Low Temperature Fracture Toughness Research Articles

Effect of welding method and heat treatment on the fracture toughness of low alloy steel deposited metal

The fracture toughness of the weld deposited metal plays an important role in the safe service of the reactor pressure vessel (RPV). Shielded melting arc welding (SMAW) and submerged arc welding (SAW) are two common welding methods in the construction of RPV. In this paper, the effects of the welding method and post-weld heat treatment (PWHT) on the index T0 of low-temperature fracture toughness of deposited metal were studied respectively. Through the characterization and observation of microstructure and cleavage crack initiation, the effect mechanism of the welding method and PWHT on the low-temperature fracture toughness of deposited metal was determined. The results show that the position in which oxide near the hardening phase is easy to become the cleavage crack initiation of deposited metal. The PWHT dissolves the M-A island to improve the fracture toughness of the deposited metal. The SMAW deposited metal has a smaller oxide inclusion diameter and less cementite than SAW, resulting in higher fracture toughness.

Investigating the ductile to brittle transition phenomenon in binary Fe-Ni systems using molecular dynamics simulation

Ductile to brittle transition (DBT) phenomenon is commonly observed in BCC Fe-based systems, which significantly deteriorates their low temperature fracture toughness. Ni is known to be an effective alloying element to improve the low temperature toughness by reducing the ductile to brittle transition temperature (DBTT). The present article investigates the role of Ni in tailoring the DBT curve of BCC Fe by employing molecular dynamics (MD) simulation. Here, we performed mode-I loading of different pre-cracked systems (pure Fe, Fe-2 wt% Ni and Fe-4 wt% Ni alloys) over a temperature range of 1–800 K to construct their DBT curves. The DBT curves are subsequently correlated with the temperature sensitivity of fracture stress, which is estimated using the concept of critical strain energy release rate (GIC). The role of Ni in reducing the fracture stress sensitivity is responsible for decreasing the DBTT as well as the slope of the DBT region. The underlying mechanism is discussed in terms of the crack blunting effect of Ni which effectively increases the GIC and hence the fracture stress, particularly at the low temperature regimes. Besides, the significantly large strain rate imposed in MD simulation is mainly responsible for the higher DBTT values when compared with the experimental results. The present paper tackles this strain rate issue by using an appropriate strain rate sensitivity factor to rescale the simulated DBTT values down to the experimental ones, resulting in a good match between the simulated and experimental DBTT.

Effect of Bain Unit Size on Low-temperature Fracture Toughness in Medium-carbon Martensitic and Bainitic Steels

Effect of Bain Unit Size on Low-temperature Fracture Toughness in Medium-carbon Martensitic and Bainitic Steels

Improving the Weld Heat-Affected-Zone (HAZ) Toughness of High-Strength Thick-Walled Line Pipes

The low-temperature fracture toughness of double-V weld seams is a well-known challenge due to the essential increased heat input for heavy-wall pipelines. A thorough investigation was conducted to explore the impact of the heat input on the grain size and precipitate coarsening, correlating the microstructure with the heat-affected-zone (HAZ) toughness. The results indicated that the actual weldments showed a toughness transition zone at −20 °C, with considerable scattering in Charpy V-notch (CVN) tests. Gleeble thermal simulations confirmed the decreased toughness of the coarse-grained HAZ (CGHAZ) with increasing heat input and prior austenite grain size (PAGS). A specially designed thermal treatment demonstrated its potential for enhancing the toughness of the CGHAZ, with the recommended thermal cycle involving peak temperatures of 700 and 800 °C, holding for 1 s, and rapid cooling. The toughness of the intercritically reheated CGHAZ (ICCGHAZ) improved with higher intercritical reheating temperatures and the removal of necklace-type M–A constituents along the PAG. Despite various thermal treatments, no significant improvements were observed in the toughness of the ICCGHAZ. Future work was suggested for optimising the use of tack welds to reduce the effective heat input (HI) associated with double-sided submerged arc welding (SAW).

