One Minute Mentor: Types of Cast Irons

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The ease with which graphite forms on cooling is determined largely by the temperature of the iron and the silicon content. High carbon and silicon are the primary alloying elements that increase the graphitization potential of the iron as well as its castability. Carbon and silicon composition ranges of common cast irons are plotted in the graph. The higher the silicon content, the lower the solubility of carbon in austenite and the more readily carbon is precipitated as graphite on slow cooling.

The carbon in cast irons can solidify into graphite with different shapes during casting. Cast irons also are classified according to the shape of graphite that forms during solidification.

There are three basic types of cast iron with distinct forms of graphite shape:

• Gray iron contains carbon in the form of graphite flakes. Gray iron exhibits a gray fracture surface because fracture occurs along the graphite plates (flakes).
• Ductile iron, also known as spheroidal or nodular graphite iron, contains rounded graphite in which the graphite flakes form into balls (like cabbage leaves). Ductile iron is so named because in the as-cast form it exhibits measurable ductility.
• Compacted graphite iron contains graphite in the form of thick, stubby flakes. Its mechanical properties are between those of gray and ductile iron.
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The shape of graphite has important effects on properties that can be easily discerned. For example, ductile iron castings have a definite ring when struck with a hammer, but it is not the clear ring of steel. Gray iron castings have a damped sound. Breathing on a fresh surface (filed, ground, or machined) of ductile iron releases the smell of acetylene gas, which is formed by moisture reacting with the magnesium carbide in ductile iron.

There are three other important categories of cast irons:

• White iron essentially is free of graphite, and most of the carbon content is present as separate grains of hard Fe₃C. White iron exhibits a white, crystalline fracture surface because fracture occurs along the iron carbide plates. It is only stress relieved, or heat treated to cause decomposition of cementite into graphite.
• Malleable iron contains compact nodules of graphite flakes. These are called “temper carbon” because they form during an extended annealing of white iron of a suitable composition.
• High-alloy (special) iron contains over 3% alloy content and is commercially classified separately. These include the wear-resisting white irons, the austenitic high-alloy irons, and the alloyed self-hardening types. The special irons may be, but usually are not, heat treated, except for precipitation hardening, stress relieving, or surface hardening.

Volume 4D, Heat Treating of Irons and Steels -> Introduction to Cast Iron Heat Treatment -> Types of Cast Irons

One Minute Mentor: Introduction to cast irons

October 12, 2018
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Cast irons, like steels, are iron-carbon alloys. The difference is that cast irons typically possess higher carbon levels than steels, so that cast irons can take advantage of eutectic solidification in the binary iron-carbon system, as shown in the diagram.

 Cast irons often are defined as ferrous alloys that contain more than 2% carbon and 1% or more silicon. Eutectic solidification occurs with carbon content above 2%, but it also is important to note that silicon and other alloying elements may considerably change the maximum solubility of carbon in austenite (γ). Therefore, in exceptional cases, alloys with less than 2% C can solidify with a eutectic structure and therefore still belong to the family of cast irons.

Nonetheless, most commercial irons have carbon contents within the range from 2.5 to 4.0% along with other essential alloying elements, of which silicon and phosphorus are the most important. 

Volume 4D, Heat Treating of Irons and Steels ->Heat Treatment of Cast Irons -> Introduction to Cast Iron Heat Treatment

Solar Atmospheres develops better cleaning process for vacuum furnace heat exchangers

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Solar Atmospheres, Souderton, Pa., announces that it has developed an improved process to clean heat exchanger fins that have become clogged with dirt and debris.

In the operation of vacuum furnaces, the quench gas heat exchanger fins can become clogged with various debris and dirt that is very difficult to completely clean with standard pressure washing and other conventional methods. The clogged heat exchanger greatly impedes quench gas flow, resulting in reduced work cooling rates.

Solar’s special process involves a 12-hour soak submerged in a suitable tub using a water base cleaning agent. Recirculated forced water jets wash out the accumulated dirt from deep inside the heat exchanger. It is then pressure washed and blown dry with nitrogen gas. The heat exchanger is cleaned to an as new condition and then returned to the user.

www.vacpumpservices.com, an affiliate of Solar Atmospheres.