Grain-Boundary Hardening in Iron and Iron Alloys

Abstract

Hardening at the grain boundaries in irons of various purities, and in certain iron-base alloys, has been investigated by means of a microhardness technique. The difference between the grain-boundary and grain-interior hardness values (∆H) increased as the load applied to the indenter was decreased. Zero values of ∆H were found for loads > ∼ 14 g. Factors affecting the grain-boundary hardening were studied: (a) Additions of tungsten or molybdenum reduced the grain-boundary hardness to that of the grain interior at ∼ 0.5 at.-% solute; manganese additions did not affect ∆H (b) Quenching from 900°C eliminated the hardening effect in all the materials investigated. Annealing to equilibrium at temperatures between 300 and ∼ 750°C caused a return of the hardening, the magnitude of which was independent of the annealing temperature. (c) A kinetic study of the rate of recovery of the hardening effect, following quenching from 900°C, showed that it occurs very rapidly. The conditions required for the observation of this effect have been defined and possible causes of the phenomenon are discussed.