Damage accumulation and failure of HY-100 steel

Strain-induced damage accumulation in the form of void volume fractions and number densities has been experimentally characterized for an HY-100 steel subjected to tensile failure over a range of temperatures (−85 °C to 25 °C), strain rates (10⁻³/s to 10³/s), and stress states (stress triaxiality ratios of 0.8 to 1.3). While the strain-induced evolution of damage is relatively insensitive to temperature and strain rate, it increases very rapidly with increasing stress triaxiality. In particular, the large body of void-growth data presented suggests the presence of an initially slow void-growth stage that can be described by a relationship with a form similar to that predicted by Rice and Tracey (but with increased dependence on stress triaxiality). The damage results also indicate a transition to rapid void growth (and imminent coalescence by a void-sheet mechanism) at a critical void volume fraction that decreases slowly with an increasing stress triaxiality ratio. A straightforward analysis, based on the experimental observations, relates the observed experimental dependence of failure strains on stress triaxiality for this steel.     

The effect of microstructural banding on failure initiation of HY-100 steel

Microstructural banding of a hot-rolled HY-100 steel plate was accentuated by cooling slowly from the austenite region, which resulted in alternating layers of soft, equiaxed ferrite, and hard “granular ferrite.” The segregation of substitutional alloying elements such as Ni and Cr was identified as the main cause for the microstructural banding. Such banding induces anisotropic flow behavior at large strains, with deformation constrained by “pancake-shaped” bands of the hard granular ferrite. Tensile tests of circumferentially notched HY-100 specimens were performed in order to explore the stress dependence of failure in the slow-cooled as well as the quenched and tempered conditions. The failure behavior of the slow-cooled, microstructurally banded material exhibited a pronounced susceptibility to a void-sheet mode of failure. However, the absence of carbides within the equiaxed ferrite delays void coalescence and material failure to higher strains than in a quenched and tempered microstructure, despite the increased susceptibility to shear localization.     

The effect of microstructural banding on failure initiation of HY-100 steel

Microstructural banding of a hot-rolled HY-100 steel plate was accentuated by cooling slowly from the austenite region, which resulted in alternating layers of soft, equiaxed ferrite, and hard “granular ferrite.” The segregation of substitutional alloying elements such as Ni and Cr was identified as the main cause for the microstructural banding. Such banding induces anisotropic flow behavior at large strains, with deformation constrained by “pancake-shaped” bands of the hard granular ferrite. Tensile tests of circumferentially notched HY-100 specimens were performed in order to explore the stress dependence of failure in the slow-cooled as well as the quenched and tempered conditions. The failure behavior of the slow-cooled, microstructurally banded material exhibited a pronounced susceptibility to a void-sheet mode of failure. However, the absence of carbides within the equiaxed ferrite delays void coalescence and material failure to higher strains than in a quenched and tempered microstructure, despite the increased susceptibility to shear localization.     

Dynamic fracture initiation behavior of an HY-100 steel

An investigation was conducted into the effects of test temperature and loading rate on the initiation of plane strain fracture of an HY-100 steel. Fracture toughness tests were conducted using fatigue precracked round bars loaded in tension to produce a quasi-static stress intensity rate of ·K₁ = 1 MPa√m/s and a dynamic rate of ·K₁ = 2 × 10⁶ MPa√m/s. Testing temperatures covered the range from -150 °C to 200 °C, which encompasses fracture initiation modes involving quasi-cleavage to fully ductile fracture. The results of toughness tests show that the lower-shelf values of fracture toughness were substantially independent of loading rate, while the dynamic values exceeded the quasi-static values by about 50 pct on the upper shelf. In analyzing these results, phenomenological fracture initiation models were adopted based on the requirement that, for fracture to occur, a critical strain or stress must be achieved over a critical distance. In separate tests, the observation of microfracture processes was investigated using fractography and anin situ scanning electron microscope (SEM) fracture technique. The layered ppearance of the fracture surfaces was found to be associated with a banded structure which generally contains many MnS inclusions, probably resulting in a reduction of the fracture toughness values.     