Design of smart transforming metal ceramic multilayers

Novel metal ceramic multilayer composites with the ability to phase transform into a single phase ultra-high temperature ceramic are described. These materials offer enhanced low-temperature fracture toughness and reliability without sacrificing high-temperature mechanical properties by eliminating the metal reinforcement layers upon reaching temperatures sufficient for the activation of carbon diffusion. At low temperatures, these materials are practically indefinitely stable because of the high activation energy for carbon diffusion in the carbide layers that control the phase transformation kinetics. Here, knowledge of the overall thermodynamics, i.e., carbon content, is combined with prediction of phase transformation kinetics and toughening in the limit of small-scale yielding to characterize the design space of these novel composites. It is proposed that the ability to control thermodynamic and mechanical properties of the post-transformation ceramic via selection of metal-to-ceramic layer ratios makes these novel composites an excellent alternative to current methods for reinforcement of ultra-high temperature ceramics.

Implications of cryogenic treatment on microstructure, phase formation, mechanical and tribological properties of tungsten carbide cutting bits with varying cobalt content for mining applications

This study aims to provide a comprehensive analysis on the effect of cryogenic treatment on two different grades of WC rock cutting bits (low cobalt grade: WC-9 wt% Co and high cobalt grade: WC-25 wt% Co). The cutting bits are exposed to cryogenic treatment at different holding time such as 12, 24 and 36 h. The treated and untreated bits are characterized for low temperature DSC, phase analysis, microstructural study, elemental composition, hardness, fracture toughness and scratch test. Phase shift was observed for both the cases due to the martensitic phase transformation from α-Co (fcc) to ε Co (hcp) during cryogenic treatment. This transition was confirmed with low temperature DSC analysis between −90 °C and − 60 °C. New η carbide particles (Co6W6C) was formed for WC-25%Co and no such carbides was found for WC-9%Co. CT 36 of WC-25%Co exhibited a higher level of precipitation of η carbides. The results show that CT has enhanced hardness for both low (14.08%) and high cobalt (23.34%) WC cutting bits compared to untreated bits whereas density is not altered evidently. CT 24 and CT 36 possessed higher hardness and lower fracture toughness for WC-9%Co and WC-25%Co respectively. Higher CoF and formation of tribo layer was observed for treated bits whereas untreated bit experiences surface fracture and cracks as a result of scratch test. As a finding, mining sectors can utilize cutting bits that have undergone sustainable cryogenic treatment for rock cutting applications.

Effect of accelerated cooling after cross-helical rolling on formation of structure and low-temperature fracture toughness of low-carbon steel

The effect of accelerated cooling after cross-helical rolling of X70 low-carbon steel on the formation of structures and mechanical properties under static tension and impact bending was investigated. The use of interrupted accelerated cooling of steel after cross-helical rolling with exposure at 530 °C (mode I) and continuous accelerated cooling (mode II) leads to the formation of different types and ratios of structures in steel. After rolling according to mode I, the structure is characterized by the presence of ferrite, troostite, granular bainite, and fine Fe3C carbides. After rolling according to mode II, the structure is characterized by the formation of lath bainite and large sections of the martensitic-austenitic (MA) component up to 1 – 2 µm in size. It is shown that a decrease in the fineness of ferrite grains in steel after cross-helical rolling in modes I and II from 12 to 4.6 – 4.3 μm, the formation of a bainitic phase, and hardening of the matrix with carbides led to an increase in the yield strength of steel up to 440 and 490 MPa and tensile strength up to 760 and 880 MPa. Carrying out helical rolling according to mode I makes it possible to significantly increase the low-temperature fracture toughness of steel (KCV–70 °С = 160 J/cm2) compared to the hot-rolled state (KCV–70 °С = 11 J/cm2) and reduce the cold brittleness of steel to the temperatures below –50 °C. The use of continuous accelerated cooling (mode II) does not allow increasing the cold resistance of steel due to the formation of the lath bainite structure and large areas of the MA component.

Evaluating Low-Temperature fracture toughness of steel slag Aggregate-Included asphalt mixture using response surface method

This study is aimed to evaluate the low-temperature (0 to −24 °C) fracture toughness of fine and coarse (0–50 % and 0–100 % inclusion) steel slag aggregate (SSA)-included asphalt mixtures in different Modes (I, II and I/II). To this end, edge-notched disk bend (ENDB) specimens were made and tested under three-point bending in different fracture modes, and response surface method (RSM) was used to evaluate and optimize the low-temperature fracture toughness of fine and coarse SSA-included asphalt mixtures. Results indicated that at temperatures between 0 and −24 °C, to increase the possibility of reaching the maximum fracture toughness, it is better to use a combination of fine and coarse SSAs. Otherwise, the desirability of only inclusion of fine/coarse SSA is significantly lower than simultaneously inclusion of them in terms of fracture toughness. Here, the highest desirability was achieved at 0 % fine SSA, 73 % coarse SSA and 0 and −24 °C.

