Interphase Precipitation Research Articles

Effect of manganese on microstructure of an isothermally transformed Fe–Nb–C alloy

The effect of manganese on the microstructure developed by isothermal transformation of an Fe-Nb-C alloy was examined over a wide range of temperatures. A well defined discontinuity was found at about 750°C in the time-temperature-transformation curve of an Fe–0·033Nb–0·07C–1·07Mn (wt-%) alloy. The discontinuity appears close to the temperature at which the interphase precipitation of NbC begins to occur, this temperature being about 50 K lower than that for a corresponding Fe–Nb–C alloy. Carbide-free ferrite is principally formed between 850 and 780°C in the Mn-containing alloy, a wider temperature range than that for the alloy without manganese. The amount of NbC dispersed in ferrite is decreased by the addition of manganese. The changes that develop in the microstructure after the addition of manganese are explained in terms of the reduction of ferrite growth rate and the decrease in supersaturation of NbC in ferrite.MST/45

Effect of manganese on microstructure of an isothermally transformed Fe–Nb–C alloy

The effect of manganese on the microstructure developed by isothermal transformation of an Fe-Nb-C alloy was examined over a wide range of temperatures. A well defined discontinuity was found at about 750°C in the time-temperature-transformation curve of an Fe–0·033Nb–0·07C–1·07Mn (wt-%) alloy. The discontinuity appears close to the temperature at which the interphase precipitation of NbC begins to occur, this temperature being about 50 K lower than that for a corresponding Fe–Nb–C alloy. Carbide-free ferrite is principally formed between 850 and 780°C in the Mn-containing alloy, a wider temperature range than that for the alloy without manganese. The amount of NbC dispersed in ferrite is decreased by the addition of manganese. The changes that develop in the microstructure after the addition of manganese are explained in terms of the reduction of ferrite growth rate and the decrease in supersaturation of NbC in ferrite.MST/45

Microstructures of isothermally transformed Fe-Nb-C alloys

AbstractThe microstructures of an Fe-Nb-C alloy, with and without boron, developed by the γ → α transformation during isothermal holding were examined mainly by metallographic techniques. Ferrite formed above about 750°C is equiaxed or polygonal, while Widmanstatten ferrite is developed at lower transformation temperatures increasingly replacing polygonal ferrite, as commonly observed in low alloy steels. The carbide precipitation changes with transformation temperature. Above about 800°C, the structure is not uniform, i.e. almost carbide free ferrite is formed at first and later coarse interphase precipitation takes place. Interphase precipitation occurs readily around 800°C, while uniform precipitation of NbC from supersaturated ferrite is the principal structural change around 730°C. At the lower temperatures, ferrite often contains a high density of dislocations and coarse cementite. The observed structural changes are explained in terms of the γ → α reaction kinetics and the partition of carbon to au...

Relationship between processing, microstructure, and properties in controlled rolled 0·45%V steel

Abstract The microstructures of a 0·45%V controlled rolled steel, subjected to various processing conditions, have been characterized by light and electron microscopy in order to correlate changes in processing with microstructure and mechnical properties. It is found that vanadium carbide can precipitate in austenite during controlled rolling to the extent that austenite recrystallization is prevented and fine grained structures are produced. Substructured, carbide free, ferrite forms during final rolling, and carbon is rejected into the surrounding austenite. The subsequent transformation to ferrite can be accompanied by interphase precipitation of carbide in a relatively dislocation free ferrite. Austenite remaining after the interphase reaction has, by virtue of its vanadium content, sufficient hardenability to allow the formation of a hard carbon rich microconstituent on subsequent cooling. This constituent is responsible for the continuous yielding behaviour of the steel. The resultant microstructur...

