Effect of increasing cooling rate from normalising temperature on structure and properties of a Mn–Cr–Mo–V steel

Abstract

The work was undertaken to study the effect of increasing the cooling rate from the normalising temperature on the microstructure and mechanical properties of a Mn–Cr–Mo–V steel. The steel was to a chemical composition suitable for grade 271 in BS 1501 Part 2 and as such would be used in the normalised and tempered condition. Three linear rates of cooling (12, 36, and 120 K min ^?1) from the normalising temperature were used. With the tenfold increase in cooling rate, yield strength increases of about 20% and tensile strength increases of about 15% were obtained in the final tempered steel. Although these strength increases resulted in a loss in ductility and toughness, the values of these properties were still relatively high. The improvements in strength with increases in cooling rate have been related to an increase in the proportion of bainite and a decrease in the amount of ferrite in the resulting microstructure.

MST/683     

Grain growth behaviour on reheating Al–Nb-containing steel in the homogenised condition

ABSTRACT

Grain size development during reheating is important to the mechanical properties of steels, and non-uniform grain growth including abnormal and bimodal grain growth are undesirable for steel manufacturing. In this paper, reheating treatments for Al–Nb-containing steel (0.057?wt-% Al and 0.019?wt-% Nb) were selected based on Thermo-Calc predictions for the dissolution temperatures of precipitates in the steel for a fully homogenised condition. Abnormal grain growth occurred in the homogenised Al–Nb sample during reheating, associated with random dissolution of the grain boundary-pinning AlN precipitates, while bimodal grain growth was not observed. So material in a homogenised condition would not be expected to provide a fully uniform grain structure during reheating treatment.     

Effect of periodic melt shearing process and cooling rate on structure and hardness of Al–0.7Fe aluminum alloy

In this investigation, the effect of periodic melt shearing process and cooling rate on an Al–Fe alloy was studied. Microstructural examinations were conducted by X-ray diffraction, optical and scanning electron microscopy coupled with energy dispersive spectrometry (EDS). Experimental results suggest that shearing above melting point can refine the alloy structure and improve the morphology of intermetallic phases and; the level of refinement is increased by increase of shearing time. Also, increasing cooling rate through reduced sections, decreases grain size and secondary dendrite arm spacing (SDAS) and improves the microstructure of Al–Fe alloy. The results of hardness test showed that increasing cooling rate and time of melt shearing reduce the hardness of Al–Fe alloy.     

Effect of temperature and displacement rate on behaviour of as-cast AA5182 and Al–3·3%Cu alloys under tensile loading near solidus temperature

Hot cracking develops in the still semi-solid casting during the last stages of solidification. The micro-mechanism of its origin is not generally accepted. There exists considerable doubt whether it is initiated by a void or develops as an instantaneous crack. The aim of this work is to study the mechanism of fracture behaviour of aluminum alloys around solidus temperature. Tensile tests were performed on notched specimens of as-cast AA5182 and Al–3·3%Cu alloys using a Gleeble 3500 $^{\textrm{{\textregistered}}}$ thermomechanical simulator. The effect of temperature and strain rate on the propagation of fracture in the semi-solid state has been studied to establish fracture mechanism. The transition from ductile to brittle mode of fracture has been observed around the solidus temperature. The fracture is intergranular and propagates through interdendritic channels.     

Fractal characteristics of dendrite and cellular structure in nickel–based superalloy at intermediate cooling rate

The fractal characteristics of dendrite and cellular structure of nickel-based superalloy K5 are investigated under directional solidification. Results show that the fractal dimension of the dendrite increases from 1.228 to 1.418 as the withdraw speeds change from 40 to 264 µm/s, whereas the fractal dimension of the cellular changes little as the withdraw speeds from 600 to 952 µm/s. The physical significance of the fractal dimension is analyzed by fractal theory. Based on this, a new idea is proposed that both the fractal dimension and the dendrite arm spacing or cellular spacing be considered to describe the evaluation of the solidification structure completely and integrally.     

