The influence of stress-controlled tensile fatigue loading on the stress–strain characteristics of AISI 1045 steel

This study analyses the influence of fatigue loading on the residual tensile properties of AISI 1045 steel. The fatigue tests were carried out under stress-controlled tensile loadings at a stress ratio equal to 0. The maximum applied stresses were within the range from 550 MPa to 790 MPa. An analysis of ratcheting strain and plastic strain amplitude evolution due to fatigue loading was performed on the experimental data. In the next stage of this study, the initial fatigue loadings were introduced. Two maximum stresses, 550 MPa and 750 MPa, and three cycle lengths, 25%, 50% and 75% of the total number of cycles required to fracture the material at a given stress, were used. The pre-fatigued specimens were subjected to tensile testing at strain rates from 10⁻⁴ to 10⁰ s⁻¹. A large number of fatigue cycles, equal to 75% of the fatigue life, induces material softening as well as a drop in elongation and a reduction of area. Pre-fatigue at maximum stress equal to 550 MPa results in the increase of the elastic limit and offset yield point as well. Both parameters reach almost constant value after number of cycles equal to 25 % of the fatigue life. The further increase in the number of cycles does not affect elastic limit and offset yield point in a clearly visible way. The increase of maximum stress of the initial fatigue loadings up to 750 MPa induces similar but stronger effect i.e. increase and stabilization of elastic limit and offset yield point values, however decrease of both parameters value is observed at large number of pre-fatigue cycles corresponding to 75% of the fatigue life.     

Corrosion behaviour of amorphous Ni–Si thin films on AISI 304L stainless steel

Abstract

Nanoscale Ni – Si thin films are widely used in commercial microelectronic devices because of their promising electrical properties as well as their chemical stability. However, their application in corrosive environment has not been frequently addressed in the literature. In this study, amorphous Ni_0.66Si_0.33, Ni_0.40Si_0.60, and Ni_0.20Si_0.80 thin films are prepared on AISI 304L stainless steel by means of ion-beam sputter (IBS) deposition and their corrosion behaviour is studied using potentiodynamic polarisation measurements. The electrochemical measurements were conducted in 0.05M HCl solution at room temperature. By means of optical interferometer, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), the surface morphology and chemical composition of the thin films were examined before and after the electrochemical measurement. The evaluated results showed that the Ni–Si thin films may exhibit improved corrosion resistance over the 304L substrate provided that Si content is high enough to facilitate the formation of a Si-rich passive film.     

Early stages of the mechanical alloying of TiC–TiN powder mixtures

The present work focuses on the alloying behavior of TiC–TiN powder mixtures submitted to mechanical processing by ball milling. Accurate X-ray diffraction analyses indicate a progressive modification of the unit cell parameters of the TiC and TiN phases, suggesting the formation of TiC- and TiN-rich solid solutions with an increasingly larger content of solutes. Once the discrete character of the mechanical treatment is taken into due account, the smooth change of the unit cell parameters can be explained by a sequence of mutual dissolution stages related to individual collisions. At each collision, the average chemical composition of small amounts of TiC- and TiN-rich phases changes discontinuously. The discontinuous changes can be tentatively ascribed to local mass transport processes activated by the mechanical deformation of powders at collisions.     

Laser powder micro-deposition of compositional gradient Ti–Cr alloy

Laser powder deposition allows preparing fully dense metallic parts in a layer-by-layer additive manufacturing manner. The method presents great flexibility in controlling the materials composition as well as its microstructure and properties. In this paper, composition gradient thin walls from Ti to Ti–60 at.%Cr were prepared by the laser powder micro-deposition process. The microstructure, phase constitution, micro hardness, and modulus were characterized by scanning electron microscope (SEM), micro X-ray diffraction (micro-XRD) and ultra micro indentation testing along the composition gradient direction. The results show that the composition along the wall height change as predetermined. Metastable β-Ti(Cr) was formed for compositions with more than 25 at.%Cr. TiCr₂ phase was formed and its percentage increased with further increase of Cr content. The hardness and modulus changed conformably with the composition gradient.     

Effect of initial composition on phase selection in Ni–Si powder blends processed by mechanical alloying

The effect of initial powder blend composition on the synthesis and formation mechanism of nickel silicide phases was investigated by mechanical alloying in Ni-60 and Ni-66.7?at.% Si powder blends. It was noted that the equilibrium NiSi phase started to form in the early stages of milling and that the amount of the NiSi phase in the milled powder increased with increasing milling time. Even though, under equilibrium conditions, a mixture of both the NiSi and NiSi₂ phases was expected to be present in the Ni-60?at.% Si composition and the stoichiometric NiSi₂ phase in the Ni-66.7?at.% Si composition, the NiSi phase was present in both the compositions investigated. However, while only the NiSi phase was present homogeneously in the Ni-60?at.% Si powder blend, both the NiSi phase and a very small amount of unreacted Si were present in the powder blend of Ni-66.7?at.% Si composition. This unexpected phase constitution in the milled powders was attributed to a partial loss of Si during mechanical alloying of the powder blends, confirmed by energy dispersive X-ray spectrometer analyses, and explained on a thermodynamic basis.     

