Microstructure and mechanical properties of low alloy steel T11–austenitic stainless steel 347H bimetallic tubes

Abstract

A low alloy steel (T11) has been bonded to an austenitic stainless steel 347H by hot coextrusion under industrial conditions. The final product was a seamless bimetallic tube with 347H cladding the exterior for corrosion resistance in severely corrosive environments at high temperatures. The microstructures of the coextruded bonding have been compared to those obtained in the laboratory, after diffusion bonding experiments, using hot isostatic pressing (hipping). In all cases both the interdiffusion of the different elements across the interface and the microstructure have been analysed by optical microscopy, SEM, and TEM. On the 347H side a profuse precipitation, mainly of NbC, was found in a region near the interface. Only in the hipped specimens, as result of nickel and chromium diffusion from the stainless steel to the T11 steel, a martensite band was observed parallel to the interface. The heat treatment performed on the bimetallic tubes, to obtain the optimum combination of mechanical properties and corrosion resistance, consisted of austenitisation between 1050–1100°C, water quenching, and a stabilisation treat ment at 850–900°C, followed by slow cooling.     

Hybrid fiber laser – Arc welding of thick section high strength low alloy steel

Hybrid laser – metal active gas (MAG) arc welding is an emerging joining technology that is very promising for shipbuilding applications. This technique combines the synergistic qualities of the laser and MAG arc welding techniques, which permits a high energy density process with fit-up gap tolerance. As the heat input of hybrid laser – arc welding (HLAW) is greater than in laser welding, but much smaller than in MAG arc welding, a relatively narrow weld and restricted heat affected zone (HAZ) is obtained, which can minimize the residual stress and distortion. Furthermore, adding MAG arc can increase the penetration depth for a given laser power, which can translate to faster welding speeds or fewer number of passes necessary for one-sided welding of thick plates. In this work, a new hybrid fiber laser – arc welding system was successfully applied to fully penetrate 9.3 mm thick butt joints using a single-pass process through optimization of the groove shape, size and processing parameters.     

Effect of tempering temperature on monotonic and cyclic properties of a high–strength low–alloy chain steel

Abstract

The influence of tempering temperature in the range 200–600°C on the monotonic and cyclic response of a high-strength, low–alloy steel used for haulage chains has been investigated. Static properties are little affected by tempering up to 400°C, with a small loss of strength arising from carbide coarsening. At higher tempering temperatures, dislocation annihilation is substantial and the strength falls markedly. On undergoing high–strain fatigue, the alloy exhibits cyclic softening at all tempering temperatures, but with a maximum of about 25% after a 400°C temper. The microstructural changes producing this are difficult to detect directly, but it is thought that dislocation unpinning has an important role. For tempering temperatures in excess of 400°C, the reduced softening can clearly be equated with a dislocation redistribution and the formation of cell substructure within the ferrite grains. Estimations of the magnitude of the various components of strength are made.

MST/199     

Evolution of interface microstructure and strength properties in titanium – stainless steel diffusion bonded transition joints

Abstract

Diffusion bonding was carried out to produce transition joints between commercially pure titanium and 304 stainless steel at a temperature of 800°C for different times ranging from 30 to 180 min. in steps of 30 min under load in vacuum. The diffusion couples thus produced were studied using optical microscopy, scanning electron microscopy, and electron probe microanalysis to characterise the reaction layers formed in the diffusion zone. The chemical compositions of these layers indicate that intermetallics like σ phase, Fe₂ Ti, Cr₂ Ti, χ phase, FeTi, β-Ti, and Fe₂ Ti₄ O are formed in the reaction zone. The presence of these intermetallic compounds was also confirmed by the X-ray diffraction technique. Maximum bond strength of ~242 MPa was obtained for diffusion welded joints processed for 120 min. At this joining time, the plastic collapse of the surface asperities reaches near completion, favouring the interdiffusion of chemical species. Reduction in the bond strength of the transition joint processed for 180 min is due to the formation of a large volume fraction of voids in the reaction zone. Under tensile loading, failure takes place through α-Fe + χ phase mixture for transition joints processed in the time range of 30 – 90 min and through β titanium for joining times greater than 120 min.     

Application of advanced analytical techniques to study structure–property relationship of hot rolled high strength low alloy steel

Abstract

The microstructure–property relationship in conventional high strength low alloy (HSLA) steel was evaluated using data obtained from transmission electron microscopy (TEM) and atom probe tomography (APT). Atom probe tomography allowed the characterisation of fine TiC particles with average radius of 3±1·2 nm that were not observed by TEM. The increase in the yield strength of steel due to the presence of fine precipitates was calculated to be 128 MPa.     

