Effect of grain boundary orientation on the sensitization of austenitic stainless steel

The effect of grain misorientation on the sensitization of grain boundaries in austenitic stainless steel was investigated by sensitizing samples consisting of a large number of 50–80 μm size grains that were sintered to flat, 10 mm2 single crystals. Seven different sensitization treatments were employed and samples were intergranulary corroded in the modified Strauss test. X-ray pole figures were obtained for each sample and were used to identify the grain misorientations that were resistant to sensitization. In general, macroscopic grain boundary geometry could not explain the sensitization behaviour of most grain boundaries. Nevertheless, the Σ = 9 boundary was found to be especially resistant to sensitization. Results suggest that grain misorientation primarily affects the growth of sensitization rather than its nucleation. Finally, the crystallographic plane of the grain boundary appears to have an effect on sensitization. This revised version was published online in November 2006 with corrections to the Cover Date.     

Structure and mechanical properties of Ni–Cr alloy produced by single roll strip casting

In the present investigation the microstructure and some mechanical properties of Ni–Cr alloy prepared by single roll strip casting (SRSC) were studied. The top surface (surface not in contact with the roll) of the as received sample was rough and lusterless. The grain size of the top surface was significantly larger compared to that of the bottom surface. Grain interior showed dendritic morphology. Etch-pits, formed by dislocation were observed on the top and bottom surfaces of the sample. Scanning electron microstructure revealed continuous corroded region along the grain boundaries. X-ray diffraction (XRD) study confirms the formation of chromium carbide at the grain boundary, which depletes Cr near the grain boundary. The hardness of the top surface is lower compared to the bottom surface. The cross-sectional surface shows much higher hardness compared to top and bottom surface, suggesting anisotropic nature of the alloy. The tensile properties of the sample were affected by chromium carbide precipitation at grain boundaries.     

A TEM investigation of M23C6 carbide precipitation behaviour on varying grain boundary misorientations in 304 stainless steels

Transmission electron microscopy (TEM) along with electrochemical potentiokinetic reactivation (EPR) testing was performed on different grades of 304 stainless steel (0.01, 0.025, 0.05, and 0.07%C) in order to assess the sensitization and precipitation behaviour on different grain boundary misorientations. The materials were heat treated at 670°C for 50 h to subject the materials to the sensitization regime. The EPR data and TEM observations revealed that when the amount of carbon was increased the degree of sensitization increased along with the density of precipitates. Large angle misorientations (>15°) were prevalent in all the carbon content materials and the {1 1 0} grain surface orientation was found to be the major texturing orientation. The steels with lower carbon contents nucleated a few small precipitates on high angle grain boundaries, while larger amounts of carbides were observed on lower angle grain boundaries for the higher carbon contents. It was deemed that higher carbon contents required lower energies to nucleate and grow precipitates. A carbon content threshold was found (above 0.05% C) in which precipitates fully saturate the grain boundary. Precipitation followed the energies of different types of boundaries. The highest energy boundary (general random grain boundary) nucleated precipitates first, then precipitation followed on non-coherent twin boundaries, and was not observed on coherent twin boundaries. A critical nucleation energy, gb(crit.), was therefore found to exist at which precipitation will occur on a boundary. This value was found to be in the range of 16 mJ m-2相似文献   

Effects of Deformation (Strain) and Heat Treatment on Grain Boundary Sensitization and Precipitation in Austenitic Stainless Steels

Sensitization, particularly the degree of sensitization (DOS) in type 316 stainless steel pipe is critically dependent upon the solution anneal of the mill-annealed or commercial material, and is particularly sensitive to low-temperature aging when the starting material is solution annealed between about 1,000°C and 1,100°C. It is observed that when the DOS is above about 10 C/cm² (quantitative electrochemical potentiokinetic reactivation units in Coulombs/cm²), noticeable carbide precipitation occurs in the grain boundaries and increases with increasing DOS. Transmission electron microscopy (TEM) examination of precipitation occurring in type 316 stainless steel pipe grain boundaries has shown them to exhibit many microstructural features that seem to be coincident with grain boundary microstructures, particularly ledges. Uniaxial, tensile strain prior to aging of solution annealed pipe was observed to have a dramatic effect on DOS of type 316 stainless steel pipe; DOS increases with increasing strain. This effect appears to be correlated with the propensity of dislocations to be generated near grain boundaries, and for grain boundary microstructural changes to be related with strain, as observed in type 304 stainless steel. The effects of strain on DOS, combined with instabilities connected with annealing of type 316 stainless steel, indicate that in the practical utilization of pipe, it is imperative to reduce deformation prior to service and during service applications, to avoid sensitization and, thus, minimize the probability of grain boundary cracking.     

