Blue Brittleness Research Articles

Kinetics of deformation fronts during serrated Lüders deformation in α-iron at high temperature

At room temperature, the deformation of most bcc metals, which contain a small amount of interstitial elements, is accompanied by the formation of a Lüders band and its monotonic propagation over the tensile yield area. Within the framework of the autowave concept, front of the Lüders band is a switching autowave, which realizes the transition from a metastable elastically deformable state to a stable plastically deformable state. However, in the temperature range of blue brittleness of mild steels of 423 – 510 K, when the interaction of atoms of the dissolved substance with mobile dislocations takes place, propagation of the Lüders band is accompanied by a discrete flow. The patterns of propagation of the Chernov-Lüders fronts in ARMCO iron in the temperature range from 296 to 503 K and strain rates from 6.67·10–6 to 3.7·10–2 s–1 are considered in this paper. It was established that under these conditions both monotonic and discrete kinetics of front movement can be realized. Regardless of the movement nature, the Lüders deformation and width of the front remain unchanged throughout the entire process. The local strain rate at the front depends on magnitude of the effective stress, and with monotonic kinetics it increases with stress according to an exponential law, and with discrete kinetics it increases according to a linear law. This difference is due to different autowave modes that are formed in this case. The autowave of localized plasticity switching corresponds to monotonic kinetics, and the autowave of excitation – to discrete kinetics.

Assessment of the Blue Brittleness Range of Steel

Assessment of the Blue Brittleness Range of Steel

Effect of Temperature on the Kinetics of Localized Plasticity Autowaves in Lüders Deformation

The paper analyzes the elastoplastic transition in Fe–0.025 wt. % C at a temperature of 296–503 K and strain rate of 6.67·10−6–3.33·10−3 s−1. The analysis shows that the lower yield stress increases by a power law with increasing the strain rate, and that its rate sensitivity decreases linearly with increasing the test temperature. At temperatures lower than 393 K, the rate sensitivity of the lower yield stress is normal, and at 393–503 K, it is zero. In the range 393–503 K, the kinetics of the Lüders bands is changed from steady to discrete, and the higher the strain rate, the higher the temperature of this transition. Using the available data on the dynamics of dislocations and diffusion of interstitial impurities in the test alloy, it is demonstrated that the kinetics of Lüders bands are controlled by the effect of dynamic strain aging. If the arrest time of mobile dislocations tw at barriers which are overcome via thermal activation is comparable with the precipitation time of interstitial atoms ta at these dislocations, the motion of a Lüders band is discrete, and the band represents an excitation wave of localized plasticity; its refractory period is determined by the time of dynamic strain aging. If ta >> tw, the band moves monotonically and represents a switching autowave. The results of the analysis suggest that the effect of serrated yielding at the lower temperature boundary of blue brittleness can be suppressed by increasing the strain rate. When the arrest time of dislocations tw decreases, the comparability of tw and ta is broken, and no excitation autowave is formed. The data reported in the paper can be used to develop warm rolling technologies for materials with a sharp elastoplastic transition.

Оперативная оценка температурного интервала синеломкости стали

A brief analysis of the blue brittleness temperature range of structural steels and the reasons for its occurrence has been carried out. It is noted that blue brittleness causes a change in all mechanical characteristics of steel. In particular, the strength characteristics increase, and the plasticity characteristics decrease in a narrow temperature zone of blue brittleness. It was established that hardness changed similarly to strength and can be used as a diagnostic parameter to identify the temperature range of blue brittleness. For three grades of structural steels, experiments were carried out to identify the temperature range of blue brittleness by tension of samples at various temperatures. A technique for express detection of the blue brittleness temperature range by changing the Brinell hardness determined by instrumented indentation is proposed. The technique eliminates the need to manufacture samples and saves material and labor costs.

Fatigue analysis of high-carbon steel at different environmental temperatures considering the blue brittleness effect

Fully-reversed bending fatigue and impulse excitation tests are performed to investigate the behavior of High Carbon Steel (HCS) during cyclic loading at different environmental temperatures. The evolution of the damping, elastic modulus, and frequency for different operating temperatures ranging from 23°C to 135°C is characterized. The results of the damping are related to the rate of damage accumulation and crack growth―both of which are shown to increase with increasing the environmental temperature. It is shown that specimens tend to become more brittle with increasing of environmental temperature due to the so-called blue brittleness effect, and consequently the fatigue life of the specimens decreases. To gain further insight, a finite element model (FEM) is developed to assess the maximum principal stress distribution. The results corroborate the experimental findings.

