A CRITERION FOR INTERGRANULAR BRITTLE FRACTURE OF A LOW ALLOY STEEL

Abstract— The intergranular brittle fracture behaviour of a A533 B Cl 1 steel was investigated by applying a temper embrittlement heat treatment to simulate the presence of local brittle zones (ghost lines) in the as-received material. This heat treatment produced an embrittled material that failed by an intergranular mode at low temperature. An increase in the Charpy transition temperature and a significant decrease in fracture toughness were observed in the embrittled steel as compared to the as-received material. Tensile tests on smooth specimens and on notched bars were carried out to determine the critical stress corresponding to brittle intergranular fracture and to investigate the scatter in the results. It is shown that the mean value of the critical stress seems to be an increasing function of temperature. A statistical model, the Beremin model, initially proposed to describe brittle cleavage fracture was applied to intergranular fracture. Modifications are introduced in this model to take into account the temperature dependence of the critical stress. It is shown that this modified model is able to predict the temperature dependence of fracture toughness and the scatter in the experimental results.     

07Cr12NiMoVNb耐热钢回火脆性研究

通过冲击试验和断口形貌分析确定07Cr12NiMoVNb耐热钢有回火脆性,并存在二个回火脆性温度区间(450～525℃和575～625℃)。低温回火脆性到高温回火脆性区间的断口形貌从准解理+解理到解理+沿晶断裂,出现沿晶断裂时,冲击值最低。为方便工程上判断是否出现回火脆性,借用了韧/脆转变的(FATT50)概念。     

Statistical scatter in the fracture toughness and Charpy impact energy of pearlitic steel

The effect of microstructure on the fracture characteristics of high carbon hypo eutectoid steel was studied under conditions of quasistatic and dynamic loading. Experimentally determined sets of fracture toughness and Charpy impact energy values were statistically treated. A relationship was found between fracture toughness and Charpy impact energy. In the very brittle domain, the fracture toughness increases slightly with increasing Charpy impact energy. In the domain where the fracture toughness is higher, the rise in fracture toughness with increasing Charpy impact energy is more pronounced. Detailed SEM examination of fractured compact tension (CT) and Charpy V-notch (CVN) specimens showed that the fracture at ambient temperature occurred almost exclusively by cleavage. There were no visible differences in the morphology of cleavage facets on the fracture surfaces of Charpy and CT specimens. Mechanisms of cleavage initiation were revealed by the fractographical investigation of fracture surfaces. Whereas the fracture surfaces of broken CT specimens exhibit a number of cleavage origins, the fracture surfaces of CVN specimens usually show only one.     

Constraint effects at brittle fracture initiation in a cast ferritic steel

The fracture resistance of a cast low carbon manganese ferritic steel intended for containers for spent nuclear fuel has been analysed by combining several approaches. Based on data from three-point bend specimens with shallow and deep cracks the effect of crack tip constraint at brittle fracture initiation has been followed. Q-parameter was used for the constraint quantification. The crack length effect on the fracture toughness–temperature diagram has been analysed and peculiarities of fracture behaviour in the lower shelf region have been explained. The role of cleavage fracture stress in brittle fracture initiation under the influence of crack tip constraint has been analysed.     

The effect of crack propagation mechanism on the fractal dimension of fracture surfaces in steels

The paper reports on the results of a fractal analysis of fracture surfaces of Ni–Cr steel in two different states of heat treatment simulating embrittlement. The change in the fractal dimension of the fracture surface demonstrates a wavy character and dispersion depending on the microstructural state of the tested steel. The results of the fractal analysis in the crack growth direction and across the entire crack front were used as the basis for a reconstruction of the geometry of the fracture surface, providing a new geometric tool for fractographic analysis. The competing effects of transgranular and intergranular brittle fracture may lead to increased roughness of the fracture surface and its fractal dimension. The threshold value of the fractal dimension of the sections perpendicular to the fracture surface, indicating the transition from transgranular to intergranular fracture, is 1.12.     

Characterization of fracture behavior of a Ti alloyed supermartensitic 12%Cr stainless steel using Charpy instrumented impact tests

In this work, the toughness of a Ti-alloyed supermartensitic stainless steel with 12%Cr was evaluated by instrumented Charpy impact tests at − 46 °C. The material was heat treated by quenching and tempering at 500 °C or 650 °C. The temper embrittlement phenomena was detected in the specimen tempered at 500 °C, while the specimens as quenched and quenched and tempered at 650 °C presented a ductile fracture with high impact energy values. The predominance of cleavage fracture instead of intergranular cracks suggests that the temper embrittlement was caused by fine and disperse precipitation observed in the specimen tempered at 500 °C. The dynamic initiation fracture toughness (J_Id) was calculated from the force versus deflection curves using three different methods suggested in the literature to obtain the initiation energy.     

