Epsilon Carbide Research Articles

Influence of the amount and morphology of retained austenite on the mechanical properties of an austempered ductile iron

High Si contents in nodular cast irons lead to a significant volume fraction of retained austenite in the material after the austempering treatment. In the present work, the influence of the amount and morphology of this phase on the mechanical properties (proof stress, ultimate tensile strength (UTS), elongation, and toughness) has been analyzed for different austempering conditions. After 300 °C isothermal treatments at intermediate times, the austenite is plastically stable at room temperature and contributes, together with the bainitic ferrite, to the proof stress and the toughness of the material. For austenite volume fractions higher than 25 pct, the proof stress is controlled by this phase and the toughness depends mainly on the stability of γ. In these conditions (370 °C and 410 °C treatments), the present material exhibits a transformation-induced plasticity (TRIP) effect, which leads to an improvement in ductility. It is shown that the strain level necessary to initiate the martensitic transformation induced by deformation depends on the carbon content of the austenite. The martensite formed under TRIP conditions can be of two different types: “autotempered” plate martensite, which forms at room temperature from an austenite with a quasi-coherent epsilon carbide precipitation, and lath martensite nucleated at twin boundaries and twin intersections.

Homogeneous formation of epsilon carbides within the austenite during the isothermal transformation of a ductile iron at 410 °C

The transformation of a ductile iron at 410 °C for different times, after austenitization for 30 minutes at 900 °C, is analyzed in detail. Upper bainite and a high volume fraction of austenite are formed for intermediate annealing times. A certain amount of martensite is observed after quenching not only for short transformation times but also for intermediate times. The formation of the martensite on cooling after intermediate transformation times is due to the decrease in carbon concentration of the retained austenite because of the homogeneous precipitation of epsilon carbides within. This homogeneous precipitation of epsilon carbide inside austenite is unambiguously observed. The epsilon carbide, pre-precipitated in austenite, which transforms to martensite on cooling, continues growing in the martensite after transformation. For long times of austempering at 410 °C, some complex large carbides or silicocarbides are formed, probably from the epsilon carbide, which result in the total decomposition of austenite.

Epsilon carbide precipitation during tempering of plain carbon martensite

Epsilon carbide precipitation in steel martensite has been investigated by means of transmission electron microscopy. The first stage of tempering initiates with the nucleation of very fine e-carbide particles on the closely spaced parallel line defects, the morphology being so-called “cross-hatched” e-carbide needles. The e-carbide particles which produce the well-defined dif-fraction patterns are related to the martensite matrix with a Pitsch and Schrader orientation relationship.[32] These particles subsequently grow into rods elongated in the direction parallel to the\(\langle 100\rangle _\alpha \parallel \langle 11\bar 20\rangle _\varepsilon \) within the matrix. The final reaction in the first stage is the rearrangement of e-carbide rods into a disklike morphology. The e-carbide rods elongated in the 〈100〉e di-rections coalesce on Open image in new window planes in a raftlike manner, as in the case of those formed in the quench-aged low-carbon ferrite, the tetragonality of martensite being completely lost. Although the deviation from hexagonal symmetry about the [0001]e axis exists, no evidence of orthorhombic η-carbide formation was obtained.

The role of secondary hardening dispersions in the ductile failure of an ultrahigh-strength steel

Ultrahigh strength (UHS) steel is often achieved only at the expense of toughness. High hardness martensitic steels used for structural applications involving significant tensile loading and therefore requiring a KIC fracture toughness of 50 MPa., are limited to a Rockwell C hardness (Rc) of 50-55. In UHS steels the failure mechanism is predominantly that of plastic shear instability in which flow localization leads to the formation of intense shear bands. The underlying cause of the flow localization is investigated here in an UHS (Rc 55-56) 4340 steel.Azrin et al. have recently investigated the critical strain for shear localization in UHS 4340 steel under conditions of isothermal and adiabatic shear. The air melt 4340 steel of composition (wt.%): Fe-1.71Ni-0.75Mn-0.4lC-0.79Cr-0.25Si-0.24Mo-0.13Cu-0.011S-0.005P, was normalized at 910°C for one hour then austenitized at either 840°C or 1100°C for one further hour. Following austenitization, the specimens were stage I tempered at l60°C for one hour to form epsilon carbide and produce a peak hardness in the range Rc 55-56. The fracture surfaces of the double linear shear test specimens were examined in a JEOL 35U SEM and the alloy microstructures were investigated using a combination of CBED, EDS and EELS on both jet electropolished thin foils and carbon extraction replicas in a Philips EM 420T S(T)EM.