铸造Mg-Gd-Y-Nd-Zr合金在时效过程中的组织与性能演变

采用金相观察、硬度测试、单轴拉伸、扫描电镜观察、能谱分析、透射电镜观察等手段,研究了铸造Mg-Gd-Y-Nd-Zr合金在时效过程中的组织与性能演变。结果表明,经固溶处理后,合金具有较强的塑性变形能力,延伸率可达10%以上,但强度较低。随时效程度增加,合金强度升高塑性降低,经225℃/3h时效处理后,合金为欠时效状态,与基体共格的β″相是主要的强化相,断口以解理面、韧窝、撕裂棱和晶界为主要特征。经峰值时效处理后,与基体呈半共格关系的β′相是主要的强化相,合金抗拉强度超过300 MPa,但塑性急剧降低,断口以解理面、撕裂棱和晶界为主要特征,与欠时效样品相比,解理面所占比例明显增加,且解理面及晶界光滑。进入过时效状态后,合金的强度降低,但延伸率有所提升,断口以晶界和解理面为主要特征。     

Grain boundary engineering of titanium-stabilized 321 austenitic stainless steel

Grain boundary engineering (GBE) primarily aims to prevent the initiation and propagation of intergranular degradation along grain boundaries by frequent introduction of coincidence site lattice (CSL) boundaries into the grain boundary networks in materials. It has been reported that GBE is effective to prevent intergranular corrosion due to sensitization in unstabilized 304 and 316 austenitic stainless steels, but the effect of GBE on intergranular corrosion in stabilized austenitic stainless steels has not been clarified. In this study, a twin-induced GBE utilizing optimized thermomechanical processing with small pre-strain and subsequent annealing was applied to introduce very high frequencies of CSL boundaries into a titanium-stabilized 321 austenitic stainless steel. The resulting steel showed much higher resistance to intergranular corrosion after sensitization subsequent to carbon re-dissolution heat treatment during the ferric sulfate–sulfuric acid test than the as-received one. The high CSL frequency resulted in a very low percolation probability of random boundary networks in the over-threshold region and remarkable suppression of intergranular corrosion during GBE.     

Study on microstructure of low carbon 12% chromium stainless steel in high temperature heat-affected zone

Coarsening, embrittlement and corrosion sensitization in high temperature heat-affected zone (HTHAZ) are the major problems when low carbon 12% chromium stainless steel is being welded, which induce deterioration of the impact toughness at low temperature and intergranular corrosion resistance. This study investigated the corresponding microstructures in HTHAZ with different chemical compositions and heat inputs through thermal simulation tests. There are several primary conclusions: (1) When ferrite factor (FF) is above 9.0, the microstructure in HTHAZ is fully ferrite or a small amount of martensite net likely distributing along delta ferrite grain boundaries. On the other hand, if FF is below 9.0, the martensite content increases with the decreasing of FF. (2) Heat input influences the microstructure of high FF steel in HTHAZ. The martensite content and its distribution of low FF steel are not sensitive to heat inputs, but the grain size grows up with the increase of heat inputs. (3) The coarse Ti-rich particles in low FF steels containing Ti can promote intragranular austenite formation inside delta ferrite resulting in packet morphology of martensite. On the other hand, martensite of low FF steels only stabilized with Nb is characterized by grain boundary allotriomorphs, Widmanstätten structures and secondary sawteeth. This martensite reticularly distributes along ferrite grain boundaries.     

Influence of the starting condition on the kinetics of sensitization and loss of toughness in an AISI 304 steel

This work describes the investigation of the embrittlement of AISI 304 steel sensitized at 650°C by Charpy impact test, comparing two starting conditions: (1) mill annealed and machined (MA-M); and (2) solution treated at 1050°C by 1 h followed by oil quenching (ST). The degree of sensitization for both samples was assessed by Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and by Double Loop Electrochemical Potentiodynamic Reactivation test. The results showed that MA-M samples undergo more severe and rapid embrittlement than ST ones and a higher kinetics of sensitization due to small strains concentrated in grain boundaries and martensite phase produced during the machining operations. The martensite phase is found to be quite stable at the sensitization treatment at 650°C. The increase of microvoids nucleation at the grain boundaries seems to be the mechanism of embrittlement in the sensitized 304 steel.     