Post fire tensile properties of S355 J2 structural steel welded connections for construction industrial applications

Constructional steel structure stability during its service and post fire effect on steel after major fire exposure is very detrimental and critical on the parent metal (PM) un-welded region and welded connections. Existing literatures on residual material properties of various constructional steels have exhaustive focused only on the mechanical properties of PM. This paper presents the comparative results of S355 J2 structural steel joints made by GMAW and FCAW after fire exposure up to 900°C. The residual properties were evaluated after elevated temperature fire exposures to 300°C, 600°C and 900°C, respectively. Experimental results shows at 300°C, the residual mechanical properties such as yield strength (YS), ultimate tensile strength (UTS) and hardness revealed small increment, while after 300°C trend follows a detrimental pattern in YS, UTS and hardness irrespective of process used. The increment in strength and hardness properties at 300°C could attribute to the strain hardening and blue brittleness effect on PM and welded connections. Moreover, experimental results confirmed that welded connections obtained from high strength steel grade S355–J2 begin to deteriorate strength at temperatures above 300°C. The reduction in tensile and hardness of post fire condition is analyzed with respect to post fire microstructural changes. The results observed in this study may be useful for constructional steel industries.

Dynamic strain aging in DP1000: Effect of temperature and strain rate

The present study reports on an extensive experimental campaign performed to identify the occurrence of dynamic strain aging (DSA) in a fine-grained DP1000 steel. Tensile tests are performed covering 7 orders of strain rate, i.e., 0.0001 s-1 to 1080 s-1, and temperatures ranging from −40 °C to 400 °C. The test results are thoroughly analysed with particular attention to typical features of DSA, such as serrated flow, thermal hardening, negative strain rate sensitivity and blue brittleness. In addition to the plastic strain, DSA is sensitive to both temperature and strain rate. DSA effects are most dominant at higher temperatures and lower strain rates. However, even at the highest strain rate, traces of DSA are found in DP1000. At temperatures below 100 °C, no DSA is observed. The way and extent to which DSA manifests itself depend on the specific combination of temperature and strain rate. Analysis of the serrations stimulate the hypothesis that quasi-periodic dislocation cells might lie at their origin. In addition, scanning electron microscopy images of the fracture surfaces reveal finer and shallower dimples with flat features in the dynamic strain aging regime.

Proposition and research progress of the third-type strain aging

The traditional knowledge of plastic flow behavior of metal, dislocation activation theory, and thermoviscoplastic constitutive model of metal needs a further perfection due to the occurrence of third-type strain aging. Third-type strain aging leads to a bell-shaped flow stress-temperature curve, that is, the flow stress first increases as temperature increases, and after a peak value is reached, it decreases with further increase of temperature. Third-type strain aging occurs at both low and high strain rates. The bell-shaped segment induced by third-type strain aging on the flow stress-temperature curve shifts to higher temperature region as strain rate increases. In order to develop a systematic understanding of the third-type strain aging, firstly, macro characteristics of third-type stain aging effect that was distinguished from static strain aging effect and Portevin-Le Chatelier dynamic strain aging effect were introduced. Then, the micro mechanism of third-type strain aging and correlation of third-type strain aging, Portevin-Le Chatelier dynamic strain aging, blue brittleness phenomenon, and internal friction were systematically concluded. Third-type strain aging, Portevin-Le Chatelier dynamic strain aging, blue brittleness phenomenon, and mechanical spectroscopy are all resulted from the interaction of mobile dislocations with diffusion atoms. Third-type strain aging, Portevin-Le Chatelier dynamic strain aging, and blue brittleness phenomenon are different manifestations of dynamic strain aging. Third-type strain aging can be considered as another mode of mechanical spectroscopy. The common constitutive models are able to capture the plastic behavior of many metals under the coupling influence of strain rate and temperature in some cases. However, these thermoviscoplastic constitutive models do not take the effect of third-type strain aging into consideration, and they cannot describe the thermoviscoplastic behavior including the third-type strain aging effect. To accurately describe the plastic behavior of metals, some constitutive models including the effect of third type strain aging were proposed. The development of the thermoviscoplastic constitutive model of metal considering the effect of third type strain aging was finally introduced.

