Bainitic Plates Research Articles

CARBON REDISTRIBUTION UNDER DEFORMATION OF STEELS WITH BAINITE AND MARTENSITE STRUCTURES

In recent years, application of high duration steels, first of all, martensitic and bainitic steels used in manufacturing of parts and structures of crucial function has increased significantly. It is possible to achieve a high duration state by means of effective deformation hardening of steels of various classes at rational use. Understanding of quantitative laws and mechanisms of deformation hardening of steels of different structural classes under active plastic deformation is necessary in terms of targeted formation of structure-phase states and mechanical properties of material. In this work, comparative analysis of structure evolution, phase composition and state of defective substructure of steel with martensitic and bainitic structures under active plastic deformation prior to fracture was performed using transmission electron diffraction microscopy. It was shown that after austenitization at temperature of 950 °C (1.5 hours) and subsequent quenching in oil of 38CrNi3MoV steel and normalization of 30Cr2Ni2MoV steel, a multiphase structure (α phase, γ phase, cementite) is formed, based on martensite of packet morphology (38CrNi3MoV steel) and lower bainite (30Cr2Ni2MoV steel). Obtained quantitative regularities of changes in parameters of steel structure in process of plastic deformation made it possible to carry out studies aimed at analyzing the distribution of carbon atoms in structure of deformed steel. Localization of carbon atoms in martensite structure (38CrNi3MoV hardened steel) and bainite (30Cr2Ni2MoV normalized steel) are revealed. It was established that steels deformation is accompanied by destruction of cementite particles. For hardened martensitic steel, with an increase in degree of deformation, the total number of carbon atoms located in solid solution based on α- and γ-iron decreases, and on structural defects – increases. Redistribution of carbon atoms in steel with bainitic structure with increase in deformation degree consists in growth of number carbon atoms located in α-iron, in defects of the crystal structure, and in intraphase boundaries cementite and its’ subsequent decrease in cementite particles within bainite plates and in γ-iron.

Correlation of Fatigue Limit and Crack Growth Threshold Value to the Nanobainitic Microstructure

The recently developed nanobainitic steels show high ultimate tensile strength (UTS) as well as high ductility. Although this combination seems to be desirable for fatigue design, fatigue limit of nanostructured bainite is often disappointingly low. To improve fatigue properties we tried to earn a fundamental understanding of the microstructural parameters governing fatigue behavior.Therefore our hypothesis to improve the fatigue behavior was not necessarily avoiding the initiation of a fatigue crack – which could lead to failure of the material – but to improve the ability of the present microstructure to slow down or stop growing cracks. Thus, the key to understand the fatigue behavior of nanostructured bainite is to understand the role of the microstructural features which could act as barriers for growing cracks.We tried to correlate our results of fatigue tests and analysis of fracture surfaces to the size of microstructural features like bainitic ferrite plates, crystallographic bainite blocks and packets or prior austenite grains, as well as cracks induced at nonmetallic inclusions. Thereby we found that the crystallographic bainite block size governs fatigue behavior. Additionally, threshold values were determined from crack growth experiments and related to the characteristic microstructural features.

Transformation behavior and microstructure feature of large strain ausformed low-temperature bainite in a medium C - Si rich alloy steel

Large-strain ausforming and low-temperature bainite transformation were carried out in a medium C - Si rich alloy steel on a thermomechanical simulator. Then, temperature–time–transformation curves of the bainite transformation after ausforming at different temperatures were obtained by dilatometry. The effects of ausforming on transformation kinetics, microstructure, and hardness of samples were studied. Results show that the entire process of bainite transformation can be accelerated by ausforming. With decreasing ausforming temperature, the transformation rates greatly increased at the initial transformation stage but slightly decreased at the final stage. Compared with non-ausformed sample, not only was the maximum growth rate in the ausformed samples larger, but the maximum growth rate also appeared earlier. In addition, the maximum rate of the ausformed bainite transformation was increased and the time of reaching the maximum rate was shortened with decreasing the ausforming temperature. The effect of ausforming on bainite transformation kinetics depended not only on the ausforming temperature and strain but also on the isothermal transformation temperature. The reduction in transformation time was increased under decreased isothermal transformation temperature. Ausforming refined the bainite plates and increased the fraction of retained austenite formed in the subsequent low-temperature isothermal transformation. Consequently, nanostructured bainite with enhanced hardness was prepared in the medium C - Si rich alloy steel.

