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.     

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.     

An energy‐based damage parameter for the life prediction of AISI 304 stainless steel subjected to mean strain

Abstract

This study extends the plastic strain energy approach to predict the fatigue life of AISI 304 stainless steel. A modified energy parameter based on the stable plastic strain energy density under tension conditions is proposed to account for the mean strain and stress effects in a low cycle fatigue regime. The fatigue life curve based on the proposed energy parameter can be obtained directly by modifying the parameters in the fatigue life curve based on the stable plastic strain energy pertaining to fully reversed cyclic loading. Hence, the proposed damage parameter provides a convenient means of evaluating fatigue life on the mean strain or stress effect. The modified energy parameter can also be used to explain the combined effect of alternating and mean strain/stress on the fatigue life. In this study, the mean strain effects on the fatigue life of AISI 304 stainless steel are examined by performing fatigue tests at different mean strain levels. The experimental results indicate that the combination of an alternating strain and a mean strain strongly influences the fatigue life. Meanwhile, it is found that the change in fatigue life is sensitive to changes in the proposed damage parameter under the condition of a constant strain amplitude at various mean strain levels. A good agreement is observed between the experimental fatigue life and the fatigue life predicted by the proposed damage parameter. The damage parameter proposed by Smith et al. (1970) is also employed to quantify the mean strain effect. The results indicate that this parameter also provides a reasonable estimate of the fatigue life of AISI 304 stainless steel. However, a simple statistical analysis confirms that the proposed damage parameter provides a better prediction of the fatigue life of AISI 304 stainless steel than the SWT parameter.     

Deformation mechanism transition in Fe–17Mn–0.4C–0.06V TWIP steel with different strain rates

The dynamic tensile behaviour and deformation mechanism of the Fe–17Mn–0.4C–0.06V twinning-induced plasticity (TWIP) steel were investigated over a wide range of strain rates from 10^?4 to 10³ s^?1. With increasing strain rate, the stacking fault energy increased due to the increase of adiabatic heating temperature, ΔT. At 10^?4 to 10¹ s^?1, the transformation-induced plasticity (TRIP) effect coexisted with the TWIP effect and weakened with increasing strain rate. With the increase of strain rate in the range of 10^?1 to 10¹ s^?1, the TWIP effect strengthened gradually and intersected deformation twins were formed. When the strain rate was higher than 10¹ s^?1, the TRIP effect disappeared and the twinning was inhibited since the adiabatic heating effect elevated.     

Cyclic stress–strain response and crystal plasticity finite element analysis of AISI 9310 steel in biaxial fatigue loading

There are still many gaps in the research on the multiaxial fatigue failure mechanism of the gear shaft. In this paper, cyclic stress–strain response and biaxial fatigue damage characteristics of gear steel AISI 9310 were investigated. The specimens showed obvious cyclic softening characteristics at all phase angles, and the softening rate was directly associated with the initiation and propagation of cracks. The fractographies at different phase angles revealed that the specimens under out-of-phase loading suffered fatigue failure caused by a single crack source on the surface, while the fatigue crack under in-phase loading was gathered together by the propagation of different crack sources. Finally, the established crystal plastic finite element model showed a good prediction of the plastic strain energy density at different phase angles, and the maximum error was 13.03%. Furthermore, a biaxial fatigue life prediction method was proposed, with a maximum error of 39.5%.     

Grain boundary and fracture surface analysis of Fe–C–P steel

Abstract

The present paper investigates the distribution of grain boundary types and fracture surface crystallography in an Fe–C–P alloy. It is shown that electron backscatter diffraction (EBSD) is an effective technique with which to conduct these investigations. The proportions of both Σ1 and particularly Σ3 (in coincidence site lattice notation)present after various heat treatments were higher than would have been expected for random generation. There was limited evidence that both higher annealing temperatures and longer annealing times promoted generation of Σ3 type boundaries. The standard EBSD technique was modified and extended to encompass both the novel ‘matched fracture’ specimen approach and direct mapping from fracture surfaces to provide crystallographic information. A correlation was noted between higher aging temperatures and proportions of cleavage fracture. Furthermore, there was a strong correlation between cleavage fracture surfaces exhibiting river markings and an {001} surface orientation.     

