Hardening and Tempering
Steels can be heat treated to high hardness and strength levels. The reasons for doing this are obvious. Structural components subjected to high operating stress need the high strength of a hardened structure. Similarly, tools such as dies, knives, cutting devices, and forming devices need a hardened structure to resist wear and deformation.
As-quenched hardened steels are so brittle that even slight impacts may cause fracture. Tempering is a heat treatment that reduces the brittleness of a steel without significantly lowering its hardness and strength. All hardened steels must be tempered before use.
QUENCH AND TEMPERING PROCESSES:
(1) Conventional Heat, Quench and Temper process
Conventional Heat, Quench and Temper Process:
In this process, Austenite is transformed to Martensite as a result of rapid quench from furnace to room temperature. Then, martensite is heated to a temperature which gives the desired hardness. One serious drawback is the possibility of distorting and cracking the metal as a result of severe quenching required to form Martensite without transforming any of the austenite to pearlite. During quenching process, the outer area is cooled quicker than the center. Thinner parts are cooled faster than parts with greater cross-sectional areas. What this means is that transformations of the Austenite are proceeding at different rates. As the metal cools, it also contracts and its microstructure occupies less volume. Extreme variations in size of metal parts complicate the work of the heat treater and should be avoided in the designing of metal parts. This means there is a limit to the overall size of parts that can be subjected to such thermal processing. Figure 1 shows the conventional hardening, tempering process.
Figure 1. Conventional quenching and tempering process.
To overcome the restrictions of conventional quenching and tempering , Martempering process can be used. Martempering or marquenching permits the transformation of Austenite to Martensite to take place at the same time throughout the structure of the metal part. This is shown in Figure 2. By using interrupted quench, the cooling is stopped at a point above the martensite transformation region to allow sufficient time for the center to cool to the same temperature as the surface. Then cooling is continued through the martensite region, followed by the usual tempering.
Figure 2. Martempering process.
This is the second method that can be used to overcome the restrictions of conventional quench and tempering. The quench is interrupted at a higher temperature than for Martempering to allow the metal at the center of the part to reach the same temperature as the surface. By maintaining that temperature, both the center and the surface are allowed to transform to Bainite and are then cooled to room temperature.
Advantages of Austempering:
(1) Less distortion and cracking than martempering,
(2) No need for final tempering (less time consuming and more energy efficient)
(3) Improvement of toughness (impact resistance is higher than the conventional quench and tempering)
(4) Improved ductility
Limitations of Austempering:
Austempering can be applied to parts where the transformation to pearlite can be avoided. This means that the section must be cooled fast enough to avoid the formation of pearlite. Thin sections can be cooled faster than the bulky sections. Most industrial applications of austempering have been limited to sections less than 1/2 in. thick. The thickness can be increased by the use of alloy steels, but then the time for completion of transformation to bainite may become excessive.
Figure 3. Austempering process.
In Austempering process, the end product is 100% bainite. It is accomplished by first heating the part to the properr austenitizing temperature followed by cooling rapidly in a slat bath which is maintained between 400 and 800 oF. The part is left in the bath until the transformation to bainite is complete. The steel is caused to go directly from austenite to bainite.
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Last Update: November 11, 1999
Prepared by: Serdar Z. Elgun