Delayed Failure Tests Research Articles

Hydrogen embrittlement characteristics of hot-stamped 22MnB5 steel

Hydrogen embrittlement (HE) characteristics of 22MnB5 steel (U-bent specimen) manufactured using hot-stamping process at various temperatures were experimentally and numerically investigated. Steel resistance to HE was examined through delayed failure tests under static and cyclic loading during hydrogen charging. First, the low cyclic loading caused severe HE, in which a clear difference in the extent of HE was obtained depending on the hot-stamped sample, which directly affected the microstructural characteristics and stress–strain distribution. The hot-stamped samples with large martensite phase showed low resistance to HE compared with those with small martensite phase because of the high concentration of hydrogen trapped in the phase boundaries. Moreover, the dual phase (ferrite and martensite) of the hot-stamped samples reduced their resistance to HE, which is caused by the hydrogen trapped in the laminar-shaped pearlite phase. The resistance to HE was improved by low-temperature heating at 200 °C for 1 h because of the generation of ε-carbides as trap sites as they render the hydrogen non-diffusible. Furthermore, the internal strain in the U-bent sample could accelerate HE because of the high concentration of hydrogen. These results were verified by experimental and numerical analyses. Thus, the hydrogen trapping mechanism was proposed as a valid mechanism for HE in 22MnB5.

Managing Welded Joint Formation at Electron-Beam Welding of Aluminum Alloys of Increased Thicknesses

Abstract—The technology of electron beam welding of homogeneous and dissimilar welds of aluminum alloys 1561, 1560М, and 1980T1 has been developed. Beam scanning along the semielliptical trajectory has secured the quality of the welded parts with fine mechanical properties. The welding scheme and optimal modes have been determined. The results of mechanical and corrosion tests, as well as the results of the delayed failure tests, are presented. The technology is implemented for hydrofoil devices and for the manufacture of shipbuilding machinery components.

Effect of Load Variation on the Transition of Crack Path in Delayed Failure of 12Cr Steel

The crack path of 12Cr martensitic stainless steel tempered at low temperature is intergranular in the case of delayed failure and transgranular in the case of corrosion fatigue. The effect of varying load on the transition of crack path in delayed failure was examined. Delayed failure test was done in pure water under triangularly varying load. The crack path was dependent on load frequency. Higher frequency resulted in transgranular crack and lower frequency resulted in intergranular crack. There was a threshold frequency below which intergranular crack could appear. An insertion of a few cycles of rapid loading into the low frequency loading which usually resulted in intergranular crack caused the transition into a transgranular crack.

Effect of heat treatments on the hydrogen embrittlement susceptibility of API X-65 grade line-pipe steel

Delayed failure tests were carried out on hydrogen charged API X-65 grade line-pipe steel in as received (controlled rolled), normalized, and quenched and tempered conditions. The resistance to hydrogen embrittlement was found in the order of controlled rolled > quenched and tempered > normalized. The fracture mode in the hydrogen embrittled steel was ductile.

Evaluation of threshold stress intensity in high strength hydrogen charged steel in low strain rate tension tests

AbstractThe fracture ductility of high strength steel is strongly influenced by the presence of hydrogen, although hydrogen does not significantly affect the yield strength. The deterioration of fracture ductility is particularly evident in low strain rate tension tests, but less pronounced at conventional crosshead speeds. Low concentrations of hydrogen in high strength steels do not substantially affect the fracture toughness, but result in the appearance of a threshold stress intensity. The threshold values can be obtained from low strain rate tension tests at a crosshead speed of 0·1 mm min−1. These values are practically the same as those obtained from hydrogen charged peripherally notched pre cracked specimens subjected to a constant static load in a delayed failure test. Microfractographic investigations of fracture surfaces of hydrogen charged steel from low strain rate tension tests indicate that the growth and the coalescence of voids in the final stages of the fracture process are partly assist...

