Ferrite Laths Research Articles

Study on Microstructure and Low‐Temperature Impact Toughness in Coarse Grain Heat‐Affected Zone of EH460 Steel

Herein, the effects of peak temperature on the microstructure and low‐temperature impact toughness of coarse grain heat‐affected zone (CGHAZ) of EH460 steel are investigated by Gleeble simulation welding. The low‐temperature impact toughness of CGHAZ decreases with the increase of peak temperature. With the increase of peak temperature, the microstructure of CGHAZ gradually changes from bainite to a mixed structure composed of bainite, intragranular ferrite, side lath ferrite, and a small amount of grain boundary ferrite. The inclusions size decreases as the peak temperature increases. The existence of large‐sized inclusions is conducive to the initiation and propagation of cracks, reducing the low‐temperature impact toughness of the material.

Induction assisted autogenous plasma arc welding of HSLA steel

In this work, a high frequency induction heating (HFIH) assisted plasma arc welding (IPAW) technique is proposed to weld 6 mm thick S690QL high strength low alloy steel plates in square butt joint configuration and without using any filler material. A 3D finite element based coupled electromagnetic-thermal analysis is carried out to ascertain the weld's thermal characteristics. Microstructure evolution and mechanical performance are investigated using scanning electron microscopy, electron back scattered diffraction, transmission electron microscopy, tensile test, Charpy impact test and micro hardness test. The results demonstrate that 15 s initial static heating using a co-directional current coil with magnetic flux concentrator predominantly improves the weld penetration by 48 % and joint efficiency reaches 90 % to the base plate strength. Microstructural study shows that the addition of HFIH promotes low angle grain boundaries (LAGB) with bainite ferrite and granular bainite micro-constituents in the fusion zone, which causes localised yielding and failure in this zone during tensile test. Further investigation reveals that the HFIH encourages the formation of BI-type bainite ferrite laths of 1.3–2.2 μm width, combined with slender martensite-austenite (M-A) island at the bainite ferrite lath boundaries. The results also reveal that the induction heating promotes the formation of M23C6 precipitates and B2 structured Cu-enrich nano precipitates in the fusion zone (FZ). The microhardness distribution indicates that the FZ and coarse grain heat affected zone are significantly reduced due to the assistance of the HFIH. The impact test result shows a lower energy absorption by the IPAW weldments due to dominance of the LAGB with unfavourable strain distribution.

Superior combination of strength and ductility in Fe–10Mn-0.6C steel trigged by austenite reversion transformation

Austenite reversion transformation process was employed on Fe–10Mn-0.6C steel to adjust the content and stability of austenite and the transformation-induced plasticity effect was effectively improved. With increasing annealing temperatures from 550 °C to 590 °C and 620 °C, the content and proportion of lath-like austenite significantly increased. Austenite exhibited different stability after annealing at different temperatures, mainly related to the distribution of Mn and C. The higher the local Mn and C concentration, the more stable austenite, vice versa. After annealing at 620 °C for 6 h, the highest austenite content of 61 % was achieved, and half of that had a lath-like morphology. The steel had a superior combination of ultimate tensile strength of 1500 MPa and elongation of 59 %, resulting from the enhanced transformation-induced plasticity effect. The key reason was the alternating distributions of austenite and ferrite laths, accompanied with the formation of nanotwins in the austenite with high stability.

The effect of microstructure on the initial corrosion behavior of low carbon steel in simulated coal solution

Abstract The service life of weathering steels in wagon body was determined by their corrosion resistance directly. This study investigated the influence of microstructure on the initial corrosion behavior of low carbon steels, systematically. The initial corrosion behavior of ferritic-bainitic (F+B) steel, bainitic (B) steel and ferritic-pearlitic (F+P) steel are thoroughly analyzed using coal leach solution immersion test, macroscopic and microcircuit electrochemical methods. The results revealed that F+B steel exhibited the highest corrosion resistance, with the potential of M-A islands surpassing that of ferrite. The initial corrosion initiates from the dissolution of the ferrite matrix, followed by detachment of the M-A islands. The potential of M-A islands is higher than that of bainitic ferrite lath, and the corrosion originates from ferrite dissolution in B steel. Moreover, F+P steel exhibited the largest potential difference between pearlitic and ferrite, leading to initiation of corrosion from the pearlitic corrosion of internal ferrite. Additionally, the multiphase characteristics of P in F+P steel exacerbates their corrosion susceptibility. Overall, the influence of microstructure on the initial corrosion behavior of low carbon steels can be attributed to the potential difference between different phases. Ferrite is the preferentially dissolved phase due to its negative potential difference.

