Bainite Formation Research Articles

Study on Microstructure and Property in Heat-Affected Zone of X80 Pipeline Steel

Experimental methods, such as OM, TEM and property testing, were used to study the HAZ microstructure and property of X80 pipeline steel. HAZ can be divided into three regions, such as coarse grain zone, transition zone and fine grain zone. Granular bainite of CGHAZ consists of bainite ferrite (BF) lath and M/A constituents. Charpy impact energy of the HAZ is much lower than that of base metal, for the grain size of CGHAZ is much larger than that of BM, and crack formation and propagation of granular bainite is easy.

On the Stress Development in SA508 Autogenous Weld

Considering the significant role that residual stresses play in determining the lifetime-service of materials, it is mandatory to have a good understanding of and a means of predicting those that develop during welding processes. For this purpose, a User MATerial subroutine (UMAT) is developed to study the effects of various parameters that influence solid state phase transformations and residual stress evolution during welding of SA508 ferritic steel. The temperature dependent elastic and kinematic hardening parameters for each of the individual phases that can potentially develop during cooling from elevated temperatures are measured and are used for calculating stress development during low (75 mm/min) and high (300 mm/min) speed gas-tungsten arc welding (GTAW) on SA508 grade 3. These two speeds are selected to cover a wide range of cooling rates in the heat affected zone so that different phase proportions would be present. The results of the numerical simulations for residual stresses are compared against those measured by neutron diffraction. It is shown here that a low speed weld results in bainite formation whereas a high speed weld results in bainitic as well as subsequent martensitic phase transformations where each welding rate results in different residual stress development.

The Influence of Thermomechanical Controlled Processing on Bainite Formation in Low Carbon High Strength Steel

Thermomechanical controlled processing is a very important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes the control of the processing important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the roles of austenite conditioning and cooling rates on the bainitic phase transformation. Specimens were compared with and without deformation, and followed by seven different cooling rates ranging between 0.5°C/s and 40°C/s. The CCT curves showed that the transformation behaviors and temperatures are very different. The different bainitic microstructures which varied with austenite deformation and cooling rates will be discussed.

Effects of Coarsening of Cementite in Pearlite before Austenitization on Formation of BWING and Mechanical Property in a Steel

Effect of pre-treatment before austenitization on mechanical property in a steel has been investigated. Pre-treatment was done at 500°C for 600, 1200 and 1800s before austenitization at 1440°C for 120s. This pre-treatment was performed in order to introduce dislocation network into austenite, which acts as nucleation site for “bainite laths within austenite grain (BWING)”. BWING was formed with isothermal holding at 500°C for 10s after austenitization. Micro-Vickers hardness is measured in a steel with and without pre-treatment. The steel with appropriate pre-treatment exhibits high hardness, because large amounts of BWINGs are formed. In the case of cementites in pearlite being coarsened and the distance between cementites becoming large, dislocation network is apt to be introduced into γ. The dislocation network acts as nucleation site for BWING, resulting in the suppression of formation of intergranular bainite. BWINGs strengthen the steel.

Effects of ausforming on isothermal bainite transformation behaviour and microstructural refinement in medium-carbon Si–Al-rich alloy steel

The effects of ausforming temperature, strain and strain rate on isothermal bainite transformation behaviour and microstructural refinement in medium-carbon Si–Al-rich alloy steel were determined by thermomechanical simulation. Results show that ausforming decreased the martensite start temperature and enabled isothermal transformation at low temperatures, resulting in the formation of nanostructured bainite. Decreasing the ausforming temperature and increasing the ausforming strain did not only reduce the incubation of isothermal transformation but also refined bainite lath and enhanced the hardness of the alloy steel. Strain rate showed a weak influence on the transformation behaviour and microstructure of the obtained nanostructured bainite.