An improved constitutive model for high-temperature flow behaviour of the Fe–Mn–Al duplex steel

The high-temperature flow curves of the Fe–Mn–Al duplex steel showed an uncommon yield-like behaviour and an abnormal dynamic recrystallisation behaviour that occurred at low temperatures rather than high temperatures. The interaction of strain partitioning and unsynchronised softening behaviour in δ-ferrite and austenite caused this peculiar flow behaviour. By discussing the stress exponent and apparent activation energy, respectively, at low and high temperatures, a modified hyperbolic sine function was developed to predict the characteristic stresses. By simplifying the material constant θ and compensating the microstructural evolution in the exponential saturation work-hardening law, an improved constitutive model was developed to predict the transient stress. The comparison between the experimental and calculated values confirmed a high prediction accuracy of this improved model.     

Stress–strain behaviour and abrupt loss of stiffness of geopolymer at elevated temperatures

This paper reports stress versus strain curves of geopolymer tested while the specimens were kept at elevated temperatures, with the aim to study the fire resistance of geopolymer. Tests were performed at temperatures from 23 to 680 °C and after cooling. Hot strengths of geopolymer increased when the temperature increased from 290 to 520 °C, reaching the highest strength at 520 °C, which is almost double that of its initial strength at room temperature. However, glass transition behaviour was observed to occur between 520 and 575 °C, which was characterised by abrupt loss of stiffness and significant viscoelastic behaviour. The glass transition temperature is determined to be 560 °C. Further, the strength reductions occurred during cooling to room temperature. This is attributed to the damage due to brittle nature of the material making it difficult to accommodate thermal strain differentials during cooling phase.     

Vacuum plasma spray deposition of YSZ–NiO–Ni coatings at different Ar and H2 gas flow rates

The work presents results of the experimental investigation of vacuum sprayed yttria stabilized zirconia, nickel oxide, nickel (YSZ–NiO–Ni) ceramic composite coatings deposited on Al₂O₃ ceramic and stainless steel substrates produced at different Ar and H₂ gas flow rates. The Ar and H₂ gas flow was varied according to the factorial plan design. It is shown that for the used vacuum plasma spray YSZ and NiO powder mixture the produced coatings were composed of three phases mainly: cubic YSZ (c-YSZ), cubic NiO (c-NiO), and cubic Ni (c-Ni). The quantitative X-ray diffraction (XRD) analysis was used to evaluate each phase amount in the coatings. It was found that the vacuum spray technique enables formation of composite layers with a variable composition and that phase content in the coatings can be controlled choosing the Ar and H₂ gas flow rates. The electrical conductivity measurements revealed that a variation of the phase content in the YSZ–NiO–Ni composites is responsible for the existence of different electrical conduction mechanism and rapid change in the conductivity of coatings with the used powder content. The surface morphology and the cross-section analysis by scanning electron microscope (SEM) have shown porous structures of the deposited coatings.     

Structural characterization of lead–bismuth eutectic corrosion of stainless steel by ultrasonic and metallographic analysis

Metallic test coupons subjected to corrosion in a lead–bismuth eutectic (LBE) were analyzed by both ultrasound and scanning electron microscope (SEM). The advantages and disadvantages of each method are given, and the possibility of using ultrasound as a screening process for SEM is presented. Visual data from each method are given, and the data derived from each method are compared and contrasted. Use of both ultrasound and SEM is recommended for future analysis of corrosion coupons, and development of a better methodology will increase the portion of the analysis workload obtainable by ultrasound.     

