Stress Corrosion Resistance Research Articles

Friction stir welding: An alternative to fusion welding for better stress corrosion cracking resistance of maraging steel

Friction stir welding (FSW), a solid state welding process, is generally used for welding soft materials like aluminium and magnesium alloys. In this work, maraging steel, an ultra high strength steel, is welded using FSW process. Major problems associated with fusion welded maraging steel, i.e segregation of alloying elements and formation of reverted austenite pool, were suppressed by FSW. Slow strain rate stress corrosion characteristic of 18%Ni maraging steel (grade-250) and its weldments, by gas tungsten arc welding (GTAW) and FSW, has been investigated in air and 3.5% NaCl solution. It was observed that FSW joints demonstrated higher resistance to stress corrosion cracking (SCC) compared to both, base metal and gas tungsten arc weldments. Improved stress corrosion resistance of friction stir weldments was correlated to microstructural differences and residual stress. Fine grain structure, absence of segregation of alloying elements and high compressive residual stress in friction stir weld resulted in better SCC resistance compared to that of fusion weldment. It is brought out from the present study that FSW process can be used as an alternative to fusion welding process for better SCC resistance.

Effect of Heat Treatments on Stress Corrosion Behavior of 7050 Al Alloys in 3.5% NaCl Solution

Effect of heat treatments on the stress corrosion behavior of 7050 Al alloys in 3.5% NaCl solution has been investigated using slow strain rate tensile (SSRT) test. During the slow strain rate tensile process, electrochemical impedance spectroscopy (EIS) in real time was carried out to characterize the electrochemical behavior for different tempers 7050 Al alloys. The investigation shows that both the stress corrosion resistance of 7050 Al alloys is controlled by heat treatments due to the different precipitates state. The improvement of stress corrosion resistance is contributed to the tiny precipitates in matrix which are beneficial to corrosion potential and maintain passivation, and precipitates discontinuous distribution at grain boundary which obstruct intergranular crack connection. Moreover, base on the results, we find out retrogression and re-aging (RRA, i.e., T6 + 200 °C/ retrogression + water quench + T6) increases both tensile strength and stress corrosion resistance. The optimized of retrogression time is 30 minutes.

Microstructure and Properties of 7075Al Alloy Fabricated by Directly Combined of Spray Forming and Continuous Extrusion Forming under Different Atomization Gas Pressures

A new process consisting of the spray forming and the continuous extrusion forming for manufacturing 7075Al alloy was proposed. The microstructure evolution, mechanical properties and the resistance to stress corrosion cracking of the alloy were studied. The results indicate that the spray forming process induces obviously grain refinement and greatly lower segregation microstructure. Besides, the Conform process produces finer grains and conduces to more uniform distribution of the precipitates of AlCu and MgZn2 phases. The fabricated alloy shows good comprehensive mechanical properties and superb performance of stress corrosion resistance. Moreover, a better combination of the mechanical properties and the resistance to stress corrosion cracking could then be obtained under a certain condition of atomization gas pressure of 0.19 MPa. The enhanced properties are attributed to the following factors, which include the grain refinement, the fine and homogeneous distribution of AlCu and MgZn2 phases, the high density of the extruded products, as well as the discontinuous distribution of the grain boundaries after retrogression and reaging (RRA) heat treatment.

An Approach to Maximize Weld Penetration During TIG Welding of P91 Steel Plates by Utilizing Image Processing and Taguchi Orthogonal Array

P-91 is modified 9Cr–1Mo steel. Fabricated structures and components of P-91 has a lot of application in power and chemical industry owing to its excellent properties like high temperature stress corrosion resistance, less susceptibility to thermal fatigue at high operating temperatures. The weld quality and surface finish of fabricated structure of P91 is very good when welded by Tungsten Inert Gas welding (TIG). However, the process has its limitation regarding weld penetration. The success of a welding process lies in fabricating with such a combination of parameters that gives maximum weld penetration and minimum weld width. To carry out an investigation on the effect of the autogenous TIG welding parameters on weld penetration and weld width, bead-on-plate welds were carried on P91 plates of thickness 6 mm in accordance to a Taguchi L9 design. Welding current, welding speed and gas flow rate were the three control variables in the investigation. After autogenous (TIG) welding, the dimension of the weld width, weld penetration and weld area were successfully measured by an image analysis technique developed for the study. The maximum error for the measured dimensions of the weld width, penetration and area with the developed image analysis technique was only 2 % compared to the measurements of Leica-Q-Win-V3 software installed in optical microscope. The measurements with the developed software, unlike the measurements under a microscope, required least human intervention. An Analysis of Variance (ANOVA) confirms the significance of the selected parameters. Thereafter, Taguchi’s method was successfully used to trade-off between maximum penetration and minimum weld width while keeping the weld area at a minimum.

