The fracture toughness of borides formed on boronized cold work tool steels

In this study, the fracture toughness of boride layers of two borided cold work tool steels have been investigated. Boriding was carried out in a salt bath consisting of borax, boric acid, ferro-silicon and aluminum. Boriding was performed at 850 and 950 °C for 2 to 7 h. The presence of boride phases were determined by X-ray diffraction (XRD) analysis. Hardness and fracture toughness of borides were measured via Vickers indenter. Increasing of boriding time and temperature leads to reduction of fracture toughness of borides. Metallographic examination showed that boride layer formed on cold work tool steels was compact and smooth.     

Measuring toughness and the cohesive stress-displacement relationship by the essential work of fracture concept

The complete fracture behaviour of ductile double edge notched tension (DENT) specimen is analysed with an approximate model, which is then used to discuss the essential work of fracture (EWF) concept. The model results are compared with the experimental results for an aluminium alloy 6082-O. The restrictions on the ligament size for valid application of the EWF method are discussed with the aid of the model. The model is used to suggest an improved method of obtaining the cohesive stress-displacement relationship for the fracture process zone (FPZ).     

Assessment of the fracture toughness of cast steels

Estimates of the fracture toughness in terms of the critical stress intensity factorsK _C andK _IC are made for a 1Cr steel, a 1/2Cr-1/2Mo-1/4V steel, a 1 1/2Mn-Ni-Cr-Mo steel and a 1 1/2 Ni-Cr-Mo steel all in cast form. The methods used are linear elastic fracture mechanics,J-integral and crack opening displacement methods. The last two methods are applied in combination with an electrical potential method to detect the initiation of fracture.     

HAZ fracture toughness of titanium containing steels

Abstract

Steels containing various combinations of microalloying elements (Nb, V, and Ti) were welded at heat inputs from 3 to 6 kJ mm^?1. It was shown by detailed crack tip opening displacement fracture toughness testing of coarse grained heat affected zone (HAZ) regions in single pass weld deposits that the poorest toughness properties were exhibited by steel containing a combination of Nb, V, and Ti. Steel microalloyed with only titanium had the best HAZ fracture toughness at all heat input levels. Detailed microstructural analysis, grain size measurement, hardness, and precipitation in HAZ regions were evaluated to explain the fracture toughness properties observed.

MST/887     

Use of the acoustic emission method for determination of the fracture toughness of ductile steels

Siberian Metallurgical Institute, Novokuznetsk. Translated from Problemy Prochnosti, No. 1, pp. 26–30, January, 1989.     

Model for fracture toughness alteration due to cyclic loading

A new model that is capable of predicting and explaining the effect of cyclic loading on the apparent fracture toughness of materials was developed. The model combines macroscopic fracture criteria with the assumption that transient flow properties of material in the cyclic plastic zone can be simulated by those of macroscopic low cycle fatigue specimens, tested in reversed strain control. Little or no changes in the cleavage fracture toughness due to cyclic loading is predicted or observed for materials that cycle strain harden (e.g., rail steel) and in the fracture toughness of other materials that cycle strain harden (e.g., the commercial 2000 series Al-Cu alloys) and fracture by rupture. However, an increase in the fracture toughness is predicted and observed for materials that cycle strain soften (e.g., 1Cr-Mo-V and 18 Ni 300 maraging steels), irrespective of fracture mode (cleavage or rupture). The changes in the fracture toughness are predicted and observed to increase with both the number of cycles of applied load and the reversed plastic strain range (or stress intensity range for precracked specimens).

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Résumé On a développé un modèle à même de prédire et d'expliquer l'effet de mise en charge cyclique sur la ténacité apparente des matériaux. Ce modèle combine les critères de rupture macroscopique avec l'hypothèse que les propriétés d'écoulement transitoire d'un matériau dans une zone où se produit une déformation plastique cyclique peut être simulée par les propriétés d'éprouvettes de fatique olygocyclique macroscopique essayées en déformation alternée. On ne prédit ni n'observe que peu ou pas de changement de la ténacité à la rupture, par clivage dû à une sollicitation cyclique dans le cas de matériaux qui font état d'un écrouissage cyclique, par exemple les aciers à rail. Il en est de même en ce qui regarde la ténacité à la rupture pour les autres matériaux qui durcissent par écrouissage cyclique, par exemple la série commerciale 2000 des alliages AlCu et qui se rompent de manière ductile. Toutefois, on prédit et on observe un relèvement de la ténacité à la rupture dans le cas de matériaux qui s'adoucissent sous l'effet de déformations cycliques, par exemple les aciers 1 CrMoV et les aciers maraging 18Ni300, quel que soit le mode de rupture, c'est-à-dire par clivage ou par rupture ductile. On peut prédire et l'on observe effectivement un accroissement de la ténacité à la rupture avec le nombre de cycles de la charge appliquée et avec l'étendue de la déformation plastique alternée (ou l'étendue de l'intensité de contrainte dans le cas des échantillons préfissurés).

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(The authors wish to dedicate this paper in memory of the late Professor Alan S. Tetelman.)     

