Advances in Physical Metallurgy and Processing of Steels. The Limits of Strength and Toughness in Steel.

Abstract

The ideal structural steel combines high strength with excellent fracture toughness. In this paper we consider the limits of strength and toughness from two perspectives. The first perspective is theoretical. It has recently become possible to compute the ideal shear and tensile strengths of defect-free crystals. While the ferromagnetism of bcc Fe makes it a particularly difficult problem, we can estimate its limiting properties from those of similar materials. The expected behavior at the limit of strength contains many familiar features, including cleavage on {100}, ͗111͘ slip on multiple planes, "conditionally" brittle behavior at low temperature and a trend away from brittle behavior on alloying with Ni. The behavior of fcc materials at the limit of strength suggests that true cleavage will not happen in austenitic steels. The results predict an ideal cleavage stress near 10.5 GPa, and a shear strength near 6.5 GPa. The second perspective is practical: how to maximize the toughness of high-strength steel. Our discussion here is limited to the subtopic that has been the focus of research in our own group: the use of thermal treatments to inhibit transgranular brittle fracture in lath martensitic steels. The central purpose of the heat treatments described here is grain refinement, and the objective of grain refinement is to limit the crystallographic coherence length for transgranular crack propagation. There are two important sources of transgranular embrittlement: thermal (or, more properly, mechanical) embrittlement at the ductile-brittle transition, and hydrogen embrittlement from improper heat treatment or environmental attack. As we shall discuss, these embrittling mechanisms use different crack paths in lath martensitic steels and, therefore, call for somewhat different remedies.

The Nature and Consequences of Coherent Transformations in Steel.

“…24,25) The connection between T B and the cleavage fracture stress can be understood on the basis of a model that was originally suggested by the Russian physicist, Yoffee, in the early 20th century. The modern version can be stated as follows (Fig.…”

“…9, which shows how multivariant transformations can be used to refine grain size by breaking up lath alignment within a packet. 25,35,37) The steel shown in the transmission electron micrograph is "9Ni" steel that has been given an "LQ" treatment: an intercritical anneal (L) followed by an austenite reversion and quench (Q). The intercritical anneal creates a "dual phase" structure in which laths of high-Ni fresh martensite alternate with laths of relatively low-Ni well-tempered martensite.…”

Improving Toughness of PH13-8 Stainless Steel through Intercritical Annealing

“…Detailed discussion of the four treatments can be referred to in Ref. 13). Intercritical annealing is used to denote a treatment just below A c3 , in the upper part of the two-phase region.…”

“…12,13) The alloy is quenched to martensite, and then reheated to accomplish a partial or complete reversion to the austenite phase. The re-heating causes one of four characteristic reactions, which are labelled in the schematic phase diagram in Fig.…”

Effect of Pre-Quenching Process on Microstructure and Mechanical Properties in a Nb-Microalloyed Low Carbon Q-P Steel

“…A recent study indicated that cyclic intercritical tempering heat treatment was beneficial to getting both fine microstructure and stable retained austenite due to the refinement of austenite grains [14]. A reduction in the austenite grain size is well known to increase the austenite stability by suppressing the martensite transformation.…”