The clustering and coarsening of carbon multiplets during the aging of martensite from mössbauer spectroscopy: The preprecipitation stage of epsilon carbide

Mossbauer spectra of as-quenched and aged martensite (9.5 at. pct C) are analyzed in terms of seven Fe environments, the abundances of which evolve with aging time. Besides the atoms of the matrix (A), Fe atoms which are first nearest neighbors of an isolated interstitial can only be distinguished by their quadrupole shift depending upon whether they lie in a direction perpendicular (Ba) or parallel to thec axis (Bc); classC atoms have two carbon neighbors whileD orE belong to an ordered phase; classF atoms have three carbon neighbors. The carbon multiplet model, based on interstitials residing exclusively in octahedral sites, an elastic stability of theC-C 1/2«111” doublet and a diffusion path for carbon in (001)α' can explain all the features observed: a first step attributed to the clustering of isolated multiplets, and a second step to the coarsening into extended multiplets which leads to the emergence of a new ordered iron carbide (B2 monoclinic with parallelepiped cell of 5.73, 6.74, and 8.60 A) which transformsin situ into e-carbide (~Fe9C4) (η carbide as long as it is coherent with the matrix). The positions of carbon atoms in Fe9C4 are specified and a synopsis with related phenomena is drawn.

Isothermal martensite transformation in a 1.80 Wt Pct C steel

The linear products formed isothermally at 373 K in a 1.80 wt pct C steel (Ms = 346 K) were examined by means of transmission electron microscopy. They were first reported as “black line products” by Greninger and Troiano. The isothermal product was of a thin plate with about 0.5 μm width, and it contained {112}b transformation twins and revealed a habit plane of {3 15 10}f. The orientation relationship between austenite and product was close to the Nishiyama relationship. These crystallographic data were in good agreement with those calculated by the phenomenological theory of martensitic transformation. Consequently, the product was determined not to be lower bainite, but isothermal martensite. The black color of isothermal martensites resulted from the fact that they were easily etched by the precipitates of epsilon carbide formed during the isothermal holding.

Transformation behaviour of laser processed Fe-5% Cr, Fe-5% Ni and Fe-6% Cr-2% Ni alloys

The effects of rapid cooling, of the order of 104‡ C sec−1, achieved through the laser surface alloying process, on the transformation behaviour of Fe-5% Cr, Fe-5% Ni and Fe-6% Cr-2% Ni alloys have been investigated. The solidification structures and microstructures were characterized by optical and transmission electron microscopy. The significant findings are that Fe-5% Cr alloy undergoes a massive transformation to ferrite while Fe-5% Ni and Fe-6% Cr-2% Ni alloys undergo a transformation to a structure consisting of both ferrite and martensite. The substructures of ferrite and martensite were observed to contain essentially dislocations. These alloys also exhibited two carbide precipitations namely epsilon carbide and cementite. Solidification studies revealed that cellular solidification structures were present in 5% Ni and 6% Cr-2% Ni alloys with a cell spacing of 3 Μm but no solidification structures were observed in 5% Cr alloy. The influence of nature of solute and cooling rate on solidification behaviour, transformation mode and morphology of structures have been discussed.

Retained austenite and tempered martensite embrittlement in medium carbon steels

Electron microscopy, diffraction and microanalysis, X-ray diffraction, and auger spectroscopy have been used to study quenched and quenched and tempered 0.3 pct carbon low alloy steels. Somein situ fracture studies were also carried out in a high voltage electron microscope. Tempered martensite embrittlement (TME) is shown to arise primarily as a microstructural constraint associated with decomposition of interlath retained austenite into M3C filMs upon tempering in the range of 250 °C to 400 °C. In addition, intralath Widmanstatten Fe3C forms from epsilon carbide. The fracture is transgranular with respect to prior austenite. The sit11Ation is analogous to that in upper bainite. This TME failure is different from temper embrittlement (TE) which o°Curs at higher tempering temperatures (approximately 500 °C), and is not a microstructural effect but rather due to impurity segregation (principally sulfur in the present work) to prior austenite grain boundaries leading to intergranular fracture along those boundaries. Both failures can o°Cur in the same steels, depending on the tempering conditions.

