Microstructural evolution in bainite,martensite, and δ ferrite of low activation Cr–2W ferritic steels

Abstract

The microstructural evolution in (2–15)Cr–2W–0·1C (wt-%) firritic steels after quenching, tempering, and subsequent prolonged aging was investigated, using mainly transmission electron microscopy. The steels examined were low induced radioactivation ferritic steels for fusion reactor structures. With increasing Cr concentration, the matrix phase changed from bainite to martensite and a dual phase of martensite and δ ferrite. During tempering, homogeneous precipitation of fine W₂C rich carbides occurred in bainite and martensite, causing secondary hardening between 673 and 823 K. With increasing tempering temperature, dislocation density decreased and carbides had a tendency to precipitate preferentially along interfaces such as bainite or martensite subgrain boundaries. During aging at high temperature, carbides increased in size and carbide reaction from W₂C and M₆C to stable M₂₃C₆ occurred. No carbide formed in δ ferrite. The precipitation sequence of carbides was analogous to that in conventional Cr–Mo steels.

MST/1049     

Influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steel

The influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steels was analyzed by means of scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The microstructure of laser melted high chromium steel is composed of austenite with supersaturated carbon and alloy elements and granular interdendritic carbides of type M₂₃C₆. Secondary hardening of the laser melted layer begins at 450 °C after tempering, and the hardness reaches a peak of 672HV at 560 °C and then decreases gradually. After tempering at 560 °C, a large amount of lamellar martensite was formed in the laser melted layer with a small quantity of thin lamellar M₃C cementite due to the martensitic decomposition. The stripy carbides precipitating at the grain boundaries were determined to be complex hexagonal M₇C₃ carbides and face centered cubic M₂₃C₆ carbides. In addition, the granular M₂₃C₆ carbides and fine rod-like shaped M₇C₃ carbides coexisted within the dendrites. As a result, the combined effects of martensitic transformation, ultrafine carbide precipitations, and dislocation strengthening result in the secondary hardening of the laser melted layer when the samples were tempered at 560 °C.     

Precipitation in 9Cr–1Mo steel after creep deformation

The carbides present after creep testing a 9Cr–1Mo steel at 566 °C over a range of stress levels giving rupture times of up to 7300 h have been characterized and identified using a transmission electron microscopy, energy-dispersive X-ray spectroscopy and electron diffraction. The initial carbide precipitates present were M₇C_3, (NbV)C and VC and it was determined that M₆C carbide precipitates were present in all specimens after elevated temperature exposure for greater than approximately 1700 h. No precipitation of M₂₃C₆ was detected. The evolutionary sequence from the initially present carbides during high temperature exposure involved the formation of the stable M₆C carbide directly, without the intermediate formation of M₂₃C₆, as is reported to occur in other Cr–Mo steels.     

Morphology and microstructure of M<Subscript>2</Subscript>C carbide formed at different cooling rates in AISI M2 high speed steel

In as-cast structure of AISI M2 high speed steel, M₂C carbide prevails, the morphology of which has crucial influence on distribution and dimension of carbides in final products. In this study, the morphology and microstructure of M₂C formed at different cooling rates have been investigated by scanning electron microscope, X-ray diffraction, transmission electron microscope, and electron back-scatter diffraction. The results show that the morphology of M₂C carbide changes from the plate-like type to the fibrous one with increasing cooling rates. Surprisingly, the microstructure between plate-like and fibrous M₂C is significantly different. Twining and stacking faults are observed in the plate-like M₂C, which is supported by great misorientations between adjacent carbides. However, no planar faults are identified in fibrous M₂C and the carbides in one colony have almost identical orientation. It is expected that plate-like M₂C grows as a faceted phase, while fibrous M₂C formed at high cooling rates is likely a non-faceted phase. The difference of liquid/solid interface structure is supposed to result in the morphology change of M₂C.     

Isothermal transformations in advanced high strength steels below martensite start temperature

Abstract

The transformation products in advanced high strength steels have been studied during the isothermal decomposition of austenite, subsequent to initial martensite formation. Rapid cooling to various temperatures below martensite start was carried out in a dilatometer with the intention to form controlled volume fractions of initial martensite and austenite, followed by isothermal holding. The transformation kinetics was monitored by means of dilatometry and microstructural characterisation by scanning electron microscopy, electron backscatter diffraction and X-ray diffraction. Hardness measurements of the resulting microstructures were analysed. The results revealed that the microstructures formed below M_S are mainly composed of different fractions of tempered martensite, isothermal bainite with carbide precipitation and retained austenite.     

