Austenite grain coarsening in low–carbon manganese steels containing niobium and aluminium

Abstract

The effect of microadditions of niobium, aluminium, and a combination of the two on the austenite grain coarsening behaviour of 0·08C–0·25Si–1·5Mn steels whose nitrogen content was varied/from 0·002 to 0·02 wt-% has been studied. Modified methods of surface oxidation and thermal grooving were used in order to determine the precise prior austenite grain size in these steels with low interstitial elements. The optimum concentration of niobium which could be effectively used for restraining austenite grain growth when steels are austenitized at high temperatures has been determined. It is also demonstrated that steels containing. NbC or NbN particles exhibit similar austenite grain coarsening behaviour. Increasing the nitrogen content from 0·002 to 0·02 wt-% to give nitrogen–rich Nb(CN) particles does not help restrain austenite grain growth. However, when nitrogen was added to steels containing aluminium or aluminium and niobium, significant improvement in grain restraining behaviour was observed.

MST/120     

Isothermal embrittlement of Fe–8Mn alloys at 450°C

Abstract

Two Fe–8Mn alloys, one of which is alloy 193, stabilised with 0·17%Ti and 0·18%Al, were austenitised at 900°C, ice brine quenched and their DBTTs determined. In this condition, brittle fractures were predominantly cleavage, and thermodynamic calculations on alloy 193 showed that there were 0·0025 wt-%C and <0·03 ppm N in solid solution. Alloys were tempered for 6 min, 1 h and 10 h at 450°C and their DBTTs again determined; in this case, brittle fractures were mainly intergranular. In alloy 193, DBTT rose from 27 to 125°C in 6 min. Hardness values at 450°C were also monitored and the variation of hardness with time is discussed. It is thought that brittle fracture in alloy 193 is due to segregation of Mn per se to prior austenite grain boundaries, unlike an earlier investigation of a pure Fe–8Mn alloy (K1525), where embrittlement was due to a Mn–N and to a lesser extent a Mn–P interaction at prior austenite grain boundaries. The driving force for Mn segregation to prior austenite grain boundaries is thought to be the initial formation of reverted austenite at such sites.     

Effect of austenite grain boundary mobility on hardenability of steels containing vanadium

Abstract

A correlation has been established between the rate of grain boundary migration during austenitisation and the hardenability of steels containing 0·2–0·3%C, 1·5–1·7%Mn, up to 0·35% V, and small additions of Al or Ti. Interaction between the austenite grain boundaries and pinning particles was investigated using transmission electron microscopy and segregation to the austenite grain boundaries was examined using Auger electron spectroscopy. It has been concluded that the velocity of grain boundary migration during austenitisation influences the extent of equilibrium segregation to the austenite grain boundaries which, in turn, affects the hardenability. Pinning of the austenite grain boundaries enabled the potential hardenability effect of the alloying elements to be increased. Mechanisms have been discussed for the ways in which segregation, particularly of V, occurs to pinned or immobilised austenite grain boundaries, and the conditions by which most effective grain boundary pinning can be achieved have been considered. Some technological implications have been suggested.

MST/804     

Fe–AI–Zn ternary phase diagram at 450°C

Abstract

Several Fe–Al and Fe–Al–Zn alloys were vacuum melted, annealed at 450°C under atmospheric pressure, and then quenched in iced water. The structure and composition of phases were determined using microstructural, X-ray diffraction, and X-ray energy dispersive analyses. The maximum solubility of zinc was found to be 5·3 wt-% in FeAl₃, 20·9 wt-% in Fe₂Al₅, and 2·0 wt-% in FeAl at 450°C. Based on these results, an isothermal section for the aluminium rich corner of the Fe–Al–Zn ternary phase diagram is proposed.