The effects of hydrogen on dynamic fracture toughness of high-strength low-carbon medium manganese steel

The effect of retained austenite (RA) stability and dislocation density of martensite matrix on low-temperature dynamic fracture toughness and the hydrogen embrittlement (HE) susceptibility was investigated in a high-strength low-carbon medium manganese steel subjected to three different heat treatments, i.e., the inter-critical annealing at 630 °C (QHA), inter-critical annealing at 610 °C (QLA), and direct quenching (DQ). Dynamic cracked three-point bending tests were performed at 20 °C and −40 °C with and without hydrogen charging. The RA volume fraction and the martensite matrix's dislocation density significantly affected the dynamic fracture toughness and HE behavior. All QHA and QLA tested at 20 °C fractured in the ductile mode, but all DQ and QLA tested at −40 °C fractured in the brittle mode. Dynamic fracture toughness decreases with RA volume fraction under the same tested temperature. The HE mechanism in the ductile region is HELP, and that in the brittle region is HEDE.

Fundamental insights on ductile to brittle transition phenomenon in ferritic steel

Ductile to brittle transition (DBT) is a well-known phenomenon responsible for deteriorating the low temperature fracture toughness of ferritic steels. It is unambiguous to state that the transition phenomenon is a direct consequence of the strong temperature and strain rate sensitivity of flow stress, which in turn is regulated by the thermally activated motion of screw dislocation at the atomic scale. Due to very high Peierls stress, screw dislocation motion is accompanied by the nucleation and propagation of kink-pairs along the dislocation line. Thus, the activation energy required for kink-pair nucleation is an essential ingredient for correctly predicting the flow stress sensitivity and hence the ductile to brittle transition temperature (DBTT) of ferritic steel. However, experimental determination of kink-pair activation energy is rather tedious and there exists a strong discrepancy between atomistic simulation results (using Density Functional Theory and empirical potentials). Therefore, the present review is aimed at understanding the mechanism of screw dislocation motion in BCC Fe lattice and comparing the results obtained for kink-pair activation energy through experiments and simulations. The effect of different alloying elements on the overall flow stress sensitivity of Fe is also discussed with reference to the interaction of screw dislocation with solute atoms or clusters. Finally, the importance of screw dislocation on predicting the DBTT is explored to identify the present knowledge gaps and the future research directions.

Using two and three-parameter Weibull statistical model for predicting the loading rate effect on low-temperature fracture toughness of asphalt concrete with the ENDB specimen

Loading rate can affect significantly the load bearing capacity and cracking resistance of asphalt mixtures that are basically visco-elastic materials. On the other hand, the scatter of mechanical test results obtained from the asphalt mixtures (as multi ingredients and randomly distributed heterogeneous materials) would be high. Therefore, in this research the effect of loading rate on mode I fracture toughness of a fixed hot mix asphalt (HMA) mixture is investigated experimentally using several test replicates and the obtained data are analyzed using two and three parameter Weibull statistical models. The Edge Notch Disc Bend (ENDB) specimen, (that is a circular disc containing an edge crack along the diameter and loaded by three point bending) is utilized for conducting the KIc experiments under three loading rates of 0.5, 5 and 10 mm/min. For each loading rate, twenty five ENDB specimens are tested at a fixed low temperature of −12 °C and the probability of failure and Weibull distribution curves are obtained. It is shown that both two and three parameter Weibull models can be utilized successfully for predicting the mode I fracture behavior of tested asphalt material in the considered range for the loading rate. By increasing the loading rate from 0.5 to 10 mm/min, the mean KIc values and other Weibull parameters are increased. The Weibull distribution parameters obtained experimentally from the loading rates of 5 and 10 mm/min, are also predicted in terms of the Weibull parameters of the ENDB specimen tested at loading rate of 0.5 mm/min considered as a reference case.

Minimum design metal temperature of Q345R determined by exemption curve in ASME VIII-2 and its derived methods

In order to prevent the brittle fracture accident, minimum design metal temperature of ferrite steel should be limited. After the minimum design metal temperature curve in American Society of Mechanical Engineers VIII-2 (2007) was proposed, much related research has been done in recent years. In this paper, firstly the theoretical basis of four methods used to determine the minimum design metal temperature was introduced. Secondly, the mechanical properties of Q345R was measured by tensile test, Charpy v-notch impact test and fracture toughness test Thirdly, minimum design metal temperature curve of Q345R that determined by four methods were obtained. There are obvious difference between the curves of Q345R that determined by four methods. It can be concluded that low temperature fracture toughness of Q345R is underestimated when classifying Q345R into exemption curve A in American Society of Mechanical Engineers VIII-2 (2007).

