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.     

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.     

Effect of specimen size in determining ductile to brittle transition temperature

Ductile to brittle transition temperature (DBTT) for 9Cr–1Mo steel has been determined from Charpy impact testing for full size and subsized specimens. DBTT was obtained at various percentage of upper shelf energy (USE). Assuming that most of the energy is spent in crack initiation, notch root volumes of subsized specimens (V_NS) were normalised with full size specimen (V_NF), and a power law relationship between DBTT and notch root volume has been established. From finite element method, it is observed that the sum of von Mises stress (σ_eq) and hydrostatic stress (σ_h) reaches ～2400 MPa (fracture stress, σ_f*) as the specimen dimension decreases at a temperature corresponding to 33% USE. This corresponds to ～68 J of full size specimen used in the determination of nil ductility transition temperature.     

Competing risks models for fracture in the ductile to brittle transition temperature region

When fracture toughness testing is carried out over the ductile to brittle transition temperature region cleavage instability may be observed at the initiation of cracking or after some prior ductile crack growth. The amount of precleavage ductile crack growth increases with increasing temperature. At the lower test temperatures, it is possible to assume that all tests will result in cleavage instability. However, as the test temperature increases, at some limiting temperature, the failure mode during the final instability changes from cleavage to ductile. These two different types of behaviour can be accommodated in a statistical analysis which is based on the method of competing risks. A statistical approach is presented for the analysis of data by competing risks and a procedure is given for the estimation of the probability of cleavage failure.     

Control of ductile to brittle transition temperature in ferritic pressure vessel steel

Abstract

Controlled amounts of cold work are shown to cause a minimum in the ductile to brittle transition temperature (DBTT) in a ferritic steel at a critical level of ～1·5%. Mechanical property assessments show that the hardness values exhibit the same trend. A theory is advanced for explanation of these effects, based on work hardening and Cottrell–Bilby locking models. Consideration is given to an alternative Ashby–Embury model, but it is concluded that the former approach is most successful in predicting the observed DBTT shift behaviour. Although independent of fracture surface type, the degree of plastic deformation shows some dependency on the grain boundary character. This leads to the conclusion that the matrix yield strength is the primary factor in determining the DBTT in these steels. Discussion focuses on methods for exploiting the effect to give higher toughness steels utilising knowledge of how to control matrix hardening and cleavage fracture strength.     

Hydrogen effects on the ductile to brittle transition behaviour of 21-6-9 stainless steel

Charpy V-notched impact test studies on 21-6-9 austenitic stainless steel at 293 and 77 K demonstrated that hydrogen charging promoted the formation of larger microvoids at 293 K, promoted the formation of facets at 77 K, and reduced the energy absorbed by the material at both temperatures. These observations suggest that the role of hydrogen in the impact behaviour of this material is to enhance whatever crack-growth mechanism is operating at a given temperature. Further, the observation that embrittlement exists even at liquid nitrogen temperatures indicates that little or no localized rearrangement of hydrogen during the test is required or that relatively high strain-rate effects on hydrogen embrittlement need not be necessarily attributed to enhanced transport of hydrogen atmospheres by mobile dislocations. The data presented in this paper are consistent with a model in which the mechanism of hydrogen embrittlement is affected by the extent of plastic deformation.     

Effects of carbon and niobium on microstructure and properties for Ti bearing steels

The objective of the study described here is to elucidate the effect of carbon and niobium on the microstructure, precipitation behaviour, and mechanical properties of 0·09C–0·11Ti (%) steel and 0·05C–0·025Nb–0·11Ti (%) steel under ultra fast cooling condition. The strengthening mechanisms are also discussed. The ferrite grains size and the size of precipitates in Ti and Nb–Ti steels were measured respectively. The mechanical properties obtained in Ti steel were similar to Nb–Ti steel with yield stress >700 MPa, elongation >20%, and good low temperature impact toughness. The study underscores that addition of higher carbon content by 0·04% under controlled rolling and ultra fast cooling conditions, we can achieve similar strength in the absence of micro-alloying element, niobium.     

Modelling of fracture toughness data in the ductile to brittle transition temperature region by statistical analysis

A fracture toughness database for a ferritic 22NiMoCr37 steel forging for 12.5, 25, 50 and 100 mm thick specimens tested at nine different temperatures has been analysed statistically. The method employed uses a statistical procedure based on competing risks to evaluate the fracture toughness and quantify the probability of cleavage fracture as a function of temperature, specimen thickness and ductile crack growth. This paper describes the application of the competing risks statistical methods to the fracture toughness database obtained from the joint European Project.     

