Microstructure and abrasive wear behaviour of iron base hardfacing

Abstract

The influence of the composition and heat treatment of overlays on the abrasive wear resistance of iron base hardfacing alloy overlays is reported. Overlays were deposited using a shielded metal arc (SMA) welding process on structural steel using two commercial hardfacing electrodes, i.e.Fe – 6%Cr – 0.7%C (H₁) and Fe – 32%Cr – 4.5%C (H₂). Abrasive wear resistance of overlays in as welded and heat treated conditions was tested using a pin on disc system against a 300 grade waterproof SiC polishing paper at different normal loads (1 – 4 N) and constant sliding speed 2.0 m s^-1. Optical microscopy was used to study the microstructure of overlays in as welded and heat treated conditions. SEM studies of wear surfaces were carried out to analyse wear mechanisms. It was found that the wear resistance of the high Cr – C coating is better than the low Cr – C hardfacing under identical conditions. Significant variation in hardness was noticed across the interface, indicating the effect of dilution. Hardness of the coating adjacent to the interface was found to be comparatively lower than the coating further away from the interface. Post-weld heat treatment enhanced the abrasive wear resistance.     

Processing and Microstructural Evolution of Superalloy Inconel 718 during Hot Tube Extrusion

The processing parameters of tube extrusion for superalloy Inconel 718 (IN 718), such as slug temperature, tools temperature, choice of lubricant, extrusion ratio and extrusion speed, were determined by experiment in this paper. An appropriate temperature range recommended for the slug is 1080～1120℃, and the temperature range recommended for the tools is 350～500℃. The microstructural evolution of superalloy IN 718 during tube extrusion was analyzed.With the increase of the deformation the cross crystal grains were slightly refined. While the vertical crystal grain is elongated evidently and the tensile strength increased along the axial rake. Glass lubricants have to be spread on the slug surface after being heated to 150～200℃, vegetable oil or animal oil can be used as the lubricant on the surface of the tools to reduce the extrusion force remarkably.     

Scalable,continuous variable,cellular automaton model for grain growth during homogenisation of vacuum arc remelted Inconel* 718

Abstract

A scalable, continuous variable, cellular automaton (CA) model for the quantitative simulation of normal grain growth is presented. The CA model is based on a discrete solution of the classical Turnbull rate equation for grain boundary motion on a mesoscopic scale. The domain is discretised using a regular cubic lattice considering the first and second nearest neighbourhoods. CA rules were usedto determine the state of each cellbased on the local driving force. The effects of both the boundary curvature and the misorientation of grains were incorporated. The driving force was used to determine the direction of the movement of each boundary cell, forming the basis of a continuous variable cell transition rule. The use of experimental grain boundary characteristics (e.g. energy and mobility) allows one to make predictions on industrially applicable spatial and temporal scales. The model was applied to quantitatively predict grain growth during the homogenisation heat treatment of vacuum arc remelted Inconel 718.     

Prediction and analysis of process failures by ANN classification during wire-EDM of Inconel 718

Wire breakages and spark absence are two typical machining failures that occur during wire electric discharge machining (wire-EDM), if appropriate parameter settings are not maintained. Even after several attempts to optimize the process, machining failures cannot be eliminated completely. An offline classification model is presented herein to predict machining failures. The aim of the current study is to develop a multiclass classification model using an artificial neural network (ANN). The training dataset comprises 81 full factorial experiments with three levels of pulse-on time, pulse-off time, servo voltage, and wire feed rate as input parameters. The classes are labeled as normal machining, spark absence, and wire breakage. The model accuracy is tested by conducting 20 confirmation experiments, and the model is discovered to be 95% accurate in classifying the machining outcomes. The effects of process parameters on the process failures are discussed and analyzed. A microstructural analysis of the machined surface and worn wire surface is conducted. The developed model proved to be an easy and fast solution for verifying and eliminating process failures.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-020-00327-w     

