Cleavage Initiation Sites Research Articles

Effect of V, Nb, and Ti microalloying on low–temperature impact fracture behavior of non–quenched and tempered forged steel

In this study, three kinds of non-quenched and tempered forged steels with V, V–Nb and V–Nb–Ti microalloying elements were designed. The microstructure and low–temperature impact fracture behavior of the three steels were comprehensively compared and analyzed. The results showed that the addition of Nb and Ti elements can significantly refine the grains and form precipitated particles to improve the strength of the steel. The fracture behavior of the three steels types at a low temperature of −28 °C was dominated by brittle cleavage fractures. V–Nb–Ti steel exhibited the worst low–temperature impact toughness. The SEM analysis result showed that coarse (V, Nb, Ti) (C, N) particles present in the steel acted as local cleavage initiation sites. No cleavage fracture caused by (V, Nb) (C, N) particles occurred in the V–Nb steel. The results showed that the high impact energy (70.4 J) at room temperature and low ductile–brittle transition temperature (DBTT, −20 °C) of the V–Nb steel were mainly attributable to the grain refinement of Nb. The micron–sized coarse (V, Nb, Ti) (C, N) particles were the main factor responsible for the low impact energy (35.5 J) at room temperature and high DBTT (−7 °C) of V–Nb–Ti steel and can act as the local cleavage initiation sites to dominate the cleavage fracture at low temperature and decrease the impact toughness.

Effects of microstructure and specimen size on the fracture toughness of EH47 shipbuilding steel at low temperature

This study investigated the effects of microstructure and specimen size on the fracture behavior of EH47 shipbuilding steel at low temperature. The fracture toughness (characterized by the Crack Tip Opening Displacement, i.e., CTOD) was evaluated using compact tensile specimens with various microstructures and sizes at − 60 °C. The results revealed that due to inhomogeneous microstructure along the steel plate thickness direction, fracture toughness decreased as the sampling location shifted from the surface to the center of the steel plate. Furthermore, the fracture toughness values of 19, 25, and 38 mm-thick specimens taken from the center of an 80 mm-thick steel plate remained almost constant. Based on the analysis of fracture surfaces, metallography and the characteristics of cleavage initiation site, it was found that grain size had significant effect on the cleavage fracture toughness. When the microstructure was strongly inhomogeneous distributed along the thickness direction and tested at the properly low temperature range, the influence of the microstructure on the fracture toughness was more significant than that of the specimen size. In this condition, small specimens taken from the central location of steel plate can be used to estimate the fracture toughness of specimens with full thickness.

Effect of heat treatment on the impact toughness and brittle fracture initiation mechanism of a quenched and tempered nuclear Pressure Vessel Steel

The effect of heat treatment conditions on the tensile properties, impact toughness properties and the brittle fracture initiation mechanisms of a low-alloy steel were investigated. Only one type of location for the cleavage initiation sites, common to all cooling rates, was identified. Cleavage crack initiation always occurred in the very vicinity of a high angle boundary. MnS inclusions and Ti(CN) precipitates may play a role, but they did not seem to be necessary for cleavage initiation. The critical stress for cleavage increased with the cooling rate and decreased with the tempering parameter. This is consistent with the observed differences in the ductile-to-brittle transition behavior with the heat treatment conditions.

Fracture behavior of reactor steel 20MnMoNi 55 in the transition temperature region

The fracture represents the final phase of crack propagation, and the causes of their occurrence or partial crack propagation in the real constructions made of ferritic steels can be different. Along with cracks, influences such as stress concentration and low temperature contribute to fracture being “faster” and have an impact on the fracture mechanics parameters, as well as on the fact that fracture is more likely to occur due to cleavage. The parameters discussed in this study are Jc and rc (parameter, i.e. distance between fatigue crack tip and cleavage initiation site) by testing of C(T)50 specimen in the transition temperature region. Grooved C(T) specimens were made of reactor steel 20MnMoNi 55 with two thickness values, since an additional goal of this study was to present the influence of thickness of tested specimens. Along with it, other parameters that have influence on the understanding of transition temperature region and fracture in this region of ferritic steel are presented.

