Electron beam surface treatment. Part II: microstructure evolution of stainless steel and aluminum alloy during electron beam rapid solidification

Surface rapid solidification microstructures of AISI 321 austenitic stainless steel and 2024 aluminum alloy have been investigated by electron beam remelting process and optical microscopy observation. It is indicated that the morphologies of the melted layer of both stainless steel and aluminum alloy change dramatically compared to the original materials. Also, the microstructures were greatly refined after the electron beam irradiation.     

Application of Taguchi method in optimizing the microstructure of 316LN austenitic stainless steel

The hot deformation behavior of AISI 316LN austenitic stainless steel was studied through isothermal hot deformation tests conducted on Gleeble 1500 thermo‐simulation machine. By using Taguchi method with an L16 (4³) orthogonal array, the hot deformation conditions including deformation temperature, strain rate and strain were optimized for obtaining minimum mean grain size of the steel. Analysis of variance was used for determining the effect of the hot deformation parameters on the mean grain size. The results suggested that the strain had the most significant effect on the mean grain size among the three factors. The percent contributions of deformation temperature, strain rate and strain to the mean grain size were 0.80 %, 2.45 % and 93.32 %, respectively. Finally, the confirmation experiment under the optimum conditions, i.e., 1050 °C temperature, 0.01 s^–1 strain rate and 0.7 strain was carried out. The observational mean grain size (11.4 μm) is approximately twelve times smaller than that of the initial microstructure.     

Effects of aluminum additions on the microstructure and mechanical properties of alumina-forming austenitic stainless steels

The mechanical properties, including tensile and impact properties at different testing temperatures of alumina-forming austenitic steels (25 % nickel, 20 % chromium) with different aluminum contents (0, 2.5 %, 5 % and 8 %) were investigated. Scanning and transmission electron microscopy together with tensile and impact properties tests were conducted. The results showed that the tensile strength of steels at 298 K increased obviously along with aluminum contents increasing, while plasticity decreased at the same, which attributed to the higher volume fraction and number density of spherical NiAl precipitation together with main ferrite in matrix. In addition, spherical NiAl particles dispersed easily in ferrite. In particular, the ultimate tensile strength of the sample with 8 % aluminum could reach 1398 MPa, with the elongation of 14 % at 298 K. However, NiAl precipitations would lose strengthening effects at high temperatures, but the plasticity could be improved. In addition, the sample with 5 % aluminum showed better comprehensive properties by comparison to other samples, and the ultimate tensile strength was 1018 MPa and 491 MPa at 298 K and 973 K with the elongation of 26 % and 43 %, respectively, enabling it to be promising material for industrial application in advanced nuclear systems.     

Influence of the microstructure on the corrosion resistance of plasma‐nitrided austenitic stainless steel 304L and 316L

Austenitic stainless steels are widely used in medical and food industries because of their excellent corrosion resistance. However, they suffer from weak wear resistance due to their low hardness. To improve this, plasma nitriding processes have been successfully applied to austenitic stainless steels, thereby forming a thin and very hard diffusion layer, the so‐called S‐phase. In the present study, the austenitic stainless steels AISI 304L and AISI 316L with different microstructures and surface modifications were used to examine the influence of the steel microstructure on the plasma nitriding behavior and corrosion properties. In a first step, solution annealed steel plates were cold‐rolled with 38% deformation degree. Then, the samples were prepared with three kinds of mechanical surface treatments. The specimens were plasma nitrided for 360 min in a H₂–N₂ atmosphere at 420 °C. X‐ray diffraction measurements confirmed the presence of the S‐phase at the sample surface, austenite and body centered cubic (bcc)‐iron. The specimens were comprehensively characterized by means of optical microscopy, scanning electron microscopy, glow discharge optical emission spectroscopy, X‐ray diffraction, surface roughness and nano‐indentation measurements to provide the formulation of dependencies between microstructure and nitriding behavior. The corrosion behavior was examined by potentio‐dynamic polarization measurements in 0.05 M and 0.5 M sulfuric acid and by salt spray testing.     

