Cleavage Fracture Stress Research Articles

Effect of Tempforming Temperature on the Impact Toughness of an HSLA Steel

The fracture behavior of a high-strength low-alloy steel subjected to tempforming at 873 K, 923 K, or 973 K was studied by means of impact and bending tests. A decrease in tempforming temperature promoted the grain refinement resulting in the ultrafine grained lamellar-type microstructure with finely dispersed particles that led to significant strengthening along with an increase in the impact toughness. The tempformed steel samples exhibited Charpy V-notch impact energy well above 100 J at temperatures of 183–293 K due to delamination along the rolling plane during bending, which was attributed to high anisotropy of cleavage fracture stress. Delamination owing to easy cleavage crosswise to the impact direction blunted the primary crack and resulted in the zigzag crack propagation, leading to high impact toughness. Depending on tempforming temperature, three types of the delamination behavior were observed in the V-notch specimens upon three-point bending tests at room temperature. Namely, the early, restrained, and late delaminations took place in the samples after tempering at 873 K, 923 K, and 973 K, respectively. On the one hand, a decrease in the test temperature promoted delamination, and on the other the strengthening by lowering tempforming temperature is accompanied by a suppression of ductile fracture. The sample tempformed at 873 K exhibited the highest impact toughness at room temperature, whereas the samples tempformed at 923–973 K were characterized by the higher impact toughness at 183–233 K.

Elastic-plastic deformation behavior of sapphire M-plane under static loading using nano-indentation

Elastic-plastic response of M-plane single crystal sapphire was explored via a nano-indenter with a Berkovich tip. Elastic-plastic transition was observed with eight different points (ranging from 0.30 to 0.55 mN) subject to respective peak load. Mechanical properties i.e., Oliver-Pharr hardness and elastic modulus were also determined in the onset elastic and elastic-plastic regions. Stable value of elastic modulus estimated from Oliver-Pharr nanoindentation experiments was around 430 ± 15.0 GPa. However, Oliver-Pharr hardness in purely elastic and elastic–plastic (ISE) regions was approximately 2.19 and 2.0 times greater than the non-ISE hardness values respectively. Values of hardness in the non-ISE region were also in compliance with the depth independent hardness calculated through Nix-Gao and proportional specimen resistance models. Additionally, principal stresses and maximum shear stress were estimated on pop-in burst using Hertzian contact theory. Values of the critical resolved shear stress (CRSS) and maximum possible shear strength were also calculated at the first pop-in burst. Moreover, plastic zone size enhanced 1.36 times by shifting of critical load from 0.30 to 0.55 mN. Multiplication of Schmid factor and interplanar spacing indicated two slip systems i.e., prism {011‾1‾} <101‾1‾> and pyramidal {112‾0} <1‾100>, which are verified in the Transmission electron microscopy (TEM) images. Estimated values of maximum contact pressures at respective critical loads were much lower in comparison to phase transformation pressure of sapphire. Maximum tensile stress was also calculated using Hertzian contact theory relations. Obtained value of maximum tensile strength was 3.58 times lower than cleavage fracture stress at the first pop-in.

Stress triaxiality effect on cleavage fracture stress

In this work the effect of stress triaxiality on cleavage fracture stress was investigated by means of micromechanical modelling. A unit-cell model was used to determine macroscopic conditions for brittle fracture, in accordance with the slip-induced cleavage fracture model, because of included particle fracture and consequent crack initiation in the matrix. This approach allowed to correlate the critical fracture stress to the particle strength and the ability of the matrix to undergo plastic deformations. Numerical simulations results indicate that the critical fracture stress is strongly dependent on the stress triaxiality, which justify the variability observed in experiments on different specimen geometry. Additionally, the unit-cell modelling allowed to establish a simple relationship between the expected cleavage fracture stress and stress triaxiality, also considering temperature and strain rate. Finally, the use of the plastic constrain factor (defined as the ratio of the critical fracture stress and the yield stress) allows to obtain a master curve that can be used to predict the expected critical fracture stress at different stress triaxiality, temperature and strain rate in different steel grades and alloys.

