Ferrite Bainite Martensite Research Articles

Microstructural evolution and stability during strain-controlled fatigue in a multiphase microalloyed steel

Multiphase ferrite-bainite-martensite (F-B-M) microstructure processed through rolling and two-step cooling in a V-microalloyed steel exhibited excellent stability under strain-controlled fatigue. The steel exhibited a steady state response and even cyclic hardening up to a total strain amplitude of 0.6% and displayed a cyclic softening response at total strain amplitudes >0.7%. To understand and determine the degradation mechanisms, a detailed transmission electron microscopic investigation was carried out in fatigue tested samples as a function of total strain amplitude. At low strain amplitudes (<0.4%), microstructural constituents such as the bainite/martensite colonies, and the inter-lath films of retained austenite remained stable. The deformation occurred only in the polygonal ferrite and the lamellar retained austenite, which was sheared into fine, smaller strips. Only some boundaries of bainite/martensite colonies were subjected to shearing. At an intermediate strain amplitude (0.6%), all the coarse bainite/martensite colonies exhibited shear deformation with very fine retained austenite segments distributed over large regions. The bainite/martensite lath interiors were filled with dislocation substructures and sheared inter-lath austenite layers. Austenite grain rotation was noticed, and the grain interior exhibited sheared or segmented layers. Selective inter-lath austenite layers transformed into acicular martensite. At higher amplitudes (0.8%), the F-B-M microstructure displayed homogeneous deformation and cyclic softening. Carbides and lath debris were distributed throughout the matrix and consisted of decomposed bainite and martensite. Formation of acicular and twinned martensite was present due to the TRIP effect. The F-B-M microstructure comprising small colonies of bainite/martensite with fine lath morphology, polygonal ferrite (∼20%), retained austenite with lamellar morphology and inter-lath films (∼5%) along with a fine dispersion of carbonitride precipitates in the matrix appears to be highly resistant to fatigue damage. Hot rolling processes aimed at promoting these microstructural features in the steel, which is also accompanied by texture strengthening, seem to be effective against cyclic softening in fatigue.

On a New Two‐Step Air Cooling Method Following Thermomechanically Controlled Rolling: Microstructural Evolution and Mechanical Properties of a High‐Strength Multiphase Medium Carbon Microalloyed Steel

A new two‐step air cooling (TSAC) process following finish rolling is developed for producing a multiphase (ferrite–bainite–martensite [FBM]) microstructure in a medium carbon V‐microalloyed steel without any post‐forming heat treatment. In TSAC, step 1, designated as natural air cooling (NAC − 4.2 and 5.1 °C s−1); step 2, representing two different cooling methods, namely, forced air cooling (FAC − 7 and 7.71 °C s−1) and controlled air cooling (CAC − 1.63 and 1.75 °C s−1), are adopted from two critical transformation temperatures. Both cooling methods result in multiphase (FBM) microstructures which consist of polygonal ferrite (PF), bainite (B), martensite (M), retained austenite (RA), and vanadium carbonitride (V (C, N)) precipitates. In both steels, the fraction of RA lies in the range of 8–11%. The FAC steel exhibits a tensile strength (TS) in the range of 1652–1671 MPa and ≈4–7% of total elongation, whereas the CAC steel displays a maximum TS of 1100 MPa and 17% elongation at fracture. The FAC steel exhibits a higher work‐hardening rate compared to a CAC steel. A transmission electron microscopy (TEM) analysis of tested samples reveals that stable RA plays a crucial role in improving the work‐hardening capability of these multiphase steels.

Influence of Microstructure on Low-Cycle and Extremely-Low-Cycle Fatigue Resistance of Low-Carbon Steels

The goals of this study were to quantify and explain the effects of microstructure on the resistance of low-carbon steels to low-cycle fatigue and to extremely low-cycle fatigue (ELCF). Three different microstructures (ferrite–pearlite, ferrite–martensite, and ferrite–bainite–martensite) were tested, and their fatigue properties were analyzed using the strain-based Coffin–Manson model and an energy-based model. According to the Coffin–Manson model, ferrite–pearlite showed the best ELCF resistance, whereas in the energy-based model that considers the effect of tensile strength ferrite–bainite–martensite revealed the highest ELCF resistance. At similar tensile strength, ferrite–bainite–martensite had longer ELCF life than ferrite–martensite; the difference may be a result of the smaller strain incompatibility between bainite and ferrite than between ferrite and martensite. In all three microstructures, cracks initiated at the surface and propagated into the interior; this result indicates that fracture mode was not altered during cyclic loading at high strain amplitudes. Ferrite–martensite microstructure developed many sub-cracks surrounding a main crack; they could facilitate propagation of a main crack, and thereby degrade fatigue life at high strain amplitudes.