MST/1196     

Influence of peritectic phase transformation on hot ductility of high aluminium TRIP steels containing Nb

Abstract

Increasing Al from 0·05 to 1% in Nb containing transformation induced plasticity steel resulted in deepening and considerable widening of the hot ductility trough. Further increase in the Al level to 1·5% produced a trough similar to the low Al steel but having better ductility in the temperature range of 650–800°C. This improved ductility could be ascribed to its finer austenite grain size. Nb(CN) was able to precipitate readily in these steels and was important in influencing the hot ductility of the 0·05 and 1·5%Al steel in the temperature range of 750–1000°C, with ductility improving as the particle size increased with test temperature. No AlN was found in 0·05%Al containing steel, and there was no significant dendritic precipitation of AlN in 1·5%Al containing steel, although precipitation of AlN in plate form was readily observed. In 1%Al steel, copious dendritic precipitation of AlN was present at the γ grain boundaries, leading to rock candy fracture. The poor ductility shown in 1%Al containing steel is due to a combination of this dendritic precipitation, which took place only in a steel of peritectic carbon composition, and its coarse grain size. Both low and 1·5%Al containing steels had compositions outside the peritectic range. It is strongly advised that for this type of steel, the composition should be designed to fall outside the peritectic carbon range.     

Thermal stability of Al/Al11 Ce3 and Al/Al11 La3 /Al3 Ni eutectics obtained by Bridgman growth

Abstract

The effects of extended treatment at 500°C on microstructure and microhardness of Al/Al₁₁ Ce₃ and Al/Al₁₁ La₃ /Al₃ Ni eutectics in Al–12·1Ce and Al–11·3La–5·4Ni (wt-%) alloys Bridgman solidified at 0·1 mm s^-1 are reported. Coarsening of Al/Al₁₁ Ce₃ occurs more rapidly in some eutectic cells than others such that areas coarsening at a lower rate still form 10% of the microstructure even after 3024 h. It was found that Al/Al₁₁ La₃ /Al₃ Ni showed accelerated coarsening at eutectic cell boundaries but association between Al₁₁ La₃ and Al₃ Ni still remained within cells and at cell boundaries even after 3024 h. Associated decreases in hardness for Al/Al₁₁ Ce₃ were similar to fibrous Al/Al₆ Fe of similar initial spacing subjected to the same heat treatment. The 30% higher initial hardness of Al/Al₁₁ La₃ /Al₃ Ni largely persisted even after 3024 h at 500°C.     

Microstructural characterisation and thermal stability of in situ TiB2/60Si–Al composite synthesised by displacement reactions and spray forming

Abstract

A novel in situ reactive technique has been employed for preparing 2·0 wt-%TiB₂/60Si–Al composite. The kinetic equations and the Arrhenius type equation were applied to compute the coarsening rate constant and the activation energy for grain growth for the composite when it was heated at semisolid state for partial remelting. Experimental results have shown that the in situ TiB₂ particles can refine effectively the primary Si phase and restrain the Si phase growth. The cubic coarsening rate constant for the composite was computed to be in the range of 75–148 μm³ min^?1 at temperatures in the range of 600–700°C, which was much less than that for the 60Si–Al alloy (1323–4523 μm³ min^?1). The value of activation energy for grain growth for the composite was about twice of that for the 60Si–Al alloy. The composite exhibited a higher thermal stability than that of the 60Si–Al alloy, suggesting that the in situ TiB₂ particles can effectively pin the grain boundaries and arrest the migration of liquid film in the semisolid state of the composite.     

Austenite and ferrite grain sizes in interstitial free steel

Abstract

An oxidation method has been employed to reveal prior austenite grain boundaries in C–Mn and interstitial free (IF) steels. The ability of this technique to reveal prior austenite grain boundaries is assessed by comparing its results with those of an etching method applied on the C–Mn steel. Optimum conditions were established by trial and error. The conditions varied with different steels and with heat treatment temperature. In the IF steel rapid grain growth at high temperatures in the ferrite range made a significant contribution to the prevention of grain refinement through transformation. Attempts to obtain the smallest prior austenite grain size in the IF steel to assess the ability of the oxidation technique to reveal fine austenite grains led to an average austenite grain size of 80 μm in warm rolled samples after the shortest holding time at 950°C.

