Uptake of iron and its effect on grain refinement of pure magnesium by zirconium

Abstract

The uptake of iron by molten magnesium from uncoated new mild steel crucibles at temperatures 680°C, 730°C, and 780°C has been investigated. It was shown that the uptake of iron was sluggish at 680°C and the use of 0.05% zirconium addition could effectively suppress the increase in iron content within the first 2 h of holding at temperature. Rapid and severe uptake of iron was observed at 780°C. As a consequence, it was found that the grain refinement of pure magnesium achieved by 1% zirconium addition nearly vanished after 60 min hold at 780°C due to the depletion of soluble zirconium. The uptake of iron at 730°C was conspicuous but it was still controllable by use of 0.05% zirconium addition within the first 60 min of holding at temperature. The work conducted using an aluminium titanite crucible and a boron nitride coated mild steel crucible at 730°C further confirmed the highly detrimental influence of the uptake of iron on the grain refinement of pure magnesium by zirconium. The characteristic zirconium rich coring structures developed from circular to rosette like when the melt was held at 730°C in an uncoated mild steel crucible, while no such evolution was observed when held in an aluminium titanite crucible at the same temperature. Recommendations to minimise the consumption of zirconium by the uptake of iron were made based on the results obtained from this investigation. The mechanism of grain refinement of magnesium by a low concentration of zirconium is discussed.     

Influence of intercritical annealing on strength and impact behaviour of niobium containing steels

Abstract

The influence of inter critical annealing at 730°C on the impact properties and strength of C–Mn–Al–Nb steels has been examined. For low Mn (0·56%), Nb steels, intercritical annealing resulted in improved impact performance and the impact transition temperature (ITT) was reduced by as much as 35 K with no change in strength. The improvement in impact performance is considered to be due to Mn segregating to the α/γ boundaries leading to refinement of the grain boundary carbides. This refinement increased with holding time at 730°C in accordance with an increased grain boundary segregation of Mn. Strength was not influenced because grain size remained unchanged on intercritical annealing. The improvement in impact behaviour was greater the longer the holding time at 730°C but was significant even after 15 min. Improvements occurred both on cooling from the austenitising temperature (9·20°C) to 730°C and on heating from room temperature to 730°C, the latter heat treatment being the more beneficial. For higher Mn (1·4%), Nb steels, improvements in impact performance resulting from intercritical annealing depended on cooling rate. Again, the Mn build-up in the y increases with time of intercritical annealing. Owing to the initial overall higher Mn level and finer grain size, the steels were susceptible to martensite formation if the cooling rate was too high. At a cooling rate of 40 K min - 1, improvements in impact behaviour occurred only after short intercritical annealing times (30 min) when only a small amount of martensite had formed. Long times caused a serious deterioration in impact behaviour due to the presence of high volume fractions of martensite. Slow cooling (1 K min^?1), however, ensured ferrite–pearlite structures and significant improvements in impact behaviour (20–60 K reductions in ITT) were noted on intercritical annealing with no change in strength. The short holding times required to achieve an improvement in impact behaviour in these fine grained steels are encouraging for the possible commercial exploitation of this heat treatment.

MST/1382     

Effect of precipitation on hot ductility of niobium and aluminium microalloyed steels

Abstract

The precipitation reactions occurring in C–Mn–Al and C–Mn–Nb steels before and after hot deformation have been examined and their influence upon hot ductility is discussed. Precipitation has been studied at 850°C, when ductility is poor, and also at 1100°C, when the ductility is good. Rapid intergranular precipitation occurred at 1100°C, but the precipitation present before deformation did not prove to be detrimental to ductility and grain boundary mobility at this temperature. Although only a limited amount of precipitation occurred at 850°C before deformation, intergranular precipitation continued during deformation resulting in embrittlement of the steels. At this temperature, strain induced transgranular precipitation of Nb(CN) occurred in the C–Mn–Nb steel and this is thought to be a major cause of poor hot ductility in this steel. By holding the steels for 15 min at 800–850°C before reheating to 1100°C, the area fraction of intergranular precipitation at 1100°C was increased. This produced a decrease in ductility at this temperature in the C–Mn–Al steel but had a less marked effect in the C–Mn–Nb steel.

