Tempering (metallurgy) | EPFL Graph Search

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures. Introduction Tempering is a heat treatment technique applied to ferrous alloys, such as steel or cast iron, to achieve greater toughness by decreasing the hardness of the alloy. The reduction in hardness is usually accompanied by an increase in ductility, thereby decreasing the brittleness of the metal. Tempering is usually performed after que

Mechanisms of void initiation at small hard particles and implications for ductile fracture

Daniele Cannizzaro

Ductile fracture in steels is essentially due to cavity nucleation, growth and coalescence. Once voids are initiated in the metallic matrix, and the deformation continues to increase, voids grow in size and finally coalescence to fracture. This dissertation investigates ductile fracture initiating in the bulk of material for relatively low stress triaxialities. Particularly zero stress triaxiality conditions, i.e. under shear dominated loading, provided for an isolated study of single mechanisms such as void nucleation, and shear localization. In this dissertation, we show that the idealized division of ductile fracture in a sequence of elementary processes is not appropriate for the selected quenched and tempered VAR steel. Ductile fracture is nucleation-controlled, i.e without meaningful void growth. Void nucleation occurs by debonding of tempering carbides from the surrounding metallic matrix. The carbide-matrix interface strength is estimated by a dislocation-based stress decohesion model. By means of FEM computations with a unit cell simulations comprising a single carbide, the role of the carbide-matrix interface strength is emphasized. The results of FE computations demonstrate that the carbide-matrix interface strength is the key parameter controlling damage initiation and its evolution to fracture.

EPFL2006

High-Temperature Water Effects on the Fracture Behaviour of Low-Alloy Reactor Pressure Vessel Steels

Zaiqing Que

The structural integrity of the reactor pressure vessel (RPV) of light water reactors (LWR) is of utmost importance regarding operation safety and lifetime. High-temperature water (HTW) and hydrogen absorbed from environment, in synergy with irradiation embrittlement, dynamic strain aging (DSA), environmentally-assisted cracking (EAC) or temper embrittlement (TE) may reduce the fracture resistance of RPV steels. The fracture behaviour in the upper shelf region of low-alloy RPV steels with different microstructures and DSA, EAC and TE susceptibilities in various simulated LWR environments was evaluated by elastic-plastic fracture mechanics tests. In the reference RPV steels with low sulphur and phosphorus contents and low DSA, EAC and TE susceptibilities, environmental effects on fracture resistance are absent or marginal in both oxygenated and hydrogenated HTW. However, moderate but clear reduction of fracture initiation resistance occurred in: a) Simulated coarse grain heat-affected zone material with high yield stress, since a higher yield stress facilitates the hydrogen enrichment in the fracture process zone; b) Low-sulphur RPV steels with high DSA susceptibility, where the reduction of fracture initiation resistance increased with decreasing loading rate at 288 °C and was most pronounced in hydrogenated HTW due to the localization of plastic deformation by the interaction between DSA and hydrogen; c) High-sulphur RPV steels with high EAC susceptibility in aggressive occluded crevice environment (oxygenated HTW with addition of impurities) with preceding EAC crack growth, resulting from the enhancement of hydrogen availability and uptake; and d) High-phosphorus RPV steel with high TE susceptibility, where the reduction of fracture initiation resistance was most pronounced in hydrogenated HTW, indicating that TE effect dominates over effects of occluded crevice chemistry. In hydrogenated HTW, the reduction of fracture resistance correlated fairly well with the DSA susceptibility of different steels and sulphur content played no or a minor role. In oxygenated HTW, the reduction of fracture resistance increased with steel sulphur content for steels with low DSA susceptibility. Stable ductile transgranular tearing by micro-void coalescence dominates in both air and HTW environments. The fracture surface of specimens tested in air was mainly ductile. In contrast, specimens tested in HTW environments, varying amounts (a few %) of secondary cracking, macro-voids, quasi-cleavage and intergranular cracking were observed on fracture surface. The observed fracture modes and morphology suggest a combination of hydrogen-enhanced local plasticity and hydrogen-enhanced strain-induced vacancies mechanisms with minor contributions of hydrogen-enhanced decohesion embrittlement mechanism. The main reason for the moderate degradation effects is the low hydrogen availability in HTW together with a high density of (fine-dispersed and strong) hydrogen traps in RPV steels. In addition, the environmental reduction and softening by hydrogen and HTW environments was partially compensated by the toughness increase due to DSA. Although the environmental effects are moderate, effects can be more critical for RPV materials with low initial upper shelf toughness under unfavourable combinations (high sulphur content, increased strength, high EAC, TE and DSA susceptibilities) or old plants with small margins regarding irradiation embrittlement.

EPFL2019

Dynamic study of contact damping in martensitic stainless steels using nano-indentation

Hans-Peter Seifert, Philippe Spätig

The study was undertaken to gain insight into the micro-mechanisms controlling plasticity at the micrometer scale of elastic-plastic metallic alloys. Dynamic nano-indentation tests, where a small harmonic force amplitude is superimposed during loading, referred to as continuous stiffness measurement (CSM), were performed to study contact damping in martensitic stainless steels with different heat treatments. The nano-indentation experiments were analyzed according to a specific protocol developed in this study, which results from a careful dynamic characterization done over a wide range of materials with significant different plastic flow properties, to separate the effect of the instrument from those of the material. The method was then applied to the tempered martensitic steel Eurofer97 in different tempering conditions. Based on the dynamic contact analysis developed for visco-elastic materials, we propose a new interpretation of the damping coefficient for materials like Eurofer97 dominated by an elastic-plastic behavior. The loss factor and the nano-hardness were correlated with the propensity of the material to plastic deformation, characterized by the average flow stress determined from tensile tests.

ELSEVIER2020

High-Temperature Water Effects on the Fracture Behaviour of Low-Alloy Reactor Pressure Vessel Steels

Zaiqing Que

EPFL2019