Degree Of Cold Work Research Articles

Microstructure and fracture toughness of 850°C quenched and aged Zr-2.5Nb alloy

The Zr-2.5Nb alloy pressure tubes manufactured by Water Quenched and Aged (WQA) fabrication route has reportedly exhibited superior resistance against in-reactor dimensional changes. WQA route comprises three consecutive thermo-mechanical treatments viz., solution heat treatment (SHT), cold working and vacuum aging, which govern the final microstructure and therefore fracture toughness of the pressure tube material. This work describes the influence of SHT soaking duration, degree of cold working and aging temperature on the microstructure and fracture toughness of WQA Zr-2.5Nb alloy. WQA pressure tubes are expected to have better resistance to in-reactor dimensional changes. However, if proper care of hydrogen absorption during thermo-mechanical processing is not taken, though resistance to creep will improve expectedly but fracture toughness at ambient temperature will deteriorate.

Correlations Between XRD Peak Broadening and Elastic and Plastic Deformation for Mild Steel Sheets Under Tensile Loading

The polycrystalline nature of steel sheets plays a fundamental role in their plastic behavior during forming, especially under complex stresses, such as the deep drawing process. Heterogeneous microstructures cause variable internal stress levels, which strongly impact the material’s macroscopic mechanical properties. X-ray diffraction (XRD) technique can be applied to determine microscopic internal stresses by peaks broadening analysis, which can be related to the degree of cold work at the microscopic level. As deep drawing is a complex deformation mode, an important motivating factor to perform such a study is to find the correlation between anisotropy and XRD peak broadening through tensile deformation tests. Therefore, the initial mechanical state and different deformed states were analysed by in-situ XRD peak broadening under tensile tests in various directions in the rolling plane of mild steel sheet, for both the elastic and plastic deformation domains. The results showed that the XRD peak broadening gives accurate estimations of deformation degrees. In the elastic domain, under realistic assumptions, a relation between the relative variation of peak broadening and the applied stress σ, like Hooke’s law, was obtained by exploiting the linearity between the peak broadening and the elongation measured by the strain gauge. This formula may be used to deduce The Young’s modulus. Interesting empirical relations between peak broadenings, macro deformations and hardness have been also established for the studied sheet in the plastic deformation region. Moreover, XRD peak broadenings were able to characterize the anisotropy of deformation in the rolling plane of the sheet. They are like the anisotropy coefficients in indicating the expected behavior under complex processes such as deep drawing by using ordinary tensile tests.

Root cause analysis for cracked pipe in auxiliary system in nuclear power plant

Penetrated cracks were found in succession in 316LN stainless steel elbows in auxiliary system in a Chinese nuclear power plant after about 19 years’ service. In order to clarify the root cause of the cracking, the cracked elbow with nominal diameter of 12 in. was cut and its chemical composition, hardness, microstructure and residual strain were investigated. The results show that this elbow was heavily cold worked during push-bending and the degree of cold work was within 25∼40%. The manufacturer did not obey RCC-M M3317 to solution treat this elbow before delivery. The heavily cold-worked elbow suffered stress corrosion cracking in occluded primary water.

Evaluation of Hydrogen Gettering Rates Correlated to Surface Composition and Texture of Nickel-Plated Zircaloy Getters of Different Heat Treatment Procedures.

Coatings of metal specimens are known to have an impact on hydrogen gettering (hydrogen absorption). The coating can have one or more functions, such as enhancing gettering, preventing gettering and/or preventing oxidation of the metal substrate. It is known that contaminants and surface texture can impact hydrogen gettering/absorption performance, but has not previously been thoroughly explored. This study evaluated the role of different post-plating heat treatments of nickel-plated zircaloy-4 getters (NPGs) and the role of the heat treatments on gettering rates, surface composition and texture. Nickel plating is applied to prevent oxidation of the Zircaloy-4 surface and also enhances gettering. The nickel plating must be heat treated before desirable gettering can occur. Our NPG getters with historically known satisfying performance were pre-heat treated in air followed by activation heat treatment in a vacuum at a higher temperature. In this study, we were interested in finding out if both heat treatment steps were necessary to obtain a desirable gettering performance, or if one step could be omitted. XPS analysis showed that if the nickel surface is not heat treated before bonding the nickel to the zirconium in the activation step, there will be carbon contaminants on the surface, which significantly reduces gettering. We studied the texture of Zircaloy-4 using SEM/EBSD to compare NPGs with both heat treatment steps with NPGs that had no post-plating heat treatment to learn if the degree of cold work could be impacted by the heat treatment steps. We did not observe any differences in texture between them. We measured gettering rates of both pretreated and activated NPGs and NPGs that had been activated without first being pre-heat treated. We found that the NPGs without the first post-plating heating step had up to a seven times slower gettering rate and obtained higher plateau pressures due to the contaminated surface. Thus, the pre-heat treatment in air before activation is necessary to avoid slower gettering rates and higher plateau pressures.