Enhancement of Mechanical Properties in Bainitic Steel Processed from Different Austenitization Temperatures

A novel high strength steel has been thermo‐mechanically controlled processed (TMCP) with 750 °C finish rolling temperature followed by air cooling. Subsequently, the air‐cooled steel is partially and fully austenitised separately prior to isothermal bainitic treatment (IBT) followed by air cooling. TMCP steel reveals a congregation of ferrite–bainite–martensite microstructure resulting in an effective combination of strength and ductility. After IBT, multiphase microstructures comprising ferrite, carbide‐free lath microstructure (martensite and bainite), and retained austenite up to 12% are obtained which may be due to combined addition of Si and Al. The attractive combination of yield strength (575–699 MPa), tensile strength (1252–1262 MPa), and total elongation (14–16%), as well as continuous yielding behavior, are suitable for structural and automotive applications.

Effect of Vibration on Surface Texture during Machining Multiphase Microalloyed Steel

Multiphase ferrite-bainite-martensite (FBM) microalloyed steel produced through two step cooling procedure was turned and compared with ferrite-pearlite (FP) microstructure and tempered-martensite (TM) microstructure to study the effect of vibration on surface finish. The cutting parameters like cutting speed, feed and depth of cut were varied to understand the parameter influence on surface finish due to vibration. The result shows that FBM microstructure steel gives better performance in terms of lower surface roughness and lower vibration compared to FP and TM microstructure steel.

Effect of cutting parameters on cutting force and surface roughness during machining microalloyed steel: Comparison between ferrite–pearlite, tempered martensite and ferrite–bainite–martensite microstructures

Three different microstructures, namely ferrite–pearlite, tempered martensite and ferrite–bainite–martensite of 38MnSiVS5 microalloyed steel, were produced using controlled thermomechanical processing. The properties are comparable to quenched and tempered steel. The developed microstructures were turned to evaluate their machinability. Mixed modes of response were observed while ferrite–bainite–martensite microstructure exhibits better machinability by way of good surface texture/finish, the ferrite–pearlite microstructure of least strength encounters smaller cutting force.

A study on the influence of cutting parameters on forces during machining the multiphase V-microalloyed steel

The ferrite-bainite-martensite (FBM) multiphase V-microalloyed steel with a yield strength of 1384 MPa was subjected to turning test in order to study the effect of cutting parameters such as cutting speed, feed, and depth of cut on cutting force, feed force, and radial force. The cutting forces are found to decrease with increase in the cutting speed for various feed with constant depth of cut. The analysis of variance (ANOVA) for all the three forces shows that feed rate and depth of cut are the significant parameters at 95 % confidence level, and also, the interaction between cutting parameters is insignificant. The optimum parameter for machining multiphase (FBM) microalloyed steel is found to be cutting velocity 80 m/min, feed rate 0.05 mm/rev, depth of cut 0.1 mm. The multiphase microalloyed steel requires less force as compared to quenched and tempered, microalloyed, and other existing high strength low alloy (HSLA) steels like AISI 4340. The easy machinability of multiphase microalloyed steel is promoted by soft polygonal ferrite (PF) and bainite phases present in the microstructure of the steel.

Effect of microstructure on the surface finish during machining of V-microalloyed steel: Comparison between ferrite–bainite–martensite and ferrite–pearlite microstructures

Multiphase ferrite–bainite–martensite microalloyed steel produced through a two-step cooling followed by annealing route and a ferrite–pearlite steel obtained through air-cooling after forging were subjected to turning operation. The influence of process parameters such as cutting speed, feed and depth of cut on surface roughness on both materials was compared. The results show that the multiphase microalloyed steel exhibited high surface finish than air-cooled steel. The analysis of variance shows that the contribution of cutting speed and depth of cut on surface roughness are insignificant for both ferrite–bainite–martensite and ferrite–pearlite microstructures.

