What is Spheroidization?


Spheroidization phenomena

Spheroidization or also called softening, is a change in the microstructure of steels resulting from exposure, in the temperature range of 850 °F to 1400 °F, or 440 °C to 760 °C. At this temperature range, the carbide phases in carbon steels and Carbon- half Moly steels are unstable, and may agglomerate from their normal plate-like form to a spheroidal form or from small, finely dispersed carbides in Chrome-Moly steels to large agglomerated carbides. Spheroidization may cause a loss in strength and, or creep resistance. The first picture shows the unaffected base metal of a carbon steel having pearlite and ferrite microstructure. When this material is exposed to high temperature above 850-degree Fahrenheit, the Spheroidization takes places as visible by the black area in the below picture.

Watch this Youtube video for full classroom training on API 571 Damage Mechanism- Spheroidization or Softening.

Materials being affected by the Spheroidization

Spheroidization takes place in all common grades of carbon steel used in the refineries. The low alloy steel such as carbon- half moly, and chrome moly steel from 1 chrome-half moly to 9 chrome 1 moly are included. So basically, full range of chrome moly steels are prone to Spheroidization.

Critical Factors for the Spheroidization

The primary factor affect Spheroidization are the:

a. Chemistry of base metal,

b. Microstructure, temperature and time exposure.

If we talk about types of steel, here at first, Annealed steels are more resistant to spheroidization than normalized steels.

Second, Coarse-grained steels are more resistant than fine-grained.

And third, Fine grained silicon-killed steels are more resistant than aluminum-killed steel.

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