CAST IRON

Lab Session (6) Cast Iron

Objective
The aim of this experiment is to:
    1. Focusing on the cast iron zone in the iron-iron carbide system.
    2. Getting acquainted with the different types of cast irons, and the differences between them.
    3. Studying the different microstructures of the different types of cast iron.

Introduction

Cast irons family is one of the most important classes of iron-carbon alloys, with wide range of applications. The name "cast" was given due to the fact that such a type of alloys was designed to be cast-able, rather than experiencing thermal-mechanical treatment in the solid state. There are several types of cast iron, according to the alloying elements composition, which consequently differ in its mechanical, physical properties and microstructure.
There are three basic reasons for such a wide spectrum of cast iron classes, which are:
1 . The chemical composition of the iron; the existence of certain alloying elements.
2 . The cooling rate of the casting in the mould.
3 . The type of graphite formed and its percent composition.
Generally speaking, cast irons contain normally 2%-4% of carbon content. Such a region is included in the phase diagram as shown in figure (1), and labeled in cyan color.

Figure 1 Cast Iron and carbon content on the Fe-Fe3C phase diagram

Variations in the carbon content has a decisive effect on the alloy strength, ductility, machineability, grain size, residual stresses, hardness and toughness. As shown on figure (2), the carbon content has a decisive effect on the yield strength, tensile strength, and hardness.

Figure 2 Effect of carbon content on steel alloys properties

In addition to iron, cast irons have 1%-3% silicon content which increases the molten metal fluidity to be cast-able, in addition to other alloying elements, such chromium, nickel or molybdenum, which increase strength and corrosion resistance. Cast irons score several material advantages over certain types of steels due to several reasons; they are easily melted which provided lower tooling and production cost when compared to steel alloys, they provide wide range of hardness and strength, good machinability without burring, excellent wear resistance and high hardness (particularly white cast iron), and high vibration damping properties. They can be alloyed to increase their corrosion resistance, and wear resistance properties.

On the other side, they have certain disadvantages that are all attributed to the high carbon content. The larger the carbon content, the lower the ductility and the higher the brittleness. This leads to poor impact resistance. Therefore, this provides a limitation on their use. Still, their low cost and good properties trade off for such disadvantages.

There are major four kinds of cast irons, from the point of their frequent uses in the industry. In addition, there are other types that stem from the basic four types, but were exposed to certain additions of alloying elements, or certain heat treatment.

CAST IRON TYPES

The basic four types of cast iron are: gray CI, white CI, malleable CI, and ductile CI. The basic difference between the four types lies in the carbon and other alloying elements content. The following tables shows these differences:

Table (1) Types of Cast Iron

1. Gray Cast Iron (graphite flake cast iron):
This type has the highest carbon content among the cast iron classes. It is formed when the carbon in the alloy exceeds the amount that can be dissolved. Therefore, carbon starts to precipitates in the form of graphite flakes. The high silicon content increases fluidity, and stabilizes the graphite structure by promoting the formation of graphite. The solidification rate determines the extent at which graphite forms.

In fact, the name gray comes from the fact that the fracture surface of gray cast iron is gray due to the exposed graphite.
-Properties: good wear resistance, excellent vibration damping capacity and good machineability, resistance to galling under restricted motion.
-Microstructure: A pearlitic matrix in thin sections, and carbon precipitates in form of graphite flakes in ferrite matrix.

2. White Cast Iron:
White cast iron forms when iron in the molten cast iron changes into iron carbide instead of forming graphite upon solidification. The microstructure is composed of iron carbide embedded in a pearlitic matrix. While the name "white" is given because the fracture surface of this type is white. The white part of the matrix being cementite, while the black part is pearlite.

3. Ductile (Nodular or Spherulitic Graphite Cast Iron):

In this type, carbon precipitates in the form of black nodules. It combines the high fluidity, with enhanced mechanical properties. It has a good ductility, strength, corrosion resistance, and wear resistance. Addition of alloying elements such as: Mg, Ce, Ca, Li & Na are to produce spherodization (spherical carbon precipitates).

4. Malleable Cast Iron:

This type is formed initially as white cast iron. Then it is heat treated to produce it in a melleablizing furnace to dissociate the iron carbide into rounded clumps of graphite and iron. It shows good ductility, wear resistance, toughness. It is easily machineable and castable.

5. Other types of CI:

There are other types of cast iron, that depend on the composition of the alloying element in producing different microstructure, such as: Silicon Flake Cast Iron, High Silicon Flake Cast Iron, compact cast iron, Low Alloy Nodular Cast Iron, Austenitic Flake, Acicular Cast iron, and Austempered Ductile Iron.

Procedures

Three specimens were used:

Common preparation procedures such as grinding and polishing were taken, followed by etching using 3ml of HNO₃ + 100 ml of CH₃OH (methyl alcohol), etched by immersing for 15 seconds.

Testing Devices

Optical Microscope

Results/Discussion

Answer the following questions in the body of your discussion (refer to your textbook pages 551-560) 1. Describe the microstructure

Further References/Images

--The following sites and books contain useful information and images for several articles that we have encountered in this experiment. Make sure to check them.

Academic Page @ MIT http://me.mit.edu/2.01/Taxonomy/Characteristics/Iron.html

Kalpakjian, Serope. Manufacturing Process for Engineering Materials. Addison Wesley, 3^rd Ed., 1997.

Smith, William F. . Principles of Materials Science and Engineering. McGraw Hill, 3^rd Ed., 1996.

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