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 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.     

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.     

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.     

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.     

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 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.     

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.     

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.     

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.     

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     

Influence of aluminium on solidification structure and initial deformability of continuously cast C–Mn–Si–N steel

Abstract

The solidification structure and the initial deformability of continuously cast steel were investigated by assessment of cracks on billets and on rolled product. Some billets were rolled directly off the casting machine and some cooled to ambient temperature, then reheated to rolling temperature. On direct rolled steels, the number of defects increases with increasing aluminium content, while virtually no defects are found on steel rolled after reheating. By increasing the aluminium content, the solidification structure of steel is highly modified and a columnar structure obtained over the entire section of the billet. It was shown by chemical analysis and fracture examination that the increased hot shortness is not related to the effect of AIN. It is concluded that the hot shortness is related to the effect of aluminium on the solidification structure.

MST/761     

Low cycle fatigue and creep–fatigue interaction behaviour of 2.25Cr1MoV steel at elevated temperature

This study reports the fatigue behaviour of 2.25Cr1MoV steel under low cycle fatigue (LCF) loading and creep-fatigue interaction (CFI) loading at 355, 455 and 555 °C. Various hold durations up to 600 s were introduced in the CFI tests at the peak/valley strain under strain or stress control. In LCF tests, the steel exhibited remarkable strengthening at 455 °C, which can be ascribed to the effect of dynamic strain aging. In CFI tests, tensile holds were found more damaging than compressive holds but considerably less harmful than the combined tensile-compressive holds. A modified plastic strain energy approach based on the damage mechanisms was proposed to predict fatigue life under LCF and CFI conditions. The predictions obtained compared very favourably with the experimental results.     

Effect of nanoclays on physico-mechanical properties and adhesion of polyester-based polyurethane nanocomposites: structure–property correlations

Polyester–polyurethane nanocomposites based on unmodified and modified montmorillonite clays were compared in terms of their morphology, mechanical, thermal, and adhesive properties. Excellent dispersion of the modified nanoclay in polymer with 3 wt% loading was confirmed from X-ray diffraction, and low-, and high-magnification transmission electron micrographs. The properties of the clay-reinforced polyurethane nanocomposites were a function of nature and the content of clay in the matrix. The nanocomposite containing 3 wt% modified clay exhibits excellent improvement in tensile strength (by ~100%), thermal stability (20 °C higher), storage modulus at 25 °C (by ~135%), and adhesive properties (by ~300%) over the pristine polyurethane.     

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 1—Mechanical behaviour

Abstract

A split Hopkinson bar is used to investigate the effects of prestrain and strain rate on the dynamic mechanical behaviour of 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is prestrained to strains of 0·15, 0·3, and 0·5, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 10² to 5 × 10³ s^-1 at room temperature, with true stains varying from 0·1 to 0·3. It was found that 304L stainless steel is sensitive to applied prestrain and strain rate, with flow stress increasing with increasing prestrain and strain rate. Work hardening rate, strain rate sensitivity, and activation volume depend strongly on the variation of prestrain, strain, and strain rate. At larger prestrain and higher strain rate, work hardening rate decreases rapidly owing to greater heat deformation enhancement of plastic flow instability at dynamic loading. Strain rate sensitivity increases with increasing prestrain and work hardening stress (σ-σ_y). However, activation volume exhibits the reverse tendency. Catastrophic fracture is found only for 0·5 prestrain, 0·3 strain, and strain rate of 4·8 × 10³ s^-1. Large prestrain increases the resistance to plastic flow but decreases fracture elongation. Optical microscopy and SEM fracture feature observations reveal adiabatic shear band formation is the dominant fracture mechanism. Adiabatic shear band void and crack formation is along the direction of maximum shear stress and induces specimen fracture.     

Effects of temperature on interface microstructure and strength properties of titanium–niobium stainless steel diffusion bonded joints

Abstract

The effects of temperature on interface microstructure and strength properties of Ti/stainless diffusion bonded joint using Nb interlayer, processed in the temperature range 800–950°C for 1·5 h in vacuum were investigated. The stainless steel/Nb interface is free from intermetallic phase up to 900°C; however, Fe₂Nb+Fe₇Nb₆ phase mixture has been observed at 950°C processing temperature. The Nb/Ti interface is free from intermetallic for all processing temperatures. The maximum tensile strength of ～287 MPa (～90% of Ti) and shear strength ～222 MPa (～75% of Ti) along with 6·9% ductility have been achieved in the diffusion bonded joints, when processed at 900°C. The bonded samples failure takes place through the stainless steel/Nb interface for all processing temperatures during the loading.     

