The circumstances under which steels exhibit intergranular fracture can be classified into four general categories: (1) owing to the presence of certain secondary phases at the grain boundaries; (2) owing to thermal treatments which cause impurity segregation to the grain boundaries without the precipitation of an observable second phase; (3) owing to the action of certain environments; and (4) owing to a combination of stress and high temperatures. In this paper each of these categories and their effects on material properties are reviewed. How the chemical composition of the grain boundary influences and induces intergranular cracking is discussed in detail for all four cases.

Intergranularfailure in steel: the role of grain-boundary composition

It is considered that all forms of graphite, precipitating from a metallic solution, must evolve from a basic hexagonal ring structure, growing into an open monolayer sheet. Subsequent growth of this precursor can lead to multi-layer sheets—flake graphite, to rolled or wrapped concentric shells—spheroidal graphite, or, possibly, to saturated fullerene units. The molecular attachment kinetics which might favor these alternatives are briefly discussed and compared with experimental evidence. Spheroidal graphite is the preferred morphology in a clean melt, flake graphite is an impurity modified form and growth to form fullerenes is presently unpredictable, but is considered to be improbable.

The nucleation and growth of graphite—the modification of cast iron

Confirmation of sulfur embrittlement in nickel alloys

Maraging steels based on iron–nickel martensite constitute a very important family of high-strength steels, which distinguishes itself by demonstrating an unparalleled combination of excellent fabricability and high strength and fracture toughness after heat treatment. Heat treatment of these steels has now been perfected to ensure consistently high levels of strength, ductility, and toughness in a variety of product shapes and sizes. Cobalt-free variants have been commercialized as part of efforts to save production costs. Further knowledge has been generated on 18% nickel maraging steels regarding phases precipitating during aging, thermal embrittlement, thermal cycling and austenite reversion/retention and their effect on mechanical properties. The paper reviews the progress made over the last one and half decades.

Progress in understanding the metallurgy of 18% nickel maraging steels

The interphase boundaries in gray and ductile cast iron were studied with a scanning Auger microprobe (SAM). Sulfur and oxygen were found to be adsorbed at the flake/ metal interfaces in the gray iron, while the nodule/metal and intercrystalline graphite interfaces in the ductile iron were free of foreign elements. The only magnesium detected in the magnesium modified ductile iron was combined with phosphorus and sulfur as a compound. A model is presented which proposes that Fe−C−Si eutectic alloys in the absence of surface active impurities (such as in vacuum casting of high purity materials) produce nodular graphite due to the inherent instability of the graphite/melt interface. The sulfur and oxygen always present in commercial alloys adsorb at the graphite/melt interface, effectively “stabilizing” the active sites on the graphite basal planes, and preventing spherulitic growth. The purpose of modifiers is to getter these impurities.

The role of interphase boundary adsorption in the formation of spheroidal graphite in cast iron

Auger electron spectroscopy was employed to directly determine the cause of thermal embrittlement of 250 series maraging steel. A drop in CVN energy with introduction of an embrittling intermediate anneal was directly correlated with a build-up of both Ti and C on the prior austenite grain boundaries. By following grain boundary concentration of Ti as a function of time at a given temperature, the diffusion coefficient for Ti in the maraging steel at 1600°F was determined. Finally, the further drastic drop in CVN energy on aging an embrittled specimen was shown to be due to a segregation of B to the already embrittled prior austenite grain boundaries, causing a further loss of cohesion.

D. F. Stein

Papers

A study of grain boundary segregants in thermally embrittled maraging steel