Intercrystalline Cracking Research Articles

低濃度塩化マグネシウム溶液中のＳＵＳ３０４鋼の応力腐食割れ発生

In order to examine SCC of SUS 304 in industrial usage, the double bend-beam tests (four point bending) were carried out in low concentrated MgCl2 solutions under O2 flow, and the relation between SCC and crevice corrosion was examined. The results obtained are summarized as follow:In the double bend-beam test, cracks appeared on both of the sensitized SUS 304 and the solution-treated one under low MgCl2 concentration and low temperature conditions. The concentration and temperature in these experiments were low enough to be comparable to the SCC initiation condition in industries. The inter-crystalline cracks was observed on the sensitized SUS 304.Corrosion took place within the crevice between the double bend-beams. The initiation points of SCC were concentrated near the boundary between the crevice corrosion area and the non-corrosion area, where the passive state film seems to be very unstable. The crevice corrosion is considered to induce SCC initiation by concentration of chlorides and destabilization of passive state film in the crevice.

Behaviour of granites and limestones subjected to slow and homogeneous temperature changes

We have studied the evolution of the texture in two granites and two limestones subjected to slow and uniform temperature change. Each granite has a different grain size and each limestone a different texture: Carrare crystalline limestone and Crepey oolitic limestone. Temperature was varied from 200°C to 700°C. Scanning Electron Microscope observations of different rock samples show that during thermal cycling intercrystalline boundaries in granites widen out progressively and porosity increases. New microcracks appear in crystals between 500°C and 600°C. In Carrare crystalline limestone, intercrystalline cracks appear at temperatures as low as 200°C. Due to its heterogeneous cracks appear at temperatures as low as 200°C. Due to its heterogeneous increasing temperature. Longitudinal wave velocity and intrinsic permeability show good correlation with the Scanning Electron Microscope observations.

Intercrystalline cracking, grain-boundary sliding, and delayed elasticity at high temperatures

The hypothesis of an interrelation between grain-boundary sliding and delayed elasticity in polycrystalline materials at high homologous temperatures is used to investigate the conditions conducive to microcracking. It is known that a material may exhibit cracking activity on attaining a critical delayed-elastic strain corresponding to a critical grainboundary sliding displacement. Experimental data on ice at temperatures >0.9T m are used to verify this concept. The new criterion is then extended to develop simple, selfconsistent equations describing the interdependence of stress, strain, time, temperature, and grain size in predicting the onset of structural degradation due to microcracking and hence possible failure by fracture or rupture. The merit of the theory lies in its ability to forecast explicitly a large number of commonly observed high-temperature phenomena, including superplasticity, brittle-ductile transition, and the stress and temperature dependence of the apparent activation energy for fracture. One derivation makes it clear that cracking occurs when a critical stress depending only on temperature (and independent of grain size) is exceeded. The near constancy of fracture strain in the quasi brittle range can also be predicted

Intercrystalline cracking, grain-boundary sliding, and delayed elasticity at high temperatures

The hypothesis of an interrelation between grain-boundary sliding and delayed elasticity in polycrystalline materials at high homologous temperatures is used to investigate the conditions conducive to microcracking. It is known that a material may exhibit cracking activity on attaining a critical delayed-elastic strain corresponding to a critical grainboundary sliding displacement. Experimental data on ice at temperatures >0.9T m are used to verify this concept. The new criterion is then extended to develop simple, selfconsistent equations describing the interdependence of stress, strain, time, temperature, and grain size in predicting the onset of structural degradation due to microcracking and hence possible failure by fracture or rupture. The merit of the theory lies in its ability to forecast explicitly a large number of commonly observed high-temperature phenomena, including superplasticity, brittle-ductile transition, and the stress and temperature dependence of the apparent activation energy for fracture. One derivation makes it clear that cracking occurs when a critical stress depending only on temperature (and independent of grain size) is exceeded. The near constancy of fracture strain in the quasi brittle range can also be predicted