Slow crack growth in Si3N4 at room temperature

Tests of Si3N4 hot pressed with various types and levels of oxide additives show evidence of room temperature slow crack growth in delayed failure tests (using natural flaws), but not in fracture mechanics (e.g. DCB or DT) tests consistent with more limited literature data for these two types of tests. Neither type of test showed slow crack growth in either CVD Si3N4 or RSSN. Further the fracture mode in the latter two materials was essentially all transgranular, while it was predominantly (e.g. 80%) intergranular in the hot-pressed materials. It was thus postulated that (1) the oxide grain boundary phase is responsible for slow crack growth and; (2) varying distribution of the oxide boundary phase and grain boundary character result in sufficient boundaries not susceptible to slow crack growth to pin cracks with macroscopic crack lengths (i.e. as in DCB and DT tests). Both the much smaller crack front lengths and the large number of small (natural, e.g. machining) flaws allows some of these small flaws to grow to critical size, thus leading to delayed failure in the hot-pressed materials.

Slow crack growth in Si3N4 at room temperature

Tests of Si3N4 hot pressed with various types and levels of oxide additives show evidence of room temperature slow crack growth in delayed failure tests (using natural flaws), but not in fracture mechanics (e.g. DCB or DT) tests consistent with more limited literature data for these two types of tests. Neither type of test showed slow crack growth in either CVD Si3N4 or RSSN. Further the fracture mode in the latter two materials was essentially all transgranular, while it was predominantly (e.g. 80%) intergranular in the hot-pressed materials. It was thus postulated that (1) the oxide grain boundary phase is responsible for slow crack growth and; (2) varying distribution of the oxide boundary phase and grain boundary character result in sufficient boundaries not susceptible to slow crack growth to pin cracks with macroscopic crack lengths (i.e. as in DCB and DT tests). Both the much smaller crack front lengths and the large number of small (natural, e.g. machining) flaws allows some of these small flaws to grow to critical size, thus leading to delayed failure in the hot-pressed materials.

Effect of Residual, Impurity Elements and Microstructure, on the Mechanical Properties and the Crack Growth Rate in Delayed Failure Test of Martensitic Fe-Ni Alloy

Effect of Residual, Impurity Elements and Microstructure, on the Mechanical Properties and the Crack Growth Rate in Delayed Failure Test of Martensitic Fe-Ni Alloy

Effect of boriding surface treatment on hydrogen embrittlement of UNI 30NiCrMo12

The influence of boriding on hydrogen embrittlement was studied by means of delayed failure tests performed on borided UNI 30NiCrMo12 steel. The samples were borided at 900°C for 8 hours with powders ( B 4 C=30%, KBF 4=5%, SiC=65% and quenched at the end of the thermochemical treatment. Hydrogen was introduced into the samples under galvanostatic conditions (d.c. 10 mA/cm 2) by means of cathodic charge in a 0.1 N sulphuric acid solution kept at a constant temperature of 22° C±1. The tests carried out on borided specimens showed an appreciable increase in both the threshold load value and the crack incubation times with respect to the untreated steel. This behaviour is due to the barrier effect of the borided layer. With this coating, hydrogen entry into the base metal was delayed. As a consequence the critical hydrogen concentration in a potential fault was reached later and the crack nucleation and the fracture propagation were delayed. Furthermore the load needed to cause failure within a fixed time was higher for borided samples than for untreated ones.

Factors in hydrogen embrittlement of high strength steels

The effect of pH and current density on a hydrogen induced fracture on UNI 39NiCrMo3 steel was studied using the delayed failure test at a constant load. The hydrogen was introduced into the material by means of cathodic polarization in a sulphuric acid solution at different pH values. The current density imposed on the samples during the embrittlement test ranged from 5 to 20 mA/cm 2. The results obtained showed a noticeable influence of the pH and current density on threshold load values and crack incubation times. This behaviour was attributed to the effect of the pH and current density on rate at which atomic hydrogen adsorbs on the material surface.

Study of the effect of heat treatment on hydrogen embrittlement of AISI 4340 steel

Delayed failure tests were performed on fully-quenched AISI 4340 steel tempered at 500 and 700° C, subjected to sustained tensile loads and cathodically charged with hydrogen (current density 10 mA cm−2) in an aqueous solution of 0.1 N sulphuric acid. The aim was to study the effect of the microstructure on the behaviour of steel towards the embrittling action of hydrogen. The tests were carried out with two different research techniques in order to highlight this behaviour better. This was done with a view to making the use of the steel safer. With the methods used the results obtained, although substantially different from each other, showed that thermodynamically more stable structures are less sensitive to the phenomenon of hydrogen embrittlement.