Feasibility analysis and process characteristics of selective laser ablation assisted milling Inconel 718

Laser-assisted machining (LAM), as one of the most efficient ways, has been employed to improve the machinability of nickel-based superalloys. However, the conventional LAM process usually used high power laser with large spot size, easily leading to high processing costs and overheating of bulk materials. In this paper, a new approach of selective laser ablation assisted milling (SLA-Mill) process for nickel-based superalloys was proposed, in which low power laser with small spot size was used to selectively ablate the uncut surface in front of the cutting tool, resulting in plentiful surface defects emerging. Such defects would significantly weaken the mechanical strength of difficult-to-cut materials, which was different from the thermal "softening" principle of conventional LAM. Thus, the laser ablation effect with low power and small spot size was first studied. The relationship between process parameters (e.g., laser power, cutting speed and cutting depth) and process characteristics of SLA-Mill (e.g., chip morphology, tool wear and surface integrity) was systematically discussed. Moreover, the chip formation mechanism in the SLA-Mill process was indepth analyzed. Results show that the SLA-Mill process is an effective approach for enhancing the machinability of nickel-based superalloys. The resultant cutting force has a reduction of about 30% at laser power of 60 W, cutting speed of 90 m/min, and cutting depth of 0.1 mm. Furthermore, the chip formation, tool wear, and surface integrity have improved significantly. In general, this paper provides a new route for the application of LAM technology.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-021-00384-9     

Formation of microlayers of clad residue on aluminium brazing sheet during melting and resolidification in brazing process

Abstract

The present paper is devoted to an analysis of clad residue formation during a controlled atmosphere brazing (CAB) process applied to composite aluminium brazing sheets. Evolution of the microstructure of the clad residue, and in particular the mass of resolidified clad formed, were studied. Observations confirmed that, even under optimal brazing conditions, a residue layer (formed away from the joint zone) always appears after brazing. It was established that the peak brazing temperature plays an important role in the process responsible for formation of the residue mass. However, dwell time at the peak brazing temperature does not have a significant influence on clad residue mass accumulation beyond its known influence on substrate dissolution and core metal erosion in the joint zone.     

Microstructural evolution of carbon fiber reinforced SiC-based matrix composites during laser ablation process

In this work, four different carbon fiber reinforced SiC-based matrix composites (C/SiC) were prepared, and microstructure evolution during laser ablation process was characterized. Laser irradiation provided a special high-temperature environment up to 3500 °C. For all four composites, different morphologies can be obtained in the transition region due to the oxidation of different matrices. While only needle-shaped carbon fiber and nanolayered carbon without any matrix remained in the central region, indicating that graphitization process occurred in the center, resulting from the high-temperature and low-oxygen environment in the laser process. Therefore, the laser ablation of C/SiC composites is controlled by chemical and physical erosion, and mainly by the physical erosion in the center.     

Electron beam melting of Inconel 718: effects of processing and post-processing

ABSTRACT

This study reports the effect of process temperature on microstructure evolution of electron beam melted Inconel 718. Samples fabricated at 915°C had fine grain boundary δ (～200?nm) along with coarse intragranular δ spanning through the length of the grains. On the other hand, samples fabricated at 990°C, only had grain boundary δ along with secondary carbides. During hot isostatic pressing, the distribution of carbides governs the grain growth vs. lack of it. The samples fabricated at 990°C having grain boundary carbides had no grain growth owing to the pinning effect of carbides. In contrast, the sample processed at 915°C had significant grain growth owing to dissolution of grain boundary δ phase and absence of grain boundary carbides.     