Interface characterisation of diffusion bonded Ti–6Al–4V alloy and austenitic stainless steel couple

Abstract

In this study, the Ti–6Al–4V alloy was diffusion bonded to austenitic stainless steel at temperatures of 820, 885, 930 and 980°C, under a pressure of 5 MPa for 30 min. The effect of temperature on interface formations and microstructure was investigated using a scanning electron microscope (SEM), energy dispersive spectrograph (EDS) microanalyses, X-ray diffraction and shear strength of bonded specimens. The results showed that intermetallic phases and σ-phase formed in the interface region.     

Cryorolling effect on microstructure and mechanical properties of Fe–25Cr–20Ni austenitic stainless steel

Microstructure and mechanical properties of the Fe–25Cr–20Ni austenitic stainless steel after cryorolling with different reductions were investigated by means of optical, scanning and transmission electron microscopy, X-ray diffraction and mini-tensile testing. High density tangled dislocations and a small amount of deformation twins formed after 30% deformation. After 50% strain, a large amount of deformation twins was generated. Meanwhile, interactions between the twins and dislocations started to happen. As the strain increased to 70%, many deformation twins were produced and the interactions between the twins and dislocations were significantly enhanced. When the cryorolling was 90%, the grain size was refined to the nanometer scale. XRD analysis indicated that the diffraction peaks of the samples became broader with the strain increase. The yield strength and the ultimate strength increased from 305 MPa and 645 MPa (before deformation) to 1502 MPa and 1560 MPa (after 90% deformation), respectively. However, the corresponding elongation decreased from 40.8% to 6.4%. The tensile fracture morphology changed from typical dimple rupture to a mixture of quasi-cleavage and ductile fracture. After 90% deformation, the microhardness was 520 HV, which increased by 100% compared with the original un-deformed sample.     

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.     

Zr–Si biomaterials with high strength and low elastic modulus

In order to develop new biomaterials for hard tissue replacements (HTR), the Zr–8.8Si–xNb (x = 0.0, 0.3, 0.6 and 0.9) alloys with required properties were designed and prepared for the first time. Experimental results show that these alloys can provide excellent mechanical compatibility for the special demands for substitution of human bones. The highest compression strength of the alloys is 1189.30 MPa, while the highest yield strength of alloys is 850.25 MPa. The elastic energy is determined to be 5.001–12.01 MJ/m³, and the Young’s modulus is in the range of 25.08–29.63 GPa. The composition of high strength and low elastic modulus of Zr–8.8Si–xNb alloys offer potential advantages for biomedical applications.     

Evolution of microstructure and mechanical properties during quenching and tempering of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel

Effects of quenching and tempering treatments on the development of microstructure and mechanical properties of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel were investigated. Samples were austenitized at 1123–1323 K for 2400 s and oil quenched (OQ) to produce mixed microstructures. Tempering was carried out at 473–773 K for 2–3 h. Phase transformation temperatures were measured using dilatometer. The microstructures were characterized using optical and scanning electron microscope. SEM–EDS analysis was carried out to determine the type and size of non-metallic inclusions. Volume percent of retained austenite was measured by X-ray diffraction technique. Hardness, tensile properties, and impact energies were also determined for all heat treated conditions. Fractography of impact specimens were done using stereomicroscope and SEM. The results showed that newly developed steel exhibited peak hardness, yield strength, and tensile strength of about 600 HV, 1760 MPa, and 1900 MPa, respectively, when OQ from 1203 K and tempered in between 473 and 573 K, combined with adequate ductility and impact toughness. Decrease in hardness and strength was observed with increasing tempering temperature whereas the impact energy was stable up to 623 K, however, impact energy was found to decrease above 632 K due to temper martensite embrittlement.     

Tensile stress–strain and work hardening behaviour of 316LN austenitic stainless steel

Abstract

The true stress (σ)–true plastic strain (?) data of a type 316LN austenitic stainless steel tested at nominal strain rates in the range 3×10^-5–3×10^-3 s^-1 and temperatures of 300–1123 K were analysed in terms of flow relationships proposed by Hollomon, Ludwik, Swift, Voce, and Ludwigson. The applicability of the particular flow relationship is discussed in terms of ‘complete’ and ‘applicable’ range fits of the experimental σ–? data. At all strain rates, in the case of the complete range fit, the Ludwigson equation followed the stress–strain data most closely at 300 K, while in the temperature range 523–773 K, the flow behaviour was described equally well by both the Ludwigson and Voce equations. In the temperature range 823–1023 K, the Voce equation described the flow behaviour most accurately in the case of the complete range fit of σ–? data at all strain rates. The analysis of σ–? data employing the Ludwigson equation in the applicable range fit covering low and intermediate strains, and the Hollomon equation at high strains provided close simulation of the observed flow behaviour in the temperature range 823–1023 K. At high temperatures of 1073 and 1123 K, the Ludwigson equation reduces to the Hollomon equation. The variations in different flow parameters of the Ludwigson and Voce equations with temperature and strain rate exhibited anomalous behaviour at intermediate temperatures because of dynamic strain aging.     

