Granular Bainite Research Articles

Correlation of Microstructure Feature with Impact Fracture Behavior in a TMCP Processed High Strength Low Alloy Construction Steel

The present article aims at elucidating the effect of thermo-mechanical controlled processing (TMCP), especially the finish cooling temperature, on microstructure and mechanical properties of high strength low alloy steels for developing superior low temperature toughness construction steel. The microstructural features were characterized by scanning electron microscope equipped with electron backscatter diffraction, and the mechanical behaviors in terms of tensile properties and impact toughness were analyzed in correlation with microstructural evolution. The results showed that the lower finish cooling temperature could lead to a considerable increase in impact toughness for this steel. A mixed microstructure was obtained by TMCP at lower finish cooling temperature, which contained much fine lath-like bainite with dot-shaped M/A constituent and less granular bainite and bainite ferrite. In this case, this steel possesses yield and ultimate tensile strengths of ~ 885 MPa and 1089 MPa, respectively, and a total elongation of ~ 15.3%, while it has a lower yield ratio of ~ 0.81. The superior impact toughness of ~ 89 J at −20 °C was obtained, and this was resulted from the multi-phase microstructure including grain refinement, preferred grain boundaries misorientation, fine lath-like bainite with dot-shaped M/A constituent.

Effect of Heat Treatment on Microstructure and Mechanical Properties of High-Strength Steel for in Hot Forging Products

High-strength steel is widely used in hot forging products for application to the oil and gas industry because it has good mechanical properties under severe environment. In order to apply to the extreme environment industry requiring high temperature and high pressure, heat treatments such as austenitizing, quenching and tempering are required. The microstructure of high-strength steel after heat treatment has various microstructures such as Granular Bainite (GB), Acicular Ferrite (AF), Bainitic Ferrite (BF), and Martensite (M) depending on the heat treatment conditions and cooling rate. Especially in large forged products, the difference in microstructure occurs due to the difference in the forging ratio depending on the location and the temperature gradient according to the thickness during post-heat treatment. Therefore, this study attempted to quantitatively analyze various phases of F70 high-strength steel according to the austenitizing temperature and hot forging ratio using the existing EBSD analysis method. In addition, the correlation between microstructure and mechanical properties was investigated through various phase analysis and fracture behavior of high-strength steel. We found that various microstructures of strength steel depend on the austenitizing temperature and hot forging ratio, and influence the mechanical properties and fracture behavior.

Effects of prior austenite grain size and phase transformation temperature on bainitic ferrite formation in multi-constituent microstructures of a strong ultra-low-carbon steel

Bainitic ferrite is a key microstructural constituent in High-Strength Low-Alloy (HSLA) steels, influenced by the prior austenite grain size and heat treatment conditions. This study investigates the decomposition of coarse- and fine-grained austenite under rapid step quenching heat treatments around the martensite-start (MS) temperature in an ultra-low-carbon HSLA steel. An isothermal-to-nonisothermal transition occurred in the austenite phase transformation behavior after the grain size refinement from ~69 μm to ~10 μm resulting in the formation of a complex combination of various ferritic products consisting of quasi-polygonal ferrite, granular bainite, bainitic ferrite, and martensite in the microstructure of the below-MS-treated samples. Fine bainitic ferrite laths with an average thickness of ~150 nm formed, which were about 70% thinner than those of the above-MS-treated samples, contributing in part to an ultimate tensile strength of 791 MPa along with a uniform elongation of 6.4% for the below-MS-treated samples. The results can give insight into the methodologies for transferring the concept of nanoscale bainite to ultra-low-carbon steels.

