Brittle-to-ductile transition in the fracture of steels hardened by thermomechanical treatment

This article describes the results of a comparative study of the brittle-to-ductile transition observed during the fracture of steel after quench-hardening and after thermomechanical treatment, with respect to the tempering and test temperatures. Substantial differences in the kinetics of the increase in ductility during the brittle-to-ductile transition were revealed by mechanical torsion tests and electron microscopic examination of fracture surfaces.     

Determination of the critical ductile-brittle transition temperature in testing the cracking resistance of steels

Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 1, pp. 7–11, January, 1988.     

Brittle-to-ductile transition temperature and its strain rate sensitivity in a two-phase titanium aluminide with near lamellar microstructure

The temperature dependence of tensile properties of a two-phase titanium aluminide with nearly lamellar microstructure has been investigated and brittle-to-ductile transition (BDT) temperatures (T_BDS) have been determined under different strain rates from 10^–5 to 10^–1 s^–1. It is found that T_BD rises with the increase of strain rate. From the positive strain rate sensitivity of T_BD, the apparent activation energy of BDT is determined to be 324 kJ/mol by means of Zener-Hollomon factor. The determined activation energy approximates to the activation energies of self-diffusion of Ti atoms, and inter-diffusion of Ti and Al atoms in TiAl phase. The approximation, fractography analysis and theoretical calculation using the Nabarro Model add up to the speculation that the BDT of the alloy is controlled by dislocation climbing.     

Model for static recrystallisation critical temperature in microalloyed steels

Abstract

By means of hot torsion tests, the static recrystallisation critical temperature (SRCT) has been determined for 18 microalloyed steels classified into two groups. In one group the metallic microalloying element is vanadium, and in the other it is niobium. In both groups the microalloying element, carbon, and nitrogen contents vary from one steel to another. Tests have been carried out at various strains and strain rates, and recrystallisation–precipitation–time–temperature (RPTT) diagrams have been drawn for each steel in each condition. The SRCT is the asymptote of strain induced precipitation start P _s and end P _f curves, and its determination has permitted the construction of a model that quantifies the effects of all the external variables implicit in hot working such as strain and strain rate, and the internal variables such as austenite grain size and chemical composition of the steel. Hence, the influence of each of these variables has been quantified, and the model's prediction, comparing experimental values with calculated values, gives a correlation index of ～0.9.     

Ductile-to-brittle transition temperature as a measure of the crack resistance of steels

Conclusion In 1971 there was developed in collaboration with G. S. Pisarenko a model [19] of fracture of materials representing this process as a successive series of acts of origin of a microcrack at the tip of the main crack and their subsequent merging. In [20] the proposal was made for determination of the ductile-to-brittle transition temperature as the temperature corresponding to the appearance at the crack tip at the moment of fracture of a plastic zone of maximum size in which plane strain conditions are still satisfied. In it on the basis of the earlier proposed model there was obtained a determining relationship relating the transition temperature to the basic parameters influencing it, specimen dimensions, loading rate, and structure of the material. As the result of fruitful collaboration with Professor D. Francois and the presentation by him of broad experimental data on the scale effect in the ductile brittle transition in various steels it was possible to experimentally base the relationships obtained and to propose certain relationships useful for practical purposes [7]. Since [7] is not yet known to Soviet readers, it was appropriate to first give it a brief presentation and then to develop some of its proposals to other cases not touched upon in this work.Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 10, pp. 89–95, October, 1985.     

Determination of charpy transition temperature of ferritic steels using miniaturized specimens

Miniaturized specimen technology permits mechanical behaviour to be determined using a minimum volume of material. A method for obtaining the ductile-brittle transition temperature of ferritic steels was developed using a miniaturized notched bar test. Comparisons between conventional and miniaturized specimen ductile-brittle transition temperatures are encouraging. Fracture toughness values were calculated for the miniaturized notched specimens and compared with large-specimen data. The miniaturized specimen values were high, even after appropriate adjustments had been made. Further development may yield valid data when an optimum combination of specimen size, shape, and notch acuity is determined.The work was performed during employment of Battelle.     

Anomalies of temperature dependences of the elastic moduli of steels of the austenitic and transition classes

Data on the temperature dependence of the moduli of longitudinal elasticity E of a number of commercial steels of the ferritic, martensitic, austenitic, and transition classes in the temperature interval from room temperature to the temperature of liquid helium are presented. It is shown that for the austenitic and transition classes of steels, the elastic modulus E does not increase, but decreases when the test temperature is lowered within certain temperature ranges. It is suggested that this anomaly is associated with antiferromagnetic ordering in the austenite.Translated from Problemy Prochnosti, No. 1, pp. 20–23, January, 1992.     