Combined Martensite and Bainite Formation from Austenite Decomposition in HSLA Steel

Recent studies have shown the possibility to induce time-dependent phase transformations during isothermal treatment between the martensite start (MS) temperature and martensite finish (Mf,) temperature, i.e. after initial martensite formation. Such treatments result in specific complex microstructures consisting of bainite, martensite and retained austenite, depending on the holding temperature and time. However, the nature of the isothermal transformations below MS is not completely understood and issues like isothermal formation of martensite and bainite formation are still under discussion. The purpose of this study is to investigate the phase transformations from austenite, subsequent to initial martensite formation, during isothermal treatments at different temperatures of HSLA steel. The microstructure development was monitored by means of dilatometry and microstructural characterization of the transformation products by Optical Microscopy, Scanning Electron Microscope, Electron Backscatter Diffraction and X-ray diffraction. The phase transformations and complex competition and interactions between the different transformation mechanisms are discussed.

A Phase-Field Model for the Formation of Martensite and Bainite

The phase field method is rapidly becoming the method of choice for simulating the evolution of solid state phase transformations in materials science. Within this area there are transformations primarily concerned with diffusion and those that have a displacive nature. There has been extensive work focussed upon applying the phase field method to diffusive transformations leaving much desired for models that can incorporate displacive transformations. Using the current model, the formation of martensite, which is formed via a displacive transformation, is simulated. The existence of a transformation matrix in the free energy expression along with cubic symmetry operations enables the reproduction of the 24 grain variants of martensite. Furthermore, upon consideration of the chemical free energy term, the model is able to utilise both the displacive and diffusive aspects of bainite formation, reproducing the autocatalytic nucleation process for multiple sheaves using a single phase field variable. Transformation matrices are available for many steels, one of which is used within the model.

Effects of Cooling Processes on Microstructure Evolution of X80 Pipeline Steel

With the development of pipeline industry, the pipeline steels with higher strength and plasticity, better low-temperature toughness and weldability are the main development trend. For bainitic pipeline steels, M/A constituent is the main hard phase. Although the M/A constituent can enhance the strength, the larger block-form M/A constituent can deteriorate low-temperature toughness. Therefore, it is essential to further investigate how to refine the M/A constituent. In the present paper, X80 pipeline steel was cooled to room temperature with various cooling paths after hot compression deformation at the temperature of 800oC. The evolution of microstructure of X80 pipeline steel has been analyzed by optical microscope (OM) and scanning electron microscope (SEM). The experimental results show that increasing the cooling rate can significantly refine M/A constituent and promote the formation of granular bainite, and the bainitic ferrite can be also greatly refined. In addition, the effects of five final temperatures of fast cooling were also investigated.

Effect of carbon on the density, microstructure and hardness of alloys formed by mechanical alloying

This work aimed to produce iron-based alloys containing resistant microstructures to improve the mechanical properties of the resulting alloy. The effects of both carbon content and compaction pressure on the microstructure, density and hardness of the alloys were examined. Iron-based alloys with initial carbon contents of 0.5%, 1%, 2% and 3% were produced by powder metallurgy following a process that involved ball milling elemental powders, cold pressing and sintering. The composition, density, microstructure, porosity, hardness and ductility of the alloys depended on both compaction pressure and carbon content. As the carbon content increased, the amount of the resistant microstructure bainite in the alloys also increased, as did their hardness. In contrast, the density and ductility of the alloys decreased with increasing carbon content. This study shows that formation of the resistant microstructure bainite in alloys fabricated by powder metallurgy is influenced by both the initial carbon content of the alloy and compaction pressure during cold pressing.

Effect of Austenitization Temperature on Formation of Low Temperature Bainite

The influence of austenitization temperature on the incubation period and bainitic transformation behaviours of the high-carbon silicon steel has been investigated. It was found that the nose temperature of bainite transformation and incubation period decreases with increasing austenitization temperature. The microstructure characteristics of the bainitic transformation products have been also observed. After isothermal heat treatment at 230°C for 20 mins, all samples austenitized at different temperatures produced a bainitic structure, which consists of packets of parallel ferrite laths. The major difference lies in the edge boundary morphology. Bainitic laths formed in low-temperature austenitization conditions has sharp saw-tooth edge boundaries, whereas bainite transformed from high-temperature austenitization conditions, have smooth wedge boundaries. Key Words: austenitization temperature; low-temperature bainite; incubation period;edge boundary