Laser surface remelting of Cu–Cr–Fe contact material

Abstract

Laser surface remelting/resolidifying treatment on a powder metallurgically manufactured Cu–Cr–Fe contact material was studied. Test results showed that a compact remelting/resolidifying layer was obtained with appropriate laser treatment conditions and a suitable surface absorption coating. After such treatment, the Cu–Cr–Fe microstructure was greatly refined and the Cr phase was uniformly dispersed in the Cu rich matrix with fine spherical or near spherical form. Improved compactness and microstructure of the laser remelted Cu–Cr–Fe material yielded increased hardness (by ~80%), wear resistance, and a reduced friction coefficient compared with the base material. The mechanism of laser strengthening is discussed in relation to the microstructural features of the Cu–Cr–Fe material.     

Preparation of TiAl–Cr surface alloy by plasma-surface alloying technique

A plasma-surface technique was used to form a TiAl–Cr alloy on the surface of a TiAl-based alloy. The feasibility of the alloying process was evaluated by a first principle calculation method. The preparation process was optimized by investigating the effects of processing temperature and discharge pressure on the formation of surface alloy. The calculation using the first-principle method showed that the addition of Cr into the TiAl lattice would increase the stability of the system, and improve mechanical properties of the alloy. The processing temperature was selected at 1100 °C, slightly below the eutectic temperature of the Ti–Al system, ensuring diffusion efficiency and also avoiding the degradation of substrate microstructure. The optimum discharge pressure was 25 Pa within the range of 12–55 Pa. Sputtering and diffusion were well coordinated at this pressure and the obtained alloyed layer had the largest thickness.     

Development of nanocrystalline Fe–Co alloys using mechanical alloying

Abstract

Nanostructured alloys have considerable potential as soft magnetic materials. In these materials a small magnetic anisotropy is desired, which necessitates the choice of cubic crystalline phases of Fe, Co, Ni, etc. In the present work, Fe–50 at.-%Co alloys were prepared using mechanical alloying (MA) in a planetary ball mill under a controlled environment. The influence of milling parameters on the crystallinity and crystal size in the alloys was studied. The particle size and morphology were also investigated using SEM. In addition, a thermal treatment was employed for partial sintering of some of the MA powders. The crystal size in both MA powders and compacted samples was measured using X-ray diffraction. It was shown that the crystal size could be reduced to less than 15 nm in these alloys. The nanocrystalline material obtained was also evaluated for magnetic behaviour.     

Sol–gel autocombustion synthesis of Co–Ni alloy powder

Sol–gel autocombustion is confirmed to be an efficient method in the synthesis of Co–Ni alloy powder. Addition of adequate amount of ethanol can make the reduction reaction thorough and increase the purity of the samples. X-ray diffraction measurement indicates that the obtained samples consist of a single phase with bcc structure. Transmission electron microscopy study shows the grain size is about 10 nm. The magnetic measurements show that the samples are a soft magnetic material with the coercive field smaller than 100 Oe and the saturation magnetization about 95% of the theoretical value.     

Shear strength prediction of Ni–Ti alloys manufactured by powder metallurgy using fuzzy rule-based model

Powder metallurgy is an important manufacturing method and developing models that can predict the characteristics of the products is remarkable for researchers. There are many models discussed in the literature for prediction of the product properties however, nonlinear modeling methods including artificial neural networks (ANNs) and fuzzy models have shown better performance.     

Assessment of localized corrosion in carbon steel tube-grade AISI 1045 used in output oil–gas separator vessel of desalination unit in oil refinery industry

In this research, we aimed to investigate the level of damage occurred in the steel tube material grade CK45 (AISI 1045) after a short period of service in an output desalination unit of an oil refinery industry. Visual examinations revealed that the material of the failed tube had significant thickness reduction and also localized corrosion damage. Energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction spectrum show that the corrosion products mostly consist of oxygen as the main element and slightly sulfur. Metallographic observations on the failed sample show that the observed pinholes were initiated on the inner surface of the tube sample where the surface was exposed to a possible mixed corrosive gas of H₂S, CO₂, and naphthenic acid. It is assumed that a severe thickness reduction has been initially taken place from the inner surface of the tube, and consequently the condensation of byproducts led to development of a localized corrosion and formation of pinholes due to flow stream of corrosive exhausted gases in output oil–gas separator vessel of the desalination unite. Presence of metallic elements in the EDS analysis such as calcium, magnesium, potassium, aluminum, manganese, silica, and zinc confirmed the possibility of condensation of corrosive compounds on the inner surface of the carbon steel tube grade CK45 in output oil–gas separator vessel of desalination unit. Microhardness measurements confirm that the steel tube has no hardness change in its core and outer surface. However, slight reduction in hardness is noticed near the inner surface of the tube sample which can be attributed to the presence of the pinholes. Electrochemical corrosion studies were carried out in the corrosive media filled up with a NACE ID 182 solution. Electrochemical investigations revealed that the corrosion products formed are typically porous which cannot provide a protective layer on the surface of the steel tube sample. Finally, recommendations mainly include application of protective coatings on the inner surface of the tube sample, and/or substitution of more resistive alloys with lower susceptibility to corrosive environments. Moreover, operational conditions such as temperature, water to oil ration and addition of any emulsifiers should be precisely controlled in order to decline any unpredicted fluctuations in output desalination unit’s products.     