Texture Evolution in Thermomechanically Control Processed Multiphase Medium Carbon Microalloyed Steel: Forged, Rolled and Low Cycle Fatigued Microstructures

A ferrite-bainite-martensite (F-B-M) microstructure was produced in a medium carbon microalloyed steel through two routes, namely, low temperature finish forging or rolling, followed by a two step cooling and annealing. The texture formed in control forged and rolled material after two step cooling followed by annealing (TSCA) was examined. Texture investigation was also carried out after low cycle fatigue testing at low and high total strain amplitudes. Transmission electron microscopy was employed to study the microstructural evolution. Fatigue tested F-B-M microstructure obtained through the rolling route was stable up to a total strain amplitude of 0.6%. This paper reports the evolution of texture and microstructure in two-step cooled F-B-M microstructure and their stability during fatigue loading.

Influence of martensite volume fraction on tensile properties of triple phase ferrite–bainite–martensite steels

► Triple phase microstructures were obtained by step-quenching heat treatment. ► Higher dislocation density in ferrite, adjacent to martensite, was observed by TEM. ► This evidence is due to higher localized plastic strains of martensitic transformation. This paper presents the effect of martensite volume fraction ( V M ) on mechanical properties of AISI 4340 steel with ferrite–bainite–martensite microstructures. The steel bars were austenitized at 900 °C for 1 h followed by intercritical annealing at 740 °C for 100 min and quenching into a salt bath with temperature of 300 °C and holding at different times to obtain triple phase (TP) microstructures with 34 vol.% fraction ferrite and various martensite (or bainite) contents. Presence of three phases in adjacent to each other was confirmed by metallographic analysis and TEM studies. TEM observation also indicates higher density of dislocations within ferrite near martensite colonies compared to that near the bainitic phases. Results of the tensile and hardness tests showed that with increasing martensite volume fraction, yield strength, ultimate tensile strength and hardness increase which is related to higher martensite volume fraction, higher strength of ferrite and higher residual stresses. The increment of martensite volume fraction increased ductility in terms of uniform and total elongation and tensile fracture energy. This evidence is due to the change of martensite morphology, increase of dislocation density at interphase boundaries and increment in residual stresses of the ternary phase steel with increasing V M . Fracture surface micrographs of tensile specimens, examined by SEM and stereo optical microscopy, confirmed the reduction in ductility of TP steels with increasing V M .

Fatigue behavior of a multiphase medium carbon V-bearing microalloyed steel processed through two thermomechanical routes

In view of their capacity to develop high strength following limited alloying and ease of processing, medium carbon microalloyed (MA) steels are very cost effective compared with quenched and tempered (Q&T) steels for the production of automotive components. Recently, a two-step cooling (TSC) procedure with an additional anneal following low temperature forging/rolling was developed to obtain multiphase (ferrite–bainite–martensite), microstructures in a microalloyed steel. The microstructure predominantly contained granular/lower bainite, lath martensite and polygonal ferrite, with inter-lath films as well as blocks of retained austenite. Vanadium carbide precipitates were observed only in the polygonal ferrite and the ferrite in the bainite region. The F–B–M (R) exhibited (rolling route) significantly higher yield and tensile strength values than the F–B–M (F) microstructure. Under low cycle fatigue (LCF) loading, the F–B–M (R) microstructure exhibited a cyclically stable response at total strain amplitudes ≤0.6% and continuous cyclic softening till failure. However, the F–B–M (R) microstructure exhibited a greater fatigue life than the F–B–M (F) microstructure when the comparison was based on the total strain amplitude, (Δ ɛ t/2). Fatigue tested microstructure of F–B–M (F) at a total strain amplitude of 0.4% and the microstructure of F–B–M (R) at a total strain amplitude of 0.55% were stable. The inter-lath retained austenite strips/films played a significant role in preventing the softening during fatigue loading.

A study on fracture properties of multiphase microstructures of a CrMo steel

An investigation into the fracture properties of tempered martensite (TM) and ferrite–bainite–martensite (FBM) microstructures was carried out for a CrMo steel, with a particular focus on the role of inter-phase FBM microstructure boundaries. The fractographic investigations showed the fracture mechanism of transgranular cleavage for the FBM microstructures. The obvious presence of bright facets on broken sample surfaces was indicative of the effect of inter-phase boundaries on the cleavage fracture. The appearing of bright facet surfaces in FBM microstructures was attributed to the high concentration of ferrite/bainite and ferrite/martensite inter-phase boundaries.