Role of bainitic microstructures with M-A constituent on the toughness of an HSLA steel for seismic resistant structural applications

Fracture toughness of the coarse-grained heat-affected zone (CGHAZ) of fusion welded HSLA steel is severely reduced at low temperatures with the combination of deleterious microstructures such as bainitic ferrite (BF), granular bainite (GB), polygonal ferrite (PF), and the martensite-austenite (M-A) constituents. Toughness is one of the basic requirements for seismic resistant steels. The three heat input conditions (different cooling rate) were thermo-mechanically simulated, and the combined effect of M-A constituents with bainitic microstructures on toughness was examined. As the heat input changes from low to high, a combination of acicular ferrite (AF) and BF microstructures changes to a combination of PF, GB and M-A constituent. The former microstructure combination provides better toughness (ductile), whereas the latter makes the samples more brittle. The coarse BF, GB, and PF microstructures with low angle grain boundaries (LAGBs) make them brittle by easy crack propagation. Moreover, harder microstructures such as M-A can potentially act as cleavage initiation sites. However, a high frequency of high angle grain boundaries (HAGBs) in the AF microstructure can suppress the crack propagation, which turns the fracture to completely ductile. Also, it is observed that the M-A constituent in CGHAZ was abundant in martensite rather than the austenite.

Effect of minor Zr element on microstructure and properties of Fe-16Cr-2.5Mo damping alloys

The Fe-16Cr-2.5Mo damping alloy was microalloyed with different amount of Zr element ranging from 0% to 0.5%. In this study, the effects of the different amount of Zr on the microstructure, mechanical properties and damping property of Fe-16Cr-2.5Mo alloys were investigated. The results suggested that the mean grain sizes of the Fe-16Cr-2.5Mo alloys with 0%, 0.1%, 0.3% and 0.5% Zr addition were 440 μm, 285 μm, 155 μm and 98 μm, respectively. The grain refinement strengthening was caused by Zener pinning and it leaded to improving the strength and elongation of the alloys. In addition, the Fe-16Cr-2.5Mo alloy with 0.1% Zr amount exhibited an impact toughness of 296J, which was much higher than those (<12J) of the alloys with more Zr element or without. On the other hand, Zr (Fe, Cr)2 or Zr carbide precipitations (>0.5 μm) formed in the alloys notablely, based on the results of TEM and EDX. The large precipitations were apt to establish the cleavage initiation sites, thus damaging the impact toughness of the alloy. The maximum internal friction (Q−1) of the alloys fluctuated within 0.016–0.014 with the increase of Zr content at a low strain amplitude (9 × 10−6∼3 × 10−4). However, their damping properties are still marvelous.

Weibull distribution of brittle failures in the transition region

The Weibull stress is a well-known means of predicting the likelihood of weakest link brittle fracture that has been shown to accurately model the behavior of ferritic steels in the lower transition region. Weibull stress is based on its use of a two parameter Weibull distribution, a commonly used distribution in probabilistic engineering. The distribution is defined by a shape parameter, the Weibull modulus, and a scaling parameter.In the lower transition region, the Weibull modulus is relatively insensitive to temperature and the likelihood of failure can readily be defined by assuming it is constant and scaling the distribution with a ‘measured’ scaling parameter. This assumption, however, does not hold as the temperature increases into the upper transition zone and becomes less accurate as the upper shelf is approached.This manifests itself as a broadening of the failure distribution that can be attributed to the increased size of the plastic zone ahead of a defect, which in turn ‘samples’ more potential failure sites, while simultaneously increasing the likelihood of blunting these sites and initiating ductile tearing. Thus, while more potential cleavage initiation sites are sampled, the likelihood of an individual defect causing failure is reduced.This paper details the changes in the Weibull modulus and Weibull stress calculated from the ‘Euro’ fracture toughness data. The differences in the Weibull modulus and the mechanistic reason for these differences are explored to enable greater understanding of the factors that influence fracture toughness in the upper transition regime.

Effect of TiN Particles and Grain Size on the Charpy Impact Transition Temperature in Steels

The toughness of ferritic steels is influenced by the grain size distribution, second phase, precipitates and coarse inclusions. In this work an examination of the effect of coarse TiN particles (>0.5 μm) and ferrite grain size on the Charpy impact transition temperature in high strength low alloyed steels has been carried out. Steels with high Ti content (up to 0.045 wt%), have been heat-treated and furnace cooled to obtain a ferrite-pearlite microstructure with different ferrite grain sizes. Coarse TiN particle size and ferrite grain size distributions have been measured and Charpy impact testing has been carried out. Scanning electron microscopy (SEM) analysis has been used to measure the grain boundary carbide thickness and to determine if the coarse TiN particles are acting as cleavage initiation sites by fractographic analysis. The Charpy ductile-brittle transition temperatures (DBTT) have been predicted using standard literature equations, and compared to the measured values. The relationship between the ferrite grain size and coarse TiN particle size and number density in terms of whether the coarse TiN particles act as effective cleavage initiation sites is discussed in this paper.