Effect of pre‐processing annealing and ram speed on mechanical properties for low carbon steel processed by constrained groove pressing

Constrained groove pressing (CGP) has emerged for producing ultra‐fine‐grained materials with distinguished properties. Low carbon steel sheets were subjected to severe plastic deformation by constrained groove pressing process. The effect of pre‐processing annealing temperature, ram speed and number of passes on microstructure, mechanical properties and wear behaviour of the sheets were investigated. The 3 mm thick sheets were deformed by a constrained groove pressing die at ram speeds: 5 mm/min, 10 mm min^?1 and 20 mm min^?1. Furthermore, the as received sheets were annealed at 600 °C and 900 °C, then deformed at ram speed 20 mm min^?1. The annealing temperature 900 °C led to slightly coarser grains, lower strength and larger ductility compared to those obtained after annealing at 600 °C. With lowering the ram speed to 5 mm min^?1, the number of passes could be increased to 10 passes while increasing ram speed from 5 mm min^?1 to 20 mm min^?1 improved the mechanical properties; after 3 constrained groove pressing passes, the ultimate tensile strength increased from 420 MPa to 490 MPa, the hardness from 174 HV 1 to 190 HV 1 and the elongation from 7.6 % to 9.5 %. Finer grains were also obtained by increasing ram speed. Wear resistance was greatly enhanced by constrained groove pressing and by the increase in ram speed.     

Effect of interlayer addition on microstructure and mechanical properties of NiTi/stainless steel joint by electron beam welding

NiTi/Stainless Steel(SS) sheets have been welded via a vacuum electron beam welding process, with three methods(offsetting electron beam to SS side without interlayer, adding Ni interlayer and adding Fe Ni interlayer), to promote mechanical properties of the Ni Ti/SS joints. The joints with different interlayers are all fractured in the weld zone near the Ni Ti side, which is attributed to the enrichment of intermetallic compounds including Fe2 Ti and Ni3 Ti. The fracture mechanisms of different joints are strongly dependent on the types of interlayers, and the joints without interlayer, adding Ni interlayer and adding Fe Ni interlayer exhibit cleavage fracture, intergranular fracture and mixed fracture composed of cleavage and tearing ridge, respectively. Compared with the brittle laves phase Fe2 Ti, Ni3 Ti phase can exhibit certain plasticity, block the crack propagation and change the direction of crack propagation. The composite structure of Ni3 Ti and Fe2 Ti will be formed when the Fe Ni alloy is taken as the interlayer, which provides the joint excellent mechanical properties, with rupture strength of 343 MPa.     

Influence of carbon diffusion on microstructure and wear behaviour of duplex stainless steel surface layers on lamellar grey cast iron

Surface welding with duplex stainless steel was performed to enhance the wear and corrosion properties of grey cast iron, which is used as material for applications as pump components in maritime and chemical environments. The method used for surface welding and the corresponding process parameters determine the chemical composition and microstructure, which both determine the corrosion and wear properties of the surface layer. High heat input leads to high chemical dilution and thus, reduced corrosion resistance. Slow cooling rates, which are recommended for welding of grey cast iron components, facilitate the formation of carbides in the fusion zone of the chromium‐rich duplex stainless steel surface layer. On the one hand, carbides lead to increased hardness and thus, improved wear resistance of the surface layers. On the other hand, carbides and high chemical dilution rates reduce the corrosion resistance and therefore should be avoided. Under high cooling rates, the risk of cracking in the heat affected zone of the grey cast iron increases due to martensitic phase transformations. The paper describes the correlation of process parameters, microstructure and chemical composition with a focus on carbon diffusion and carbide formation, ever considering the effect on the wear behaviour in an oscillation tribometer and under erosion‐corrosion conditions.     

Influence of nitriding on corrosion resistance of martensitic X17CrNi16‐2 stainless steel

Nitriding increases surface hardness and improves wear resistance of stainless steels. However, nitriding can sometimes reduce their corrosion resistance. In this paper, the influence of nitriding on the corrosion resistance of martensitic stainless steel was investigated. Plasma nitriding at 440 °C and 525 °C and salt bath nitrocarburizing were carried out on X17CrNi16‐2 stainless steel. Microhardness profiles of the obtained nitrided layers were examined. Phase composition analysis and quantitative depth profile analysis of the nitrided layers were preformed by X‐ray diffraction (XRD) and glow‐discharge optical emission spectrometry (GD‐OES), respectively. Corrosion behaviour was evaluated by immersion test in 1% HCl, salt spray test in 5% NaCl and electrochemical corrosion tests in 3.5% NaCl aqueous solution. Results show that salt bath nitrocarburizing, as well as plasma nitriding at low temperature, increased microhardness without significantly reducing corrosion resistance. Plasma nitriding at a higher temperature increased the corrosion tendency of the X17CrNi16‐2 steel.     

