Bainitic Carbide Research Articles

Effect of nano-bainite microstructure and residual stress on friction properties of M50 bearing steel

By means of the measurement of the residual stress and friction properties, the observation of the microstructure by the SEM, the X-ray diffraction (XRD) was used to phase analysis and the nano-bainite structure was determined by transmission electron microscopy (TEM). The effect of nano-bainite microstructure and residual stress on friction properties of M50 bearing steel was analyzed after austempered at 200 °C for 2 h, 16 h and 32 h. Results show that nano-bainite structure is composed of thin film austenite and bainite ferrite, the mean thickness of bainite ferrite lath is about 90 ± 10 nm. The contents of nano-bainite are 15.772%, 33.951% and 50.332% respectively after austempered for 2 h, 16 h and 32 h. The hardness increases from 2 to 16 h, which is related to the precipitation of secondary carbides in bainite. And the hardness decreases after 16 h due to the fact that the bainite content is the largest. The fluctuation range of the residual stress of the sample austempered for 32 h + tempering is the smallest and the distribution is the most uniform. Wear resistance of the sample gradually increases with the extension of austempering time which is mainly related to hardness, residual stress state and thermal stability.

Application of bending creep for examining effect of service conditions on creep response of steel

We demonstrate that creep testing of cantilevers in bending is suitable for evaluating steady-state creep parameters, including the stress exponent. In particular, a low alloy bainitic boiler tube steel, namely T11, in as-fabricated condition and after 27 years of service in a coal-fired power-plant were tested in bending at 600 °C at effective steady-state stresses of 100–200 MPa. Steady-state strain rates obtained from the bending tests matched reasonably well with those obtained from uniaxial compression tests performed at equivalent stresses, thereby unambiguously validating the bending test methodology for the entire range of stress. All types of samples showed a stress exponent of 6; however, the steady-state strain rates of the in-service samples were ~7 times larger than those of as-fabricated samples. Although this suggests a substantial loss in the creep resistance of the tubes at the end of the design-life, the “to-be-discarded” tube may still have considerable residual life. Microstructural evaluation of the samples revealed coarsening of carbides in bainite and metal enrichment of carbides, with excess of M6C, with exposure to the service conditions. A structure-property correlation is established to explain the experimental observations, and results are discussed in light of existing literature.

Austempering Experiments of Production Grade Silicon Solution Strengthened Ductile Iron

Austempered ductile iron provides a feasible way to produce high strength components. However, in heat treatments resulting in highest strengths some of the ductility is lost due to formation of bainitic carbides. The role of silicon in inhibiting the formation of iron carbides in as-cast ductile irons as well as its solution strengthening effect is well known and acknowledged in industry. The effect of silicon on austemperability, resulting microstructures, and mechanical properties of austempered ductile irons with silicon contents with 3.4-3.8 w-% was researched. Quenching and austempering heat treatments were carried out for production grade silicon solution strengthened ductile irons EN GJS 500-14. Results indicate, that it is possible to manufacture a fully ausferritic structure into a silicon solution strengthened matrix and indeed good ductility can be achieved in combination with ultimate tensile strength of 1600 MPa. Segregation of silicon reduces the solubility of carbon into the matrix especially close to the graphite nodules which reduce the stability of carbon stabilized austenite and leads into compromised machinability.

贝氏体相变时原子的位移

研究贝氏体相变过程中原子的位移具有重要理论意义。本文综合试验事实理论地分析了过冷奥氏体在中温区发生贝氏体相变时原子位移的方式，并指出在孕育期内，依靠浓度涨落，碳原子扩散位移形成了贫碳区和富碳区。在贫碳区中原子以新旧相化学势之差为驱动力，依靠热激活跃迁形成贝氏体铁素体晶核。从热力学上分析，贝氏体铁素体的形成不是切变过程，从相变动力学和扩散速度分析，也非扩散过程。贝氏体相变中，碳原子扩散，界面铁原子以非协同热激活跃迁方式形成贝氏体铁素体和贝氏体碳化物。It is significant theoretically to study the atomic displacement during bainite transformation. The atomic displacement mode during bainite transformation in medium temperature region was synthetically analyzed in theory. It is pointed out that, in incubation period, the carbon-poor areas come into being by carbon atoms diffusion displacement for the concentration fluctuation. In the carbon-poor area, the bainite ferrite crystal nucleus is generated by heat activation transition of the iron atoms under the driving force of the difference between the old and new phase chemical potential. From the view of thermodynamics, the formation of bainite ferrite is not shear. From the aspects of transformation kinetics and diffusion velocity, the formation of bainite ferrite is not diffusion either. In bainite transformation, the displacement mode of carbon atoms is diffusion, but that of the interface iron atoms is non-synergic thermal activation transition, which results in the formation of bainite ferrite and bainite carbide.