Influence of boss-backing welding to ERW pipe

Station and valve chamber design often encounter the situation of drilling hole at the main pipeline and welding boss-backing to connect the branch pipe. Boss hole location should generally be at least 100 mm away from the longitudinal weld or spiral weld. However, because the electric resistance weld (ERW) is difficult to distinguish in practice, some bosses mounting position coincide with ERW or close to. In this paper, the influence of boss-backing welding directly on the longitudinal weld to the original residual stresses of ERW pipe was studied. The microstructure of pipe body and longitudinal weld after welding was also analysis. The testing results showed that the overall residual stresses of ERW pipe were relatively small. Residual stress at the longitudinal weld region were smaller than those at the pipe body region. After the boss-backing welding, the axial residual stress at the longitudinal weld and the circumferential residual stress at the pipe body region near the intersection increased sharply to 2.5 (444 MPa) and 3.8 (433 MPa) times, respectively. The invaded width and depth to the ERW pipe after welding were about 15.167 mm and 3.376 mm. Granular bainite with necklace type M-A constituents could be observed at the invaded zone. It is suggested that small welding heat input should be adopted for boss-backing welding.

Effect of Tempering Temperature on Microstructure and Uniformity of LP Steel Plate under HOP Process

Longitudinal profiled steel plate (LP steel plate) is a green steel product, which is widely used in steel structure fields such as ships and bridges. In this paper, the effect of Heat-treatment Online Process (HOP) on microstructure and uniformity of LP steel plate was investigated by thermal simulation test. The results show that the microstructure consists of grain boundary allotriomorphs, upper bainite, acicular ferrite and carbon-enriched austenite at 550°C. During the isothermal process, the austenite transforms into bainite, the acicular ferrite grows and coarsens, and the bainite laths gradually “fuse” to form large granular bainite. After online tempering at 650°C, the austenite transforms into ferrite, acicular ferrite and bainite undergo tempering transformation, in which the size of bainite decreases and some acicular ferrite transforms into polygonal ferrite. After tempering at different temperatures (650∼750°C), the microstructure consists of grain boundary allotriomorphs, coarsened acicular ferrite and a small amount of granular bainite. With the increase of tempering temperature, the tempering transformation degree of acicular ferrite and bainite is intensified. Elongated carbides at grain boundaries become short rods and particles, which decreases the size and aggregation of carbides. Compared with the traditional process, the difference of microstructure and hardness of LP steel plate with different thickness specifications after HOP process is reduced, and the hardness is higher, which is beneficial to improve the performance and uniformity.

Interpretation of distribution of microstructural heterogeneity across the plate steel thickness

The relationships of the anisotropy behavior and other characteristics of structural heterogeneity over the thickness of the plate have been established and their microstructural interpretation has been presented using the example of low-alloy plate steel. The features of structure formation from the surface to the center of 100 mm thick plate metal have been considered with a detailed description of the structure evolution and the processes that caused these changes. It was found that a fine quasi-homogeneous structure consisting of lath bainite and granular bainite provides anisotropy Ka100 <1 and occupies a narrow layer from 5 to 17 mm from the plate surface. In the near-surface layer, due to the abnormal structure consisting of large austenite grains elongated along the rolling direction and decorated with allotriomorphic ferrite, the anisotropy is Ka100 > 1. In a quarter of the plate thickness, due to weak fragmentation of the structure, the anisotropy is Ka100 > 1. In the center of the plate, quasi-polygonal and polygonal ferrite absorbs all extended boundaries of the former austenite grains; therefore, Ka100 becomes again less than unity.

Effect of finish rolling temperature on microstructures and mechanical properties of 1000 MPa grade tempered steel plate for hydropower station

With the decreasing of finish rolling temperature, the microstructure of the hot rolled steel was changed from lath martensite to complex microstructures composed of lath martensite, ferrite, granular bainite and lath bainite, unrecrystallized prior austenite grains are the main reason for the formation of complex microstructures. After tempering, the steels are mainly composed of ferrite and cementite. For the steel with high finish rolling temperature, the proportion of high misorientation boundary is less, and it has rod-like precipitates with the length of 25−85 nm and punctate precipitates with the size of 4−16 nm, the intensities of {111}<110> and {001}<110> textures are enhanced. The experimental steel has the best combination with strength and toughness as yield strength of 996 MPa, tensile strength of 1023 MPa, total elongation of 19.1 %, impact value at -60 ℃ of 125 J. Grain refinement, higher density of dislocation, fine and dispersed precipitates could enhance the yield strength and tensile strength. Stronger intensity of {332}<113> texture and weaker intensity of {001}<110> texture could improve the impact toughness.