应力与氢对40Cr钢断裂特征的影响

本文利用悬臂梁试验机,测定了40Cr 钢的断裂韧性 K_(1c)和在3.5%NaCl溶液中,充氢达饱和条件下的氢脆临界应力场强度因子 K_(IH),以及在不同的应力场强度因子 K_(Ii)下的裂纹扩展速率 da/dt,并结合 SEM 对断裂形态进行了分析。     

Transferability of Charpy Absorbed Energy to Fracture Toughness Based on Weibull Stress Criterion

The relationship between Charpy absorbed energy and the fracture toughness by means of the (crack tip opening displacement (CTOD)) method was analyzed based on the Weibull stress criterion. The Charpy absorbed energy and the fracture toughness were measured for the SN490B steel under the ductile-brittle transition temperature region. For the instrumented Charpy impact test, the curves between the loading point displacement and the load against time were recorded. The critical Weibull stress was taken as a fracture controlled parameter, and it could not be affected by the specimen configuration and the loading pattern based on the local approach. The parameters controlled brittle fracture are obtained from the Charpy absorbed energy results, then the fracture toughness for the compact tension (CT) specimen is predicted. It is found that the results predicted are in good agreement with the experimental. The fracture toughness could be evaluated by the Charpy absorbed energy, because the local approach give     

STATISTICAL MODELLING OF INTERGRANULAR BRITTLE FRACTURE IN A LOW ALLOY STEEL

The intergranular brittle fracture (IBF) behaviour of a low alloy steel 16MND5 (A508 Cl. 3) was investigated. A temper embrittlement heat treatment was applied to the material to simulate the effect of local brittle zones (ghost lines) which can be found in the as-received material condition. An increase in the Charpy V toughness transition temperature and a significant decrease in the fracture toughness measured on CT-type specimens were observed in the embrittled material, as compared to the reference material which was submitted to the same austenitizing and tempering heat treatment, but which was not subjected to the temper embrittlement treatment. Tensile tests on notched specimens were carried out to measure the Weibull stress and scatter in the results. A statistical model, the Beremin model, originally proposed for brittle cleavage fracture was applied to IBF. It is shown that this model is not able to fully account for the results, in particular for the existence of two slopes in a Weibull plot. Systematic fractographic observations showed that the low slope regime in this representation was associated with the existence of MnS inclusions initiating brittle fracture, while the larger slope was related to microstructural defects. Initiation of IBF from MnS inclusions can occur when the material is still elastically deformed while the second population of microstructural defects is active in the plastic regime. A modified statistical model based on the Beremin model and taking into account these specific aspects is proposed in the framework of the weakest link theory. The parameters of this model are identified from test results on notched specimens. It is shown that this model is able to predict the temperature dependence of fracture toughness and the scatter in the experimental results.     

Fracture behaviour and cleavage initiation in hypoeutectoid pearlitic steel

The paper reports on an investigation of the micromechanism of cleavage fracture in hypoeutectoid pearlitic R7T steel, commonly used for producing railway wheels. The steel possesses extensive Lüders deformation, which somewhat complicates finite element (FE) modelling and analyses of fracture behaviour. Standard Charpy V-notch specimens were used in order to analyse the fracture behaviour at quasistatic and impact loading. Finite element 3D calculations were performed and the elastic-plastic behaviour of notched bars up to the fracture was simulated. Detailed fractographic analysis was carried out on a number of Charpy V-notch specimens in order to investigate the origin site of cleavage fracture initiation and its distance from the notch root. The suitability of the three-criterion micromechanical model (Chen et al. Acta Materialia 51:1841–1855, 2003) for cleavage initiation was verified. The R7T steel under investigation exhibited a cleavage fracture stress of 1,837 MPa. Its independence on temperature evidenced the micromechanism of cleavage fracture to be microcrack propagation-controlled. For the investigated blunt-notched bend bars, an active volume exists ahead of the notch root in which pearlite colony-associated initiation sites are located. The cleavage fracture initiation of the steel is thus governed by the sites lying in the active volume. The active volume is determined by the values of three parameters. A plastic strain lying in interval from to (for the steel investigated from 0.033 to 0.108) is necessary to create a cleavage crack nucleus at any location within the active volume depending on the local pearlite properties. A stress triaxiality parameter ranging from h _min to h _max (from 0.93 to 1.39) is supposed to prevent the blunting process at the site of the cleavage nucleus. Once the main principal stress σ ₁ exceeds the local cleavage fracture stress σ _CFmin, an unstable global cleavage fracture occurs in a blunt-notched bar.     

