An investigation of mechanical and metallurgical properties of explosive welded aluminum-dual phase steel

The aim of this paper is investigation of microstructure and property relationship in aluminum-HSLA steel and aluminum-dual phase steel bimetals fabricated by explosive welding technique. Dual phase steel was produced by intercritical annealing and water quenching from 1.45Mn-0.2Si-0.186C HSLA steel. Hardness, tensile shear strength, tensile strength, toughness and microstructure of explosively welded aluminum-HSLA steel and aluminum-dual phase steel were evaluated. Both bimetals have a straight bonding interface. It was also seen that plastic deformation of dual phase steel was higher than HSLA steel near interfaces of bimetals. The hardness was increased near the bond interface of bimetals. Tensile and tensile shear strength tests showed that aluminum-dual phase steel is superior than aluminum-HSLA steel. Also, impact toughness of aluminum-dual phase steel was found significantly higher than that of aluminum-HSLA steel.     

Comparative evaluation of strain controlled low cycle fatigue behaviour of solution annealed and prior cold worked 316L(N) stainless steel

The effect of 20% prior cold work on low cycle fatigue (LCF) behaviour of type 316L(N) stainless steel (SS) was studied at 873 K by conducting total axial strain controlled tests in air with strain amplitudes in the range ±0.25% to ±1.0%. The cyclic deformation behaviour of 20% prior cold worked (PCW) material was compared with the LCF response of solution annealed (SA) alloy tested under similar conditions. The cyclic stress response (CSR) of 316L(N) SS in the PCW condition was characterized by a short period of hardening followed by prolonged softening prior to failure, whereas SA material exhibited a significant hardening regime followed by stress saturation. Interrupted tests on PCW material were carried out at different stages of CSR in order to determine the underlying mechanisms as reflected in substructural changes. The fatigue life in the solution annealed condition was similar to that of the PCW material at higher strain amplitudes of testing (≥±0.5%) while at lower strain amplitudes, the PCW material exhibited longer life.     

Microstructural evolution and mechanical properties of friction stir welded joint of Fe–Cr–Mn–Mo–N austenite stainless steel

2 mm thick Fe–18.4Cr–15.8Mn–2.1Mo–0.66N high nitrogen austenite stainless steel plate was successfully joined by friction stir welding (FSW) at 800 rpm and 100 mm/min. FSW did not result in the loss of nitrogen in the nugget zone. The arc-shaped band structure, consisting of a small amount of discontinuous ferrite aligning in the bands and fine austenite grains, was a prominent microstructure feature in the nugget zone. The discontinuous ferrite resulted from newly formed ferrite during welding and the remained ferrite, whereas the fine austenite grains were formed due to dynamic recrystallization of the initial austenite during FSW. The fine dynamically recrystallized grains in the nugget zone significantly increased the hardness compared to that of the base material. The strength of the joint was similar to that of the base material, with the joint failing in the base material zone.     

Development of new cyclic plasticity model for 304LN stainless steel through simulation and experimental investigation

Cyclic plastic deformation characteristics of 304LN stainless steel material have been studied with two proposed cyclic plasticity models. Model MM-I has been proposed to improve the simulation of ratcheting phenomenon and model MM-II has the capability to simulate both cyclic hardening and softening characteristics of the material at various strain ranges. In the present paper, strain controlled simulations are performed with constant, increasing and decreasing strain amplitudes to verify the influences of loading schemes on cyclic plasticity behaviors through simulations and experiments. It is observed that the material 304LN exhibits non Masing characteristics under cyclic plastic deformation. The measured deviation from Masing is well established from the simulation as well as from experiment. Simulation result shows that the assumption of only isotropic hardening is unable to explain the hardening or softening characteristics of the material in low cycle fatigue test. The introduction of memory stress based cyclic hardening coefficient and an exponentially varying ratcheting parameter in the recall term of kinematic hardening rule, have resulted in exceptional improvement in the ratcheting simulation with the proposed model, MM-II. Plastic energy, shape and size of the hysteresis loops are additionally used to verify the nature of cyclic plasticity deformations. Ratcheting test and simulation have been performed to estimate the accumulated plastic strain with different mean and amplitude stresses. In the proposed model MM-I, a new proposition is incorporated for yield stress variation based on the memory stress of loading history along with the evolution of ratcheting parameter with an exponential function of plastic strain. These formulations lead to better realization of ratcheting rate in the transient cycles for all loading schemes. Effect of mean stress on the plastic energy is examined by the simulation model, MM-I. Finally, the micro structural investigation from transmission electronic microscopy is used to correlate the macroscopic and microscopic non Masing behavior of the material.     