Identification of constitutive model parameters for nickel aluminum bronze in machining

The material of nickel aluminum bronze (NAB) presents superior properties such as high strength, excellent wear resistance and stress corrosion resistance and is extensively used for marine propellers. In order to establish the constitutive relation of NAB under high strain rate condition, a new methodology was proposed to accurately identify the constitutive parameters of Johnson–Cook model in machining, combining SHPB tests, predictive cutting force model and orthogonal cutting experiment. Firstly, SHPB tests were carried out to obtain the true stress–strain curves at various temperatures and strain rates. Then, an objective function of the predictive and experimental flow stresses was set up, which put the identified parameters of SHPB tests as the initial value, and utilized the PSO algorithm to identify the constitutive parameters of NAB in machining. Finally, the identified parameters were verified to be sufficiently accurate by comparing the values of cutting forces calculated from the predictive model and FEM simulation.

Effect of precipitate state on the stress corrosion behavior of 7050 aluminum alloy

The effect of precipitate size and state on stress corrosion property of 7050 aluminum alloy has been investigated for the first time. The results suggest that 7050 aluminum alloy with fine precipitates in matrix and discontinuous copper-rich precipitates at grain boundary exhibits excellent stress corrosion resistance. The different precipitates in matrix lead to electrochemical heterogeneity and different corrosion susceptibility, due to the different coherence with matrix and alloy element matrix solute content. The discontinuous grain boundary precipitates can hinder corrosion cavities connecting into a continuous crack, and copper-rich grain boundary precipitates can retard hydrogen embrittlement, during the stress corrosion process. Furthermore, the in-situ electrochemical impedance spectroscopy (EIS) in real time suggests that precipitate state has obvious influence on passivation during the stress corrosion process. Fine nanoscale precipitates in matrix are beneficial to maintaining continuous passivation, and the copper-rich grain boundary precipitate is beneficial to repassivation.

Evaluation of the residual stresses and corrosion resistance of shot-peened 7075 aluminium alloy under different states

This paper investigates the effect of pitting corrosion and residual stresses on different shot-peened ENAW 7075 aluminium alloys under different states. The objective of the research was to establish the parameters of the shot peening treatment of the aluminium alloy in different precipitation-hardened states concerning residual stress profiles and corrosion resistance. The experimental characterisation is primarily based on the evaluation of the residual stress profiles caused by shot-peening treatments and on the measurement of corrosion potentials as well as of cyclic polarisation curves in an NaCl solution for different material conditions. The obtained results show a favourable influence of shot peening treatment on induced compressive residual that retarded the initiation of fatigue cracks. The analysis also indicated that particles such as Al7Cu2Fe and MgZn2 play an important role in pitting corrosion and that the numbers and sizes of surface pits were reduced after shot peening.

Mg–6%Al–1%Zn–2%Ca合金押出材の応力腐食割れと耐食性

Mg–6%Al–1%Zn–2%Ca合金押出材の応力腐食割れと耐食性

Regulation Mechanism of Novel Thermomechanical Treatment on Microstructure and Properties in Al-Zn-Mg-Cu Alloy

Scanning electron microscopy, transmission electron microscopy, tensile test, exfoliation corrosion test, and slow strain rate tensile test were applied to investigate the properties and microstructure of Al-Zn-Mg-Cu alloy processed by final thermomechanical treatment, retrogression reaging, and novel thermomechanical treatment (a combination of retrogression reaging with cold or warm rolling). The results indicate that in comparison with conventional heat treatment, the novel thermomechanical treatment reduces the stress corrosion susceptibility. A good combination of mechanical properties, stress corrosion resistance, and exfoliation corrosion resistance can be obtained by combining retrogression reaging with warm rolling. The mechanism of the novel thermomechanical treatment is the synergistic effect of composite microstructure such as grain morphology, dislocation substructures, as well as the morphology and distribution of primary phases and precipitations.