Tempering characteristics of a vanadium containing dual phase steel

Dual phase steels are characterized by a microstructure consisting of ferrite, martensite, retained austenite, and/or lower bainite. This microstructure can be altered by tempering with accompanying changes in mechanical properties. This paper examines such changes produced in a vanadium bearing dual phase steel upon tempering below 500 °C. The steel mechanical properties were minimally affected on tempering below 200 °C; however, a simultaneous reduction in uniform elongation and tensile strength occurred upon tempering above 400 °C. The large amount of retained austenite (≅10 vol pct) observed in the as-received steel was found to be essentially stable to tempering below 300 °C. On tempering above 400 °C, most of the retained austenite decomposed to either upper bainite (at 400 °C) or a mixture of upper bainite and ferrite-carbide aggregate formed by an interphase precipitation mechanism (at 500 °C). In addition, tempering at 400 °C led to fine precipitation in the retained ferrite. The observed mechanical properties were correlated with these microstructural changes. It was concluded that the observed decrease in uniform elongation upon tempering above 400 °C is primarily the consequence of the decomposition of retained austenite and the resulting loss of transformation induced plasticity (TRIP) as a contributing mechanism to the strain hardening of the steel.

The isothermal austenite-ferrite transformation in some deformed vanadium steels

The effect of vanadium on the isothermal austenite-ferrite transformation, between 725 °C and 775 °C, of a hot-deformed microalloyed steel has been studied by examination of the microstructure and measurement of the volume fraction of ferrite in specimens quenched from the reaction temperature. The accompanying precipitation was studied by transmission electron microscopy of thin foils and carbon extraction replicas and by electron energy-loss spectroscopy. Very early in the transformation a continuous band of fine-grained ferrite forms at austenite grain boundaries. After some time some of these grains coarsen to form large equiaxed ferrite grains. It is found that vanadium has no effect on the time to the start of coarsening but thereafter accelerates the rate of formation of ferrite. Interphase precipitation of VN occurs throughout the transformation in the vanadium steels and this is thought to influence the rate at which the ferrite coarsens at the lower temperatures (750 ° and 725 °C) in the range studied.

Microstructure of weld heat-affected zone of 12Cr–Mo steel

Detailed studies have been made of the transformations and microstructure of the weld heat-affected zone (HAZ) of a class 212Cr–Mo steel, particular attention being given to δ-ferrite. The investigation covered a wide range of welding heat inputs and involved real and simulated HAZ material. To obtain the phasetransformation data extensive use was made of a weld simulator and dilatometer. The weld H AZ consisted of three regions: secondary-hardened martensite, homogeneous martensite, and duplex δ-ferrite and martensite adjacent to the fusion line. The amount of δ-ferrite increased with increasing weld heat input. It was shown that δ-ferrite forms above 1320°–1360°C, depending on the heat rate and remains untransformed on cooling to room temperature for the normal range of weld cooling rates. Detailed metallography showed that δ-ferrite forms at austenite grain boundaries, involves redistribution of allo ying elements and is associated with interphase precipitation of carbides. However, in amounts up to 16·6% it does not affect the subsequent decomposition of austenite. From these findings a representative C–C–T diagram is produced to describe the transformation occurring in the HAZ of this 12Cr-Mo steel.

On the dispersion hardening potential of interphase precipitation in micro-alloyed niobium steel

In an experimental high-strength bw-alloy (HSLA) steel micro-alloyed with 0.13wt% Nb, microhardness of the ferrite phase has been measured as a function of the particle spacing across interphase precipation sheets of niobium carbonitrides. The dispersion geometry was controlled by isothermal transformation treatments using different temperatures and time intervals, and evaluated quantitatively by dark-field transmission electron microscopy. The results were in reasonable agreement with the Orowan—Ashby model of dispersion hardening, suggesting that the sheet spacing may represent the effective geometrical parameter responsible for dispersion hardening due to interphase precipitation.

Solution and precipitation of carbide phase in medium-carbon steels

1. Carbide phase is almost completely dissolved in austenitic vanadium steel at 1200°. 2. Alloying of vanadium steel with nickel or chromium accelerates the solution of vanadium carbides at 1000–1150° but has almost no effect on the temperature at which the solution of the carbide particles is fairly complete. 3. With isothermal holding of supercooled austenite in the range of subcritical temperatures there are two possible mechanisms of transformation: precipitation of vanadium carbide directly from supercooled austenite and, with longer holding times, formation of a ferrite-carbide structure. 4. Under conditions where dispersed vanadium carbide is precipitated, chromium and, especially, nickel increase the stability of supercooled austenite and nickel also lowers the temperature at which the stability of austenite is minimal. 5. Precipitation of vanadium carbide from supercooled austenite in the process of isothermal holding is accompanied by an increase in hardness, and the formation of a ferrite-carbide structure by a reduction of the hardness. 6. When vanadium steel is alloyed with chromium or nickel there is a change in the morphology of the precipitating carbides from stringers (interphase precipitation) to fibers.

