Heat treating iron castings: part 2.

In the 1930s, a fine, feathery, needle-like microstructure unlike ferrite, pearlite and martensite was found in steels when they were cooled rapidly from austenitizing temperatures and held at intermittent temperatures. This structure, called bainite, formed above the martensite start temperature (M) but below the pearlite formation region found in isothermal transfomation (IT) diagrams. Years later, this same heat treating process, called "austempering," was applied to cast iron. A similar needle-like microstructure was formed. However, x-ray diffraction determined this structure to be acicular ferrite and carbon-enriched austenite or ausferrite. Austempering is valuable because it is stronger and tougher than tempered martensitic structures obtained with conventional heat treatment of cast iron.

Austempering How-To

The procedure consists of three basic steps.

1. Heat the part to austenitize in the temperature range of 1,550-1,700F (843-927C) for a time sufficient to produce a fully austenitic matrix that is saturated with the equilibrium carbon content.

2. Cool the part rapidly enough (quench) to avoid the formation of ferrite and pearlite to a temperature above M in the range of 450-750F (230-400C).

3. Hold at the austempering temperature to produce ausferrite. In the higher-silicon gray and ductile irons, the resultant structure is ausferrite. If low quench temperatures are used, then a small amount of martensite may be present with the ausferrite. In the lower-silicon malleable irons, the structure may contain some bainite along with the ausferrite.

Key Factors

The austenitizing temperature is a function of the chemical composition of the iron. The austenitizing tinge is largely a function to the section size of the part, though the chemical composition of the iron and the graphite morphology play small roles as well.

The final properties determine the choice of austempering (quench) temperature. Higher austempering temperatures result in coarser structures that exhibit good ductility and dynamic properties. Lower austempering temperatures produce freer structures that have higher tensile and yield strengths and superior wear resistance. The austempering time is dependent upon austempering temperature. Longer times require lower temperatures.

It should be noted that austempering is not a band-aid for poor-quality cast iron. Because ausferrite is stronger than conventional cast iron microstructures, it is more sensitive to defects within the iron. In order to successfully austemper cast iron, it should have a consistent chemical analysis, consistent graphite shape and distribution, and a matrix structure essentially free of porosity and carbides.

The casting and austempering processes are specific to the casting supplier and the heat treater. To achieve the desired properties in a component, both the casting process and the type of austempering process must be established and documented at the outset of production.

Martempering--A Cousin

Since the 1950s, hot oil (410F [210C]) quenching has been routinely applied to cast irons to minimize distortion and cracking in cast iron parts. This process, known as marquenching, or martempering, also may require a subsequent tempering operation. The highest temperature quench oils are used to restrict the quenching process to temperatures just at or below [M.sub.s]. Thus, mixed microstructures of martensite, bainite, and ausferrite are produced.

When austempering, components are quenched from the austenitizing temperature to a temperature above the martensite start temperature (M) and then held for a time sufficient to form the desired microstructure (ausferrite). Because ausferrite forms over many minutes or hours at one temperature, no cracking occurs. Furthermore, unlike martempering, no subsequent tempering operation is required.

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