Fracture Toughness Of High Strength Steels Research Articles

Experimental study on fracture toughness of quenched and tempered and TMCP high strength steels

In this study, the fracture toughness of five kinds of high strength steel (HSS) with the nominal yield strength of 550 MPa, 690 MPa and 890 MPa were experimentally studied. Two steel delivery conditions, i.e., quenched and tempered HSS and thermo-mechanical control process HSS, were included and compared. The effect of heat input on the fracture toughness of HSS was investigated. A comparative study was organized for the fracture toughness of base material (BM), heat-affected zone (HAZ) and weld metal (WM). Results indict that the fracture toughness of the heat-affected zone of QT steels (QT550 and QT690) first increases and then decreases slightly with the increase of heat input. There is an optimal heat input for quenched and tempered HSS welding to achieve satisfactory fracture toughness. Either too small or large heat input is not beneficial to the resistance to crack propagation. With increase of steel strength, it seems the optimal heat input for quenched and tempered HSS becomes smaller. The effect of heat input on fracture toughness of TMCP HSS is different with quenched and tempered HSS: higher heat input would normally produce higher fracture toughness for HSS TMCP550. With the increase of yield strength, the fracture resistance increases for both quenched and tempered HSS and TMCP HSS.

The relationship between fracture toughness and tensile property in high-strength steels

In the present work, the fracture toughness and tensile property of 40CrNiMoA high-strength steel were systematically investigated. The experimental results reveal that the ultimate tensile strength of 40CrNiMoA steel decreases gradually with improving the tempering temperature, while the fracture toughness reveals a reverse trend to the tensile strength. The fractography shows the consistency of quasi-static fracture characteristic in the tensile and KIC specimens. The complicated fracture mechanism of 40CrNiMoA steel is mainly attributed to the various microstructures: grain size, dislocation density and precipitated carbides. Additionally, based on the macro/micro- fracture mechanisms and energy principle, we proposed the relation between fracture toughness and macroscopic tensile parameters. The new model would provide an effective strategy to predict the fracture toughness of high-strength steels, which can be further applied to other engineering materials such as aluminum and titanium alloys.

(Invited) H-Mat: Enhancing the Durability and Affordability of Metals and Polymers in Hydrogen Service

The H-Mat national laboratory consortium is conducting cross-cutting, foundational R&D to enhance the durability of key polymers and metals in hydrogen service. Technologies utilized in hydrogen service commonly face stresses that influence materials compatibility, such as cryogenic temperatures, fatigue loading, and corrosive environments. Industries in which hydrogen compatibility can heavily influence materials selection are diverse, and include fuel cell electric vehicles (FCEVs), nuclear energy technologies, oil & gas sectors, and vehicle lightweighting applications. H-Mat R&D aims to enhance fundamental understanding of materials’ behavior in hydrogen, with a current focus on materials used onboard FCEVs and at fueling stations. Key research thrusts include: 1) development of novel experimental polymers with enhanced durability in hydrogen pressure cycles, 2) microstructural engineering to enhance the fracture toughness of high-strength steels in hydrogen, 3) development of higher-fidelity predictions of crack initiation in steels in hydrogen service, and 4) evaluation of the effects of moisture on hydrogen induced degradation in aluminum alloys. These R&D thrusts are targeted to enhance durability of hydrogen seals by 50%, fracture resistance of pressure vessel steels by 50%, and life of pressure vessels by 50%. H-Mat is led by Sandia National Laboratories and Pacific Northwest National Laboratory, with participation from Oak Ridge National Laboratory, Savannah River National Laboratory, and Argonne National Laboratory. Members of industry and academia are encouraged to engage with the consortium through cooperative research and development agreements (CRADAs) and funding opportunity announcement (FOA) awards. For more information, please see: https://www.energy.gov/eere/fuelcells/h-mat-hydrogen-materials-consortium

The quantitative relationship between fracture toughness and impact toughness in high-strength steels

In this paper, the fracture toughness and impact toughness were systematically studied in high-strength steels. The results show that the two types of toughness are both the representative of the competition between flat fracture and shear fracture, which follows the minimum energy density criterion. Accordingly, we established the quantitative relation between impact toughness and fracture toughness in high-strength steels through theoretical derivations. The relation is affected by two aspects: the energy consumption pattern and specimen size. It would provide a more economical and faster way to quantitatively predict the fracture toughness of high-strength steels, which can be further applied to some new materials such as high-entropy alloys, nanostructured materials and metallic glasses.

Fracture Toughness of High-Strength Steels Within the Temperature Range of Ductile-to-Cleavage Transition. Master Curves

The fracture toughness of high-strength ferritic steels within the temperature range of ductile-tocleavage transition is analyzed. We tested two steels: S960QC and Hardox-400. The experimental results are compared with the results obtained by using the standard Master Curve (MC) equation. It turns out that the MC equation can be adopted to predict the fracture toughness of the high-strength steels after certain modifications. The shape of the formula can be preserved but some coefficients should be changed. The relationship between the fracture toughness and the thickness of an element included in the MC formula does not exist in the case of high-strength steels. One formula cannot be proposed for the MC of the tested steels, in contrast to the steels with a yield stress below 825 MPa.

Specific Features of Crack Propagation Under the Conditions of Static and Low-Cycle Loading

We study the influence of elliptic fatigue cracks of the fracture toughness of high-strength steel and the influence of static and low-cycle loading on the process of crack growth.

Fracture toughness of high-strength steel

1. The fracture toughness of high-strength steels after tempering at low temperatures is increased by alloying with ∼4% Ni, Cr, and Cu, and also 1%W, 1.6% Si, and 2% Co. Alloying with molybdenum impairs the fracture toughness. 2. The resistance to fracture of high-strength steel depends to a considerable extent on structural factors — the size of martensite crystals and the evenness of carbon distribution in microvolumes of the martensite.