Cleavage fracture toughness of tempered martensitic Ni–Cr–Mo low alloy steel with different martensite fraction

Abstract This study investigates the relationship between fracture toughness behavior and phase fraction using the master curve approach for SA508 Gr.4N Ni–Cr–Mo low alloy steel. The different phase fractions of the model alloy were obtained by controlling the cooling rate from the austenitization temperature to transformation temperature. The fracture toughness tests were carried out using ASTM E1921-09c, standard master curve method. Yield strength and the impact toughness at room temperature are improved when increasing tempered martensite fraction, but the transition region narrowed in a fully martensitic microstructure. Fracture toughness test results indicated that transition properties are improved when increasing the tempered martensite fraction by reducing the probability of discovering carbide larger than critical size, which can act as cleavage initiation site. The exponential fitting for datasets of the model alloys showed that the temperature dependency of the measured fracture toughness is steeper with higher fraction of the tempered martensite. The T 0 values determined from the adjusted curves for the data distribution well reflect the variation of transition property with the phase fraction when compared to the T 0 values from the standard master curves.

Investigation of Cleavage Fracture Behavior in Notched Specimens of a Low Alloy Hot Rolled Steel 16MnR

The cleavage fracture behaviors are studied in notched specimens of a low alloy hot rolled steel 16MnR. The results show that two types of cleavage initiation sites are existed in notched specimens, one being related to the inclusions ahead of notch root (IC type) and the other related to inclusions located ahead of string cracks far from the notch root (SIC type). The types of initiation sites are influenced strongly on temperature, changing from IC type at -196°C to SIC type at -130°C. In both IC and SIC initiation mechanisms, the crack nucleation is induced by inclusions and the final fracture is controlled by propagation of a ferrite grain-sized crack into matrix grain. The cleavage fracture of IC initiation type in notched specimens satisfies a dual-criterion model, i.e. a critical plastic strain ep ³epc for initiating a crack nucleus and a critical tensile stress syy³sf for its propagation. While for SIC initiation type, the dual-criterion model is evolved with the expression of ep+eps³epc and syy+syys³sf.

Prediction of cleavage fracture from non-sharp defects using the Weibull stress based toughness scaling model

The structural integrity assessment of engineering plant is based on the principles of fracture mechanics that assume defects to be sharp cracks. Whilst conservative, this assumption may be overly conservative in some situations, e.g. for defects that are non-sharp. This paper describes the prediction of cleavage fracture initiation from blunt notches of varying root radii using the Weibull stress based toughness scaling model. Failure predictions are compared with the results of experiments performed on single edge notch bend SE(B) specimens containing both cracks and notches of varying notch root radii. Cleavage initiation sites were located close to the peak tensile stress ahead of the notch, implying that a tensile stress criterion is the main controlling factor for cleavage fracture. The cleavage fracture predictions from the toughness scaling model correlate well with the experimental data; but care needs to be taken to ensure that calibration of the Weibull parameters references fracture toughness data from constraint levels that span that of the defect of interest. This ensures the model interpolates between the constraint states used for calibration, rather than extrapolating outside the range of applicability.

Introducing Heterogeneity into Brittle Fracture Modeling of a 22NiMoCr37 Ferritic Steel Ring Forging