MST/107     

Influence of grain size and precipitation on hot ductility of microalloyed steels

Abstract

The influence of grain size on the hot ducility of microalloyed steels (C–Mn–Al, C–Mn–V–Al, and C–Mn–Nb–Al) has been determined by heating them above their solution temperatures and cooling to the test temperature of 850°C. The C–Mn–Al steel showed excellent hot ductility which was independent of grain size. Dynamic recrystallization readily occurred and there was no evidence for AlN precipitation. Marked dynamic precipitation occurred during the tensile test for vanadium- and niobium-containing steels but this did not vary significantly with reheating temperature, provided complete dissolution of the precipitates had occurred. Isolating the influence of grain size from that of precipitation in these steels showed that a change in grain size from 150 to 300 μm reduced the reduction of area values by 15–20%. Precipitate distribution was also varied by heating to temperatures in the range 850–1330°C and tensile testing at 850°C. When present before testing at the γ grain boundaries in the form of a fine grain-refining precipitate, AlN reduced the hot ductility in the C–Mn–Al steel and delayed the onset of dynamic recrystallization. Coarser precipitates produced by raising the reheating temperature allowing dynamic recrystallization to occur gave improved ductility. For the niobium- and vanadium-containing steels, precipitate distributions which were in a coarse randomly precipitated form gave the best hot ductility. These occurred with the niobium-containing steel when heated to 1100°C and more generally in the vanadium-containing steel throughout a wide temperature range. The worst precipitate distribution occurred in the niobium containing steel when the NbCN was taken into solution before testing and reprecipitated in a fine form at the γ grain boundaries and within the matrix during the test.

MST/490     

Coarsening of equiaxed microstructure in the semisolid state of aluminum 7075 alloy through SIMA processing

In the present study, the coarsening mechanism of equiaxed grains in the semisolid state of aluminum 7075 alloy, treated via strain induced melting activation process, was investigated. The kinetics of equiaxed grain growth in the semisolid state of the experimental alloy was determined. The results revealed that when the holding temperature increased, the coarsening rate constant (K) showed a precipitously increasing character in the range of 590–610 °C. This was attributed to the extensive effect of the coalescence mechanism on the grain growth at the high solid fractions. By further increasing the holding temperature to 620 and 625 °C (increasing the liquid fraction), the effect of coalescence on the grain growth appeared to be weakened, that is, although there was a slight decrease at 620 °C, a gently increasing character could be generally supposed. Severe segregation of Zn and Cu alloying elements at grain boundaries and intragranular droplets was detected at 620 and 625 °C after 15 and 10 min, respectively.     

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     

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     

Solid–liquid structural break-up in M2 tool steel for semi-solid metal processing

The success of semi-solid metal processing mostly depends on the formation of suitable starting microstructure, which must consist of solid metal spheroids in a liquid matrix. Various methods of obtaining this structure have been established; they include recrystallisation and partial melting (RAP), strain-induced melt-activated (SIMA), or simple mechanical stirring, to name a few. These methods, as widely discussed, have mostly been applied with light alloys, mainly aluminium based. This article discusses solid–liquid structural break-up in M2 tool steel subjected to a direct re-melting procedure from the as-annealed condition. The role of carbide dissolution in the grain boundary liquation of the steel is described. This leads to the production of near spheroidal solid grains in a liquid matrix, a microstructure suitable for the thixoforming process. Microstructural examination revealed that carbide particles contained in bands at 1220 °C slowly disappeared with temperature. At 1300 °C, the solid grains seemed to be free from carbides. Most of the carbides had now re-precipitated at the grain boundaries. Thixoforming carried out at 1340 and 1360 °C revealed the thixotropic properties of the semi-solid metal slurries. The results indicate a widening of the range of potential routes to thixoformable microstructures.     