Effect of polyvinyl caprolactam–polyvinyl acetate–polyethylene glycol graft copolymer on bioadhesion and release rate property of eplerenone pellets

Abstract

The present study involved the design and development of oral bioadhesive pellets of eplerenone. A solid dispersion of eplerenone was developed with a hydrophilic carrier, polyvinyl caprolactam–polyvinyl acetate–polyethylene glycol graft copolymer (Soluplus^®). Bioadhesive pellets were prepared from this solid dispersion using a combination of HPMC K4M and Carbopol 934P. Both the solid dispersion and the pellets were evaluated for various physicochemical properties such as solubility, entrapment efficiency, drug content, surface morphology, mucoadhesion and swelling behavior. Analysis carried out using FT-IR, DSC and XRD found no interaction between the eplerenone and excipients. The solid dispersion had irregular-shaped smooth-surfaced particles of diameter 265?±?105.5?μm. In TEM analysis, eplerenone particles of size 79–120?nm were found. The solubility and dissolution of eplerenone in the Soluplus^®-based solid dispersion were 5.26 and 2.50 times greater, respectively. Investigation of the swelling behavior of the pellets showed that the thickness of the gel layer increased continuously over the duration of the study. Moreover, a correlation was observed between the thickness and strength of the gel layer and the percentage release. The mechanism of drug release was found to be non-Fickian (anomalous), with the release kinetics approaching first-order kinetics. The bioavailability of the eplerenone bioadhesive pellet formulation was studied using Wistar rats and was found to be improved. An in vivo mucoadhesion study showed that the pellets are retained for 24?h in rabbits. It was concluded that Soluplus^® had a positive effect on the solubility and dissolution of pellets without affecting the bioadhesion.     

Experimental investigations on welding behaviour of sintered and forged Fe–0.3%C–3%Mo low alloy steel

Tungsten Inert Gas (TIG) welding is considered as one of the cleanest welding methods. It is generally adopted for thinner materials with moderate weld joint strengths. Welding of sintered porous materials continues to be a challenge due to the inherent porosity of the parent metals. The present research work attempts to address some of the issues relating to the welding behaviour of sintered and forged Fe–0.3%C–3%Mo low alloy steels under TIG welding. Rectangular strips of size 70 mm × 15 mm × 5 mm, obtained by blending, compacting and sintering of elemental powders of iron, graphite and molybdenum, were upset forged – both hot and cold in order to obtain alloy steel strips of various porosities. Two identical alloy steel strips of equal density were then welded both along longitudinal and transverse directions, by TIG welding, employing filler metal of suitable composition. The welded strips were then subjected to tensile test, hardness test, microstructural and Scanning Electron Microscope (SEM) fractography studies. Cold/hot upsetting of the sintered alloy preforms has led to enhanced density. As a result of improved density, their tensile strength and hardness values were also found to be enhanced. The welded alloy exhibited higher tensile strength compared to the un-welded base metal, due to strengthening by residual stress. Similarly, the strength and hardness of the welded alloy strips were found to be enhanced with increase in density. The tensile strength of welded joint is found to be higher compared to that of the base metal due to alloy metals segregation, rapid cooling and formation of acicular ferrite at the weldment of welded joint. No porosity was observed in the weld metal or Heat Affected Zone (HAZ) of the weld joint. However, the base metal had numerous micro pores, though pore migration towards weldment has not been observed.     

Erosive–corrosive wear behaviour of Fe–TiC and cobalt based layers separately deposited on En31 steel

Abstract

Observations pertaining to the erosive–corrosive wear behaviour of samples of En31 steel hardfaced separately with Fe–TiC and a commercial cobalt based material are reported in the present investigation. Erosion–corrosion tests were carried out by the sample rotation technique in 3·5%NaCl solution for varying test durations of 8–32 h (corresponding traversal distances 136–544 km) at a fixed traversal speed of 4·71 m s^?1. The effect of the hardfaced layer on the wear behaviour was assessed through testing one set of each of the hardfaced and substrate steel samples under identical test conditions. The wear rate increased initially with traversal distance, attained the maximum and decreased thereafter. In the case of the steel substrate, the wear rate increased once again towards the final stage of testing. Hardfaced layers exhibited substantially decreasing wear rate compared with the substrate. Moreover, the performance of the Fe–TiC composite layer was intermediate between those of the cobalt based hardfacing material and the substrate. The observed response of the samples has been substantiated through their affected surface and subsurface characteristics.