IGSCC crack growth in simulated BWR environment – Effect of nitrogen content in non-sensitised and warm rolled austenitic stainless steel

Effect of nitrogen level in strain hardened stainless steel (SS) on crack growth rate (CGR) in simulated boiling water reactor conditions has been the focus of this study. Type 304 LN stainless steel has been used in a warm rolled condition containing two different levels of nitrogen. Clear intergranular (IG) fracture was observed in both the stainless steels. The CGR increased 3 times in the stainless steel with higher level of nitrogen at all levels of dissolved oxygen and this was related to the increase in yield strength due to rolling and dynamic strain aging (DSA).     

Study on edge cracking and texture evolution during 150 °C rolling of magnesium alloys: The effects of axial ratio and grain size

Pure Mg, AZ31 (Mg-3 wt%Al-1 wt%Zn) and experimental alloys, Mg-1 wt%Zn-1 wt%In, Mg-2 wt%Li-1 wt%Zn, Mg-2 wt%Li-1 wt%Zn-1 wt%In were rolled at 150 °C to 0.3 and 0.55 strain. The samples rolled to 0.3 strain were subsequently annealed for 10 min at 400 °C. The texture was evaluated in rolled and in rolled/annealed conditions. The axial ratio (c/a) of the alloys strongly influenced edge cracking during rolling (expressed as cracking index, I_C), which was explained via the influence of c/a on the twinning mode. The as-cast grain size did not correlate to I_C (edge cracking). Texture intensity was strongly influenced by the as-cast grain size (which was attributed to plastic compatibility at grain boundaries), showed weak correlation to the lattice parameter a, but did not depend on c/a. The Mg-2 wt%Li-1 wt%Zn alloy, with fine grain size and small c/a and a, exhibited the optimum combination of weak texture and crack-free rolling at 150 °C.     

Microstructure and mechanical properties of AM31 magnesium alloys processed by differential speed rolling

Ingot casted AM31 alloys were rolled at a warm temperature of 350 °C and subsequently rolled at 300 °C using equal speed rolling (ESR) and differential speed rolling (DSR) with speed ratios of top roll to bottom roll, 1.2 and 1.5, respectively. Microstructures, textures and mechanical properties of the as-rolled AM31 sheets were examined. Ductility was improved by DSR due to inclination of basal poles and weakened texture. The sheets produced by DSR with a speed ratio of 1.2 showed highest strength and ductility at room temperature, which can be attributed to homogeneous fine grain distribution and tilted basal texture.     

An electrochemical framework to explain the intergranular stress corrosion behavior in two Al-Cu-Mg-Ag alloys as a function of aging

A modified version of the Cu-depletion electrochemical framework was used to explain the metallurgical factor causal to intergranular stress corrosion cracking (IGSCC) susceptibility in two Al-Cu-Mg-Ag alloys (i.e., Al-5.0%Cu-0.8%Mg-0.5%Ag and Al-5.4%Cu-0.5%Mg-0.5%Ag). This framework was also used to explain the specific cases of IGSCC susceptibility in the under-aged condition and increased IGSCC resistance with over-aging. Susceptibility in the under-aged and T8 (peak-aged) conditions is consistent with the grain boundary Cu-depletion mechanism and high concentrations of Cu in solid solution in grain interiors. Improvement in resistance to IGSCC of the over-aged T8+ condition (i.e., T8 temper followed by an additional thermal exposure of 5000 h at 107.2 °C) can be explained by elimination of the differential Cu concentration adjacent to the grain boundary compared to that of the grain interior. The resultant leveling of Cu concentration leads to the suppression of a preferential dissolution path at the grain boundaries associated with IGSCC.     