Failure behavior of heat-affected zones within HSLA-100 and HY-100 steel weldments

The deformation and fracture behavior of simulated heat-affected zones (HAZ) within HSLA-100 and HY-100 steel weldments has been studied as a function of stress state using notched and unnotched axisymmetric tensile specimens. For the case of the HSLA-100 steel, the results for fine-grained, as well as coarse-grain HAZ (CGHAZ) material, show that, despite large differences in the deformation behavior when compared to base plate or weld metal, the failure strains are only weakly dependent on the thermal history or microstructure. Ductile microvoid fracture dominates the failure of the HSLA-100 steel with small losses of ductility occurring in the HAZ conditions only at high stress triaxialities. In contrast, the HY-100 steel is susceptible to a large loss of ductility over all of the stress states when subjected to a severe, single-pass simulation of a CGHAZ. The ductility loss is greatest at the high stress triaxiality ratio in which case failure initiation occurs by a combination of localized cleavage and ductile microvoid fracture.     

Structure-property correlation of submerged-arc and gas-metal-arc weldments in HY-100 steel

Structure-property relationships of two HY-100 steel weldments prepared by submerged arc (SAW) and gas metal arc (GMAW) welding processes using identical heat input (2.2 kJ mm^-1) have been studied. It has been found that submerged arc welded (SAW) HY-100 steel weldments have a lower weld toughness than welds produced by the gas metal arc welding (GMAW) process. Optical, scanning, and transmission electron microscopy were used in conjunction with microhardness traverses to characterize and compare the various microconstituents that are present in the last weld pass of both weldments. TEM examination revealed the presence of coarse upper bainite, B-II bainite, and carbides in a highly dislocated ferrite matrix as well as in ferrite laths in the SAW weldment, while the GMAW weldment exhibited a typical fine low carbon lath martensite, autotempered martensite, and mixed B-II and B-III bainites which occasionally contained small regions of twinned martensite. The measured cooling rate in the SAW was found to be about 40 pct slower than that in GMAW. It was also found in the SAW that the weld metal inclusion number density was about 25 pct greater than that in GMAW. Micro-hardness traverses exhibited significantly lower hardness (about 50 HV) in the SAW weldment compared with GMAW, but the tempered weld metal microhardness in both the weldments was measured about the same, at 250 HV. The ductile-to-brittle transition temperature (DBTT) of both weldments was determined by Charpy impact test. Based on an average energy criterion, the DBTT of the SAW weldment was 323 K (50 °C) higher than that of the GMAW weldment. This difference in fracture resistance is due to the different weld metal microstructures. The different microstructures most probably result from differences in cooling rate subsequent to welding; however, the SAW weld also has a higher inclusion number density which could promote a higher transformation temperature for the austenite.     

Structure-property correlation of submerged-arc and gas-metal-arc weldments in HY-100 steel

Structure-property relationships of two HY-100 steel weldments prepared by submerged arc (SAW) and gas metal arc (GMAW) welding processes using identical heat input (2.2 kJ mm^-1) have been studied. It has been found that submerged arc welded (SAW) HY-100 steel weldments have a lower weld toughness than welds produced by the gas metal arc welding (GMAW) process. Optical, scanning, and transmission electron microscopy were used in conjunction with microhardness traverses to characterize and compare the various microconstituents that are present in the last weld pass of both weldments. TEM examination revealed the presence of coarse upper bainite, B-II bainite, and carbides in a highly dislocated ferrite matrix as well as in ferrite laths in the SAW weldment, while the GMAW weldment exhibited a typical fine low carbon lath martensite, autotempered martensite, and mixed B-II and B-III bainites which occasionally contained small regions of twinned martensite. The measured cooling rate in the SAW was found to be about 40 pct slower than that in GMAW. It was also found in the SAW that the weld metal inclusion number density was about 25 pct greater than that in GMAW. Micro-hardness traverses exhibited significantly lower hardness (about 50 HV) in the SAW weldment compared with GMAW, but the tempered weld metal microhardness in both the weldments was measured about the same, at 250 HV. The ductile-to-brittle transition temperature (DBTT) of both weldments was determined by Charpy impact test. Based on an average energy criterion, the DBTT of the SAW weldment was 323 K (50 °C) higher than that of the GMAW weldment. This difference in fracture resistance is due to the different weld metal microstructures. The different microstructures most probably result from differences in cooling rate subsequent to welding; however, the SAW weld also has a higher inclusion number density which could promote a higher transformation temperature for the austenite. Formerly Adjunct Research Professor with the Materials Engineering Group, Naval Postgraduate School Formerly Graduate Student at NPS     

