Morphology and aging of the martensite induced by cathodic hydrogen charging of high-carbon austenitic steels

Abstract

The transformation by cathodic hydrogen charging of a 1.95 wt pct C austenite is studied by X-ray diffraction and Mössbauer spectroscopy, and the morphology of the induced martensite, analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), is compared to that of the same alloy quenched at 78 K. Optimal charging current density is found to be at 2200 A/m₂, and charging time is varied. Martensite crystals have three different shapes: long thin plates, “spear-like,” and “palm frond.” The habit plane is essentially {225}_γ, and the serratures of the palm frond are obtained by (111)_γ accommodation slip. The state of aging of the hydrogen-induced martensite at room temperature is similar to that of a martensite obtained by quench at 78 K and subsequently aged at much higher temperature, e.g., 97 °C for 1 hour, and displays the “tweed-like” or “salt and pepper” morphology of Génin’s Fe₆C “carbon extended multiplets” which transform in situ into ε- or η-carbide precipitates at the same tem-perature, i.e. 117 °C, for 1 hour. The role of hydrogen in catalyzing the martensitic transfor-mation and affecting the aging kinetics is explored.