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 Widmanstätten 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 Widmanstätten 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.     

Pearlite phase transformation in Si and V steel

Systematic research has been undertaken on the effects of single and combined additions of vanadium and silicon on the phase transformation and microstructure of pearlitic steels. Both alloy additions were found to result in the formation of nonlamellar products in the vicinity of austenite grain boundaries in hypereutectoid compositions (0.77 to 0.95 wt pct C). The products comprise discrete initial cementite particles and grain boundary ferrite, which is embedded with interphase precipitates of vanadium carbide. As the carbon content is increased further (up to 1.05 wt pct), the amount of grain boundary ferrite gradually decreases without any dramatic change in the morphology of the initial cementite particles. No continuous embrittling grain boundary cementite network was formed. The aspect ratios of the grain boundary cementite particles were decreased from 60:1 to 25:1 by the addition of the alloy elements. A compre-hensive model has been suggested to explain these effects. Other effects of these alloy elements on the microstructure of pearlitic steels have also been examined. For given austenitization conditions, an increase in carbon and vanadium content produced a decrease in austenite grain size. Silicon was found to increase the rate of interphase precipitation of vanadium carbides. Formerly Graduate Student, Department of Materials, Oxford University Formerly University Lecturer, Department of Materials, Oxford University     

Three-dimensional analysis and classification of grain-boundary-nucleated proeutectoid ferrite precipitates

The three-dimensional (3-D) shapes and distributions of grain-boundary-nucleated proeutectoid ferrite precipitates have been obtained by computer-aided 3-D reconstruction of serial sections of an Fe-0.12 wt pct C-3.28 wt pct Ni alloy. Isothermal transformation for short times at 650 °C was used to produce a low volume fraction of ferrite, which appeared as both Widmanstätten shapes and more-equiaxed grain-boundary precipitates. While the two-dimensional (2-D) cross sections of these precipitates appeared to fit into the previously accepted categories of precipitate morphologies, 3-D reconstructions revealed important aspects of connectivity and shape that were not observed earlier. A partially revised morphological classification based on these 3-D observations is provided here. Significant differences between the 3-D morphology of proeutectoid ferrite and that of proeutectoid cementite are also discussed, indicating (among other things) that the 3-D morphological classifications of ferrite are generally more complex than those of proeutectoid cementite.     

A transmission electron microscopy study of copper precipitation in the cementite phase of hypereutectoid alloy steels

The precipitation of copper has been detected and studied in three of the main decomposition products of austenite: allotriomorphic grain-boundary cementite, pearlitic cementite, and Widmanstätten cementite plates. The investigation has been carried out on two high-alloy hypereutectoid steels containing copper contents of 1.0 and 2.5 wt pct. The main advantage of these high-alloy steels is that the parent austenite phase remains stable upon cooling to room temperature, thus preserving the parent phase and the parent/product interfaces in the microstructure for subsequent examination. Transmission electron microscopy (TEM) revealed that the copper precipitation occurs in proeutectoid allotriomorphic grain-boundary cementite in association with the transformation interface. The copper particles were dispersed in the form of rows (or sheets) within the allotriomorphs of cementite. Evidence for copper precipitate particles nucleated at structural features imaged at the growth interface was also obtained. Copper precipitation was found to occur in both the ferrite and cementite lamellae of pearlite, and again, examination of partially decomposed structures revealed copper particles nucleated at the austenite/pearlite transformation interface. In addition, copper particles were also observed at the ferrite/cementite interface of pearlite. Copper precipitation observed in Widmanstätten cementite plates revealed a precipitate-free midrib region in the plates and a higher concentration of copper particles toward the broad faces of the plate. Copper particles were also found located at coarse linear interface defects at the broad faces of the plate.     