オーステナイトステンレス鋼における応力腐食初期形態の応力レベル依存性について

There have been many published papers about stress corrosion cracking (SCC) of austenitic stainless steels in chloride aqueous solutions. By paying too much importance to acceleration of cracking, however, many of these studies were made at stress levels high enough to cause plastic deformation. Thus, few were concerned in the nucleation mechanism and propagation characteristic of stress corrosion cracking at low stress levels that were practically important in considering the materials resistance in corrosive environments of austenitic stainless steels used for structural materials. Furthermore, to elucidate the actual mechanism of fracture, it is necessary to have a good understanding of the process of failure itself. But, for example, the fracture morphology that is related to such process has not been studied fully systematically. Moreover, in the previous studies concerning the fracture morphology, the extremely early initial stage of stress corrosion cracking that is substantially important has not been fully discussed and many points are left to be studied in future.Therefore in this paper, stress corrosion cracking behavior at the initial stage of austenitic stainless steel SUS 304 was investigated in boiling 42% magnesium chloride aqueous solution. Nucleation and initial propagation of stress corrosion micro cracks were analyzed both in original surface and in fracture surface from microstructural aspect in a wide stress range from near threshold stress to 0.2% proof stress, where no plastic deformation occurred macroscopically.The main results were summarized as follows:(1) Stress corrosion cracking behavior at the initial stage was dependent upon the stress level. Namely, at higher stress levels, intercrystalline cracking originating from triple grain boundaries was observed, but as the stress level was lowered, it was replaced with transgranular one accompanied with corrosion grooves.(2) Micro crack growth from the small corrosion grooves was an important factor determining the value of the threshold stress of material used.(3) At a stress level near 0.2% proof stress, the stress corrosion crack nucleates from the broken point of the passive film on triple grain boundaries where the material is concentrically strained and it propagates so as to interconnect triple grain boundaries.

高炭素鋼のガスシールドアーク溶接に関する研究 ＩＩＩ 溶接部の機械的性質に及ぼすＳの影響

Mainly mechanical properties of high carbon steel (0.65-0.70%C, 0.008-0.026%S) welds for narrow gap (gap; 10 mm) butt welding in the flat position (build up process; intermittent, continuous) and vertical position (continuous) have been investigated by CO2+20%O2 gas shielded arc welding.The results obtained are as follows:(1) In case of tensile tests of welded joints by butt welding in the flat position (intermittent), excellent tensile properties with break down in weld metal and base metal are obtained for 0.008%S base metal. Tensile properties decrease as sulphur contents increase from 0.014%S to 0.026%S and Fractured position are almost in heat affected zone. For different base metals, tensile properties of welded joints by continuous welding are lower than by intermittent welding and fractured position by continuous welding are almost in heat affected zone. In case of vertical welding, excellent tensile properties with break down in base metal are obtained for different base metals.(2) In case of rotating bending fatigue tests of welded joints by butt welding in the flat position and vertical welding, excellent fatigue strength are obtained for 0.008%S, 0.020%S base metals. In case of welded joints in the flat position for 0.026% S base metal, low fatigue strength are obtained and fractured origin exists at intercrystalline crack in heat affected zone for continuous welding.

Ａｌ‐５％Ｍｇ合金鋳物の電子ビーム溶接に関する一研究

In electron beam welding of Al-5%Mg alloy casting (JISH5202, Class 7 AC7A), effects of factors such as welding conditions, base metal thickness (6, 16 mm), Ti addition (Ti free, 0.12%Ti) and etc. on welding phenomena, metallurgical and mechanical properties of weld zone have been investigated by microscopic observation, thermal cycle measurement, grain size measurement, fatigue test and etc. The results obtained are as follows:(1) In welding of AC7A with 6 mm thickness, arcing phenomena are reduced by proper welding condition and weld zone with sound mechanical properties are obtained. In welding AC7A (Ti) with 6 mm thickness, arcing phenomena are prevented by beam current at 5% down of filament current and weld zone with sound mechanical properties are obtained.(2) In welding of AC7A and AC7A(Ti) with 16 mm thickness, arcing phenomena are prevented by combined conditions such as low power density of beam (ab; 1.2), preheating (130°C) by EB passes and beam current at 5% down of filament current and weld zone with sound mechanical properties are obtained.(3) Intercrystallrne cracks in HAZ are observed with AMA, 16 mm thickness welded joints. Eutectic compositions (mainly Al3Mg2) with low melting point are recognized in intercrystalline cracks in HAZ.(4) Al-5%Mg alloy casting with 0.12%Ti addition shows good weldability owing to refined grain size of base metal and HAZ.

