Bainitic Ferrite Microstructure Research Articles

Microstructural evolution during tempering of an ausformed carbide-free low temperature bainitic steel

The influence of tempering at 400 and 500 °C on two bainitic ferrite microstructures obtained via pure isothermal transformation and ausforming followed by the isothermal transformation was studied in a medium C-Si rich bainitic steel. To understand the dilatometric behaviour and mechanisms of carbide-free bainite decomposition, extensive microstructural characterization using scanning electron microscopy, X-ray diffraction, and microhardness measurements were conducted. The softening due to tempering at 400 °C was negligible for both microstructures. Tempering at 500 °C led to a remarkable contraction in dilatometric signal in an ausformed sample, while a pure isothermal sample showed slight contraction. The main mechanism governing microstructural changes during tempering at 500 °C was the decomposition of as-ausformed highly dislocated bainitic ferrite plates and/or film-like retained austenite into cementite and ferrite as a result of enhanced carbon diffusion. While tempering at 400 °C did not result in changes in the size of the bainitic plates, the higher tempering temperature of 500 °C led to a thickening of the bainitic ferrite plates for both testing conditions. However, the thickening was restricted in the ausformed bainite, which was related to the pinning effect of very small cementite precipitates, which moderate the tempering effects on hardness reduction.

Role of bainitic microstructures with M-A constituent on the toughness of an HSLA steel for seismic resistant structural applications

Fracture toughness of the coarse-grained heat-affected zone (CGHAZ) of fusion welded HSLA steel is severely reduced at low temperatures with the combination of deleterious microstructures such as bainitic ferrite (BF), granular bainite (GB), polygonal ferrite (PF), and the martensite-austenite (M-A) constituents. Toughness is one of the basic requirements for seismic resistant steels. The three heat input conditions (different cooling rate) were thermo-mechanically simulated, and the combined effect of M-A constituents with bainitic microstructures on toughness was examined. As the heat input changes from low to high, a combination of acicular ferrite (AF) and BF microstructures changes to a combination of PF, GB and M-A constituent. The former microstructure combination provides better toughness (ductile), whereas the latter makes the samples more brittle. The coarse BF, GB, and PF microstructures with low angle grain boundaries (LAGBs) make them brittle by easy crack propagation. Moreover, harder microstructures such as M-A can potentially act as cleavage initiation sites. However, a high frequency of high angle grain boundaries (HAGBs) in the AF microstructure can suppress the crack propagation, which turns the fracture to completely ductile. Also, it is observed that the M-A constituent in CGHAZ was abundant in martensite rather than the austenite.

The effect of fluid velocity and microstructure on erosion corrosion of two-phase CK45 steel

The essential factor to reduce the overall cost in many industries which facing erosion-corrosion phenomenon is the choice of correct design and material selection. The effect of fluid flow on corrosion and erosion–corrosion of materials is a well-recognized phenomenon in chemical and process industries and oil and gas production. The most effective parameter of flow is fluid velocity and best method of determination of the effect of fluid velocity as well as the magnitude of erosion-corrosion synergistic interaction is based on the weight loss measurements. In this study, the effect of fluid velocity on erosive corrosion of two-phase steel CK45 with different microstructures was investigated. These microstructures were formed by austenitizing the samples at 740C° for 2 ​min and then by air cooling and quenching in water or by austempering at 400C° for 2 ​h. Erosion corrosion test was carried out on the water dissolved in 3 ​wt% sodium chloride and 1 ​wt% alumina particles at velocities of 5, 9 and 16 ​m/s. According to the results, the kinetics of erosion-corrosion was linear. Increasing fluid velocity from 5 ​m/s to 16 ​m/s erosion-corrosion rate increased 4 to 6 times depending on the microstructure. SEM examination revealed presence of cavitation phenomenon on the surface of the samples. The results confirm that the bainitic ferrite microstructure exhibited the least damage in the Condition which contain sand particles or cavitation in a corrosive medium.