Strain rate analyses during elliptical vibration cutting of Inconel 718 using finite element analysis,Taguchi method,and ANOVA

The high strain rate in metal cutting significantly affects the mechanical properties of the work piece by altering its properties. This study outlines the material strain rates during elliptical vibration cutting. The finite element analysis, Taguchi method, and analysis of variance (ANOVA) were employed to analyze the effects and contributions of cutting and vibration process parameters (feed rate, rake angle, tangential amplitude, and frequency of vibration) on the variation of strain rates during machining of Inconel 718. Taguchi signal-to-noise analysis on an L₁₈ (2¹×3³) orthogonal array was used to determine the optimum parametric combination for the maximum strain rate, and ANOVA was applied to evaluate the significance of control parameter factors on the strain rate. The results of the finite element analysis under different conditions illustrated that the feed rate and rake angle were negatively related to the strain rate, whereas the tangential amplitude and frequency had a positive response. Furthermore, ANOVA results indicated that the effect of the feed rate, tool rake angle, vibration frequency, and tangential amplitude on the strain rate were all statistically significant, with a reliability level of 95%. Of these, the dominant parameter affecting the strain rate was the feed rate, with a percentage contribution of 40.36%. The estimation of the optimum strain rate and confirmation tests proved that the Taguchi method could successfully optimize the working conditions to obtain the desired maximum strain rate.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-020-00315-0     

Microstructural characteristics and evolution of Ti2AlN/TiAl composites with a network reinforcement architecture during reaction hot pressing process

The microstructural evolution of TiAl matrix composites with a novel network distribution of Ti₂AlN particle reinforcement was studied. The composites were synthesized by reaction hot pressing method using pure Al and nitrided Ti powders as initial materials. Pure Ti powders nitrided at 600 °C for a certain time in an atmosphere of flowing nitrogen turned into new compound Ti(N) powders, which have a shell of titanium nitrides (such as TiN, Ti₂N and TiN_0.3) and a core of Ti–N solid solution. Within the composites synthesized, Ti₂AlN particles, produced by in situ reaction, exhibit a network distribution. The special shell/core structure of the compound Ti(N) powders contributes to this architecture. Nitriding time of the Ti powders greatly affects the microstructure of the composites. Increasing the nitriding time is beneficial to the distribution of Ti₂AlN particles in a continuous network form. However, too long nitriding time can result in the aggregation of Ti₂AlN particles and thus destroy the uniformity of the network structure. The in-situ synthesized Ti₂AlN/TiAl composites with uniform network structure have a superior mechanical property, and their compressive strengths at 800 °C and 1000 °C are 1112 MPa and 687 MPa, respectively.     

Microstructural evolution of injection molded gas- and water-atomized 316L stainless steel powder during sintering

The present study investigates the microstructural evolution and densification behavior of water- and gas-atomized 316L stainless steel powder. Dilatometry and quenching studies were conducted to determine the extent of densification and corresponding microstructural changes. Results indicate that water-atomized powder could be sintered to 97% of theoretical density, while gas-atomized powders could be sintered to near-full density. The difference in the densification behavior is examined in terms of the particle morphology, initial green density and the particle chemistry.     

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy

Abstract

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy were characterised by employing deformation temperatures of 300, 350 and 400°C and strain rate ranging from 10^?3 to 100 s^?1. When compressed at 10^?3 s^?1, all stress–strain curves at different temperatures (300, 350 and 400°C) showed a flow softening behaviour due to active dynamic recrystallisation. When compressed at 10^?2 s^?1 and elevated temperatures (300, 350 and 400°C), all stress–strain curves showed a flow stress drop after peak stress due to twinning for 300 and 350°C deformation and recrystallisation for 400°C deformation. The balance between shear deformation and recrystallisation resulted in a steady flow behaviour after the true strain reached 0·22. When strain rate increased to 10^?1 s^?1, a small fraction of dynamic recrystallisation in shear deformation region was responsible for slight flow softening behaviour during compression. A flow hardening appeared due to basal and non-basal slips when deformed at 100 s^?1. It is suggested that the flow behaviour during hot compression of twin roll cast ZK60 alloy depends on the separating effect or combined effects of shear deformation, twinning and recrystallisation.     

