Austempering Heat Treatment
The unique Fisher Barton austempering heat treatment process produces high hardness parts that are extremely ductile. The ductility is achieved through the elimination of quench cracks (macro and micro) that are common in most conventional quench and temper heat treat processes. When large (macro) cracks occur it is usually obvious. However, the micro cracks in the atomic structure are virtually impossible to detect. These defects inhibit ductility by setting up barriers in the atomic structure, preventing the material to flow plastically.
When steel is heat treated we expect: high hardness, wear resistance, strength and toughness. A transformation of the metals’ atomic structure occurs. During heat treating the metal is raised to a high temperature of 843 degrees Celsius (1550 degrees Fahrenheit) and is transformed to austenite (FCC), the highest density phase. The metal is cooled causing the austenitic phase to be transformed into three possible atomic arrangements or phases: BCC, BCT and Orthorhombic. In all of these transformation phases, expansion and a decrease in density of the metal occurs. It is this expansion that results in quench crack problems.
The conventional quench and temper process produces martensite (BCT), a low density, hard, and very brittle phase. There are two factors that can cause quench cracks during the formation of martensite. They are reduced plasticity at low temperatures and the instantaneous shear characteristics of the transformation. Because martensite is not ductile at low quench temperatures and because of its rapid shear formation nature, the continuing transformation of austenite (FCC) to martensite (BCT) sets up very high stress factors in the initially transformed martensitic structure. When the martensitic phase can no longer absorb additional expansion stresses, cracks (macro and micro) can occur.