Influence of prior cyclic hardening on high temperature deformation and crack growth in type 316L (N) stainless steel

For power generating equipment subjected to cyclic loading at high temperature, crack growth could arise from the combinations of fatigue and creep processes. There is potential for the material to undergo hardening (or more generally changes of material state) as a consequence of cyclic loading. Results of an experimental study to examine the influence of prior cyclic hardening on subsequent creep deformation are presented for type 316L(N) stainless steel at 600°C. Experiments were also carried out to explore creep crack growth at constant load, and crack growth for intermittent cyclic loading. For the as-received material there is substantial primary creep (hardening) at constant load, while for the cyclically hardened material at constant load the creep curves show recovery, and increasing creep rate with increasing time. Specimens subjected to prior cyclic hardening were also used for a series of creep and creep-fatigue crack growth tests. These tests demonstrated that there was accelerated crack growth compared to crack growth in as-received material.     

A comparative analysis of metallurgical and mechanical properties of friction welded and post weld heat treated (oil quenched) duplex stainless steel joints

The effects of post weld heat treatment (PWHT) and oil quenching on the metallurgical and mechanical properties of the duplex (UNS S31803) welded joints were evaluated at three different temperatures namely 1080, 1150 and 1200 °C. The microstructural variation, austenite/ferrite phase changes, grain size measurements and microhardness aspects of the welded joint were observed. The fraction of ferrite and austenite phases was equivalent at 1150 °C. Nickel element was more efficient in controlling the twin phase balance. Finer grain structure was achieved at 1150 °C due to recrystallization effect. Twin phase presence and absence of precipitates were confirmed through XRD and TEM which followed Kurdjumov–Sachs relationship. At a heating pressure of 40 MPa, heating time of 4 s, an upsetting pressure of 80 MPa, and an upsetting time of 2 s during a PWHT at 1150 °C, a 50/50 balance between the duplex phases, fine grains, and increased microhardness were obtained.     

Characterization of microstructure,mechanical properties and corrosion resistance of dissimilar welded joint between 2205 duplex stainless steel and 16MnR

The joint of dissimilar metals between 2205 duplex stainless steel and 16MnR low alloy high strength steel are welded by tungsten inert gas arc welding (GTAW) and shielded metal arc welding (SMAW) respectively. The microstructures of welded joints are investigated using scanning electron microscope, optical microscope and transmission electron microscopy respectively. The relationship between mechanical properties, corrosion resistance and microstructure of welded joints is evaluated. Results indicate that there are a decarburized layer and an unmixed zone close to the fusion line. It is also indicated that, austenite and acicular ferrite structures distribute uniformly in the weld metal, which is advantageous for better toughness and ductility of joints. Mechanical properties of joints welded by the two kinds of welding technology are satisfied. However, the corrosion resistance of the weldment produced by GTAW is superior to that by SMAW in chloride solution. Based on the present work, it is concluded that GTAW is the suitable welding procedure for joining dissimilar metals between 2205 duplex stainless steel and 16MnR.     