Effect of Microstructure on Rolling Contact Fatigue of Bearings

There has been proposed an innovative thermal treatment of bearing steel 100CrMnSi6-4, where the existing standard heat treatment has been replaced by austempering. The structure of low-temperature tempered martensite has been replaced by a microstructure composed of martensite and lower bainite with midrib. The kinetics of bainitic transformation and isothermal martensitic transition at selected austempering temperatures was controlled by acoustic emission. The research on contact strength was made under the conditions of rolling-sliding friction. The microstructure was revealed with the use of a light microscope and the forms of pitting wear were displayed by a scanning electron microscope. It was found that the optimum microstructure providing the best used contact strength of the tested steel is conditioned by the formation of a lower bainite with midrib at the temperatures near MS. A plausible cause of the increased resistance to pitting is bifurcation of fatigue cracks on dispersion bainitic carbides in combination with primary carbides, in bainitic-martensitic matrix.

Structure–property correlation in austempered alloyed hypereutectic gray cast irons

The austempering behavior of a series of hypereutectic alloyed gray iron compositions with carbon equivalent from 4.37 to 5.14 was studied to understand the influence of microstructure on its mechanical and wear properties. The alloying elements in the alloys included Ni, Mo, Cr and inoculation by micro-constitution of Ti, Nb and Ce. The alloys were austempered at 360 °C and upper bainitic type feathery ferrite was observed in the matrix. While the graphite content determined by optical metallography varied between 16 and 24 vol%. The volume of austenite determined by XRD analysis showed values between 20 and 26%. The ferrite lath size was determined using XRD peak broadening. The tensile property varying between 188 and 270 MPa, showed no significant variation with volume percentage of carbon or austenite in the ausferrite. However the wear rate varying between 0.5 and 2.6 × 10 −7 g/Nm, showed a decreasing trend with graphite content attributed to the higher lubricating effect of released carbon during sliding wear. The specific wear rate of hypereutectic alloys, increased with increasing ferrite lath size due to enhanced softer ferrite phase on the sliding surface. The wear rate was found to increase with volume of austenite, austenite carbon content and austenite lattice parameter, which is attributed to increased stability of austenite against strain induced martensite formation and the increased formation of bainitic carbides in the second stage tempering. The various technical aspects in correlating the microstructure with the mechanical and wear properties of hypereutectic austempered gray iron are described.

Precipitation behavior of the lower bainitic carbide in a medium-carbon steel containing Si, Mn and Mo

The fine microstructure, crystallographic features and ɛ-carbides precipitation behavior of lower bainite produced by isothermal transformation in a medium-carbon steel containing Si, Mn and Mo were investigated using transmission electronic microscopy. It was found that the microstructure produced by isothermal reaction at 320 °C was composed of a large amount of plate-like lower bainite with retained austenite embedded between the plates, and ɛ-carbides precipitated within them. Midrib and subunits were readily visible in the lower bainite plate. The bainite plate kept a G-T orientation relationship (OR) with the austenite. Selected area electron diffraction patterns of “three phases in four variants” and analysis indicated that two variants of ɛ-carbides precipitated in a single bainitic ferrite plate. The two (or three) variants of ɛ-carbides can simultaneously keep a Jack OR with its “bainite matrix”, while keeping no fixed OR with the austenite. The crystallographic features of ɛ-carbides precipitated within the bainite were the same as those observed in tempered martensite. The results indicated that the bainitic transformation bore an analogy to the martensitic one in carbide precipitation.

