Fracture toughness and fatigue resistance of quenched and tempered steels with microstructures deriving from a slant quench. Consequences on technical standards

Abstract

RIASSUNTO.Componenti meccanici di acciai allo stato bonificato, caratterizzati da microstrutture miste derivanti da condizioni di tempra incompleta, si incontrano frequentemente nella pratica costruttiva, a causa della tendenza verso acciai progressivamente meno legati o a causa di componenti con sezioni piuttosto elevate. La bassa resistenza all'intaglio di microstrutture miste è stata messa in evidenza già negli anni '50, ma da allora non è stata più oggetto di indagine e si è trascurato, anche nella normativa, di metterne in evidenza la pericolosità. Quando alla martensite rinvenuta ed alla bainite si accompagnano anche perlite e ferrite, la tenacità alla frattura scende a livelli estremamente pericolosi. I risultati di una campagna di ricerche condotte sulla tenacità a frattura di acciai per stampi di materie plastiche caratterizzati da microstrutture miste vengono presentati, individuando le combinazioni di microstrutture con tenacità a frattura troppo basse. Anche la propagazione delle cricche di fatica risulta negativamente influenzata dalla presenza di più costituenti. Le norme di uso dei manufatti dovrebbero tenere presenti le considerazioni sviluppate.ABSTRACT. Mechanical components fabbricated with quenched and tempered steels, exhibiting mixed microstructures as derived from slant quench conditions, are frequently encountered in the industrial practice, owing to a tendency to employ quite low alloy steels or due to quite large sections. The low notch strength of mixed microstructure steel samples was already emphasized in the 1950s; yet, it has never been investigated again. Also, technical standards have not addressed the risk deriving from the use of steel components with mixed microstructures. When pearlite and ferrite are present alongside tempered martensite and bainite, the fracture toughness of steel pieces diminishes to very dangerous levels. Results of an experimental program on the fracture toughness of plastic mould steels are reported, singling out microstructure mixtures with too a low toughness. In addition, the fatigue crack propagation rate is adversely affected by inhomogeneous metallographic structures. It is inferred that experimental results and ensuing considerations should be taken into account when formulating technical norms.

Thermomechanical warm forging of Ti–V, Ti–Nb, and Ti–B microalloyed medium carbon steels

“…Toughness could be increased by lowering carbon content and reducing the amount of pearlite in the microstructure, thus limiting the sensitivity of microalloyed medium carbon steels to cleavage fracture. In any case, mixed microstructures tend to decrease fracture toughness in medium carbon low alloy steels, as shown by the study of Firrao and Matteis [24]. Moreover, the distribution and size of pre-existent precipitates, not evaluated in this study, could affect the final mechanical properties of microalloyed medium carbon steels.…”

Influence of the microstructure on fatigue and fracture toughness properties of large heat-treated mold steels

“…Thus, the observed low toughness of the as-received specimens must be attributed to the mixed microstructures caused by the steelwork heat treatment, and in general to the large size of the blooms, as already signaled elsewhere [21].…”