Effects of strain rate and temperature on dynamic mechanical behaviour and microstructural evolution in aluminium–scandium (Al–Sc) alloy

Abstract

The present study applies a compressive split Hopkinson bar to investigate the mechanical response, microstructural evolution and fracture characteristics of an aluminium–scandium (Al–Sc) alloy at temperatures ranging from ? 100 to 300°C and strain rates of 1·2 × 10³, 3·2×10³ and 5·8 × 10³ s^?1. The relationship between the dynamic mechanical behaviour of the Al–Sc alloy and its microstructural characteristics is explored. The fracture features and microstructural evolution are observed using scanning and transmission electron microscopy techniques. The stress–strain relationships indicate that the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, but decrease with increasing temperature. Conversely, the activation volume and activation energy increase as the temperature increases or the strain rate decreases. Additionally, the fracture strain reduces with increasing strain rate and decreasing temperature. The Zerilli–Armstrong fcc constitutive model is used to describe the plastic deformation behaviour of the Al–Sc alloy, and the error between the predicted flow stress and the measured stress is found to be less than 5%. The fracture analysis results reveal that cracks initiate and propagate in the shear bands of the Al–Sc alloy specimens and are responsible for their ultimate failure. However, at room temperature, under a low strain rate of 1·2 × 10³ s^?1 and at a high experimental temperature of 300°C under all three tested strain rates, the specimens do not fracture, even under large strain deformations. Scanning electron microscopy observations show that the surfaces of the fractured specimens are characterised by transgranular dimpled features, which are indicative of ductile fracture. The depth and density of these dimples are significantly influenced by the strain rate and temperature. The transmission electron microscopy structural observations show the precipitation of Al₃Sc particles in the matrix and at the grain boundaries. These particles suppress dislocation motion and result in a strengthening effect. The transmission electron microscopy analysis also reveals that the dislocation density increases, but the dislocation cell size decreases, with increasing strain rate for a constant level of strain. However, a higher temperature causes the dislocation density to decrease, thereby increasing the dislocation cell size.     

Investigation on influence of dynamic strain ageing on fatigue crack growth behaviour of modified 9Cr–1Mo steel

Fatigue crack growth behaviour of modified 9Cr–1Mo steel is examined in the temperature range 300–823 K. An improvement in fatigue crack growth resistance is observed in the dynamic strain ageing regime. The activation energy for the process leading to this is estimated from the temperature-dependence of crack tip strain rate as 55–80 kJ/mole. This indicates that dynamic strain ageing due to interaction of dislocations with interstitial solute elements is responsible for the improved fatigue resistance in this range.     

Combined effects of Ag content and cooling rate on microstructure and mechanical behavior of Sn–Ag–Cu solders

Sn–0.7 wt%Cu–1.0 wt%Ag and Sn–0.7 wt%Cu–2.0 wt%Ag alloys were directionally solidified under transient conditions undergoing cooling rates varying from 0.1 to 25 K/s. The microstructure was characterized along the castings lengths and the present experimental results include the secondary dendrite arm spacing (λ₂) and its correlation with: the tip cooling rate ($\dot{T}$) during solidification and microhardness (HV), yield tensile strength (σ_y), ultimate tensile strength (σ_u) and elongation to fracture (δ). The aim is to examine the effects of Ag content and tip cooling rate on both the microstructure and mechanical properties. The initiation of tertiary branches within the dendritic arrangement, as well as the distinct morphologies of the intermetallic compounds (IMC) related to the solidification cooling rate was also assessed for both examined alloys. While the Cu₆Sn₅ phase appeared as large faceted crystals along the entire casting length, very fine Ag₃Sn spheroids prevailed at higher cooling rates (>7.5 K/s and > 4.0 K/s for 1.0 wt%Ag and 2.0 wt%Ag alloying, respectively) with a mixture of Ag₃Sn coarser spheroids and fibers predominating at lower cooling rates. The Sn–0.7 wt%Cu–2.0 wt%Ag alloy exhibited smaller dendritic spacings and HV of about two times higher than the corresponding values of the Sn–0.7 wt%Cu–1.0 wt%Ag alloy. A single Hall–Petch equation is proposed relating δ to λ₂ for both alloys, which means that the increase in Ag content from 1.0 to 2.0 wt% does not affect the elongation. It is shown that δ decreases with the increase in λ₂.     

Hall–Petch behaviour of 316L austenitic stainless steel at room temperature

Abstract

The room temperature plastic deformation behaviour of two different batches (with differences in chemical composition) of 316L austenitic stainless steel has been studied. By thermomechanical treatments, a wide range of grain sizes varying from 2·7 to 64·0 νm was obtained in this study. The different microstructural parameters, such as grain size, distribution of grain size and shape, dihedral angle distribution, and grain aspect ratio were measured for annealed and deformed specimens of the two batches. The Hall–Petch behaviour of batch 1 showed two distinctly different linear regions, one in the fine grain size range (d≤6νm) and the other in the coarse grain size range (d6νm). The Hall–Petch parameter K _H (?) was significantly higher in the fine grain regime than coarse grain regime at all strains. Hardness measurements were also performed across the grain at different strain levels. The applicability of the Hall–Petch relationship was assessed in batch 1 and batch 2. It was observed that the Hall–Petch relationship was applicable in the coarse grain regime and Kocks composite relationship in the fine grain regime of batch 1. In batch 2 of 316L austenitic stainless steel, a single linear Hall–Petch relationship could describe the deformation behaviour over the entire range of grain size (from 2.9 to 46 νm) studied. The variation of the Hall–Petch and Kocks composite parameters with strain was discussed in terms of changes in the microstructural parameters.     