Twinning, grain orientation and texture variation of AZ31 Mg alloy during compression by EBSD tracing

In order to investigate the micro-mechanism of warm forming of Mg alloys, three specimens cut from a rolled AZ31 sheet were chosen to be compressed along the Rolling Direction (RD) at 100 °C, 170 °C and 230 °C, separately. During compression, an in situ measurement of grain orientation in the plane of RD × TD (Transverse Direction) was carried out with EBSD method. Experimental and analytical results show that temperature has remarkable impact on activation of twinning and variation of texture. As the temperature was raised from 100 °C to 230 °C, the number of grains with twins activated decreased substantially during deformation, and rolling texture varied from quick vanishing at 100 °C to always existing at 230 °C. Tracing for orientation of individual grains during deformation shows that there are obvious different orientation changes between grains with twins activated and those without twins activated. Twinning plays a significant effect on texture variation during compression. The extension twin variant really activated during deformation is the one with maximal Schmid factor.     

Effect of vertical rolling at various temperatures on subsequent multi-pass severe rolling of AZ31B alloy sheet

AZ31B alloy was subjected to vertical rolling at various temperatures prior to multi-pass severe rolling processing including initial rolling including one 80% reduction pass and finish rolling at 300 °C and 350 °C, respectively. The depth and number of edge crack, microstructure evolution and tensile properties were examined. The results indicate that pre-vertical rolling at low temperature before severe rolling can significantly restrain edge crack, change relative frequency distribution of edge-crack depth, increase microstructure homogeneity and sharply change the intensity and distribution of basal texture of initial-rolled sheets. The level of edge crack increases with increased vertical rolling temperature above 100 °C mostly due to the combination of shear band density, microstructure homogeneity, grain size and texture of rolled sheets. Compared with conventional rolling, the effect of vertical rolling on final mechanical properties depends on the finish rolling temperature due to the combination of shear bands, twins and grain size. The variation trend of mechanical properties with increased vertical rolling temperature is also sensitive to finish rolling temperature. For as-rolled sheets, the severe rolling route with vertical rolling at 100 °C and finish rolling at 300 °C should be required.     

Effects of hot rolled shear bands on formability and surface ridging of an ultra purified 21%Cr ferritic stainless steel

An ultra purified 21%Cr ferritic stainless steel (FSS) was hot rolled at a low finisher entry temperature (FET) of 750 °C to generate shear bands through large shear flow localizations in grain interiors. The effects of shear bands on the formability and surface ridging have been studied as compared to a conventional hot rolled band at the FET of 970 °C with few shear bands. The results showed that as compared to the final sheet produced with the FET of 970 °C, the average r-value for the final sheet with the FET of 750 °C was increased by 25%, and the surface roughness of the final sheet strained by 15% along the rolling direction was decreased by 40%. It has been observed that the existence of shear bands could enhance the nucleation for recrystallization during hot rolling and annealing for microstructure refinement, and modify the texture by intensifying the {1 1 1} textures in the final sheet. The influence of shear bands on sheet formability and resistance against surface ridging was discussed.     

Effect of ageing on microstructure and mechanical properties of bulk, cryorolled, and room temperature rolled Al 7075 alloy

The effect of ageing on mechanical properties and microstructural characteristics of a precipitation hardenable Al 7075 alloy subjected to rolling at liquid nitrogen temperature and room temperature are has been investigated in the present work employing hardness measurements, tensile test, XRD, DSC, and TEM. The solution-treated bulk Al 7075 alloy was subjected to cryorolling and room temperature rolling to refine grain structures and subsequently ageing treatment to simultaneously improve the strength and ductility. The solution treatment combined with cryorolling up to a true rolling strain of 2.3 followed by low temperature ageing at 100 °C for 45 h has been found to be the optimum processing condition to obtain fine grained microstructure with improved tensile strength (642 MPa) and good tensile ductility (9.5%) in the Al 7075 alloy. The combined effect of suppression of dynamic recovery, partial grain refinement, partial recovery, solid solution strengthening, dislocation hardening, and precipitation hardening are responsible for the significant improvement strength-ductility combination in the cryorolled Al 7075 alloy subjected to peak ageing treatment. The cryorolled and room temperature rolled Al 7075 alloy, upon subjecting to peak ageing treatment, have shown higher strength and ductility in the former than the latter. It is due to presence of high density of nanosized precipitates in the peak aged cryorolled sample.     