Stress-state dependence of dynamic strain aging: Thermal hardening and blue brittleness

This study aims to discover the stress-state dependence of the dynamic strain aging (DSA) effect on the deformation and fracture behavior of high-strength dual-phase (DP) steel at different deformation temperatures (25–400°C) and reveal the damage mechanisms under these various configurations. To achieve different stress states, predesigned specimens with different geometric features were used. Scanning electron microscopy was applied to analyze the fracture modes (e.g., dimple or shear mode) and underlying damage mechanism of the investigated material. DSA is present in this DP steel, showing the Portevin-Le Chatelier (PLC) effect with serrated flow behavior, thermal hardening, and blue brittleness phenomena. Results show that the stress state contributes distinctly to the DSA effect in terms of the magnitude of thermal hardening and the pattern of blue brittleness. Either low stress triaxiality or Lode angle parameter promotes DSA-induced blue brittleness. Accordingly, the damage mechanisms also show dependence on the stress states in conjunction with the DSA effect.

Effect of Implanted Helium on the Physicomechanical Properties of Armco Iron at the Temperatures of Manifestation of the Blue Brittleness Effect

Effect of Implanted Helium on the Physicomechanical Properties of Armco Iron at the Temperatures of Manifestation of the Blue Brittleness Effect

Influences of Stress State, Temperature, and Strain Rate on Ductility of Pure Iron

In this study, the plastic response of pure iron DT8 at different stress states, temperatures, and strain rates is investigated. Specimens with different structures designed for different stress states are subjected to quasi-static uniaxial tension tests to investigate the effect of stress state on the fracture behavior of pure iron. Standard specimens at temperatures ranging from 100 to 900 °C are tested by a quasi-static uniaxial tension machine. Double-notch shear specimens are subjected to a universal electronic tensile testing machine and an improved split-Hopkinson pressure bar device with strain rates ranging from 0.0033 to 227450/s to investigate the effect of strain rate on the fracture behavior of pure iron. Results show that the flow stress of pure iron DT8 slightly increases with the increase in stress triaxiality before the material breaks. With the increase in stress triaxiality from 0.333 to 0.550, the fracture strain of pure iron DT8 decreases almost linearly from 1.949 to 0.898. With the increase in temperature, the fracture strain increases almost linearly from 1.43 at 100 °C to 3.19 at 900 °C. A blue brittleness effect occurs at about 300 °C, where the yield and tensile strengths of pure iron increase slightly. In the low strain rate range from 0.0033 to 800/s, the fracture strain of pure iron is almost stable at approximately 2.0. Then, the fracture strain increases with the further increase in strain rate. When the strain rate reaches 120850/s, the fracture strain reaches its peak value of 3.64. Thereafter, the fracture strain decreases rapidly as the strain rate increases. The fracture surfaces of the specimens take on different fracture morphologies with the increase in strain rate. The nature of the strain rate’s effect on material ductility is discussed.

High-temperature behaviour of thermo-mechanically treated and cold-drawn twisted steel bars

This paper presents the results of experimental testing at elevated temperatures of two types of steel rebars locally available in Pakistan. These include cold-drawn twisted (CDT) bars and hot-rolled thermo-mechanically treated (TMT) bars. Two rebar diameters (12 and 16 mm) of both types of bars were included in the testing programme. The bars were heat treated in an electrical furnace at 100−900°C in increments of 100°C. The yield strength of the 12 mm dia. hot-rolled TMT bar was less than the recommended strength of 500 MPa at ambient temperature. The yield phenomenon and strain hardening were not observed in the CDT bars either at ambient or high temperatures. The yield plateau of the hot-rolled TMT bars also disappeared after 300°C of heating. The ultimate strength of the hot-rolled TMT bars up to 300°C was higher than the strength at ambient temperature due to the precipitation of iron nitride in these bars (termed as blue brittleness effects). The reduction coefficients for yield strength and elastic modulus for the investigated bars were similar at all exposure temperatures. The behaviour of bars tested in this study are compared with those reported in the literature and differences in the bar behaviour are discussed.