Effect of Carbide Morphology on the Susceptibility to Type IV Cracking of a 1.25Cr-0.5Mo Steel

Type IV cracking is considered to be the likely failure mode of ferritic steel welds when operated for long duration. The carbide morphology of the service-exposed 1.25Cr-0.5Mo steel weldment, which is composed of a forged flange and pipe fabricated from plates, has been examined before and after a creep test. Higher susceptibility to Type IV cracking was observed at the Intercritical HAZ (ICZ) on the flange side despite higher creep resistance of the parent material compared with a pipe. The change in carbide morphology during the creep exposure was the most pronounced at the flange ICZ. The coarse bainitic carbide originally existing depleted the intragranular carbides and significant variation in carbide density inside the ICZ was generated. In contrast, carbides at the pipe ICZ were more uniformly distributed. It was interpreted that higher susceptibility of the flange ICZ was accelerated by the heterogeneous distribution of carbide density and the resultant variation of creep strength and would enhance grain boundary sliding associated with creep strain accumulation. It was proved that the susceptibility to Type IV cracking was highly dependent upon the characteristics of a parent material by experiments using simulated ICZ specimens. Significant difference in the effect of heat treatment to simulate the microstructure at the ICZ upon the creep strength was observed between a flange and pipe. The simulated ICZ specimen generated by a pipe parent showed no apparent change due to the heat treatment compared with a parent material. On the contrary, it reduced the time to rupture and changed the fracture mode from transgranular to intergranular for a flange material. The feature of grain boundary cracking was similar to that of actual weldment which took place preferentially at the inclined grain boundaries to the tensile direction, that was to be considered the evidence of grain boundary sliding.

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.

Failure of a super heater tube

The failure of two adjacent platen super heater tubes of a thermal power plant has been analysed. One tube fractured with a fish mouth opening and another thinned down considerably. A significant amount of oxide layer was observed in the inner side of the failed tube. In the other tube the inner oxide layer was very small and the tube was thinned down due to fireside corrosion. The unexposed tube from the same lot revealed a ferritic bainitic microstructure. The failed tube also showed a ferrite bainite microstructure but most of the bainitic carbide had transformed to globular form. Considerable carbide precipitation both in grain boundary (GB) and inside grains were observed. A large number of cavities and micro-cracks were observed along the grain boundaries. A similar microstructure was observed on the same tube 10 cm away from the failed region. The micrograph at the fracture location showed a similar microstructure but the grains are very much elongated. The formation of globular oxide on the other tube was compared to that of the failed tube. Due to breakage of oxide scale of the failed tube, the metal surface of the failed tube was exposed to steam at a higher temperature and this accelerated the oxide growth. This led to creep damage of the material. Ultimately, failure occurred due to rapid overheating of the tube material.

TEM of the tempering reactions in 2.25 Cr 1Mo steel

The precipitation of carbides in the ferritic steels, has been the subject of several investigations. This paper discusses the precipitation of carbides in 2.25Cr-1Mo steels. The sequence of changes that occur in the morphology, structure and composition of carbides as a function of tempering temperatures and carbon content, 0.06%C(A) and 0.11%C(B) has been examined in detail.The steels were austenitized at 1323K for 1h and quenched in flowing argon gas(AQ), followed by tempering in the range 823-1023K for varying durations of time. Structural and chemical information of carbides were obtained using electron diffraction and analytical microscopy respectively. The Cliff-Lorimer ratio method, incorporating theoretically calculated correction factors was employed in the analysis of spectra.Following AQ, both steels exhibited a mixture of bainite and polygonal ferrite. Fig.1 shows the presence of coarse distribution of carbides in bainite and relatively carbide-free ferrite matrix in steel B. The repartitioning of carbon takes place during AQ and transforms carbon rich regions into bainite. The inset in Fig.1 is the SAD pattern obtained from the bainite, the analysis of which confirms that the precipitates are Fe3C.