Influence of vanadium on transformation characteristics of high-strength line-pipe steels

The mechanical properties ofa new line-pipe steel containing 0·45% V are strongly dependent upon the ability of the V to influence transformation characteristics in addition to providing strengthening from precipitation of V(CN). The V addition depresses the bainite start temperature and in this respect is similar to Mo. It is also shown that, although V increases the ferrite start temperature, it stabilizes the high-C austenite remaining at the end of the ferrite reaction. This stabilized austenite subsequently transforms to a martensite–bainite (M–B) constituent which is responsible for the continuous yield behaviour in a tensile test. The volume fraction of the M–B phase is strongly affected by the extent of interphase precipitation of V(CN) concurrent with ferrite formation and factors which increase the amount of interphase precipitation, such as N content, cause a reduction in the amount of M–B phase.

The effect of intercritical annealing temperature on the structure of niobium microalloyed dualphase steel

The structures produced in a Nb-microalloyed steel by oil quenching after intercritical anneals at 760 and 810 °C have been examined by light and transmission electron microscopy. After both anneals, the periphery of the austenite pool transforms on cooling to ferrite in the same orientation as the ferrite retained during intercritical annealing. Thus the ferrite forms by an epitaxial growth mechanism without the formation of a new interface or grain boundary. The new ferrite is precipitate-free in contrast to the retained ferrite which develops a very dense precipitate dispersion during intercritical annealing. In the carbonenriched interior of the austenite pool beyond the epitaxial ferrite only martensite forms in specimens annealed at 760 °C but various mixtures of ferrite and cementite form in specimens annealed at 810 °C. The latter structures include lamellar pearlite, a degenerate pearlite, and cementite interphase precipitation. All Nb is in solution in the austenite formed at 810 °C, and therefore the low hardenability of the specimens annealed at that temperature is best explained by the effect of low austenite carbon content.

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.

29th HATFIELD MEMORIAL LECTURE: Ferrite

The morphology of ferrite grains formed from austenite is first considered, then the refinement of ferrite grain size by controlled rolling as exemplified by HSLA steels. The influence of alloying elements is briefly examined, followed by a detailed discussion of the precipitation of carbides during and after the formation of ferrite. It is shown that the γ/α interfaces are preferred sites for the precipitation of alloy carbides (interphase precipitation), which occurs on both planar and curved boundaries. In the former, the precipitation is associated with the movement of small ledges across the γ/α interface, whereas the curved bands of precipitate are formed on high-energy boundaries which move by bowing around the particles. A change in the nature of the γ/α interface often leads to fibrous carbide growth, while more rapid rates of cooling lead to the formation of supersaturated ferrite and subsequent precipitation of carbides on dislocations. The dislocations in the ferrite are shown to play an important role in the subsequent coarsening of the carbide dispersion, whether they form at the interfaces or on the dislocations. Finally, the observations on the formation and growth of carbide dispersions are related to the structure and properties of microalloyed steels.

Effects of deformation on decomposition of austenite: Part II – Carbide precipitation

AbstractCarbide precipitation has been studied in three alloy steels subjected to plastic deformation in the austenitic condition prior to transformation. A 1½Ni-1Cr-¼Mo steel is shown to precipitate M23C6 in austenite when deformed both in the stable (750°C) and metastable (550°C) austenitic state prior to transformation between 600 and 700°C. Again, in a 5Cr-0.2C alloy, M23C6 was found to precipitate at 550°C in the austenite, but subsequent transformation led to complex structures containing M7C3 interphase precipitation and fibres. A 1V-0.2C alloy revealed precipitation of VC in stable austenite at 875°C, which caused solute-depleted zones in which no further precipitation occurred during the γ → α transformation; the bulk of the structure exhibited interphase precipitation and fibrous growth of VC, the latter morphology being encouraged by prior plastic deformation of the austenite.