MST/3203     

Superplasticity of Fe3Al(Cr)

Abstract

The superplasticity of an Fe₃Al based intermetallic alloy with 3 at.-% chromium has been investigated in the strain rate range 10^-5-10^-2 s^-1 at test temperatures between 700 and 900°C. The composition of the iron aluminide was Fe–28Al–3Cr (at.-%) with additions of titanium and carbon. After thermomechanical processing the material possessed a coarse grained microstructure with an average grain size of 55 ± 10 μm. Strain rate exponents of 0·33≤m≤0.42 were recorded at strain rates of approximately 10^-5-10^-3 s^-1 in the temperature range 750-900°C. Superplastic elongations of 350% and more were achieved. From thermal activation analysis of superplastic flow, an activation energy of 185 ± 10 kJ mol^-1 was derived. This value is comparable to activation energies of superplastic flow in Fe₃Al(Ti) alloys. However, in unalloyed Fe₃Al the activation energy is higher, ~ 263 kJ mol^-1. Optical microscopy showed grain refinement to ~ 30 ± 5 μm in size in superplastically strained tensile specimens. Transmission electron microscopy gave evidence of the formation of subgrains of 0·3–0·5 μm in size. Superplasticity in this iron aluminide is mainly attributed to viscous dislocation glide, controlled by solute drag in the transformed B2 lattice at the deformation temperatures. During superplastic deformation, subgrain formation and grain refinement in the gauge length were revealed. From this it is concluded that dynamic recrystallisation makes an important contribution to the deformation mechanism of superplastic flow in this material.     

Influence of grain size on hot ductility of plain C–Mn steels

Abstract

The influence of grain size on the hot ductility of 0·19 and 0·65wt-%C steels of the C–Mn type has been determined. For the low-carbon steel, a gram Size increase from 70 to 180 μm had only a small influence on hot ductility, as measured by tensile reduction in area values. However, increasing the grain size to 290 μm raised the temperature at which ductility started to fall by 50°C. In the finer grained steels it is believed that the ductility trough starts at the Ar₃ temperature when films of ferrite form round the stronger austenite grains. Ductility soon recovers as the temperature is lowered because of a thickening of the ferrite and a consequent reduction of strain concentration at the boundaries, so that only a narrow trough is observed. In coarser grained steels it is considered that deformation induced ferrite can have a pronounced influence on hot ductility over a wide range of temperatures leading to a wide ductility trough. Refining the grain size had an even greater influence on the hot ductility of the 0·65wt-%C steel. Intergranular tensile fracture at coarse grain size was by grain boundary sliding in the austenite resulting in a very wide ductility trough. Refining the grain size prevented intergranular failure occurring in the γ down to the lowest temperature examined: 700°C. Although the main influence of grain size is in controlling the width of the trough, the depth also increased with an increase in grain size.

MST/420     

Structure–property relationships for quenched and tempered flanges made to ASTM A350 LF2 specification

Abstract

The structure–property relationships for quenched and tempered flanges made to the ASTM A350 LF2 specification have been determined. Samples from Al containing and Al free forged flanges have been heated to temperatures in the range 900–1000°C to produce a wide range in γ grain size and quenched in oil or iced water to produce a variety of commercially obtainable microstructures from fine grained ferrite–pearlite to fully bainitic. After quenching, the samples were tempered at 600°C, and the Charpy V-notch impact values at ?46°C and room temperature hardness values were obtained. Raising the quenching temperature reduced the impact energy values and increased the hardness. Increasing the cooling rate also increased hardness, but there was little change in impact behaviour. Impact behaviour was found to be mainly dependent on the γ grain size: the coarser the grain size, i.e. the higher the quench temperature, the lower the impact value. The facet size was also found to be related to γ grain size, indicating that the high angle grain boundaries, i.e. γ, bainite colony, and ferrite boundaries, were the main obstacles to crack propagation. Increasing the cooling rate from the austenitising temperature increased hardness without significantly affecting the impact behaviour. It is believed that the expected deterioration in impact behaviour associated with an increase in hardness was offset by a refinement of the carbides and the removal of the long carbides situated at the interlath ferrite boundaries. The impact energy in joules at ?46°C was given by the equation: impact energy absorbed at ?46°C=82+18·6d^?1/2?0·8H+0·05CR, where d is austenite grain size in millimetres, H is hardness HV10, and CR is cooling rate in K min^?. To meet the ASTM A350 LF2 impact requirement, the γ grain size should be below 40 μm, and this necessitates a grain refining addition.