The morphology,crystallography, and mechanism of carbide precipitation in an Fe-0.12 Pct C-3.28 Pct Ni alloy

Carbide precipitation during the eutectoid decomposition of austenite has been studied in an Fe-0.12 pct C-3.28 pct Ni alloy by transmission electron microscopy (TEM) supplemented by optical microscopy. Nodular bainite which forms during the latter stages of austenite decomposition at 550 °C exhibits two types of carbide arrangement: (a) banded interphase boundary carbides with particle diameters of about 20 to 90 nm and mean band spacings between 180 and 390 nm and (b) more randomly distributed (“nonbanded”) elongated particles exhibiting a wide range of lengths between 33 and 2500 nm, thicknesses of approximately 11 to 50 nm, and mean intercarbide spacings of approximately 140 to 275 nm. Electron diffraction analysis indicated that in both cases, the carbides are cementite, obeying the Pitsch orientation relationship with respect to the bainitic ferrite. The intercarbide spacings of both morphologies are significantly larger than those previously reported for similar microstructures in steels containing alloy carbides other than cementite (e.g., VC, TiC). Both curved and straight cementite bands were observed; in the latter case, the average plane of the interphase boundary precipitate sheets was near {110}_α//{011}_c consistent with cementite precipitation on low-energy {110}_α//{111}_γ ledge terrace planes (where α,β, andc refer to ferrite, austenite, and cementite, respectively). The results also suggest that the first stage in the formation of the nonbanded form of nodular bainite is often the precipitation of cementite rods, or laths, in austenite at the α:γ interfaces of proeutectoid ferrite secondary sideplates formed earlier. Although these cementite rods frequently resemble the “fibrous” microstructures observed by previous investigators in carbide-forming alloy steels, they are typically much shorter than fibrous alloy carbides. The bainitic microstructures observed here are analyzed in terms of a previously developed model centered about the roles of the relative nucleation and growth rates of the product phases in controlling the evolution of eutectoid microstructures.     

The effect of phosphorus content on grain boundary cementite formation in AISI 52100 steel

Intergranular fracture surfaces of high phosphorus (0.023 wt pct P) and low phosphorus (0.009 wt pct P) AISI 52100 steels were investigated by Auger Electron Spectroscopy (AES). Cementite, identified by composition and Auger peak shape, was found to form on austenite boundaries in specimens oil quenched from 960 °C to room temperature as well as in specimens quenched from 960 °C and isothermally held at temperatures between A_cm and A₁. Phosphorus segregates to austenite boundaries during austenitizing and accelerates cementite formation on the austenite boundaries. Concentration profiles obtained by AES during ion sputtering showed that phosphorus may be incorporated in the first-formed cementite and concentrates at cementite/matrix interfaces in later stages of cementite growth. The amount of interphase P segregation in the later stages is proportional to bulk alloy P concentration in accord with McLean’s theory of grain boundary segregation in dilute alloys and appears to approach equilibrium at high reaction temperatures (785 °C). At lower reaction temperatures (740 °C), the interphase segregation is lower than expected, a result that may be attributed to reduced diffusivity of P at the lower reaction temperature.     

Aging behavior of Fe-30 Ni alloys containing niobium

A study has been made of the precipitation reactions in Fe-30 wt pct alloys containing up to 5 wt pct Nb. The as-quenched structures of these alloys consist, of austenite, martensite in twinned as well as in massive form, and Ni₃Nb and Fe₂Nb precipitates. On aging at 700° and 800°C the main precipitation reaction results in the formation of hexagonal Laves phase Fe₂Nb, but Ni₃Nb in both bct and orthorhombic structures also precipitates. The precipitation of Fe₂Nb is a heterogeneous process and results in a considerable increase in the hardness of the alloy.     