Constraint effects on the ductile-to-brittle transition temperature of ferritic steels: a Weibull stress model

This study examines crack front length and constraint loss effects on cleavage fracture toughness in ferritic steels at temperatures in the ductile-to-brittle transition region. A local approach for fracture at the micro-scale of the material based on the Weibull stress is coupled with very detailed three-dimensional models of deep-notch bend specimens. A new non-dimensional function g(M) derived from the Weibull stress density describes the overall constraint level in a specimen. This function remains identical for all geometrically similar specimens regardless of their absolute sizes, and thus provides a computationally simple approach to construct (three-dimensional) fracture driving force curves _w vs. J, for each absolute size of interest. Proposed modifications of the conventional, two-parameter Weibull stress expression for cumulative failure probability introduce a new threshold parameter _w–min. This parameter has a simple calibration procedure requiring no additional experimental data. The use of a toughness scaling model including _w–min>0 increases the deformation level at which the CVN size specimen loses constraint compared to a 1TSE(B) specimen, which improves the agreement of computational predictions and experimental estimations. Finally the effects of specimen size and constraint loss on the cleavage fracture reference temperature T ₀ as determined using the new standard ASTM E1921 are investigated using Monte Carlo simulation together with the new toughness scaling model.     

Modeling ductile to brittle transition temperature of functionally graded steels by fuzzy logic

In this article, a model based on fuzzy logic (FL) for predicting ductile to brittle transition temperature of functionally graded steels in both crack divider and crack arrester configurations has been presented. Functionally graded steels containing graded ferritic and austenitic regions together with bainite and martensite intermediate layers were produced by electroslag remelting. For purpose of building the model, training and testing using experimental results from 140 specimens produced from two basic composites were conducted. The used data as inputs in FL models are arranged in a format of six input parameters that cover the FGS type, the crack tip configuration, the thickness of graded ferritic region, the thickness of graded austenitic region, the distance of the notch from bainite or martensite intermediate layer, and temperature. According to these input parameters, in the FL, the ductile to brittle transition temperature of each FGS specimen was predicted. It has been found that FL model will be valid within the ranges of variables. The training and testing results in the FL model have shown a strong potential for predicting the ductile to brittle transition temperature of each FGS specimen.     

An experimental investigation of the effect of biaxial loading on the master curve transition temperature in RPV steels

During the 1990s considerable work was conducted to characterize the effect of biaxial loading on the ductile to brittle transition temperature. The work centered on a series of tests using large cruciform bend specimens from an experimental A533B test plate denoted as HSST Plate 14 (Heavy Section Steel Technology Plate 14). Recently a series of similar biaxial cruciform tests has been conducted on the steel used for an extensive European Round Robin that investigated the ductile-to-brittle transition master curve and associated T₀ reference temperature. The results of these tests have been used to promote the concept of a “Biaxial Effect” which corresponds to a shift in the shallow crack transition master curve of +20 °C or more when biaxial stresses are present, in comparison with the master curve for uniaxially loaded shallow crack specimens. A comprehensive analysis of the all of the available HSST Plate 14 data and data from two other structural steels was performed to investigate the extent of a biaxial effect on the reference temperature, T₀. The analysis included many additional biaxial cruciform test results on three different materials. The results of all three materials discussed in this paper fail to clearly demonstrate that biaxial loading, imposed through the use of a cruciform specimen geometry, has an effect on the fracture toughness, characterized using a master curve approach and reference temperature T₀. The analysis utilized in this paper assumes that the toughness distribution and temperature dependence of shallow cracked specimens can be modeled by using the master curve approach. This assumption has not been rigorously validated and would benefit from further study. Additional detailed stress analysis of the constraint evolution in the cruciform specimens may better define the precise conditions under which a biaxial effect on the fracture toughness could be realized.     

Cleavage to quasicleavage fracture transition in steels

Abstract

The cleavage behaviour of plain carbon steels containing from 0· 2 to 0·8%C has been investigated. It is observed that each steel displays two characteristic temperatures at which a transition in the mode of fracture occurs. These are the transition temperatures for cleavage T_c and for general yielding T_g. At temperatures below T_c, the steels fail by pure cleavage. This involves the generation of a cleavage crack nucleus in a carbide particle followed by cleavage crack propagation. The cleavage fracture stress σ _f is independent of temperature. Between temperatures T_c and T_g, the steels fail by quasicleavage. This involves the generation of a crack nucleus by a localised fibrous process followed by cleavage crack propagation. The crack nucleation stage is shear stress controlled and therefore the quasicleavage fracture stress σ_q increases with decreasing test temperature. Above temperature T_g, failure occurs at or after general yielding.