Effect of Microstructure on Low-Temperature Fracture Toughness of a Submerged-Arc-Welded Low-Carbon and Low-Alloy Steel Plate

This study investigated the low-temperature fracture behavior of an 80-mm-thick low-carbon steel plate welded by submerged arc. The relationship between impact absorbed energy and ductility–brittle transition temperature (DBTT) based on the microstructures was evaluated through quantitative analysis on grain size and complex constituent phases using advanced EBSD technique. The microstructure formed differently depending on the heat affections, which determined fracture properties in a low-temperature environment. Among the various microstructures of the heat-affected zone (HAZ), acicular ferrite has the greatest resistance to low-temperature impact due to its fine interlocking formation and its high-angle grain boundaries.

Deterioration of HAZ toughness by residual Sn and Determination of its Allowable Content for Electric Furnace Steels

One promising direction in steelmaking to respond to global climate change issue is to increase the use of electric furnace manufacturing methods using zero-carbon electricity, which have received more and more attention recently. Considering the replacement from blast furnace steel, the temper embrittlement problem induced by some tramp elements, which is unique behavior for electric furnace methods, must be resolved.Sn is considered as a tramp element that cannot be removed in steel refining process, and it has been reported that the toughness of the base metal and the welded heat affected zone can be significantly degraded. In the case of electric furnace steels with degraded toughness, segregation of Sn is observed at grain boundaries and the resulting fracture mode shifts to intergranular fracture mode. In this study, in order to quantify the allowable amount of Sn added to thick steel plates that is used for large-heat input welding, experiments were conducted on specimens with different amounts of Sn and heat-treated for different holding times at 400℃, which is a typical embrittlement temperature and investigated the effect of the amount of Sn on the change in low-temperature fracture toughness. As a result of fracture toughness tests, clear evidence of temper embrittlement was observed. Apart from previous famous study by McMahon, there was no intergranular fracture in fracture surface but transgranular and cleavage fracture was observed. Therefore, the mechanism by which Sn tends to cause cleavage fracture in the weld heat affected zone was proposed. Based on our newly proposed mechanism, a model was developed to estimate the transition temperature change for various conditions of tempering time, tempering temperature and Sn content.Finally, the authors propose allowable maximum content of Sn based on the model for the establishment of the rational ore sorting system from scraps in near future society.

Laminated composites using potassium doped tungsten

Abstract Highly deformed tungsten (W) thin foils could show excellent low temperature ductility, even at room temperature. A laminated composite using pure W foils has been proposed for the application to the plasma-facing material of divertors, which is a composite material consisting of stacked pure W thin foils and interlayer materials. However, the degradation of low temperature ductility attributed to the recrystallization and neutron irradiation of pure W foils is one of the concerns of the laminated composite using pure W foils. Potassium (K) doping is known as one of the methods to suppress recrystallization and to improve neutron irradiation resistance, in addition to improving the low temperature fracture toughness. Therefore, a laminated composite using K-doped W foils was developed in the present study, which has the possibility to show improved mechanical properties even after the neutron irradiation compared to the pure W foil laminated composite. The K-doped W foil laminates using an interlayer made of pure cupper (Cu) and pure vanadium (V) were fabricated at 900 and 1250 °C in the present study. An enough ductility at room temperature was observed in these laminates, even after the fabrication at 1250 °C.

Prediction of fracture toughness of SA738Gr.B steel in the ductile-brittle transition using master curve method and bimodal master curve method

The low temperature fracture toughness of SA738Gr.B steel is as important as its strength, but the low temperature fracture toughness tends to be more complicated due to some deterministic and random inhomogeneity, such as sampling locations. In this paper, the fracture toughness of the 60 mm thick SA738Gr.B sheet in DBT region from the surface to the center was characterized by MC method and BMC method. Considering the sampling position, a distinct trend is found that the toughness of the surface is significantly higher than that of the center and T0 is lower from center to the surface due to the faster quenching rate. Special attention was paid on the discussion of the applicability of China-produced SA738Gr.B steel to the master curve (MC) method. It was found that the inhomogeneity caused the actual distribution of the experimental data to deviate from the three-parameter weibull distribution, and the weibull slope was lower than the theoretical value of 4. Characterizing the fracture toughness of materials containing heterogeneous components by the MC method can overestimate the fracture toughness and may pose a potential hazard. The BMC method can effectively solve the problem of material inhomogeneity. It is an evaluation method based on the actual toughness distribution of materials. The BMC method takes into account the influence of material inhomogeneity on the toughness estimation, and more accurate assessment of material scatter than the standard MC method yields a slightly conservative but more appropriate prediction.