Influence of Interfacial Structure upon Carbide Precipitation at Austenite : Ferrite Boundaries in an Fe-C-Mo Alloy

During conventional isothermal transformation of an Fe-0.11 pct C-1.95 pct Mo alloy, eutectoid decomposition occurs by the interphase boundary carbide precipitation and the fibrous carbide mechanisms at 770° to 825 °C. When proeutectoid ferrite is formed and then recrystallized within the α + γ region, and subsequently further transformed at 770° to 825 °C, however, both of these eutectoid decomposition mechanisms are rendered inoperative. Carbide precipitation occurs instead entirely as isolated particles. This result supports the deduction that carbide precipitation at austenite : ferrite boundaries can occur only when these boundaries are locally immobilized by a partially coherent interfacial structure. A general approach to explaining the development of planar and curved interphase boundary precipitation, fibrous structure, and pearlite is developed in terms of two crystallographic factors. Formerly Research Associate in the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 and the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213 Formerly Graduate Student, Michigan Technological University, and Visiting Graduate Student, Camegie-Mellon University Formerly Professor at Michigan Technological University     

New insights into the widmanstätten proeutectoid ferrite transformation: Integration of crystallographic and three-dimensional morphological observations

The crystallography and three-dimensional (3-D) morphology of Widmanstätten proeutectoid ferrite precipitates are examined in an Fe-0.12 wt pct C-3.28 wt pct Ni steel isothermally reacted at 650 °C, 600 °C, and 550 °C. This article integrates new orientation mapping (OM) results with the findings of a companion article to this one on the 3-D morphology of proeutectoid ferrit^[1] and an earlier transmission electron microscopy (TEM) study which is reanalyzed here in light of the new OM and 3-D results. All of these studies were performed for the same alloy and heat treatments. The 3-D morphologies and distributions of proeutectoid ferrite precipitates are now known to often be quite different from those deduced by conventional two-dimensional (2-D) microscopy techniques. The present crystallographic studies indicate that “primary” ferrite (nucleated directly on prior austenite grain boundaries) forms monolithic single crystals and can be approximated as elongated triangular pyramids. “Secondary” ferrite morphologies can be described as laths and plates branching into the austenite from a thick and/or broad allotriomorphic ferrite base. These secondary Widmanstätten branches are composed of many misoriented crystals with ferrite: ferrite boundaries between them and appear to approach a common orientation as they extend into the austenite grain. Implications of the current findings on existing growth and crystallography models are discussed, and a preliminary hypothesis or mechanism of ferrite formation has been proposed to account for the present observations.     

Influence of interfacial structure upon carbide precipitation at austenite: Ferrite boundaries in an Fe-C-Mo alloy

During conventional isothermal transformation of an Fe-0.11 pct C-1.95 pct Mo alloy, eutectoid decomposition o°Curs by the interphase boundary carbide precipitation and the fibrous carbide Mechanisms at 770° to 825 °C. When proeutectoid ferrite is formed and then recrystallized within the α+ γ region, and subsequently further transformed at 770° to 825 °C, however, both of these eutectoid decomposition Mechanisms are rendered inoperative. Carbide precipitation o°Curs instead entirely as isolated particles. This result supports the deduction that carbide precipitation at austenite: ferrite boundaries can o°Cur only when these boundaries are locally immobilized by a partially coherent interfacial structure. A general approach to explaining the development of planar and curved interphase boundary precipitation, fibrous structure, and pearlite is developed in terM_s of two crystallographic factors. T. OBARA, formerly Research Associate in the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 and the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. G. J. SHIFLET, formerly Graduate Student, Michigan Technological University, and Visiting Graduate Student, Carnegie-Mellon University. H-I AARONSON, formerly Professor at Michigan Technological University.     

