Bainite Grains Research Articles

Effect of Mo on the Phase Transformation, Microstructure, and Hardness of Q500qENH Weathering Steel of Bridge Construction during Continuous Cooling

This article investigates the precipitation behavior of Mo element in the continuous cooling phase transformation process of weathering steel of bridge construction and its influence on microstructure and hardness. The results demonstrate that the addition of Mo leads to the narrowing of the phase transformation interval of proeutectoid ferrite and pearlite, the expansion of the bainite transformation zone, a delayed bainite phase transformation process, and the refinement of bainite microstructure. Moreover, theoretical calculations of bainite phase transformation kinetics reveal that the average activation energy for bainite transformation in Mo‐free and Mo‐containing steels is 94.8 and 134.0 kJ mol−1, respectively. The new grains of Mo‐free steel mainly grow in a 2D mode, while Mo containing steel mainly grows in a 1D mode, which confirms the retarding effect of Mo on bainite grain growth. Additionally, Mo significantly increases the number density and refines the size of precipitated phases in weathering steel of bridge construction. Within the cooling rate range of 0.1–30 °C s−1, the hardness of weathering steel of bridge construction is enhanced by Mo, with the magnitude of hardness increment decreasing as the cooling rate increases. This behavior is primarily governed by the combined influence of phase transformation strengthening and precipitation strengthening.

Simultaneous improvement of strength and uniform elongation of ferrite-bainite steel with heterogeneous networked microstructure

A novel heterogeneous networked dual-phase structure, composed of bainite grains surrounding the ferrite grains and forming a networked structure, was successfully introduced into the low-carbon ferrite-bainite steel through a two-step heat treatment process. The ferrite-bainite steel with the networked structure exhibited a high ultimate tensile strength of 872 ± 39 MPa, a good uniform elongation of 15.59 %, and a high total elongation of 22.79 %. Meanwhile, it possessed excellent work hardening behaviors compared to the ferrite-bainite steel with an island feature. The improvement of the comprehensive mechanical properties was attributed to the influence of hetero deformation induced (HDI) hardening and the synergistic deformation of the dual-phase structure.

Micromechanical properties and deformation behavior of the constituent phases in 3rd generation complex phase AHSS: In-situ neutron experiment and crystal plasticity simulation

This paper investigated the micromechanical properties and deformation behavior of the constituent phases in 3rd generation (GEN) complex phase advanced high-strength steel (AHSS). The hybrid method was a comprehensive combination of uniaxial tensile test with digital image correlation (DIC), electron backscatter diffraction (EBSD), in-situ neutron diffraction (NED) and crystal plasticity finite element method (CPFEM). Results reveal that the microstructure of 3rd GEN AHSS exhibits a complex phase of ferrite (46.2 %), bainite (36.1 %), martensite (9.4 %) and austenite (8.3 %), which would be promising microstructure in the development of new-generation complex phase AHSS. The significant findings are: (1) excellent ductility is obtained while high strength is preserved despite small martensite volume fraction, which can reduce the tendency of martensite cracking; (2) macroscopic stress-strain relationships and microscopic hardening properties of constituent phases are successfully determined, which are very important for predicting the overall performance of complex phase AHSS; (3) presence of bainite with high volume fraction plays a critical role in inducing plastic strain localization in ferrite, resulting in the heterogeneous stress/strain distribution. It is disclosed that pronounced strain localization is mostly initiated in ferrite grains while the deformation bands are mainly located in ferrite/bainite grain boundaries. Furthermore, strain partitioning between ferrite and bainite results in the high-stress concentration in bainite grains. As a result, these “hot zones”, i.e., ferrite grains, ferrite/bainite grain boundaries and bainite grains, experience high strain, strain gradient and stress, respectively, which can be a primary origin of ductile fracture initiation in the 3rd GEN complex phase AHSS. The findings reported in the present investigation would be of great interest in the development of new-generation complex phase automotive steels.