MST/1045     

Effect of grain size and carbide thickness on impact transition temperature of low carbon structural steels

Abstract

A model previously developed by Petch enables an estimate of the impact transition temperature (ITT) of low carbon structural steels to be made from measurements of grain size and carbide thickness. In the present work, various laboratory heat treated and commercially processed structural steels were assessed in order to ascertain the validity of the Petch model. Laboratory heat treated steels, with carbon contents of <0.1 wt-%, with a range of grain and carbide sizes, showed that accurate predictions of ITT could be made provided that the original Petch model was modified. The revised model incorporated a measure of the largest grains in the material rather than an average grain size, and necessitated a change to the expression used by Petch for the variation in yield stress with temperature at high strain rates appropriate to a Charpy test. The present work also showed that cleavage is associated with the largest carbides in the steel. The modified model also successfully estimated, to within ±10 K, the ITT of some commercially processed structural steels, of similar carbon content to the laboratory treated steels, but containing much thicker carbides. The revised model was slightly less successful in predicting the impact behaviour of a laboratory processed structural steel having a slightly higher carbon content (0.16 wt-%). It is believed that this is primarily because pearlite plays a more significant role in the cleavage fracture process in this steel compared to the lower carbon steels, but its role is ignored in the model.     

Effects of Ti and a twice-quenching treatment on the microstructure and ductile brittle transition temperature of 9CrWVTiN steels

The effects of Ti and a twice-quenching treatment on the microstructure and ductile brittle transition temperature (DBTT) of 9CrWVTiN steels have been studied. The results show that Ti addition reduces austenite grain size and martensitic lath, and moderate Ti (< 0.018%) content reduces the precipitates size. Microstructure, especially the coarse M₂₃C₆ precipitates is remarkably refined by twice quenching and by consuming C through preferential precipitation of MX precipitates in the furnace cooling period, thus contributing to the decrease of DBTT compared with quenching–tempering process. However, DBTT increases with increasing Ti content, and the increased DBTT reaches to as high as 36 °C compared with steel without Ti regardless of the refinement of microstructure. Through precipitate analyses, we find that, Ti strongly interferes with the precipitation of V(C,N). Ti(C,N) hardly exists in matrix alone. Instead, it acts as the core of quadrate (Ti,V)(C,N) particle. Big stress concentrations at the corners of coarse quadrate (Ti,V)(C,N) precipitates make them as crack initiators during impact tests, thus deteriorating the toughness. Also, formation of complex (Ti,V)(C,N) particles reduces the amounts of V, N, and C available for adequate fine V(C,N) particle precipitation. These two factors are the main reasons for high DBTT generated by Ti addition.     

The critical temperature for brittle-to-ductile transition of intermetallic compound based on NiAl

The brittle-to-ductile transition (BDT) behavior of NiAl–9Mo alloys was firstly investigated to occur in the small temperature range of about 10 °C using tensile test method. The characteristic temperature T_AB was defined for BDT in the alloy. It associates with the temperature at which the resistance to cleavage starts to sharply increase and the massive dislocation suddenly generates. Therefore, T_AB is the critical temperature which symbolizes that the material undergoes the brittle-to-ductile transition.     

The correlation of ultrasonic attenuation,microstructure and ductile to brittle transition temperature in very low carbon steels

A study of the frequency dependence of the ultrasonic attenuation and the ductile to brittle transition temperature as a function of microstructure has been carried out for a very low carbon (0.02 wt%) steel. The ultrasonic attenuation was found to be anomalously high when compared to normal low carbon (0.2 wt%) steels and could not be interpreted in terms of simple scattering models. This is thought to be due partly to large hysteresis losses (dislocation and magnetic domain-wall damping) and partly to large grain-size distributions. An empirical correlation between the ductile to brittle transition temperature and an ultrasonic attenutation parameter has been found for this particular steel.     

Suppression of the critical current and the superfluid transition temperature of3He in a single submicron cylindrical channel

We report on an investigation into confined geometry effects and critical currents of superfluid³He in a single circular cylindrical channel. The diameter of the channel, 0.7 µm, is of the order of the (temperature-dependent) coherence length and its aspect ratio is 10. The reduction of the critical temperature demonstrates diffuse scattering on the solid walls of the microchannel. Using the Ginzburg-Landau formulation, we derive a model for the critical current and the critical temperature in a small, infinitely long, cylindrical channel with a circular cross section. The measured reductions of these quantities are in reasonable agreement with the predictions of the model.     

Influence of the dynamic-to-static stress ratio on the second critical transition temperature of a low-alloy steel

The results of bending tests are presented for cracked plane specimens of steel 09G2 under combined loading, i.e., under simultaneous action of static and dynamic stresses at room and low (down to −80°C) temperatures. It has been shown that, at a given temperature, the dynamic component of the critical stress decreases linearly with increase in the static stress. The influence of the static-to-dynamic stress ratio upon critical stresses and the second critical transition temperature has been studied. It is shown that with increase in the coefficient of dynamic capacity which is equal to the critical-to-static stress ratio, the critical stresses decrease and the second critical transition temperature increases. The conclusion has been drawn regarding the necessity of determining the critical stress and the second critical transition temperature of the material under study for the coefficient of the load dynamic capacity of a real structure. Translated from Problemy Prochnosti, No. 2, pp. 41–47, March, 1997.