Design of novel high-performance Al–Cu–Mg alloys containing Zn and Li

To design novel high-performance aluminium alloys, the properties and microstructure of Al–4.2Cu–1.4Mg alloy containing Zn and Li have been investigated by tensile tests, fatigue crack propagation test, slow strain-rate tensile test, Kahn tear test, scanning electron micrography and transmission electron micrography. The stress corrosion cracking resistance and toughness of Al–4.2Cu–1.2Mg alloy can be markedly improved by small Zn addition. Independent Li addition has no significant effect on the corrosion resistance of Al–4.2Cu–1.2Mg alloy, but the tensile strength is improved and the fatigue crack propagation is restrained. Small Zn addition promotes the precipitation of S′ phase during age treatment, and the grain boundary precipitates are scarcer than those of the base alloy. The co-effect of Li and Zn addition promotes the fine and dispersed precipitation of the S′ (non-equilibrium Al2CuMg) phases in Al–4.2Cu–1.2Mg alloy. The comprehensive performance (the fracture toughness, tensile properties, stress corrosion resistance and fatigue crack propagation resistance) of Al–4.2Cu–1.2Mg alloy with 0.25% Zn and 1.0%Li is outstanding. This alloy could lay the foundation for the design of new aluminium alloys.

Rheological, Mechanical, Environmental Stress Corrosion and Gamma Resistance Properties of Semi-Crystalline PLA: Effect of Catalysts on the Reactivity of a Chain Extender

Rheological, Mechanical, Environmental Stress Corrosion and Gamma Resistance Properties of Semi-Crystalline PLA: Effect of Catalysts on the Reactivity of a Chain Extender

Study on crack density of concrete exposed to stress corrosion

A new quantitative method is introduced to depict the micro-cracks in the concrete—crack density, which is closely related to the number and the size of the cracks in concrete. According to the experimental data and relative corrosion depth, the data analysis and numerical simulation are conducted by the scientific plotting and data processing software SigmaPlot 12.0 to the damage process, which can produce the three-dimensional distribution map of propagation of inner crack density of concrete. From the map we can see the formula and character on the propagation of micro-crack density of concrete in corrosive medium. In this paper, three types of concrete under the action of external stress and attack of sulfate was investigated. By analyzing the crack density, we can conclude: GHDC2 has the best stress corrosion resistance in four kinds of concrete; chloride can accelerate the chemical corrosion of magnesium and sulfate brine to concrete; tensile stress and compressive stress can obviously improve the appearance and propagation of micro-cracks.

Improved stress corrosion cracking resistance of a novel biodegradable EW62 magnesium alloy by rapid solidification, in simulated electrolytes.

The high corrosion rate of magnesium (Mg) and Mg-alloys precludes their widespread acceptance as implantable biomaterials. Here, we investigated the potential for rapid solidification (RS) to increase the stress corrosion cracking (SCC) resistance of a novel Mg alloy, Mg-6%Nd-2%Y-0.5%Zr (EW62), in comparison to its conventionally cast (CC) counterpart. RS ribbons were extrusion consolidated in order to generate bioimplant-relevant geometries for testing and practical use. Microstructural characteristics were examined by SEM. Corrosion rates were calculated based upon hydrogen evolution during immersion testing. The surface layer of the tested alloys was analyzed by X-ray photoelectron spectroscopy (XPS). Stress corrosion resistance was assessed by slow strain rate testing and fractography. The results indicate that the corrosion resistance of the RS alloy is significantly improved relative to the CC alloy due to a supersaturated Nd enrichment that increases the Nd2O3 content in the external oxide layer, as well as a more homogeneous structure and reduced grain size. These improvements contributed to the reduced formation of hydrogen gas and hydrogen embrittlement, which reduced the SCC sensitivity relative to the CC alloy. Therefore, EW62 in the form of a rapidly solidified extruded structure may serve as a biodegradable implant for biomedical applications.