Improvement of stretch formability of Mg-3Al-1Zn alloy sheet by high temperature rolling at finishing pass

The effects of increasing rolling temperature from 723 K to 828 K at the last rolling pass on microstructure, texture, mechanical properties and stretch formability of a Mg-3Al-1Zn magnesium alloy previously rolled at 723 K were investigated. In the as-rolled condition, the basal texture strengthens slightly with increasing the rolling temperature whereas it weakens more remarkably after static recrystallization during annealing for the sheets rolled at higher temperatures. Only by increasing the rolling temperature from 723 K to 798 K, the Erichsen value is significantly increased from 4.5 to 8.6 due to the weakened texture for the annealed sheets. Further increasing the last rolling temperature does not appear to further improve the stretch formability.     

Effect of rolling and annealing processes on the hardness and electrical conductivity values of Cu-13.5%Mn-4%Ni alloy

It was found that the dendritic microstructure of an as-cast alloy was changed to an almost equiaxed alloy after 480 min of annealing at 700 °C. The electrical conductivity of as-cast and hot rolled samples increased from 27.36 and 30.51% IACS to 30.67 and 32.1% IACS after 480 min of annealing at 700 °C. The dendritic microstructure and the electrical conductivity values of an as-cast alloy annealed for 60 min remained almost unchanged when the annealing temperature was increased to 800 °C. The hardness values of the samples that were hot rolled and annealed for 60 min were higher than those of the as-cast samples for all annealing temperatures. The electrical conductivity values of the hot rolled and as-cast samples were almost the same after annealing at 800 °C for 60 min. It was shown that after 20% cold work, the electrical conductivity value of the as-cast sample decreased from 30.5% IACS to 22.6% IACS. The electrical conductivity values of the samples were not significantly changed when the cold work was increased from 20% to 60%. The electrical conductivity values of the 20%, 40% and 60% cold worked samples increased from 22.62, 24.69 and 26.63 to 27.14,28.36 and 30.55% IACS, respectively, after 30 min annealing at 400 °C.     

Microstructure and mechanical properties of Mg-Al-Mn-Ca alloy sheet produced by twin roll casting and sequential warm rolling

Microstructural evolution and mechanical properties of twin roll cast (TRC) Mg-3.3 wt.%Al-0.8 wt.%Mn-0.2 wt.%Ca (AM31 + 0.2Ca) alloy strip during warm rolling and subsequent annealing were investigated in this paper. The as-TRC alloy strip shows columnar dendrites in surface and equiaxed dendrites in center regions, as well as finely dispersed primary Al₈Mn₅ particles on interdendritic boundaries which result in the beneficial effect on microstructural refinement of strip casting. The warm rolled sheets show intensively deformed band or shear band structures, as well as finely and homogeneously dispersed Al-Mn particles. No evident dynamic recrystallization (DRX) takes place during warm rolling process, which is more likely attributed to the finely dispersed particle and high solid solution of Al and Mn atoms in α-Mg matrix. After annealing at 350 °C for 1 h, the warm rolled TRC sheets show fine equiaxed grains around 7.8 μm in average size. It has been shown that the present TRC alloy sheet has superior tensile strength and comparative elongation compared to commercial ingot cast (IC) one, suggesting the possibility of the development of wrought magnesium alloy sheets by twin roll strip casting processing. The microstructural evolution during warm rolling and subsequent annealing as well as the resulting tensile properties were analyzed and discussed.     

Grain refinement and mechanical properties of asymmetrically rolled low carbon steel

The formation of fine ferrite grains by the asymmetric rolling of low carbon steel and their mechanical properties were studied. Super-cooled low carbon austenite was deformed by asymmetric rolling at 750 °C with a roll size ratio of 1.5 and immediately cooled at various cooling rates ranging from 3 °C/s to 15 °C/s. Fine ferrite grains (∼2 μm) were formed after asymmetric rolling, preferentially at the prior austenite grain boundaries. The volume fraction of the fine ferrite grains increased with increasing rolling reduction. A ferrite plus pearlite microstructure was obtained at smaller strains and slower cooling rates. However, after heavy deformation, a fine ferrite grain structure with carbide particles dispersed at the ferrite grain boundaries was obtained and the pearlite structure was not observed even after very slow cooling, which implies that most of the ferrite grains were formed dynamically, i.e. during deformation. The yield strength of the asymmetrically rolled steel plates increased with increasing deformation; however, the yield ratio also increased with increasing rolling reduction. The best combination of strength and yield ratio was obtained by using a low level of deformation and a high cooling rate, in which case a portion of the untransformed austenite transformed to martensite.     