Cleavage fracture and microstructural effects on the toughness of a medium carbon pearlitic steel for high-speed railway wheel

The fracture behavior and microstructural effects on the toughness of a medium carbon wheel steel were systematically investigated by using instrumented impact tests. The microstructure of the steel consisted of lamellar pearlite and a small amount of proeutectoid ferrite. The results showed that with the refinement of prior austenite grain size, the pearlite block size decreased and the ferrite content increased, while the pearlite interlamellar spacing kept almost unchanged. The impact toughness was improved in the temperature range from −80 °C to room temperature. Observation of fracture surfaces revealed that the fracture occurred in a brittle manner, which was characterized by cleavage and a small proportion of ductile fracture. However, the toughness was positively correlated to the ductile fracture area. Cleavage cracks were found to be initiated at the sites of proeutectoid ferrite, pearlite colony or pearlite/ferrite interface. The values of macroscopic cleavage fracture stress measured were similar (~1833 MPa) for the specimens with different pearlite block size and ferrite content but the same pearlite interlamellar spacing. The toughness improvement by microstructural refinement in the transition temperature region was attributed to the increase in cleavage crack initiation energy. The cleavage crack propagation energy was, however, relatively low and mainly unchanged. With decreasing the test temperature, the difference in crack initiation energy was decreased, causing the difference in total impact absorbed energy to decrease. The results also suggested that in the lower shelf temperature region where complete cleavage fracture occurred, the toughness was insensitive to the microstructural parameters.

Influence of Nb Addition on Impact Toughness of As-Quenched Martensitic Stainless Steel for Automotive Applications

This study presents how Nb addition allowed improving the Charpy impact toughness of a martensitic stainless steel by comparing a conventional AISI410 (12%Cr-0.1%C) and a 12%Cr-0.1%C-0.1%Nb steel after the same austenitization and quenching heat treatment. Adding niobium decreased the ductile-to-brittle transition temperature by 100°C with respect to the Nb-free steel. To identify quantitative fracture criteria for the two materials, the values of critical cleavage fracture stress were determined by the local approach to fracture, combining low temperature tensile tests on notched specimens and mechanical analysis by the finite element method. The main effects of niobium were to refine the grain size and to promote retained austenite films, resulting in a similar resistance to cleavage crack initiation but in a strong improvement of the ductile-to-brittle transition behavior by increasing the resistance to cleavage crack propagation.

Effects of cooling path and resulting microstructure on the impact toughness of a hot stamping martensitic stainless steel

The present study examined the effect of microstructural characteristics on the toughness properties of a hot stamping martensitic stainless steel. Moderately slow cooling during the martensitic transformation leads to the auto-tempering of the martensite laths and the stabilization of thin austenite films. The amounts of retained austenite and cementite precipitates were quantified for various cooling conditions. Charpy impact toughness tests were performed over a large range of temperatures to characterize the ductile-to-brittle transition. Decreasing the cooling rate from 300 °C/s down to 3 °C/s increased the retained austenite fraction from 0.6% up to 2.6% and decreased the ductile-to-brittle transition temperature by 140 °C. The critical cleavage fracture stress was determined to be around 2400 MPa whatever the cooling rate, by applying the local approach to fracture. However, it has been demonstrated that a higher retained austenite fraction modifies incipient plasticity and decreases the yield stress by 60 MPa. As a result, retained austenite delays cleavage fracture by increasing the strain necessary to reach the critical cleavage fracture stress required to trigger cleavage initiation in the ductile-to-brittle transition domain. In this way, retained austenite plays a determining role to decrease the ductile-to-brittle transition temperature. It is thus beneficial to design cooling rates in order to increase the retained austenite fraction and to improve impact toughness at low temperatures.