Investigation of the formation of Al,Fe, N intermetallic phases during Al pack cementation followed by plasma nitriding on plain carbon steel

Plain carbon steels are not suitable for nitriding as they form an extremely brittle case that spalls off readily, and the hardness increment of the diffusion zone is small. In this research, the effect of plasma nitriding time and temperature variation on the microstructure of the pack cemented aluminized plain carbon steel is investigated. All samples were aluminized at 900 °C for 2 h; the aluminized samples were subsequently plasma nitrided at 500 °C, 550 °C and 600 °C for 2.5, 5, 7.5 and 10 h. The phases formed on the sample surface were detected by X-ray diffraction (XRD). The cross section and samples surface were investigated by optical and scanning electron microscopy (SEM). Microhardness test was conducted to determine hardness change from the surface to the sample core. Results showed that by aluminizing the steel, Fe₃Al phases as well as Fe–Al solid solution were formed on the surface and some aluminum rich precipitates were formed in solid solution grain boundaries. Plasma nitriding of the aluminized layer caused the formation of aluminum and iron nitride (AlN, Fe₄N) on the sample surface. Consequently, surface hardness was improved up to about eight times. By increasing the nitriding temperature and time, aluminum-rich precipitates dissociated. Moreover, due to the diffusion of nitrogen through aluminized region during ion nitriding, iron and aluminum nitrides were formed in aluminized grain boundaries. Increasing nitriding time and temperature lead to the growth of these nitrides in the grain boundaries of the substrate. This phenomenon results in the increment of sample hardness depth. Plasma nitriding of aluminized sample in low pressure chamber with nitrogen and hydrogen gas mixture reduced surface aluminum oxides which were formed in aluminizing stage.     

ON THE NUCLEATION OF FATIGUE CRACKS IN PURE POLYCRYSTALLINE α-IRON

Abstract— This paper presents the results of an investigation of fatigue crack initiation in pure polycrystalline α-iron as a function of testing frequency (0.01–1000 Hz). Three distinct types of fatigue crack initiation mechanisms were identified:
(i) At low frequencies, <0.5 Hz, cracks were observed to nucleate at intrusions, extrusions and grain boundaries. However over the frequency range studied the cracks at the intrusions and extrusions were not associated with the final fatigue failure. The results suggest that in pure iron frequencies <0.01 Hz are required for fatigue failure to be initiated at intrusions and extrusions.
(ii) At frequencies ≤0.01 Hz at high strain amplitudes, cracks nucleated at the intersection of grain boundaries with the free surface as a result of the incompatable deformation of surface grains. The subsequent propagation of these cracks resulted in fatigue failure.
(iii) Tests performed at low strain amplitude at 5 Hz and 1000 Hz initiated surface grain boundary cracking without any significant associated change of grain shape.
The results of the present investigation indicate that fatigue failure in pure iron alloys (at frequencies >0.01 Hz) is initiated not at intrusions and extrusions but at grain boundaries.     

U-Ti合金的奥氏体晶粒度检测技术

探索了淬火态U-Ti合金的奥氏体晶界显示方法,并依据GB/T 6394—2002和GB/T21865—2008,利用光学金相图像分析仪测定了典型制备工艺下U-Ti合金的平均奥氏体晶粒度。结果表明:采用先将U-Ti合金在500℃保温1h进行晶界析出敏化处理,再使用体积分数为95%的磷酸溶液可以清晰完整地显示U-Ti合金的奥氏体晶界;平均晶粒面积法和截线法测得的U-Ti合金的晶粒度级别指数近似;以截线法作为仲裁法,典型制备工艺下U-Ti合金的奥氏体晶粒度级别指数为2.84±0.16,平均截距为(119.74±6.97)μm。     

Grain boundary and microstructure engineering of Inconel 690 cladding on stainless-steel 316L using electron-beam powder bed fusion additive manufacturing