Study on the Fracture Properties of HTPB Propellant at Low Temperature

AbstractIn order to study the fracture behaviour of hydroxyl‐terminated polybutadiene (HTPB) propellant, the low temperature fracture toughness experiment of HTPB propellant was carried out by using centrally cracked sheet tensile specimens. The fracture toughness values of HTPB propellant with mode I crack at different temperature and loading rate were obtained, and the effect of specimen thickness on the fracture toughness was investigated. The experimental results show that the fracture toughness value continuously increases with the decreasing of temperature and the increasing of loading rate, and a linear relationship exists between the fracture toughness and the logarithm of loading rate. The effect on the fracture toughness by the change in loading rate is more notable at a lower temperature. Both at 25 °C and −40 °C, the fracture toughness of thick specimens is higher than that of thin specimens, and at low temperature the change is more obvious. A master curve of quadratic function for the fracture toughness was obtained. The influence of temperature on the fracture toughness is more obvious than that of the strain rate with double factor analysis of variance.

Improved microstructure heterogeneity and low-temperature fracture toughness of C–Mn weld metal through post weld heat treatment

The Crack Tip Opening Displacement (CTOD) test was used to investigate the effect of post-weld heat treatment (PWHT) on the low-temperature fracture toughness of C–Mn weld metal. The microstructure observations, instrumented nano-indentation test and microstructure-based finite element simulation were conducted to analyze the toughening mechanisms. The study found that the CTOD value increased significantly from 0.241 mm to 1.754 mm after PWHT. The hardness and strength between acicular ferrite (AF) and proeutectoid ferrite (PF) both get closer, which improves the extent of microstructure heterogeneity. The simulation indicated that the distributions of the strain and stress on AF and PF are homogenized with the improvement of microstructure coordination after PWHT, making the level of plastic strain localization within PF decreased and the stress concentration on the interphase of AF and PF lessened. As a result, the resistance of crack initiation and propagation increases, and the low-temperature fracture toughness is improved. Moreover, the dislocation densities in AF and PF both decrease, the dislocation morphology evolves from dislocation tangles into movable dislocation lines, and amounts of fine carbides precipitate and spheroidize, which also contribute to the toughening of microstructures.

A method to determine minimum design metal temperature of pressure vessels made from ferritic steel by Master Curve approach

The four impact exemption curves in ASME are convenient to estimate the minimum design metal temperature (MDMT) of ferritic steel. However they can’t distinguish low temperature fracture toughness of ferritic steel well. Materials with similar fracture toughness, divided into different impact exemption curves will have different MDMT and conservative degree. And the MDMT of ferritic steel are estimated by the steel grade. There are differences in toughness for steel with same steel grade from different factories and different heats and batches of the same factory. In this paper, the seven MDMT curves, identified with labels I, II, III, IV, V, VI and VII are proposed based on assumed yield strength and seven reference temperatures, (T0MC = 20 °C, 0 °C, −20 °C, −40 °C, −60 °C, −80 °C and −100 °C) which can cover the temperature range for commonly used pressure vessels made from ferritic steel. The MDMT for a ferritic steel can be determined once the reference temperature and yield strength are measured. SA508-3 and Q345R, as a representative of ferritic steel are used to illustrate the applicability of the MDMT curves.

BISALLOY STRUCTURAL 80 PRESSURE VESSEL STEEL

Abstract Bisalloy Structural 80 Pressure Vessel Steel (80 ksi minimum yield strength) is a low-carbon, low-alloy, high-strength structural steel for pressure vessel applications. It exhibits excellent cold formability and low-temperature fracture toughness. This datasheet provides information on composition, physical properties, hardness, and tensile properties. It also includes information on forming, machining, and joining. Filing Code: SA-844. Producer or source: Bisalloy Steels Group Limited.