In-situ investigation on the fatigue crack propagation behavior in ferrite-pearlite and dual-phase ferrite-bainite low carbon steels

2020

Metals

The effect of a ferrite/mixed phase banded structure on the fatigue crack growth behavior of pipeline steel has been investigated in this work. Constant stress-controlled fatigue tests were carried out on unilateral notch specimens in the longitudinal direction (LD) and the transverse direction (TD). The strain accumulation was measured by digital image correlation (DIC) method and the fatigue fracture morphology was analyzed. The results show that the banded structure in the LD would cause a wide range of crack deflections, especially in the early stage of fatigue crack growth, which results in a lower fatigue crack growth rate compared to the TD. At the later stage, due to the increase of the plastic zone of the crack tip, more ferrite in the crack growth path of the TD specimen is also beneficial for crack tip blunting, thereby slowing down the increase of the crack growth rate.

Section: Introductionmentioning

confidence: 99%

Experimental Investigation of Fatigue Crack Growth Behavior in Banded Structure of Pipeline Steel

Zhan

2020

Metals

“…It is noted that the da/dN of QT steel in the near‐threshold regime is slightly greater than that of AC steel at the same ΔK. This is because the low crack driving force may lead to a uniform crack front in AC steel; therefore, the crack becomes more tortuous or irregular 4 . Then, a cumulative plastic strain between slip planes blunts the crack tip 20 .…”

Section: Resultsmentioning

confidence: 99%

“…However, the tear ridge and secondary crack appear in QT steel which correspond to the quasi‐cleavage fracture mode. In addition, the trend of crack growth has a certain directivity in AC steel because the deformation mainly occurred in ferrite, and the fatigue cracks tend to start along the ferrite‐pearlite boundary 4,13,22 . However, the appearance of fatigue step in QT steel has no preferred orientation.…”

Section: Resultsmentioning

confidence: 99%

“…Fatigue failure in metallic components and structures is a general technical problem, especially in vehicle gearings that are subjected to cyclic load. The pursuit of strength improvement and weight (or thickness) reduction, the improvement of fatigue strength, and fatigue crack growth (FCG) resistance of vehicle gearings, as well as the development of safety design, are strongly required to control fracture and assure the safety of structures 1–4 . FCG is the essential process in the total life of components with notches and is unavoidable in most structures and machines.…”

Section: Introductionmentioning

confidence: 99%

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Fatigue behaviour of a multiphase medium carbon steel: Comparison between ferrite/pearlite and tempered microstructures

Fatigue Fract Eng Mat Struct

Susmel

et al. 2020

Fatigue crack growth is the essential process in the total life of components and is unavoidable in most structures and machines. In this study, the effects of cementite particles on the fatigue properties of medium carbon steel were investigated. A quenching and tempering treatment was developed to obtain uniformly distributed cementite particles with dozens of nanometres. To evaluate the fatigue properties, fatigue strength, fatigue crack growth rate and dynamic mechanical behaviour were examined. It was found that the fatigue strength of the tempered steel is much higher than that of air cooling ferrite/pearlite steel because the cementite particles act as barriers and hinder crack propagation during cyclic loading. More importantly, the high sensitivity of the damping peak to prior fatigue may play an important role on the fatigue history of multiphase medium carbon steel.

“…It is well known that the FCP behavior is closely correlated with the phase constituents in the microstructure of steel. Guan et al 14,15 reported that the bainite phase in the bainite-ferrite steel can effectively impede the FCP compared with ferrite-pearlite steel because of the interlocking and branching effect of crack tips. Kumar et al 16 investigated the bainite-ferrite steel and also found that the FCP rate decreases with the increasing of bainite content, most likely due to the difference in strength of the phase constituents and the interactions effect between each other during loading.…”

Section: Introductionmentioning

confidence: 99%

Fatigue crack propagation behavior of multiphase microstructure in a quenched and partitioned medium‐carbon bainitic steel

Fatigue Fract Eng Mat Struct

Luo

2022

A medium‐carbon steel was treated by the bainitic isothermal transformation plus quenching and partitioning (B‐QP) process to obtain bainite/martensite/retained austenite multiphase microstructure, and its fatigue crack propagation (FCP) behavior was evaluated in contrast with BAT (bainite austempering) sample with fully bainite microstructure. Results show that B‐QP sample exhibits a lower FCP rate and higher fatigue threshold ΔKth (12.6 MPa·m1/2). Moreover, the FCP path of B‐QP sample displays a strongly tortuosity and more crack branching due to more filmy retained austenite (7.2%) and higher percentage of high angle misoriented boundaries (68%). The larger crack tortuosity and the secondary cracks as result of crack branching are primarily responsible for the lower FCP rate of B‐QP sample. In addition, the FCP rate curve of B‐QP sample shows a pronounced small plateauing at the near‐threshold zone, which can be ascribed to the mechanical twinning that occurred in the filmy retained austenite.