Equal-channel angular sheet extrusion of interstitial-free (IF) steel: Microstructural evolution and mechanical properties

Interstitial-free steel (IF-steel) sheets were processed at room temperature using a continuous severe plastic deformation (SPD) technique called equal-channel angular sheet extrusion (ECASE). After processing, the microstructural evolution and mechanical properties have been systematically investigated. To be able to directly compare the results with those from the same material processed using discontinuous equal channel angular extrusion, the sheets were ECASE processed up to eight passes. The microstructural investigations revealed that the processed sheets exhibited a dislocation cell and/or subgrain structures with mostly low angle grain boundaries. The grains after processing have relatively high dislocation density and intense micro-shear band formation. The electron backscattering diffraction (EBSD) examination showed that the processed microstructure is not fully homogeneous along the sheet thickness due probably to the corner angle of 120° in the ECASE die. It was also observed that the strengths of the processed sheets increase with the number of ECASE passes, and after eight passes following route-A and route-C, the yield strengths reach 463 MPa and 459 MPa, respectively, which is almost 2.5 times higher than that of the initial material. However, the tensile ductility considerably dropped after the ECASE. The limited ductility was attributed to the early plastic instability in the tensile samples due to the inhomogeneous microstructure. The specimen orientation with respect to the ECASE direction did not have a considerable effect on the stress-strain response. Appropriate low temperature annealing of ECASE-processed IF-steel resulted in a good strength-ductility balance.     

(Ti,Cr)N and Ti/TiN PVD coatings on 304 stainless steel substrates: wear-corrosion behaviour

When hard nitrides coatings work in aggressive environments, wear phenomena occur in conjunction with corrosion processes; hence the characterisation of wear and corrosion resistance, considered as two distinct aspects, cannot be regarded as representative of the real degradation process, where the interactions between the two phenomena become extremely important. In this work wear-corrosion tests were performed by means of a laboratory-made apparatus, where the electrochemical characterization can be easily carried out. TiN coatings (with and without the presence of a Ti underlayer) and (Ti,Cr)N coatings were tested under different wear conditions in a sodium chloride solution. The results were compared and the different behaviour was interpreted in terms of film texture and residual stresses due to the PVD process. The (Ti,Cr)N deposits, which have the best intrinsic corrosion resistance, turned out to be the less resistant coatings to the wear-corrosion process, probably because residual stress was much higher than in TiN, making the coating more brittle and poorly adherent to the substrate. The TiN/Ti samples showed the best behaviour under wear-corrosion, which can be explained by a better adhesion of the coating, enhanced by the titanium underlayer. Moreover, the presence of the Ti underlayer proved to be effective in reducing the residual stresses in the TiN layer.     

Microstructures and mechanical behavior of the bimetallic additively-manufactured structure (BAMS) of austenitic stainless steel and Inconel 625

Bimetallic additively manufactured structures (BAMSs) can replace traditionally-fabricated functionally-graded-components through fusion welding processes and can eliminate locally-deteriorated mechanical properties arising from post-processing.The present work fabricates a BAMS by sequentially depositing the austenitic stainless-steel and Inconel625 using a gas-metal-arc-welding (GMAW)-based wire + arc additive manufacturing (WAAM) system.Elemental mapping shows a smooth compositional transition at the interface without any segregation.Both materials being the face-center-cubic (FCC) austenite,the electron backscattered diffraction (EBSD) analysis of the interface shows the smooth and cross-interface-crystallographic growth of long-elongated grains in the <001> direction.The hardness values were within the range of 220-240 HV for both materials without a large deviation at the interface.Due to the controlled thermal history,mechanical testing yielded a consistent result with the ultimate tensile strength and elongation of 600 MPa and 40 ％,respectively,with the failure location on the stainless-steel side.This study demonstrates that WAAM has the potential to fabricate BAMS with controlled properties.     

Microstructural,mechanical and tribological properties of Al–7Si–(0–5)Zn alloys

A series of Al–7Si–(0–5)Zn alloys were produced by permanent mould casting and their microstructure, mechanical and tribological properties were investigated in as-cast state. The microstructure of Al–7Si alloy consisted of α-Al dendrites surrounded by eutectic Al–Si mixture and a small amount of primary silicon particles. Addition of zinc into Al–7Si alloy resulted in the formation of α-solid solution and an increase in size and volume fraction of primary silicon particles. Moreover, these particles gathered inside interdendritic regions of the ternary Al–7Si–Zn alloys. The density, strength and hardness of Al–7Si–Zn alloys increased continuously with increasing zinc content, but their elongation to fracture and impact energy showed a reverse trend. It was also observed that zinc had no significant effect on the friction coefficient of the alloys, but their wear volume decreased with increasing zinc content up to 4%, above which the trend reversed. The wear surfaces of the alloys were characterized mainly by smearing layer with some degree of oxidation. In addition, delamination and fine scratches were observed on the worn surface. It was concluded that the addition of zinc up to 4% improves both mechanical and wear behaviour of Al–7Si alloy.     