Structure and wear resistance of the composite layers produced by glow discharge nitriding and PLD method on AISI 316L austenitic stainless steel

The rapid technical development enhances the demands on constructional materials in terms of their resistance to frictional wear, resistance to corrosion and erosion, high hardness, high tensile and fatigue strength. These demands can be satisfied by e.g. applying various surface engineering techniques that permit to modify the microstructure, phase and chemical composition of the surface layers of the treated parts. A prospective line of the development of surface engineering is the production of composite layers by combining various surface engineering methods. The paper presents the results of examinations of the phase composition and frictional wear resistance of the layers produced by hybrid processes, i.e. such that combined glow discharge assisted nitriding performed at 450 °C and 550 °C with a pulsed laser deposition of boron nitride coatings (PLD method). It has been shown that the boron nitride coatings formed on nitrided AISI 316L steel increase its frictional wear resistance.     

Effect of process parameters and niobium carbide addition on microstructure and wear resistance of laser cladding nickel-based alloy coatings

Based on the background of the engineering application of automobile mold repair and surface strengthening, the effects of process parameters on the formation and microstructure of laser cladding nickel(Ni)-based alloy coating were studied. The optimal parameters were: laser power 2000 W, powder feeding rate 15 g/min, scanning speed 4 mm/s. Under this process, the cladding layer and the substrate can exhibit good metallurgical bonding, and the cladding layer has fine crystal grains and a low dilution ratio. On this basis, different mass fractions of niobium carbide (NbC) powder were added to the nickel-based powder and laser cladding was carried out on the surface of die steel. The phase composition, microstructure, hardness and wear resistance of the coating were studied. The results show that with the increasing of niobium carbide addition, the hardness of the cladding layer decreases, and the wear loss of the cladding layer decreases first and then increases. When the niobium carbide addition reaches 6 wt.%, the wear loss of the cladding layer is the least, and the wear resistance is the best.     

Investigation of wear resistance and microstructure of a newly developed chromium and vanadium containing iron‐based hardfacing alloy

Plasma transferred arc (PTA) welded Ni and Co‐based alloys have gained high acceptance in many industrial applications for the wear protection of components. Recently, the cost of nickel and cobalt is rising drastically. This paper presents the development of a cost‐effective high chromium and vanadium containing iron‐based hardfacing alloy with high hardness and wear resistance. The welding processing of the alloy is carried out by PTA welding of atomized powders. Investigations on powder production as well as on weldability are presented. The coatings are metallographically studied by optical microscopy, SEM, EDX and micro‐hardness measurements. The wear resistance properties of the coatings are examined using pin on disk, dry sand rubber wheel and Miller testing, the corrosion properties are determined by immersion corrosion tests. The newly developed iron‐based alloy has nearly the same wear resistance as Ni‐based alloys with fused tungsten carbides at a higher level of corrosion resistance and much lower cost.     

Effect of rare earth cerium on the impact toughness,corrosion and wear resistance of CrNiMnMo alloyed lining steel

The effects of the addition of rare earth cerium on the impact corrosion and wear resistance of CrNiMnMo alloyed lining steel were studied using metallurgical microscopy, scanning electron microscope, x-ray powder diffraction, a hardness tester, an impact tester, and an impact corrosion abrasion tester. The properties of CrNiMnMo alloyed lining steel with the addition of cerium were compared with that of CrNiMnMo steel without the addition of cerium after being heat treated by same processes. The results show that CrNiMnMo alloyed lining steel with the addition of rare earth cerium consisted of lath martensite with a low residual austenite phase after being quenched and tempered. The lath of the martensite was fine and uniform. The impact toughness and the corrosion resistance of the steel lining board improved greatly with the addition of cerium and the impact toughness increased by 22 % with the fracture mode of being ductile. The CrNiMnMo alloyed lining steel with cerium showed the highest resistance to impact corrosion abrasion compared with the alloy without cerium.     