Feasibility study for thixoforming nanostructured bainitic steels

Super Bainite Steel (0.74% C, 1.64% Si, 1.82% Mn, 1% Ni, 0.36% Mo, 0. 21% Cr, 0.047% Al bal. Fe) after cold plastic deformation was used as the starting material for bainitic treatment preceded by controlled cooling from the thixoforming temperature range. Differential Scanning Calorimetry was used to determine the amount of liquid phase fraction vs temperature in the solidus–liquidus range. The steel was heated up to 1430°C, which corresponded to about 30% of liquid fraction. It was then rapidly cooled in oil at three different temperatures: 240°C, 270°C and 300°C. The samples were then held at those temperatures for 5h. The microstructure of the samples after treatment at 240°C showed globular grains (average size of 115µm), containing carbide-free bainite as a mixture of ferrite plates with average thickness of 63nm, and retained austenite plates of 40nm in thickness. Plates of martensite and blocks of austenite were visible in some grains. X-ray studies confirmed the presence of 20.1% of austenite and 79.9% of ferrite with martensite. The sample revealed the highest compression strength of 6651MPa, at yield strength of 1780MPa and compression strain ε=38%. With the increase of isothermal treatment temperature to 270°C and 300°C, yield strength decreased to 1370MPa, 1375MPa and compression strength was 5243MPa, 4138MPa, respectively, while plastic strain reached 38.5% and 25%, respectively. Higher temperature of bainitic treatment led to coarsening of super bainite plates and as a consequence, a decrease in mechanical properties. Initiation of cracks was observed inside grains. They propagate through the globules and eutectic, unlike in thixoformed tool steels, in which the crack path runs along the eutectic mixture between the globular grains. Preliminary studies of the corrosion resistance of Super Bainite steels containing a carbide-free bainite structure were carried out. Results indicate slightly a higher resistance in chlorides environment of the sample isothermally treated at 240°C/5h preceded by controlled cooling from thixoforming temperature range than the sample treated in the conventional way (1000°C/15min/240°C/20h).

Below-Ms austempering to obtain refined bainitic structure and enhanced mechanical properties in low-C high-Si/Al steels

A novel heat-treatment process, below-Ms austempering, was performed on two low-C high-Si/Al steels, and accordingly the resultant microstructures and mechanical properties were investigated. The results showed that bainitic transformation occurs at isothermal temperatures below Ms and unexpectedly, the transformation process is accelerated significantly by the prior formation of martensite. Furthermore, as compared with the conventional above-Ms austempering process, the below-Ms austempering refines effectively the bainitic plates and reduces the size and volume fraction of blocky martensite/austenite phases, giving rise to much enhanced impact toughness and improved tensile properties.

Effect of Single and Multiple Thermal Cycles on Microstructure and Mechanical Properties of Simulated HAZ in Low Carbon Bainitic Steel

The correlation of microstructure and mechanical properties in simulated heat affected zone (HAZ) of a low carbon bainitic steel plate of thickness 28 mm using single and multiple thermal cycles was investigated in the Gleeble 3800 Thermo Mechanical Simulator. Optical microscopy, field emission scanning electron microscopy, hardness, and impact strength measurements were done to characterize the weld HAZ simulated steel samples. Peak temperatures (Tp) of 1300, 1150, 1000, 900, 800, 700, and 600°C with heat input 50 kJ/cm were used for single pass weld. In two pass weld cycles, peak temperature (Tp1) 1300°C, and (Tp2) of 1100, 1000, 900, 800, and 700°C were used. This heat input is in line with the submerged arc welding (SAW) process extensively used in the fabrication of ship building industries. The impact toughness of weld HAZ simulated specimens was determined at −50°C by using a Charpy impact tester. Best impact toughness values (minimum 75 J) in weld HAZ simulated specimen were observed at the peak temperatures from 600 to 1150°C in single pass weld and from 1000 to 1100°C in two pass weld HAZ thermal cycles.