Effect of heat treatment on microstructure and mechanical properties of a new β high strength titanium alloy

Microstructure and mechanical properties of a new β high strength Ti–3.5Al–5Mo–6V–3Cr–2Sn–0.5Fe titanium alloy were investigated in this paper. Both the α/β and β solution treatment and subsequent aging at temperatures ranging from 440 °C to 560 °C for 8 h were introduced to investigate the relationship between microstructures and properties. Microstructure observation of α/β solution treatment plus aging condition shows that the grain size is only few microns due to the pinning effect of primary α phase. The β solution treatment leads to coarser β grain size and the least stable matrix. The size and volume fraction of secondary α are very sensitive to temperature and strongly affected the strength of the alloy. When solution treated at 775 °C plus aged at 440 °C, the smallest size (0.028 μm in width) of secondary α and greatest volume fraction (61%) of α resulted in the highest yield strength (1624 MPa). And the yield strength decreased by an average of 103 MPa with every increase of 40 °C due to the increase of volume fraction and decrease of the size of secondary α. In β solution treatment plus aging condition, tensile results shows that the strength if the alloy dramatically decreased by an average of 143 MPa for every increase of 40 °C because of larger size of secondary α phase than α/β solution treated plus aged condition.     

Comparison of high temperature wear behaviour of plasma sprayed WC–Co coated and hard chromium plated AISI 304 austenitic stainless steel

The wear behaviour of plasma sprayed coating and hard chrome plating on AISI 304 austenitic stainless steel substrate is experimentally investigated in unlubricated conditions. Experiments were conducted at different temperatures (room temp, 100 °C, 200 °C and 300 °C) with 50 N load and 1 m/s sliding velocity. Wear tests were carried out by dry sliding contact of EN-24 medium carbon steel pin as counterpart on a pin-on-disc wear testing machine. In both coatings, specimens were characterised by hardness, microstructure, coating density and sliding wear resistance. Wear studies showed that the hard chromium coating exhibited improved tribological performance than that of the plasma sprayed WC–Co coating. X-ray diffraction analysis (XRD) of the coatings showed that the better wear resistance at high temperature has been attributed to the formation of a protective oxide layer at the surface during sliding. The wear mechanisms were investigated through scanning electron microscopy (SEM) and XRD. It was observed that the chromium coating provided higher hardness, good adhesion with the substrate and nearly five times the wear resistance than that obtained by uncoated AISI 304 austenitic stainless steel.     

The wear and corrosion resistance of shot peened–nitrided 316L austenitic stainless steel

316L austenitic stainless steel was gas nitrided at 570 °C with pre-shot peening. Shot peening and nitriding are surface treatments that enhance the mechanical properties of surface layers by inducing compressive residual stresses and formation of hard phases, respectively. The structural phases, micro-hardness, wear behavior and corrosion resistance of specimens were investigated by X-ray diffraction, Vickers micro-hardness, wear testing, scanning electron microscopy and cyclic polarization tests. The effects of shot peening on the nitride layer formation and corrosion resistance of specimens were studied. The results showed that shot peening enhanced the nitride layer formation. The shot peened–nitrided specimens had higher wear resistance and hardness than other specimens. On the other hand, although nitriding deteriorated the corrosion resistance of the specimens, cyclic polarization tests showed that shot peening before the nitriding treatment could alleviate this adverse effect.     

High strength low carbon hot rolled δ-ferritic steel: microstructure and mechanical properties

Abstract

The present study concerns the development of high strength low carbon hot rolled bainitic and martensitic δ transformation induced plasticity steels. Equilibrium and para-equilibrium phase evolution have been examined by carrying out thermodynamic calculation using MT-DATA software. Microanalysis demonstrates that both manganese and aluminium partition between liquid and solid phases. Isothermal treatment and tempering at 350°C for bainitic and martensitic microstructures respectively have yielded the best combination of strength and ductility. All the steels have exhibited the continuous yielding behaviour and favourable ratio of yield and tensile strength, which are desirable for formability. The annealed steel has yielded a high level of tensile strength with the static toughness value in between the conventional transformation induced plasticity assisted and dual phase steels.     