Workability of Ti–6Al–4V alloy at high temperatures and strain rates

Hot workability of Ti–6Al–4V has been investigated by means of hot compression tests carried out in the 880–950 °C temperature range and 1–50 s⁻¹ strain rate range. The effect of microstructural characteristics of the deformed specimens have been studied and correlated with the test temperature, total strain and strain rate. A constitutive equation for the flow stress has been defined and the test conditions for a homogeneous deformation evaluated. The machine employed for testing allowed to reach very high strain rates by means of a uniform compression for long strains (until 0.9), whereas data extracted from the scientific literature are significantly limited in comparison. In this way, a higher accuracy could be obtained in material behaviour modelling for forging process simulation.     

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 2—Microstructural study

Abstract

The morphologies and characteristics of microstructure, including dislocations, mechanical twins and α' martensite, in 304L stainless steel deformed under various strain, strain rate range from 10² to 5 × 10³ s^-1 for different prestrain levels at room temperature were examined by a split Hopkinson bar and TEM. The evolution of microstructure correlated with dynamic mechanical behaviour are presented and discussed in terms of prestrain and applied strain rate. The results show that characteristics of dislocations, mechanical twins and α' martensite varied with prestrains, strains and strain rates. They dominate the strengthening effects on the 304L stainless steel. Dislocation cell structures can be observed in all tested specimens. At larger prestrain under dynamic loading, the formation of elongated dislocation cells becomes evident. The presence of elongated dislocation cells leads to different work hardening behaviour. Twinning occurred at all testing conditions except for the 0·15 prestrain specimen deformed at 0·1 strain and 8 × 10² s^-1 strain rate. The formations of α' martensites were found to be confined to the microshear bands and were barriers of dislocation movement. As the heavy loading is imposed, irregular and blocky α' martensites were observed. Quantitative measurement revealed that dislocation and twin density, as well as the volume fraction of α' martensite increase with the prestrain, strain and applied strain rate, but a decay of twin density occurred as the prestrain of 0·5 is applied. These microstructrual changes can be related to the different work hardening stress (σσ_y and strengthening nature. The observed strengthening effect resulted from the dislocation multiplication, twin formation and α' martensite seems to reflect an enhancement of hardness. However, the increased hardness is less sensitive to the twin formation.     

Deformation behaviour of ultrafine grained Mg–7Gd–5Y–1.2Nd–0.5Zr alloy under high strain rates

The mechanical behaviour of Mg–7Gd–5Y–1.2Nd–0.5Zr (wt. %) alloy with ultrafine grains was measured by split Hopkinson pressure bar method under the strain rates of 1000, 1500, and 2000 s^?1 at room temperature. Dynamic tests were carried out along extrusion direction (ED), transverse direction (TD), and normal direction (ND). The results demonstrated that the flow stress increased with the increase of strain rate, showing a positive strain rate strengthening effect. There was no obvious anisotropy in dynamic compression along ED, TD, and ND, which was caused by rare earth elements and multi-pass deformation. This led to the adoption of plastic deformation mode dominated by non-basal slip and participated by tension twinning.     

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.     

Effect of Ni interlayer on partial transient liquid phase bonding of Zr–Sn–Nb alloy and 304 stainless steel

Zr–Sn–Nb alloy and 304 stainless steel were joined by means of partial transient liquid phase bonding. The effects of Ni interlayer on the microstructure and properties of the joints were investigated. The reaction layers are formed in both joints and which are mainly composed of σ-FeCr layer, Zr(Cr, Fe)₂ + α-Zr layer and α-Zr + Zr₂(Ni, Fe) layer. The intermetallic compounds are compact relatively and cracks are formed in the reaction layer of the direct bonded joint. In the joint with Ni interlayer, many α-Zr phases dispersedly exist in the reaction layer and the thickness of the reaction layer is distinctly larger than that without Ni interlayer. As a result of lower residual stresses and wider crack-free reaction layer, the bonding strength of the joint increases by using Ni interlayer.     