Structure-property correlation of submerged-arc and gas-metal-arc weldments in HY-100 steel

Metallurgical and Materials Transactions A - Structure-property relationships of two HY-100 steel weldments prepared by submerged arc (SAW) and gas metal arc (GMAW) welding processes using...     

The correlation of microstructure and stress corrosion fracture of HY-130 steel weldments

Microstructure is among the most important factors known to affect the stress corrosion cracking (SCC) susceptibility of medium and high strength steels. Multipass welding can produce various microstructures in the weld and heat affected zone on quite a fine scale, so that intimately mixed fracture modes are observed on SCC specimens of such welds. Performance of these welds mirrors the different SCC susceptibilities of the different microstructures. Detailed metallographic observations have been carried out to demonstrate the correlation between the microstructural features and SCC fracture of HY-130 steel weldments. It is shown that the refined microstructures were most resistant to SCC and the accompanying fracture mode was microvoid coalescence. Those microstructures giving rise to the more brittle fracture modes and thus less resistant to SCC were associated with the columnar/coarse equiaxed grain structures of untempered (or slightly tempered) martensite and/or bainite. These results were used as the basis for suggested welding practice to improve the SCC resistance of HY-130 and other medium strength steel weldments. Formerly Post-doctoral Associate at Carnegie-Mellon University     

The tensile failure of mild steel oxides under hot rolling conditions

The failure of oxide scales formed on mild steel (mass contents of 0.17% C, 0.13% Si, 0.72% Mn) was investigated using a high-temperature tensile test technique over the temperature range 830 – 1150°C. Strain and strain rates used were 1.5 – 20% and 0.02 – 4.0 s^?1, respectively. The scales were 10 – 300 μm thick. The variations of these test parameters were chosen to approximate the tensile loading of the oxidised surface layer at the slab faces just before roll contact at the upper or lower surfaces. Oxide scales formed cannot be assumed either to be perfectly adhering during tensile loading, in the sense of slipping, or to be fully brittle. Two limit modes leading to oxide spallation have been observed which are strongly influenced by the temperature, strain rate and strain. For the first mode initial through-scale cracking occurs followed by initiation and propagation of a crack along the oxide-metal interface between adjacent cracks. The second mode corresponds to the slipping of the oxide scale relative to the metal surface before spallation. The delamination and following slipping of the oxide raft can take place along the interface within the non-homogeneous oxide scale.     

Effect of overaging on mechanical properties and stress corrosion cracking of HY-180M steel

The tensile properties, fracture toughness and stress corrosion cracking (SCC) behavior of HY-180 M steel at 22 °C were studied after final 5 h overaging treatments >510 ≤650 °C. SCC tests were conducted for 1000 h with compact tension specimens in aqueous 3.5 pct NaCl solutions at a noble (anodic) potential of −0.28 V_SHE ( −0.48 V_Ag/AgC1) and a cathodic protection potential of −0.80 V_SHE (−1.0 V_Ag/AgC1). The SCC resistance improved at aging temperatures >565 °C, the most significant improvement being at −0.80 V_She, especially after 650 ° aging whereK _ISCC was raised to at least 110 MPa · m^1/2. However, this was at the expense of mechanical properties. Provided low crack propagation rates of ∼3 X 10⁻¹¹ m/s at −0.80V _SHEmay be tolerated, the best compromise between strength, toughness, and SCC resistance was obtained after 594 °C aging. Under these conditions, stress intensities as high as ∼ 110 MPa · m^1/2 can be used, with a yield strength of ∼ 1150 MPa and fracture toughness of ∼ 170 MPa · m^1/2. The retained austenite content after aging increased with aging temperature up to 25 pct by vol at 650 °C. It appeared to correlate with improved SCC resistance, but other microstructural effects associated with aging may be involved. Formerly Research Associate with theDepartment of Metallurgical Engineering , University of BritishColumbia     