MST/771     

Division of character zones and elements distribution of Fe3Al/Cr–Ni alloy fusion bonded joint

Abstract

A Fe₃Al/Cr–Ni alloy fusion bonded joint was divided into four character zones of a homogeneous mixture zone, a partial mixture zone, a partially fused zone and a heat affected zone. The microstructures, elements distribution and phase constitutions of the various character zones were analysed via metalloscope, SEM, electron probe microanalysis and X-ray diffraction. The results indicated that the microstructures were dissimilar in the different character zones. A 0·04–0·05 mm austenite rich band existed in the partial mixture zone. The diffusion of Fe, Al, Cr, Ni and C mainly occurred in fusion zone where Cr and Ni diffused into Fe₃Al to substitute some Fe on α ₁, α ₂, and β sublattices to form substitutional solid solution. The phase constitutions of Fe₃Al/Cr–Ni joint were Fe₃Al, γ-Fe, FeAl, NiAl, an unidentified Fe–C compound and an Fe–Cr–C compound (Cr₉Fe)₇C₃.     

Kinetics of martensite formation in plain carbon steels: critical assessment of possible influence of austenite grain boundaries and autocatalysis

Abstract

The kinetics of the martensitic transformation in Fe–0·80C has been determined from dilatometry data and shows no significant variation when the cooling rate is changed by two orders of magnitude. All kinetic data can be adequately simulated by the Koistinen and Marburger (KM) equation using a specific start temperature T_KM and rate parameter α_m. This finding supports the suggestion that the transformation is athermal, and moreover, the absence of a time dependence strongly indicates that autocatalytic nucleation does not contribute to the transformation kinetics in plain carbon steels on measurable time scales. Furthermore, dilatometry experiments with different austenitising conditions were conducted to examine the effect of the prior austenite grain size on the overall kinetics of martensite formation. The present results indicate that the progress of martensite formation beyond a fraction f?=?0·15 is independent of the prior austenitising treatment. It is therefore concluded that austenite–austenite grain boundaries have no significant effect on the overall nucleation and growth of athermal martensite, which is consistent with a model proposed by Ansell and co-workers.     

Combined quenching and tempering induced phosphorus segregation to grain boundaries in 2·25Cr–1 Mo steel

Abstract

Combined quenching and tempering induced phosphorus segregation to prior austenite grain boundaries in α 0·077 wt-%P doped 2·25Cr–1Mo steel was examined using field emission gun scanning transmission electron microscopy. The results indicate that combined equilibrium and non-equilibrium phosphorus segregation may play an important part in temper embrittlement of the steel caused by direct tempering after quenching. Non-equilibrium segregation requires the formation of sufficient quantities of vacancy–impurity complexes and their migration to grain boundaries is of great importance in the segregation. For this reason, the mechanism for migration of the complexes is discussed in detail.

MST/3419     

High temperature tensile properties of oxide dispersion strengthened T91 and their correlation with microstructural evolution

Abstract

In the present work, high temperature deformation behavior of oxide dispersion strengthened T91 was investigated and linked to the corresponding microstructure. First, tensile properties are presented and discussed in terms of yield strength, tensile stress and total elongation as a function of temperature. The results are compared to the matrix material and other ODS alloys. Second, transmission electron microscopy was applied to as received and deformed tensile test specimens. It is shown that the Y₂O₃ particle diameter increases slightly upon deformation at elevated temperatures. Additionally, distinctive coarsening of M₂₃C₆ carbides occurs at prior austenite grain boundaries. At temperatures above 500°C, dislocations are straight and pile up at grain boundaries due to thermally activated climbing. Oxide dispersion strengthened T91 provides high strength due to strong particle/dislocation interactions and good toughness properties.     

Welding of rapidly solidified Alloy 8009 (Al–8·5Fe–1·7Si–1·3V): preliminary study

Abstract

Alloy 8009 is a rapidly solidified, dispersion strengthened Al–8·5Fe (wt-%) alloy designed for high temperature (up to 400°C) aerospace applications. Both fusion and solid state joining techniques were shown to produce bonds. Fusion techniques destroyed the base metal microstructure with primary Fe₃Al, loss of solute, formation of larger aluminium grains, and the formation of grain boundary FeAl₃ and intermetallics enriched with silicon and vanadium. Solid state friction stir welding did not cause a significant modification to the dispersoid population but there was a loss of solute to dispersoid/matrix interfaces.

