The occurrence of retained austenite is a cause of concern among many companies that are conducting the process of carburizing. We will discuss this over the course of a few blogs, dealing with the cause of retained austenite, the problem of retained austenite itself and the control of retained austenite.
Causes of Retained Austenite
The causes of the formation of retained austenite are well known and well documented. Causes include:
- Too high a surface carbon content after carburizing has been completed. This is generally caused by a lack of control of the carbon potential of the furnace atmosphere or carburizing medium (such as with pack carburizing).
- Too slow of a cooling rate (quenching) of the carburized case.
- Quenching from too high of a case austenitizing temperature.
The cause of the formation of retained austenite can usually be attributed to a lack of carbon potential within the process furnace and the surface of the part being carburized. This can be observed microscopically or by a cross-traverse microhardness test procedure.
Nature of Austenite
In order to form a martensitic case, it is necessary to have three conditions in existence within the steel:
- Carbon present at the appropriate percentage in the surface of the steel to form martensite (generally 0.60-1.10%, depending on the steel analysis)
- The appropriate case austenitizing temperature to transform from austenite to martensite that will produce both the required surface hardness and the core hardness, which is required to support the formed case
- The critical rate of cooling to form martensite, yet at the same time not causing surface cracking by cooling too quickly
Without the formation of austenite it will be difficult to create martensite. This is achieved by raising the temperature of the steel to an economically selected carburizing temperature and to diffuse carbon into the surface of the steel from the furnace atmosphere or the carburizing medium. Austenite has a high limit of solubility of carbon in iron for the surface carbon to be introduced into the steel, resulting from the elevated temperature and carburizing atmosphere.
As a direct result of the diffusion of the surface carbon at austenitic carburizing temperatures, it will then become necessary to rapidly cool down the carburized surface (at the appropriate critical cooling rate). The cooling rate will pass through what is known as the martensitic start line. It is the austenite phase that will influence the final microstructure of the carburized steel in many ways. The grain structure of the created austenite (grain growth as a result of time at temperature) will affect the martensite structure on quenching simply because the austenite grain size will limit the formation of the martensite needles (plate martensite).
In some instances of carburizing-steel chemistries, aluminum is added. The purpose of the addition of aluminum is to assist in the refining of the prior austenite grain to fine-grain steel. The small amounts of aluminum are added at the melting operation and will dissolve into austenite as the steel is further processed.
At elevated carburizing temperatures (which are typically 1650-1750°F) the limit of solubility of aluminum in austenite is very low, and the aluminum will tend to restrict the carburizing grain growth. The aluminum can – if sufficient aluminum is present in the steel analysis – react with nitrogen (from the furnace atmosphere) and form very hard aluminum nitrides within the case.
Formation of the Martensite
The carburized austenite grain and formed case require the appropriate austenitizing temperature (sometimes known as the case-hardening temperature) from which rapid cooling is required for transformation of the case to fresh, untempered martensite. This can be determined from the Iron-Carbon equilibrium diagram (ICE diagram).
It can be expected that the case austenitizing temperature is within a temperature range of 1450-1525°F for the selected austenitizing temperature. This temperature range is not where the martensite begins to form. The martensite will begin to form at a temperature called the Ms (martensitic-start line on the critical Time-Temperature-Transformation diagram). This temperature can be anywhere from 450°F up to 650°F, depending on the steel chemistry.
Further, the Ms temperature will be dependent on the steel chemistry and the diffused carbon-content percentage at the steel surface. Once the steel cools through the Ms line, martensite will begin to form progressively until the transformation from austenite is complete. The martensite formation is a progressive and not an instantaneous transformation.