Evaluation of Shear-Punched Surface Layer Damage in Three Types of High-Strength TRIP-Aided Steel

The damage properties in the shear-punched surface layer, such as the strain-hardening increment, strain-induced martensite fraction, and initiated micro-crack/void characteristics at the shear and break sections, were experimentally evaluated to relate to the stretch-flangeability in three types of low-carbon high-strength TRIP-aided steel with different matrix structures. In addition, the surface layer damage properties were related to the mean normal stress developed on shear-punching and microstructural properties. The shear-punched surface damage of these steels was experimentally confirmed to be produced under the mean normal stress of negative to 0 MPa. TRIP-aided bainitic ferrite (TBF) steel had the smallest surface layer damage, featuring a significantly suppressed micro-crack/void initiation. This was due to the fine bainitic ferrite lath matrix structure, a low strength ratio of the second phase to the matrix structure, and the high mechanical stability of the retained austenite. On the other hand, the surface layer damage of TRIP-aided annealed martensite (TAM) steel was suppressed next to TBF steel and was smaller than that of TRIP-aided polygonal ferrite (TPF) steel. The surface layer damage was also characterized by a large plastic strain, a large amount of strain-induced martensite transformation, and a relatively suppressed micro-crack/void formation, which resulted from an annealed martensite matrix and a large quantity of retained austenite. The excellent stretch-flangeability of TBF steel might be caused by the suppressed micro-crack/void formation and high crack propagation/void connection resistance. The next high stretch-flangeability of TAM steel was associated with a small-sized micro-crack/void initiation and high crack growth/void connection resistance.

Microstructure and properties of Mn–Si–Cr alloy steel modified by quenching and partitioning

Abstract This study investigates the influence of modification on the microstructure and properties of Mn–Si–Cr alloy steel. The results indicate that the as-cast microstructure of Mn–Si–Cr alloy steel is composed of black acicular bainitic ferrite lath and white retained austenite. The microstructure of the alloy steel changes to martensite, austenite, and carbide after quenching and partitioning treatment. After rare-earth magnesium modification and compound modification, the as-cast microstructure of Mn–Si–Cr steel becomes more refined and displays a more regular arrangement. Furthermore, the martensite and austenite grains in the modified samples show refinement, and the arrangement of martensite is more systematic. Additionally, the amount of austenite decreases, and the amount of carbides increases after quenching and partitioning heat treatment. In comparison with the unmodified samples, the modified samples show negligible changes in hardness. However, the impact toughness of modified quenched and partitioned steel increases by 20 %. Moreover, the wear resistance of compound modified quenched and partitioned steel is 38 % higher than that of the unmodified sample. The compound modified sample steel exhibits excellent wear resistance and comprehensive mechanical properties.

Influence of microstructure on slurry erosion resistance of medium carbon high silicon steels

The present study compares slurry erosion resistance of austempered and tempered medium carbon high silicon steels. A 0.62C-1.91Si-0.85Mn-0.23Cr steel was subjected to austempering at 300, 350 and 400 °C for 10 min to develop a microstructure comprising of bainite and retained austenite. With the increase in the austempering temperature, the mechanical stability of retained austenite decreased, and the size of bainitic ferrite laths increased. As a result, strain hardening ability and toughness of the steels decreased with austempering temperature. Another set of steel with the same composition was quenched and tempered at 300, 450 and 550 °C for 60 min. An increase in the tempering temperature led to an increase in the size and fraction of carbides and a decrease in the mean free path and dislocation density. Strain hardening ability and toughness of tempered steels was substantially small than austempered steels. Both the austempered and tempered steels were subjected to slurry erosion. Austempered steels exhibited higher slurry erosion resistance as compared to tempered steels. Slurry erosion resistance of austempered steels has been noted to depend on the mechanical stability of retained austenite and the size of bainitic ferrite laths. Due to the difference in the extent of deformation-induced transformation of retained austenite into martensite in the steels austempered at different temperatures, number of mobile dislocations generated (to accommodate the dilatation due to transformation) in bainitic ferrite laths differed. A large number of mobile dislocations in the specimens austempered at 300 °C blunted the propagating crack and delayed the material loss during erosion. The erosion resistance of tempered steels was observed to decrease with an increase in tempering temperature. A large fraction of coarse carbides led to the formation of many decohesion sites in steels tempered at 450 and 550 °C. Further, a small number of mobile dislocations present per unit volume in the steels tempered at high temperatures could not resist the propagation of cracks nucleated from the decohesion sites leading to a high erosion rate in these steels.