Mechanism of Decrease in Impact Toughness in a Low-Carbon MnCrMoNiCu Plate Steel with Increasing Austenitizing Temperature

In order to reveal how microscopic factors affect the toughness and the occurrence of cleavage fracture of a low-carbon MnCrMoNiCu alloyed steel, a series of thermal treatments was performed on the steel employing a thermomechanical simulator. These involved reheating samples at different temperatures (950-1250 °C), producing different prior austenite sizes, followed by a continuous cooling transformation process. The Charpy V-notch toughness was determined, and the effect of austenite grain size on the ductile-to-brittle transition temperatures of the steel was investigated. The microstructural evolution on the austenite sizes was studied, fracture features were characterized, the critical event for cleavage fracture was identified, and the local cleavage fracture stress σf was calculated. The impact toughness decreased as the austenitizing temperature increased. A quantitative relationship between σf and the size of the initial cleavage fracture facet (microcrack nucleus) af in the lathy martensite + bainite microstructure has been developed.

Microstructural Evolution and Ductile-to-Brittle Transition in a Low-Carbon MnCrMoNiCu Heavy Plate Steel

Low carbon MnCrMoNiCu alloyed steels are typically used to produce highly ductile thick plates for offshore structures and bulk shipbuilding. The current study revealed how microscopic factors affect the toughness and the occurrence of cleavage fracture of the steel. In this regard, a series of thermal treatments was performed on the test steel by employing a thermomechanical simulator. These involved reheating samples at different temperatures (1168 K to 1623 K (895 °C to 1350 °C)) producing different prior austenite grain sizes, followed by a continuous cooling transformation process. The Charpy V notch (CVN) toughness was determined, and the effect of the austenite grain size on the ductile–brittle transition-temperatures of the steel was investigated. The microstructural evolution of the austenite grain sizes was studied, fracture features were characterized, the critical event for cleavage fracture was identified, and the local cleavage fracture stress σf was calculated. The CVN toughness and σf were maximized in the steel which was reheated at 1273 K (1000 °C) and containing refined lathlike bainite.

Quantification and microstructural origin of the anisotropic nature of the sensitivity to brittle cleavage fracture propagation for hot-rolled pipeline steels

This work proposes a quantitative relationship between the resistance of hot-rolled steels to brittle cleavage fracture and typical microstructural features, such as microtexture. More specifically, two hot-rolled ferritic pipeline steels were studied using impact toughness and specific quasistatic tensile tests. In drop weight tear tests, both steels exhibited brittle out-of-plane fracture by delamination and by so-called “abnormal” slant fracture, here denoted as “brittle tilted fracture” (BTF). Their sensitivity to cleavage cracking was thoroughly determined in the fully brittle temperature range using round notched bars, according to the local approach to fracture, taking anisotropic plastic flow into account. Despite limited anisotropy in global texture and grain morphology, a strong anisotropy in critical cleavage fracture stress was evidenced for the two steels, and related through a Griffith-inspired approach to the size distribution of clusters of unfavorably oriented ferrite grains (so-called “potential cleavage facets”). It was quantitatively demonstrated that the occurrence of BTF, as well as the sensitivity to delamination by cleavage fracture, is primarily related to an intrinsically high sensitivity of the corresponding planes to cleavage crack propagation across potential cleavage facets.

The effects of Silicon and Copper on microstructures, tensile and Charpy properties of weld metals by refined X120 wire

This paper analyzes the micro-mechanism of decrease of impact toughness with increasing Silicon and increasing Copper in weld metal of a low-alloy high-strength X120 steel. By comparing the microstructures, measuring the extending length of the fibrous crack, and calculating the local cleavage fracture stress σf, and then using the basic principles of the micro-mechanism of cleavage fracture, the essential causes of change of toughness in the weld metal of high-strength steel welded joints are revealed in this work.