Continuous Heating/Cooling Transformation Kinetics of a Novel CrNiMoV Steel for Large Structural Parts

Continuous heating transformation (CHT) and continuous cooling transformation (CCT) characteristics as well as phase transformation kinetics of a novel Cr–Ni–Mo–V steel for large structural parts are studied using a Gleeble‐3500 thermal–mechanical simulator. CCT curves are obtained. The results show that carbides in initial structure can be completely dissolved during CHT as the heating rates are not greater than 0.2 °C s−1. The final microstructure exhibits obvious sensitivity to cooling rate. It contains martensite with or without bainite. Besides, kinetic models of austenitization are established, and it is confirmed that diffusion is the major mechanism of austenitic transformation in steel during CHT. Meanwhile, bainitic phase transformations kinetic model is developed based on the J–M–A model. The fitted Avrami indexes n combined with transmission electron microscope reveal that the bainite nucleation sites of the steel are mainly located at the prior austenite grain boundaries, and the growth of bainite grains mainly follows a 2D mode. The kinetics of martensitic transformation is better modeled based on the K–M equation. A deep understanding of kinetics and microstructure characteristics during CHT and CCT is of great significance to regulate the final performance of the novel steel under development.

Influence of microstructure on the micro-region fracture toughness of the 30Cr2Ni4MoV turbine rotor welded joint

The fracture toughness of each micro-region in 30Cr2Ni4MoV steam turbine rotor welded joint was studied, including the central zone of the SAW bead (SAW-C), the overlapping zone of the SAW bead (SAW-Z), and the central zone of the TIG weld metal (TIG), coarse-grained heating affected zone (CGHAZ), fine-grained HAZ (FGHAZ) and base metal (BM). The microstructure, fracture morphologies and cracking paths were observed by OM and SEM microscope, and the influence of microstructure on fracture toughness was analyzed. The research results show that the fracture toughness of the weld metal is lower than that of BM and HAZ, and the fracture toughness of the weld metal is non-uniform. TIG and SAW-C are composed of columnar tempered bainite, intergranular tempered lath martensite and carbides, and the fracture toughness is the worst. SAW-Z is zigzag columnar tempered bainite and equiaxed granular tempered bainite between weld passes, which has good fracture toughness. CGHAZ and FGHAZ are equiaxed tempered martensite with the best fracture toughness. By observing the fracture morphology and cracking path, SAW-C and TIG are dominated by brittle cleavage fracture mode, and SAW-Z, FGHAZ, CGHAZ and BM are in the ductile fracture mode. The straight columnar grains of SAW-C and TIG are prone to crack propagation, but the tempered bainite acts as a bridge. SAW-Z has a zigzag tempered bainite grains, and the crack propagation path is tortuous. For FGHAZ, CGHAZ and BM with equiaxed grains, the fracture toughness is related to the grain size and the dimple size around the crack tip. The crack propagation process is that the dimples are first formed in a certain range around the crack tip, but only the dimples in the lower strength micro-regions grow and merge preferentially, and rapidly form new cracks.

Microstructure and Mechanical Properties of MIG Welds between 6252 Armor Steel and Q550D HSLA Steel.

The microstructure and mechanical properties of the welded joints of 6252 armor steel and Q550D high-strength low-alloy (HSLA) steel welded by MIG welding were studied. ER70S-G and ER140S-G were used as fillers to obtain welded joints with good formation and no faults. The joint microstructure (OM) analysis showed that a large Widmanstätten structure was observed at the fusion line on the Q550D side, and the apparent grain sizes changed on the 6252 side. Cylindrical ferrite growth along the bainite grain boundary was observed in the ER70S-G filler weld zone, while the ER140S-G filler weld zone was occupied by lower bainite structures. The XRD phase analysis showed that more Fe-Ni-Cr compounds and less ferrite were formed in the ER140S-G filler weld. The hardness test showed that the hardness of the HAZ on the 6252 side was significantly higher than that of the BM and the WZ, and the welded joint obtained by the ER140S-G filler had a higher hardness. The tensile strength test showed that WZ (>772 MPa) had a higher strength than Q550D BM, and the tensile fracture (SEM) was primarily a ductile fracture. The impact test results showed that the welded joint had better impact resistance at room temperature, but the impact absorption energy of the weld and the heat-affected zone was strongly affected by changes in temperature, and brittle fracture occurred easily at low temperatures.

Influence of the Heat Input on the Dendritic Solidification Structure and the Mechanical Properties of 2.25Cr-1Mo-0.25V Submerged-Arc Weld Metal

The alloy 2.25Cr-1Mo-0.25V is commonly used for heavy wall pressure vessels in the petrochemical industry, such as hydrogen reactors. As these reactors are operated at elevated temperatures and high pressures, the 2.25Cr-1Mo-0.25V welding consumables require a beneficial combination of strength and toughness as well as enhanced creep properties. The mechanical properties are known to be influenced by several welding parameters. This study deals with the influence of the heat input during submerged-arc welding (SAW) on the solidification structure and mechanical properties of 2.25Cr-1Mo-0.25V multilayer metal. The heat input was found to increase the primary and secondary dendrite spacing as well as the bainitic and prior austenite grain size of the weld metal. Furthermore, it was determined that a higher heat input during SAW causes an increase in the stress rupture time and a decrease in Charpy impact energy. This is assumed to be linked to a lower number of weld layers, and therefore, a decreased amount of fine grained reheated zone if the multilayer weld metal is fabricated with higher heat input. In contrast to the stress rupture time and the toughness, the weld metal’s strength, ductility and macro-hardness remain nearly unaffected by changes of the heat input.