Mechanical behavior of CrMo steel with tempered martensite and ferrite–bainite–martensite microstructure

This study is concerned with a correlation between the microstructure and mechanical properties of 42CrMo4 steel, with two different microstructures. Quench tempering and step quenching heat treatments produced tempered martensite and ferrite–bainite–martensite (FBM) microstructures, respectively. Tensile test results indicated a yield-drop effect in FBM microstructure with ferritic matrix. This was attributed to dislocation generation in ferrite phase during bainitic and martensitic transformations. Fractographic investigations indicated transgranular cleavage and microvoid in FBM and tempered martensite microstructures, respectively. This can be attributed to high density of interphase boundary in FBM microstructure and carbide formation in tempered martensite microstructure.

Transmission electron microscopy studies of thermomechanically control processed multiphase medium-carbon microalloyed steel: Forged, rolled, and low-cycle fatigued microstructures

A ferrite-bainite-martensite (F-B-M) microstructure was produced in a medium-carbon microalloyed (MA) steel through two routes, namely, low-temperature finish forging and rolling, followed by a two-step cooling (TSC) and annealing. Transmission electron microscopy (TEM) was employed to study the microstructural evolution in control forged and rolled material after TSC followed by annealing (TSCA). A TEM investigation was also carried out on samples low-cycle fatigue (LCF) tested at low and high total strain amplitudes of 0.4 and 0.7 pct in case of the forged steel (F-B-M(F)TSCA) and 0.55 and 0.8 pct for the rolled steel (F-B-M(R)TSCA), respectively. Microstructural changes accompanying the LCF testing were identified. The two-step cooled microstructure processed through forging (F-B-M(F)TSC) as well as rolling (F-B-M(R)TSC) revealed a complex multiphase microstructure, along with films and blocks of retained austenite. In both microstructural conditions, vanadium carbide precipitates were too fine to be identified after the TSC treatment. Annealing after TSC produced a stress-free microstructure. The F-B-M(F)TSCA microstructure predominantly consisted of granular/lower bainite, lath martensite, and polygonal ferrite with interlath films as well as blocks of retained austenite, while the F-B-M(R)TSCA microstructure predominantly consisted of lath martensite, granular/lower bainite, and polygonal ferrite with interlath strips/films of retained austenite. Lath martensite content was higher in the F-B-M(R)TSCA condition than in the F-B-M(R)TSCA condition. In both conditions, vanadium carbide precipitates could be seen after annealing. Fatigue-tested F-B-M(F)TSCA microstructure up to a total strain amplitude of 0.4 pct and F-B-M(F)TSCA microstructure up to a total strain amplitude of 0.55 pct were stable. Lath martensite did not undergo deformation and in both microstructural conditions dislocation cell structures were not observed in the ferrite or bainite regions. The interlath retained austenite strips/films played a significant role in preventing the softening during fatigue loading. First, it was stable up to a total strain amplitude of 0.4 and 0.55 pct in the respective microstructures. Second, it underwent heavy deformation during fatigue loading at high total strain amplitudes, thereby accommodating the strain. Fatigue-tested F-B-M(F)TSCA microstructure at a total strain amplitude of 0.7 pct and F-B-M(R)TSCA microstructure at a total strain amplitude of 0.8 pct revealed deformed bainite/martensite laths, dislocation cells, and slip bands in the ferrite regions, which are characteristic features of cyclic softening. The retained austenite transformed to martensite through a strain-induced transformation mechanism and, at that stage, the microstructure contained in addition dislocation-rich bainite and ferrite.