Investigation of Thermally Induced Deterioration Processes in Cold Worked SAF 2507 Type Duplex Stainless Steel by DTA

Thermally induced deterioration processes were studied in cold worked (up to 60% deformation) SAF 2507 type super-duplex stainless steel (SDSS) by differential thermal analysis (DTA). DTA results revealed two transformations. Parent and inherited phases of these transformations were examined by other methods too, such as micro-hardness tests, optical metallography and X-ray diffraction (XRD). Finally, these transformations were identified as the formation of α’- and σ-phases. Formation of strain-induced martensite (SIM) and recrystallization were not experienced until 1000 °C, despite high degree of cold working. Activation energies of the σ-phase precipitation and α’-phase formation were determined from the Kissinger plot, through DTA measurements—they are 275 and 220 kJ/mol, respectively—in good agreement with the values found in the literature.

Effect of cold working on crack growth rate of environmentally assisted cracking of 316L SS

Environmentally assisted cracking (EAC) is a main failure mode of key structural materials of nuclear power pressure vessels and pipelines in service. The key structural materials of nuclear power will undergo plastic deformation due to cold working during manufacturing, assembly and welding, and cold working will affect the EAC crack growth rate. In order to understand effect of cold working on crack growth rate of EAC, a combination of theory, experiment and finite element simulation is used. There are experiments to obtain the mechanical properties of 316L stainless steel at different degrees of cold working, and the creep characteristics of 316L stainless steel at low temperature and high stress were obtained by experiment. Based on experiment and theoretical analysis, creep rate of crack tip is used to replace strain rate of crack tip in Ford-Andresen model, and effect of cold working on crack growth rate of EAC is analyzed with hardening at crack tip and without hardening at crack tip. The results show that cold working accelerates crack growth rate of EAC, and the hardening of crack tip micro zone restrains crack growth rate of EAC, causing the “hysteresis” phenomenon in the EAC crack growth process. It is more reasonable to use creep rate of crack tip instead of strain rate of crack tip to calculate crack growth rate of EAC.

Effects of Cold-Work Degree on Stress Corrosion Cracking Behavior of Alloy 600 in Simulated Boiling Water Reactor (BWR) Water Environments

The growth behavior of stress corrosion cracking (SCC) of Alloy 600 with different cold-work levels was investigated in simulated boiling water reactors water environments. In addition, a correlation of cold-work levels, grain boundary characteristic, and the SCC growth behavior of Alloy 600 were studied. The results show that grains with high residual strain caused by cold work provide transgranular crack growth paths. The SCC growth rates of the specimens also increase with an increase in the degree of cold work and decrease remarkably after switching to the hydrogen water chemistry environment. Grain boundary character proves to be a factor more important than the localized strain concentration at the grain boundary in terms of its role in the intergranular crack growth rate of the Alloy 600 with a cold-work degree from 20% to 30%.

Gas Gun Flyer Plate Impact Testing of Stainless Steel Deposited by Low Pressure Plasma Spraying and Cold Gas Dynamic Spraying

Understanding shock wave propagation behavior in coatings is valuable for predicting their performance in high-velocity impacts. Samples of stainless steel were deposited by low pressure plasma spray and cold gas dynamic spraying. The sound speeds of the deposits were measured, and the deposits were tested in flyer plate impact tests in a gas gun to determine the shock propagation and porosity compaction properties. The most porous sample was tested to determine its equation of state, while the other samples were tested to measure shock wave profiles as a function of deposition parameters. Comparison of the results between the coatings and to reference wrought stainless steel shows the effect of deposition conditions on the dynamic behavior of the deposits. Higher deposit density leads to higher sound speed. The shock speed for the highest porosity plasma sprayed deposit was below that of wrought stainless steel. The Hugoniot elastic limit stress for cold sprayed deposits is higher than that for plasma sprayed deposits due to the higher degree of cold work. Porosity crush up times were found to vary with the total porosity of the deposit. Better understanding of the behavior of these deposits under extreme impact conditions is demonstrated.