Retained Austenite and Tempered Martensite Embrittlement in Medium Carbon Steels

Electron microscopy, diffraction and microanalysis, X-ray diffraction, and auger spectroscopy have been used to study quenched and quenched and tempered 0.3 pct carbon low alloy steels. Some in situ fracture studies were also carried out in a high voltage electron microscope. Tempered martensite embrittlement (TME) is shown to arise primarily as a microstructural constraint associated with decomposition of interlath retained austenite into M3C films upon tempering in the range of 250 °C to 400 °C. In addition, intralath Widmanstätten Fe3C forms from epsilon carbide. The fracture is transgranular with respect to prior austenite. The situation is analogous to that in upper bainite. This TME failure is different from temper embrittlement (TE) which occurs at higher tempering temperatures (approximately 500 °C), and is not a microstructural effect but rather due to impurity segregation (principally sulfur in the present work) to prior austenite grain boundaries leading to intergranular fracture along those boundaries. Both failures can occur in the same steels, depending on the tempering conditions.

Microstructural Contributions to Stress Corrosion Cracking in High Strength Steels

The effects of microstructural variables upon stress corrosion cracking resistance of 300M steel in 3.5% NaCl solution have been studied. Table 1 lists KIsccvalues for three heat treatment conditions. The martensite substructure was predominantly twinned plates for conventional heat treatment and dislocated laths for both high temperature and step heat treatments. A typical twinned region observed in the conventional heat treatment is shown in Fig. 1. Such twinned regions were less frequent in the other heat treatments. Both cementite and epsilon carbide were seen in all cases, as illustrated in Figs. 2 and 3 for conventional heat treatment. Epsilon carbide was usually observed within large grain boundary nucleated laths (autotempered martensite), with definite habit planes and growth directions. The formation of such laths have been discussed previously. Retained austenite, mostly in the form of interlath films, was observed in all cases with increased amounts present in coarser grained structures (Fig. 4).

The Bainite reaction in Fe-Si-C Alloys: The primary stage

The morphology, crystallography, and substructure of bainitic ferrite formed in silicon alloyed high-carbon steels in the temperature range 290 to 380 °C have been studied. The bainite exhibits a plate shaped morphology, an irrational habit plane, and an irrational orientation relationship. The bainitic ferrite is heavily dislocated, while the surrounding austenite contains thin twins, the density of which is highest in the austenite between closely spaced ferrite plates. The bainite plates can cross these twins in such a manner that the twinned region remains in a crystallographic orientation, which is quite different from that of the other regions of the bainitic ferrite plates. Epsilon carbide subsequently precipitates on the austenite twin/bainitic ferrite boundaries. The bainitic ferrite shape strain direction and magnitude are estimated from displacements of austenite twins inherited in the ferrite. All results, including measurement of the austenite carbon content, are consistent with a shear mode of transformation.

A study of interstitial atom configuration in fresh and aged iron-carbon martensite by mossbauer spectroscopy

Mössbauer spectra of Fe-1.5 ∼ 7.2 at %C alloys without and with a 5 T longitudinal external magnetic field were measured to clarify the structure of iron carbon martensite. The spectra of freshly quenched martensite are decomposed into three magnetic hyperfine patterns having different fields. The 1st component with a hyperfine field of H ∼- 27 T is identified as the absorption caused by 1st neighboring iron atoms of octahedral carbon atoms. The 2nd component ( H ∼- 31.5 T). the intensity of which rapidly decreases during aging, is due to 1st neighboring iron atoms of tetrahedral carbon or possibly 2nd nearest neighbors of octahedral carbon atoms. The 3rd main component ( H ∼- 34 T) is due to iron atoms not strongly perturbed by more distant carbon neighbors. The appearance of three additional components ( H ∼- 37. 26.5 and 18 T) in the aged martensite indicates the formation of the ordered non-stoichiometric Fe 4C x(x < 1) phase. The Mössbauer spectra of the epsilon carbide formed after aging at 413 K are decomposed into three hyperfine components corresponding to different iron environments in the carbide, which suggests the existence of vacant sites of carbon in the Fe 2C structure.