Prediction of the temperature dependence of fracture toughness of 12Cr–2Ni–Mo refractory steel

We study the influence of temperature and the size of the specimens on the characteristics of static crack resistance of 12Cr–2Ni–Mo refractory steel. It is shown that, in the temperature range 20–450°C, the increase in the thickness of specimens leads to an insignificant increase in fracture toughness obtained along a 5% secant line according to the standards of evaluation of the characteristics of crack resistance. The evaluation of the characteristics of crack resistance of 12Cr–2Ni–Mo steel with regard for the scale effect according to an earlier developed numerical-experimental model reveals the existence of satisfactory agreement with the experimental data in the entire investigated temperature range. Translated from Problemy Prochnosti, No. 4, pp. 78–88, July–August, 2009.     

Microstructure and wear resistance enhancement of cast steel rolls by laser surface alloying NiCr–Cr3C2

The laser surface alloying technique was used to form wear resistant layers on 70MnV cast steel rolls with NiCr–Cr₃C₂ powders. The objective was to investigate the effects of the scanning speed on microstructure, phases, microhardness and wear resistance. Results indicate that the alloyed layers had dense, pore and crack free and homogeneous structures, as well as a metallurgical bonding with the substrates. With the increase of scanning speed, volume of retained austenite in the alloyed layer increased, microhardness and wear resistance increased and the microstructure refined. Wear results indicated that the wear resistance of the alloyed layer was enhanced by 7.8 times compared with that of the cast steel substrate. The improvement in wear resistance was attributed to the combined results of the grain refining effect, the solution strengthening effect, the tough γ-Fe matrix of the layer, the distribution of the hard Cr₇C₃, Fe₃C and martensite phases, and the good bonding between these hard phases and the matrix.     

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.     

Influence of CrB2 Powder on the Physicomechanical Properties of Fe–Cu–Ni–Sn Composites Sintered By Vacuum Hot Pressing

Materials Science - We study the influence of admixtures of CrB2 powder on the mechanical (microhardness H and modulus of elasticity E) and tribological (wear resistance) characteristics of...     

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.     

Synthesis of manganese–zinc ferrite by powder mixing using ferric oxide from a steel mill acid recovery unit

With the aim of producing fine-grained manganese–zinc (Mn–Zn) ferrite at the end of a calcination process at moderate temperatures, this study consisted, at first, of an “electrochemically designed” powder mixing by wet-ball milling a mixture of manganese (MnO₂), zinc (ZnO), and iron (Fe₂O₃ granules produced by an acid recovery unit of a Brazilian steelmaker, milled to fine sizes using alkaline media) –based raw materials. This mixing/milling resulted in improved size reduction when compared to milling without any alkali addition. Further, noticeable size reduction was achieved when elemental Zn was used in place of ZnO, especially when ammonia was used as the medium. Calcination of the alkaline-milled mixture of MnO₂ + ZnO + Fe₂O₃ at 1200 °C allowed obtaining well-crystallized single-phase Mn–Zn ferrite, whereas calcination of the MnO₂ + ZnO + Fe₂O₃ mill-mixed in 100% NH₄OH at 1200 °C produced the highest saturation magnetization in the as-calcined state.     

Finite element method simulation of residual stresses in Al2O3–AISI 304 steel joints

Abstract

Thermal residual stresses are very detrimental to the mechanical resistance of metal–ceramic joints and thin metallic foils acting as stress relieving interlayers have been used to reduce their effect. The present work presents finite element method simulations of the residual stress field in Al₂O₃–AISI 304 steel joints using interlayers. Different interlayer materials (Ti, Ni, Mo, and Cu) were considered, either separately or in combination. Calculations show that among the different interlayer materials considered, Cu and Ti/Cu are most effective in reducing the thermal stresses and that this role is determined mainly by the ductility of the interlayer material. The calculated results were validated by shear tests performed on real joints obtained by diffusion bonding and it was concluded that residual stresses control the mechanical resistance of the joints.