The effect of nitrogen on precipitation and transformation kinetics in vanadium steels

The isothermal decomposition of austenite has been studied in a series of vanadium steels containing varying amounts of carbon and nitrogen, (in approximately stoichio-metric proportions), in the temperature range 700 to 850°C. In the basic alloy, Fe-0.27V–0.05C (composition in wt pct), below 810°C the austenite to polygonal ferrite trans-formation is accompanied by interphase precipitation of vanadium carbide, the finer dis-persions being associated with the lower transformation temperatures. However, below 760°C there is an additional precipitation reaction where dislocation precipitation of vanadium carbide predominates; this is shown to occur in association with Widmanstatten ferrite. Above 810° C, a proeutectoid ferrite reaction results, the ferrite being void of precipitates; evidence is provided to show that partitioning of vanadium from ferrite to austenite occurs during the transformation. In the two steels containing nitrogen, namely Fe-0.26V-0.022N-0.020C and Fe-0.29V-0.032 N the basic interphase precipitation re-action is unchanged, but the resultant precipitate dispersions are finer at a given trans-formation temperature. The temperature range over which interphase precipitation oc-curs is expanded by the presence of nitrogen, since the Widmanstatten start tempera-ture is depressed and the proeutectoid ferrite reaction is inhibited. Precipitation in austenite prior to transformation and twin formation during transformation are both en-couraged by the presence of nitrogen.

The observation of interphase precipitation in association with the lateral growth of Widmanstätten ferrite

This paper describes the observation of interphase precipitation in association with the isothermal decomposition of austenite to ferrite in two particular iron-base alloys. Specifically, it is shown that the interphase precipitates are arranged in sheets parallel to the broad faces of the Widmanstatten plates such that it may be inferred that this precipitate dispersion nucleates on the low-energy austenite/ferrite interface. It is also shown that precipitation from supersaturated solid solution often occurs in the Widmanstatten ferrite plates.

The observation of interphase precipitation in association with the lateral growth of Widmanstätten ferrite

The observation of interphase precipitation in association with the lateral growth of Widmanstätten ferrite

Cleavage of ferrite strengthened by interphase precipitation of vanadium carbide

AbstractCleavage fracture has been studied in a series of steels strengthened by vanadium carbide dispersions. It has been shown that the impact transition temperature is strongly dependent on the precipitation hardening contribution to the yield strength, this being more important than the precipitate volume fraction. Ageing of non-random dispersions shows that there is no significant difference between the mechanical properties of random and non-random (‘interphase precipitation’) dispersions. Fractographic investigation reveals that the interphase precipitate dispersion can change the path of a cleavage crack during fracture at room temperature, although this does not have any significant effect on the mechanical properties.

Structural and textural aspects of deformation and recrystallization of low-carbon steels containing dispersions of Nb(CN)

AbstractThe effect of variation of second-phase particle size and spacing on the structure and crystallographic texture of two 80% cold-rolled and recrystallized low-carbon steels containing dispersions of Nb(CN) has been studied by a variety of techniques. Alloy compositions, based on Fe-0.5wt-%Mn, were chosen to give ∼ 0.12 and ∼ 0.18 vol.-%Nb(CN). The dispersions were produced by interphase precipitation. The behaviour of these materials was compared to that of a niobium-free steel. Parallel kinetic measurements had shown that the recrystallization rates of the two-phase steels could vary by a factor of ∼ 106 with change of interparticle spacing over the range studied, the effect being mainly caused by change of the nucleation kinetics of recrystallized grains. In the present work a detailed investigation of the cold-worked materials showed that the changes in dispersion caused significant substructural changes which in turn caused textural variations. These factors were then responsible for causing th...

The application of photoemission electron microscopy to the study of diffusional phase transformations in steels

The photoemission electron microscope has been used to examine continuously the decomposition of austenite in an Fe-4Mo-0.2C low-alloy steel. Excellent phase contrast and resolution were obtained, and allowed close examination of several microstructural features of the austenite → ferrite transformation occurring by continuous cooling, or isothermally, in the temperature range 850–600°C. Of particular interest was the precipitation of alloy carbides at the austenite-ferrite transformation interface (interphase precipitation), which was clearly revealed. Rigorous examination of the transformation microstructure was also carried out by conventional heat treatment practice and metallographic techniques and the results compared with the observations made in the photoemission electron microscope. It was possible to show that the observed surface behavior was representative of the bulk reactions. From this preliminary study it was concluded that the photoemission electron microscope could be used to advantage in the examination of phase transformations in steels.