Microstructural evolution and mechanical integrity relationship of dissimilar metal welding between 2205 duplex stainless steel and composite bimetallic plates

In this paper, shielded metal arc welding on the dissimilar joint between 2205 duplex stainless steel and composite bimetallic plates (304 L stainless steel/10CrNi3MoV steel) with a filler metal E2209 was performed. Furthermore, the microstructure, phase, mechanical properties and intergranular corrosion resistance of the joints were investigated and element distributions of the interfaces were characterized. The results show that austenite transformed to ferrite under the influence of welding thermal cycle, and then a large amount of ferrite appeared in heat affected zone (HAZ) of 2205 duplex stainless steel. Coarse bainite grains were formed in HAZ of the 10CrNi3MoV steel near the fusion line with high temperature welding thermal cycle. Fine granular bainite was also generated in HAZ of 10CrNi3MoV steel due to the relatively short exposure time to the active temperature of grain growth. Local peak temperature near the base 10CrNi3MoV steel was still high enough to recrystallize the 10CrNi3MoV steel to form partial-recrystallization HAZ due to phase change. The filler metal was compatible with the three kinds of base materials. The thickness of the elemental diffusion interfaces layers was about 100 µm. The maximum microhardness value was obtained in the HAZ of 2205 duplex stainless steel (287 ± 14 HV), and the minimum one appeared in HAZ of SS304L (213 ± 5 HV). The maximum tensile strength of the welded joint was about 670 ± 6 MPa, and the tensile specimens fractured in ductile at matrix of the composite bimetallic plates. The impact energy of the weld metal and HAZ of the 10CrNi3MoV steel tested at –20 °C were 274 ± 6 J and 308 ± 5 J, respectively. Moreover, the intergranular corrosion resistance of the weldment including 304 L stainless steel, weld metal, HAZs and 2205 duplex stainless steel was in good agreement with the functional design requirements of materials corrosion resistance.

Recrystallisation before austenitization for improved microstructure and properties in automotive steel sheets with high product of strength and elongation

This study investigated the effect of recrystallization before austenitizing transformation (i.e., pre-recrystallization) on the microstructure and properties of 22Mn2Cr cold-rolled automotive steel sheets based on continuous annealing at 700 °C for various holding times. Subsequently, the samples were heated to an austenitizing temperature of ∼850 °C, which led to the formation of a fine austenite grains via nucleation at the dislocations during reheating. The austenite grain size was more refined when samples were pre-recrystallized at 700 °C for several minutes compared to those directly heated to 850 °C, which was attributed to combined grain refinement from recrystallization and austenite phase transformation. This refined austenite grain size led to a finer bainite grain size in the cooled final product. The strength of the steel decreased slightly (∼1200 MPa), but the elongation (∼19%) and product of strength and elongation (∼23 GPa%) were significantly enhanced by the pre-recrystallization treatment.

Relationship between the Microstructure and Impact Toughness of the Coarse‐Grained Heat‐Affected Zone for Offshore Engineering Steels with Different Mg Contents

The relationship between the microstructure and impact toughness of coarse‐grained heat affected zone for offshore engineering steels with 0.0002 and 0.0042 wt% Mg, termed as 2 and 42Mg, respectively, is clarified. With increasing the Mg content from 0.0002 to 0.0042 wt%, the average impact energy at −40 °C decreases from 201 to 59 J. The prior austenite grain size (PAGS) increases from ≈92 to ≈163 μm, the average thickness of lath bainites increases from ≈234 to ≈361 nm. The addition of Mg makes the particle increase in number and refine in size, but the amount of the effective pinning particles at the temperature close to 1350 °C in 42Mg is smaller than that in 2Mg, which leads to the larger austenite grain growth rate and larger PAGS in 42Mg. The larger PAGS with the higher bainite transformation temperature and smaller PAG boundary density in 42Mg makes the bainite coarser. So, in 2Mg, the effective bainitic grain size with misorientation above 45° is smaller and the density of high‐angle grain boundaries (HAGBs) is larger, resulting in its higher toughness. Besides, the coalesced bainite containing cementite colonies in 42Mg makes the bimodal bainitic lath thicknesses and heterogeneous microhardness distribution, reducing the toughness.

