Bainitic Low-alloy Steels Research Articles

Effect of Carbon Content on Variant Pairing in Bainitic Low Alloy Steel

Variant pairing in bainite was evaluated in four different commercial low alloy steels with medium to high carbon content. The steels investigated were austempered in the temperature range 275 °C to 450 °C to obtain a bainitic microstructure. It was found that the V1–V6 is the most frequent variant pairing at lower temperature, while it gradually decreases towards intermediate temperatures, and at the highest austempering temperatures the variant pairing with low misorientation boundaries such as V1–V4 and V1–V8 is the most frequent. The preferred variant pairing is the least pronounced after austempering of steels with higher carbon contents at intermediate temperature. Nonetheless, a continuous increase of variant pairing within the same Bain group was observed with increasing austempering temperature for all steels. Furthermore, it was observed that the deviation from the theoretical Kurdjumov–Sachs orientation relationship increases with increasing austempering temperature for all steels.

Parameter evolution in a continuous Masing approach for cyclic plasticity and its physical interpretation

A continuous Masing model for the description of the elastic–plastic constitutive behaviour of a bainitic low-alloy steel was recently introduced. Its four model parameters can be directly linked to the shape of the stress–strain characteristic and to the microstructure of the material on the basis of the Masing concept. In this study the temperature, strain rate and strain amplitude dependent cyclic evolution of these parameters is presented for a bainitic 2CrMoNiWV steam turbine rotor steel loaded under low-cycle fatigue conditions. It is observed that the parameters develop differently for tensile and compressive loading and that the parameter evolution reflects the observed dislocation density reduction and sub-grain size increase. A statistical correlation analysis indicates that the parameters characterising the element yield strain distribution function significantly correlate with the measured sub-grain size and dislocation density of the material.

Determination of the Key Microstructural Parameter for the Cleavage Fracture Toughness of Reactor Pressure Vessel Steels in the Transition Region

The effects of the microstructural parameters, such as the prior austenite grain size and carbide size, on the cleavage fracture toughness were investigated in the transition region of Mn-Mo-Ni bainitic low alloy steels. Cleavage fracture toughness was evaluated by the ASTM standard E 1921 Master curve method. In order to clarify the effects of each microstructure, the grain size and carbide size of the test materials were independently controlled by modifying the heat treatment process. Firstly, the grain sizes were changed from 25㎛ to 110㎛ without any significant changes in the carbide size and shape. Secondly, the average carbide sizes were changed from 0.20 ㎛ to 0.29㎛ but maintaining the initial grain sizes. As a result, the fracture toughness in the transition region did not show any significant dependency on the austenite grain size, while the carbide size showed a close relation to the fracture toughness. Fracture toughness was decreased with an increase of the average carbide size. From the microscopic observation of the fractured surface, the cleavage initiation distance (CID) from the original crack tip showed no direct relationship to the prior austenite grain sizes but a strong relationship to the carbide sizes. However, the measured cleavage fracture toughness was strongly related to the distance from the crack tip to the cleavage initiation site. From the viewpoint of the weakest link theory, the particle size and their distribution in front of the crack tip is probably more important than the grain size in the transition temperature range where the fracture was controlled by the cleavage crack initiation.

The Geometric Compatibility Factor Determining Brittle Crack Propagation Across Boundary in Bainitic Low Alloy Steels

Boundary properties between two adjacent grains with different crystallographic orientation relation are important factors in deciding the mechanical properties of materials. in our previous reports, we introduced the geometric compatibility equation as a factor determining the brittle crack propagation condition into the adjacent grain in banitic low alloy steel as follows:M = cos α · cos θwhere α is the angle between crack normal in the cracked grain-I and normal to (001) plane in the adjacent grain-II and θ is the angle between normal to (001) plane in the grain II and loading axis.(see FIG. 1) Higher M gives greater possibility to the crack propagation. The geometric compatibility factor (M) could be easily calculated from the crystallographic data obtained from Electron Backscatter Diffraction (EBSD) analysis, and successfully predicted actual crack propagation plane. We also found that there was a critical M value to arrest the brittle crack propagation at the boundaries.

Effects of Crystallographic Orientation Relatiotionships on the Propagation of Brittle Crack in Mn-Mo-Ni Low Alloy Steel

Abstract Boundary properties between two adjacent grains with different crystallographic orientation relation are important factors in deciding the mechanical properties of materials. The concept of the geometric compatibility factor has been introduced in order to explain the relationship between transfer of deformation mode and crystal orientation. In the same manner, when the brittle crack propagates, grain boundary also obstructs crack propagation. Therefore it is also possible to introduce the concept of geometric compatibility considering the crystallographic orientation in analyzing the crack propagation. In this study, the characteristics of the brittle crack propagation in bainitic low alloy steel with compositions shown in table 1 have been investigated using EBSD analysis. A 3-point bending test was performed at -100°C and the secondary crack below fracture surface was analyzed in relation to the crystallographic orientation. In order to explain the behavior of crack propagation, modified geometric compatibility factor, M was introduced considering loading axis and crystallographic orientation relationship.