This research explores the prospect of fabricating a face-centered cubic(fcc) Ni-base alloy cladding(Inconel 690) on an fcc Fe-base alloy(316 L stainless-steel) having improved mechanical properties and reduced sensitivity to corrosion through grain boundary and microstructure engineering concepts enabled by additive manufacturing(AM) utilizing electron-beam powder bed fusion(EPBF). The unique solidification and associated constitutional supercooling phenomena characteristic of EPBF promotes[100] textured and extended columnar grains having lower energy grain boundaries as opposed to random, high-angle grain boundaries, but no coherent {111} twin boundaries characteristic of conventional thermo-mechanically processed fcc metals and alloys, including Inconel 690 and 316 L stainless-steel.In addition to [100] textured grains, columnar grains were produced by EPBF fabrication of Inconel 690 claddings on 316 L stainless-steel substrates. Also, irregular 2–3 μm diameter, low energy subgrains were formed along with dislocation densities varying from 108 to 109 cm~2, and a homogeneous distribution of Cr_(23)C_6 precipitates. Precipitates were formed within the grains(with ~3 μm interparticle spacing),but not in the subgrain or columnar grain boundaries. These inclusive, hierarchical microstructures produced a tensile yield strength of 0.527 GPa, elongation of 21%, and Vickers microindentation hardness of 2.33 GPa for the Inconel 690 cladding in contrast to a tensile yield strength of 0.327 GPa, elongation of 53%, and Vickers microindentation hardness of 1.78 GPa, respectively for the wrought 316 L stainlesssteel substrate. Aging of both the Inconel 690 cladding and the 316 L stainless-steel substrate at 685?C for50 h precipitated Cr_(23)C_6 carbides in the Inconel 690 columnar grain boundaries, but not in the low-angle(and low energy) subgrain boundaries. In contrast, Cr_(23)C_6 carbides precipitated in the 316 L stainless-steel grain boundaries, but not in the low energy coherent {111} twin boundaries. Consequently, the Inconel690 subgrain boundaries essentially serve as surrogates for coherent twin boundaries with regard to avoiding carbide precipitation and corrosion sensitization.     

<Emphasis Type="Italic">In-situ</Emphasis> TEM observations of the coarsening of a nanolamellar structure in a cobalt based magnetic alloy

The thermal stability and coarsening of nanostructures is of both scientific interest and of engineering significance in order to produce thermally stable nanomaterials. Real time observations were carried out using ultra high vacuum (UHV) in situ TEM to investigate the coarsening process of a highly modulated nanolamellar structure obtained by crystallization of a Co based Co₆₅Si₁₅B₁₄Fe₄Ni₂ amorphous magnetic alloy. The coarsening process consisted of three steps: (a) precipitation of spherical fine precipitates; (b) continuous coarsening of the nanolamellar structure at the surface and precipitation at the grain boundaries; and (c) formation of a stable multiphase structure. Due to surface effects, continuous coarsening of nanolamellar structure was observed during in-situ annealing; this mechanism was different from that of the coarsening process found during conventional annealing. Discontinuous coarsening from grain boundaries, which dominates the coarsening process in the conventional annealing of bulk sample, also occurred in in-situ annealing of thin sample. The driving force for coarsening of the nanolamellar structure from interlamellar interfaces, grain boundaries and surfaces is discussed.     

Measurements of spacing of sliding grain boundaries

The spacing of grain boundaries at which grain boundary sliding (GBS) had occurred during superplastic (SP) deformation was determined by measuring the length of segments of marker lines inscribed on the pre-polished surface in Pb-62%Sn after superplastic deformation in shear. Statistical distribution of this segment length (L) was bimodal at low strain levels, but became unimodal at high strain levels. The concept of cooperative GBS, i.e. sliding of groups of grains as an entity, has been invoked to explain the evaluation of the L-distribution with strain. This investigation suggests that the real spacing of sliding grain boundaries should be taken into account for modelling of SP flow.On leave from Ufa Aviation Institute, Ufa 450025, Russia.     

Electron microscopic study of sputter-deposited Ir films

The microstructural development of iridium (Ir) film deposited on isotropic graphite by a sputtering method was investigated using a transmission electron microscope. A columnar structure was developed in which the diameters of columnar grains were distributed from 3 to 50 nm at the substrate temperature of 330 K. Directions of grains were almost perpendicular to the substrate surface, and grain boundaries were wavy. Grains forming the columnar structure indicated different orientations of a growth direction, though growth orientations of grains showed the weak preferred orientation of a [111] direction. Non coincident related boundaries, as well as low angle grain boundaries and twin boundaries tend to be observed more frequently than coincidence lattice related boundaries.     

Magnetically induced abnormal grain growth in pure nickel

The evolution of the microstructure of cold-rolled pure Ni during annealing at 300–800°C for 120?min with and without a high magnetic field of 10?T was investigated. When the direction of the field was either parallel or perpendicular to the sample rolling direction (RD), the average grain size of the cube grains decreased, and abnormally grown grains with random orientation were observed. The migration of different kinds of grain boundaries, including Cube–Cube (C–C) grain boundaries, Cube–Non-cube (C–N) grain boundaries, and Cube–Abnormal (C–A) grain boundaries, was also investigated. The driving force produced by the magnetic field accelerated the migration of C–A grain boundaries, which reasonably explained the formation of abnormal growth.     