Quantitative microstructural characterization of nuclear grade bainitic steel: Influence of macrosegregations in full-scale forgings

Large forgings used for the fabrication of nuclear reactor components often present chemical macrosegregations as a result of the solidification processes in the initial ingot. This paper presents an extensive microstructural characterization of both the macrosegregated area (up to 0.28 wt% of carbon) and a reference non-segregated area (0.18 wt% of carbon) from 16MND5 steel full-scale large forgings after the final heat treatments and tempering. Inside the macrosegregated areas, significant microhardness variations are observed, corresponding to millimetric microsegregations observed by optical microscopy. At a finer scale, the microstructures of the reference non-segregated area and the macrosegregated area are quite similar, as revealed by Scanning Electron Microscope (SEM) and Electron BackScattered Diffraction (EBSD). In both cases, the microstructure is mostly made up of granular bainite and upper bainite. Various types of precipitates were observed by either SEM or Transmission Electron Microscopy (TEM), such as Fe(Cr,Mn)3C, Mo2C, AlN and AlMnSiN precipitates. The main difference between unsegregated and segregated areas is the significantly higher precipitate number density in the segregated area, as a result of the increase in C, Cr, Mn and Mo content, which can explain the lowered fracture toughness in these areas. Thermodynamic calculations confirm that Cr and Mn enrich cementite precipitates after tempering.

Microstructure evolution and impact toughness variation for high strength steel multi-pass weld metals with various cooling rates

Thermally simulated experiments on high-strength weld metals were performed. Based on the results, an extended continuous cooling transformation (CCT) curve with microstructure, hardness, and impact toughness can be proposed. The microstructures, hardness, high-angle grain boundaries, impact toughness, and fracture surfaces of the weld metals treated with different cooling rates were systematically compared and analyzed. The microstructure of the original weld metal corresponds to a cooling rate of 5 °C/s and is composed of uniform granular bainite and a small amount of acicular ferrite, which is similar to that of the specimens with cooling rates of 3−30 °C/s. The increase in the cooling rates leads to the evolution of the microstructure from quasi-polygon ferrite to granular bainite. The proportion of large angle grain boundaries increases with the increasing cooling rate, which can increase the resistance of crack propagation and induce a higher impact toughness. The impact toughness at room temperature is in the range of 80–131.81 J. Various cooling rates lead to various impact toughness. Except for the cooling rate of 5 °C/s, the other cooling rates decrease the impact toughness. Moreover, at the lowest cooling rate, the decrease in the impact toughness reaches 63 %. The impact toughness at -50 °C is in the range of 34–109.86 J. The impact toughness of the weld metals without thermally simulated processes at -50 °C is 88.6 J. The reheating process decreases the impact toughness from 88.6 J to 34 J, and the decrease rate of the impact toughness at -50 °C reaches 38 %.

Microstructure and properties of laser hybrid welding joint of AH36 marine steel

AH36 Marine steel 10mm thick was welded by Laser-MIG hybrid welding process, and the structure and properties of the arc zone and laser zone of the Laser-MIG hybrid welding joint were studied respectively. The results show that the width of welding seam and HAZ in arc action zone is obviously larger than that in laser action zone. The microstructure of the weld was mainly acicular ferrite, bainite, martensite and grain boundary ferrite, and the content of martensite and bainite in the laser action area was relatively higher. The microstructure of the coarse grain zone is mainly thick lath martensite, granular bainite and acicular ferrite, and the microstructure of the arc action zone is relatively larger. The microstructure of the fine crystal region is mainly fine ferrite and pearlite. The hardness of arc zone and laser zone is distributed symmetrically in the weld center, the lowest hardness occurs in the weld center, and the high hardness occurs in the weld HAZ. The tensile specimens in the arc zone were fractured near the HAZ and the laser zone was fractured on the base metal far away from the weld. The lowest value of impact absorption work at 0°C in arc action area and laser action area appeared at the fusion line +0.5; the highest value of impact absorption work at 0°C in arc action area appeared at the fusion line +1.0, the fusion line +1.5, and the fusion line +2.0; the highest value of impact absorption work at 0°C in laser action area appeared at the fusion line. The impact absorption at 0°C in the laser action area is higher than that in the arc action area.