Abstract Microstructural observations of the 22NiMoCr37 “EURO” reactor pressure vessel (RPV) steel ring forging reveal that there is a banded structure along the radial direction, composed of alternate layers rich in bainite and ferrite of wavelength ∼1.5 ±0.75 mm. Heterogeneity at this meso (millimetre)-scale as well as at the micro (micrometre)-scale is currently not considered by conventional fracture mechanics. This paper describes the development of two numerical approaches aimed at incorporating such heterogeneity into the Beremin local approach model of cleavage failure, a model that has been used extensively for predicting the brittle fracture of ferritic RPV steels. The approaches developed combine the crystal plasticity finite element method (CPFEM) with continuum finite element analysis (FEA). CPFEM is applied to predict stress distributions at the microscale and to obtain phase-specific yield and flow properties for continuum FEA that derives stresses at the mesoscale. The results confirm that deformation heterogeneity on the micro- and mesoscales influences the local development of stress. At the microscale, the stress distribution within a representative volume of material located within the crack-tip plastic zone is shown to follow a normal distribution with a ratio of mean stress to standard deviation tending towards 0.1. These results indicate local stress levels that are within approximately ±20 % of those derived using continuum FEA. At the mesoscale, a periodic variation of stress is predicted within the larger representative volume. This variation is less dramatic than that observed at the microscale, though it still gives a spatial variation in maximum principal stress of approximately ±7 % between bainite- and ferrite-rich microstructural regions. These results suggest a significant influence of deformation heterogeneity on local stress levels, particularly at the microscale. However, the conventional Beremin cleavage fracture model, modified to account for microscale stress distribution, predicts only a modest influence of deformation heterogeneity on cleavage fracture probability, increasing Pf by just 5 %. This highlights the need to account for both the spatial variation in cleavage initiation sites as well as the distribution in stress throughout the microstructure. The paper describes one approach for this development.

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.

A probabilistic model for cleavage fracture with a length scale – Parameter estimation and predictions of growing crack experiments

A probabilistic model for the cumulative probability of failure by cleavage fracture was applied to experimental results where cleavage fracture was preceded by ductile crack growth. The model, introduced by Kroon and Faleskog [Kroon M, Faleskog J. A probabilistic model for cleavage fracture with a length scale – influence of material parameters and constraint. Int J Fract 2002;118:99–118], includes a non-local stress with an associated material related length scale, and it also includes a strain measure to account for the number of nucleated cleavage initiation sites. The experiments were performed on single edge cracked bend test specimens with three different crack lengths at the temperature 85 °C, which is in the upper transition region for the steel in question. The ductile rupture process is modelled using the cell model for nonlinear fracture mechanics. The original cleavage fracture model had to be modified in order to account for the substantial number of cleavage initiators being consumed by the ductile process. With this modification, the model was able to accurately capture the experimental failure probability distribution.

Effects of Notch Tip Geometry on the Cleavage Fracture of RPV Steel

A series of fracture tests and elastic-plastic FEM analysis were carried out for three-point bend specimens with different notch root radius in the cleavage temperature region of a RPV low alloy steel. The cleavage initiation distance (CID) from the notch tip was measured by SEM photographs on the fracture surface of each specimen. The local cleavage fracture stress σf * was defined as the stress σyy at the cleavage initiation site. The σf * increased with decreasing notch root radius. The CID’s were larger in specimens with bigger root radius. This implies that a larger volume of material and possibly bigger microcracks should be involved in the cleavage fracture process of those specimens. The σf * is considered to have a certain relationship with the level of stress-strain concentration in different geometry specimens.

A critical stress–critical area statistical model of the [formula omitted] curve for MA957 in the cleavage transition

We model the temperature ( T) dependent fracture toughness K J c ( T ) of MA957 based on a statistically modified critical stress–critical stressed area ( σ ∗– A ∗) local fracture criteria. The K J c ( T ) of MA957 is strongly dependent on the specimen orientation. Analysis of cleavage initiation in the longitudinal–radial (L–R) orientation, with the highest K J c ( T ) , yielded the highest σ ∗ ≈ 3600 MPa. In contrast, the σ ∗ for the circumferential–longitudinal (C–L) orientation, with the lowest K J c ( T ) , yielded the lowest σ ∗ ≈ 2850 MPa, while for the circumferential–radial (C–R) orientation with intermediate K J c ( T ) , σ ∗ ≈ 3000 MPa. The A ∗ ranged from ≈ 30 to 400 μm 2. The cleavage initiation sites are μm-scale Al 2O 3 particles aligned in the textured low toughness extrusion direction.

Evaluation of the Cleavage Fracture Toughness of RPV Steels Using Small Bend Specimens

The cleavage fracture toughness of several reactor pressure vessel (RPV) steels was measured by the ASTM E 1921 master curve method using small bend (pre-cracked Charpy V-notched) specimens. The applicability of the method was experimentally evaluated by using different types of specimens and materials. Micro-structural and fracto-graphical observations were performed on cleavage fracture surfaces and polished-and-etched specimens as well. The measured cleavage fracture toughness was strongly related to the distance from the crack tip to the cleavage initiation site, cleavage initiation distance (CID), which was slightly beyond the local peak stress region. Based on the microscopic observations, the cleavage initiation site was not influenced by the grain size of the material. Metallurgical particles, such as inclusions and/or carbides, may play an important role in the cleavage fracture of low alloy RPV steels in the transition range.