Modeling the brittle–ductile transition in ferritic steels. Part II: analysis of scatter in fracture toughness

A dislocation simulation model has been proposed to predict the brittle–ductile transition in ferritic steels in Part I. Here we extend the model to address the problem of inherent scatter in fracture toughness measurements. We carried out a series of Monte Carlo simulations using distributions of microcracks situated on the plane of a main macrocrack. Detailed statistical analysis of the simulation results showed the following: (a) fracture is initiated at one of the microcracks whose size is at the tail of the size distribution function, and (b) the inherent scatter arises from the distribution in the size of the critical microcrack that initiates the fracture and not from the variation of the location of the critical microcrack. Utilizing the weakest-link theory, Weibull analysis shows good agreement with the Weibull modulus values obtained from fracture toughness measurements.     

Assessment of effect of ductile tearing on cleavage failure probability in ductile to brittle transition region

The fracture behaviour of ferritic and ferritic martensitic steels in ductile to brittle transition (DBT) region has been extensively studied in recent years and a probabilistic approach of master curve method is generally used to describe the fracture toughness of BCC steels in DBT region as a function of temperature. The assessment of cleavage failure probability however is still untouched in the upper region of ductile to brittle transition, although various extensions of master curve approach and various local approaches has been explored. Additionally the geometry and loading in tension and bending also adds up to the difficulties when cleavage failure is assisted with prior ductile tearing. In this work the cleavage fracture is investigated in upper region of DBT and a modified master curve approach is presented which can satisfactorily describe the fracture toughness as a function of temperature as well as amount of ductile tearing preceded by cleavage.     

Fatigue analysis of ductile and brittle behaving steels under variable amplitude multiaxial loading

The analysis of fatigue behavior under multiaxial variable amplitude stress states, despite its wide applicability, has not been fully studied. Issues such as varying degrees of nonproportionality of the load history, cycle counting, damage accumulation, failure behavior of the material, and mean stress fluctuations which can significantly affect the results of these analyses have not been well understood. In this study, a methodology for the analysis of fatigue behavior under multiaxial variable amplitude loading conditions is employed which accounts for the aforementioned issues. At its core, the applied methodology uses critical plane analysis based on the failure behavior of each material to assess the fatigue damage. In order to evaluate the performance of the analysis method, axial, torsional, and combined axial‐torsional variable amplitude tests were performed on one ductile and one brittle behaving steel. The applied methodology resulted in close estimation of the experimental fatigue life for both ductile and brittle behaving steels.     

Brittle-to-ductile transition in steels and the critical transition temperature

Specimens of different thickness made of a low-alloy steel were tested to determine fracture toughness under static and impact loading at room and low temperatures. It was found that for the specimen of each thickness there was a specific brittle-to-ductile transition temperature T_psdefined as the upper temperature boundary up to which plane-strain conditions were valid at fracture. The temperature T_ps, as well as the brittleness transition temperature T ₅₀ (determined via the 50% ductile component of the fracture surface), rose with the increase in the thickness B of a specimen. The value of T_pswas found to be in a linear dependence on lgB for both static and impact loading.

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Résumé On a procédé à l'essai d'éprouvettes de différentes épaisseurs réalisées en acier faiblement allié en vue de déterminer la ténacité à la rupture sous charge statique et d'impact à température ambiante et à basse température. On a trouvé que pour les éprouvettes correspondantes à chaque épaisseur, il existait une température de transition spécifique ductile-fragile T_ps, définie comme la limite supérieure de température sous laquelle les conditions d'état plan de déformation sont atteintes à la rupture. La température T_psainsi que la température de transition de fragilité T₅₀, correspondant à une surface de rupture présentant 50% de ductilité, se relèvent avec un accroissement de l'épaisseur B de l'éprouvette. La valeur de T_ps a été trouvée en dépendance linéaire du lgB à la fois en contrainte statique et en contrainte d'impact.

     

Comparative study of multiaxial fatigue damage models for ductile structural steels and brittle materials

Fatigue damage prediction under a general multiaxial service loading consists of three main steps: multiaxial cycle counting, damage evaluation for an identified cycle (or reversal), and damage accumulations. The accuracy of fatigue life predictions depends on all the above steps. This paper reviews the evolutions of various multiaxial fatigue damage models, a comparative study is conducted about the physical basis, the computational efficiency, and the application range of the approaches. Based on the comparative studies, a new procedure is proposed to evaluate fatigue damage under general multiaxial random loading, which uses the Wang and Brown´s multiaxial cycle counting method for identifying cycles (or reversals), the modified procedure of the minimum circumscribed ellipse (MCE) approach for fatigue damage evaluation for an identified cycle (or reversal), and the Miner´s linear damage law for fatigue damage accumulations. By comparisons of the predicted life results with experimental results and with other approaches, it is shown that the proposed procedure is very efficient and suitable for computer aided structural optimization against fatigue.