Hydrogen trap states in ultrahigh-strength AERMET 100 steel

Hydrogen (H) trap states and binding energies were determined for AERMET 100 (Fe-13.4Co-11Ni-3Cr-1.2Mo-0.2C), an ultrahigh-strength steel using thermal desorption methods. Three major H desorption peaks were identified in the precipitation-hardened microstructure, associated with three distinct metallurgical trap states, and apparent activation energies for desorption were determined for each. The lattice diffusivity (D _L ) associated with interstitial H was measured experimentally and verified through trapping theory to yield H-trap binding energies (E _b ). Solid-solution elements in AERMET 100 reduce D _L by decreasing the pre-exponential diffusion coefficient, while the activation energy for migration is similar to that of pure iron. M₂C precipitates are the major reversible trap states, with E _b of 11.4 to 11.6 kJ/mol and confirmed by heat treatment that eliminated these precipitates and the associated H-desorption peak. A strong trap state with E _b of 61.3 to 62.2 kJ/mol is likely associated with martensite interfaces, austenite grain boundaries, and mixed dislocation cores. Undissolved metal carbides and highly misoriented grain boundaries trap H with a binding energy of 89.1 to 89.9 kJ/mol. Severe transgranular hydrogen embrittlement in peak-aged AERMET 100 at a low threshold-stress intensity is due to H repartitioning from a high density of homogeneously distributed and reversible M₂C traps to the crack tip under the influence of high hydrostatic tensile stress.     

The influence of the stress state on the plasticity of transformation induced plasticity-aided steel

The influence of the stress state on the plastic deformation of CMnSi, CMnSi(Nb), and CMnAlSi transformation induced plasticity (TRIP)-aided steel has been analyzed. Imposing hydrostatic pressures up to 800 MPa during tensile deformation made it possible to change the stress state of the tensile testing specimens. It was found that the ratio of normal to shear stresses has a pronounced effect on the evolution of the microstructure, the austenite volume fraction change during straining, and the fracture surface morphology. The CMnAlSi TRIP steel, which has the largest uniform elongation and the smallest equivalent strain at fracture in the absence of the hydrostatic pressure, had a more pronounced improvement of all plastic characteristics at increasing hydrostatic pressure. An increased austenite stabilization, brought about by the high hydrostatic pressure, was clearly observed. The austenite stabilization results in a decrease of 20 °C to 25 °C of M _s for an increase of 100 MPa of the hydrostatic pressure. The implications of the observations could be far-reaching for new sheet forming technologies, such as hydroforming, as the full transformation potential is available for crashsensitive structural parts by avoiding the formation of the martensite during forming operations.     

The influence of multiaxial states of stress on the hydrogen embrittlement of zirconium alloy sheet

The ductility of ZIRCALOY *-2 sheets containing 21-615 wt ppm hydrogen has been investigated at room temperature over a range of stress states from uniaxial to equibiaxial tension. Data based on locally determined fracture strains show a decrease in ductility with both increasing hydrogen content and increasing degree of biaxiality of the stress state. Metallographic and fractographic examinations indicate that the embrittlement is a consequence of void nucleation (due to hydride fracture), void growth, and void link-up. The influence of hydrogen content and stress state on each of the sequential stages of ductile fracture is determined. These results indicate that the primary cause for the influence of stress state on the hydrogen embrittlement of the ZIRCALOY sheet is that void link-up is initiated at a much lower critical void density in equibiaxial tension than in uniaxial tension. This appears to be a result of equibiaxial deformation enhancing (a) direct participation of previously unfractured hydrides in providing a fracture path linking up voids and (b) a localized shear instability process which is triggered by the nucleation of voids.     