High-temperature transmission electron microscopyin situ study of lower bainite carbide precipitation

In situ observation of the bainite carbide precipitation processes in 40CrMnSiMoV steel by means of high-temperature transmission electron microscopy (TEM) is conducted. It is evident that carbides can precipitate either in bainitic ferrite or from austenite when carbide-free bainite (meta-bainite) obtained by isothermal transformation is tempered at higher temperatures. In view of the quantity of carbides precipitated from ferrite in combination with the result of an X-ray diffraction analysis of the bainitic ferrite carbon content, it can be concluded that bainitic ferrite growth involves supersaturation of carbon content to some degree.

Microstructure of ASTM A-36 Steel Laser Beam Weldments

The effect of the laser welding process on the microstructure of ASTM A-36 steel is to produce a bainitic fusion zone and a refined ferrite/pearlite heat-affected zone. The formation of these microstructural constituents results in improved mechanical properties such as in strength and toughness as compared to the base plate. The stress-relief heat treatment of 1175°F/1 h breaks up and spherodizes the carbide films present at the bainite lath boundaries. This significantly increases the impact energy the material can absorb down to low temperatures. The results of this study indicate that laser welding can be used to produce weldments with good mechanical properties in low-carbon steels such as A-36 where the high cooling rates refine the microstructure without the formation of the embrittling martensite that would be found in high-carbon steels. The toughness of the A-36 laser beam weldments can be increased further through the use of a stress relief anneal which coarsens and spherodizes the bainitic carbides.

Kinetics of formation of different structures in steel 25Kh2NMFA

1. The investigation of austenite decomposition in steel 25Kh2NMFA showed that the intermediate transformation is the basic transformation both under isothermal and continuous cooling conditions. 2. In the process of the intermediate transformation the austenite partially decomposes with formation of martensitic α phase (αM), i.e., "bainite without carbides" is formed. The dispersed carbides in bainite observed by electron microscopic analysis are precipitated due to secondary processes associated with selftempering of αM phase. The quantity of carbides increases substantially when the intermediate transformation temperature is lowered. 3. Upper bainite has a distinct granular structure. The mother phase of this bainite is αM phase almost free of self-tempering carbides, with regions of Are or M+Are evenly distributed, which are visible in a light microscope at large magnifications. Under isothermal conditions upper bainite is formed at temperatures above 350°C, but in a broad range of cooling rates during continuous cooling (1000-1 deg/min). 4. Lower bainite is characterized by a small quantity (<10%) of evenly distributed regions of Are or M+Are and, as a consequence, a somewhat granular structure. In the mother α solid solution of lower bainite there are large numbers of dispersed, evenly distributed carbides from self-tempering.

Metallography of bainitic transformation in silicon containing steels

The formation of carbide in lower bainite was studied in two silicon containing carbon steels by transmission electron microscopy and diffraction techniques. Epsilon carbide was identified in the low temperature isothermally transformed bainite structure. The crystallographic relationship between epsilon carbide and bainitic ferrite was found to follow the Jack orientation relationship,viz, (0001)ε l l(011)α, (101l)ε l 1(101)α. The cementite observed in lower bainite was in the shape of small platelets and obeyed the Isaichev orientation relationship with the bainitic ferrite,viz, (010) cl 1(1-11)α, (103) cl 1 (011)α. Direct evidence showing the sequence of carbide formation from aus-tenite in bainite has also been obtained. Based on the observations and all the crystallo-graphical features, it is strongly suggested that in silicon containing steels the bainitic carbide precipitated directly from austenite instead of from ferrite at the austenite/fer-rite interface as has been proposed by Kinsman and Aaronson (Ref. 1). The uniformity of the carbide distribution is thus envisaged to be the outcome of precipitation at the aus-tenite-ferrite interphase boundary.

