Basics of Heat Treatment
The purpose of heat treatment is to cause desired changes in the structure of a metallic material, thereby affecting its properties. Heat treatment can be employed to change the properties of most metals and alloys, with ferrous alloys typically undergoing the most dramatic changes.
Generally, the most desirable properties of steel relate to its high toughness, strength and hardness. These properties are easily modified by means of suitable heat treatments, which allow the material properties to be altered to meet the in-service requirements after machining and forming operations have been completed. During such heat treatment, the microstructure of the material is altered as a function of, inter alia, the material’s chemical composition, the heat treatment temperature and the cooling rates that can be achieved during the heat treatment process (and hence the component’s geometry).
In steels, heat treatments are typically employed to either harden or soften the material, but also cause changes in ductility and toughness. When a steel component is heat treated, it is typically heated to above the alloy’s upper critical temperature (T >723°C depending on the alloy) in order to convert the microstructure to the austenite (a face centred cubic (FCC) structure of iron and carbon, which is stable at high temperatures). If the steel is cooled slowly (in near equilibrium conditions), the austenite typically transforms to the body centred cubic (BCC) ferrite and pearlite (a mixture of ferrite and cementite that forms in a lamellae structure). Depending on the degree of mechanical restraint and thermal gradients, annealing treatments of this nature produce a material with low levels of internal/residual stress and a microstructure that can be predicted from the phase equilibrium diagram for the specific alloy.
However, if the steel is cooled rapidly (i.e. quenched in oil, water, brine, salt bath, or even air for some alloys), a very hard and brittle body centred tetragonal (BCT) microstructure know as martensite forms. Owing to this brittleness, untempered martensite is typically tempered by reheating the material to a temperature well below the austenising temperature, which on cooling results in a tempered martensitic microstructure which is not as hard and is less brittle. Changing the temperature at which the material is tempered changes the final microstructure and the properties of the material. If quenching from above the austenising temperature is interrupted and the component is held at an intermediate, material dependent temperature for a period of time, bainite is formed. Bainite is a structure in which fine carbide particles are distributed in a ferritic matrix and is typically softer and more ductile than martensite.
Internal stresses of significant magnitudes, that are associated with volume expansion that occurs during the change from the FCC to BCT structure, are often developed during the austenite to martensite phase transformation. As this expansion is in direct opposition to the cooling contraction it can often lead to cracking in large items, or those with complex geometries, due to the thermal gradients developed within these components. When the austenite to martensite phase transformation is prevented from proceeding to completion (typically when the transformation occurs below room temperature), such quench cracking can be delayed and can occur days (or even weeks) after the heat treatment process.
The heat treatment of steel can afford beneficial changes to material properties, such as toughness, strength and hardness, but requires i) a thorough understanding of both the phase equilibrium diagram and time temperature transformation (TTT) curves of the alloy, and ii) careful control of the heat treatment temperatures and cooling rates.
Published in Technical Tips by Origen Engineering Solutions on 1 March 2018