Hot workability of stainless steels: influence of deformation parameters, microstructural components, and restoration processes

The literature on the hot ductility of stainless steels is surveyed. The role of different fracture mechanisms is analysed with the aid of the fracture-map concept, proposed recently by Ashby and co-workers. A low hot ductility is practically always associated with a large portion of intercrystalline cracking. The hot ductility is, in general, lower in the temperature interval 900°-1000°C than 1100°-1200°C owing to the presence of dynamic recrystallization in the latter interval, which prevents grain -boundary cracks from growing. At still higher temperatures, the ductility decreases with increasing temperature owing to a loss of grain -boundary cohesion. In addition to a general tendency of reducing the ductility, the influence of a large grain size and high contents of the alloying elements Mo, N, and Ni is characterized by an increased ‘transition temperature’ from the low- to the high-ductility regime. The dependence of ductility on strain rate is less pronounced than the effect of temperature but, in general, the ductility increases with increasing strain rate owing to the suppression of inter crystalline cracking mechanisms. The strong influence of Mo and N can be explained in term s of their effect on flow stress, recovery, and static and dynamic recrystallization, or the presence of eutectic phases and δ-ferrite. This latter phase gives rise to cracking in γ/δ grain boundaries, and its presence, in most cases, has been reported to reduce the ductility.

Variation de la tension superficielle avec l'ordonnance

A model which relates the surface energy to the degree of long-range order in a ternary alloy has been developed on the basis of the known effect of ordering on intercrystalline cracking during tempering (‘fire cracking’). The precise case of type (A 2B)C has been examined. In this pseudo-binary system all the α sites are occupied by either A or B atoms and the β sites by C atoms when ordering is perfect. The relations developed from the model allow only qualitative deductions to be made on the dependence of surface energy on the degree of ordering given that knowledge of the binding energy of the atoms in contact is either incomplete or non-existent.

Liquid-metal embrittlement in an Al 6%Zn3%Mg alloy

Metallographic and fractographic studies of fracture in an Al6% Zn3% Mg alloy in inert and several low melting-point liquid-metal environments (Hg, Ga, BiPbInSnCd) are described. The liquid-metal environments caused rapid sub-critical intercrystalline cracking in polycrystals. and transcrystalline ‘cleavage’ fracture in single crystals. For ‘cleavage’, fracture surfaces were macroscopically parallel to ¦100¦planes, crack growth occurred in 〈 110〉 directions and extensive slip occurred on ¦111¦ planes intersecting cracks. On a microscopic scale, shallow dimples were observed on ‘cleavage’ fractures, and the size of dimples depended on the distribution of precipitates in a manner which was consistent with nucleation of voids by precipitates ahead of cracks. It is proposed that ‘cleavage’ occurs by localised plastic flow at crack tips and that the effects of liquid-metal environments can be explained on the basis that chemisorption reduces the shear strength of interatomic bonds at crack tips and thereby facilitates nucleation of dislocations at crack tips.

A study of the mechanisms of hydrogen embrittlement in molybdenum

The mechanical properties of polycrystalline molybdenum samples in both the “as annealed” and hydrogenised conditions were studied. The results indicate that hydrogen does not alter the yield stress of samples significantly although it reduces both the ultimate tensile strength and the ductility considerably. Fractographic study of samples shows a tendency to intercrystalline cracking of the hydrogenised molybdenum at low temperatures. In the light of the results, a mechanism involving grain boundary weakening is suggested.

Zn-22%Al合金における結晶粒径の常温塑性挙動への影響

The plastic behaviour at room temperature is investigated on the superplastic Zn-22%Al alloys with grain sizes of 0.1∼4.9 μm. The optimum grain size giving the maximum value of total elongation existed for each strain rate.The strain rate sensitivity m have the maximum value at grain size of about 0.6∼0.8 μm, and the strain rate sensitivity disappears above 4 μm (m\fallingdotseq0). The total elongation increases with increase in m-value at the same grain size.The maximum tensile stress is reached at 2∼5% strain, and the grain boundary sliding of mother phase α′ is observed in these strain ranges. The local elongation after the occurrence of the maximum tensile stress depends mainly on the α-β grain boundary sliding and on the deformation of the grains.In the specimen of optimum grain size, at low strain rate, the total elongation is large because the α-β grain boundary sliding is remarkable, but at high strain rate, the specimen is brittle because the α-β grain boundary sliding does not occur and the intercrystalline cracks of α′-boundary is observed. In the large grain size alloy, the deformation of grains is observed at any strain rate.