Effects of strain rate and temperature on the stress–strain response of solder alloys

To ensure reliable design of soldered interconnections as electronic devices become smaller, requires greater knowledge and understanding of the relevant mechanical behavior of solder alloys than are presently available. The present paper reports the findings of an investigation into the monotonic tensile properties of bulk samples of three solder alloys; a lead–tin eutectic and two lead-free solders (tin–3.5 copper and a tin–3.5 silver alloy). Temperatures between–10 and 75°C and strain rates between 10^–1 and 10^–3 s^–1 have been studied. Both temperature and strain rate may have a substantial effect on strength, producing changes well in excess of 100%. Strength is reduced by lowering strain rate and increasing temperature, and Sn–37 Pb is usually most sensitive to the latter. Expressions for strain and strain rate hardening have been developed. The Sn–0.5 Cu alloy is usually the weakest and most ductile. Sn–37 Pb is strongest at room temperature but with increasing temperature and lower strain rates it becomes inferior to Sn–3.5 Ag. Ductility changes with temperature and strain rate for all three alloys are generally small with inconsistent trends. The role of such data in stress analysis and modeling is considered and the paramount importance of employing data for conditions appropriate to service, is emphasized.     

Effect of strain rate on microstructures and mechanical properties of Fe–18Cr–8Ni steel

The effect of strain rate on deformation microstructures and mechanical properties of Fe–18Cr–8Ni austenitic stainless steel was investigated at strain rates of from 10^?3 to 100?s^?1. The results indicated that the deformation mechanism of steel changes from transformation induced plasticity (TRIP) to TRIP?+?twinning induced plasticity (TWIP) effect when the strain rate is increased from 10^?3 to 100?s^?1. The yield strength of steel increases gradually with strain rate increased, while the tensile strength and elongation first decreases and then increases slowly. The changes in tensile strength and elongation are due to the change of deformation mechanism with the strain rate increased.     

Effect of deformation of austenite and cooling rates on transformation microstructures in a Mn–Cr gear steel

The effect of austenite deformation and cooling rates on continuous cooling transformation microstructures for a Mn–Cr gear steel were investigated using a Gleeble 1500 thermomechanical test system. The experimental results show that the deformation of austenite promotes the formation of proeutectoid ferrite and pearlite, leading to the increase of critical cooling rate of proeutectoid ferrite plus pearlite microstructure. The deformation enhances the stability of austenite against bainite transformation, which results in an increase in amount of martensite/austenite (M/A) constituent with deformation at some cooling rates studied. Moreover, cooling rate also affects amount of M/A constituent. With decrease of cooling rate, amount of M/A constituent increases at first, but decreases subsequently till disappears eventually.     

Effects of cooling rate on latent heat released mode of near pure aluminium and aluminium–silicon alloys

Abstract

The information concerning solid fraction with respect to temperature in the mushy range is very important to solidification models which employ the effective specific heat method. The computer aided cooling curve analysis (CA–CCA) method is used in this study to measure the relationships between solid fraction (f_s) and temperature/time for aluminium alloys of different composition for various cooling rates. The present study examined near pure aluminium, A356.2, A390, and A413 (near eutectic) alloys. The results of the measurements and analyses show that a rather large temperature range is observed near the end of solidification for both near pure aluminium and A413 alloys. This mushy range becomes longer as cooling rate increases. For the A356.2 and A390 alloys, the solidus temperature, liquidus temperature, eutectic temperature, and maximum solid fraction at the eutectic temperature decrease as cooling rate increases. This is not true, however, for cooling rates higher than 9·5 K S^?1. It is also known that afunctional relationship of f_s with temperature is very convenient when it is applied to a solidification model. Two non-linearity factors n_e and n_p are required to define the function; n_e and n_p are found to increase as cooling rate increases. The relationship between n_e and n_p and cooling rate can also be obtained. A reasonable estimation of the solid fraction datafor the cooling rate not measured can then be made.