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.

Study of the nature of destruction of castings made of 35HGSL steel after a long service life

Today, castings of their alloy steels are an integral part of modern foundry production. In the process of obtaining castings, any defects inevitably appear. This paper deals with the issues of stone-like destruction of castings made of 35HGSL steel after a long service life. The paper compares the macro-and microstructures of the rock-like fracture and based on the analysis revealed that its causes are large grain and the presence of impurities in steel in the form of sulfides or nitrides, which are located on the grain boundaries and weaken them. The influence of ferrite layers around bainite grains on the presence of the studied defect was also revealed.

Investigation of Hydrogen Embrittlement Susceptibility and Fracture Toughness Drop after in situ Hydrogen Cathodic Charging for an X65 Pipeline Steel.

The present research focuses on the investigation of an in situ hydrogen charging effect during Crack Tip Opening Displacement testing (CTOD) on the fracture toughness properties of X65 pipeline steel. This grade of steel belongs to the broader category of High Strength Low Alloy Steels (HSLA), and its microstructure consists of equiaxed ferritic and bainitic grains with a low volume fraction of degenerated pearlite islands. The studied X65 steel specimens were extracted from pipes with 19.15 mm wall thickness. The fracture toughness parameters were determined after imposing the fatigue pre-cracked specimens on air, on a specific electrolytic cell under a slow strain rate bending loading (according to ASTM G147-98, BS7448, and ISO12135 standards). Concerning the results of this study, in the first phase the hydrogen cations’ penetration depth, the diffusion coefficient of molecular and atomic hydrogen, and the surficial density of blisters were determined. Next, the characteristic parameters related to fracture toughness (such as J, KQ, CTODel, CTODpl) were calculated by the aid of the Force-Crack Mouth Open Displacement curves and the relevant analytical equations.

Tailoring the strength and ductility of T91 steel by partial tempering treatment

T91 ferritic/martensitic steel (modified 9Cr-1Mo steel) with high strength and high ductility is considered one of the promising structural materials for various industrial applications. Recent studies show that equal channel angular pressing can improve the yield strength of T91 steel to 1.6 GPa but degrade the uniform elongation to ∼1%. Here, we report that by using a modified quenching and tempering treatment, T91 steel can achieve a high yield strength of 1.4 GPa and a uniform elongation of ∼5.5%. Extensive microstructure analyses show the formation of transition carbides and ultra-fine martensite within bainite grains. This study provides an alternative approach to obtain a combined enhancement of strength and ductility for T91 steel.

Effect of banding on micro-mechanisms of damage initiation in bainitic/martensitic steels

Multiphase bainitic steels, as a third generation of advanced high strength steels, show promising properties for automotive applications. Understanding the micro-mechanisms of damage initiation during plastic deformation is a key to further mechanical properties enhancement. The topological effect of martensite as a second phase constituent on local damage nucleation activity is significant. This effect has been studied in two different martensite banded microstructures produced by two various annealing cycles. The postmortem damage analysis by a scanning electron microscope on uniaxial loaded samples, revealed more damage nucleation along the dispersed and fragmented martensite phase within martensite banded regions. More pronounced strain partitioning was observed in coarse bainitic grains between adjacent martensite blocks. It is shown that the fracture strain is not controlled by local damage activities, implying that earlier damage initiation or an increased volume fraction of voids does not give rise to a reduced ductility.

Investigation of the Microstructure of Laser-Arc Hybrid Welded Boron Steel

The microstructure of boron steel for automotive driving shaft manufacturing after laser-arc hybrid welding was investigated. Laser-arc hybrid welding technology was applied to 3-mm-thick plates of boron steel, ST35MnB. The temperature distribution of the welding pool was analyzed using the finite element method, and the microstructure of the welded boron steel was characterized using optical microscopy and scanning and transmission electron microscopies. The microstructure of the weld joint was classified into the fusion zone, the heat-affected zone (HAZ), and the base material. At the fusion zone, the bainite grains exist in the martensite matrix and show directionality because of heat input from the welding. The HAZ is composed of smaller grains, and the hardness of the HAZ is greater than that of the fusion zone. We discuss that the measured grain size and the hardness of the HAZ originate from undissolved precipitates that retard the grain growth of austenite.