Simulating toughness properties under varying temperatures with micromechanical and phenomenological damage models

The sustainable and efficient use of high-strength micro-alloyed steels is of particular interest in today’s industry. Using modern steels with high strength and excellent toughness enables thinner constructions, for example in infrastructure applications, for pressure vessels or pipelines. Sufficient toughness is of special importance to avoid catastrophic brittle failure. Yet, the simplified, conventional design rules prevent a full exploitation of the properties of modern high strength steels. Modern simulation models are able to represent the actual failure characteristics of high strength steels and thereby help to fully exploit the potential of these materials. To include toughness properties, it is essential that the models are able to describe the transition behavior of these steels. The materials’ behavior in the transition region is characterized by a competition of ductile and cleavage failure mechanisms. To simulate the corresponding behavior, it is necessary to adapt damage mechanics models for considering both effects. While numerous studies on the simulation of either ductile or cleavage failure exist, only a limited selection is available on investigations in the transition range. Therefore, the presented study investigates two representatives of the most popular groups of ductile failure models. The Gurson-Tvergaard-Needleman model (GTN) is investigated as a micromechanical model while the Modified-Bai-Wierzbicki model (MBW) is selected as a phenomenological model. Both are combined with the Orowan cleavage fracture model. Investigations are performed on a thermomechanical rolled S355, for which the cleavage fracture stress and the parameters for the ductile failure models were experimentally determined. The study compares simulations of Charpy impact toughness tests to experimental results to determine which model class delivers results that are more accurate. The focus of the investigation is hereby set on the simulation of the lower shelf and the lower transition area. In addition, the efficiency of the simulations performed is also evaluated.

Intergranular/transgranular fracture in the liquid metal embrittlement of polycrystalline zinc

Tensile tests were performed on polycrystalline Zn specimens in contact with liquid Ga, In, In–Bi eutectic alloy, or Sn–Bi eutectic alloy below 450 K. These low-melting-point metals and alloys embrittled Zn specimens when they were in the liquid state. Although single-crystalline Zn is known to be embrittled by liquid Ga, intergranular cracking occurred in polycrystalline Zn in the presence of liquid Ga, while transgranular cracking occurred in the presence of In and Sn. Just above the melting point of Ga, only Ga atoms were found to penetrate along the grain boundaries of polycrystalline Zn. Although all these embrittlers reduced the cleavage fracture stress, only Ga atoms in the grain boundaries significantly reduced the strength of the grain boundaries, resulting in intergranular cracking. For the specimens in contact with liquid Ga, the incidence of transgranular cleavage fractures increased at high temperatures due to the reduction in the number of Ga atoms in the grain boundaries by diffusion into the grains.

High Strength-High Ductility Combination Ultrafine-Grained Dual-Phase Steels Through Introduction of High Degree of Strain at Room Temperature Followed by Ultrarapid Heating During Continuous Annealing of a Nb-Microalloyed Steel

Ultrafine-grained dual-phase (UFG-DP) steel consisting of ferrite (1.2 μm) and martensite (1 μm) was uniquely processed via combination of hot rolling, cold rolling and continuous annealing of a low-carbon Nb-microalloyed steel. Room temperature tensile properties were evaluated and fracture mechanisms studied and compared to the coarse-grained (CG) counterpart. In contrast to the CG-DP steel, UFG-DP had 12.7% higher ultimate tensile strength and 10.7% greater uniform elongation. This is partly attributed to the increase in the initial strain-hardening rate, decrease in nanohardness ratio of martensite and ferrite. Moreover, a decreasing number of ferrite grains with {001} orientation increased the cleavage fracture stress and increased the crack initiation threshold stress with consequent improvement in ductility UFG-DP steel.

