Upper shelf temperature dependence of fracture toughness for four low to intermediate strength ferritic steels

To examine the inadequacy and expose the potential consequences of utilizing transition temperature range fracture toughness values when performing fracture mechanics analyses on structures operating at upper shelf temperatures, the upper shelf fracture toughness behavior of three rotor forging steels (ASTM A471 NiCrMoV, ASTM A470 CrMoV and AISI 403 modified 12 Cr) plus an ASTM A217 $\text{2}\text{1}\text{4}\text{Cr}$-lMo cast steel was investigated. Elastic-plastic fracture toughness values were obtained via the $\text{J}{}_{\mathrm{IC}}$ resistance curve test technique. Except for the CrMoV rotor steel, upper shelf fracture toughness peaked near the temperature where zero percent brittle fracture first occurred in the Charpy V-notch impact tests after which decreases of 25–45% in toughness were realized. Fracture toughness tests were conducted up to 800°F.     

Development of fracture toughness of ultrahigh strength low alloy steels for aircraft and aerospace applications

Abstract

Recently, ultrahigh strength low alloy steels, e.g. AISI 4340 and 300M, have been used increasingly for critical structural aircraft and aerospace applications. These steels can be employed successfully at yield strengths of ≥1400 MN m^?2 but their use has often been limited in commercial practice because of low fracture toughness compared with other types of ultrahigh strength steel. The results of studies carried out over the past two decades to improve the fracture toughness are presented. Particular emphasis is placed on improvements obtained by microstructural control via thermal and thermomechanical treatments, sulphide inclusions, and new alloying design. The major metallurgical factors controlling fracture toughness are discussed for each of these techniques.

MST/1413     

The use of stretch zone width measurements in the determination of fracture toughness of low strength steels

The evaluation of fracture toughness from stretch zone measurements on test piece fracture surfaces using selective backscattered electron imaging in the scanning electron microscope is described. Results are compared with the initiation toughness values obtained using single specimen $\text{R}$ curve procedures.     

Hardness and strength of steels at low temperatures

Volgograd Polytechnic Institute. Translated from Problemy Prochnosti, No. 10, pp. 112–115, October, 1988.     

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.     

Deformation induced ferrite transformation in low carbon steels

Deformation induced ferrite transformation (DIFT) is a kind of solid state transformation induced through deformation, which can be applied to be as the effective method to produce fine or ultrafine ferrite grains. This paper reviews the research progress in the theory and application of DIFT from five aspects: evidence and study methods, thermodynamics and kinetics, transformation mechanisms, factors influencing DIFT, application of DIFT in production of fine grained C–Mn steel and ultrafine-grained microalloyed steel.     

Strain rate effects on dynamic fracture and strength

An experimental procedure and accompanying theoretical analysis is presented to produce a well-characterized technique for quantifying dynamic fracture properties of quasi-brittle materials. An analytical and experimental investigation of mode I fracture of concrete was conducted under the dynamic loading of a split Hopkinson pressure bar. Fracture specimens in the form of notched-cavity splitting tension cylinders were subjected to stress wave loading that produced strain rates nearing 10/s. Fracture parameters were extracted by the application of the two-parameter fracture model, a nonlinear fracture model for quasi-brittle materials. Finite element analysis verified the experimental configuration and addressed inertial contributions within the dynamic environment. Ultra-high-speed digital photography was synchronized with the fracture process to provide additional validation and insight to the experimental technique. Results show that the effective fracture toughness and specimen strength both increase significantly with loading rate. The numeric and photographic results validate the experimental technique as a new tool in determining rate dependent material properties.     

Comparison of mode I and mode II elastic-plastic fracture toughness for two low alloyed high strength steels

In the present work, mode I and mode II tests were carried out on two low alloyed high strength steels. An asymmetrical four point bend specimen and J _II-integral vs. crack growth resistance curve technique were used for determining the mode II elastic-plastic fracture toughness, J _IIc · J _II-integral expression of the specimen was calibrated by finite element method. The results indicate that the present procedure for determining the J _IIc values is easy to use. Moreover, the mode I fracture toughness J _Ic is very sensitive to the rolling direction of the test steels, but the mode II fracture toughness J _IIc is completely insensitive to the rolling direction of the steels, and the J _IIc /J _Ic ratio is not a constant for the two steels, including the same steel with different orientations. Finally, the difference of the fracture toughness between the mode I and mode II is discussed with consideration of the different fracture mechanisms.     

