Bainite Packet Research Articles

Microstructure and Mechanical Properties of Hot Rolled TRIP Steel Based on Dynamic Transformation of Undercooled Austenite

The microstructure characteristics and tensile properties in a 0.2C-1.5Mn-1.0Al-0.50Si, high strength hot rolled TRIP steel obtained by a new processing based on dynamic transformation of undercooled austenite(DTUA) were investigated. The results show that the main feature of the new technology is that the ferrite was produced by the applied strain during DTUA. Characterization by means of optical and scanning electron microscopy, transmission electron microscopy and X-ray diffraction has shown that the microstructure of the investigated steel contained a ferrite matrix with fine grain size, bainite with small bainitic packets, and high volume fraction of retained austenite with a large number of granular retained austenite. Tensile testing indicates the steels produced by this processing have higher strength (790MPa) and total elongation (35%) as well as low yield ratio..

Hot-Rolled TRIP Steels Based on Dynamic Transformation of Undercooled Austenite

A novel thermomechanical process to manufacture hot-rolled transformation-induced plasticity (TRIP) steels was developed based on dynamic transformation of undercooled austenite (DTUA). Between DTUA and the isothermal bainitic treatment, only one-step controlled-cooling was required. The microstructure evolution of hot-rolled C-Mn-Si and C-Mn-Al-Si TRIP steels based on DTUA was investigated by hot uniaxial compression tests using a Gleeble1500 simulation test machine. The results indicated that during DTUA, the kinetics of ferrite formation was fast, the volume fraction of ferrite formed was determined by applied strain. In comparison with the process based on static transformation of austenite, a more uniform multiphase microstructure with fine ferrite grains was formed, the bainite packets were small and had relatively random orientations, the retained austenite distributed uniformly and had relatively high volume fraction. Hot-rolled TRIP steels based on DTUA demonstrated better mechanical properties, especially for C-Mn-Al-Si TRIP steel.

Effect of acicular ferrite formation on grain refinement in the coarse-grained region of heat-affected zone

The microstructure of acicular ferrite and its formation for the grain refinement of coarse-grained region of heat-affected zone of high strength low-alloy bainite steels were studied using three-dimensional reconstruction technique. Crystallographic grain size was analyzed by means of electron backscatter diffraction. It was revealed that the microstructure in the coarse-grained region of the heat-affected zone consisted of predominantly bainite packets and a small proportion of acicular ferrite. Acicular ferrite was of lath or plate-like rather than needle or rod-like morphology. Tempering of the coarse-grained region of heat-affected zone showed that the acicular ferrite was more stable than the bainite, indicating that the acicular ferrite was formed prior to bainite. The acicular ferrite laths or plates divided the prior austenite grains into smaller and separate regions, and confining the bainite transformed at lower temperatures in the smaller regions and hence leading to the grain refinement in the coarse-grained region of the heat-affected zone.

MICROSTRUCTURE CONTROL OF HOT ROLLED TRIP STEEL BASED ON DYNAMIC TRANSFORMATION OF UNDERCOOLED AUSTENITE I. Prior Austenite Grain Size

Low-alloy multiphase transformation-induced plasticity (TRIP) steels offer excellent mechanical properties combining high-strength levels with a well ductility. This results from the com- plex synergy between different phases, i.e., ferrite, baintie and retained austenite in them. An in-depth understanding of the novel hot rolled TRIP processing based on dynamic transformation of undercooled austenite (DTUA) is required to study the microstructure evolution under different treatment condi- tions. The parameters determining the stability of the metastable austenite and mechanical property of TRIP steel were revived and investigated experimentally, with a special attention paid to the effect of prior austenite grain size and cooling rate after DTUA on its microstructure and mechanical property. The results show that the kinetics of ferrite during DTUA is accelerated when the prior austenite grain size is small. In addition, reducing the grain size of austenite, the distribution of ferrite, bainite and retained austenite becomes more uniform. Moreover, the size of bainite packets and bainitic ferrite becomes smaller, the volume fraction and carbon content of retained austenite higher, and the finer granular retained austenite vast and distributed uniformly, which result in the investigated steel having

Quantitative Analysis of Variant Selection and Orientation Relationships during the γ-to-α Phase Transformation in Hot-Rolled TRIP Steels

The orientation relationships that apply to the fcc (γ) – bcc (α) phase transformation in high-performance hot-rolled TRIP-aided steels were characterised by EBSD techniques. A statistical treatment of the experimental data allows the mean orientation relationship to be determined. This mean orientation relationship was compared to the models commonly proposed in the literature and confronted qualitatively to the predictions of the phenomenological theory of martensite crystallography (PTMC). The variant selection phenomenon was also characterized quantitatively at the level of individual austenite grains. The reconstruction of the EBSD maps evidences that bainite grows by packets in which the bainite laths share a common {111}γ plane in the austenite. This growth mechanism is not influenced by the prior hot deformation of the austenite. The hot deformation has a critical influence on the number of packets that forms. The analysis of the crystallographic features of the bainite packets reveals that all possible variants are formed in a packet, though in different proportions.