Kinetics of Austenite Phase Transformations in Newly-Developed 0.17C-2Mn-1Si-0.2Mo Forging Steel with Ti and V Microadditions.

This work presents the results of phase transformation kinetics during continuous cooling in newly developed high strength low-alloy steel (HSLA). Initial theoretical calculations for the determination of heat treatment parameters were conducted. To determine the structural constituents formed due to the austenite decomposition the dilatometry approach was used. The material was cooled down from the austenitization temperature of 1000 °C with cooling rates between 0.1 °C/s to 60 °C/s. Then, light and scanning electron microscopy investigations were carried out. The microstructure after cooling at rates between 0.1 °C/s up to 1 °C/s is mainly ferritic with some fraction of granular bainite. Increasing the cooling rate led to formation of a higher fraction of bainitic ferrite. At 60 °C/s the microstructure was mainly bainite with some fraction of ferrite. To determine the presence of retained austenite, color etching using Klemm solution was used. The results show that the increase of cooling rate decreases the amount of retained austenite in the microstructure of the steel. Hardness measurements were made to determine the changes in the mechanical properties as a function of the cooling rate.

A Comparison Study of the Austenite Grain Growth and Its Transformation Behavior during Uniform Continuous Cooling of a Wrought and Selective Laser Melting 4340 Steels

Abstract This study compares austenite grain growth and its continuous cooling transformation (CCT) behavior between selective laser melting (SLM) 4340 and conventional wrought 4340 steels. Standard dilatometry tests were used to determine the austenite decomposition behavior at different cooling rates. The analysis of the data from the controlled cooling paths was used to generate CCT diagrams for the two steels investigated in this study. Advanced microstructural characterization techniques, Scanning Electron Microscopy and Orientation Imaging Microscopy, were employed to support the CCT diagrams. Electron Backscatter Diffraction (EBSD) and a special EBSD-Image Quality characterization technique were used to assess the percentage of the microstructural components observed. Three important observations were made: (1) the hardenability of the wrought 4340 steel was higher than the SLM 4340 steel; (2) the presence of granular bainite was observed after slow cooling conditions in both steels; (3) the SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied. MTEX, a MatLab program, was used to reconstruct the prior austenite grains. The SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied.

Microstructure evolution and properties of direct laser deposited 24CrNiMoY alloy steel assisted by non-contact ultrasonic treatment

To solve the problems of high porosity and uneven microstructure of high-performance alloy steel parts, 24CrNiMoY samples were prepared by non-contact ultrasonic cavitation-assisted (UC-A) direct laser deposited (DLD) process. The samples in the UC-A DLD process were treated with ultrasonic application at three angles of 30°, 45°, and 60° to the horizontal plane and without ultrasonic treatment. The ultrasonic frequency and amplitude were 40 kHz and 30 μm, respectively. The laser energy density of the deposited samples was 83.3 J/mm2. Characterization tests such as OM, XRD, SEM, EBSD, TEM, hardness, wear and tensile tester were performed to study the structural evolution and mechanical properties of the samples under different ultrasonic treatments. The results showed that the microstructure of the prepared samples were a large amount of lath bainite (LB) and a small amount of granular bainite (GB), and the introduction of ultrasound could significantly reduce the porosity in the sample. When the angle of ultrasonic application was 60°, the dense sample with fine-grained and better mechanical properties was obtained. With the ultrasonic application angle increased to 60°, the average strip width of LB was reduced from 34.9 μm to 14 μm, and the grain size of alloy sample was decreased from 0.87 μm to 0.47 μm. The average microhardness, wear loss, tensile strength, and elongation of the samples treated with ultrasonic application angle of 60° were 381.97 HV0.2, 1.0×10-3 g, 1125.1 MPa, and 11.5%, respectively, which increased by 10.6%, 33.3%, 31.7%, 88.5% compared with the sample untreated. Finally, the evolution mechanism between the angle of ultrasonic application and the pores in 24CrNiMoY alloy steel sample was established, and the effect of ultrasonic cavitation on the microstructure and performance of the samples were analyzed through the experimental procedure. Therefore, it is an effective method to reduce porosity, refine microstructure, and improve the mechanical properties of DLD 24CrNiMoY alloy steel by applying non-contact UC-A with a proper application angle (60°).