Effects of Deformation on the Properties of Emulsion Grains

Emulsion grains have been deformed by bending an emulsion layer coated on film, and the effects of deformation on the dispersity ol photolytic silver and on the number and distribution of development centres have been studied. The observations show thai a major effect of such treatment is to increase the number of sues within a grain at which silver specks can be nucleated.

The usual sensitomelric effect of such changes is to increase sensitivity on internal-image development at the expense of sensitivity on surface-image or conventional development. However, it is shown that surface desensitizalion may be accompanied sometimes by internal desensitization rather than sensitization. The internal desensitization can be attributed to competition between the increased number of internal sensitivity centres. Support for this view was obtained from the fact that changes in the effect of deformation on storage prior to exposure (decrease of surface desensitization. decrease of internal sensitization or desensitization I were associated with a reduction in the number of centres ,il which internal silver could be nucleated.

A further effect of deformation noted with coatings of lodohromide emulsions (but probably general with respect to halide) is to decrease the luminescence intensity at low temperatures, particularly at low intensities of excitation, and to increase the time required for luminescence to reach maximum intensity. These observations show that deformation introduces new recombination centres and electron traps into emulsion grains The connection between the effects of deformation on sensitomelric properties and luminescence properties have been studied as a function of grain size and chemical sensitization in a series of animoniacal emulsions. The findings confirmed that damage to the grains is responsible for the loss of sensitivity on conventional development, and that chemical sensitization, though reducing desensitization hv deformation, has no influence on the grain disorder introduced by this latter treatment. 11 is thought unlikely that the new recombination centres play any significant part in the desensitization by deformation of emulsions chemically sensitized so as lo be al or near optimal speed, but some contribution would appear feasible in the case of emulsions at a low level of sensitization.     

Effects of solution heat treatment on grain growth and degree of sensitization of AISI 321 austenitic stainless steel

This work investigates the influence of solution temperature on grain growth and degree of sensitization of AISI 321 steel. Samples were solution treated at temperatures between 800 and 1,200°C for 80 min and sensitized at 600°C for 105 h. Optical microscopy and double loop electrochemical potentiodynamic reactivation (DLEPR) techniques were used to characterize and evaluate the degree of sensitization. The grain coarsening temperature (T_gc) found was 1,080°C, with occurrence of abnormal or discontinuous grain growth. Samples submitted to solution heat treatment below 1,080°C presented average grain diameter approximately equal to those presented by non-heat treated samples. The sensitization process at 600°C for 105 h became null when the samples were previously solution treated at 800 or 900°C, for 80 min. Sensitized and previously solution treated samples for temperatures greater than 1,075°C presented a decrease in sensitization intensity and an increase in transgranular precipitation.     

Microstructural and electrochemical characterization of the low temperature sensitization of AISI 321stainless steel tube used in petroleum refining plants

This work describes the behavior of ASTM A312 TP321 tubes heat treated at 600°C for periods of 1, 3, 10, 30, 40, 50, 60, 70, 80, 90, and 100 hours. The degree of sensitization that had occurred was assessed by Scanning Electron Microscopy (SEM) and by the Double Loop Electrochemical Potentiodynamic Reactivation test (DLEPR). The results showed that exposure at 600°C for 80 hours or longer caused severe precipitation of chromium carbides along the grain boundaries characterizing the sensitization of the material despite it being titanium stabilized. In order to minimize the sensitization process, solution annealing in the temperature range of 900–1100°C was studied. Solution annealing at 900°C was the best heat treatment in order to prevent sensitization at 600°C. The solution annealing at 900°C was also effective for samples from tubes that were sensitized after one year of operation in a refining plant.     

The origin of varistor property of SrTiO3-based ceramics

The role of oxygen in the heat-treatment process of SrTiO₃ varistor ceramics has been investigated in this paper. The varistor voltage of SrTiO₃ ceramics has been found to be independent of the sample thickness and it increases with the heat-treatment temperature. It has been further revealed that the dielectric property is mainly governed by a highly resistive surface layer. The XPS results of Mn 2p and O 1s suggest that the surface layer is formed by oxygen diffusion and surface chemisorption at grain boundaries during the heat treatment in air. The chemisorption of oxygen in grain boundaries, which leads to the highly resistive surface layer, is the origin of the varistor property of SrTiO₃ ceramics.