FE modelling of bainitic steels using crystal plasticity

Models classically used to describe the probability of brittle fracture in nuclear power plants are written on a macroscale. Physical phenomena are not naturally captured by this type of approach, so that application of these models far from their identification domain (such as temperature history, loading path) may become questionable. To improve the quality of the prediction of resistance and life time, microstructural information describing the heterogeneous character of the material and its deformation mechanisms has to be taken into consideration. The objective of this work is to propose a model able to describe local stress and strain fields in an A508 Cl3 bainitic steel. This information will then be used to introduce critical variables for multiscale failure models. The microstructure of A508 Cl3 steel is made of bainitic packets (coming from former austenitic grains), which are not randomly oriented. An accurate model must take the actual microstructure into account, in order to provide realistic local stress and strain fields. Starting from experimental observations of the bainitic microstructure (texture measurements, morphological analysis), the paper first proposes a numerical model able to produce quantitatively representative numerical aggregates, then discusses the results obtained by a finite element treatment of the aggregates involving crystal plasticity.

The Ultra-Fine Bainitic Steels and Refinement Technology

By employing the new developed relaxation-precipitation-controlling phase transformation (RPC) technique in large scale production the bainitic steels with ultra fine bainite has been obtained. These bainitic steels have good synergistic properties. With the aid of thermal simulation the refinement mechanism of RPC technique has also been investigated. The optical microscope, scanning electron microscope, transmission electron microscope and Electron back scattering diffraction technique were employed to study the features of microstructure produced by RPC technique, precipitation and the evolution of dislocation configuration during the relaxation. The results show that when produced by RPC technique the microstructure of the steel is mainly ultra-fine lath bainite packets, and these bainite packets block each other. It is also found that during the relaxation the dislocation cells form and strain induced precipitation occurs, the dislocation cell pinned by the precipitates can confine the bainite transformation. After the relaxation during the cooling the acicular ferrite forms at first and in succeeding the bainite transformation is blocked by the acicular ferrite and the bainite is refined effectively.

The Development of Ultra-Fine High Performance Low Carbon Bainitic Structural Steel

For the microalloyed low carbon steel, after the steel was hot deformed in the non-recrystallization temperature, the relaxation of the austenite effected the refinement of intermediate transformation phase. The phase transformation start temperatures rose with the relaxation time increasing. By refining the bainitic packet and controlling multi-intermediate transformation phases, the high performance ultra-fine low carbon bainitic steel has been developed.

Effect of Niobium on HAZ Microstructure and Toughness of HSLA Steels

Experimental low alloy steels containing different levels of C, Nb and Mn were employed, to each of which a simulated thermal cycle was applied to produce CG-HAZ microstructure. Effect of Nb on the microstructure was observed and it was found that it has some beneficial effects, such as increasing the hardenability, which lead to retardation of GB ferrite and enhancement of intragranular ferrite formation in low and medium carbon steels. For high carbon steels, it enhances the carbides precipitation and as a result the MA area fraction decreases. The initiation site of brittle fracture was found to be dependent on the microstructure. Three categories of microstructure are described; namely grain boundary ferrite with Widmnastatten ferrite plus areas of degenerated pearlite, which is denoted as category I; grain boundary ferrite plus intragranular acicular ferrite which is called category 11; and finally the lath structure which is called category III. The brittle fracture was observed to be initiated at the pearlite colonies in category I, to be associated with the grain boundary ferrite in category II, and to be initiated at the intersection of the bainitic packets in category III. The area fraction ofMA increases by Nb addition for low and medium Carbon steels but for high Carbon steels, MA area fraction shows a slight increase due to carbides precipitation. MA is not likely to play an essential role in the brittle fracture initiation. Mn is the most dominant element affects the formation of MA at the same carbon equivalent. Therefore the beneficial effect ofNb could be obtained through the optimization of alloy design mainly Mn content.