On the Influence of Cold Working and Electrochemical Nitridation on the Corrosion Behaviour of 316L Austenitic Stainless Steel in Acidic Environment

The present paper investigates the effect of cold working and electrochemical nitridation on corrosion resistance of 316L austenitic stainless steel (ASS). The X-ray diffraction results showed that the strain-induced martensite is formed on cold work. Also, the volumetric fraction of strain induced martensite and dislocation density increased with increase in degree of cold work. The cold working process adversely affects the corrosion resistance of 316L ASS due to the formation of the strain-induced martensite and increased dislocation density. On electrochemical nitridation, the electrochemical results show the formation of stable passive film, thus improving the corrosion resistance of 316L ASS attributed to the formation of nitrides on the surface. Moreover, the beneficial effect of electrochemical nitridation is more pronounced in cold worked samples, thus the adverse effects of cold working can be overcome by electrochemical nitridation.

Synergistic effect of cold work and hydrogen charging on the pitting susceptibility of 2205 duplex stainless steel

Solution-treated 2205 duplex stainless steel (DSS) was cold-worked to different degrees and then charged with hydrogen. The pitting susceptibility and passivation of cold-worked 2205 DSS samples in neutral chloride solutions were investigated by chemical immersion, potentiodynamic polarization, potentiostatic polarization, and electrochemical impedance spectroscopy measurements. The dislocation densities in austenite and ferrite inside 2205 DSS samples both increased as the degree of cold work increased from 0% to 80%. After hydrogen charging, the hydrogen content in 2205 DSS samples increased from 13.52 ppm to 41.49 ppm as the degree of cold work increased from 0% to 80%, accompanied by an increased tendency to form hydrogen blisters. Deformation alone had little effect on the corrosion performance of 2205 DSS samples, but the deterioration effect of hydrogen charging on pitting resistance has been promoted by cold work, which could be ascribed to the increased hydrogen content as the degree of cold work increased. The change in pitting potential before and after hydrogen charging was linearly dependent on the hydrogen content in 2205 DSS samples. Pits preferably nucleated within austenite or at the austenite/ferrite boundaries for the hydrogen-charged 2205 samples regardless of the degree of cold work.

Hardening effect and fatigue behavior enhancement through ball burnishing on AISI 1038

The objective of this paper is to present a comprehensive study of the effects of ball burnishing on the low cycle fatigue endurance of cylindrical specimens subjected to alternative bending stress, by constructing fatigue S-N curves to evaluate the impact of the process on the general fatigue behavior of AISI 1038. The specimens were burnished through five different force-number of passes couples, displacing the as-machined-material S-N curve towards higher lifespans as the degree of cold work is increased, either by effect of a higher force and more burnishing passes. The derived surface hardening effect also proved to be correlated analytically with the fatigue lifespan increase at all tested maximum-stress levels, as prove the correlation of the parameters that represent the potential equation associated to the Wöhler model with the hardening effect. It is concluded that the process performed with 120 N and 7 passes results in the most favorable results, represented by a 41% increase of hardness and a 77% increase in the lifespan in the best of cases.

Effect of cold work and slow strain rate on 321SS stress corrosion cracking in abnormal conditions of simulated PWR primary environment

Abstract In this paper, stress corrosion cracking (SCC) behavior of 0%, 5%, 10% cold-worked (CW) 321 stainless steel (321 SS) in abnormal conditions of simulated pressurized water reactor (PWR) primary water was studied at two different slow strain rates of 4.2 × 10−6, 4.2 × 10−7. The experimental results show that with the increase of the degree of cold work, the internal grain of the sample is refined. The slip zone and deformation twins occur inside some grains. The SCC sensitivity of the strain rate of 4.2 × 10−7 S−1 is higher than the strain rate of 4.2 × 10−6 S−1. And the higher the degree of cold work, the higher the SCC sensitivity of the sample. Dynamic strain aging (DSA) occurs at 321 SS with different cold work degrees with a slow strain rate of 4.2 × 10−7 S−1. DSA makes the tensile zone near the fracture of 5% CW and 10% CW samples have higher vacancy defect density, which promotes the initiation and expansion of SCC crack.