FRACTURE CHARACTERISTICS OF BOROSILICATE GLASSES REINFORCED BY METALLIC PARTICLES

Abstract— The fracture behaviour of borosilicate glass reinforced by molybdenum and/or vanadium particles has been investigated. For the addition of 5 vol% molybdenum particles, two processing procedures have been tested and the influence of volume fraction of vanadium particles (in the range 2 to 30 vol%) on fracture resistance has been assessed. The use of chevron-notched specimens in three-point bending has been shown to be a reliable method for the evaluation of fracture toughness even at toughness levels of order 0.7 to 1.3 MPa √m. The existence of subtle differences in fracture behaviour of glass-composites having comparable volume fractions of molybdenum particles but obtained by two different processing procedures has been established by statistical treatment of the fracture toughness data. An increase in the volume fraction of metallic particles results in an increase of the fracture resistance and the measured fracture toughness level. Toughening mechanisms which have been identified include both the plastic deformation of particles and the bridging of cracks by ductile particles. Some particle cleavage and debonding has been observed, which indicates that a decrease in particle plasticity, probably induced by processing or due to constraints imposed by the rigid matrix, is responsible for a smaller than expected enhancement of the fracture toughness.     

Effects of loading rate, notch geometry and loading mode on the local cleavage fracture stress of a C–Mn steel

In this work, notched specimens with two notch geometries were tested in two loading modes (four-point bending (4PB) and three-point bending (3PB)) at various loading rates at a temperature of − 110°C for a C–Mn steel. An elastic–plastic finite-element method (FEM) is used to determine the stress distributions ahead of notches. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σ _f is measured. The results obtained and combining with previous studies by the authors show that the local cleavage fracture stress σ _f is closely related to the cleavage fracture mechanism (critical events) in steels. The σ _f values do not change with loading rate, notch geometry and loading mode, as long as the critical event of cleavage fracture does not change at various testing conditions. The σ _f is mainly determined by the steel microstructure, and its scatter is mainly caused by the size distribution of the weakest constituent in steels (ferrite grain or pearlite colony with large sizes and large second phase particles) and the change of the critical events in cleavage process. The σ _f can characterize the intrinsic toughness of steels and may be used in a “local approach” model for assessing integrity of flawed structures. The σ _f values could be measured by both 4PB and 3PB tests.     

Ductile to brittle transition of an A508 steel characterized by Charpy impact test: Part I: experimental results

This study is devoted to the ductile-brittle transition behavior of a French A508 Cl3 (16MND5) steel. Due to its importance for the safety assessment of PWR vessels, a full characterization of this steel with Charpy V-notch test in this range of temperature was undertaken. The aim of this study is to provide a wide experimental database and microstructural observations to supply, calibrate and validate models used in a local approach methodology. Mechanical and fracture properties of the steel have been investigated over a wide range of temperatures and strain-rates. Effects of impact velocity on ductile-brittle transition curve, on ductile tearing and on notch temperature rise are presented and discussed. A detailed study of ductile crack initiation and growth in Charpy specimens is also carried out. From fractographic investigations of the microvoids nucleation around carbide second phase particles, a plastic strain threshold for nucleation is determined for this material. A508 Cl3 steels undergo a transition in fracture toughness properties with temperature, due to a change in fracture mode from microvoids coalescence to cleavage fracture. A systematic investigation on the nature and the position of cleavage triggering sites and on any change in the ductile to brittle transition (DBT) range has been carried out. This leads to the conclusion that manganese sulfide inclusions do not play an increasing role with increasing test temperature as recently mentioned in other studies on A508 Cl3 steel with a higher sulfur content. In a companion paper [Tanguy et al., Engng. Fract. Mech., in press], the numerical simulation of the Charpy test in the ductile-brittle transition range using fully coupled local approach to fracture is presented.     

Fracture behaviour in tempered martensite embrittlement of medium-carbon alloy steels

The effect of alloying additions (nickel and silicon) on the fracture behaviour in tempered martensite embrittlement (TME) has been studied in commercial alloy steels. The fracture behaviour is analysed using the fractographs of the impact specimens tested at various temperatures. In 4140-Ni(4340) steel, where nickel-addition increases the intrinsic matrix toughness, the intergranular brittle type of THE is observed. In 4140-Si(4140 + 2Si) steel, where silicon-addition decreases the intrinsic matrix toughness, the intergranular brittle type of TME is also observed. The occurrence of the intergranular brittle type of TME is attributed to the activation of coarse grain-boundary carbides at the grain boundaries which the relatively high impurity content of commercial alloy steel renders impurer (i.e. weaker), despite relatively low intrinsic matrix toughness in 4140-Si steel.     