Evaluating the susceptibility of steels to hydrogen embrittlement using statistical analyses

Delayed failure tests of hydrogen-charged specimens result in a large scatter of failure times and failure stresses, analogous to the brittle behaviour exhibited by ceramics. Because of the considerable variation in results, a large number of specimens have to be tested in order to gain an indication of the performance of a particular material; when two materials of similar susceptibility to hydrogen embrittlement are tested, interpretation of the results is often difficult. In the present investigation, seven similar steels were tested in the delayed failure mode to determine their relative susceptibilities to hydrogen embrittlement. These steels were of similar composition and microstructure and showed a low susceptibility to hydrogen embrittlement. The results were analysed using established statistical analysis methods, and by a modified statistical analysis which was developed to describe the failure behaviour. The results of the delayed failure tests and the statistical analyses are presented.

The effect of temperature on hydrogen induced fracture in UNI 40NiCrM07 steel

The influence of the temperature on hydrogen embrittlement in UNI 40NiCrMo7 steel was evaluated by means of delayed failure tests. The tests were carried out in the temperature interval 2°–60°C using samples which had undergone two different heat treatments. Hydrogen was introduced into the samples in intensiostatic conditions (d.d.c. 10mA/cm 2) by means of cathodic charge in a 0.1N sulphuric acid solution thermostatically maintained at the test temperatures. The results obtained showed a noticeable influence of the temperature on threshold load values and crack incubation times in the tests carried out on quenched material, whereas this influence was found to be negligible in the tests performed on tempered material. This different behaviour was attributed to the effect of the temperature on movement kinetics in the dislocations, responsible for the accumulation and transport of hydrogen into the zones of maximum stress, and on hydrogen diffusion in the metallic lattice.

Bending fatigue tests of sintered silicon nitride at room temperature.

For strength design of ceramic parts, static bending fatigue tests, namely delayed failure tests, and cyclic bending fatigue tests were carried out at room temperature on smooth specimens, hourglass type specimens and notched specimens of sintered Si3N4. (1) Cyclic bending fatigue strength (maximum stress) at stress ratio R=-1 is lower than static bending fatigue strength. It is considered that this is due to the effect of cyclic stress rather than the effect of compressive stress in alternating stress and the difference of effective volume. (2) Both static bending strength and rotating bending fatigue strength of a notched specimen reduced almost at the rate of a stress concentration factor. (3) In case of cyclic fatigue, the mark made by rubbing against each other was observed on the fracture surface of some specimens.

Influence of impurities and microalloying on the delayed failure resistance of steels

The mechanical properties and delayed failure resistance of high-strength bolts made of steels of different degrees of purity and microalloyed with additions of titanium, boron, and niobium were studied. Bolts made of 40Kh steel, 40Kh-PV steel, and 20Kh2NMTRB steel were used for the investigation. Three-point bend tests were conducted and fracture toughness and the work for crack origin and propagation were determined from the test results. Delayed failure tests were made under conditions of cathodic polarization and hydrogen impregnation. Results of the investigations were comparatively analyzed and relations between the failure resistance, structure, composition, and impurity content of the microalloyed steels were established.

The effect of prior-austenite grain size on the hydrogen embrittlement susceptibility of UNI 40NiCrMo7

Delayed failure tests were carried out on UNI 40NiCrMo7 steel subjected to sustained tensile loads and cathodically charged with hydrogen ( d. d. c. = 8 mA/cm 2), in a 0.1 N sulphuric acid solution. Our aim was to study the effect of austenitising heat treatment for different times and temperatures on the failure of UNI 40NiCrMo7 steel. The grain size obtained with different heat treatments does not influence the threshold load, whereas a noticeable influence is recorded for the failure times.