A debate on the bainite reaction

The authors debate three topics central to the controversies which have enveloped the bainite reaction ever since it was first recognized as a distinctive mode of austenite decomposition. These include: “what is bainite?”, “the growth mechanism of the ferritic component of bainite”, and “the sources of bainitic carbide precipitation.” RFH concludes that bainite is the product of a shear transformation. Individual bainite plates are suggested to grow substantially more rapidly than volume diffusion-control allows, but a constraint such as the build-up of volume strain energy limits the extent of their growth. This mechanism of growth ensures extensive supersaturation of bainitic ferrite with respect to carbon. Whether or not carbides precipitate in association with bainite plates and whether the carbide is cementite ore, however, is a complex question in competitive reaction kinetics. New experimental evidence is presented to demonstrate thate carbide precipitated in lower bainite dissolves upon heating above the kinetic-B stemperature in an alloy steel containing 1.5 pct Si. This result is taken to support the existence of the metastable eutectoid reactionγ ⇌ α + e atca 350°C. HIA and KRK define bainite as the product of a nonlamellar eutectoid reaction. On this view, carbide precipitation thus plays an essential, rather than an ancillary role. Development of the Widmanstatten morphology by the ferritic component of bainite is shown to be inessential to the classification of a eutectoid structure as bainite. When this morphology is present, however, it is concluded to grow by the ledge mechanism, without the participation of shear, at rates of the order of or less than those allowed by volume diffusion-control. New experimental evidence is presented to show that the lengthening and thickening kinetics of individual plates within sheaves of upper bainite are consistent with this description. The results of a new calculation indicate that the initial carbon content of bainite plates lies between theα/α + Fe3C) and the extrapolatedα/(α+ γ) phase boundaries, in agreement with expectation from the ledge mechanism of growth.

Transmission electron microscope study of the bainite transformation in iron-chromium-carbon alloys

Thin foil electron microscope studies of lower bainite formed in two iron-chromium-carbon alloys have been carried out. One of these alloys (Fe-7.9% Cr-1.11% C) has been used previously for the X-ray measurements of the habit plane and the specific variant of the austenite-ferrite orientation relationship. These results have been confirmed in the present thin foil studies. Selected area diffraction and trace analysis studies of carbides in this bainite indicate that there are two types of carbides. One of these carbides precipitates on striations parallel to the major growth direction of the plate. The other carbide appears in the form of short transverse rods between these striations at an angle of 60–70° to the longitudinal direction in a manner characteristic of bainitic carbide in low alloy steels. While the structure of the carbides on the striations has not been uniquely determined, the short rods appearing between the striations were found to exhibit the usual orthorhombic structure of cementite. Trace analysis of the cementite rods shows that they appear to precipitate on a definite (112) ferrite plane. Electron diffraction patterns showing spots from austenite, ferrite, and cementite show that these structures are uniquely related. The ferrite-cementite relationship in this bainite is the standard one exhibited by tempered martensite and lower bainite in other alloys. No evidence of internal twinning was observed in this bainite. The above results have been analysed in terms of a shear model of step-wise lateral growth of the bainite plate. Comparative thin foil studies of lower bainite in a lower chromium alloy (Fe-3.3 Cr-0.66 C) were also made, which show that the mode of carbide formation is different in the two alloys.

数種の低合金鋼の連続冷却変態特性

Continuous cooling transformation characteristics have been determined for four grades of steels which are being used as rotor shaft materials. Data on continuous cooling transformation characteristics of these low alloy steels were collected from literatures and were summarized together with the present result.Critical cooling rates for the formation of pearlite and bainite were obtained respectively for twenty kinds of low alloy steels including four steels in the present paper, and steels were classified according to the critical cooling rate in consideration of actual cooling rates for larger mass.These steels have a so-called bainitic hardenability.Remarkably larger differences in critical cooling rate among steels were observed in the formation of pearlite rather than in the formation of bainite.The prediction of the critical cooling rate for the pearlite formation from the ideal critical diameter calculated by the conventional empirical formula was difficult for these steels with relatively large alloy contents.The effect of Ni on the critical cooling rate for the pearlite formation was remarkable. In mainly bainitic structures, the appearance of bainite was different among steels and this seemed to have a profound influence on hardness.The structural appearance of bainite was significantly dependent on the temperature of formation.Addition of Cr lowered this temperature, resulting in a fine distribution of carbides in bainite.

Nucleation sites of bainitic carbides in alloy steels

Nucleation sites of bainitic carbides in alloy steels