Plasma Nitriding Behavior of Fe-C-M (M = Al, Cr, Mn, Si) Ternary Martensitic Steels

Change in surface hardness and nitrides precipitated in Fe-0.6C binary and Fe-0.6 mass pct C-1 mass pct M (M = Al, Cr, Mn, Si) ternary martensitic alloys during plasma nitriding were investigated. Surface hardness was hardly increased in the Fe-0.6C binary alloy and slightly increased in Fe-0.6C-1Mn and Fe-0.6C-1Si alloys. On the other hand, it was largely increased in Fe-0.6C-1Al and Fe-0.6C-1Cr alloys. In all the Fe-0.6C-1M alloys except for the Si-added alloy, fine platelet alloy nitrides precipitated inside martensite laths. In the Fe-0.6C-1Si alloy, Si-enriched film was observed mainly at a grain boundary and an interface between cementite and matrix. Crystal structure of nitrides observed in the martensitic alloys was similar to those in Fe-M binary ferritic alloys reported previously. However, there was a difference in hardening behavior between ferrite and martensite due to a high density of dislocations acting as a nucleation site of the nitrides and partitioning of an alloying element between martensite and cementite changing the driving force of precipitation of the nitrides.     

Supersaturation and precipitation of carbon in austenite as a result of electrotransport

Supersaturation and precipitation of cementite and/or graphite in austenite due to electrotransport were studied. Two types of experiments were made with alloys of relatively high initial carbon concentration (1.3 wt. pct); one in the two-phase (austenite plus cementite) region at 850° and the other in the single phase (austenite) region at 927° C. In the two-phase experiments the formation of cementite and/or graphite was observed at the high-carbon end of the specimen. In the single-phase case, only a continuous layer of graphite was formed at the cathode end of the specimen. The present research suggests that electrotransport can be used to investigate processes involving supersaturation, such as precipitation from solid solution. T. Okabe, formerly Postdoctoral Fellow, Department of Metallurgical and Materials Engineering, University of Florida, Gainesville, Florida     

Precipitation of VC in ferrite and pearlite during direct transformation of a medium carbon microalloyed steel

Transmission electron microscopy of an air-cooled medium carbon (0.5 wt pct) steel containing 0.1 wt pct vanadium has shown that VC precipitates by the interphase mecha-nism during transformation to both proeutectoid and pearlitic ferrite. Depending upon the rate of transformation, a considerable proportion of the available vanadium may remain in supersaturated solid solution and can be precipitated as VC upon subsequent aging at 700°C. It was found that the proportion of proeutectoid ferrite, the interlamellae pearlite spac-ings and the VC precipitate dispersion parameters all decreased with increasing cooling rate in as-transformed material. G. FRIMODIG were formerly undergraduate students     

Volume Fractions of Proeutectoid Ferrite/Pearlite and Their Dependence on Prior Austenite Grain Size in Hypoeutectoid Fe-Mn-C Alloys

It has been generally believed that pearlite transformation in hypoeutectoid steels starts when the average carbon concentration in untransformed austenite reaches the A_cm line after the formation of proeutectoid ferrite. To test this concept experimentally, volume fractions of proeutectoid ferrite/pearlite and carbon contents in the austenite being transformed into pearlite were measured for the Fe-2Mn-0.3C alloy isothermally transformed in the temperature range 848 K to 898 K (575 °C to 625 °C). It was found that lamellar pearlite can form even when the average carbon content in untransformed austenite is much lower than the A_cm line. This peculiar observation is probably due to the two-dimensional diffusion of carbon, i.e., parallel to and normal to the austenite/pearlite interface, which enables lamellar cementite to grow continuously by supplying carbon atoms to its growth front. This results in proeutectoid ferrite fractions with respect to pearlite being much lower than those predicted by the lever rule. With decreasing prior austenite grain size, proeutectoid ferrite fractions with respect to pearlite were found to increase, but the thickness of proeutectoid ferrite was constant within the range of grain size investigated. This is due to the existence of the critical α/γ interface velocity only below which pearlite (actually cementite) can be nucleated at the migrating α/γ interface. Furthermore, the upper limit temperatures for pearlite formation in the Fe-1Mn-0.33C and Fe-2Mn-0.3C alloys were found to be well between the PLE/NPLE and PE A_e1 temperatures.     