Damage effect of steel circular tube subjected to fire and blast

Q345B steel circular tubes are widely applied in structural engineering applications, such as offshore platforms, bridges, and other structures, and they face potential risks of being subjected to the external loads of fires, explosions, or both during their service lives. In this study, the damage of Q345B steel circular tubes due to the combined action of fire and blast loading were analyzed. A specialized experimental platform was built, and experiments were performed on Q345B steel tubes with a range of wall thicknesses (a), fire loading durations (t), and stand-off distances (R) which means the distance between the bottom of the explosive and the upper surface of tube. The experimental local deflection, compressed degree, and local deformation volume are presented and studied in a quantitative manner. As the fire duration increased, the specimens underwent more noticeable deformation and damage. The local deflection amounts of the specimens treated with fire increased by 20%–50% compared to that of the specimen not exposed to fire at R = 17 cm. When R = 12 cm, the local deflection increased by 20%–120%. In addition, based on the stress–strain curves, the effects of the blue brittleness of the Q345B steel materials on the anti-blast performances of the specimens were examined. Finally, scanning electron microscopy was employed to explore the rupturing mechanism under a microscope.

Impact fracture energy of steel welded connections in fire and post-fire

Charpy impact tests were conducted on steel welded connections at elevated temperatures and at the ambient temperature after heating and cooling processes. The elevated temperature tests were conducted to examine the Charpy impact energies under the blue brittleness temperature (100–300 °C) of steel. V-notch specimens quarried from steel base metal, weld metal, and heat-affected zone of the welded connection were heated and subjected to Charpy impact tests. In the ambient temperature tests after the heating and cooling processes, the heated specimens were either air cooled in a furnace or quenched with water. The test results show that the Charpy impact energies do not decrease in the blue brittleness temperature, and the quenched welded connections demonstrated very low Charpy impact energies.

Incorporating dual BCC/FCC Zerilli-Armstrong and blue brittleness constitutive material models into Oxley’s machining shear zone theory

This work presents an extension of Oxley's shear zone theory by substituting the original velocity-modified power form by the widely used constitutive material model developed by Zerilli and Armstrong (ZA). Used are variations of ZA constitutive laws capable of accounting for operating strains, strain rates, and temperatures as well as the appropriate crystal structure such as body-centered cubic (BCC) and face-centered cubic (FCC). This latter feature is useful in cutting simulations of metal alloys with complex phase diagrams (e.g., carbon steels) having temperature-dependent crystal structures. Loss of ductility encountered at elevated temperatures, dubbed Blue Brittleness (BB), adds another challenge to the simulations.For validation purposes, cutting and thrust forces of single-phase Aluminum 6061-T6 (FCC crystal structure) were investigated. Additionally, and in order to illustrate the utility of the methodology for dual phase materials, AISI 1045 steel was utilized as a metal with temperature-dependent crystal structure that evolve from BCC to FCC as temperatures increase due to increasingly aggressive cutting parameters. The simulations utilized publicly available cutting force values from orthogonal machining tests conducted at different cutting speeds and feeds. For both 6061-T6 Aluminum and AISI 1045 steel, the methodology of extending Oxley’s thick shear zone analysis by incorporating the widely used Zerilli-Armstrong constitutive laws (dual BCC/FCC) was shown to produce accurate estimates of forces and chip thickness over a wide range of cutting conditions.

Влияние имплантированного гелия на физико-механические свойства армко-железа при температурах проявления эффекта синеломкости

Влияние имплантированного гелия на физико-механические свойства армко-железа при температурах проявления эффекта синеломкости

The effect of heating and thermocycling on the mechanical properties of carbon steel rope wire