Effect of a novel controlled thermomechanical treatment on the microstructure and mechanical properties of a high-carbon nanobainitic steel

The effect of the novel controlled thermomechanical treatment, including torsion components in the elastic strain range during the isothermal holding on the microstructure and mechanical properties of the high-carbon nanobainitic steel, was investigated. TEM observations of the thermo-mechanically treated steel revealed bainitic ferrite laths with an average size of 68 ± 40 nm and films of retained austenite with an average size of 34 ± 17 nm, along with the blocky morphology of retained austenite in sub-micron scale. The XRD synchrotron diffraction allows estimating the amount of retained austenite at 43.1 ± 1.2% volume fraction with a carbon concentration of 1.17 ± 0.09 wt.%. Furthermore, the deconvolution of (200) Fe-γ reflections corresponding to two different low-carbon and high-carbon retained austenite peaks and, simultaneously, the blocky and film-like retained austenite was performed. In addition, the Nishiyama–Wassermann (N–W) crystallographic orientation relationship between bainitic ferrite and retained austenite was described as dominant using the misorientation distribution function (MDF). The crystallographic texture results indicated that the main growth of bainitic ferrite plates occurred after removing external stress during isothermal holding. The tensile tests and hardness measurements showed a high tensile strength achieved mainly by nano-metric bainitic ferrite plates and a high dislocation density. The high level of elongation is most likely attained due to a high amount of retained austenite in steel and both TRIP and TWIP effects during tensile deformation.

Development of advanced cementite-free bainitic steel using Ni, Al and Cu additions – Design concept and phase transformations study

A novel concept, consisting of the stabilization of retained austenite with Ni, Al, and Cu additions instead of using Si as the major alloying element to prevent carbide formation, is proposed to design nanocrystalline, cementite-free bainitic steels. In this work, the novel steel (wt.%) Fe-0.40C-0.50Mn-8.0Ni-2.65Al-0.8Cu was designed. Dilatometry investigations were performed to obtain the experimental CCT and TTT diagrams for this material. It was concluded that the bainitic reaction occurs between 400 and 225 °C during a transformation time ranging from 2 h to 66.5 h, respectively. An in-depth analysis performed on the resulting materials confirmed the presence of an ultrafine structure free of cementite consisting of carbon supersaturated laths of tetragonal bainitic ferrite and retained austenite with thin film and block morphologies. The use of Ni, Al and Cu as alloying elements leads to high carbon content in both populations of retained austenite, which prevents, the formation of fresh martensite during cooling to room temperature. The results of this research constitute a novel insight into the design of ultra-fine, cementite-free bainitic steels.

Effect of Austempering Processes on the Tensile Properties and the Work-Hardening Behavior of Austempered Bainitic Steels Below the Martensite Start Temperature.

The tensile properties and work-hardening behavior of austempered bainitic steels below martensite start temperature (Ms) were investigated and compared with those of bainitic steel austempered above Ms. The results show that the tensile strength and yield strength increased from 1096 MPa and 734 MPa to 1203 MPa and 951 MPa, respectively, when the austempering temperature was decreased from 400 °C to 300 °C. However, the total elongation decreased from 23% to 16%. The martensite-retained austenite blocks and bainitic ferrite laths are significantly refined. With a decrease in the austempering temperature, the volume fraction of retained austenite decreased from 15.4 vol% to 6.2 vol%. The carbon content in retained austenite increased from 1.12 wt% to 1.69 wt%. All tensile specimens exhibited three stages of deformation in the differential Crussard-Jaoul (C-J) models. The difference in ductility is mainly attributed to the transformation of the retained austenite blocks into strain-induced martensite during deformation. The initial content of retained austenite is the main factor affecting the ductility of bainitic steels. Therefore, the work-hardening ability of austempered bainitic steel above Ms is higher than that of bainitic steel below Ms.