Static and dynamic fracture toughness of a medium carbon microalloyed steel of three different microstructures

A multiphase ferrite-bainite-martensite (F-B-M) microstructure was developed in an automotive grade V-bearing medium carbon microalloyed steel, 38MnSiVS5. It was characterized using optical, scanning, and transmission electron microscopy. The tensile, Charpy impact, and static and dynamic fracture toughness behaviors were evaluated. The results are compared with those of ferrite-pearlite (F-P) and tempered martensite (T-M) microstructures of the same steel. Although the tensile properties of the multiphase microstructures were superior, the Charpy impact and static and dynamic fracture toughness properties were inferior compared with those of the other two microstructures. The F-P condition displayed the highest plane strain fracture toughness value (KIC), while the T-M condition was characterized by the highest dynamic fracture toughness (conditional) value (KIDQ). The Charpy impact energy of the T-M condition was greater than that for the other two conditions. An examination of the surfaces of fractured samples revealed predominant ductile crack growth in the F-P microstructure and a mixed mode (ductile and brittle) crack growth in the T-M and the F-B-M microstructures. Although the Charpy impact energy, plane fracture toughness (KIC), and conditional dynamic fracture toughness (KIDQ) of the multiphase microstructure were inferior to those of the T-M and the F-P microstructures, the toughness properties were comparable to those of medium carbon low alloy steels having bainite-martensite (AISI 4340) or tempered martensite microstructures.

Microstructural evolution and tensile behaviour of medium carbon microalloyed steel processed through two thermomechanical routes

A multiphase microstructure was obtained in a medium carbon microalloyed steel using two step cooling (TSC) from a lower than usual finish forging/rolling temperature (800–850°C). A low temperature anneal was then used to optimise the tensile properties. A multiphase microstructure (ferrite–bainite–martensite) resulted from forging as well as rolling. These were characterised using optical and scanning and transmission electron microscopy. X-ray diffraction, transmission electron microscopy and hardness measurements were used for phase identification. Tensile properties and work hardening curves were obtained for both the forged and the rolled multiphase variants. A Jaoul–Crussard (J–C) analysis was carried out on the tensile data to understand the basic mode of deformation behaviour. Rolling followed by the TSC process produced a uniform microstructure with a very fine grain boundary allotriomorphic ferrite, in contrast to the forged variety, which contained in addition coarse idiomorphic ferrite. The volume fraction of ferrite and its contiguity ratio in the rolled microstructure were greater than in the forged grade. The rolled microstructure exhibited a better combination of strength and toughness than that of the forged material. The rolled steel work hardened more than the forged variety owing to its fine, uniform (bainite–martensite and ferrite) microstructure. Retained austenite present in these steels underwent a strain induced transformation to martensite during tensile deformation. The J–C analysis of the work hardening rates revealed typical three stage behaviour in both varieties during tensile deformation.

Low cycle fatigue behaviour of a multiphase medium carbon microalloyed steel processed through rolling

A multiphase (ferrite–bainite–martensite) microstructure was developed in an automotive grade medium carbon microalloyed steel 38MnSiVS5 through a two-step cooling and annealing process following controlled rolling. The tensile and low cycle fatigue properties of the multiphase steel are reported and compared with those of multiphase and tempered martensite microstructures obtained by forging.

Low cycle fatigue behavior of a multiphase microalloyed medium carbon steel: comparison between ferrite–pearlite and quenched and tempered microstructures

In an attempt to improve fatigue and fracture resistance, a multiphase (ferrite–bainite–martensite) microstructure was developed in a V-bearing medium carbon microalloyed steel using a two-step cooling and annealing (TSCA) treatment following finish forging. The monotonic, cyclic stress–strain and low cycle fatigue behavior of this steel are reported. These results are compared with those of ferrite–pearlite and tempered martensite microstructures obtained by air cooling (AC) and quenching and tempering (Q&T), respectively. The tensile properties of the multiphase microstructure are superior to those of the ferrite–pearlite and the Q&T microstructures. Under cyclic loading, the ferrite–pearlite microstructure showed hardening at higher total strain amplitudes (≥0.7%) and softening at lower total strain amplitudes (<0.7%). The quenched and tempered and the ferrite–bainite–martensite (TSCA) microstructures displayed cyclic softening at all total strain amplitudes employed. Despite the cyclic softening, the ferrite–bainite–martensite structure was cyclically stronger than the ferrite–pearlite and the Q&T microstructures. Bilinearity in the Coffin–Manson plots was observed in Q&T and the multiphase TSCA conditions. An analysis of fracture surface provided evidence for predominantly ductile crack growth (microvoid coalescence and growth) in the ferrite–pearlite microstructure and mixed mode (ductile and brittle) crack growth in Q&T and the multiphase TSCA microstructures.