A Study on the Characteristics of the Boundaries in Bainitic Low Alloy Steels Using Electron Back-Scatter Diffraction (EBSD) Technique

Bainitic low alloy steel has a complex microstructure exhibiting several types of boundaries. The boundaries in bainitic steel, although certain boundaries are absent with respect to the alloy composition and the manufacturing process, could be typically divided into 4 types; dislocation cell boundary, lath boundary, packet boundary, and prior austenite grain boundary, in increasing order of size. The size and distribution of the respective boundaries are an important factor which controls the mechanical properties of the steels, including brittle fracture. In the present research, the characteristics of the boundaries in the bainitic low alloy steels were investigated in view of misorientation between grains enclosed by the respective boundaries.The alloys investigated were Mn-Mo-Ni low alloy forging steels having chemical compositions shown in TABLE 1. Steel-A was manufactured by the Vacuum Carbon Deoxidation(VCD) process. For the finer prior austenite grain size, Steel-B was produced by the aluminium addition and the silicon killing process. Before EBSD analysis, the microstructures of the alloys were observed using SEM and TEM. EBSD measurements were obtained using a Link OPAL system(Oxford) linked to a JEOL JSM 6300 SEM operating at 15KeV with the sample tilted at 70°.

The role of microstructure in the propagation of ultrasound in bainitic low-alloy steels

The propagation characteristics of ultrasound in two types of bainitic Cr-Mo-V steels including the Soviet type reactor pressure vessel steel 15Kh2MFA have been examined. In order to find out the factors affecting the sound velocity the steels have been exposed to different heat treatments and to neutron irradiation. It turned out that the sound velocity is considerably changed by tempering but only slightly by neutron irradiation. The effect of tempering can be explained on the basis of the decomposition of retained austenite and carbide precipitation. The main mechanisms of ultrasonic attenuation are Rayleigh scattering at prior austenite grains and thermoelastic absorption in the vicinity of packet boundaries. Both effects have been shown to depend on the orientation relationship between bainitic ferrite and parent austenite.

Mechanisms of ultrasonic attenuation in a bainitic low-alloy steel

Mechanisms of ultrasonic attenuation in a bainitic low-alloy steel

Tensile deformation of two experimental high-strength bainitic low-alloy steels containing silicon

AbstractTwo silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~400–250°C) have been deformed in tension. The bainitic microstructures in these steels contain appreciable amounts of retained austenite (instead of interlath cementite), and the behaviour of this phase during tensile deformation, and its apparent influence on the mechanical properties, has been examined. In particular, it is shown that provided the retained austenite exists in an interlath, thin-film morphology it has appreciable mechanical stability. Larger volumes of retained austenite have less mechanical and thermal stability, forming plate martensite structures and also undergoing deformation twinning. The effects of these variations on tensile strength and ductility are discussed.MST/527

Microstructural examination of two experimental high-strength bainitic low-alloy steels containing silicon

A detailed microstructural characterization of two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~ 400–250°C), has been carried out using principally electron microscopy, X-ray diffraction, and dilatometry. Upper bainite in these silicon-containing steels consists of bainitic ferrite laths and interwoven thin films of retained austenite instead of cementite. Coarser granular regions of retained austenite may also be obtained. The bainitic ferrite laths (or plates) in lower bainitic structures contain intralath carbides, but the interlath morphology of retained austenite still occurs. The variations in these microstructures with isothermal transformation temperature, and the thermal stability of the retained austenite phase is described and discussed.MST/526

Tensile deformation of two experimental high-strength bainitic low-alloy steels containing silicon

Two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~400–250°C) have been deformed in tension. The bainitic microstructures in these steels contain appreciable amounts of retained austenite (instead of interlath cementite), and the behaviour of this phase during tensile deformation, and its apparent influence on the mechanical properties, has been examined. In particular, it is shown that provided the retained austenite exists in an interlath, thin-film morphology it has appreciable mechanical stability. Larger volumes of retained austenite have less mechanical and thermal stability, forming plate martensite structures and also undergoing deformation twinning. The effects of these variations on tensile strength and ductility are discussed.MST/527

Fracture toughness of two experimental high-strength bainitic low-alloy steels containing silicon

AbstractThe fracture toughness of two experimental silicon-containing steels in the bainitic condition has been measured and related to the microstructural state of the steels. The optimum bainitic microstructure for high strength and high toughness combinations consists of bainitic ferrite and thin interwoven laths of retained austenite instead of cementite, this condition being achieved through the silicon addition to the steels. The thin films of retained austenite are thermally and mechanically stable and act to reduce the effective fracture grain size and also possibly help to blunt propagating microcracks; blockier volumes of retained austenite are unstable and hence not beneficial to toughness. The two experimental steels achieved strength and toughness values equal to, or better than, some commercial steels in the martensitic condition.MST/528

Fracture toughness of two experimental high-strength bainitic low-alloy steels containing silicon

Fracture toughness of two experimental high-strength bainitic low-alloy steels containing silicon

Microstructural examination of two experimental high-strength bainitic low-alloy steels containing silicon

A detailed microstructural characterization of two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~ 400–250°C), has been carried out using principally electron microscopy, X-ray diffraction, and dilatometry. Upper bainite in these silicon-containing steels consists of bainitic ferrite laths and interwoven thin films of retained austenite instead of cementite. Coarser granular regions of retained austenite may also be obtained. The bainitic ferrite laths (or plates) in lower bainitic structures contain intralath carbides, but the interlath morphology of retained austenite still occurs. The variations in these microstructures with isothermal transformation temperature, and the thermal stability of the retained austenite phase is described and discussed.MST/526