Corrosion Resistance of The Bearing Steel 67SiMnCr6-6-4 with Nanobainitic Structure

AbstractThe paper describes a comparative study of the corrosion resistance of bearing steel 67SiMnCr6-6-4 after two kinds of nanostructuring treatments and two kinds of conventional quenching and tempering treatments. The nanostructuring treatment consisted of austempering with an isothermal quenching at 240°C and 300°C. The conventional heat treatment consisted on quenching and tempering at 350°C for 1 h and quenching and tempering at 550°C for 1 h. Time and temperature of tempering was chosen so that the hardness of both samples (nanostructured as well as quenched and tempered) was similar. The microstructure of steel after each heat treatment was described with the use of transmission electron microscopy (TEM). It was shown, that the austempering conducted at 240°C produced homogenous nanobainitic structure consisting of carbide-free bainite plates with nanometric thickness separated by the layers of retained austenite. The austempering at 300°C produced a sub-micrometric carbide-free bainite with retained austenite in form of layers and small blocks. The conventional heat treatments led to a tempered martensite microstructure. The corrosion resistance study was carried out in Na2SO4acidic and neutral environment using potentiodynamic and electrochemical impedance spectroscopy (EIS) methods. The corrosion resistance of nanostructured steel samples were compared to the steel samples with tempered martensite. The obtained results indicate, that the corrosion resistance of bearing steel with nanobainitic structure is similar to steel with tempered martensite in both acidic and neutral environment. This means that the high density of intercrystalline boundaries in nanobinite does not deteriorate the corrosion properties of the bearing steel.

Effect of a quenching–long partitioning treatment on the microstructure and mechanical properties of a 0.2C% bainitic steel

A quenching–long partitioning (Q–LP) heat treatment has been proposed for large-scale industrial production of steel components. Compared to the conventional quenching–tempering (Q–T) and quenching–partitioning (Q–P) process, the steel treated by air-quenching from austenitizing temperature and 60min-partitioning at 200°C exhibits the best combination of strength and toughness. With a nearly unchanged product of strength and elongation of around 18GPa%, the room-temperature impact toughness has been significantly improved from 84J/cm2 for the air Q–T treatment to 104J/cm2 for the Q–LP treatment, which is attributed to the more retained austenite and the effective relief of the microstresses. The multi-phase microstructure of the Q–LP treated steel contains martensite, bainite and retained austenite exhibiting two different morphologies, i.e., one is the inter-lath plates, and the other is the austenite films within the bainite plates.

Evolution of microstructure in 100Cr6 steel after cooling from a thixoforming temperature to bainitic transformation ranges

Abstract A new concept for the isothermal heat treatment of thixo-elements, consisting of controlled cooling from a semi-solid metal processing (SSM) range, was proposed. 100Cr6 steel (0.97% C, 1.4% Cr, 0.4% Si, 0.4% Mn bal. Fe) after forging was used as the starting material. The DSC analysis was used to determine the liquid phase fraction vs temperature in the solidus–liquidus range. The temperatures and kinetics of bainite transformation after cooling the steel in the semi-solid state were calculated based on the Chester and Bhadeshia models. The steel was heated up to 1425 °C in order to obtain about 25% of the liquid fraction. Then it was cooled in oil at three different temperatures: 135 °C, 235 °C and 335 °C, at which the samples were held for 5 h. The microstructure of the samples annealed at 135 °C consisted of globular grains (the average size of 323 µm), where coarse needles of martensite were observed. They were surrounded by an eutectic mixture of chemical composition different from that of the globular grains. The electron diffraction pattern (SAEDP) showed reflections from α′-Fe and from Fe3C carbides. The X-ray diffraction confirmed the presence of martensite, austenite and carbides to the amount of 74.5%, 22% and 3.5% respectively. The average hardness of samples was 735HV10, while the compression strength attained 3810 MPa at the plastic strain of 8.6%. The samples cooled down to 235 °C also showed globular grains surrounded by the eutectic mixture. The TEM studies allowed researchers to identify lower bainite with a plate thickness of about 500 nm (SAEDP from that area showed reflections from α-Fe and Fe3C carbides). X-ray diffraction confirmed the presence of 90% ferrite, 6% austenite and 4% carbide. The average hardness of samples annealed at 235 °C decreased to 627HV10, while the compression strength decreased to 3100 MPa. The plastic strain increased to 32%. The microstructure of the samples after cooling and treatment at 335 °C consisted of globular grains, inside which upper bainite plates were present. The average hardness was 519HV10, while the compression strength decreased to 1420 MPa. The obtained experimental results correspond to the assumed model, in which the range of the bainitic transition terminates at the temperature from 135 to 370 °C.