The lower bainite transformation and the significance of carbide precipitation

The nature and significance of carbide precipitation accompanying the lower bainite transformation in steels has been examined. It is found that lower bainitic cementite nucleates and grows from within supersaturated ferrite, and that such precipitation is strongly inconsistent with the concept of 'interphase precipitation' at the α/γ transformation interface. Crystallographic determinations suggest that the lattice invariant shear of lower bainite may not be directly responsible for the occurrence of the characteristic single carbide variant. It is suggested that it is the transformation strain associated with a lower bainite platelet that stimulates the development of a particular carbide variant which is compatible with the relief of this strain. The transition temperature from upper to lower bainite has been tentatively rationalised on thermodynamic grounds and in the context of the B s and M s temperatures. It seems that the appearance of epsilon carbide in some lower bainites but cementite in others can be understood in terms of a theory of martensite tempering due to Kalish and Cohen.

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.

The tempering of a low-carbon internally twinned martensite

The first stage of tempering of high-carbon martensites results in the precipitation of the metastable epsilon carbide; in low-carbon steels, it is generally considered that epsilon carbide does not form during tempering and only cementite is observed. Since high-carbon steels have an internally twinned martensite morphology, whereas low-carbon martensite (with the exception of high-nickel steels) consists of laths, it is not clear whether the first stage of tempering is controlled by carbon content or by martensite substructure. Therefore, a study was made of the tempering characteristics of a low-carbon steel in which the martensite was internally twinned. The steel chosen for this investigation contained 28 per cent nickel and 0.1 per cent carbon. Dark-field transmission electron microscopy revealed the presence of carbide precipitates in specimens that had been tempered for 48 hr at 100°C. Selected-area diffraction established that these carbides were the h.c.p. epsilon carbide. This indicates that the first stage of tempering of martensites containing carbon may be more directly related to the martensite morphology than to the carbon content alone.

The structure of cementite and its relation to ferrite and epsilon carbide—II

An explanation is provided for the crystallographic relationship between ferrite and cementite suggested by the mutual orientation {125}α ∥ (001) Fe 3C with 〈311〉 ∥ [010]. The interpretation is similar to that already provided for the commonly occurring alternative based on {112} ∥ (001) with 〈111〉 ∥ [010]. In the {125} case, zig-zag rows involving the closest and second closest interatomic distances, are already present and small atom movements equivalent to rotations of the rows and other minor adjustments would give atomic configurations identical with those of cementite or the corresponding planes in ε-carbide. Although orientations based on such systems might also appear to be (nearly) twin-related, both are well substantiated and appear to occur independently. Twinning in the ferrite phase might, however, provide an explanation for some possible subsidiary orientation relationships.

Morphology and crystal structure of carbides precipitated from solid solution in alpha iron

The carbides precipitated from solid solution in high-purity iron-carbon alloys, in two low-carbon steels and in an iron-carbon-3.25% silicon alloy were studied by electron microscopy and diffraction. In the iron-carbon alloy and in the two low-carbon steels the carbide precipitates in a dendritic form during aging following quenching. In specimens quenched directly to the aging temperature oblong plates appeared, along with dendrites. Both of these forms are cementite (Fe 3C) and no diffraction evidence of precipitation of epsilon carbide was found. The time required for formation of identifiable particles at 250°C corresponds to the time reported by others for 70 per cent completion of the precipitation reaction, as measured by internal friction. At 150°C the time required for formation of such particles was much longer than the time for completion of precipitation, as measured by internal friction. The dendritic carbides in Fe-C alloys appear to precipitate on {110} planes of ferrite, with branches along 〈111〉 directions. In the iron-3.25% silicon alloy the carbides take the form of lenticular disks precipitated on {100} planes of the ferrite matrix; the crystal structure of this carbide was not determined but it is neither epsilon nor cementite. The disks are displaced by grain boundary films of cementite during prolonged aging.