New energy partitioning approach to the measurement of plane-strain fracture toughness of high-density polyethylene based on the concept of essential work of fracture

This study explores three approaches to determining plane-strain fracture toughness of high-density polyethylene (HDPE), based on the concept of essential work of fracture (EWF). The first approach adopted the traditional method that uses total energy consumed in the testing to determine the plane-strain fracture toughness. The second and the third approaches used energy-partitioning principles to exclude the energy consumption in a later stage of the testing from the calculation of the EWF value. The 2nd approach used a new energy partitioning scheme that excluded the energy consumed at the last stage of plastic deformation; and the 3rd approach was based on an existing energy partitioning scheme proposed by another group, to exclude the energy consumption after the maximum load was reached. The results show that only the 2nd approach generates the EWF value that is independent of the specimen thickness. The paper recommends that the 2nd approach be the most suitable method for determining the plane-strain fracture toughness for ductile polymers like HDPE.     

Weibull master curves and fracture toughness testing Part IV Dynamic fracture toughness of ferritic-martensitic steels in the DBTT-range

Ferritic-martensitic steels are considered to be promising candidates for structural materials in fusion technology. However, they are sensitive to irradiation embrittlement, being characterized by a shift of the ductile-to-brittle transition temperature (DBTT) to higher values. It has already been shown in an earlier publication, that dynamic Weibull master curves are useful in estimating the capacity of materials to undergo stable (micro)cracking prior to brittle failure, if Charpy impact tests are performed in the DBTT-range. Thus, experimental dynamic Weibull master curves of three ferritic-martensitic steels, having been obtained by performing series of instrumented Charpy impact tests at a defined temperature in the DBTT-range, have been evaluated for subsize and, in the case of the reference ferritic-martensitic steel of the European Fusion Technology Program MANET II, for normal-size specimens. It has been observed, that the capacities of stable (micro)cracking of MANET II are nearly optimal and clearly superior to those of the other two considered steels. On the other side, the capacity of stable (micro)cracking prior to failure has been found to be highly specimen-size-dependent, nevertheless it is thought to be an important factor in predicting DBTT-shifts due to irradiation embrittlement.     

Plane strain fracture toughness of modern ferritic structural steels

Abstract

The temperature dependence of the plane strain fracture toughness of a low carbon, fine grain, ferritic steel for structural applications is investigated. The ductile–brittle transition is found to occur in the interval between 160 and 184 K. The experimental results are interpreted by an analytical model which permits calculation of the plane strain fracture toughness K _1c in the brittle domain as a function of the tensile properties and the cleavage fracture stress, making use of a piecewise approximation for the distribution of tensile stress on the crack axis and applying a deterministic fracture criterion at the stress peak. A similar criterion, which consists of equating the severest strain on the crack axis to a critical strain for cavity nucleation, provides the upper shelf fracture toughness. A relatively simple figure for predicting the transition temperature of steels in this family as a function of material properties can be obtained in this way.     

Microstructural features affecting fracture toughness of high strength steels

The fracture toughness of quenched and tempered steels, such as AISI 4340, AISI 4130 and 300M, can be increased by 50–100% by minor changes in heat treating procedures. Certain microstructural features, particularly blocky ferrite, upper bahnte and twinned martensite plates, are deleterious to fracture toughness. Similarly, the presence of undissolved carbides and sulfide inclusions, which act as crack nuclei, can lower fracture toughness by 25–50%. Other microstructural constituents, such as lower bainte, autotempered martensite, and retained austenite can enhance fracture toughness. By controlling the amounts and distributions of the microstructural constituents, the fracture toughness values of AISI 4340, AISI 4130 and 300M can be raised to the fracture toughness level of 18Ni maraging steel at equivalent values of yield strength.     

Process zone in fracture toughness testing of HSLA steels

Materials Science - The method of statistic fractography used after the fracture toughness tests provided the following information:     

The influence of manganese on the fracture toughness of nickel steels

The effect of manganese content on the fracture toughness of a high-purity martensitic 0.6% C, 5% Ni steel has been determined after tempering below 350°C. The intergranular mode, characteristic of the brittle fracture of material with only trace manganese content, was eliminated by the addition of between 0.1% and 0.6% of that element. However, the purposeful addition of manganese introduced slow crack growth along an intergranular path prior to fast fracture which could in turn, be eliminated by testing in a dry environment. Although this environmental sensitivity prevented the full potential toughness of the material being realised in normal atmospheres the manganese addition effectively improved the toughness, and it is suggested that this is consequential to a change in the fracture criterion.

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Résumé L'effet de la teneur en manganèse sur la ténacité à la rupture d'un acier martensitique de haute pureté à 0,6% C et 5% Ni a été déterminé après revenu à 350°C.En procédant à des additions de manganèse de 0,1% à 0,6%, on a pu éliminer le mode intergranulaire qui caractérisait la rupture fragile de l'acier ne comportant que des traces de cet élément.Toutefois, ces additions intentionnelles de manganèse provoquent une tendance à la croissance lente de fissures le long d'un parcours intergranulaire, suivie d'une rupture brutale, qui peut, à son tour, être éliminée en travaillant dans une atmosphère sèche.En dépit du fait que la sensibilité à l'environnement pénalise la ténacité potentielle du matériau en atmosphère normale, des additions de manganèse améliorent effectivement la ténacité, et l'on suggère que ceci est dû à une modification du critère de rupture.