The effect of surface roughness on the efficiency of the cyclic potentiodynamic passivation (CPP) method in the improvement of general and pitting corrosion resistance of 316LVM stainless steel

The influence of surface roughness on the efficiency of a cyclic potentiodynamic passivation (CPP) method employed to increase the general and pitting corrosion resistance of 316LVM stainless steel was investigated. The results show that a decrease in surface roughness of both the surface on which the passive film was formed naturally and by the CPP method, results in an increase in general corrosion resistance of the material, while for the CPP-modified surface, a notable increase in pitting corrosion resistance was also observed. It was further demonstrated that the CPP method is highly effective in increasing the general and pitting corrosion resistance of 316LVM, and that its efficiency does not depend on the surface roughness.     

Selective laser melting (SLM) of CX stainless steel: Theoretical calculation,process optimization and strengthening mechanism

In the present work,selective laser melting (SLM) technology was utilized for manufacturing CX stainless steel samples under a series of laser parameters.The effect of laser linear energy density on the microstructure characteristics,phase distribution,crystallographic orientation and mechanical properties of these CX stainless steel samples were investigated theoretically and experimentally via scanning electron microscope (SEM),X-ray diffraction (XRD),electron backscatter diffraction (EBSD) and transmission electron microscope (TEM).Based on the systematic study,the SLM CX stainless steel sample with best surface roughness (Ra =4.05 ± 1.8 μm) and relative density (Rd =99.72 ％±0.22 ％) under the optimal linear density (η=245 J/m) can be obtained.SLM CX stainless steel was primarily constituted by a large number of fine martensite (α'phase) structures (i.e.,cell structures,cellular dendrites and blocky grains) and a small quantity of austenite (γ phase) structures.The preferred crystallographic orientation (i.e.,<111 > direction) can be determined in the XZ plane of the SLM CX sample.Furthermore,under the optimal linear energy density,the good combinations with the highest ultimate tensile strength (UTS =1068.0 ％±5.9 ％) and the best total elongation (TE =15.70 ％±0.26 ％) of the SLM CX sample can be attained.Dislocation strengthening dominates the strengthening mechanism of the SLM CX sample in as-built state.     

Low cycle fatigue and creep–fatigue interaction behavior of 316L(N) stainless steel and life prediction by artificial neural network approach

Low cycle fatigue (LCF) behavior of solutionized 316L(N) stainless steel (SS) has been studied at various temperatures, strain amplitudes, strain rates, hold times and in 20% prior cold worked condition. The alloy in general showed a reduction in fatigue life with, increase in temperature, increase in strain amplitude, decrease in strain rate, an increase in duration of hold time in tension and with prior cold work. The LCF and creep–fatigue interaction (CFI) behavior of the alloy was explained on the basis of several operative mechanisms such as dynamic strain ageing, creep, oxidation and substructural recovery. The capability of artificial neural network (ANN) approach to life prediction under LCF and CFI conditions has been assessed by using the data generated in the present investigation. It is demonstrated that the prediction is within a factor of 2.     

多弧离子镀(Ti,Al,Zr,Cr)N多组元氮化物膜的研究

采用多弧离子镀技术,使用Ti Al Zr合金靶和Cr靶,在W18Cr4V高速钢基体上沉积(Ti,Al,Zr,Cr)N多组元氮化物膜.利用扫描电镜(SEM)、电子能谱仪(EDS)和X射线衍射(XRD)对薄膜的成分、结构和微观组织进行测量和表征;利用划痕仪、显微硬度计测评薄膜的力学性能.结果表明,获得的多组元氮化物膜仍具有B1 NaCl型的TiN面心立方结构;薄膜的成分除-50V偏压外,其它偏压下的变化均不明显;增大偏压可减少薄膜表面的液滴污染,提高薄膜的显微硬度及膜/基结合力,最高值可分别达到HV3300和190N.     