Effect of The Degree of Cold Work and Sensitization Time on Intergranular Corrosion Behavior in Austenitic Stainless Steel

Abstract Present paper deals with the influence of a wide range of cold rolling (5, 10, 15 and maximum 40% cold deformation) and the sensitization time (aging at 700°C for 0.12, 0.5, 1, 4, 16 and 32 hours) on intergranular corrosion (IGC). Intergranular corrosion of commercial stainless steel type X6CrNiTi18-10 (1.4541, AISI 321) is frequently observed in several process environments. These localized attacks are normally attributed to the carbide precipitation and concomitant depletion of chromium near grain boundary due to steel exposure to sensitization temperature. Such undesirable microchemistry is expected to be changed further if the material undergoes deformation prior to sensitization. The consequences of deformation on IGC have been investigated by using EN ISO 3651-1methods (Huey test – Corrosion test in nitric acid medium by measurement of loss in mass). Introducing deformation to the investigated stainless steel seems to change the kinetics of carbide precipitation M23C6 and thereby changes it resistance to IGC. Cold deformation before sensitization reduces the intensity of intergranular corrosion of this steel. The deformed structure created during the cold work process, numerous slip planes and the twins boundaries are just like the grain boundaries and the places where the chromium carbides preferentially precipitates. Due to the more evenly occurring precipitation processes within the whole deformed grains, there is no phenomenon of local grain boundary carbide precipitation, and thus there is no decrease in the resistance of this steel to intergranular corrosion. The assessment of the degree of intergranular corrosion was based on the measurement of mass loss and observation of corroded surfaces on optical and electron transmission and scanning microscopes.

ПІДВИЩЕННЯ МІЦНОСТІ ТА НАДІЙНОСТІ БАРАБАНІВ КОЛІС ЛІТАКІВ ЗМІЦНЮВАЛЬНОЮ ОБРОБКОЮ

Technology, equipment, and results of stand tests of plane wheel hubs and flanges strengthened by surface plastic deformation are described. The new method suggested by the authors is called vibrational-centrifugal strengthening treatment. It belongs to the method group of dynamic strengthening of revolutional shape parts. It is based on impact interaction of the part processed with a massive tool which is rolled over the strengthened surface of the part when under vibration. Moreover, the impact contact between the part and the tool occurs through a small number of deformable bodies. This provides formation of compressive residual stress in the contact places in the part material. For magnesium wheel hubs (alloy ML-12) residual compressive stress is within 110 MPa, for aluminium ones (alloy AK6) it is within 250 MPa. The degree of strengthening of outer zone material for magnesium wheel hubs is 45...59 % with surface micro-hardness increasing up to 1150 N/m2 and the thickness of the strengthened layer being 0.9...1.0 mm. When strengthening aluminium wheel hubs and flanges, the thickness of the strengthened layer is to be 0.6...0.9 mm with a degree of cold work being 25...30 %. Fatigue studies of a party of KT-141 type wheel hubs strengthened by the method (magnesium alloy MT-12) demonstrated their service life increasing up to 1000 take-offs and landings at the safety coefficient of n = 3.5. The lifetime of this type of wheel hubs strengthened by roller burnishing did not exceed 750 take-offs and landings; for unstrengthened ones, it made 500 take-offs and landings at lower values of the safety coefficient. Strengthening the wheel hub web KT-150K (aluminium alloy AK6) increases their lifetime by 28...30 % on average. Apart from plane wheel hubs and flanges, the method of vibrational-centrifugal strengthening treatment can be applied for increasing the lifespan of various parts of chassis components of circular section, for strengthening nonferrous metal webs of car wheels, radius blends and steel shaft journals etc.

Effect of Cold Working on the Residual Stress around the Notch front of Stress Corrosion Cracking

Failures show that the austenitic stainless steel (SS) pipes with produced high weld residual stresses are more susceptible to stress corrosion cracking in the simulated high temperature water environments of pressurized water reactor. To understand the effect of cold working on the residual stress and strain of stress corrosion cracking in 304SS, the stress-strain field near the crack front of 304SS under different degree of cold working was simulated by ABAQUS. Results show that the higher tensile residual stress and the larger compressive residual stress simultaneously occurs in the lower cold working material, and cold working has greater effect on the residual stress than on the residual strain. The residual compressive stress and residual compressive strain both decrease rapidly in a small range around the notch fronts and tend to be consistent in a more larger distance. But cold working has little effect on the residual strain.