Effect of High Temperature Stays Process on Ultra-Low-Carbon Bainitic Steel without Nb, Ti and other Metal Compounds Forming Elements

The influence of high temperature stays process on the directly cooling experimental ULCB steel had been investigated. We found that the process of high temperature stays had effect on the ULCB steels phase transformation point, and made the CCT curve move to top left. When the cooling rate increased, the formation and growth of granular bainite would be inhibited, but the formation and growth of bunchy bainite would be promoted. And the high temperature stays process had a weaker effect on the specimens structure when specimens were cooled slowly. Key Words: high temperature stays; cooling rate; bunchy bainite; granular bainite

Effect of Welding Heat Input on Microstructure and Mechanical Properties of HSLA Steel Joint

Gas tungsten arc welding of HSLA steel was conducted with different welding heat inputs, and the influences of welding heat input on the microstructure, Vickers hardness, and impact toughness of heat-affected zone (HAZ) in prepared joints were systematically investigated. The microstructure in HAZ with low welding heat input mainly consisted of martensite, and the microhardness of coarse grain HAZ was measured higher than that of fine grain zone. The results show that increasing the welding heat input could suppress the formation of martensite and reduce the microhardness of HAZ. However, the impact toughness of HAZ was not monotonously improved with the increase of welding heat input. It is deemed that the coarsened grain, the formation of upper bainite, and the non-uniform distribution of carbides and inclusions in HAZ could degrade the impact toughness. Tests demonstrate that the optimum comprehensive properties of HAZ were obtained when the welding heat input was 0.67 kJ/mm.

Predicting the Effect of Mo, Ni, and Si on the Bainitic Stasis

The “transformation stasis” phenomenon during the isothermal bainitic ferrite formation in a series of Fe-C-X (X = Mo, Ni, Si ) alloys has been analyzed. Both the Gibbs energy balance (GEB) approach and the \(T_0\) model have been applied to model the “transformation stasis” phenomenon, and their predictions are compared with experimental results. The \(T_0\) model failed in predicting the transformation stasis (TS) phenomenon for the alloys investigated here, while the GEB predictions are in very good agreement with experimental data. It is found that Mo has a very strong effect on the TS phenomenon, while the effect of Si is found to be negligible. Ni has an intermediate effect.

Influence of transformation temperature on carbide precipitation sequence during lower bainite formation

The nature of different carbides formed during the lower bainite reaction at different transformation temperatures was determined by atom probe tomography in three steels with different carbon and silicon contents. It is known that steel composition and temperature alter the carbide precipitation sequence during low-temperature bainite formation. However, present results confirm that dislocations in lower bainite that are more prominent at the lower the transformation temperature, trap a substantial amount of carbon and hence, can influence the carbide precipitation sequence and, in particular, can determine where ɛ-carbide forms before the growth of cementite.

High-efficiency fast-heating annealing of a cold-rolled dual-phase steel

In the present work, fast-heating annealing was performed on a cold-rolled Fe–0.07C–1.7Mn–0.429Si dual-phase steel. In contrast to commercial conventional continuous annealed steel, 6.6% higher ultimate tensile strength and 14.1% greater elongation were obtained. Transmission electron microscopy observations reveal the incomplete ferrite recrystallization, the formation of bainite and fine fiber-like martensite in fast-heating processed dual-phase steel. High-efficiency simplified fast-heating annealing process demonstrates great potential for large-scale production.

Metallurgical Interpretation on Grain Refinement and Synergistic Effect of Mn and Ti in Ti-microalloyed Strip Produced by TSCR

The grain refinement mechanism and synergistic effect of Mn and Ti involved in the Ti-microalloying technology of thin slab casting and direct rolling (TSCR) were elucidated. Because the inevitable precipitation of TiN in high Ti-containing liquid steel decreases the volume fraction of TiN precipitated from austenite and the rapid coarsening rate leads to a large size of TiN particles, a relatively weak inhibition effect on the recrystallized grain growth was obtained compared with that in the low Ti-containing steel. However, the ferrite grain size in high Ti-containing steel can be refined by the so-called non-recrystallization rolling. The complex addition of Mn and Ti can improve the strength and toughness of strip remarkably, and the mechanisms are that Mn decreases the transformation temperature, refines the ferrite grains, and enhances the formation of bainite and TiC precipitation in ferrite.