Kinetics of Reverse Transformation from Pearlite to Austenite in an Fe-0.6 Mass pct C Alloy and the Effects of Alloying Elements

Substitutional alloying effects on reversion kinetics from pearlite structure at 1073 K (800 °C) in an Fe-0.6 mass pct C binary alloy and Fe-0.6C-1 or 2 mass pct M (M = Mn, Si, Cr) ternary alloys were studied. Reverse transformation in the Fe-0.6C binary alloy at 1073 K (800 °C) was finished after holding for approximately 5.5 seconds. The reversion kinetics was accelerated slightly by the addition of Mn but retarded by the addition of Si or Cr. The difference of acceleration effects by the addition of the 1 and 2 mass pct Mn is small, whereas the retardation effect becomes more significant by increasing the amount of addition of Si or Cr. It is clarified from the thermodynamic viewpoint of carbon diffusion that austenite can grow without partitioning of Mn or Si in the Mn- or Si-added alloys. On the one hand, austenite growth is controlled by the carbon diffusion, whereas the addition of them affects carbon activity gradient, resulting in changes in reversion kinetics. On the other hand, thermodynamic calculation implies that the long-range diffusion of Cr is necessary for austenite growth in the Cr-added alloys. It is proposed that austenite growth from pearlite in the Cr-added alloys is controlled by the diffusion of Cr along austenite/pearlite interface.     

Effects of Residual Elements Arsenic,Antimony, and Tin on Surface Hot Shortness

Scrap-based electric arc furnace (EAF) steelmaking is limited by a surface cracking problem in the recycled steel products, which is known as surface hot shortness. This problem originates from the excessive amount of copper (Cu) in the steel scrap, which enriches during the oxidation of iron (Fe) and consequently melts and penetrates into the austenite grain boundaries. In this article, the effects of arsenic (As), antimony (Sb), and tin (Sn) on surface hot shortness were investigated. A series of Fe-0.3 wt pct Cu-x wt pct (As, Sb, or Sn) alloys with x content ranging from 0.06 to 0.10 wt pct was oxidized in air at 1423 K (1150 °C) for 60, 300, and 600 seconds inside the chamber of a thermogravimety analyzer (TGA) where heat is supplied through infrared radiation. Scanning electron microscopy (SEM) investigations show that (1) the presence of Sb and Sn results in severe grain boundary cracking, whereas the presence of As does not, (2) open cracks with Fe oxides were found beneath the oxide/metal interface in the Sb and Sn alloys, and (3) the oxide/metal interfaces for all As, Sb, and Sn alloys are planar. Penetration experiments of pure Cu and Cu-30 wt pct Sn liquid were also conducted in the chamber of a hot-stage confocal laser scanning microscopy (CLSM) in nonoxidizing atmosphere: (1) on the Fe-35 wt pct manganese (Mn) alloys to study the correlation between cracking and grain boundary characters, and (2) on the pure Fe substrates to exclude the bulk segregation effects of Sn on grain boundary cracking. It was found that grain boundary cracking rarely took place on low-energy grain boundaries. The results also suggest that the bulk segregation of Sn in the substrate is not necessary to promote significant grain boundary cracking, and as long as the liquid phase contains Sn, it will be highly embrittling.     

An investigation of the generality of incomplete transformation to bainite in Fe-C-X alloys

The overall kinetics of the isothermal transformation of austenite to bainite and to pearlite in high-purity Fe-C-3 at. pct X alloys (X = Mn, Si, Ni, or Cu) containing 0.1 wt pct C and 0.4 wt pct C were investigated with quantitative metallography and transmission electron microscopy (TEM) to ascertain the presence or absence of the incomplete reaction phenomenon. The incomplete transformation of austenite to bainite was not observed in the Fe-C-Si, Fe-C-Ni, Fe-C-Cu, or Fe-0.4C-Mn alloys. It was found, however, in the Fe-0.1C-Mn alloy. Transmission electron microscopy results indicate that sympathetic nucleation of ferrite without carbide precipitation is a necessary but not a sufficient condition for the development of the incomplete reaction phenomenon. Transformation resumes following stasis in the low-carbon Fe-C-Mn alloy with the formation of a nodular bainite. The results support the view that the incomplete transformation of austenite to bainite is a characteristic of specific alloying elements and is not an inherent trait of the bainite reaction. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. Formerly Visiting Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. Formerly Undergraduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Age hardening and mechanical properties of a 2400 MPa grade cobalt-free maraging steel