MST/3262     

Effect of brazing temperature on the microstructure of martensitic–austenitic steel joints

Dissimilar joining of reduced activation ferritic–martensitic steel to AISI 316LN austenitic stainless steel is carried out by brazing in inert atmosphere at three different temperatures, i.e. 980, 1020 and 1040°C using AWS BNi-2 powder. The braze joints are characterised by scanning electron microscopy, X-ray diffraction, micro-hardness measurement. With increasing brazing temperature from 980 to 1040°C, the approximate width of the braze layer decreases from 350 to 80?µm and hardness reduces from 600 to 410?VHN. However, not much difference is found in microstructure and hardness between braze joints produced at 1020 and 1040°C. With increasing brazing temperature, morphology and volume fraction of intermetallics formed in the braze layer change, thereby reducing the hardness variation between the braze layer and the base metal.     

Effect of consolidation route on structure and property control in rapidly solidified Al–Cr–Zr–Mn povvder alloy for high temperature service

Abstract

In this paper the authors describe a new rapidly solidified alloy which is capable of meeting the projected requirements of the aerospace industry. Initial studies using splat quenched particulates were carried out on the Al–Cr–Zr system. Microhardness tests indicated that these alloys age–hardened between 350 and 400°C and showed excellent thermal stability. Further alloy development and studies of fabrication behaviour were carried out using air atomized powder. Powders were consolidated by either conventional or hydrostatic extrusion. Microstructural changes during fabrication were identified and correlated with mechanical property data. The alloy can achieve the requirements of the aerospace industry provided microstructural development is controlled during fabrication.

MST/233     

Effect of strain rate on the fracture behaviour of epoxy–graphene nanocomposite

In this study, epoxy-based nanocomposite was fabricated by the addition of graphene nanosheet via a solution casting method. To investigate the effect of strain rate on tensile properties of epoxy, tensile tests were done on standard samples at different strain rates (0.05–1 min^?1). The role of strain rate and presence of graphene on fracture behaviour of epoxy were also studied by investigation of the fracture surfaces of some samples by scanning electron microscopy (SEM). Finally, Eyring’s model was performed to clarify the role of strain rate on activation volume and activation enthalpy of epoxy. The results of tensile tests showed a maximum strength of epoxy–graphene nanocomposite at the graphene wt% of 0.1%. Tensile strength of epoxy obviously improved with increasing strain rate, but tensile strength of epoxy/graphene nanocomposite sample was less sensitive. Fracture micrographs showed that the mirror zone of the fracture surface of epoxy diminished by increasing strain rate or addition of graphene; and final fracture zone also became rougher. Finally, by investigation of the activation enthalpies, it was showed that much higher enthalpy was needed to fracture the nanocomposite sample, as the activation enthalpy changed from 41.54 for neat epoxy to 67.34 kJ mol^?1 for EP–0.1% GNS sample.     

Studies on electrical conductivity and dielectric behaviour of PVdF–HFP–PMMA–NaI polymer blend electrolyte

Polymer blend electrolytes composed of poly(vinylidene fluoride-co-hexafluoro-propylene), poly(methyl methacrylate) and 1·0 M NaI as salt have been synthesized using solution caste technique by varying the PVdF(HFP)–PMMA blend concentration ratio systematically. A.c. impedance studies were performed to evaluate the ionic conductivity of the polymer electrolyte films. The highest ionic conductivity at room temperature for [PVdF(HFP)–PMMA(4:1)](20 wt%) – [NaI(1·0 M)](80 wt%) system is found to be 1·67 × 10^???2 S cm^???1. XRD studies reveal complete complexation of the salt in the polymeric blend systems. The temperature dependence conductivity has been performed in the range of 303–373 K and it is observed that it obeys the Arrhenius behaviour. It has been observed that the dielectric constant, ε _r and dielectric loss, ε _i, increases with temperature in the lower frequency region and is almost negligible in the higher frequency region. This behaviour can be explained on the basis of electrode polarization effects. Plot of real part, M _r and imaginary part, M _i vs frequency indicates that the systems are predominantly ionic conductors. The phenomenon suggests a plurality of relaxation mechanism.     

Effects of post-aging cooling condition on structure and tensile properties of aged Ti–7.5Mo alloy

The present study investigated the effects of post-aging cooling condition on tensile properties of an aged Ti–7.5 wt% Mo alloy. Experimental results indicated that the solution-treated (ST) Ti–7.5Mo samples were comprised substantially of α″ phase, while all aged samples demonstrated the co-existence of α′ and β phases therein. The relative amount of β phase retained in the aged alloy was found sensitive to post-aging cooling condition. Water-quenched (WQ) alloy had the highest β phase content, while furnace-cooled (FC) alloy had the lowest β phase content. The Mo concentration in β phase was also sensitive to post-aging cooling condition. The average Mo concentration in β phase was found highest in FC samples and lowest in WQ samples. Compared to ST samples, all aged samples demonstrated significant increases in yield strength and modulus values, along with decreases in elongation. The effect of post-aging cooling condition on microhardness was similar to that on strength.