Fig. 1 A time-temperature-transformation (TTT) diagram for the formation of austenite in a 1080 steel. Figure adapted from G.A. Roberts and R.F. Mehl, Trans. ASM vol. 31, pp. 613-650 (1943).
Carbides are often observed in quench hardened steel and can have an adverse effect upon the hardness and hardenability. These carbides are often referred to as residual or persistent carbides and they deplete carbon and alloying elements from the austenite. A lower carbon austenite may not obtain the maximum hardness and in alloy steels, these carbides tie up chromium and molybdenum that are critical in regard to obtaining good hardenability.
Figure 1 shows the time dependence of austenite formation in 1080 steel. As can be seen from this figure, the formation of austenite takes a relatively short time, but homogeneity takes much longer. For example, after 10 seconds at 760°C (1400°F) the steel is 99.5% austenite and contains a small amount of residual carbide. These carbides will persist for an additional 16 minutes. A carbon rich region will be left when the carbides finally dissolve and these carbon inhomogeneities will still be present even after 3 hours at temperature.
The dissolution time (tD) of carbide will be proportional to the square of the carbide radius (R2). Thus when practical, a normalization heat treatment prior to hardening is recommended to reduce the carbide size. This is of particular value when the parts are induction heated where the austenitization times are short. However, when the steels are banded, the alloy inhomogeneities may promote persistent carbides in chromium and molybdenum rich bands. The high alloy content within these bands shift the eutectoid composition to lower carbon where the carbides become thermodynamically stable and dissolution is no longer possible except by long soaking times to remove the alloy segregation.
Persistent carbides are often observed in carburized and hardened microstructures where the parts have been reheated to harden. In this case, the atmosphere of the reheat furnace can affect the near surface carbide dissolution. These carbides often appear as network carbides, originally precipitated along prior austenite grain boundaries during a slow cool from the initial carburization heat treatment, or appear as a fine distribution of smaller carbides throughout the case. These persistent carbides are a result of a competing process of carbide dissolution and continued carburization during the reheat. Typically, the reheat furnace has a gas atmosphere that is maintained to control the overall surface carbon content of the case. However upon reheating, the first austenite to form will have the eutectoid composition that is lean in carbon content relative to the gas atmosphere. As the temperature of the austenite is increased, the austenite requires more carbon to maintain equilibrium with the carbide. The carbon can either be obtained from the dissolution of carbide or obtained from the protective gas atmosphere. In alloy steels, carbon pick-up from the atmosphere is favored and the overall carbon content of the case can become in excess of the carbon solubility in austenite leaving persistent carbides in the microstructure. A simple test to show that this is occurring can be performed by copper plating the carburized part prior to reheating. Copper has a very low solubility for carbon and will effectively cut off the atmosphere as a source of carbon for the newly formed austenite. Thus, as the austenite is heated, the required carbon will come from the dissolution of carbide alone.