Brittle fracture of high-strength steels at very low temperatures

High-strength steels are used to increase the load carrying capacity of components. However, to guarantee a safe design, it is also necessary to combine high strength with adequate fracture toughness. In this paper, fracture toughness of three high-strength steels with yield strengths ranging from 460 to 890 MPa has been studied at very low temperatures. Taking into account experimental evidence, a new mechanism of cleavage at very low temperatures is proposed. This mechanism considers the possibility of reaching the ideal strength (the stress at which the lattice of a single crystal losses its stability) in the immediate vicinity of the fatigue crack tip. Moreover, a computational model able to calculate the external load needed to produce a catastrophic failure of these steels has been developed.     

Strain dependence of creep cavity nucleation in low alloy and 12%Cr steels

The nucleation of creep cavities was analysed using data from the literature for creep resistant low alloy and 12%Cr steels. The number of cavities per unit area was in most cases found to be a linear function of the creep strain, the cavity nucleation rate increasing linearly with increasing minimum creep rate. As a result, the cavity nucleation rate showed the same type of stress and temperature dependence as the strain rate, i.e. a power law function of the stress, where the stress is raised to a power close to the exponent in the creep rate equation (Norton's exponent). Simple models relating the number of cavities to strain and time as well as relating the cavity nucleation rate to time were derived. The theoretically estimated cavity numbers and cavity nucleation rate in relation to strain and time showed satisfactory agreement with the observations for both low alloy and 12%Cr steels.

MST/3353     

On the embrittlement of fracture toughness specimens of two high strength steels

Hydrogen embrittlement of two commercial materials, a 1$\text{1}\text{2}$ Ni-Cr-Mo steel and a 5 Cr-V-Mo steel both quenched and tempered to a nominal ilrength of 1.60 × 10³MN/m² has been studied. Frecture toughness testing was carried out on each steel in three conditions, (a) heat-treated and tested in air, (b) thermally charged with hydrogen and tested in air, and (c) uncharged and tested a an atmosphere of dry hydrogen. The effects of hydrogen pressure and loading rate on embrittlement were examined. The experimental results show that thermal charging had a greater embrittling effect on the 1$\text{1}\text{2}$ Ni-Cr-Mo steel than on the 5 Cr-V-Mo steel although the latter had a higher hydrogen concentration. In a hydrogen environment the 1$\text{1}\text{2}$ Ni-Cr-Mo steel was much more sensitive to embrittlement than the 5 Cr-V-Mo steel.     

Modelling coarsening behaviour of TiC precipitates in high strength,low alloy steels

Abstract

Some of the most modern automotive sheet steels rely on a dispersion of fine precipitates based on TiC, generated during the major phase changes that occur as the rolled material is cooled to the coiling temperature. The coils themselves cool extremely slowly, thus leading to the coarsening of the precipitates and a loss of strength. Beginning with a calculation of the interfacial energy, the precipitate coarsening kinetics are modelled as a function of the stoichiometry of titanium and carbon. The purpose was to assess the influences of interface energy and Ti/C stoichiometry which limit the rate at which the dispersion coarsens by the diffusion of solute from the small to the larger particles. It is found that Ti/C ratio plays a critical role; a titanium concentration which is slightly less than required to combine with carbon leads to a dramatic reduction in the coarsening rate.     

Evaluation and review of simultaneous transformation model in high strength low alloy steels

Abstract

This work briefly describes and evaluates one of the most complete transformation models, which deals with the non-isothermal decomposition of austenite. The model, that does not consider the effect of precipitation on phase transformations, has been experimentally validated in high strength low alloy steels in order to evaluate how it works for microalloyed steels, where precipitation may play an important role. It has been found that the simultaneous transformation model is able to predict with an excellent agreement in microalloyed steels the formation of microstructures consisting of ferrite plus pearlite. However, the bainite formation is not successfully described by the model. The calculations incorrectly predict the formation of martensite instead of bainite in many situations.     

Microstructure and mechanical properties of bainitic low carbon high strength plate steels

This paper reports the results of an investigation of plates produced by the advanced thermomechanical processing (TMP) schedules, which were designed using the results of a laboratory study. There were two steel compositions that corresponded to X-80 with carbon contents 0.04 and 0.07 wt.%, respectively. The variation in microstructure, hardness, tensile properties and Charpy impact properties with TMP schedule were determined, and compared with the expected requirements for X-100 linepipe steel. The relationships between microstructure and mechanical properties were experimentally obtained and discussed.