What is the difference between gray iron and ductile iron?

Willman Industries produces two different types of iron castings each working day. All of the iron we pour contains between 2.5% and 4% Carbon. It is the shape of the carbon (graphite) in the solid casting and the structure in the surrounding metal grains which are responsible for the material properties and performances achieved in the wide range of castings we produce.


Gray Iron: Lower Cost and Higher Vibration Dampening

Gray iron, the oldest commonly used form of cast iron, consists of flake graphite in a pearlitic matrix. Gray iron is weak in tension and strong in compression. It behaves like a brittle material when under stress and fails in tension without significant plastic deformation. Gray iron is easy to manufacture and machine and has excellent vibration dampening characteristics. These properties make gray iron an excellent choice for machine bases, engine blocks, motor housings, gearboxes. It is also best for other metal components where moderate strength is needed, but impact resistance and ductility are not.


Willman Industries produces a variety of grades of gray iron, from Class 20 to Class 55.


Gray Iron – Grades Produced:

(For Gray Iron, the tensile strength defines the grade.)
(Grades divided by Classes as defined by ASTM and Meehanite Metal Corp)
Class 20 / GE20 (20,000B)
Class 55/ GM60 (55,000B)


Ductile Iron: Higher Strength and Better Performance

Ductile iron, developed as recently as the middle of the 20th century, is made using a Magnesium ladle addition to condense the graphite flakes into nodules during solidification. With the graphite in this form, the resulting metal behaves (to a much greater degree) according to the properties of the ferritic or pearlitic matrix surrounding the nodules. The material properties are no longer simply dominated by brittle fracture along the graphite flakes in gray iron. The resulting metal has higher strength than gray iron and will exhibit plastic deformation before failure. Therefore, a Ductile Iron casting will behave in a manner that is in between the rigid and brittle behavior of a Gray Iron casting and the soft and very ductile behavior of low carbon steel. Ductile Iron, while being more difficult to manufacture than Gray Iron, is still much cheaper to produce than steel. This combination makes Ductile Iron castings the best choice for ferrous casting components for which the stress, strain, and fatigue requirements are met by one of its many and diverse standard grades.

Willman Industries can supply grades 60-40-18 through 100-70-03 as cast (w/o heat treatment).


Ductile Iron – Grades Produced

(Grades of Ductile Iron are defined by their Tensile Strength, Yield Strength, and Elongation.)
Grades divided by Classes as defined by ASTM and Meehanite Metal Corp
60-40-18 / SF60
65-45-12 / SF60
80-55-06 / SP80
100-70-03 / SH100

Willman Industries produces a wide variety of ductile iron castings with strict quality requirements: Clutch housings and components; knuckles, axle housings, and control arms for suspensions; high wear agricultural components; transmission housings and other heavy-duty, high fatigue housings; pipe elbows; compressor cylinders and heads; planetary gear carriers and other gear blanks; sheaves and pulleys; valve bodies; differential carriers; wheel hubs and flywheels; bearing components; ingot molds; cable drums; swash blocks and valve plates.


Ductile Iron vs Gray Iron and Steel

Ductile Iron has replaced gray iron in some applications where better performance was desired. It has replaced steel in some applications as a significant cost savings in cases where the properties of Ductile Iron were sufficient and the benefits of steel were not worth the cost and manufacturing challenges.

Choose Willman for Quality and Dependability!

Willman Industries has open capacity for new projects for both large and small castings.  We produce most of the commonly used grades of gray iron and ductile iron.  So send us the drawings and specifications of your latest project today and we will provide timely feedback and/or quotations.  If you are experiencing a crisis with your current supply chain, call us. We have the management horsepower to quickly step into the breach.  

Call us today for a quote or a tour of our facilities!

Pouring Metal at the Willman Industries Iron Foundry
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