Hydrogen embrittlement of ASTM A 203 D nuclear structural steel

The influence of hydrogen on the mechanical properties of ASTM A 203 D nuclear structural steel has been studied by tension, bend and delayed-failure tests at room temperature. While the tension tests of hydrogen charged unnotched specimens reveal no change in ultimate strength and ductility, the effect of hydrogen is manifested in notched specimens (tensile and bend) as a decrease in ultimate strength (maximum load in bend test) and ductility; the effect increases with increasing hydrogen content. It is observed that for a given hydrogen concentration, the decrease in bend ductility is remarkably large compared to that in tensile ductility. Hydrogen charging does not cause any delayed-failure upto 200 h under an applied tensile stress, 0.85 times the notch tensile strength. However delayed failure occurs in hydrogen charged bend samples in less than 10 h under an applied bending load of about 0.80 times of the uncharged maximum load. Fractographs of hydrogen charged unnotched specimens show ductile dimple fracture, while those of notched tension and bend specimens under hydrogen-charged conditions show a mixture of ductile dimple and quasi-cleavage cracking. The proportion of quasi-cleavage cracking increases with increasing hydrogen content and this fracture mode is more predominant in bend specimens. The changes in tensile properties and fracture modes can reasonably be explained by existing theories of hydrogen embrittlement. An attempt is made to explain the significant difference in the embrittlement susceptibility of bend and tensile specimens in the light of difference in triaxiality and plastic zone size near the notch tip.

Crack Propagation Assisted with Hydrogen under Cyclic Stress in Ni–Cr–Mo Steel

Hydrogen cracking tests under cyclic stress were carried out on hydrogen pre-charged specimens, and the effect of stress ratio on the relation between frequency f and the decreasing rate of crack propagation velocity 1−β was investigated. 1−β equals {(da⁄dt)S−(da⁄dt)R}⁄(da⁄dt)S, and (da⁄dt)S and (da⁄dt)R are the crack propagation velocities under static and cyclic stress, respectively. The results were analyzed by using an asymmetrical internal friction model proposed.There appear two peaks on a 1−β vs. f curve, as they do in a delayed failure test under cyclic stress in water, i.e. this phenomenon is controlled by the diffusion and concentration process of hydrogen atoms to the triaxially tensile stressed position near a crack tip. With an increase in stress ratio R=Kmin⁄Kmax, the two peaks approach each other, the height of peaks decreases, and the frequencies at which two peaks appear shift to the higher frequency range.The existence of two peaks on a 1−β vs. f curve, and the characteristic change in this curve with a change in the stress ratio can be explained by assuming the interaction between hydrogen atoms and the cyclic moving of triaxially stressed position. Furthermore, 1−β vs. f relations calculated by using the asymmetrical internal friction model were in good agreement with the experimental tendencies.

Ni-Cr-Mo鋼の水素脆化割れにおけるき裂の枝分かれ

Delayed failure tests in air were carried out on the Ni-Cr-Mo steel specimens which were hydrogen pre-charged after quenching from 1123 K and tempering at 637 K, in order to investigate the effect of the crack propagation rate da⁄dt (which expressed the concentration of hydrogen inside the specimen) on the stress intensity factor at crack branching KIB and the crack branching angle. The results were compared with those of the specimens no hydrogen pre-charged and tested in water.For the specimens pre-charged and tested in air, the crack branching occurred when the crack propagation rate da⁄dt was higher than 20×10−6 m/s and KIB decreased with increase in da⁄dt, while when da⁄dt was lower than 20×10−6 m/s, the unstable crack propagation occurred without crack branching. The branching angle of the pre-charged specimen tested in air was smaller than that of the no pre-charged specimen tested in water. These results can be explained both by a criterion which was introduced by a combination of hydrostatic tensile stress with hydrogen concentration at crack tip and by the difference in hydrogen diffusion path between the no pre-charged specimen tested in water and the pre-charged specimen tested in air.

Ni-Cr-Mo鋼の遅れ破壊におけるき裂の伝ぱと枝分かれにおよぼす水素チャージの影響

Delayed failure tests were carried out on Ni-Cr-Mo steel quenched and tempered at 673 K in a NaCl aqueous solution in hydrogen charging condition, and the crack branching as well as crack propagation behaviors were investigated.The crack propagation rate da/dt increases with increase in cathode charging current density, i, and da/dt increases to a maximum and then decreases with increase in the increasing rate of stress intensity factor, \dotK. The stress intensity factor at crack branching, KIB, decreases to a minimum and then increases with increase in i. These results can be explained by interaction between dislocations and hydrogen atoms, such as dislocation trapping and dislocation transport of hydrogen on the slip planes near crack tip.