Effects of Molybdenum on HIC Susceptibility in Normalized Pressure Vessel Steels for Sour Service Applications

This paper discusses a molybdenum-added alloy design for normalized pressure vessel steels, to reduce hydrogen induced cracking (HIC) and inhibit crack propagation. The change in microstructure produced by the modified alloy composition was analyzed to determine its effect on HIC characteristics. The microstructural change was observed by optical microscopy, hardness tests, scanning electron microscopy, and electron backscattered diffraction. The crack length ratio and crack thickness ratio were evaluated using the NACE TM 0284 standard, and ultrasonic testing was used for HIC analysis. The formation of polygonal ferrite and pearlite during the processing of the alloy creates localized areas of high stress concentration at the polygonal ferrite/pearlite interface, due to the expansion/contraction of various structures during the transformation. This results in the generation of potential hydrogen-trapping sites, subsequent HIC, and crack propagation. The addition of molybdenum leads to a decrease in the volume fraction of the pearlite structure in the steel in favor of a more beneficial bainitic ferrite microstructure, which is generated during the normalizing process. This bainitic transformation creates a more favorable expansion/contraction compatibility and reduces/breaks up the ferrite/pearlite banding. The combination of these two characteristics can result in an overall lower stress-intensity state, which can minimize hydrogen-trapping and crack propagation. This study demonstrates that the resistance of normalized pressure vessel steels to HIC can be significantly improved by incorporating molybdenum in the alloy design. Key words: sour service, molybdenum, hydrogen induced crack (HIC), normalizing steel, pressure vessel steel

Modeling of Precipitation Hardening during Coiling of Nb–Mo Steels

Nb–Mo low-alloyed steels are promising advanced high strength steels (AHSS) because of the highly dislocated bainitic ferrite microstructure conferring an excellent combination of strength and toughness. In this study, the potential of precipitation strengthening during coiling for hot-strip Nb–Mo-bearing low-carbon steels has been investigated using hot-torsion and aging tests to simulate the hot-rolling process including coiling. The obtained microstructures were characterized using electron backscatter diffraction (EBSD), highlighting the effects of Nb and Mo additions on formation and tempering of the bainitic ferrite microstructures. Further, the evolution of nanometer-sized precipitates was quantified with high-resolution transmission electron microscopy (HR-TEM). The resulting age hardening kinetics have been modelled by combining a phenomenological precipitation strengthening model with a tempering model. Analysis of the model suggests a narrower coiling temperature window to maximize the precipitation strengthening potential in bainite/ferrite high strength low-alloyed (HSLA) steels than that for conventional HSLA steels with polygonal ferrite/pearlite microstructures.

Control of Microstructures and the Practical Properties of API X80 Grade Heavy-Wall High-Frequency Electric Resistance-Welded Pipe with Excellent Low-Temperature Toughness

This paper describes development of heavy-walled API X80 grade high-frequency electric resistance-welded (HFW) line pipes and conductor-casing pipes with wall thicknesses up to 20.6 mm. A fine bainitic–ferrite microstructure, which is preferable for low-temperature toughness, was obtained by optimizing the carbon content and applying the thermomechanical controlled hot-rolling process. As a result, the Charpy ductile–brittle transition temperature (DBTT) was well below 227 K (−46 °C) in the base metal of the HFW line pipe. When the controlled hot-rolling ratio (CR) was increased from 23 to 48 pct, the area average grain size decreased from 15 to 8 μm. The dependence of CTOD properties on CR was caused by the largest grain which is represented by the area average grain size. No texture development due to the increase of CR from 23 to 48 pct was observed. In addition, because controlled in-line heat treatment of the longitudinal weld seam also produced the fine bainitic–ferrite microstructure at the weld seam, DBTT was lower than 227 K (−46 °C) at the weld portion. The developed pipes showed good girth weldability without preheat treatment, and fracture in the tensile test initiated from the base metal in all cases.