Effects of Ti/N Ratio on Coarse-Grain Heat-Affected Zone Microstructure Evolution and Low-Temperature Impact Toughness of High Heat Input Welding Steel

Coarse-grain heat-affected zone (CGHAZ) properties of steel deteriorate when it is welded using high heat input, which always restricts the promotion and use of high heat input welding steel. TiN particles significantly inhibit the growth of austenite and improve the microstructure and properties of high heat input welding steel. Effects of different Ti/N ratios on the CGHAZ microstructure and properties of high heat input welding steel were studied using welding thermal simulations and in situ observations. Results showed that a higher Ti/N ratio led to the abnormal growth of austenite grains and promoted the nucleation and growth of lath ferrite, which made the microstructure of the CGHAZ heterogeneous. In contrast, austenite grains were more uniform at a lower Ti/N ratio. Thus, the microstructure was refined, the brittle structure was reduced, and the properties of the CGHAZ were improved. In addition, when Ti/N = 5.85, the impact absorption energy of the CGHAZ obviously fluctuated. However, when Ti/N = 2.82, the impact absorption energy of the CGHAZ was higher and more stable. These results provided a new idea for the development of high heat input welding steel based on TiN theory.

Influence of austempering temperature on carbide free bainite development, a critical review

In this review paper, it is well established that high carbon high silicon steels on austempering result in carbide free bainite (CFB) which exhibits enhanced mechanical and tribological properties without adding any heavy alloying elements. Most of the research has been pointed out regarding the improvement of CFB steels that consists of the fine structure of bainitic ferrite laths and retained austenite (RA). In some cases, it was observed that the tensile strength of CFB steels was found similar to the commercial quenched and tempered steel alloys. Regarding other properties, it is estimated that out of the various failures that take place due to wear, 50% are caused by abrasive wear. To improve the lifetime of components, the selection of material and optimized processing techniques plays a pivotal role.

Two design strategies for enhancing the thermal stability of bainitic structures

In response to the growing interest of the industry in advanced high-strength steels subjected to extreme operating conditions, two novel grades BainTS and BainNiAlCu bainitic steels have been developed. Since the in-use properties of bainitic steel are directly related to the thermal stability of the bainitic structure, two different alloy design concepts to improve tempering resistance considering various strengthening mechanisms were proposed. BainTS steel follows the standard assumptions of nanocrystalline bainitic steel design, where the retained austenite is stabilized with silicon. A potential secondary hardening was also considered using Cr, Mo, and V alloying additives. On the other hand, BainNiAlCu steel was designed taking into account the precipitation strengthening with intermetallic compounds (nickel aluminide and copper particles). It was revealed that both steels after heat treatment consist of bainitic ferrite laths and a significant content of retained austenite (above 25%) with film and blocky morphologies. After the tempering process (350 °C–650 °C), there was significant structural evolution following the gradual decomposition of the metastable bainitic structure. It was proved that BainTS steel is characterized by the higher thermal stability of both bainitic ferrite and retained austenite compared to BainNiAlCu. Despite this, the hardening effect after tempering BainNiAlCu steel was higher (by about 140 HV compared to heat treatment, approx. 30% increase). The presented results suggest that bainitic steels strengthened with intermetallic phases are extremely promising in the context of future industrialization in applications where operating conditions are exposed to elevated temperatures.