Studies of precipitate rich-zones on grain boundaries of Al-Mg-Si alloys after aging

In most age-hardening aluminum alloys, metallographic studies have shown that the extent of precipitation adjacent to grain boundaries is much less than that which occurs in the interior of the grains. The width of these almost precipitate-free regions, which are sometimes called denuded zones, and the extent of solute depletion with them are dependent upon the particular alloy and its aging treatment. It has been observed that these zones are relatively soft with the result that plastic deformation takes place preferentially within them. It has been shown 2-4 that there exists a tendency for intercrystalline cracking in fatigue when such zones are present. It is of interest to note that Broom et al. were able to reduce the incidence of this type of failure in an Al- 4 wt pct Cu alloy by stretching the material 10 pct prior to aging. This was later confirmed also on Al - 10 wt pct Mg alloy.

Observation of ductile intercrystalline fracture of an Al-Zn-Mg-alloy

Localisation of strain at grain boundaries, the mechanism of initiation and propagation of cracks, plane strain elongation at fracture, fracture toughness, and fatigue growth velocities were measured with a precipitation hardening Al-Zn-Mg-alloy. The alloy was produced with grain sizes of 10 and 115 μn and investigated as solid solution, in the under-, peak- and overaged condition. It was found that in the peak aged condition localisation of strain at grain boundaries and plastic intercrystalline cracking dominates. The results are used to define the conditions under which a quantitative model for grain size dependence of fracture toughness can be applied in precipitation hardened alloys.

Low cycle fatigue behavior of inconel 718 at 298 K and 823 K

The cyclic stress-strain response and the low cycle fatigue life of conventionally heat treated Inconel 718 were studied. Fully reversed strain-controlled tests were performed at room temperature and at 823 K. Optical and electron microscopy were used to study the development of deformation and cracking during cycling. A power-law relationship between life-time and plastic strain amplitude was obtained. A substantial decrease in fatigue life occurred as the temperature was increased from 298 to 823 K and as the cycling frequency was lowered from 3 cyclesJmin to 0.3 cyclesJmin at 823 K. At 298 K, for all the strain amplitudes investigated, an initial rapid hardening was followed by softening, while at 823 K only softening occurred. Electron microscopy showed that the precipitates were sheared in the course of cyclic straining and that plastic deformation proceeded by the propagation of planar bands. These bands were identified as twins. Twinning was found to be more abundant at elevated temperatures than at room temperature, especially at lower frequencies. Cracking was generally initiated along the interfaces between these twin bands and the matrix but, at elevated temperatures and low strain rates, intercrystalline cracking took place, as well. The influence of particles shearing and twinning on the cyclic stress-strain response of the material are discussed. The importance of planar deformation and twinning on intergranular cracking is emphasized.

Influence of γ′ precipitation on the creep strength and ductility of an austenitic Fe-Ni-Cr alloy

Stress rupture tests have been carried out at 600 and 700‡C on austenitic iron-nickel-chromium alloys based on Alloy 800 characterized by different Ti/(C + N) ratios (4.5 to 9.5 :1 by weight) and also having significant differences in aluminum and titanium contents. The alloys were solution heat treated at 1150‡C for 15 min prior to testing. At 600‡C, the alloy with the higher Ti/(C + N) ratio and (Ti + Al) content displayed a significantly longer life for each stress level investigated. It is suggested that this behavior is due to homogeneous γ′ precipitation. Because of small amounts of γ′ precipitation in the low Ti/(C + N) ratio, low (Ti + Al) content alloy, the failure strain remained relatively constant with rupture time whereas the failure strain for the higher ratio material decreased with rupture time. At 700‡C, rapid overaging with insignificant amounts ofy’ occurred and the stress rupture lives for the different alloys were found to be independent of the Ti/(C + N) ratio and the (Ti + Al) content. In all cases, intercrystalline cracking was observed.