Hot rolling and annealing effects on the microstructure and mechanical properties of ODS austenitic steel fabricated by electron beam selective melting

The grain morphology, nano-oxide particles and mechanical properties of oxide dispersion strengthened (ODS)-316L austenitic steel synthesized by electron beam selective melting (EBSM) technique with different post-working processes, were explored in this study. The ODS-316L austenitic steel with superfine nano-sized oxide particles of 30–40 nm exhibits good tensile strength (412 MPa) and large total elongation (about 51%) due to the pinning effect of uniform distributed oxide particles on dislocations. After hot rolling, the specimen exhibits a higher tensile strength of 482 MPa, but the elongation decreases to 31.8% owing to the introduction of high-density dislocations. The subsequent heat treatment eliminates the grain defects induced by hot rolling and increases the randomly orientated grains, which further improves the strength and ductility of EBSM ODS-316L steel.     

The effect of pit size and density on the fatigue behaviour of a pre‐corroded martensitic stainless steel

UNS S17400 steel is used in turbines for the aerospace and utility industries. While it is generally corrosion resistant, it is susceptible to pitting when exposed to aqueous chloride environments. Effects of pitting characteristics, such as depth, width, and local density on fatigue life, have been studied in this work to better inform criteria for component replacement or repair. While pit depth correlates well with cracking, the deepest pit never initiated the crack that ultimately led to failure. The clustering of pits, or local pitting density, also correlated well with crack initiation location; however, the densest region of pitting was not always the location where cracking occurred. There is likely no single metric that directly correlates pitting with fatigue cracking, rather there is a combination of pitting characteristics that ultimately lead to cracking. The results from this work suggest that pit depth and local pitting density are among the more important metrics.     

Finite element analysis on shape deformation of the macro-structure region during hot-rolling process for gear steel FAS3420H and its modification

The rectangular macro-segregation region in the core of hot-rolled steel bar has a great influence on the heat treatment distortion of an automotive gear steel FAS3420H. In order to understand the distortion process of the segregation region during hot-rolling process, a finite element (FE) analysis of the whole hot-rolling process was carried out by software DEFORM-3D, and a constructed constitutive model was embedded into DEFORM-3D to describe the deformation process. The results show there is an inhomogeneous strain distribution at the cross-section of hot-rolled round bar, which results in uneven distortion of the segregation region in different directions. A square segregation region, which can reduce inhomogeneous distortion of gear in the subsequent heat treatment, is obtained after being rolled by simulation calculation verified through experiment.     

Design of a Medical Non‐Linear Drilling Device: The Influence of Twist and Wear on the Fatigue Behaviour of NiTi Wires Subjected to Bending Rotation

This paper considers fundamental and experimental aspects associated with the engineering design of a medical, non‐linear drilling device which exploits shape memory pseudoelasticity of NiTi wires. For this application it is important that the NiTi wires have a good fatigue resistance. This is why the present authors have previously determined the influence of various parameters on cyclic life, crack growth and stress state of pseudoelastic wires subjected to bending rotation fatigue. The actual drilling device has to withstand twist in addition to bending rotation because the free rotation is constrained by friction between the drill head and the bone material. In addition, friction between the wire and a NiTi guiding tube results in wear and this may well promote fatigue crack nucleation. In this paper, we explain the function of the medical drill. We then report results on the effect of the additional parameters (1) twist and (2) wear on the fatigue life of thin pseudoelastic NiTi wires. We finally discuss the implications of our experimental results for the design process of the medical drilling device.     

Study based on the analysis of valence electron structure for adiabatic shear sensitivity of 30CrMnMo and C45E4 steel in process of high‐speed penetration

In this paper, the influence of valence electron structure parameters on the adiabatic shearing sensitivity has been studied for two grade steels (30CrMnMo and C45E4) as a function of covalent pair number and lattice electron number by empirical electron theory of solids and molecules in high‐speed penetration process. The research shows that the adiabatic shearing sensitivity increases with the increase of the covalent electron pair number n_A, decreases with the increase of the lattice electron number n_l. The n_A of each structure unit in 30CrMnMo steel is larger than that in C45E4steel, the n_l of each structure unit in 30CrMnMo steel is smaller than that in C45E4steel. Therefore, the adiabatic shearing sensitivity is higher for 30CrMnMo targets damaged by adiabatic shear failure, and yet C45E4 targets were damaged by ductile fracture without any adiabatic shear band. The basis can be provided for appropriately selecting and designing materials with different adiabatic shearing sensitivity by studying the specific alloy elements on the influence of adiabatic shearing sensitivity in valence electron structure level.