Increased corrosion resistance of the AZ80 magnesium alloy by rapid solidification.

Magnesium (Mg) and Mg-alloys are being considered as implantable biometals. Despite their excellent biocompatibility and good mechanical properties, their rapid corrosion is a major impediment precluding their widespread acceptance as implantable biomaterials. Here, we investigate the potential for rapid solidification to increase the corrosion resistance of Mg alloys. To this end, the effect of rapid solidification on the environmental and stress corrosion behavior of the AZ80 Mg alloy vs. its conventionally cast counterpart was evaluated in simulated physiological electrolytes. The microstructural characteristics were examined by optical microscopy, SEM, TEM, and X-ray diffraction analysis. The corrosion behavior was evaluated by immersion, salt spraying, and potentiodynamic polarization. Stress corrosion resistance was assessed by Slow Strain Rate Testing. The results indicate that the corrosion resistance of rapidly solidified ribbons is significantly improved relative to the conventional cast alloy due to the increased Al content dissolved in the α-Mg matrix and the correspondingly reduced presence of the β-phase (Mg17 Al12 ). Unfortunately, extrusion consolidated solidified ribbons exhibited a substantial reduction in the environmental performance and stress corrosion resistance. This was mainly attributed to the detrimental effect of the extrusion process, which enriched the iron impurities and increased the internal stresses by imposing a higher dislocation density. In terms of immersion tests, the average corrosion rate of the rapidly solidified ribbons was <0.4 mm/year compared with ∼2 mm/year for the conventionally cast alloy and 26 mm/year for the rapidly solidified extruded ribbons.

Laser Peening Process and Its Impact on Materials Properties in Comparison with Shot Peening and Ultrasonic Impact Peening.

The laser shock peening (LSP) process using a Q-switched pulsed laser beam for surface modification has been reviewed. The development of the LSP technique and its numerous advantages over the conventional shot peening (SP) such as better surface finish, higher depths of residual stress and uniform distribution of intensity were discussed. Similar comparison with ultrasonic impact peening (UIP)/ultrasonic shot peening (USP) was incorporated, when possible. The generation of shock waves, processing parameters, and characterization of LSP treated specimens were described. Special attention was given to the influence of LSP process parameters on residual stress profiles, material properties and structures. Based on the studies so far, more fundamental understanding is still needed when selecting optimized LSP processing parameters and substrate conditions. A summary of the parametric studies of LSP on different materials has been presented. Furthermore, enhancements in the surface micro and nanohardness, elastic modulus, tensile yield strength and refinement of microstructure which translates to increased fatigue life, fretting fatigue life, stress corrosion cracking (SCC) and corrosion resistance were addressed. However, research gaps related to the inconsistencies in the literature were identified. Current status, developments and challenges of the LSP technique were discussed.

Influence of a retrogression and reaging (RRA)-treatment on the mechanical and microstructural characteristics of the aluminium alloy AlZn4,5Mg1

The investigated material is the copper-free Al–Zn–Mg-alloy EN AW-7020 (AlZn4,5Mg1) in the artificial aged T6 temper. In this temper the 7xxx series alloys show a high strength level, but also a high stress corrosion sensitivity. Therefore, the retrogression and reaging (RRA)-treatment has been developed to combine stress corrosion resistance with high strength. The aim of this study is the description of the course of the mechanical behavior influenced by a RRA treatment. The changes of the mechanical properties are represented by changes in the microstructure as recorded by thermal analysis. Therefore typical mechanical properties and thermal analysis curves are determined according to temperature and time in each of the retrogression annealing and of the RRA-treatment. The temperature for retrogression annealing was between 220 and 360 °C. Reaging was performed on material at 130 °C for 16 h and 160 °C for 5 h after a storage of 7 days at room temperature. The results of the thermal analysis are generally described on the curve of the T6 tempered material and transferred to the changes by the retrogression temperature and the reaging parameter. The results show a main influence of the precipitation structure and mechanical properties after the retrogression and reaging treatment by the temperature of the retrogression annealing. For all investigated parameters, an overaging of the material after the RRA-treatment is recognizable.