Premature failure of work-rolls in tandem mill: Some microstructural revelations

The microstructural features of prematurely spalled tandem mill work-rolls were examined in an attempt to correlate microstructure with spalling behavior and roll performance. Spalled samples were collected from work-rolls that had shown variations in roll life under similar conditions of mill usage. Optical microscopy revealed that a fine dispersion of spheroidal carbides in a matrix of tempered martensite was conducive to superior performance in terms of roll life (i.e., tonnage rolled), and that coarse angular and irregular shape carbides were detrimental to roll life. Image analysis of roll microstructures indicated that small carbide size, large carbide volume fraction, and high carbide count were characteristic of higher-life rolls, and that large carbide size, low carbide volume fraction, and less carbide density were typical of lower-life rolls. The carbides in both types of microstructure were M₇C₃ type.     

Precipitation behaviour of a sensitized AISI type 316 austenitic stainless steel in hydrogen

The purpose of this study was to characterize the precipitation behaviour of AISI type 316 steel in hydrogen. The different precipitates (M₂₃C₆, M₆C), the intermetallicχ-phase and the martensitic phase (α′,ε) were determined by using transmission electron microscopy (TEM) and X-ray diffraction techniques. All the specimens were sensitized at 650? C for 24 h. Some samples were carburized up to 2 wt% C. Additions of carbon content decrease the time required for sensitization. Short-term (24 h) exposure of this steel to sensitization temperature results in a complex precipitation reaction of various carbides and intermetallic phases. Hydrogen was introduced by severe cathodic charging at room temperature. This study indicates that by conventional X-ray techniques it is possible to detect those precipitates and their behaviour in a hydrogen environment. The zero shift as observed by X-ray diffraction from the carbides (M₂₃C₆, M₆C) and the intermetallicχ-phase, indicates that those phases absorb far less hydrogen than the austenitic matrix. TEM studies reveal that hydrogen inducesα′ martensite at chromium-depleted grain-boundary zones, near the formation of the carbides.     

Morphology and crystallographic phase of V–C particles formed in Fe–Cr–Ni–V–C alloys

Abstract

The morphology and crystallographic phase of V–C carbide particles formed in cast Fe–Cr–Ni–V–C alloys were investigated by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy (TEM). The combination of results obtained with these techniques revealed that cuboidal, cruciform and spherical carbide particles were formed, depending on the alloy composition, all having the cubic-VC_1?x structure (Fm-3m). Detailed TEM observations suggested that small carbide particles were initially cubic in shape and became spherical with increasing particle size. All cuboidal and spherical carbides were single crystallites with no grain boundary at any particle sizes, even after growing to 6 μm in diameter.     

Carbide transformation in carburised zone of 25Cr35NiNb+MA alloy after high-temperature service

Microstructure evolution due to carburising of a 25Cr35NiNb+MA ethylene pyrolysis furnace tube was investigated after service for approximately 4 years. The microstructure of 25Cr35NiNb+MA alloy was examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) equipped with energy-dispersive X-ray spectroscopy and high-resolution transmission electron microscopy (TEM). The results revealed that M₂₃C₆ carbides of the 25Cr35NiNb+MA alloy had coarsened and transformed to M₇C₃ carbides with a heavily faulted structure during carburising. The content of M₇C₃ carbide increased closer to the inner wall. A large number of dislocations had been observed surrounding the carbides and some formed dislocation walls. The carbide transformation mechanism and the effect of the dislocation walls are discussed.     

Secondary carbide precipitation in an 18 wt%Cr-1 wt% Mo white iron

High chromium (18%) white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The austenite is destabilized by a hightemperature heat treatment which precipitates chromium-rich secondary carbides. In the as-cast condition the eutectic M₇Ca₃ carbides are surrounded by a thin layer of martensite and in some instances an adjacent thicker layer of lath martensite. The initial secondary carbide precipitation occurs on sub-grain boundaries during cooling of the as-cast alloy. After a short time (0.25 h) at the destabilization temperature of 1273 K, cuboidal M₂₃C₆ precipitates within the austenite matrix with the cube-cube orientation relationship. After the normal period of 4 h at 1273 K, there is a mixture of M₂₃C₆ and M₇C₃ secondary carbides and the austenite is sufficiently depleted in chromium and carbon to transform substantially to martensite on cooling to room temperature.     