Susceptibility to Underclad Cracking in Relation to Chemical Composition

Heat-affected zone cracking under the weld-overlay-cladding has been investigated for the combination of 22 kinds of ferritic base metals and electrodes including strip electrodes of austenitic and ferritic stainless steels, Inconel and mild steel, and covered electrodes of austenitic and ferritic stainless steels and Inconel. The results indicate that (1) intercrystalline cracks form on stress relieving under the overlap area of the cladding beads, (2) the susceptibility to cracking depends markedly on the chemical composition of the base metal, and the following equation predicts cracking tendency, ΔG = [Cr] + 3.3 [Mo] + 8.1 [V] − 2, and (3) cracking is avoided or decreased by using cladding material which is similar in thermal expansion coefficient to base metal.

Behavior of a nickel-base high-temperature alloy under hot-working conditions

The behavior of a Ni--Cr--Co base alloy with significant additions of Mo, Ti and Al (Ni-monic 105) under hot working conditions was studied using hot compression tests in the temperature range of 1223 to 1523/sup 0/K and strain rates between 0.38 and 64.3 s/sup -1/. The microstructure of the Nimonic 105 is complex and the matrix contains second phases in the form of Ni/sub 3/ (Ti, Al) dispersion (..gamma..'), various Cr and Ti carbides and titanium cyanonitrides inclusions. However, the results show that above the dissolution temperature of the ..gamma..' phase, the alloy behaves like a single phase nickel-base solid solution from the point of view of steady state flow stress-temperature-strain rate relationships, and the activation energies for hot working and static recrystallization. Under deformation conditions where the ..gamma..' phase is present, as in the case of creep; the activation energy is almost doubled. The hot working temperature range giving sound product is 1280 to 1450/sup 0/K at a strain rate of 0.4 s/sup -1/ and decreases to 1400 to 1480/sup 0/K at a strain rate of 65 s/sup -1/. At temperatures above the higher limit the alloy suffers intercrystalline cracks due to hot shortness and at temperatures below themore » lower limit the alloy suffers transcrystalline cracks due to excessive strain hardening.« less

銅の疲労初期き裂の破面観察

To study the behavior of cracks in the earlier stage of fatigue in copper, observations were made on both longitudinal sections and fracture surfaces using optical and scanning electron microscopes.The observation on longitudinal sections showed that transcrystalline crack paths along the slip bands were predominant at low fatigue stresses, but intercrystalline crack paths were frequent at high fatigue stresses.Two kinds of fracture surfaces were observed beneath the specimen surface; one is lying in a slip band and the other along a grain boundary. The former was usually said to be featureless, but steps of about one micron in the direction of crack growth as well as clear slip traces were found on the flat surface. Matching of the steps on mating fracture surfaces was fairly good in such manner that hills on one surface fit into valleys on the other, but matching of the slip traces was not good. On the smooth fracture surfaces lying along grain boundaries, slip traces were observed in most areas, but clear and regular striations were found in some cases. Such topography observed on the fracture surface of intercrystalline crack was similar to that observed within the interior.The observations above suggest that both unslipping and the local plastic blunting process are responsible for the growth of the crack lying in the slip bands, and that intercrystalline cracks grow in tensile mode even in the earlier stage.

Strain components in high temperature creep of a Cu-30% Zn alloy

The surface measurements of strain components in high temperature creep of a coarse grained Cu - 30% Zn alloy were carried out in order to determine the contributions to creep strain of participating processes—grain interior deformation, grain boundary sliding and intercrystalline void formation. An equation describing the contribution of intercrystalline void formation was developed on the basis of detailed metallographic observations. Grain boundary sliding and grain interior deformation were found to be mutually independent processes, whereas intercrystalline void formation, consisting of intercrystalline cracking preceded by cavitation, represents a process accommodating stress concentrations created by grain boundary sliding and grain interior deformation. The relative importance of these three processes strongly depends on testing conditions. Thus, at applied stresses not exceeding 1.0 kg/mm 2 the contribution to creep strain of grain interior deformation is almost negligible at temperatures lower than about 500°C while it predominates at temperatures higher than about 700°C. The contribution to creep strain of intercrystalline void formation is proportional to the sum of the contributions of grain interior deformation and grain boundary sliding.