Fracture toughness analysis in transition temperature region of API X70 pipeline steels

This study is concerned with the analysis of fracture toughness in the transition temperature region of API X70 pipeline steels according to ASTM E1921 standard test method. Two kinds of steels were fabricated by varying finish rolling temperatures, and their microstructures and mechanical properties were examined. Elastic–plastic cleavage fracture toughness, K J c was determined by three-point bend tests of precracked Charpy V-notch specimens, and then the measured K J c values were interpreted by the three-parameter Weibull distribution. Fractographic observation indicated that the critical distance from a precrack tip to a cleavage initiation site linearly increased with increasing critical J value. Effects of martensite–austenite constituent (MA) and effective grain size on fracture toughness in the transition region were analyzed from the standpoint of cleavage crack initiation and propagation. The higher volume fraction of MA led to the lower resistance to cleavage crack initiation, and the larger effective grain size led to the lower resistance to cleavage crack propagation. Thus, the steel rolled in the two-phase region had the higher fracture toughness in the transition region and the lower reference temperature and energy transition temperature than the steel rolled in the single-phase region because of the lower volume fraction of MA and the smaller effective grain size.

Cleavage fracture of RPV steel following warm pre‐stressing: micromechanical analysis and interpretation through a new model

ABSTRACTIn this paper, the warm pre‐stress (WPS) effect on the cleavage fracture of an 18MND5 (A533B) RPV steel is investigated. This effect, which describes the effective enhancement of the cleavage fracture toughness at low temperature following a prior loading at high temperature, has received great interest in light of its significance in the integrity assessment of structures, such as nuclear pressure vessels, subjected to thermal transients. Several loading cycles between room temperature (RT) and −150 °C are considered: Load‐Unload‐Cool‐Fracture (LUCF), Load‐Cool‐Fracture (LCF) and Load‐Cool with Increasing K‐Fracture (LCIKF). All experiments complied with the conservative principle, which states that no fracture will occur if the applied stress intensity factor (SIF) decreases (or is held constant) while the temperature at the crack‐tip decreases, even if the fracture toughness of the virgin material is exceeded. The experimental results indicate that an effective WPS effect is present even at small pre‐load (Kwps= 40 MPa√m), and that a minimum critical slope (−ΔK/ΔT) in the LCIKF cycle has to be exceeded to induce cleavage fracture between RT and −150 °C. Numerical modelling was performed using mixed isotropic and kinematic hardening laws identified on notched tensile (NT) specimens, tested in tension to large strains (up to 40%), followed by large compressive strains. Detailed microstructural investigations on compact tensile (CT) and NT fracture test specimens were performed so as to determine the nature of the cleavage initiation sites, as well as the local mechanical conditions at fracture. Based on this local information, a new cleavage model was calibrated and applied to predict the probability of cleavage fracture after WPS: it is shown that the predictions are in good agreement with the experimental results.

Determination of the Key Microstructural Parameter for the Cleavage Fracture Toughness of Reactor Pressure Vessel Steels in the Transition Region

The effects of the microstructural parameters, such as the prior austenite grain size and carbide size, on the cleavage fracture toughness were investigated in the transition region of Mn-Mo-Ni bainitic low alloy steels. Cleavage fracture toughness was evaluated by the ASTM standard E 1921 Master curve method. In order to clarify the effects of each microstructure, the grain size and carbide size of the test materials were independently controlled by modifying the heat treatment process. Firstly, the grain sizes were changed from 25㎛ to 110㎛ without any significant changes in the carbide size and shape. Secondly, the average carbide sizes were changed from 0.20 ㎛ to 0.29㎛ but maintaining the initial grain sizes. As a result, the fracture toughness in the transition region did not show any significant dependency on the austenite grain size, while the carbide size showed a close relation to the fracture toughness. Fracture toughness was decreased with an increase of the average carbide size. From the microscopic observation of the fractured surface, the cleavage initiation distance (CID) from the original crack tip showed no direct relationship to the prior austenite grain sizes but a strong relationship to the carbide sizes. However, the measured cleavage fracture toughness was strongly related to the distance from the crack tip to the cleavage initiation site. From the viewpoint of the weakest link theory, the particle size and their distribution in front of the crack tip is probably more important than the grain size in the transition temperature range where the fracture was controlled by the cleavage crack initiation.