Microstructure evolution and densification behavior of TiC/316L composite powders during cold/warm die compaction and solid-state sintering: 3D particulate scale numerical modelling and experimental validation

To identify the microstructure evolution and densification behavior of TiC/316L composites in powder metallurgy (PM) process, 3D particulate scale numerical simulations were conducted to reproduce the cold/warm compaction and solid-state sintering of TiC/316L composite powders with corresponding physical experiments being carried out for model validation. The effects of compaction parameters and sintering temperature on the densification behavior of TiC/316L composite powders were systemically investigated. The particle deformation and morphology, stress/strain and microstructure evolutions, and grain size distribution in the whole process were characterized and compared to further illustrate the densification behavior and the underlying dynamics/mechanisms. The results show that compared with the cold compaction, the warm compaction can not only achieve higher relative density, smaller and more uniform equivalent stress, and weaker spring back effect, but also improve the friction condition among powder particles. The plastic deformation of 316L particles is the main densification mechanism during compaction. In the solid-state sintering of TiC/316L compacts, the densification is mainly indicated by shrinkage and vanishing of large residual pores along with the growth of the sintering necks, accompanied by the particle movement and growth along the boundary regions. Meanwhile, the particle displacement and grain size distribution are more uniform in the warm compacted TiC/316L component. Moreover, the equivalent (von Mises) stress in 316L particles is smaller than that in TiC particles.     

Microstructural evolution of aluminum alloy during friction stir welding under different tool rotation rates and cooling conditions

The microstructural evolution during friction stir welding (FSW) has long been studied only using one single welding parameter. Conclusions were usually made based on the final microstructure observation and hence were one-sided. In this study, we used the “take-action” technique to freeze the microstructure of an Al-Mg-Si alloy during FSW, and then systematically investigated the microstructures along the material flow path under different tool rotation rates and cooling conditions. A universal characteristic of the microstructural evolution including four stages was identified, i.e. dynamic recovery (DRV), dislocation multiplication, new grain formation and grain growth. However, the dynamic recrystallization (DRX) mechanisms in FSW depended on the welding condition. For the air cooling condition, the DRX mechanisms were related to continuous DRX associated with subgrain rotation and geometric DRX at high and low rotation rates, respectively. Under the water cooling condition, we found a new DRX mechanism associated with the progressive lattice rotation resulting from the pinning of the second-phase particles. Based on the analyses of the influencing factors of grain refinement, it was clearly demonstrated that the delay of DRV and DRX was the efficient method to refine the grains during FSW. Besides, ultra-high strain rate and a short duration at high temperatures were the key factors to produce an ultrafine-grained material.     

Fatigue of asphaltic dike revetments under wave loading: validation and improved modelling

In the Netherlands, 600 km of the sea dikes are protected by an asphaltic revetment that has to resist considerable wave loads with a significant wave height of up to 4.5 m. The subsoil is normally sandy, and the asphalt layer acts as a protection against erosion. The asphalt layer can fail as a result of fatigue due to repeated loading under storm conditions. In case of very high wave loads, the asphalt can fail after a few large waves. Ageing of the asphalt has a large effect on the resistance against fatigue. Therefore, periodic monitoring is prescribed by law. This monitoring consists of: falling weight deflection measurements, lab testing, radar measurements, visual inspection and calculations with the software program ‘Wave impact’ (‘Golfklap’) in order to determine the strength under storm conditions. The subsoil bearing capacity determines how the asphaltic revetment deforms under wave loading. It is still unsure how the soil will behave and failure mechanisms may occur that are different from fatigue due to elastic bending, the latter being part of the current approach. A first step to validation consisted of finite element modelling that was compared with experiments on medium scale (scale 1:4). In these experiments, the wave attack was simulated by a mechanical system that was placed on a 5 cm thick asphalt plate on a sand layer. The deflection of the asphalt was measured. Crack development was detected by means of strain gauges. It was found that – as a first approximation and excluding extreme wave loads – the fatigue behaviour of aged asphalt on a typical sandy base under wave attack can be described with a combination of a relatively simple elastic–plastic response to wave loads and a special fatigue line that takes into account the flexural strength of the aged asphalt.     