The analysis of bainitic ferrite microstructure in microalloyed plate steels through quantitative characterization of intervariant boundaries

Electron backscatter diffraction (EBSD) measurements were performed to investigate the bainitic ferrite microstructure in low-carbon, microalloyed steels with varying C and Mn contents. Fully austenitized samples were isothermally heat treated at temperatures ranging from 450 to 550°C to form bainitic ferrite. The bainitic ferrite microstructures and boundary characteristics obtained from the EBSD measurements were analyzed based on an inferred Kurdjumov-Sachs (K-S) orientation relationship. The heat treated samples exhibit a microstructure composed of laths and the lath aspect ratio tends to increase at lower isothermal heat treatment temperatures. High fractions of boundary misorientation angles below 5° are observed, which are due to lath boundaries in the microstructure. Additionally, misorientations of approximately 7°, 53° and 60° are observed, which are related to the sub-block, packet, and block boundaries, respectively. With decreasing isothermal heat treatment temperature, there is an increase of block boundaries; these boundaries are intervariant boundaries between different blocks within a packet, most of which have the misorientation angle of 60°. The specimens with a higher carbon level contained increased length of block boundaries, whereas the addition of Mn moderated the dependence of block boundary length on the heat treatment temperature within the experimental temperature range. Meanwhile, the length of intervariant boundaries of both packet and sub-block character did not vary much with heat treatment temperature and alloy composition.

Carbon and nitrogen effects on microstructure and kinetics associated with bainitic transformation in a low-alloyed steel

The effect on the bainitic transformation of carburizing, nitriding and carbonitriding of a 23MnCrMo5 low-alloyed steel in the austenitic field was examined by in situ high-energy synchrotron X-ray diffraction (HEXRD) and transmission electron microscopy. CrN nitrides precipitated in austenite during the enrichment in nitrogen and were quantified. Enrichment in nitrogen leads to a fine bainitic ferrite microstructure, which contains the CrN nitrides as well as some AlN, VN and MnSiN2 precipitates, with much smaller number density. The precipitates are frequently gathered into aggregates. By using HEXRD, it was possible to monitor ferrite, austenite, cementite and CrN mass fraction during the isothermal bainitic transformation, whereas HEXRD experiments in previous studies focused mostly on carbide-free bainite. Ferrite formed first, followed by cementite precipitation and, in the case of nitrided steels, increase of CrN mass fraction. Enrichment in carbon and/or nitrogen tends to slow down the bainitic transformation compared to the initial steel, as expected from their austenite-stabilizing character. However, comparison between enriched steels shows that kinetics is not simply related to the total interstitial content when both carbon and nitrogen are simultaneously involved. Possible role of CrN precipitated in austenite on ferrite formation acceleration is discussed.

Processing, Microstructures and Mechanical Properties of Ultra-high Strength Steel Sheet

An ultra-high strength steel sheet with a tensile strength over 1500MPa and excellent low-temperature impact toughness was obtained by either using controlled rolling plus air cooling (CR+AC) or laboratorial thermo-mechanical controlled processing (TMCP). The effects of CR+AC and the TMCP processes were then examined on the microstructures and mechanical properties of this steel sheet. The results showed that by using CR+AC technology a uniformly distributed bainitic ferrite microstructure could be produced in this steel sheet under the appropriate reheating and rolling condition. By using TMCP technology, a lath bainite and martensitic microstructure with superior mechanical properties could also be generated. A microstructural analysis was made to interprete the strengthening-toughening mechanisms in association with the processes.