Isothermal transformation of high-nitrogen austenite

The isothermal transformation of high-nitrogen austenite occurring at 225 °C was found to be a γ → α-Fe + γ′-Fe 4 N upper bainitic one, which inhomogeneously and preferentially occurred at the grain boundaries and at the dislocations in the interior of the prior austenite grains, producing a microstructure composed of (α-Fe + γ′-Fe 4 N) bainitic packets and discrete retained austenite patches. The (α-Fe + γ′-Fe 4 N) bainitic packets were morphologically strikingly different from the tempered martensite produced in the same material.

Crystallography of upper bainite in Fe–Ni–C alloys

The upper bainite structures were investigated in Fe–9Ni–(0.15–0.5)C (mass%) alloys transformed at temperatures between 723 and 623 K. A bainite packet similar to lath martensite is formed and is partitioned into blocks containing lath-shaped bainitic ferrite with the same parallel close-packed plane relationship of the K–S relationship. Its feature is categorized into three types: (A) a packet contains laths of two K–S variants with a small misorientation (sub-blocks), (B) a packet is divided by blocks largely misoriented and each block contains sub-blocks and (C) a packet is divided by blocks containing a single variant of laths. A packet of type (A) is formed at 723 K in each alloy whereas the packet type becomes (B) or (C) at lower temperatures. As a result, bainite blocks are refined with decreases in transformation temperature and carbon content. In contrast, blocks and packets of lath martensite are refined with increasing carbon content.

Crystallography of ausformed upper bainite structure in Fe–9Ni–C alloys

The effect of ausforming on formation of upper bainite structure was investigated in Fe–9Ni–(0.15–0.5)C (mass%) alloys, focusing on the crystallographic feature of bainitic ferrite. The specimens were ausformed at 1073 or 873 K by 50% compressive reduction after austenitizing, and transformed at 623 K. Lath-shaped upper bainite structures similar to lath martensite are formed at 623 K. A bainite packet is partitioned into blocks of all the six variants of the K–S orientation relationship satisfying the same parallel close-packed plane relationship. In both of the non-ausformed and ausformed specimens, bainite blocks are coarsened with increasing carbon content. In the Fe–9Ni–0.15C alloy, the effect of ausforming on the bainite block width is small. For higher carbon alloys (Fe–9Ni–0.3C and 0.5C), bainite blocks are remarkably coarsened by the ausforming. In contrast, the ausforming refines lath martensite blocks in the Fe–9Ni–0.15C alloy but has a small effect in higher carbon alloys.

Microstructural parameters governing cleavage fracture behaviors in the ductile–brittle transition region in reactor pressure vessel steels

The fracture behaviors in the ductile–brittle transition region of reactor pressure vessel (RPV) steels with similar chemical compositions but different manufacturing processes were examined in view of cleavage fracture stress at crack-tip. The steels typically had a variation in grain size and carbide size distribution through the different manufacturing processes. Fracture toughness was evaluated by using a statistical method in accordance to the ASTM standard E1921. From the fractography of the tested specimens, it was found that fracture toughness of the steels increased with increasing distance from the crack-tip to the cleavage initiating location, namely cleavage initiation distance (CID, Xf) and its statistical mean value (KJC(med)) was proportional to the cleavage fracture stress (σf) determined from finite-element (FE) calculation at cleavage initiating location. On the other hand, σf could also be calculated by applying the size of microstructural parameters, such as carbide, grain and bainite packet, into the Griffith’s theory for brittle fracture. Among the parameters, the σf obtained from the mean diameter of the carbides above 1% of the total population was in good agreement with the σf value from the FE calculation for the five different steels. The results suggest that the fracture toughness of bainitic RPV steels in the transition region is mostly influenced by only some 1% of total carbides and the critical step for cleavage fracture of the RPV steels should be the propagation of this carbide size crack to the adjacent ferrite matrix.

Fine carbide-strengthened 3Cr-2WVTa bainitic steel

Both the 3Cr-3WV and the 3Cr-2WVTa steels exhibit an acicular bainite microstructure under the normalized and the normalized-and-tempered condition. The addition of Ta to the 3Cr-3WV steel substantially decreases the prior austenite grain size, but it has little effect on the bainite packet size. Fine TaC precipitates are formed in the normalized 3Cr-3WVTa specimen. After further tempering of 3Cr-3WVTa steel, fine TaC particles are further precipitated and dispersed within grains. The carbides at the prior austenite grain boundaries in the Ta-containing steel are much smaller than those in the steel without Ta. Tensile tests and fracture toughness (K IC ) tests have been performed on both the 3Cr-3WV and 3Cr-3WVTa steels at room temperature. The 0.2 pct yield strength of the Ta-containing steel is higher than that of the steel without Ta, especially under the normalized-and-tempered condition. The 3Cr-3WVTa steel is primarily strengthened by a secondary-phase precipitation mechanism represented by the formation of fine carbides after tempering. The 3Cr-3WVTa steel exhibits higher fracture toughness than the 3Cr-3WV steel. The toughening mechanism is also discussed based on the dependence of the calculated fracture stress upon the carbide size and the bainite packet size.