Effects of Start and Finish Cooling Temperatures on Microstructure and Mechanical Properties of Low-Carbon High-Strength and Low-Yield Ratio Bainitic Steels

The effects of start and finish cooling temperatures on microstructure and mechanical properties of low-carbon high-strength and low-yield ratio bainitic steels were investigated in this study. Four kinds of low-carbon high-strength and low-yield ratio bainitic steels were fabricated by varying the start and finish cooling temperatures and cooling rates, and their microstructure and mechanical properties such as tensile and Charpy impact properties were measured. In the steels cooled down from the high start cooling temperature above Ar1 [978 K (705 °C)], the volume fraction of acicular ferrite is lower than in the steels cooled down from the low start cooling temperature below Ar1 [978 K (705 °C)]. The finish cooling temperatures and cooling rates affect the formation of bainitic ferrite, granular bainite, and martensite–austenite (MA) constituents. According to the correlation between microstructure and mechanical properties, the tensile strength increases with increasing the volume fractions of bainitic ferrite and MA constituents, whereas the elongation decreases. The yield ratio decreases as the volume fraction of MA constituents increases. Charpy impact absorbed energy is proportional to the volume fraction of acicular ferrite, and is inversely proportional to the volume fraction of granular bainite.

On Various Aspects of Decomposition of Austenite in a High-Silicon Steel During Quenching and Partitioning

Using a Gleeble thermomechanical simulator, a high-silicon steel (Fe-0.2C-1.5Si-2.0Mn-0.6Cr) was laboratory hot-rolled, re-austenitized, quenched into the Ms–Mf range, retaining 15 to 40 pct austenite at the quench stop temperature (TQ), and annealed for 10 to 1000 seconds at or above TQ in order to better understand the mechanisms operating during partitioning. Dilatometer measurements, transmission electron microscopy, and calculations showed that besides carbon partitioning, isothermal martensite and bainite form at the partitioning temperature. While isothermal martensite formation starts almost immediately after quenching with the rate of volume expansion dropping all the time, the beginning of bainite formation is marked by a sudden increase in the rate of expansion. The extent of its formation depends on the partitioning temperature following TTT diagram predictions. At the highest partitioning temperatures martensite tempering competes with partitioning. Small fractions of bainite and high-carbon martensite formed on cooling from the partitioning temperature. The average carbon content of the austenite retained at room temperature as determined from XRD measurements was close to the carbon content estimated from the Ms temperature of the martensite formed during the final cooling.

Transformation Characteristics of Ferrite/Carbide Aggregate in Continuously Cooled, Low Carbon-Manganese Steels

Transformation characteristics and morphological features of ferrite/carbide aggregate (FCA) in low carbon-manganese steels have been investigated. Work shows that FCA has neither the lamellae structure of pearlite nor the lath structure of bainite and martensite. It consists of a fine dispersion of cementite particles in a smooth ferrite matrix. Carbide morphologies range from arrays of globular particles or short fibers to extended, branched, and densely interconnected fibers. Work demonstrates that FCA forms over similar cooling rate ranges to Widmanstatten ferrite. Rapid transformation of both phases occurs at temperatures between 798 K and 973 K (525 °C and 700 °C). FCA reaction is not simultaneous with Widmanstatten ferrite but occurs at temperatures intermediate between Widmanstatten ferrite and bainite. Austenite carbon content calculations verify that cementite precipitation is thermodynamically possible at FCA reaction temperatures without bainite formation. The pattern of precipitation is confirmed to be discontinuous. CCT diagrams have been constructed that incorporate FCA. At low steel manganese content, Widmanstatten ferrite and bainite bay sizes are significantly reduced so that large amounts of FCA are formed over a wide range of cooling rates.

Bainitic transformation behavior of ultra-high strength 30CrNi3MoV steel after experiencing small deformation in the nonrecrystallization austenite region

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