Microstructural evolution and ultra-high impact toughness of austempered lamellar bainitic steel far below Ms temperature

The present study investigated the effects of the austempering process far below Ms (martensite start temperature) on the microstructural evolution and impact toughness of bainitic steel with ultrahigh impact toughness and compared the results with the tempered martensitic steel. The results revealed that the lamellar bainitic steel composed of bainitic ferrite laths and retained austenite films is obtained by austempering far below Ms temperature. The obtained steel exhibited the Charpy V-notch impact absorbed energy of up to 131 ± 6 J, which was 2.7 times higher than that of the tempered martensitic steel (49 ± 1 J). The ultra-high impact toughness of bainitic steel was attributed to the deflection of the crack propagation direction by the refined bainitic ferrite laths and the passivation of the crack tip by the retained austenite films. Furthermore, the proportion of V2, V3, and V5 variants increased and formed HAGBs (high-angle grain boundaries) with the V1 variant in the austempered bainitic steel B-32. These HAGBs inhibited crack propagation effectively, thereby significantly improving the impact toughness of the austempered bainitic steel.

Microstructure-mechanical properties relationships and strain hardening mechanism for medium-Mn steel with low yield and ultra-high tensile strength

Considering the problems of forming ultra-high strength steel, a medium-Mn steel with low yield strength was prepared. The steel exhibited a yield strength of 537 MPa, a tensile strength of 1780 MPa, and an elongation of 17.8%. The study aimed to identify the key factors contributing to the low yield-to-tensile ratio and strain hardening behavior of the steel. To achieve this, interrupted tensile tests, loading-unloading-reloading tensile tests, and microstructure characterization were used, along with theoretical calculations. The results indicated that the low dislocation density in lath ferrite, initial martensite, and retained austenite, as well as the high-volume fraction of retained austenite, were the primary factors contributing to the low yield strength. During tensile deformation, strong solid solution strengthening in strain-induced martensite was the main factor driving strain hardening. Additionally, the rapid multiplication of dislocations in ferrite and martensite, along with the heterogeneous deformation-induced hardening caused by the heterogeneous structure composed of strain-induced martensite and other phases, were also significant factors contributing to strain hardening. When considering grain boundary strengthening, dislocation strengthening, solid solution strengthening, and heterogeneous deformation-induced hardening, the theoretical yield strength and flow stress calculated in this study closely matched the actual measured values.

In situ TEM observations of the growth of bainitic ferrite in an Fe-0.3C-3Mn-1.5Si-0.15Mo steel

The current study reports in situ TEM observations of the growth of bainitic ferrite in an Fe-0.3C-3Mn-1.5Si-0.15Mo steel held isothermally at 300 °C with a higher spatio-temporal resolution than in previous studies. Significant variations were found in the lengthening rate, with the highest being in excess of 30,000 nm.s−1 while more common lengthening rates of 10–1000 nm.s−1 provided the highest quality observations. Both sheaves with visible sub units and individual laths were observed during growth with the lengthening behaviour of sheaves found to be discontinuous - in the most favourably oriented sheave this could be linked to sub unit behaviour. The transformation behaviour was comparable to that of HT-LSCM observations of bainitic ferrite growth for the most comparable steel compositions and to ‘textbook’ descriptions of the formation of bainite sheaves. In addition, other relevant phenomena were recorded, including the generation and movement of dislocations in the austenite during transformation, the interaction of laths with twin boundaries and the initially slow growth of bainitic ferrite laths.

Embrittlement mechanism of 460 MPa-Grade nuclear power steel deposited after heat treatment

The effects of post-welded heat treatment on microstructure evolution and embrittlement mechanism of 460 MPa-grade nuclear power steel deposited metal were studied using optical microscope, scanning electron microscope, transmission electron microscope and electron backscattered diffraction (EBSD). The results showed that there was no significant change in the type of deposited metal microstructure before and after heat treatment, except for some acicular ferrite turning into block ferrite. Further studies showed that the dispersive martensite-austenite (M-A) constituent did not worsen the toughness of the deposited metal. However, after heat treatment, the M-A constituent decomposed into ferrite and carbides, where the carbides were precipitated on the grain boundary, resulting in grain boundary embrittlement. The precipitated carbides also increased the dislocation tangle on the grain boundary, which was prone to causing stress concentration and induced crack initiation, resulting in the cleavage fracture. The analysis of the orientation characteristics of the impact fracture profile via EBSD showed that the ferrite laths were coarsened and the high-angle grain boundaries were reduced after heat treatment. Meanwhile, the plastic crack propagation was terminated earlier resulting in the early initiation of cleavage fracture. Moreover, the crack propagation path was smooth and secondary cracks formed during the propagation process. Finally, after heat treatment the inclusions easily fell off and formed micro-voids, which provided favorable channels for crack propagation and accelerated the deterioration of impact toughness. All of these were factors for the significant decline in the toughness of the heat-treated deposited metal.