Microstructural evolution and tensile property of 1Cr15Ni36W3Ti superalloy during thermal exposure

1Cr15Ni36W3Ti was thermally exposed at 580 ℃ and 680 ℃,respectively,up to 3000 h.The γ'phase and intergranular TiC carbides continuously coarsened during exposure.None of η,laves or σ phase was discovered in the exposed samples,indicating good microstructure stability under the present exposure conditions.The ripening process of the γ'phase could be well modelled utilizing the LSW theory.The evolutions of the yield and tensile strengths were monotonous during exposure at 580 ℃.However,a transition point in strengths was detected in the tensile samples exposed at 680 ℃ for 300 h.Accordingly,the critical γ'diameter was measured to be 13-14 nm.The γ'/dislocations interaction mechanism transformed from shearing to looping with the γ'diameter exceeding the critical point.The combination of the weakly coupled dislocations model and the Orowan looping model yielded a critical diameter of 13.1 nm which coincided well with the measured one,indicating the applicability of these two strengthening models for 1Cr15Ni36W3Ti.The present exposure conditions did not exert a profound effect on the fracture mode.All the tensile samples underwent a typically ductile fracture with a dimple pattern dominating the fracture surface.The dispersed deformation induced by the prevalence of dislocation looping in the over-aged tensile samples retarded the propagation of intergranular cracks.The declined precipitation hardening increment and the enhanced deformation homogeneity partially recovered the tensile ductility in the over-aged samples exposed at 680 ℃.     

Microstructural evolution and its influence on the impact toughness of GH984G alloy during long-term thermal exposure

The microstructure evolution and its effect on the impact toughness of a new Ni-Fe based alloy GH984 G,used in 700℃ ultra-super critical coal-fired power plant,were investigated during thermal exposure at 650℃-750℃ for up to 10,000 h.The results show that the impact toughness at room tempe rature drops rapidly at the early stage during thermal exposure at 700℃ and then has no significant change even if after exposure for 10,000 h.The significant decline of the impact toughness is attributed to the coarsening of M₂₃C₆ carbides at grain boundaries,which weakens the grain boundary strength and leads to the aging-induced grain boundary embrittlement.The M₂₃C₆ carbides have almost no change with further thermal exposure and the impact toughness also remains stable.Additionally,the impact toughness rises with the increase of thermal exposure temperature.The size of γ' after thermal exposure at 750℃ for10,000 h is much bigger than that at 650℃ and 700℃ for 10,000 h.There fore,the intragranular strength decreases significantly due to the transformation of the interaction between γ' and dislocation from stro ngly coupled dislocation shearing to Orowan bowing.More plastic deformation occurs within grains after thermal exposure at 750℃ for 10,000 h,which increases the impact toughness.     

Surface state of Inconel 718 ultrasonic shot peened: Effect of processing time,material and quantity of shot balls and distance from radiating surface to sample

Plates of Inconel 718 in precipitated state have been subjected to ultrasonic shot peening (USP), varying the distance from the radiating surface of the booster to the sample, the processing time and the material (WC/Co and steel) and number of shot balls, in order to study the effect of these parameters on the final state generated by the USP process. A change to more compressive residual stresses at the surface of the treated parts has been measured in all cases. For higher USP processing times and/or lower booster-sample distances, the degree of plastic deformation in the treated material increases, leading to a change to more compressive surface stresses and a higher density of impact marks in the treated surface. The same occurs when WC/Co balls are used instead of steel balls. The tendency to more compressive stresses reaches a saturation level after a certain processing time, when the system is not able to force the material to continue with more plastic deformation. If a higher quantity of balls is used, there will be less impacts of the shots with the surface and their energy will be lower (due to losses of energy after inelastic collisions). This diminishes the effect of the impacts in introducing compressive stresses and leads to less and shallower impact marks in the treated surface.