partitioning Time on Microstructures, Mechanical and Fatigue Properties of High-Strength Steel

The impact of partitioning time on microstructure,
tensile properties, and low-cycle fatigue (LCF)
behaviour of prepared steel developed through a single-step
quenched and partitioning (Q&P) process has been investigated.
The strain rate-controlled (± 0.5%) fatigue (LCF) test
was conducted until failure. The microstructure is changed
from ferrite-pearlite to lath-type martensite, bainite, temper
martensite, and retained austenite after Q&P treatment
with various partitioning times. EBSD and TEM analysis
identified film-type retained austenite in the martensitic
microstructure, which was confirmed by XRD analysis. All
quenching and partitioning treated samples show similar
cyclic softening behaviour. The hardness, strength, and lowcycle
fatigue life are improved significantly after quenching
and partitioning treatment. The hardness, strength, and
fatigue life increase up to 30 min, then decrease further,
partitioning for 60 min. The retained austenite (RA) is transformed
to strain-induced martensite under cyclic loading,
delaying the fatigue crack propagation rate, and improving
fatigue properties.