ADI Process

ADI Process Definition *Heat to austenitizing temperature *Hold to dissolve carbon in austenite *Quench fast to avoid pearlite *Hold at austempering temperature for transformation to ausferrite The chart to the right shows these processes  »   »  »

Control Must Be Absolute

• Initial austenitizing time and temperature (1525ºF to 1700ºF) must be controlled to form fine grain austenite and uniform carbon content in the matrix.
• Quench time must be controlled within a few seconds to avoid formation of pearlite.
• Quench temperature (450ºF to 750ºF) must stop short of the martensite formation (except for ASTM 897 Grade 5).
• Must stabilize at an exact temperature and hold for isothermal transformation into ausferrite.  Time controlled to avoid overtempering.
• ADI grades determined by austempering temperature

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Time and Temperature relationship is critical.  The diagram below shows the process on a time temperature transformation chart.

The process requires avoidance of pearlite formation around carbon nodules, which would lower mechanical properties.

It also requires avoidance of  martensite formation along cell boundaries, which would lower mechanical properties.

Ausferrite is the desired structure, which roughly doubles the strength of conventional ductile irons. 

Critical Process Parameters: *Austenitizing time and temperature *Quench time *Austempering time and temperature

ADI is produced by heat treating cast ductile iron to which small amounts of nickel, molybdenum, or copper have been added to improve hardenability.  Specific properties of the material are determined by the careful choice of heat treating parameters.   Parts are first austenitized to dissolve carbon, then quenched rapidly to avoid the formation of pearlite.  To develop final properties, castings are held at the appropriate austempering temperature for a suitable period of time.

Austempering involves the nucleation and growth of acicular ferrite within austenite, with the consequence that carbon is rejected into the austenite.  The resulting microstructure of acicular ferrite in carbon-enriched austenite is designated ausferrite.

Because the high-carbon austenite is a stable structure in ADI, it enhances properties.   This is in contrast to retained austenite in steel, which is metastable and could be harmful.  However, even though the high-carbon austenite is thermodynamically stable, it can undergo a strain-induced transformation to martensite when locally stressed.  The result is islands of hard martensite that enhance wear properties.

Castings are typically austenitized in either a high-temperature salt bath or a controlled-atmosphere furnace, and austempered in molten salt or oil. Advance Cast Products uses salt baths for austenitizing, quenching, and austempering because close dimensional control is facilitated by uniform salt temperatures.