MST/3500     

Aging behaviour of cobalt free chromium containing maraging steels

Abstract

This paper reports an investigation of the aging behaviour of two Co free Cr containing maraging steels (Fe–1·0Si–11·2Cr–1·3Mo–9·1Ni–1·2Al–1·0Ti and Fe–0·8Si–17·2Cr–6·1Ni–0·4Al–0·9Ti, all at.-%), using hardness measurements, electron microscopy of replicas and thin foils, atom probe field ion microscopy (APFIM), and thermochemical calculations. Two different families of intermetallic phases (Ti₆Si₇Ni₁₆G phase and η Ni₃Ti) have been found to contribute to age hardening. The composition and morphology of these precipitates were studied in deformed and undeformed alloys after aging at 420–570°C for various times. In addition, reverted austenite has been found in the aged structure. Results obtained using APFIM are compared with equilibrium thermodynamic calculations and previous APFIM studies of conventional Cr free low Al and Si maraging steels having higher Mo contents.

MST/1558     

Studies on influence of reheating temperature and cooling rate on structure–property behaviour of lean chemistry HSLA-100 steel

Abstract

Simulation studies on the influence of reheating temperature on austenite grain coarsening in lean chemistry high strength low alloy (HSLA)-100 steel were carried out to establish optimum soaking temperature before hot rolling. Experiments carried out in ‘Gleeble-3500’ dynamic thermomechanical simulator revealed that prior austenite grain sizes varied between 26 and 98 and 34 and 126 μm after soaking at 1150, 1200 and 1250°C for 1 and 5 min respectively; a soaking temperature of 1200°C was found to be optimum. Simulation experiments on the influence of cooling rate on microstructural changes and dilatometric studies indicated lowering of transformation temperature with faster cooling. Microstructural examination of dilatometric samples confirmed martensitic transformation at faster cooling rate. The martensite structure is desirable to achieve better strength and toughness. The findings of simulation studies were subsequently used for standardising thermomechanical treatments of Nb–Cu bearing lean chemistry HSLA-100 steels. One laboratory heat of Cu bearing HSLA steel containing 0·028%Nb was made. This heat was hot rolled into 12·5 mm thick plate by varying finish rolling temperature in the range of 800–1000°C. The soaking temperature was maintained at 1200°C. The rolled plates were heat treated by both conventional reheat quenching and tempering (RQT) as well as direct quenching and tempering (DQT) techniques. Evaluation of mechanical properties revealed that plates processed through DQT route were superior to those processed through RQT route. Transmission electron microscopy revealed that martensite structure and finer interlath spacing in DQT plates resulted in superior strength and impact toughness properties as compared to RQT steels.     

Thermally and stress induced changes in three phase structure of Cu–Zn–Al–Fe shape memory alloy

Abstract

In order to explain the stabilised shape memory behaviour observed during the thermal and mechanical cycling of a Cu–14·86Zn–5·81Al–0·5Fe (wt-%) alloy, X-ray diffraction and optical microscopy studies have been performed. The former have shown that the previously reported reorientation of the α brass phase crystallites from α(111) to α(200), observed during the heating up to 500°C, also occurs during a complete heating–cooling cycle up to 325°C. Optical microscopy studies confirm the increasing tendency for α phase preferred formation when the temperature is increased from 325 to 500°C. This texturing effect that occurs during thermal cycling is accompanied by an increase in the amount of α phase with a reduction in the amount of β ₂ austenite. Finally, a tensile completely recoverable pseudoelastic strain of almost 0·6%, determined during a loading–unloading cycle applied to a heat treated specimen that contained no austenite, has been ascribed to the reversible formation of a specific martensite variant.     

Effect of pearlite morphology on impact toughness of eutectoid steel containing vanadium

Abstract

The effect of a change in the morphology of the pearlite colonies on the Charpy impact energy of a fully pearlitic steel containing 0·76%C, 1·20%Mn, and 0·085% V was examined over the range of testing temperatures from ?50 to 200°C. The change from a multicolony nodular pearlite structure produced from austenite of grain size 185 μm to a structure composed of individually formed colonies produced from austenite of grain size 25 μm caused a decrease in the transition temperature of 75 K and an almost 100% increase in the Charpy impact energy measured at room temperature. It is proposed that the impact toughness of pearlitic steel can be affected by pearlite morphology, at constant interlamellar spacing, only at temperatures above the ductile–brittle transition temperature of the ferrite, when local plastic deformation in the pearlitic ferrite at high angle boundaries can arrest propagating brittle cracks.

MST/730