What is the result of applying flame hardening to steel with at least 0.4% carbon?

Flame hardening is a heat treatment process that involves heating the surface of the steel to a high temperature, typically above the critical temperature, and then rapidly cooling it, usually with water or oil. This rapid cooling transforms the microstructure of the steel, specifically the formation of martensite, which is a very hard and strong structure that increases the hardness of the treated areas.

When flame hardening is applied to steel with at least 0.4% carbon content, the process results in localized hardening. The high carbon content in the steel allows enough carbon atoms to be present during the transformation to martensite, enhancing the material's hardness in the specific areas that are heated and quenched. Since the process affects only the surface or specific areas of the material, it is highly beneficial in applications that require wear resistance in certain parts while maintaining other properties in the rest of the material.

The other outcomes such as becoming softer and more ductile, enhancing corrosion resistance, or allowing for easier machining do not accurately describe the effect of flame hardening. Instead, the primary and desired outcome from this type of treatment is to achieve increased hardness in targeted regions.