Fracture Mechanism Of Steel Research Articles

Evaluating hydrogen embrittlement susceptibility of operated natural gas pipeline steel intended for hydrogen service

Green hydrogen produced by renewable energy sources is expected to play an important role as a decarbonized energy carrier. It can be transported cost-effectively over long distances by pipelines. Hydrogen transport through existing natural gas pipeline networks is supposed to be prospective. However, a possible degradation of gas pipeline steels as a result of their long-term operation should be taken into account in assessing the prospects of using the existing gas network for hydrogen transportation safely. In this study, hydrogen embrittlement susceptibility of low-carbon pipeline steel in the as-received state and after long-term operation is investigated. The influence of long-term operation on the microstructure, mechanical properties, and fracture mechanism of steel is assessed, considering the direction of cutting the specimens relative to the pipe axis. The electrolytic hydrogen pre-charging process with different intensities is used to analyze the influence of absorbed hydrogen on the mechanical behaviour of steel by tensile tests of uncharged and hydrogen-charged specimens. The as-delivered steel is not prone to hydrogen embrittlement after moderate hydrogen charging, while the operated one is sensitive to it. Metallographic and fractographic analyses revealed a crucial role of non-metallic inclusions in the hydrogen embrittlement of steel, which depended on its technical state and hydrogen charging conditions. Moderate hydrogen charging led to intergranular embrittlement of steel, while intensive − to transgranular embrittlement, indicating different mechanisms of transport of hydrogen absorbed by the metal.

In-situ three-dimensional X-ray investigation on micro ductile fracture mechanism of a high-Mn steel with delayed necking effect

This study investigates the micro ductile fracture mechanism of a high-Mn steel with delayed necking effect (DN-HMn steel) under monotonic tension. A random-oriented micro-grain structure cooperating with intragranular twins in the DN-HMn steel contributes to its high tensile strength of 880 MPa and high uniform elongation of over 55%, accounting for 92.8% of the total elongation. Three-dimensional (3D) visualization of the ductile fracture process is presented, and characteristics of void evolution are identified. It was observed that voids distribute and grow uniformly during deformation, with larger voids forming a double-helix spatial distribution pattern at high strain levels, which fits the principal shear stress trajectory. The number and volume of voids in the DN-HMn steel showed a significant increase, with the porosity increasing from 0.071‰ (initial state) to 1.580‰ (before fracture). Intervoid necking was found to be the mode of void coalescence, which occurred close to the necking strain of the sample. The DN-HMn steel demonstrated a significant volume expansion of 4.58% throughout the elongation process, which breaks the volume conservation law of classical metal plasticity theory. This study opens new perspectives for understanding the ductile fracture mechanism of this steel.

The impact toughness and mechanism of low-carbon steel fracture at low temperatures

The goal of the work is to study the effect of severe plastic deformation (SPD) on the impact toughness and fracture mechanism of St3sp low-carbon structural steel within a test temperature range of 293 – 213 K. The issues of deformation processing of steel St3sp billets using SPD method in conditions of the equal-channel angular pressing scheme (ECAP) are considered. The results of low-temperature tests by impact bending of Charpy steel samples in various states are presented. The impact toughness decreased by ~1.3 times as a result of ECAP in 16 passes. It is shown that the temperature dependence of the impact toughness of steel subjected to ECAP differs from that for steel in the delivery condition. A fractographic study of the fracture mechanisms of the steel in the initial state and after processing by ECAP at a test temperature of 293 – 213 K is carried out. It is shown that for the steel in the initial the transition from fracture with the formation of viscous and brittle fracture zones at 293 K to brittle at 213 K occurs through successive expansion of the brittle fracture area with decreasing temperature, whereas for hardened steel, the mixed fracture area appears in the local region at 233 K and expands to the entire cross section of the sample at 213 K. The microstructure formed as a result of ECAP in 16 passes in the temperature range up to 213 K prevents pure brittle fracture and leads to a mixed fracture pattern.

Effect of Heating Rate during Continuous Annealing on Microstructure and Mechanical Properties of High-Strength Dual-Phase Steel

The study concerns the influence of heating rate during continuous annealing on microstructure and mechanical properties of high-strength ferrite–martensite dual-phase (DP) steel. The study suggests that the superheat (ΔT), carbon diffusion and distribution were strongly influenced by increasing the heating rate, leading to different microstructure and mechanical properties of DP steel. The morphology of martensite was observed to shift from network along ferrite grain boundaries to banded structure to uniformly dispersed, and the average ferrite grain size was refined from 5.3 to 2.0 µm. When the heating rate was 300 °C/s, the steel exhibited excellent mechanical properties with tensile strength (UTS) of 982 MPa, total elongation (TEL) of 17.5%, strength–elongation balance (UTS × TEL) of 17.2 GPa%, because of finer microstructure, uniform martensite distribution and low carbon content of martensite. Moreover, the precipitation strengthening effect of NbC is an important reason for the high-strength DP steels. In addition, the variation in strain-hardening behavior and fracture mechanism of steel subjected to different heating rates is discussed in relation to the microstructure.