An investigation of crack closure and the propagation of semi-elliptical fatigue cracks in Q1N (HY80) pressure vessel steel

The results of an experimental investigation of the effect of crack closure on the propagation of semi-elliptical fatigue cracks are presented. Load-shedding fatigue threshold tests were carried out at stress ratios of 0.2, 0.35, 0.5 and 0.7. Crack closure was measured at the surface and depth positions using backface strain gauges, near-tip gauges, and a clip gauge. Differences between the surface and depth growth behaviour are explained by considerations of the effects of the transition from plane stress conditions at the surface to plane strain conditions at the depth. The effects of stress ratios are attributed largely to differences in the crack opening displacement, which result in asperities coming into contact to induce roughness-induced crack closure.     

反应溅射（Al，Ti）（0，N）涂层的微结构与力学性能

采用Al—Ti镶嵌复合靶在Ar、N2和O2混合气体中反应溅射制备了一系列（Al，Ti）（O，N）涂层。并采用EDS、XRD、TEM和微力学探针研究了薄膜的化学成分、微结构和力学性能。结果表明，随氧分压的提高，涂层中氧含量逐步增加，氮含量相应减少，（Al＋Ti）：（O＋N）的化学计量比仍约为1：1，涂层保持与（Al，Ti）N涂层相同的NaCl结构。低氧含量时薄膜在（111）方向上择优生长，随着氧含量的提高，涂层生长的择优取向发生改变，高氧含量薄膜样品呈现强烈（200）织构的柱状晶。与此同时，（Al，Ti）（O，N）涂层的硬度和弹性模量也仍保持在与（Al，Ti）N涂层相当的35GPa和370～420GPa的高值。由于涂层中形成了相当含量的氧化物，这类涂层的抗氧化能力有望得到提高。     

Deposition process,microstructure and mechanical behaviours of RF magnetron sputtered (Ti,Al)N thin films

Abstract

Thin films of (Ti,Al)N with different Al contents were co-deposited using one Ti and one Al targets by radio frequency (RF) pulsed magnetron sputtering. Their composition, microstructure, nanohardness, surface morphology and deposition process were investigated by energy dispersive spectrometer system, X-ray diffraction, nanoindentation, atomic force microscopy and optical emission spectrum. A face cubic centred (fcc) TiN (B1) structure was found in the thin films when Al target power was low. When Al target power was increased, an additional hexagonal AlN (B4) structure appeared. With increasing Al content, the resulting films gradually changed from B1 structure to that of B4, accompanying with decrease of the lattice constant of B1 structure. Simultaneously, the preferred orientation of B4 structural thin films gradually transformed from (111) to (200). The mode of thin films transformed from island to fibre, subsequently to column with increasing Al target power. Optical emission spectrum analysis indicated that Al target surface reached non-metal sputtering mode earlier than that of Ti target under the same deposition parameters, which resulted in a lower sputtering rate of Al target than Ti target and loss of Al content in (Ti,Al)N thin films.     

SD成型剂对Ti(C,N)基金属陶瓷显微结构和力学性能的影响

采用低压烧结的方法制备Ti(C,N)基金属陶瓷材料,并结合C、N、O分析,XRD、BSE、EDS等测试手段研究了SD成型剂对Ti(C,N)基金属陶瓷合金的C含量、相组成及显微结构和力学性能的影响。结果表明,随着SD成型剂添加量的增加,脱胶后压坯的C含量逐渐增加,N含量逐渐减小;烧结后Ti(C,N)基金属陶瓷由(Ti,Me)(C,N)(Me=W、Mo、Ta)和Ni/Co固溶体相组成;显微组织以黑芯-白环结构为主,并伴随着少量白芯-灰环的结构。SD添加量为100mL/kg时,Ti(C,N)基金属陶瓷材料的抗弯强度达1929MPa,硬度为1588HV30,添加量为180mL/kg时,合金组织中石墨相的出现使其抗弯强度大幅度下降。