Study of the surface structure of tempered carbon steel

A CEMS and XMS study of carbon steel has shown that the microstructure of the bulk material as well as the surface depends on the temperature at which the steel was tempered. Ion milling of the surface results in no change to the surface microstructure which corresponds closely to the bulk properties. However, after polishing the surface with 3μm diamond, the surface composition is changed to a large extent. Polishing decreases the amount of retained austenite and also the fraction of interstitial carbon within the austenite on the surface of the non-tempered steel. A related increase in the amount of surface martensite is observed. The effects of polishing extends to depths of over 1000 Å. Both X- and θ-carbides are present in each of the tempered steels and only the amount of θ-carbide is seen to increase as the samples are tempered to decrease hardness. Tempering to higher temperatures decreases the martensite content and a corresponding increase is seen in the θ-carbide only. Polishing also removes a large feaction of the carbide from the surface and this may account for the anomalously high wear resistance previously determined for the steel.