Metallurgical investigation into the failure of a chopper blade used for cutting of hot-rolled steel coils

In the present work, failure investigation of a chopper blade received from an integrated steel plant has been presented. Chopper blades are used in chopping machines for cutting trimmed edges of hot-rolled coils into pieces to convert them into scrap. These blades are manufactured from hot forged or rolled billets or flats of high carbon high chromium cold work tool steel. The investigation consists of visual examination, chemical analysis, microstructural analysis through optical and scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and hardness measurement. The chemical analysis confirmed the steel as equivalent to D2 grade in AISI notation. Carbide volume fraction of the broken blade was in the normal range of 10–15% as commonly observed in D2 tool steel. Microstructural examination under light and scanning electron microscopy showed non-uniform distribution of large eutectic primary carbides of irregular morphology forming strings or bands in tempered martensite matrix preferentially aligned in a specific direction. The uneven carbide arrangement in the matrix made the structure highly anisotropic and susceptible to localized stress concentration. The carbides were identified mainly as M₂₃C₆ type. Cracks were observed to initiate at the edges of the blade and propagate to the interior through clustered zones of carbides. SEM study suggests that the crack initiation was associated with decohesion of carbide particles in the cluster which culminated into final fracture by the mechanism of void coalescence and subsequent crack growth.     

Formation of the reversed austenite during intercritical tempering in a Fe-13%Cr-4%Ni-Mo martensitic stainless steel

Formation of the reversed austenite obtained by intercritical tempering has been studied via transmission electron microscopy (TEM) in a Fe-13%Cr-4%Ni-Mo low carbon martensitic stainless steel. It is found that the precipitation of M₂₃C₆ carbides along the martensite lath boundaries will result in Ni-enrichment in the adjacent region. The reversed austenite forms with the Ni-enrichment region as the nucleation sites, keeps a cube-cube orientation relationship with the M₂₃C₆ carbides and bears the Kurdjumov-Sachs (K-S) relationship with the martensite. Moreover, the reversed austenite formed inside the martensite laths is also confirmed. The mechanism for formation of the reversed austenite is discussed in detail.     

Quantitative carbide analysis using the Rietveld method for 2.25Cr–1Mo–0.25V steel

It is usually difficult to quantitatively determine the mass fraction of each type of precipitates in steels using transmission electron microscopy and traditional X-ray powder diffraction analysis methods. In this paper the Rietveld full-pattern fitting algorithm was employed to calculate the relative mass fractions of the precipitates in 2.25Cr–1Mo–0.25V steel. The results suggest that the fractions of MC, M₇C₃ and M₂₃C₆ carbides were evaluated precisely and relatively quickly. In addition, it was found that the fine MC phase dissolved into the matrix with prolonged tempering.     

Microstructural study and wear behavior of ductile iron surface alloyed by Inconel 617

In this research, microstructure and wear behavior of Ni-based alloy is discussed in detail. Using tungsten inert gas welding process, coating of nearly 1–2 mm thickness was deposited on ductile iron. Optical and scanning electron microscopy, as well as X-ray diffraction analysis and electron probe microanalysis were used to characterize the microstructure of the surface alloyed layer. Micro-hardness and wear resistance of the alloyed layer was also studied. Results showed that the microstructure of the alloyed layer consisted of M₂₃C₆ carbides embedded in Ni-rich solid solution dendrites. The partial melted zone (PMZ) had eutectic ledeburit plus martensite microstructure, while the heat affected zone (HAZ) had only a martensite structure. It was also noticed that hardness and wear resistance of the alloyed layer was considerably higher than that of the substrate. Improvement of wear resistance is attributed to the solution strengthening effect of alloying elements and also the presence of hard carbides such as M₂₃C₆. Based on worn surface analysis, the dominant wear mechanisms of alloyed layer were found to be oxidation and delamination.     

Effect of tempering temperature on the microstructure and properties of ultrahigh-strength stainless steel

The microstructure, precipitation and mechanical properties of Ferrium S53 steel, a secondary hardening ultrahigh-strength stainless steel with 10% Cr developed by QuesTek Innovations LLC, upon tempering were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile and impact tests. Based on these results, the influence of the tempering temperature on the microstructure and properties was discussed. The results show that decomposition occurred when the retained austenite was tempered above 440 °C and that the hardening peak at 482 °C was caused by the joint strengthening of the precipitates and martensite transformation. Due to the high Cr content, the trigonal M₇C₃ carbide precipitated when the steel was tempered at 400 °C, and M₇C₃ and M₂C (5–10 nm in size) coexisted when it was tempered at 482 °C. When the steel was tempered at 630 °C, M₂C and M₂₃C₆ carbides precipitated, and the sizes were greater than 50 nm and 500 nm, respectively, but no M₇C₃ carbide formed. When the tempering temperature was above 540 °C, austenitization and large-size precipitates were the main factors affecting the strength and toughness.     