New insight into the hardenability of high strength low alloy steel from the perspective of crystallography

A Jominy specimen of low carbon low alloy steel was investigated to elucidate the relationship between crystallographic features and hardness. As the Jominy distance increased from 2 to 20 mm and then to 50 mm, the morphological structure changed from lath martensite to lath bainite and then to granular bainite. The cooling rate was lowered with increasing Jominy distance, and the density of high angle grain boundaries (HAGBs) and the hardness were decreased, showing a near-linear relationship between the HAGBs density and hardness. The high cooling rate led to a large driving force and a low transformation temperature, which contributed to producing high density of HAGBs and hardness.

Low Cycle Fatigue Behavior and Failure Mechanism of Wire Arc Additive Manufacturing 16MND5 Bainitic Steel

In this study, the low cycle fatigue behavior and failure mechanism of WAAM 16MND5 bainitic steel, which is commonly used in the nuclear reactor, were investigated thoroughly via a series of strain-controlled and stress-controlled low cycle fatigue tests. Results show that the microstructure of WAAM 16MND5 steel is granular bainite with a certain number of banded pro-eutectoid ferrite. WAAM 16MND5 steel exhibits similar cyclic deformation behavior under different strain amplitudes, i.e., a long stage of continuous cyclic softening and rapid cyclic softening till failure. The fatigue life decreases with the increasing strain amplitude. Moreover, the persistent slip markings in ferrite grains, the interface of pro-eutectoid ferrite and large Al2O3 inclusions are the preferred locations for microcrack initiation. The crack initiated from the edge of the specimen is observed to develop into a main crack. The main crack propagates mainly in transgranular mode and can also propagate along the pro-eutectoid ferrite interface.

Research on Microstructure and Properties in Welded Joint of 700 MPa grade Low Carbon Micro-alloy steel

High-strength residual heat treatment steel bar was tested with welding thermal simulation in Gleeble 3500. Effect of heat input on the microstructure, impact toughness and microhardness of weld heat-affected zone (HAZ)was studied. The results show that with the increase of t8/5, microstructure of coarse grain heat-affected zone (CGHAZ)transforms into granular bainite and pearlite in stead of ferrite, feather-like upper bainite and strip-like lower bainite, HAZ impact value increases firstly then decreases and reaches maximum at t8/5= 100s and the hardness decreases. And when t8/5 increased to 60s there were only ferrite and pearlite in the microstructure of CGHAZ. The impact energy of high-strength residual heat treatment steel bar would be at the range of a good level for physical production with the range of t8/5 from 20s to 300s.

Experimental Investigation of Fatigue Crack Growth Behavior of the 2.25Cr1Mo0.25V Steel Welded Joint Used in Hydrogenation Reactors.

In this work, the fatigue crack growth (FCG) behavior and fatigue damage mechanism of the 2.25Cr1Mo0.25V steel welded joint used in hydrogenation reactors were investigated. The multi-pass welding was carried out to manufacture the welded joint using the combined shielded metal arc welding (SMAW) and submerged automatic arc welding (SAAW) processes. The FCG behavior of different zones in the welded joint, including the base metal (BM), the heat-affected zone (HAZ) and the weld metal (WM), were studied by compact tension tests. Moreover, the acoustic emission (AE) technique was used to monitor AE signals generated from FCG process for further understanding FCG behavior and fatigue mechanisms. Additionally, the microstructures and fracture surfaces of different specimens were observed by optical microscopy (OM) and scanning electron microscopy (SEM). The results revealed that the microstructure of BM is fine granular bainite, while the WM shows coarser bainite grains. The HAZ exhibits the most significant inhomogeneity with large dispersion of grain size. FCG results showed that the HAZ exhibits much higher fatigue crack growth rate (FCGR) at low ΔK values, while the BM shows the most superior fatigue resistance. The AE technique is successful in monitoring and identifying damage evolutions during the FCG process. Moreover, an enhanced AE activity is observed in FCG of the WM specimen, which is attributed to the combined influence of the formation of numerous secondary cracks and coarse-grained microstructures.