Effect of Shock and Vibration Preloading on the Deformation and Fracture Behavior of 17G1S-U Steel

The procedure of evaluating the crack resistance of 17G1S-U sheet steel after shock and vibration loading was advanced and experimentally tested using the method of complete deformation diagrams. The technical potential was employed to provide the growth of a mixed mode (I+III) macrocrack on the specimens with an identical central circular opening, which guaranteed the self-similar macrocrack propagation. The complete deformation diagrams displayed initial almost straight descending portion. In real constructions of gas mains, rather long macrocracks can arise after this fracture mode. The advanced procedure permits of reliable assessment of energy variations spent for a (I+III) mode crack propagation under any complex combined loading. The straight descending branch slope of the deformation diagram is established to be used for evaluating crack resistance variations of pipe steel subject to thermomechanical loading. Shock and vibration loading of a high frequency (1–2 kHz) is shown to essentially influence the crack resistance of pipe steel and plastic strain in the vicinity of a stress raiser. The impact of a power pulse on the material is dependent on its prestrain level through static tension and damage of its initial structure correspondingly. The controlling factor influencing the change in mechanical properties is the intensity of the power pulse. Basic fracture mechanisms of steel were established on the basis of examination of specimen fractures with scanning electron microscopy. The shock and vibration loading is evidently accompanied by energy contribution not only to the existing damages of the material but also to the initiating ones, which causes the localization of deformation and growth of pores in their vicinity. Since the energy accumulation can contribute to the modification of the material in the vicinity of those damages, the shape and sizes of ductile tear dimples are the informative parameters for evaluating the strength and plasticity of examined steel.

The effect of aluminum alloying on ductile-to-brittle transition in Hadfield steel single crystal

The ductile-to-brittle transition (DBT) in Fe-13Mn-1.3C (Hadfield steel, I) and Fe-13Mn-2.7 Al-1.3C (Hadfield steel, II) (wt.%) single crystals oriented along \({[011], [{\bar{{1}}}44]}\), and [\({\bar{{1}}11}\)] directions was investigated under tension in the temperature interval of 77 to 673 K. The DBT temperature interval was found to be independent of single crystal orientation. The DBT temperatures were estimated (1) as the mean value between the temperature corresponding to the minimum crystal ductility and the one coinciding with the onset of the plateau of the \({\varepsilon}\)(T)-dependence (TDBT1); and (2) as the temperature where the volume fraction of brittle failure on the fracture surfaces was 50% (TDBT2). The DBT temperatures estimated this way, do not coincide for both steels. Mechanical twinning has been reported as the primary reason for the occurrence of the DBT in austenitic high-carbon Hadfield steel and appears to account for the difference in DBT temperatures as well. Alloying with aluminum partially suppresses twinning in steel (II). Twinning sets in only after a certain amount of dislocation slip, but still influences the fracture mechanism of steel (II).

Fatigue strength and fracture mechanism of steel modified by super-rapid induction heating and quenching

Four kinds of surface hardened-specimens (ordinary structural steel with carbon content of 0.45% C) having hardened thicknesses of 0.7–1.8 mm were prepared using a ‘super-rapid induction heating (SRIH) system’. Rotation bending fatigue tests were performed with special focus on the effect of a hardened thickness on fatigue properties. Measurement of residual stress and observation of the fracture surface were also carried out to investigate the fracture mechanism of the specimen with a shallow hardened layer. It was found that there is not much improvement of fatigue strength at 10 7 cycles for specimens with shallow hardened layers in spite of having a high compressive residual stress of about 1000 MPa. This is because the fatigue crack originating from inside the hardened layer leads to the final fracture of the specimen (internal fracture mode). Improvement of fatigue strength has been achieved on the specimen with thick hardened layers, such as those about 1.8 mm thick. In this case, fatigue cracks originate from inclusions located in hardened layers, which leads to final fracture (hardened-layer fracture mode).

Micromechanisms and toughness for cleavage fracture of steel

A complete understanding of the fracture mechanisms of steel in the ductile/brittle transition region requires analysis not only of crack initiation, but also of crack propagation. This paper reviews micrographic and fractographic experiments that give insight into both phenomena, and suggests a frame-work through which both may be related. Unstable cleavage crack initiation can occur after some blunting of the original fatigue precrack or after some stable crack growth. In either event, instability appears to be triggered by the fracture of a brittle micro-constituent ahead of the precrack. The large scatter in reported K Ic values within the transition region reflects the size distribution and relative scarcity of these “trigger” particles. While a large number of models have attempted to correlate toughness in the ductile/brittle transition regime to events occurring ahead of the crack tip, surprisingly little attention has been paid to events occurring behind the crack front. Fractographic evidence as well as metallographic sectioning of arrested cracks show that the mechanism of rapid crack propagation by cleavage is affected strongly by partial crack-plane deflection which leaves unbroken ligaments in its wake. The tearing of these ligaments by dimple-rupture is the dominant energy-absorbing mechanism. Etch-pit experiments using an Fe-Si alloy show that the crack-tip stress intensity based on plastic zone size is extremely low. It is suggested that the mechanism of crack arrest should be modeled using a sharp crack which is restrained by a distribution of discrete pinching forces along its faces. The same model is applied to crack initiation.