A framework for statistical modelling of plastic yielding initiated cleavage fracture of structural steels

The well established consensus that cleavage fracture is preceded by plastic deformation in structural steels implies that plastic yielding is the threshold stress state for a volume element to incur cleavage fracture. An accurate compliance with this consensus underlies the normalisation of cumulative cleavage fracture probability and the justification of constraint effect on cleavage fracture. These understandings lead to the proposal of a framework for statistical modelling of cleavage fracture in structural steels. The framework takes the spatial microcrack distribution into account to formulate the cumulative failure probability model that allows for a pertinent physical interpretation of Weibull statistics, and derives the fracture probability of an elemental volume in conformity with the yielding condition from a set of commonly adopted microcrack size or strength distributions. Alternative approaches to calibrating model parameters are suggested based on frequency analysis of brittle particles as cleavage initiators and on statistical analysis of cleavage fracture stress. The strict adherence to plastic yielding as a prerequisite to cleavage fracture also reveals the probabilistic nature of notch brittleness and ductile-to-brittle transition behaviour.

Impact toughness scattering of bainitic steel in the ductile-brittle transition temperature region

The impact toughness scattering in the ductile-brittle transition temperature (DBTT) region was experimentally examined on mixed and homogeneous grains of low alloy high strength bainitic steel under dynamic loading conditions. The results revealed that the mixed grain microstructure had larger impact toughness scattering than the homogeneous one, and the impact toughness scattering was mainly caused by the scattering in the cleavage fracture stress σf. The value of σf is related to the size of the microcrack formed in the bainitic packet. When a bainitic packet-sized microcrack propagates from one bainitic packet into the adjacent packet, cleavage fracture occurs. The cleavage fracture is controlled by the few coarse packets in the microstructures, and the σf scattering is influenced by the varied distances/relative locations between these coarse packets, and homogenizing the distribution of fine bainitic packet sizes is an effective way to reduce the impact toughness scattering in the DBTT region.

Microstructural evolution and mechanical properties of a novel FeCrNiBSi advanced high-strength steel: Slow, accelerated and fast casting cooling rates

In the current work, three different solidification routes and a two-step heat treatment process were applied to a novel FeCrNiBSi alloy system to introduce a new candidate for advanced high-strength steels. The evolution of the microstructure after solidification, heat treatment, and tensile deformation was characterized using optical and electron microscopy techniques, as well as hardness and room temperature uniaxial tensile tests. The effects of the different solidification routes and heat treatment parameters on the deformation and fracture mechanisms of this steel are discussed. Grain refinement, precipitation hardening, and solid solution as a result of the fast casting cooling rate led to an increase in strength at improved ductility. This result can be explained partly by the less severe stress/strain partitioning at the matrix grain/M2B interfaces and better interface cohesion. Moreover, the stress/strain partitioning characteristics between the matrix grains and M2B led to a higher initial strain hardening rate. The fast casting cooling rate further promoted ductile fracture mechanisms, which is a result of increased cleavage fracture stress. The higher casting cooling rate and two-step heat treatment resulted in a strong increase in formability index, from 8GPa% to 24GPa%, at which the mechanical properties occupy the TRIP envelope. Heat treatment of the fast-cooling specimens led to a small reduction in yield and tensile strength and 22% total elongation percentage improvement (from 10% to 32%).

Effect of Local Crystallographic Texture on the Fissure Formation During Charpy Impact Testing of Low-Carbon Steel

The severity of the formation of fissures (also known as splitting or delamination) on the fracture surface of Charpy impact-tested samples of a low-carbon steel has been found to increase with the decrease in finish rolling temperature [1093 K to 923 K (820 °C to 650 °C)]. Combined scanning electron microscopy and electron back-scattered diffraction study revealed that crystallographic texture was the prime factor responsible for the fissure formation. Through-thickness texture band composed of cube [Normal Direction (ND)║〈001〉] and gamma [ND║〈111〉] orientations developed during the inter-critical rolling treatment. Strain incompatibility between these two texture bands causes fissure cracking on the main fracture plane. A new approach based on the angle between {001} planes of neighboring crystals has been employed in order to estimate the ‘effective grain size,’ which is used to determine the cleavage fracture stress on different planes of a sample. The severity of fissure formation was found to be directly related to the difference in cleavage fracture stress between the ‘main fracture plane’ and ‘fissure plane.’ Clustering of ferrite grains having cube texture promoted the fissure crack propagation along the transverse ‘fissure plane,’ by increasing the ‘effective grain size’ and decreasing the cleavage fracture stress on that plane.