MC5冷轧辊钢奥氏体化过程碳化物演变的研究

为了深入研究MC5冷轧辊钢奥氏体化过程中碳化物的演变规律,采用扫描电镜、X射线衍射分析和硬度测试技术,研究了不同奥氏体化时间对MC5钢组织和硬度的影响.结果表明,奥氏体化时间从0增加到2 h时,随着奥氏体化时间的增加,试样的淬火硬度先陡然增加,后趋于平缓;当奥氏体化时间增加到4 h时,随着奥氏体化时间的增加,试样的淬火硬度有减小的趋势.对奥氏体化时间为30 min、1和2 h的试样进行微观统计和XRD分析,发现奥氏体化时间为30 min时,试样的碳化物分布最为弥散,数量最多,尺寸最为细小,其类型主要为M7C3型.     

The metallography of a cobalt-based implant alloy after solution treatment and ageing

The ageing characteristics of a commercial Co-Cr-Mo-C alloy after solution treatment were investigated using optical and electron microscopy. An M₂₃C₆-type carbide was identified by X-ray and electron diffraction after ageing treatments between 650 and 1150° C. Nucleation and growth of this carbide took place on intrinsic stacking faults by Suzuki segregation in the cobalt matrix. High stacking-fault densities gave rise to intragranular striations which were visible after etching once precipitation had occurred. Ageing temperatures of 925° C and above increased the stability of the f c c cobalt matrix and led to precipitation on undissociated dislocations. Grain-boundary carbides were evident at all ageing temperatures.     

The morphology and microtexture of M7C3 carbides in Fe-Cr-C and Fe-Cr-C-Si alloys of near eutectic composition

The microtexture of M₇C₃ carbides in undercooled 40 g samples of hyper- and hypo-eutectic Fe-Cr-C alloys was determined by electron back scatter diffraction. In the hyper-eutectic alloy the carbides were monocrystalline, while those in the hypo-eutectic alloy were polycrystalline. While in the former the preferred growth direction of the M₇C₃ carbides was [0001], in the hypo-eutectic alloy there was a relatively weak texture near [10¯11]. There was no evidence for the presence of growth twins in either the M₇C₃ carbide rods or in the branching mechanism in the joint between the carbide rods of the hypo-eutectic sample. The morphologies of the M₇C₃ carbides resulting from undercooling were used to explain the microstructure of hardfacing Fe-Cr-C weld deposits applied by the manual metal arc process. The effect of silicon additions on the morphology of M₇C₃ carbides in Fe-Cr-C-Si alloys is explained in terms of the effect of silicon on undercooling.     

Carbide changes and sulphur segregation in erMa steel during creep

An investigation was carried out into microstructural changes at the grain boundary of interrupted creep-tested and long-term heated 2sCr-1Mo steel by means of transmission electron microscopy with energy dispersive X-ray spectrometry. Grain boundary precipitates alternated their types of M₇C3 carbides with M₆C carbides, and were spheroidized during creep or long-term heating. These carbide changes and spheroidization of precipitates were found to be accelerated by creep stress. Sulphur migrated to segregate at the interface between M₆ C and the matrix, while this phenomenon was not observed at the Ms3 carbide or precipitate-free grain boundary. Consequently it was considered that the intergranular creep damage in 2sCr-Mo steel was caused by a reduction in interfacial energy due to the sulphur segregation at the interface between the matrix and M₆C carbides.     

Study on transformation characteristics of carbides in an 8?% Cr roller steel

The characteristics of carbide transformation in an 8 % Cr roller steel under the conditions of equilibrium, slow heating, and isothermal treatment were investigated using thermodynamic calculation, differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy. The carbides under equilibrium conditions were identified as MC, M₂₃C₆, and M₇C₃ types. The austenite transformation onset and finish temperatures, Ac₁ and Ac₃, were 745 and 780 °C, respectively. M₂₃C₆ decomposition took place at 820–850 °C and M₇C₃ at 1060–1130 °C, under slow heating conditions. Quantitative relationships were obtained between the isothermal conditions and average carbide size and aspect ratio, which increased with increasing holding temperature and time.