Transgranular SCC mechanism of thermally treated alloy 600 in alkaline water containing lead

This work aims to understand a SCC failure mode of thermally treated steam generator tubing materials in high temperature water containing lead. The effect of lead contents on the anodic polarization curves of alloy 600 (UNS NO6600) and alloy 690 (UNS NO6900) has been studied in a solution of pH 10 at 200 °C and 315 °C. Lead increased the active peaks of alloy 600 and alloy 690 in mild alkaline water at high temperatures. A reduction of PbO to a metallic lead in alloy 690 is easier than that of alloy 600. When lead was added into the solution, a relative ratio of Cr from among the main metallic elements (Cr, Fe, and Ni) of alloy 600 and alloy 690 decreased in the outer corrosion film. Alloy 690 TT showed a transgranular stress corrosion cracking (TGSCC) in a 10 M NaOH solution with 5000 ppm of lead. Intergranular stress corrosion racking (IGSCC) was observed in the 100 ppm lead condition, and some TGSCC was detected on the fracture surface of the alloy 600 MA cracked in the 10,000 ppm lead solution. IGSCC seemed to be retarded by a crack blunting around the grain boundaries, and the TG cracking mode of the thermally treated alloy 600 and 690 seemed to be related to a crack blunting at the grain boundary carbide and a film dissolution by lead in an alkaline solution.     

Effect of water vapor on annealing scale formation on 316 SS

The oxidation behavior of 316 stainless steel (SS) annealed in air containing 0.1 atm water vapor at temperatures ranging from 800 to 1030 °C was investigated. A kinetic study of the oxidation was made by employing thermal-gravimetric analysis (TGA). The morphology, composition and structure of the scale were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The experimental results showed that significant breakaway oxidation occurred, resulting in substantial weight increase, as the steel was annealed in moist air at temperatures above 950 °C. The scaling behavior of 316 SS in wet air at 1030 °C could be divided into two stages based on the alteration of the oxidation rate. In each stage, the scale on 316 SS exhibited a different structure and morphology. The complex process of the formation of scale in wet air was discussed and proposed.     

Effect of plane strain on microstructures and texture, through the reduction ratio of AZ31 Mg alloy

Structural evolution of warm-rolled AZ31 alloy sheets was investigated with respect to various reduction ratios. In order to examine the effect of rolling pass on deformation of the sheet, one-pass rolling was applied to the AZ31 alloys for various 6/1/2011reduction ratios. When the applied reduction ratio was ∼85% of the initial thickness, significant grain refinement and texture development were achieved with dynamic recrystallization. Moreover, with the increase of the rolling reduction ratio from 30% to 85%, the warm rolled sheets exhibit plane strain mode displaying uniform 〈0 0 0 1〉//ND basal textures throughout the whole sheet thickness. The two-dimensional finite element method simulation showed that the current lubrication rolling results in a uniform plane strain deformation through the whole warm rolled sheet.     

Effect of nitrogen addition on the stress corrosion cracking behavior of 904 L stainless steel welds in 288 °C deaerated water

This paper concerns with the effect of nitrogen addition to 904 L stainless steel (SS) welds on their stress corrosion cracking (SCC) behavior in high temperature (288 °C) and high pressure (1050 psi) water of high oxygen content (100 ppb) and high conductivity (2.5 μS/cm). For this study, 316 L SS base plate TIG welded with 904 L SS filler wire and with nitrogen contents of 0.027, 0.058 and 0.095 wt.% were used. Flat pin-loaded tensile specimens were fabricated from transverse welds, with the weld in the gauge length. Slow strain rate tests (SSRT) were carried out at a strain rate of 2.2 × 10⁻⁶ s⁻¹. The study shows that the samples, when tested in air, failed at the weld fusion zone for 0.027 and 0.058 wt.% N and at the base metal for 0.095 wt.% N. In the environment, the samples failed in the base metal except the one with least nitrogen content (0.027 wt.%). With nitrogen addition, as the failure location shifted to the base alloy, the weld seemed to acquire SCC resistance and became even more resistant than the base alloy.     