Effect of heating temperature and magnesium content on the thermal cyclic failure behaviour of ductile irons

Abstract

This study elucidates the effect of residual magnesium content and heating temperature on the thermal cyclic failure behaviour of ductile irons by applying repeated heating and cooling cycles. Five irons with different residual magnesium contents ranging from 0.038 to 0.066 wt-% were obtained by controlling the amount of nodulariser additions. The thermal fatigue cracking behaviour was investigated during thermal cycling from 25°C to 650, 700, 750, and 800°C, respectively. Experimental results indicate that the thermal fatigue cracking resistance of ductile iron decreases with increasing residual magnesium content. The maximum heating temperatures of 700°C and 750°C led to the most severe thermal fatigue cracking in the specimens containing 0.054 wt-% and 0.060 wt-% residual magnesium content. Recrystallisation of ferrite grain occurred when the thermal cycles exceeded a certain number after testing at 800°C, which deferred the initiation of thermal fatigue cracking.     

Dissolution of laves phase by re-austenitization and tempering of creep strength enhanced ferritic steel

ABSTRACT

Formation of Laves phase in creep strength enhanced ferritic steel is investigated using re-austenitization and tempering treatment. The as-received material is exposed to 620°C for 4560?h aging, and then re-austenitizated at 1050°C for 1?h, and followed by tempering at 760°C for different times (2 and 4?h). After re-austenitization and tempering, the dissolution of Laves phase is observed while grain size and microhardness have not changed significantly. A model is suggested to quantify the dissolution of W-containing Laves phase. Thermo-Calc is used to predict driving forces for precipitation of Laves and M23C6 phases.     

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

Abstract

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (～20 μm) and a coarse average grain size (～260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ～534°C.     

Influence of phosphorus on the hot ductility of 2.25Cr1Mo steel

Abstract

The effect of phosphorus at two levels (0.01 and 0.06 wt-%) on the hot ductility of 2.25Cr1Mo steel has been investigated over the temperature range 700-1200 °C using a Gleeble machine. Auger electron spectroscopy indicated that phosphorus segregation to austenite grain boundaries had occurred in the higher phosphorus steel. A trough in the ductility - temperature curve was observed for both steels with the minimum ductility occurring at about 750 °C. The higher P containing steel gave the worst ductility throughout the temperature range examined. The ductility trough was caused by the formation of a thin pro-eutectoid ferrite layer along austenite grain boundaries, and this trough was accentuated by the presence of phosphorus at the austenite grain boundaries.     

Electron beam melting of Inconel 718: effects of processing and post-processing

ABSTRACT

This study reports the effect of process temperature on microstructure evolution of electron beam melted Inconel 718. Samples fabricated at 915°C had fine grain boundary δ (～200?nm) along with coarse intragranular δ spanning through the length of the grains. On the other hand, samples fabricated at 990°C, only had grain boundary δ along with secondary carbides. During hot isostatic pressing, the distribution of carbides governs the grain growth vs. lack of it. The samples fabricated at 990°C having grain boundary carbides had no grain growth owing to the pinning effect of carbides. In contrast, the sample processed at 915°C had significant grain growth owing to dissolution of grain boundary δ phase and absence of grain boundary carbides.     

Interaction between precipitation,normal grain growth,and secondary recrystallisation in austenitic stainless steel containing particles

Abstract

With the aid of various complementary methods of microstructural analysis, the precipitation, grain growth, and secondary recrystallisation behaviour of an 15Cr–15Ni–1·2Mo–Ti–B (wt-%) austenitic stainless steel were studied over prolonged periods of time in the temperature range 600–1300°C. The experimental results showed that several types of precipitates were present in the material, and that the dissolution temperatures of each of these correlated with the type and extent of grain growth which was observed. It was, therefore, concluded that in the present study secondary recrystallisation was caused directly by the interaction of precipitates with grain boundaries. Furthermore, secondary recrystallisation produced a strong, predominantly {122{ 〈012〉 texture which has not previously been reported.     

Microstructure and mechanical properties of Mg – Al9Zn/SiCp composite produced by vacuum stir casting process

Abstract

15 vol.-% SiC particle reinforced cast Mg – 9AlZn (AZ91C) composite was produced by a vacuum stir casting process, and the microstructure and mechanical properties of the composite investigated. The stirring process was carried out at a speed of 750 – 1500 rev min^-1 with a stainless steel impeller for 25 min in a vacuum of 20 – 40 mbar. SiC particles in the composite exhibited a reasonably homogeneous distribution and were well wetted by magnesium. The Mg – Al9Zn/15SiC_p composite showed significant improvement in yield strength and elastic modulus following T4 heat treatment. The ultimate tensile strength of the composite was low, but close to that of unreinforced magnesium alloy. Mg/SiC interfacial reactions and reaction mechanisms are discussed. No evident interfacial products were found at a low process temperature of 700°C. However, significant chemical reactions at the Mg/SiC interface occurred when the composite melt was maintained at 750°C, and complex reaction products were formed. The fluidity of the composite melt deteriorated seriously after the interfacial reactions occurred.     