Effect of tempering upon the tensile properties of a nanostructured bainitic steel

The tensile properties of a nanostructured carbide-free bainitic steel formed at 200–250°C are compared against those after tempering sufficiently to remove the retained austenite. Although significant ductility is observed following tempering, a comparison of tempered and untempered samples shows that it is in fact reduced when a comparison is made at identical strength. The shape of the stress–strain curves shows clear evidence that the capacity for work hardening is reduced with the loss of austenite. The nanostructure of the steel transformed at 250°C is examined by transmission electron microscopy, to compare the as-transformed to the tempered structure. In this case after tempering at 500°C the energy absorbed during the tensile test is lower, due to the lower strength. Reduction of strength is caused by the slight coarsening of the bainite plates, and lower dislocation density after tempering. Considering the formation of carbide particles in high strength steel, impressive ductility is exhibited even in the tempered condition.

Acceleration of regeneration treatment for nanostructured bainitic steel by rotary impacting trailed welding

The method called rotary impacting trailed welding (RITW) was proposed to accelerate the regeneration treatment of nanostructured bainitic steel welded joint. Stress and strain behaviors of welding metal under this mechanical process were analyzed using finite element software. Rotary and impacting could simultaneously take effect to generate large plastic deformation in the welded joint. Detailed changes of microstructures in the welded joints were characterized by optical microscopy and scanning electron microscopy. Results show that the large deformation of austenite in the welded joint can accelerate the bainite transformation and reduce the regeneration time. Meanwhile, large shear deformation in the austenite generated during welding has a strong effect on the bainite growth, which results in curved bainite plates.

Dilatometric and microstructural response of variant selection during α′ transformation

Variant selection during displacive transformation is popularly monitored by assessing the pole figure or orientation density function of the bulk texture or pole figures from single prior austenite grains. It can also be done by measuring the physical orientation of α′ (bainite or marteniste) plates in the microstructure. It is known that when variants of α′ form randomly, i.e. there is no variant selection, the shear strain associated with each α′ plate gets canceled and transformation strain becomes equal to the volume strain. Under this condition, orientation of sheaves of α′ also shows random distribution. In this work, it has been shown how transformation strain and orientation of sheaves change when variant selection occurs. Mathematical models have already been developed to calculate the transformation strain under various loading conditions; extensive experiments have been conducted during the course of this work to show how the nature of externally applied stress influences the transformation strain. In addition to this, it has also been shown that the variant selection might leave its trace in the microstructure by affecting the alignment of the sheaves under externally applied stress and strain. Mathematical models have been developed to show that variant selection has negligible influence on physical orientation of α′ plates when transformation occurs under external stress, but prior plastic deformation can dictate the orientation to a great extent.

In situ measured growth rates of bainite plates in an Fe-C-Mn-Si superbainitic steel

The growth rates of bainite plates in an Fe-C-Mn-Si superbainitic steel were investigated by in situ observation. The lengthening rates of ferrite bainite during both cooling and isothermal holding processes were observed and the growth rates of bainite plates nucleating at grain boundaries, within grains and on preformed bainite were measured. It is indicated that the lengthening rates of bainite plates during the cooling and isothermal processes were different, and that the growth rates of bainite plates nucleating at different types of sites also demonstrated diversity. The bainite plates initiating at grain boundaries during cooling grew the fastest, while the plates nucleating on preformed bainite did the slowest. However, the growth rate of the bainite plates nucleating at grain boundaries during isothermal transformation decreased the most, whereas the bainite plates initiating within grains grew the fastest. In addition, the growth rate of ferrite bainite in the study supported the diffusion transformation mechanism of bainite from the viewpoint of growth rate.

Effect of Austenitizing Processes on Isothermal Quenching Microstructure in Bearing Steel

Austempering was applied to bearing steel to improve the performance. The effect of austenitizing temperature and time on isothermal quenching microstructure in bearing steel was investigated via microstructure observation and fracture analysis. The results show that with the increase of austenitizing temperature, the size of bainite plate and residual austenite content increase. Austenitizing at lower temperature, the samples has a fracture composite fracture morphology of polygon cleavage unit and even small dimple.The fracture morphology of sample austenitized at 1050 °C is quasi-cleavage fracture with bar cleavage unit which is corresponding to the coarse acicular microstructure. the size of bainite plate increase with increasing the austenitizing time. As the result of comprehensive effect of fine grain strengthening and dispersion strengthening, that the microhardness firstly increases, and then obviously decreases with the increase of austenitizing time.