Interface microstructure and strength properties of Ti–6Al–4V and microduplex stainless steel diffusion bonded joints

The diffusion bonding of Ti–6Al–4V alloy and micro-duplex stainless steel was carried out in the temperature range of 850–1000 °C for 45 min in vacuum. The influence of bonding temperature on the microstructural development, micro-hardness and strength properties across the joint region was determined. The layer wise σ phase, λ + FeTi and λ + FeTi + β-Ti phase mixtures were observed at the bond interface when the joint was processed at 900 °C and above temperature. The maximum tensile strength of ∼520.1 MPa and shear strength of ∼405.5 MPa along with 6.8% elongation were obtained for the diffusion couple processed at 900 °C. Fracture surface observation in scanning electron microscopy (SEM) using energy dispersive X-ray spectroscope (EDS) demonstrates that, failure takes place through λ + FeTi phase when bonding was processed at 900 °C, however, failure takes place through σ phase for the diffusion joints processed at and above 950 °C.     

Effect of rare earth element yttrium addition on microstructures and properties of a 21Cr–11Ni austenitic heat-resistant stainless steel

In this comparative study, the microstructure and the mechanical properties of a 21Cr–11Ni austenitic heat-resistant stainless steel with and without addition of rare earth (RE) element yttrium have been investigated. The results show that a number of fine spherical yttrium-rich oxide particles are not uniformly distributed in the matrix of steel with yttrium; instead, they are aligned along the rolling direction. The grains surrounding the alignment are nearly one order of magnitude smaller than those farther away from the alignment. The approximate calculation results indirectly show that the grain refinement may be mainly attributed to the stimulation for nucleation of recrystallization rather than to pinning by particles. Furthermore, the alignment has resulted in significant loss in transverse impact toughness and tensile elongation at room temperature. There is a trough in the hot ductility–temperature curve, which is located between 973 and 1173 K. The ductility trough of steel with yttrium becomes shallow within a certain temperature range, especially around 1073 K, indicating that improvement on hot ductility is achieved by yttrium addition. The results may be attributed to the increase of grain boundary cohesion indicated by the effective improvement on intergranular failure tendency, and the inhibitory effect of yttrium on sulfur segregation to grain boundaries is believed to be an important cause.     

Investigation on preparation and mechanical properties of W–Cu–Zn alloy with low W–W contiguity and high ductility

In order to improve the mechanical properties of the W–Cu alloy, the W–Cu–Zn alloys with low W–W contiguity were fabricated by three different preparation methods. For the first method, the mixed powder of copper-coated tungsten powder and Zn powder was sintered by SPS (Spark Plasma Sintering) process. For the second method, the mixed powder was processed by CIP (Cold Isostatic Pressing) before SPS. For the third method, a skeleton of the copper-coated tungsten powder was prepared by CIP, and then the skeleton was infiltrated with H70 brass. The microstructure, mechanical properties and failure mechanism of the prepared W–Cu–Zn alloys were investigated. The results show that the W–Cu–Zn alloy fabricated by the third method achieves a high relative density of 98.4% and a low W–W contiguity of 10%. The alloy exhibits a high dynamic compressive strength of 1000 MPa, with a high critical failure strain of 0.7. The Cu-Zn matrix of the alloy fabricated by the third method is composed of α-phase Cu–Zn alloy and Cu₃Zn particles. The homogeneous distribution of Zn in the matrix manifests good solution strengthening effect and the uniformly distributed Cu₃Zn particles has a strong precipitation strengthening effect, which are both responsible for the evidently enhanced mechanical properties.     

Structure and mechanical properties of Ti–5Cr based alloy with Mo addition

The effects of molybdenum (Mo) on the structure and mechanical properties of a Ti–5Cr-based alloy were studied with an emphasis on improving its strength/modulus ratio. Commercially pure titanium (c.p. Ti) was used as a control. As-cast Ti–5Cr and a series of Ti–5Cr–xMo (x = 1, 3, 5, 7, 9 and 11 wt.%) alloys were prepared by using a commercial arc-melting vacuum-pressure casting system, and investigated with X-ray diffraction (XRD) for phase analysis. Three-point bending tests were performed to evaluate the mechanical properties of all specimens and their fractured surfaces were observed by using scanning electron microscopy (SEM). The experimental results indicated that Ti–5Cr–7Mo, Ti–5Cr–9Mo and Ti–5Cr–11Mo alloys exhibited ductile properties, and the β-phase Ti–5Cr–9Mo alloy exhibited the lowest bending modulus. However, the Ti–5Cr–3Mo and Ti–5Cr–5Mo alloys had much higher bending moduli due to the formation of the ω phase during quenching. It is noteworthy that the Ti–5Cr–9Mo alloy exhibited the highest bending strength/modulus ratios at 26.0, which is significantly higher than those of c.p. Ti (8.5) and Ti–5Cr (13.3). Furthermore, the elastically recoverable angle of the Ti–5Cr–9Mo alloy (30°) was greater than that of c.p. Ti (2.7°). The reasonably high strength (or high strength/modulus ratio) β-phase Ti–5Cr–9Mo alloy exhibited a low modulus, ductile property, and excellent elastic recovery capability, which qualifies it as a novel implant materials.     

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.