The age hardening kinetics in the temperature range of 713 to 813 K of a 2400 MPa grade cobalt-free maraging steel (Fe-(18.8 ∼ 19.1) pct Ni-(4.4 ∼ 5.4) pct Mo-2.6 pct Ti, wt pct) has been studied. Study of microstructure and mechanical properties showed that a high number of Ni₃Ti and Fe₂(Mo,Ti) precipitates were formed during the ageing process, which resulted in high strength and relatively low fracture toughness. Ni₃Ti was the main precipitation phase. Fractography has shown ductile failure of tensile and fracture toughness specimens. Thermodynamic calculations showed that the equilibrium phases are Ni₃Ti, Fe₂(Mo,Ti), ferrite, and austenite.     

Precipitation and fine structure in medium-carbon vanadium and vanadium/niobium microalloyed steels

Hot-rolled and continuously cooled, medium-carbon microalloyed steels containing 0.2 or 0.4 pct C with vanadium (0.15 pct) or vanadium (0.15 pct) plus niobium (0.04 pct) additions were investigated with light and transmission electron microscopy. Energy dispersive spectroscopy in a scanning transmission electron microscope was conducted on precipitates of the 0.4 pct C steel with vanadium and niobium additions. The vanadium steels contained fine interphase precipitates within ferrite, pearlite nodules devoid of interphase precipitates, and fine ferritic transformation twins. The vanadium plus niobium steels contained large Nb-rich precipitates, precipitates which formed in cellular arrays on deformed austenite substructure and contained about equal amounts of niobium and vanadium, and V-rich interphase precipitates. Transformation twins in the ferrite and interphase precipitates in the pearlitic ferrite were not observed in either of the steels containing both microalloying elements. Consistent with the effect of higher C concentrations on driving the microalloying precipitation reactions, substructure precipitation was much more frequently observed in the 0.4C-V-Nb steel than in the 0.2C-V-Nb steel, both in the ferritic and pearlitic regions of the microstructure. Also, superposition of interphase and substructure precipitation was more frequently observed in the high-C-V-Nb steel than in the similar low-C steel.     

A study of nonisothermal austenite formation and decomposition in Fe-C-Mn alloys

Experiments using a hot-stage confocal scanning laser microscope (CSLM) have been carried out to observe phase transformations in two steels: Si-killed resulfurized Fe-0.38 wt pct C-1.43 wt pct Mn and Al-killed Fe-0.20 wt pct C-0.87 wt pct Mn. Austenite formation during continuous heating was investigated on the surface of samples that were etched to reveal the ferrite and pearlite regions. It was found that the austenite precipitated first at the pearlite colonies and subsequently in the ferrite phase. The measured advance rates of the γ/pearlite front were roughly twice those of the γ/α front and both interfaces were found to be curved. The γ/pearlite migration rate was found to be in qualitative agreement with published rate equations for isokinetic austenite formation where diffusion is the rate-limiting step. Austenite decomposition was studied during cooling. Widmanstätten ferrite laths precipitate as distinct colonies from the existing allotriomorphic ferrite phase but then also at MnS precipitates. The electron backscatter diffraction (EBSD) analysis showed that all of the laths in a particular colony exhibit similar orientation to one another but a slightly different orientation than the parent allotriomorph, supporting a sympathetic nucleation mechanism. The growth rate of the laths was found to vary widely within a range of 1.5 to 11 μm/s. The ferrite formation is finally halted by impingement with other advancing fronts. The results are presented in a phenomological discussion, with some quantitative analysis of the transformation kinetics.