New Optimum Extrusion Parameters of 9% Cr Ferritic Steel Tubes

The mechanism of austenite grain growth prior to forging of 9% Cr ferritic steels was analyzed. Pre-forging heating temperature and holding time had an obvious effect on austenite grain size and delta ferrite morphology. Hot compression experiments were conducted by using a Gleeble 1500D thermal–mechanical simulator. A hot upsetting test was performed on an HP01-500 oil press to verify the geometric dynamic recrystallization mechanism. New extrusion technology parameters of steel tubes were optimized by the Deform 2D software, and practical tests were conducted. Results indicated that the grains grew slowly at low temperatures (i.e., 1000°C and 1050°C). The kinetic curve of grain growth was close to the “S” type at 1100°C. The grains exhibited abnormal grain growth at high temperatures (i.e., 1150°C and 1200°C). The quantity of delta ferrites markedly increased at 1250°C. In addition, grains were refined by geometric dynamic recrystallization during the formation process. Mechanical properties of 9% Cr tube met standard requirements.     

Influence of extrusion temperature and process parameter on microstructures and tensile properties of a particulate reinforced magnesium matrix nanocomposite

Particulate reinforced magnesium matrix nanocomposites fabricated by semisolid stirring assisted ultrasonic vibration were subjected to extrusion. The results showed that grains of matrix in the SiCp/AZ91 nanocomposites were gradually refined while the amount of SiC nanoparticle bands was decreased with the extrusion temperature increasing from 250 to 350 °C. Under the same extrusion conditions, the grain size of the matrix was gradually decreased while the distribution of SiC nanoparticles was improved in the extruded nanocomposites fabricated by decreasing the stirring time. The yield strength and ultimate tensile strength of the nanocomposites were gradually enhanced with increasing the extrusion temperature. Significant improvement of tensile strength was obtained in the nanocomposites fabricated by decreasing the stirring time.     

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.     

Low cycle fatigue resistance of SA533 pressure vessel steels

Abstract

Low cycle fatigue (LCF) tests were conducted on SA533 steels with different levels of sulphur content at room temperature and 300 °C. The fatigue limit shows little or no dependence on sulphur content, but significantly depends on testing temperature. At 300 °C, the fatigue limit is at 0.2% strain amplitude, slightly higher than the 0.1% strain amplitude obtained at room temperature. The fatigue limit of SA533 steel subjected to LCF tests at 300 °C was improved by the combined effects of dynamic strain aging (DSA) and grain size reduction. DSA and grain size reduction increase the steel strength and, accordingly, improve the LCF limit at 300 °C. But, concurrently, the carbide/nitride precipitates in SA533 steel lead to a decrease in steel strength. Grain size reduction and precipitation compete with each other. Grain size reduction is dominant and the net effect is reflected in the increase in hardness value. Fatigue life could be predicted either by means of the strain-life equation or the SWT (Smith, Watson and Topper) parameter for specimens under low cycle fatigue conditions. The strain-life equation for SA533 steel in air is independent of the sulphur content, but significantly dependent on the testing temperature.     

Study on martensitic transformation of mechanically alloyed nanocrystalline Fe–Ni

Phase transformation of Fe–Ni powders with different nickel content during mechanical alloying was studied, as well as reverse transformation of mechanically alloyed nanocrystalline Fe–Ni upon heating. Results show that nickel content plays an important role in the phase transformation tendency during mechanical alloying. When heated at 300 °C, neither grain size nor phase changes in Fe–30 wt.% Ni milled for 80 h, indicating the nanometer-sized martensite is very stable below 300 °C. When the temperature increases to 350 °C, concurrently with grain growth reverse transformation takes place. The reverse transformation temperature of mechanically alloyed nanocrystalline Fe–Ni is higher than that of bulk alloys.