外加拉应力对13Cr马氏体不锈钢的腐蚀行为影响

采用电化学测试手段(开路电位、交流阻抗谱及动电位极化曲线测试),结合接触角测试及体视显微镜微观形貌观察探究在80 g·L-1Na Cl溶液中拉应力对L80-13Cr马氏体不锈钢钝化膜溶解与再修复机制的影响.结果表明,拉应力大小与L80-13Cr的钝化特性存在正相关关系.随着外加拉应力的增大,L80-13Cr马氏体不锈钢的开路电位负移,电子转移电阻减小,线性极化电阻减小,反应速率随着拉应力的增大而增大.而L80-13Cr马氏体不锈钢在高电位下再钝化形成的钝化区会缩短,自腐蚀电位降低,维钝电流密度增加.接触角测试和体视显微镜微观形貌观察发现,拉应力使得表面接触角减小,不锈钢表面容易发生点蚀.外加拉应力使得L80-13Cr马氏体不锈钢的表面能增加,促进钝化膜的溶解,并且抑制钝化膜的再生,导致材料耐蚀性降低.     

外加拉应力对13Cr马氏体不锈钢的腐蚀行为影响

采用电化学测试手段(开路电位、交流阻抗谱及动电位极化曲线测试), 结合接触角测试及体视显微镜微观形貌观察探究在80 g·L-1 NaCl溶液中拉应力对L80-13Cr马氏体不锈钢钝化膜溶解与再修复机制的影响.结果表明, 拉应力大小与L80-13Cr的钝化特性存在正相关关系.随着外加拉应力的增大, L80-13Cr马氏体不锈钢的开路电位负移, 电子转移电阻减小, 线性极化电阻减小, 反应速率随着拉应力的增大而增大.而L80-13Cr马氏体不锈钢在高电位下再钝化形成的钝化区会缩短, 自腐蚀电位降低, 维钝电流密度增加.接触角测试和体视显微镜微观形貌观察发现, 拉应力使得表面接触角减小, 不锈钢表面容易发生点蚀.外加拉应力使得L80-13Cr马氏体不锈钢的表面能增加, 促进钝化膜的溶解, 并且抑制钝化膜的再生, 导致材料耐蚀性降低.      

The failure of brittle solids containing small cracks under compressive stress states

Brittle materials (ceramics, rocks and ice are examples) may contain a distribution of small, grain-sized, cracks. When loaded in compression, these cracks propagate stably until they interact to give final failure. A model is developed for the growth and interaction of cracks in brittle solids under compressive stress states. A critical stress is required to initiate crack growth: it depends on the initial crack length and orientation, on the coefficient of friction and on the stress state. The cracks then grow in a stable way until they start to interact; interaction increases the stress intensity driving crack growth and leads to instability and final failure. This chain of events is modelled, and the framework of a theory of damage mechanics is suggested.     

Microstructure failure in ferrite-martensite dual phase steel under in-situ tensile test

To investigate microstructure failure in ferrite-martensite dual phase steel,in-situ observations were performed on multiple plate DP800 specimens during uniaxial tensile tests.Microstructure evolution of the observed region was investigated in details.The experimental data showed that micro-cracks in various regions differed in the initiation time,and micro-failures mainly occurred from the locations with typical characteristics of stress concentration (i.e.ferrite interiors,the interfaces of ferrite-martensite grains and the martensite-martensite interfaces).Growth of micro-crack generally experienced the following stages:cracking from martensite boundaries,tiny particles in ferrite interiors,or martensite interiors,propagating in ferrite,bypassing martensite boundaries,or passing through martensite-martensite interfaces,finally ending on martensite boundaries.Martensite was one important source of micro-failure and changed the propagation of micro-cracks significantly.Microstructure deformation was inhomogeneous in the stage of plastic deformation.