Gray Iron-A Unique Engineering Material - by D. E. Krause
The Iron Casting Research Institute
Gray iron is the most versatile of all foundry metals. The high carbon content is responsible for ease of melting and casting in the foundry and for ease of machining in subsequent manufacturing. The low degree or absence of shrinkage and high fluidity provide maximum freedom of design for the engineer.
By suitable adjustment in composition and selection of casting method, tensile strength can be varied from less than 20,000 psi to over 60,000 psi and hardness from 100 to 300 BHN in the as-cast condition. By subsequent heat treatment, the hardness can be increased to H Rc 60.
If the service life of a gray iron part is considered to be too short, the design of the casting should be carefully reviewed before specifying a higher strength and hardness grade of iron.
An unnecessary increase in strength and hardness may increase the cost of the casting as well as increase the cost of machining through lower machining rates. Although the relationship between Brinell hardness and tensile strength for gray iron is not constant, data are shown which will allow use of the Brinell hardness test to estimate the minimum tensile strength of the iron in a casting.
Gray iron is one of the oldest cast ferrous products. In spite of competition from newer materials and their energetic promotion, gray iron is still used for those applications where its properties have proved it to be the most suitable material available.
Next to wrought steel, gray iron is the most widely used metallic material for engineering purposes. For 1967, production of gray iron castings was over 14 million tons, or about two and one-half times the volume of all other types of castings combined. There are several reasons for its popularity and widespread use.
It has a number of desirable characteristics not possessed by any other metal and yet is among the cheapest of ferrous materials available to the engineer. Gray iron castings are readily available in nearly all industrial areas and can be produced in foundries representing comparatively modest investments. It is the purpose of this paper to bring to your attention the characteristics of gray iron which make the material so useful.
Gray iron is one of the most easily cast of all metals in the foundry. It has the lowest pouring temperature of the ferrous metals, which is reflected in its high fluidity and its ability to be cast into intricate shapes.
As a result of a peculiarity during final stages of solidification, it has very low and, in some cases, no liquid to solid shrinkage so that sound castings are readily obtainable. For the majority of applications, gray iron is used in its as-cast condition, thus simplifying production.
Gray iron has excellent machining qualities producing easily disposed of chips and yielding a surface with excellent wear characteristics. The resistance of gray iron to scoring and galling with proper matrix and graphite structure is universally recognized.
Gray iron castings can be produced by virtually any well-known foundry process. Surprisingly enough, in spite of gray iron being an old material and widely used in engineering construction, the metallurgy of the material has not been clearly understood until comparatively recent times.
The mechanical properties of gray iron are not only determined by composition but also greatly influenced by foundry practice, particularly cooling rate in the casting. All of the carbon in gray iron, other than that combined with iron to form pearlite in the matrix, is present as graphite in the form of flakes of varying size and shape.
It is the presence of these flakes formed on solidification which characterize gray iron. The presence of these flakes also imparts most of the desirable properties to gray iron.
Metallurgy of Gray Iron
MacKenzie in his l944 Howe Memorial Lecture referred to cast iron as "steel plus graphite." Although this simple definition still applies, the properties of gray iron are affected by the amount of graphite present as well as the shape, size, and distribution of the graphite flakes.
Although the matrix resembles steel, the silicon content is generally higher than for cast steels, and the higher silicon content together with cooling rate influences the amount of carbon in the matrix. Gray iron belongs to a family of high-carbon silicon alloys which include malleable and nodular irons.
With the exception of magnesium or other nodularizing elements in nodular iron, it is possible through variations in melting and foundry practice to produce all three materials from the same composition. In spite of the widespread use of gray iron, the metallurgy of it is not clearly understood by many users and even producers of the material.
One of the first and most complete discussions of the mechanism of solidification of cast irons was presented in 1946 by Boyles. Detailed discussions of the metallurgy of gray iron may be found in readily available handbooks[3-7]. The most recent review of cast iron metallurgy and the formation of graphite is one by Wieser et al. To avoid unnecessary duplication of information, only the more essential features of the metallurgy of gray iron will be discussed here.