Effects of void damage induced by warm prestressing (WPS) on cleavage fracture of notched steel specimens

The effects of void damage induced by warm prestressing (WPS) on cleavage fracture of notched steel specimens were studied by experiments and FEM calculations. The results show that the local stress concentration around the voids promotes the cleavage initiation and decreases the notch toughness and cleavage fracture stress. The fibrous cracks ahead of notch tips caused by the ductile tearing in the WPS obviously raise the normal stress in front of their tips and decrease fracture load and notch toughness. When the beneficial effects of WPS on improving apparent fracture toughness for specimens or structures are used, the loads in WPS need to be limited so that no obvious void damage and ductile tearing are produced in front of defects.     

Fracture toughness of a welded duplex stainless steel

The present work includes fracture toughness testing on 30 and 50 mm thick duplex stainless steel 2205 (22% Cr, 5.5% Ni, 3% Mo, 0.15% N). Base metal and submerged arc weldments (SAW) at subzero temperatures have been tested using full size three point bending. The evaluation of the results has been carried out using J-integral calculations and correlations to Charpy impact values. It is shown that the temperature dependence of the fracture toughness can be described by a transition temperature curve, the master curve. The reference temperature was evaluated to −143 and −101 °C for base and weld material respectively.     

Dynamic fracture toughness of a high strength armor steel

This paper summarizes the results of a research being carried out to determine fracture behavior both in static and dynamic conditions of high strength armor steel Armox500T. In this research, notched specimens were cut to be tested in three-point bending test. Specimens were pre-cracked by flexural fatigue. Thereafter, some specimens were tested in bending up to rupture to determine the static fracture toughness K_IC. To obtain fracture toughness in dynamic conditions, a split Hopkinson bar modified to perform three-point bending tests was used. In this device, displacements and velocities of the specimen were measured, as well as the rupture time by means of fracture detection sensors, glued to the specimens. After that, a numerical simulation of the test was performed by using LS DYNA hydrocode, obtaining stresses and strain histories around the crack tip. From these results, the stress intensity factor history was derived. By using the rupture time, measured by the sensors, the value of the fracture toughness computed was unrealistic. Therefore, the use of a numerical procedure to obtain the rupture time was decided, by comparing experimental results of velocities at the transmission bar with numerical results obtained with several rupture times. With this procedure, the computation of dynamic fracture toughness was possible. The method shows that the measurement of the dynamic fracture toughness is possible without the needs of using crack sensors or strain gauges. It can be observed that fracture toughness of this steel under static and dynamic conditions is quite similar.     

Brittle fracture local criterion and radiation embrittlement of reactor pressure vessel steels

The application of local criteria for predicting brittle fracture of reactor pressure vessel steels is discussed with an emphasis on radiation embrittlement. An association of the radiation-induced damages and the processes of initiation and propagation of cleavage microcracks is analyzed from the standpoint of the local criterion for fracture. Physical-mechanical models are put forward to describe the influence of radiation damages on the cleavage microcrack initiation. The influence of the material hardening caused by neutron irradiation and plastic deformation on the fracture toughness is studied.     

Caustic stress corrosion cracking of a spheroidal graphite cast iron: characterisation of ex-service component

Abstract

This paper discusses observations of features suggesting grain boundary embrittlement ahead of stress corrosion crack tips in ex-service cast iron components exposed to strong caustic environments during Bayers process for alumina processing. The cracks and the neighbouring areas in the ex-service specimens were examined using conventional metallography, SEM, the extraction replica technique in the TEM and Auger electron spectroscopy. In all cases, the cracking was initiated at the surface of the steel exposed to the corrosive environment and it appeared that crack nucleation may have been aided by local stress concentrations and/or zones of local residual stress concentration. The fracture mode was almost exclusively intergranular and the crack path followed ferrite grain boundaries. There was clear evidence of a local zone of grain boundary embrittlement extending ahead of the tip of the major cracks examined. The phenomenon was established by investigating the fresh fracture surfaces produced by extending pre-existing cracks under impact loading at liquid nitrogen temperature. Auger electron spectroscopy of the fresh intergranular fracture surface failed to reveal any evidence of local elemental grain boundary segregation that might account for the observed embrittlement. In the absence of evidence of any other embrittling species on the exposed intergranular facets, there arises the possibility of hydrogen being involved in the embrittlement. The paper discusses hydrogen assisted intergranular cracking, as observed in the case of similar materials, to be the possible mechanism.