Effect of Post Weld Heat Treatments on Mechanical and Stress Corrosion Cracking Behaviour of Aa7075 Friction Stir Welds

The high strength aluminium alloys exhibits low weldability due to poor solidification microstructure, porosity in the fusion zone and lose their mechanical properties when they are welded by fusion welding techniques. Friction stir welding (FSW) is a promising technique to retain the properties of the alloy as the joining take place in the solid state. Even though mechanical properties of the welds are better, corrosion resistance of welds gets affected by the microstructural changes that occur during welding. In the present work the effect of post weld treatments viz. peak ageing (T6), overaging (T73), retrogression and retrogression & reaging (RRA) on the mechanical properties and stress corrosion resistance is studied. It was observed that Retrogression and RRA treatments improved the resistance to stress corrosion cracking and maintained the mechanical properties. The resistance to stress corrosion is improved in RRA and Retrogression condition with the minimum loss of weld strength. This is attributed to the dissolution, re-precipitation and re-distribution of precipitates.

Effects of laser heat treatment on the fracture morphologies of X80 pipeline steel welded joints by stress corrosion

The surfaces of X80 pipeline steel welded joints were processed with a CO2 laser, and the effects of laser heat treatment (LHT) on H2S stress corrosion in the National Association of Corrosion Engineers (NACE) solution were analyzed by a slow strain rate test. The fracture morphologies and chemical components of corrosive products before and after LHT were analyzed by scanning electron microscopy and energy-dispersive spectroscopy, respectively, and the mechanism of LHT on stress corrosion cracking was discussed. Results showed that the fracture for welded joints was brittle in its original state, while it was transformed to a ductile fracture after LHT. The tendencies of hydrogen-induced corrosion were reduced, and the stress corrosion sensitivity index decreased from 35.2% to 25.3%, indicating that the stress corrosion resistance of X80 pipeline steel welded joints has been improved by LHT.

Corrosion Behavior of Nano/Sub‐Micron F401 Titanium Alloy

Titanium alloys are attractive as structural materials mainly due to their high specific strength and excellent corrosion resistance. Modifying conventional Ti alloys to impart nanostructural features holds promise for improving specific strength without reducing corrosion resistance. This study aimed to evaluate the corrosion behavior of a nano/sub‐micron F401 titanium alloy compared to its conventional Ti–6Al–4V counterpart. Following mechanical alloying and extrusion, the microstructure was evaluated using optical microscopy, SEM, HRSEM, and X‐ray diffraction analysis. Mechanical properties were evaluated by tension and micro‐hardness tests. The corrosion behavior was examined by an immersion test, salt spray testing, and potentiodynamic polarization analysis. Stress corrosion resistance was evaluated by slow strain rate testing (SSRT) in terms of UTS and time to failure versus strain rate. All the corrosion tests were carried out in 3.5% NaCl solution apart from the salt spray testing, which was done in 5% NaCl solution. The results indicate that the corrosion resistance of the novel F401 alloy was not different from its Ti–6Al–4V counterpart, although the mechanical strength was improved. Thus, the grain refinement in the F401 alloy increased the mechanical strength, whereas neither the grain refinement nor the porosity acquired during the preparation process impaired the excellent corrosion resistance.

Effect of Scandium Addition on Mechanical Properties and Corrosion Resistance of Medium Strength Al-Zn-Mg-Cu) Alloy

Al-Zn-Mg alloys are widely used construct material for wide range of application including automotive industry, building, and aviation. Scandium is rare earth metal, which modifies structure of semi-products and can lead to increasing of tensile and yield strength because of Al3Sc intermetallics of nanoscale size. This paper describes results of influence of small addition of scandium (0,08 wt. %) on mechanical and corrosion properties of cold-rolled sheets of 1913 alloy. 1913 is Russian medium strength corrosion resistant weldable Al-Zn-Mg alloy with addition of Cu for improvement of stress corrosion resistance and Mn with Zr for structure improvement. It is shown that addition of Sc transforms kinetics of artificial aging and accelerates decomposition of solid solution, besides tensile strength increases up to 50 MPa in artificially aged tempers. Changes in microstructure and quench sensitivity are described; results of intergranural corrosion, exfoliation and stress corrosion tests are introduced after different heat treatment including peak strength aging and overaging tempers.