Impact Toughness of 0.2 Pct C-1.5 Pct Si-(1.5 to 5) Pct Mn Transformation-Induced Plasticity-Aided Steels with an Annealed Martensite Matrix

The impact properties of 0.2 pct C-1.5 pct Si-(1.5 to 5) pct Mn transformation-induced plasticity (TRIP)-aided steels with an annealed martensite matrix which had been subjected to isothermal transformation after inter-critical annealing were investigated for potential automotive applications. The impact properties are related to the retained austenite characteristics of the steels. The products of tensile strength (TS) and Charpy impact absorbed value (CIAV) were the same for the 1.5 and 5 pct Mn steels, although the ductile–brittle transition temperature was higher for the latter. The impact properties of the 3 pct Mn steel were worse than these two steels. The high TS × CIAV value for the 5 pct Mn steel at 293 K (25 °C) was mainly caused by the TRIP effect of a larger amount of retained austenite (36 vol pct) and the hardened matrix structure; low retained austenite stability and/or a hard martensite–austenite phase reduced this value. The higher ductile–brittle transition temperature of the 5 pct Mn steel was associated with Mn segregation, a large amount of unstable retained austenite on prior austenitic grain boundaries, and decreased cleavage fracture stress owing to the high Mn content.

Further application of the cleavage fracture stress model for estimating the T 0 of highly embrittled ferritic steels

Abstract The semi-empirical cleavage fracture stress model (CFS), based on the microscopic cleavage fracture stress, σ f, for estimating the ASTM E1921 reference temperature (T 0) of ferritic steels from instrumented impact testing of unprecracked Charpy V-notch specimens is further confirmed by test results for additional steels, including steels highly embrittled by thermal aging or irradiation. In addition to the ferrite–pearlite, bainitic or tempered martensitic steels (which was examined earlier), acicular or polygonal ferrite, precipitation-strengthened or additional simulated heat affected zone steels are also evaluated. The upper limit for the applicability of the present CFS model seems to be T 41J ∼160 to 170 °C or T 0 or T Qcfs (T 0 estimate from the present CFS model) ∼100 to 120 °C. This is not a clear-cut boundary, but indicative of an area of caution where generation and evaluation of further data are required. However, the present work demonstrates the applicability of the present CFS model even to substantially embrittled steels. The earlier doubts expressed about T Qcfs becoming unduly non-conservative for highly embrittled steels has not been fully substantiated and partly arises from the necessity of modifications in the T 0 evaluation itself at high degrees of embrittlement suggested in the literature.

A cumulative failure probability model for cleavage fracture in ferritic steels

The fundamental physical assumption of plastic deformation as a prerequisite to the cleavage fracture event demands to rule out the contribution of elastic stress components to the occurrence of fracture from both the pure elastic zone and the elastoplastic zone. It also commands the threshold stress for cleavage fracture to be no less than the yielding stress. Based on this understanding, a model of the cumulative failure probability for cleavage fracture in ferritic steels is developed. Further, a self-consistent statistical model of cleavage fracture toughness is also derived. The Beremin model does not conform to the normality axiom of probability, as it defies the premise of plastic yielding before cleavage fracture by counting in the elastic stress components inside the plastic zone to calculate the cumulative probability. Examples are given in several homogeneous uniaxial and multiaxial loading conditions to highlight the differences between the Beremin model and the presented model in probability calculation, model calibration, and interpretation of the temperature and constraint dependence of microscopic cleavage fracture stress.