Microstructures - Cast Irons, High Alloy Steels, and Superalloys - Lesson-1

Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Cast Irons, High Alloy Steels, and Superalloys - Lesson 1 - Tenth specimen

This cast iron truck flywheel was locally overheated by friction against the clutch disk. The photomicrograph was made at 500X with the usual Nital etch.

It eventually broke due to the resulting heat checking.

Heat checking is the formation of a pattern of deep, craze like cracks resulting from repeated plastic deformation during radial and tangential expansion brought on by the heating and cooling cycles. The dimensional strains occur both by thermal expansion & contraction and also by transformational volume changes.

This image was made from an area closest to the heated surface and shows martensite, retained austenite, graphite flakes, and a small island of phosphide eutectic.

The frame at left, also made at 500X, shows partially spheroidized pearlite in another region, heated only up to the eutectoid temperature.

Here there is another patch of phosphide eutectic as well as a bonus - a small golden triangle of titanium nitride.

Continue to the Summary for this lesson.

SUMMARY: A piece of cast iron can have quite a variety of properties and microstructures, depending on its alloy content, original solidification and cooling history, and subsequent heat treatments. These microstructures are made all the more complex by the presence of internal sources of carbon ... the eutectic graphite or cementite particles. The local internal carbon gradients across the majority microconstituent (caused by rapid heating or cooling) can either fall or rise going away from these carbon sources, depending on whether the initial heating was rapid and intense, or, alternatively, the final cooling rate was intermediate. A wide range of mechanical properties results from the different shapes, plasticities, and strengths of the the microconstiuents.

Continue to the next lesson in Cast Irons, High Alloy Steels, and Superalloys. Return to the main Introduction.