Effect of grain shape on the texture evolution during cold rolling of Al-Mg alloys

The hot bands of continuous cast AA 5754 and high-Fe AA 5754 aluminum alloys exhibited elongated grains in the rolling direction after recrystallization annealing. The annealed hot bands were cold rolled to different reductions along the original rolling direction and transverse direction, respectively. The effect of grain shape on texture evolution was investigated by X-ray diffraction. It was found that straight-rolling resulted in a higher rate of disappearance of the r-cube + cube component and a slightly higher rate of formation of the β fiber component than cross-rolling. The elongated grains in the rolling direction were easier to rotate from initial orientations to the β fiber in straight-rolling than in cross-rolling.     

Comparison of rolled and heat treated SS304 in chloride solution using electrochemical and XPS techniques

SS304 in the rolled condition (66% cold work) and in the heat treated condition (solution quenched from 1100 °C) was assessed for its electrochemical behavior in de-aerated 3.5% NaCl by potentiodynamic polarization and AC impedance techniques. Surface analysis of films formed on the two materials in air and in solution was done using XPS. Open circuit potential (OCP) and pitting potential of the rolled SS304 were seen to be −0.25 V vs standard calomel electrode (SCE) and 0.25 V vs SCE respectively, while the heat treated sample showed −0.33 V vs SCE and 0.06 V vs SCE respectively. The rolled material showed a passive current density of 0.9 μA cm⁻², which was about one order lower than that shown by the heat treated material. The rolled material, in addition, exhibited repassivation at 0.085 V vs SCE during reverse scanning of potential. Repassivation was not observed in the heat tread sample. XPS showed that films on rolled material were richer in chromium as compared with those on heat treated alloy. Better passivation characteristics shown by the rolled material were attributed to thicker passive film with a higher Cr:Fe ratio, the higher Cr being the result of enhanced diffusion through the oriented grains in the rolled material.     

Effect of boron on microstructure and mechanical properties of thermomechanically processed near alpha titanium alloy Ti-1100

The effect of 0.2 wt.% of boron on the mechanical properties of Ti-1100, a near α titanium alloy, was evaluated at room temperature and at 600 °C in the as cast and thermomechanically processed (α-β rolled) condition after subjecting it to different heat treatments. Boron addition in Ti-1100 significantly refined the microstructure in the as cast condition but the mechanical properties did not show any improvement. However, in the thermomechanically processed (α-β rolled) and standard heat treatment condition, the yield strength (YS) and ultimate tensile strength (UTS) of the boron containing alloy increased significantly without any drop in elongation-to-failure as compared to the base alloy at both room temperature and 600 °C. No discernible trend was seen in YS and UTS in boron containing alloy with change in solution treatment temperature either at room temperature or at 600 °C.     

Influence of thermal exposure on the stability of metastable microstructures of sputter deposited nanocrystalline 304 and 310 stainless steel coatings

The effect of thermal exposure on the stability of the microstructures of magnetron sputter deposited 304 and 310 stainless steel (SS) coatings have been investigated. The coated 304SS samples were exposed to thermal cycling between 750 °C and room temperature and aged at 500 °C and 650 °C. X-ray diffraction (XRD) results revealed that the microstructures of the as-deposited 304SS and 310SS coatings consisted of αFe + σ and γFe + αFe, respectively. Thermal exposure at 500 °C and 650 °C resulted in the dissolution of σ and partial transformation of αFe into γFe in the 304SS coating and led to the precipitation of σ phase particles, at the expense of αFe, in the 310SS coating. The amount of σ phase in the 310SS coating increased with exposure time. However, both coatings showed no σ phase after thermal cycling to 750 °C for approximately 500 cycles.