INTRODUCTION TO ENGINEERING MATERIALS - FERROUS METALS - PART 3

Audio: Introduction to engineering materials – ferrous metals – part 3

In our introduction to engineering materials article series, we discussed the first group of ferrous metals. In part 1, we discussed the classification of engineering materials and why it is important to choose the proper material. We investigated the plain carbon and low alloy steels. Furthermore, in part 2, we discussed stainless, tool, and specialty steels. In this article, we will finish the discussion about ferrous metals with the second group of ferrous metals called cast iron. Without any further ado, let us dive into it.

Cast irons

Cast irons are part of the ferrous metal group because they, like steels, are based on iron. The cast irons are alloys of iron-carbon content between 2,1% – 4,0%, but in practice, most of cast irons contain between 3,0% – 4,5% carbon. Furthermore, they usually contain between 1% – 3% silicon. Silicon tends to promote the appearance of carbon as graphite rather than as carbide; thus, they have a low melting temperature.

The cast iron composition makes them highly suitable for shaping by casting (as their name implies) rather than forging in the solid state. They are very fluid in a molten state and have good casting characteristics. However, the castings are usually brittle (low impact resistance and ductility). That may limit their use.

The most used cast iron types are:

grey cast iron,
ductile cast iron,
white cast iron, and
malleable cast iron.

Grey cast iron

Grey cast iron contains carbon between 2,5% to 4,0% and 1% to 3% silicon. Due to the silicon content in grey cast irons and slow cooling, graphite flakes are formed in the cast products upon solidification. This structure causes the fracture surface of the cast iron to have a grey color (hence the name grey cast iron). The dispersion of graphite flakes accounts for properties like good vibration damping (desirable in engines and other machinery) and internal lubricating qualities (makes it machinable). Furthermore, the grey cast irons have good resistance to sliding wear, good resistance to thermal fatigue, and good thermal conductivity.

The compressive strength is significantly greater than its tensile strength. Furthermore, grey cast irons have very low ductility; they are relatively brittle. Due to the relatively low price, they are produced in quantities larger than the other cast iron types.

Products made from gray cast iron include automotive engine blocks and heads, motor housings, small cylinder blocks, cylinder heads, pistons, clutch plates, transmission cases, machine tool bases, etc.

Example of grey cast iron:

EN	Material number	AISI
GJL – 250	0.6025	A48-35 B

GJL – 250 is grey cast iron with 2,8% – 3,3% carbon and 1,2% – 1,7% silicon. The material has high tensile strength, hardness, and corrosion resistance. It is used for cylinders, gears, bases, cylinder blocks, pistons, brake wheels, etc.

Ductile cast iron

Ductile (or nodular) cast irons have the composition of gray cast irons with the addition of magnesium and/or cerium before casting. This results in different microstructure and sets of mechanical properties. Instead of flakes, the graphite is formed in the shape of nodules or spherelike particles. Ductile cast irons are stronger and much more ductile than grey ones (ductile cast iron’s ductility approaches steel). They are used for making valves, pump bodies, crankshafts, gears, etc.

Example of ductile cast iron:

EN	Material number	AISI
GJS-700-2	0.7070	100-70-03

GJS-700-2 is ductile cast iron with 3,65% – 3,9% carbon and 1,7% – 1,9% silicon. The material has high tensile strength, good wear resistance, and moderate machinability. It is used for belt pulleys, brackets, valves, etc.

White cast iron

White cast irons are low-silicon irons (less than 1% silicon content). They are cooled at rapid rates, and most of the carbon exists as cementite instead of graphite. It contains less carbon and silicon than gray cast iron. Furthermore, chromium, nickel, and molybdenum can be added to further enhance their properties.

The fractured structure color is white, hence the name white cast iron. The white cast irons are hard and brittle but with excellent wear resistance. They can be so hard to the point that they are virtually unmachinable. They are usually used in applications that require very hard and wear-resistant surfaces, like rollers in rolling mills, railway brake shoes, bearing surfaces, etc.

Example of white cast iron:

EN	Material number	AISI
GJH-X330NiCr 4-2	0.9625	A 532 I A NiCr-HC

GJH-X330NiCr 4-2 is white cast iron with 2,8% – 3,6% carbon; 3,3% – 5,0% nickel; 1,4% – 4,0% chromium, and 0,8% silicon. The material posse’s high hardness, high wear, and fracture resistance. It is used where high wear resistance is required (mining crusher liner, chute liners, rolling mills, parts used in the concrete and asphalt industry, etc.)

Malleable cast iron

Malleable cast irons are produced from white cast iron by heating them up to separate the carbon from the solution and form graphite aggregates. They possess substantially higher ductility compared to white cast irons.

Typical products made of malleable cast iron include pipe fittings and flanges, certain machine components, railroad equipment parts, connecting rods, transmission gears, and differential cases for the automotive industry, marine, and other heavy-duty services.

Example of malleable cast iron:

EN	Material number	AISI
GJMB-350-10	0.8135	32510

GJMB-350-10 is malleable cast iron with 2,3% – 2,7% carbon and 1,00% – 1,75% silicon. The material has high tensile strength, good wear resistance, and moderate machinability. It is used for wheel hubs, shift-forks, rear-axle casings, hydraulic parts, steering cases, planetary-wheel carriers for automatic transmissions, adjusting rings, cast heads, caps, wing nuts, etc.

Closing words

Cast irons are widely used in the industry. From small brackets to engine blocks, they are an extremely important part of our modern life. You probably have seen some of these materials at home, like a Dutch oven. If you haven’t tried a meal cooked in it, you should.

With this article, we finished the overview of both ferrous metal groups. We will continue discussing other materials groups so that you can familiarize yourself with materials you could encounter as a mechanical design engineer.

Now you have an excellent overview of ferrous metals you could encounter as a mechanical design engineer. However, I suggest you go through the text once more and identify areas you think need more understanding and clarity. Then, once you have identified those areas, start building up your knowledge in those areas.

To make it easier for you to find related posts, check the “Further reading” chapter below. Do you have any questions or need something to be clarified better? Leave a comment below, and I will give my best to adjust the post accordingly.

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