The tempering behavior of martensite/austenite islands on the mechanical properties of a low alloy Mn-Ni-Mo steel with granular bainite

The effects of tempering on the microstructure and mechanical properties of a low alloy Mn-Ni-Mo steel were investigated. The results indicate that the steel is characterized by granular bainite consisting of bainitic ferrite, blocky islands of martensite and/or austenite (M-A islands) and film-type retained austenite (AR) before tempering. The volume fraction of AR decreases with an increase in the tempering temperature: slowly before 280°C, rapidly at 350°C, and approaching zero at 450°C. When tempered at 280°C, the steel has optimum mechanical properties because of the internal stress relaxation and the decomposition of blocky M-A islands, especially twinned martensite islands. Additionally, the enhancement of AR stability is to some extent responsible for improving the impact toughness. When tempered at 350−450°C, tempering brittleness occurs, which results from the decomposition of the AR films into discrete flake-type carbides along the interfaces. Further increase in the tempering temperature results in high toughness and low strength of the steel, mainly due to the recovery and recrystallization of bainitic ferrite, and the precipitation of globular M3C carbides.

Effect of Heat Treatment on Microstructure and Properties of 24CrNiMo Alloy Steel Formed by Selective Laser Melting (SLM).

The 24CrNiMo alloy steel powder was used as experimental material. The microstructure and mechanical properties of as-deposited, quenched and tempered (QT) and stress-relief annealed (SR) specimens were analyzed. The X-ray diffraction (XRD) analysis showed that the specimens in the three states were mainly ferrite (α-Fe), in which the as-deposited samples had puny face-centered cubic (FCC) structure Fe (γ-Fe). The microstructure observation showed that the as-deposited specimens were made up of ferrite, granular bainite, a small amount of cementite and residual austenite. The tensile test results indicated that the tensile strength and yield strength of the as-deposited specimens were 1199 MPa and 1053 MPa respectively, and the elongation at break was 10.7%. The elongation of QT and SR specimens increased to 11.6% and 12.8%, respectively. The electron backscattered scattering detection (EBSD) analysis results showed that the small-angle grain boundary content of the as-deposited samples was 58%, and large-angle grain boundary content was 15%. After QT and SR, small-angle and large-angle grain boundaries were obtained than those in the as-deposited specimens. The high-temperature friction and wear properties and thermal fatigue performances of the QT and SR specimens were improved significantly. The QT specimens had the smallest wear and thermal fatigue crack lengths, excellent resistance to friction and wear performance and prevention of crack growth, with an ideal comprehensive properties.

Investigating the heat input effect of the GTAW process upon the microstructure and HAZ extension of HSLA-100 steel weld joints using thermal cycles

In the current research, the welding of HSLA-100 steel was accomplished with three different heat inputs using the GTAW process. The microstructure of various joint zones was investigated utilizing an optical microscope and scanning electron microscope. For the purpose of assessing the impact of thermal cycles upon the extension and alterations in the heat-affected zone, the thermocouples were placed at distinct distances from the fusion line. The results indicated that the weld metal microstructure comprised of acicular ferrite and a number of polygonal and quasi-polygonal Ferrite with M/A constituent. An epitaxial growth was observed in the fusion line in all heat inputs. It was determined that the heat-affected zone microstructure consisted of two regions, namely a coarse-grained heat-affected zone with a granular and lath bainite microstructure and a fine-grained heat-affected zone with the prevailing microstructure of granular Bainite. The results indicated that increasing the heat input leads to a decrease in the amount of acicular ferrite and M/A constituent volume fraction in the weld metal and an increase in lath bainite in coarse-grained zone. It was found that by increasing the heat input from 0.78kJ/mm to 1.42 kJ/mm, the grain size increases from 29.2μm to 76.5 μm and from 15.7μm to 32.5μm in coarse-grained and fine-grained zone, respectively. The thermal cycles evaluation pertinent to the implanting of a thermocouple disclosed that the heat-affected zone width is extended from 3.6 mm to 4.5mm by increasing the heat input from 0.78kJ/mm to 1.42 kJ/mm.