Study of the Effect of Thermomechanical Processing on Grain Refinement in HSLA Steels

The present study highlights the approach to multiscale analysis of the grain refinement during thermomechanical processing of microalloyed steels. HSLA steels for pipe-line use are introduced in which strain-induced precipitation and strain-induced transformations are controlled to produce ultra-fine grained microstructures. Multiscale modeling combining Digital Material Representation and Crystal Plasticity enables to gain a better fundamental understanding of mechanical response and microstructure evolution of precipitation strengthened austenite and ferrite of three microalloyed steel grades. The opportunities for the development of new thermomechanical processing schedules, focused on the grain refinement, are enhanced and the proposed models identifications procedure on the basis of the torsion tests at various forming conditions are presented and discussed. In the pipe-line grade steels, the combination of microalloying elements, accelerated cooling and transformation temperature, has led to much higher grain refinements and increased amount of bainitic ferrite microstructures than in the standard thermomechanical processing. Sensitivity analysis of studied microalloyed steel grades in the light of the discussed problems using microstructural analysis of obtained microstructures was also performed. Finally, the main challenges related to the multiscale modeling of proposed ideas are addressed.

Effect of microstructure and composition on hydrogen permeation in X70 pipeline steels

The influence of microstructure and composition on permeation of hydrogen in 1.2 and 0.5 wt.% Mn X70 pipeline steels after different processing was investigated using an electrochemical permeation technique.For 1.2 wt.% Mn (standard Mn) steel, the microstructure of normalised transfer bar was coarse equiaxed ferrite grains. This sample exhibited the highest diffusivity, followed by transfer bar, with a mixed ferrite–bainitic ferrite microstructure; and hot rolled strip, with fine elongated ferrite grains.The 0.5 wt.% Mn (medium Mn) strip displayed lower diffusivity than the 1.2 wt.% Mn strip, due to hydrogen trapping by finer ferrite grains and a higher density of carbonitride precipitates. Moreover, the medium Mn strip exhibited a uniform microstructure and consequently similar diffusion coefficients for the edge and centreline regions, whereas the finer grains of the edge region of the standard Mn strip resulted in a lower diffusivity than the centreline region.

Mechanical characteristics of spheroidal graphite cast irons containing Ni and Mn with mixed ferrite and bainitic ferrite microstructure

This study aims to clarify the influence of additive elements of Ni and Mn on tensile and impact properties of three kinds of spheroidal graphite cast irons (SG irons), which are as cast, annealed and austempered samples. Spheroidal graphite cast irons with Ni (0–4˙5 mass-%) and Mn (0–0˙5 mass-%) melted by a high frequency induction furnace and cast into a Y block CO2 mould with 30 mm in thickness. From the viewpoint of heat treatment, tensile strength and hardness of SG irons become larger in the order of ferritised<as cast<austempered ones. Matrix structures of SG irons, which are conducted to austempering treatment from α and γ mixture range, consists of bainitic ferrite with high toughness. Austempered SG iron with 3%Ni in 0˙1%Mn series is found to become higher tensile strength compound with elongation and toughness of 901 MPa, 17% and 915 kJ m−2.

ベイニティック・フェライト組織高強度熱延鋼板の伸びフランジ性

ベイニティック・フェライト組織高強度熱延鋼板の伸びフランジ性

Sulfide Stress Corrosion Cracking in Welded Joints of Welded Linepipes.

The sulfide stress cracking (SSC) behavior of welded linepipes has been studied using some SSC tests, such as tensile SSC test (NACE-TM 0177-90 Method A), full thickness SSC tests, four points bending SSC tests and full scale SSC tests. Detailed metallographic examinations have been performed in order to understand the influences of steel chemistries and heat inputs in welding on the SSC resistance of the heat affected zone. The effects of the geometry of the SSC specimen and hydrogen concentration on the SSC have also been investigated. The results have indicated that homogenized bainitic ferrite microstructure, which can be attained by reducing carbon content and by applying accelerated cooling after controlled rolling in plate rolling process, improves the resistance of steel plates to the SSC. Meanwhile, most specimens from submerged arc welding welded joints fail at the heat affected zone, regardless of differences in the microstructures of base materials, and they tend to show nearly the same level of the ratio of threshold stress to yield strength in the tensile SSC test. Specimen geometry and the quantity of hydrogen concentration in steel affect on the threshold stress, and hence reducing the hydrogen concentration improves the SSC resistance of the welded joints especially in lower hardness heat affected zone. Relations between the results of the laboratory tests and full scale tests are also discussed.