The Effects of One and Double Heat Treatment Cycles on the Microstructure and Mechanical Properties of a Ferritic–Bainitic Dual Phase Steel

In this study, samples with ferritic–bainitic dual phase structures consisting of 62 pct bainite were obtained from the AISI 4140 steel by applying one and double heat treatment cycles. Microstructural investigations by electron and optical microscopy indicated that the sample heat treated through double cycle benefited from finer ferrite and bainite grains. Additionally, results obtained from mechanical tests implied that the double-cycle heat-treated sample not only has a higher tensile strength as well as ultimate strength but also benefits from a higher ductility along with a higher impact energy than the one-cycle heat-treated sample. Moreover, fractography results showed that the type of fracture in both samples is a combination of the brittle and the ductile fracture. Besides, the ratio of the ductile fracture is higher for the double-cycle heat-treated sample than for the one-cycle sample, due to the lower aggregation of sulfur at grain boundaries.

Promoting of bainitic transformation in pre-alloyed Fe-Cr-Mo steels with different compositions by nickel additions

Nickel is one of the alloying elements promoting the formation of acicular ferrite at the expense of proeutectoid ferrite. The Ni addition reduces the steady-state nucleation rates of allotriomorphic ferrite in Fe-C-Ni alloys. Due to such reasons, Ni was added to modify the microstructure of two sintered steels with different compositions-with the aim of improvement in mechanical properties. Two pre-alloyed Fe-3.0Cr-0.5Mo and Fe-1.5Cr-0.2Mo powders were employed as starting powders for experimental works. The sintered steels were produced under the same cooling rate of 0.1 °C/s. The pre-alloyed powder compositions, differentiated by alloying element contents of Cr and Mo, showed clear influence on the final microstructures of the sintered steels. For the sintered Fe-3.0Cr-0.5Mo-0.3C-xNi steels, additions of ≥ 1 wt.% Ni could change the microstructure from dual-phase type to bainitic structure. Ni additions in the sintered Fe-1.5Cr-0.2Mo-0.3C-xNi steels could also promote bainitic transformation. However, bainitic grain portion in the sintered Fe-1.5Cr-0.2Mo -0.3C-xNi steels was lower than that of the sintered Fe-3.0Cr-0.5Mo-0.3C-xNi steels. The microstructures with dual-phase nature still existed in the sintered Fe-1.5Cr-0.2Mo-0.3C-xNi steels, even Ni additions of up to 3.0 wt.% were conducted. The Ni addition not only altered the sintered steel microstructure but also improved the hardness of sintered steels.

Experimental and numerical investigation on combined girth welding of API X80 pipeline steel

API X80 pipeline steel is widely used in natural gas and oil transportation. In this study, X80 pipeline steels with 18.4 mm thickness and 1219 mm diameter, made in China, were welded by a combined girth welding technology. Experimental results showed that microstructures in weld metal and coarse grain heat affected zone are mainly composed of coarsening bainite grains with dimensions of 20–50 μm, and Vickers hardness in weld seam is lowered by the seven-pass welding process. Simulated results illustrated that repeatedly thermal cycles imposed by the combined multipass girth welding process easily result in softened weld joints. The predicted microstructures, phase compositions and hardness in weld metal are in agreement with the measured ones. The results could be applied to optimise this combined girth welding process and improve the weld quality of API X80 pipeline steel and even higher grade pipeline steels.

Comparative study of the SCC behavior of E690 steel and simulated HAZ microstructures in a SO2-polluted marine atmosphere

Simulated microstructures of the typical CGHAZ, FGHAZ, and ICHAZ of an E690 welded joint were prepared with heat treatment at peak temperatures of 1300, 850, and 750°C, respectively. Slow strain rate tensile (SSRT) tests were implemented to investigate the stress corrosion cracking (SCC) behavior of E690 steel and the simulated HAZ microstructures in a SO2-polluted marine atmosphere. Results revealed that E690 steel and its simulated HAZ microstructures all had a high SCC susceptibility in the simulated SO2-polluted marine atmosphere with a combined mechanism of AD and HE. The SCC susceptibility gradually increased in the following order: BM, CGHAZ, FGHAZ, and ICHAZ. The cracking mode of BM and CGHAZ was transgranular, whereas that of FGHAZ and ICHAZ was intergranular. The lower SCC susceptibility of BM and CGHAZ was probably due to the impeding effect of lath bainitic grain boundaries on SCC propagation, whereas the higher SCC susceptibility of FGHAZ and ICHAZ was attributed to the facilitating effect of M-A islands on crack initiation and propagation.