Effect of cold rolling on the microstructural, magnetic, mechanical, and corrosion properties of AISI 316L austenitic stainless steel

This study has evaluated the effect of different levels of cold rolling (from 0 to 50%) on the microstructural, magnetic, and mechanical properties and the corrosion behavior of 316L austenitic stainless steel in NaCl (1 mol/L) + H2SO4 (0.5 mol/L) solution. Microstructural examinations using optical microscopy revealed the development of a morphological texture from coaxial to elongated grains during the cold-rolling process. Phase analysis carried out on the basis of X-ray diffraction confirmed the formation of the ferromagnetic α′-martensite phase under the stresses applied during cold rolling. This finding is in agreement with magnetic measurements using a vibrating sample magnetometer. Mechanical properties determined by tensile and Vickers microhardness tests demonstrated an upward trend in the hardness-to-yield strength ratio with increasing cold-rolling percentage, representing a reduction in the material’s work-hardening ability. Uniform and localized corrosion parameters were estimated via potentiodynamic polarization corrosion tests and electrochemical impedance spectroscopy. In contrast to the uniform corrosion, wherein the corrosion current density increased with increasing cold-working degree because of the high density of microstructural defects, the passive potential range and breakdown potential increased by cold working, showing greater resistance to pit nucleation. Although pits were formed, the cold-rolled material repassivation tendency decreased because of the broader hysteresis anodic loop, as confirmed experimentally by observation of the microscopic features after electrochemical cyclic polarization evaluations.

Variations in Hardness with Position In One Dimensionally Recovered Shock Loaded Metals

The microstructural and mechanical response of materials to shock loading is of the utmost importance in the development of constitutive models for high strain-rate applications. However, unlike a purely mechanical response, to ensure that the microstructure has been generated under conditions of pure one dimensional strain, the target assembly requires both a complex array of momentum traps to prevent lateral releases entering the specimen location from the edges and spall plates to prevent tensile interactions (spall) affecting the microstructure. In this paper, we examine these effects by performing microhardness profiles of shock loaded copper and tantalum samples. In general, variations in hardness both parallel and perpendicular to the shock direction were small indicating successful momentum trapping. Variations in hardness at different locations relative to the impact face are discussed in terms of the initial degree of cold work and the ability to generate and move dislocations in the samples.

Influence of Cold Work on the Microstructural Evolution and Hardness During Aging of AA6061 Alloy

It is well known that the kinetics of precipitation is altered with the degree of cold working and this study aims to bring out the effect of cold working on the aging response of the alloy AA6061 along with the studies of microstructural evolution. The as-received samples were initially solutionized at 530 °C and subsequently aged at 160 °C for varying time from 2 to 28 h at intervals of 2 h. A peak hardness of 124 HV was observed at 15 h beyond which the hardness decreased due to overaging. The solutionized samples were also subjected to various degrees of cold working and the increase in hardness by virtue of strain hardening was observed. The samples that were cold worked to various levels (10, 20, 30% etc.) were subsequently aged at 160 °C for 4 h. A peak hardness of 134 HV was obtained for the sample cold worked to 75% and subsequently aged for 4 h and this was attributed to the increase in the dislocation density that improved the kinetics of precipitation. The evolution of microstructure for various samples during the course of aging was observed using transmission electron microscopy and the changes in the properties were correlated to the obtained microstructure.

Nitriding Process Characterization of Cold Worked AISI 304 and 316 Austenitic Stainless Steels

The nitriding behavior of austenitic stainless steels (AISI 304 and 316) was studied by different cold work degree (0% (after heat treated), 10%, 20%, 30%, and 40%) before nitride processing. The microstructure, layer thickness, hardness, and chemical microcomposition were evaluated employing optical microscopy, Vickers hardness, and scanning electron microscopy techniques (WDS microanalysis). The initial cold work (previous plastic deformations) in both AISI 304 and 306 austenitic stainless steels does not show special influence in all applied nitriding kinetics (in layer thicknesses). The nitriding processes have formed two layers, one external layer formed by expanded austenite with high nitrogen content, followed by another thinner layer just below formed by expanded austenite with a high presence of carbon (back diffusion). An enhanced diffusion can be observed on AISI 304 steel comparing with AISI 316 steel (a nitrided layer thicker can be noticed in the AISI 304 steel). The mechanical strength of both steels after nitriding processes reveals significant hardness values, almost 1100 HV, on the nitrided layers.

Influence of Cold work on aging response of C17200-Beryllium Copper Alloy C17200

Progress in materials science requires deeper understanding of the correlation of properties to internal structure and how they get altered to tailor the required properties. The paper aims to explore the variation in the mechanical properties on the degree of cold work in Beryllium copper. Hardness increased with the increase in the degree of cold working and was observed that the hardness saturated beyond 40% cold work. The samples of various percentages of cold work were subjected to ageing treatment at 330°C for one hour and the trend of hardness is explained. The microstructures revealed the evolution of precipitates during aging that attributed to increase in the hardness. Dislocation density is predicted to increase during cold work and thus improving the kinetics of precipitation. Results show a very good agreement between reported experimental observations and predictions.