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

The effect of the welding cycle on the fracture toughness properties of high-strength low alloy (HSLA) steels is examined by means of thermal simulation of heat-affected zone (HAZ) microstructures. Tensile tests on notched bars and fracture toughness tests at various temperatures are performed together with fracture surface observations and cross-sectional analyses. The influence of martensite-austenite (M-A) constituents and of “crystallographic” bainite packets on cleavage fracture micromechanisms is, thus, evidenced as a function of temperature. Three weakest-link probabilistic models (the “Master-curve” (MC) approach, the Beremin model, and a “double-barrier” (DB) model) are applied to account for the ductile-to-brittle transition (DBT) fracture toughness curve. Some analogy, but also differences, are found between the MC approach and the Beremin model. The DB model, having nonfitted, physically based scatter parameters, is applied to the martensite-containing HAZ microstructures and gives promising results.

Austenite to bainite phase transformation in the heat-affected zone of a high strength low alloy steel

The austenite to bainite phase transformation was investigated in a low alloy structural steel after simulated welding heat treatment, by means of light microscopy, electron backscatter diffraction and transmission electron microscopy. Upper bainite packets result from the growth of groups of laths having close crystallographic orientations but highly misoriented habit planes. Self-accommodation of the transformation eigenstrain was evaluated for various bainite configurations using a micromechanical model. The observed pairs of variants seem to help limiting plastic strain in the austenite phase, thus enhancing growth of the bainite phase during cooling.

Correlation between Charpy impact energy and J fracture toughness for thermally embrittled reactor pressure vessel steel

The Charpy impact energies of a reactor pressure vessel steel in the as received and several thermally embrittled conditions have been tentatively correlated to three J fracture toughness parameters derived under quasi-static loading regimen. Very good correlation has been achieved over the whole fracture resistance range of the structural steel, as obtained by the application of special heat treatments. It has been established that the parameters controlling the impact energy absorption capacity of the materials are the equivalent grain size of dual phase (ferrite/bainite) annealed microstructures and the bainite packet size of single phase quenched and tempered materials. The dependence of the Charpy impact energy of precracked, side grooved bend bars on the representative grain size of the microstructures tested has been disclosed as a Hall-Petch relationship.

Grain size effects in the charpy impact energy of a thermally embrittled RPV steel

The Charpy impact energies of a reactor pressure vessel (RPV) steel in the as-received and several thermally embrittled conditions have been assessed on the basis of microstructural parameters. It has been concluded that the parameters controlling the impact-absorbed energy of pre-cracked and side-grooved Charpy test specimens are the equivalent grain size of dual-phase annealed microstructures, and the bainite packet size of single-phase quenched and tempered materials. The Charpy energy has been correlated very well with the reduction in area and true fracture strain of tensile specimens, which could be inferred as grain size governed mechanical properties.

Electron backscattering diffraction analysis of secondary cleavage cracks in a reactor pressure vessel steel

An analysis of secondary cleavage cracks in the A 508 C1.3 bainitic steel has been carried out using electron backscattering diffraction (EBSD) and scanning electron microscopy (SEM). The crystallographic orientation in the vicinity of the secondary cracks was studied. Secondary cleavage crack propagation planes were identified to be {100}, {110}, {112} and {123}. The secondary cracks propagated mostly in the range of one crystallographic grain (bainitic packet) and were arrested on high-angle twist type boundaries or in the upper bainite carbide colonies. The size of cleavage facets on the fracture surface corresponds to the size of the bainitic packets.

Grain-Size Effects in the Quasi-Static Fracture Resistance of a Thermally Embrittled RPV Steel (Quenched and Tempered Microstructures)

The elastic-plastic fracture toughness and crack extension behavior under the quasi-static loading regimen of several thermally embrittled conditions of a nuclear reactor pressure vessel (RPV) steel were assessed on the basis of microstructural parameters. It was discovered that the bainite packet size is the fracture properties controlling parameter of single-phase quenched and tempered microstructures. Results were found in close agreement to those obtained in a parallel study with dual-phase annealed microstructures derived from the same low alloy steel. Similarly, it was concluded that a Hall-Petch type relationship correlates J-fracture mechanics criteria to the grain size.