Laser and GTAW torch processing of Fe–Cr–B coatings on steel

A comparison has been made of the relationship between microstructure and microhardness developed by surface melting Nanosteel SHS 7170 Fe–Cr–B alloy powder onto a plain carbon steel surface. This powder was initially developed as a high velocity oxyfuel sprayed coating, giving a strength 10 times that of mild steel, and is particularly suitable for surface protection against wear and corrosion. In the present study, the alloy powder was injected into the laser melted surface, while a preplaced powder was melted using the gas tungsten arc welding (GTAW) technique. The laser track consisted of fine dendrites and needle-like microstructures, which produced a maximum hardness value of over 800 HV, while the GTAW track produced a mixture of equiaxed and columnar grain microstructures with a maximum hardness value of 670 HV. The lower hardness values are considered to be associated with dilution and grain size.     

XANES and XPS study of electronic structure of Ti-enriched Nd–Fe–B ribbons

Electronic structure of rapidly quenched ribbons of nominal composition Nd_yFe_(86−y−x)B₁₄Ti_x (x = 0, 2, 4; y = 7, 8 at.%) was studied by X-ray absorption (Fe and Ti K-edge) and X-ray photoelectron spectroscopy. It was found by XANES that Ti addition promotes modifications of the electronic structure of Fe sub-band. From the analysis of differences in the pre-edges structure areas, coming from variations in the 3d density of states near the Fermi level, the evidences of acceptor nature of Ti and Fe atoms in NdFeB alloys are present. Using photoelectron spectroscopy, we found that neodymium, iron and titanium are in both, metallic and oxidized, states on the surface of the ribbons. A higher Ti⁰ content (at high vacuum) is characteristic of the annealed samples. The presence of oxidized titanium states was corroborated by XANES.     

Improvement of corrosion resistance of Cu and Nb co-added Nd–Fe–B sintered magnets

Cu and Nb powders are co-added as intergranular modifiers to improve the corrosion resistance of Nd–Fe–B sintered magnets. For the magnet co-added with 0.2 wt.% Cu and 0.8 wt.% Nb, mass loss of accelerated corrosion test in 120 °C, 2 bar and 100% relative humid atmosphere for 96 h drops from 2.47 mg/cm² to 0.49 mg/cm² in comparison with the Cu/Nb free one. The corrosion potential E_corr in 3.5% NaCl aqueous solution increases from −1.115 V to −0.799 V, which indicates the better resistance against electrochemical corrosion. The improved corrosion resistance is ascribed to the enhanced stability of the intergranular phase by forming high electrode potential Cu-containing phase and reduced Nd-rich phase at triple junctions. Besides, the distribution of (Pr, Nd)-rich phases along the grain boundaries becomes more clear and continuous through Cu/Nb co-addition, maintaining fairly good magnetic properties of B_r = 13.6 kGs, H_cj = 11.4 kOe, (BH)_max = 46.2 MGOe. Further investigation demonstrates that Nb is effective to refine the grains of hard magnetic Nd₂Fe₁₄B phase and Cu is beneficial for optimizing the distribution of the intergranular phases.     

Effects of Sintering Temperature on Microstructure and Magnetic Properties of Nd–Fe–B Magnets

The sintered Nd–Fe–B (neodymium–iron–boron) magnet has been used for many applications in various fields such as acoustics, communications, and automation due to its excellent properties including high remanence, high coercivity, and large energy product. Especially high-coercivity sintered Nd–Fe–B magnets have been extensively applied in the field of permanent magnet motors. In the present work, the effects of sintering temperature on the structural and magnetic properties of a Nd₁₅Fe₇₇B₈-type magnet have been investigated. Sintered permanent magnets were produced from a Nd₁₅Fe₇₇B₈ commercial alloy. The magnetic properties were evaluated using an Automatic Magnet Tester. The magnets were successfully produced at different temperatures. It was seen that the best magnetic properties were obtained for the magnet produced at 1050 ^°C for 1 h. The structural evolution of the magnets has also been examined by means of X-ray diffraction (XRD) and polarized optical microscope. Nd₂Fe₁₄B, Fe₃B and some α-iron phases were observed by X-ray diffraction results.