Continuously Cooled Ultrafine Bainitic Steel with Excellent Strength–Elongation Combination

Serious efforts have been made to simultaneously improve the strength and ductility of steels for different applications. However, steel with the ultimate tensile strength (UTS) above 1200 MPa with minimum elongation of 20 pct is still difficult to produce. In the current work, an effort has been made to design such a steel that could be directly produced in any hot strip mill, after accelerated cooling on the runout table followed by coiling. Basically this steel consisted of C, Mn, Si, and Cr and the intended final microstructure at room temperature was about 80 pct carbide-free bainite and 20 pct retained austenite. The steel was exposed to a thermal treatment which is generally experienced by a hot strip coil. This newly developed steel possesses an UTS of minimum ~1370 MPa with minimum ~21 pct elongation. The combination of such encouraging mechanical properties can be primarily attributed to the formation of ultrafine bainite plates (~100 to 130 nm) and a high density of dislocations arising out of the bainitic transformation. The presence of sufficient quantity of retained austenite (minimum 21 pct) in the final microstructure could be the reason for the attainment of outstanding ductility values at such a high strength level.

The Influence of Ti/N Ratio on the Effective Boron and Mechanical Properties of Low Carbon Boron Alloyed Bainitic Steel

The influence of Ti/N ratio on the effective boron and mechanical properties was investigated by analyzing data from low carbon boron alloyed bainitic steel plates. The result shows Ti/N ratio varies with effective boron value. Less than 50% effective boron was obtained when Ti/N ratio is below 3.3, nearly 90% effective boron is obtained when ratio Ti/N is more than 4; Adding enough Titanium is an effective and economic way to improve qualified ratio of bainitic steel plate. The Ti content between 0.010% and 0.030% does not have obvious effect on the toughness of the bainitic steel;

Multi-step isothermal bainitic transformation in medium-carbon steel

The effect of multi-step low-temperature bainitic transformation on the microstructure and mechanical properties of a medium-carbon steel was investigated. Compared with the microstructure obtained by conventional bainitic transformation, the blocky microstructure was almost eliminated due to the formation of higher volumes of nanoscale bainitic–ferrite plates and film-like austenite, which lead to a refinement in the average thickness of the bainite plates. The samples with refined microstructure showed superior mechanical properties.

Evaluation of potential of high Si high C steel nanostructured bainite for wear and fatigue applications

The present study is concerned with the potential of high carbon, high silicon steel grades isothermally transformed to bainite at low temperature (<300°C). The first part gives an overview of the design principles, allowing very high strength and ductility to be achieved while minimising transformation duration. Wear and fatigue properties are then investigated for over 10 variants of such materials, manufactured in the laboratory or industrially. The results are discussed against published data. Tensile strengths above 2 GPa are routinely achieved, with, in one case, an exceptional and unprecedented total elongation of over 20%. Bainite plate thickness and retained austenite content are shown to be important factors in controlling the yield strength, though additional, non-negligible parameters remain to be quantified. Rolling–sliding wear performances are found to be exceptional, with as little as 1% of the specific wear rate of conventional 100Cr6 isothermally transformed to bainite. It is suggested that this results from the decomposition of retained austenite in the worn layer, which considerably increases hardness and presumably introduces compressive residual stresses. Fatigue performance was slightly improved over 100Cr6 for one of the two industrially produced materials but significantly lower otherwise. Factors controlling fatigue resistance require further investigations.

In Situ Observations of the Formation of Fine‐Grained Mixed Microstructures of Acicular Ferrite and Bainite in the Simulated Coarse‐Grained Heated‐Affected Zone

The formation of fine‐grained mixed microstructure of acicular ferrite and bainite in the simulated coarse‐grained heated‐affected zone was investigated by means of in situ observation and EBSD analysis. Single, multiple, and sympathetic nucleation of acicular ferrite were observed. Acicular ferrite laths or plates were formed prior to bainite and effectively partitioned austenite grains into many smaller and separated regions. The later formed bainite at lower temperature was confined in those smaller regions, thus resulting in the fine‐grained mixed microstructures of acicular ferrite and fine bainite plate.

Influence of Co and Al on Bainitic Transformation in Super Bainitic Steels

AbstractThe effect of cobalt and aluminum addition on the bainitic transformation of super bainitic steels was investigated. Nanostructured bainitic ferrite and retained austenite microstructures were obtained when the steels were transformed at low temperatures. The bainitic transformation was accelerated by the addition of Co and Al. The thickness of bainite plate was reduced and the volume fraction of bainite was increased in the Co and Al containing steel, compared with the parent steel.