The impacts of M/A constituents decomposition and complex precipitation on mechanical properties of high-strength weathering steel subjected to tempering treatment

The impact of microstructure evolution on the mechanical properties of a typical 500 MPa-grade weathering steel produced by an identical thermo-mechanical control process and different tempering treatments at 450–650 °C were thoroughly investigated mainly using scanning electron microscopy (SEM) equip with an electron backscatter diffraction (EBSD), high-resolution transmission electron microscopy (HRTEM), and X-ray diffractometer (XRD). Results indicated that the as-rolled steel consists of granular bainitic ferrite (GBF) and a considerable amount of martensite/austenite (M/A) constituents, exhibiting unsatisfied mechanical properties. As the tempering temperature increased from 450 to 550 and 650 °C, the massive twin-type M/A constituents decomposed preferentially into carbides following the sequence of Fe3C→(Cr, Mn, Fe)3C→(Cr, Mn, Fe)3C + (Cr, Mn)23C6, with a small amount of fine lath-type M/A constituents remained. The matrix recovered with the bainitic ferrite laths merged, the dislocation density decreased and the proportion of high-angle grain boundary (HAGB) increased. Nanoscale (Ti, Nb)C and ε-Cu particles also precipitated simultaneously, leading to an increase of yield strength. The strain hardening capacity and hence the tensile strength reduced resulting from the decomposition of M/A constituents. Moreover, the impact toughness was first enhanced by tempering at 450–600 °C with the decreasing twin-type M/A constituents content and increasing HAGBs proportion, and then deteriorated by tempering at 650 °C due to the precipitation of necklace-like M23C6 carbides at the grain boundaries. An optimum mechanical property combination was achieved via tempering at 550–600 °C.

A Novel Continuous Cooling Treatment Based on the Carbon Diffusion Process During Nanobainite Transformation in High‐Carbon Steel

A novel methodology of carbon‐diffusion‐dominated continuous cooling treatment (CC) during the bainite transformation is proposed and implemented on a high‐carbon nanobainitic steel. Nanobainite (NB) microstructure composed of bainitic ferrite lath with a thickness of ≈36 nm is obtained by successive heat treatment processes, including cooling the sample from 240 to 216 °C at a rate of 0.1 °C min−1, then cooling down to 120 °C at a rate of 0.2 °C min−1 followed by austempering at 120 °C for 12 h. Several technological routines involving oil quenching + tempering, single‐stage austempering + tempering, and two‐stage austempering + tempering are compared. The effects of heat treatment processes on the microstructure and mechanical properties are unveiled. Importantly, after the CC process, the tensile strength is 2049 MPa, the yield strength is 1552 MPa, the elongation is 2.5%, the impact toughness is 11.5 J, and the hardness is 60.7 HRC. Notably, the comprehensive mechanical properties of CC‐processed samples are superior to that of other heat‐treated samples. Microstructure analyses reveal that the CC process significantly refines the bainitic ferrite lath thickness while increasing the volume fraction of NB. Thereby, effectively shrink blocky retained austenite, and finally enhancing the overall mechanical properties.

Effect of microstructure transformation below MS temperature in bainitic steels on the impact-abrasive wear behavior

The bainitic steel austempered below MS (martensite start temperature) with good strength and toughness has a great potential in the field of wear resistance. In this paper, the impact abrasive wear resistance of bainitic steel austempered below MS temperature was studied and compared with those of bainitic steel austempered above MS temperature. The results show that the wear resistance of bainitic steel austempered below MS is better than that of bainitic steel austempered above MS. The wear mechanism of bainitic steel austempered below MS is dominated by fatigue wear. This is attributed to the refinement of bainitic ferrite laths and martensite-retained austenite blocks by austempering processes below MS. The bainite steel austempered below MS with higher initial hardness can effectively resist the impact load and the damage of abrasive particles to the wear surface, thereby reducing the wear weight loss.