Heating and thermal cycling of carbon steel rope wire significantly affect the mechanical properties and durability of the entire rope under operating conditions. However, despite a large amount of studies aimed at determination of the reasons for destruction of the ropes of foundry cranes at metallurgical plants and development of measures for their elimination, accidents attributed to sudden rupture unfortunately occur. Therefore, the problem is still urgent and requires further research. Nowadays, new exhaust air suction systems for hot air streams mounted in converter shops of metallurgical enterprises reduce dangerously high temperatures to T = 240 – 300°C thus providing the possibility to avoid overheating of the ropes. However, the rope metal is exposed to a strong impact of blue brittleness – phenomenon, which causes reduction of the metal plasticity in this temperature range. We present the results of studying the impact of the heating temperature and number of cycles in thermocycling on changes in the characteristics of the mechanical properties of steel 70KK wire ropes in tensile tests. Tests of the wire samples are carried out at room temperature and after their exposure to high-intensity heating and thermal cycling at different temperatures and number of cycles. confirmed Presence of zones of blue brittleness is proved experimentally for the wire rope with a diameter of 1.65 mm in a temperature range of 200 – 240°C. It is shown, that the largest reduction in the relative narrowing (21%) and increase in the ultimate tensile stress by 4.8% occurred at a temperature of 240°C. This phenomenon should be taken into account when assessing the structural strength of steel. It is shown that there are reserves to increase the life cycle of ropes, taking into account the data on diagnostics of the mechanical and magnetic characteristics in industrial conditions.

Constitutive parameters identification based on DIC assisted thermo-mechanical tensile test for hot stamping of boron steel

Hot stamping of Boron steel is a thermal-mechanical and phase transformation coupled process. The reliable testing and parameter identification method of the constitutive behavior is crucial for accurate simulation on hot stamping. The imprecise strain and strain rate calibration due to the inevitable temperature gradient along the gauge length in the Gleeble tensile test has been a great challenge to the accurate description on constitutive behavior at elevated temperatures. In the present work, a novel design of grips is developed to flexibly regulate the length of Uniform Temperature Zone (UTZ) along the specimen gauge length. A system with new speckle preparation method and great optical anti-interference ability is developed for the accurate strain calibration at elevated temperature and high-speed deformation based on digital image correlation (DIC). The influence of the length of UTZ and the strain measurement methods on the accuracy of parameters identification is further investigated. The DIC based calibration method on full-field strain and strain rate makes the parameter identification more robust on the length variation of UTZ, that is, the identified parameters are nearly the same using different UTZ length of specimen. The appropriate UTZ length should be satisfied to prevent the ends of gauge length from blue brittleness and the undesirable end failure. The identified constitutive parameters are verified by comparison between simulation and experiments of the hot stamping hat-type parts. It shows that the newly developed DIC-based method is the most precise within the context of the tested approaches to determine constitutive parameters of hot stamping process.

The Effect of Prestrain Temperature on Kinetics of Static Recrystallization, Microstructure Evolution, and Mechanical Properties of Low Carbon Steel

In this research, the samples of a low carbon steel sheet were rolled up to a thickness prestrain of 67% at three different temperatures consisted of room, blue brittleness, and subzero temperature. Microhardness, SEM, and tensile tests were carried out to evaluate the static recrystallization kinetics defined by the Avrami equation, microstructural evolution, and mechanical properties. It was found that the Avrami exponent is altered with change in prestrain temperature and it achieves the value of 1 to 1. 5. Moreover, it was indicated that prestraining at subzero temperature followed by annealing at 600 °C leads to considerable enhancement in tensile properties and kinetics of static recrystallization compared to room and blue brittleness temperatures. The prestraining at blue brittleness temperature followed by annealing treatment caused, however, a higher strength and faster kinetics compared with that at room temperature. It was concluded that although from the steel ductility point of view, the blue brittleness temperature is called an unsuitable temperature, but it can be used as prestraining temperature to develop noticeable combination of strength and ductility in low carbon steel.

High strain rate response of S355 at high temperatures

In this paper the high strain rate behaviour in tension and in a wide range of elevated temperatures of the S355 structural steel is presented. A Split Hopkinson Tensile Bar for the mechanical characterisation at high strain rates, equipped with a water-cooled induction heating system is used. These data are collected with the purpose of evaluating the extreme combined effect of dynamic loadings and elevated temperatures (200°C, 400°C, 550°C, 700°C and 900°C), e.g., a fire load followed by an explosion. The reduction factors for the main mechanical properties are reported. The novelty of our data is the addition of the strain rate dependency to the temperature. High strain rate tests at 550°C highlighted the phenomenon known as blue brittleness where an increase of strength and a decrease of ductility were ascribed to the dynamic strain ageing. Focusing the attention on the thermal softening parameter, m, the widely used constitutive law proposed by Johnson and Cook during the eighties is critically reviewed highlighting some weaknesses. The results can be of great interest for the assessment of robustness in structures where a fire induced progressive collapse should be evaluated focusing the attention to the extreme combined effects.