Basic Metallurgy

By:  Arthur A. Avedisian, P.E.

Editor's note ­

     The following article was written by a long time friend of the Ductile Iron Society.  
     I am sure that Art Avedisian is remembered by many of the present members.  He began his association with the DIS back in the 1960's and has continued to the present time.  He has served the Society on the Research Committee, as technical chairman at our T&O meetings and received the DIS Annual Award in 1976.  His most important contribution to the Society my be the initiation of our present "Ductile Iron News."  Art can truly be referred to as the "father" of the present magazine.
     Art is doing well in his retirement and continues to be active in teaching metallurgy.  The following is the chapter on ductile iron that will be included in the book he is writing.  He stresses the point that this is for the "metallurgically uneducated" and should be taken as such.
     For those of you who would ask, "How old is Art now?", I recommend that you give him a call.  I will only tell you that he qualifies as the oldest DIS alumni member.
     Art can be contacted at home.  His address is 810 Bookbinder, Windsor, CT  06095-2035.

The production of ductile irons

A few years ago, in my lifetime, ductile iron (nodular iron) was discovered by a wonderful friend named Keith Millis, (now deceased) of the International Nickel Company.

To set the stage of our story, we know that gray cast iron with its flake graphite exhibits no useful yield in tension, or elongation, and is considered a brittle material. (That is if the student has learned his lessons).

Ductile iron, even with its high graphite content, combines the useful properties of gray irons, among them, casting ability, with the high strengths of steels. A new industry was born that has grown to tremendous proportions all over the world!

Actually Keith was looking for way to reduce the sulfur in cast irons by treating the molten metal with magnesium. He inadvertently found when looking at a specimen under a microscope, that the graphite instead of forming a flake was present as a perfect round nodule, which eliminated almost all of the ill effects of flake graphite.

This started the greatest advance in improvement of cast irons in a hundred years! In all fairness we must tell the student of the incredible coincidence that occurred at the same time.

In England, at the same time, a Dr. Morrows discovered that the same thing occurred with the use Cerium, but unfortunately the use of magnesium proved to be more efficient and prevailed! The poor guy, while recognizing that he had to play second fiddle to Keith Millis, he accepted his role with the grace of an Englishman!

Getting back to the metallurgical aspects, and to help the student in being aware of the significance of this discovery we will put photomicrographs of ductile (nodular) iron on the screen, "as cast and annealed".

As Cast	Annealed

Here we have a metal that combines the properties of both gray irons and steels! The international Nickel Company licensed it, but the patents have run out. This means that everyone today that wishes can try to produce the metal! The problem is that the production of ductile iron requires careful control! The only policing today, is the Ductile Iron Society, but that means that the producer must belong to the society in order to participate.

This is a relatively new metal and there is still much to be learned about its production. As this primer on metallurgy is not designed to show how the metal is made, we can only give precautions on what to look out for and some of the benefits.

1. Ductile iron is a very versatile metal that has replaced steel and malleable iron in great many applications. There were many instances where a design remained on the drawing board because the part was too thin to be cast in steel. Ductile iron has both the castability of gray iron and the strength of steel.  The same equipment that gray iron producer's use, produces it! No wonder every producer of gray iron wanted to get in the act! There were problems!
2. The large automotive and agricultural equipment foundries have metallurgists and laboratories to control quality. This luxury is not affordable in many small foundries, but is absolutely necessary to control the production of ductile iron! There are no shortcuts!  Today there are many small foundries that that invested in the equipment necessary to produce the metal. Make sure your supplier is one of these!
3. As ductile iron is one leading metal in the casting industry we are quite sure that the majority of our students recognized the name and use it. There is no way of knowing by looking at a casting whether or not it's made of ductile as against gray cast iron. The student can take this first instance of connecting the dots!  Do you remember way back when we discussed damping properties and how gray iron damped out the energy because of the occurrence of flake graphite? Hit the part with a hammer dummy! If it doesn't ring, it's not ductile! If it does ring it is ductile! Gosh! Will you ever learn?
4. Ask for a certificate of compliance, if it's a very important job that needs certification.  Here's the difference! In a certificate of compliance you're asking the foundry to use the same controls that they use for all customers. It should cost you nothing! If you ask for a certification it means that the foundry must pour a test bar out of the same ladle of metal that was used for your part. That test bar follows the casting through any heat treating procedures and is tested to see that it is in compliance. That will cost you and arm and a leg! So, put it in the price of the product!
5. The various grades of ductile iron can be changed from one to the other by heat treating, which we will discuss later.
   The various grades of ductile iron listed by ASTM A536 will be described. 
   The first two numbers indicate the tensile strength 
   The second two numbers indicate the yield strength
   The third two numbers indicate the amount of elongation
   60-40-18
   65-45-12 
   80-55-06 
   120-90-02

We are losing sight of the fact that we are studying the action of the atoms, which, if understood, will help the student solve problems much easier. Let's get back to how the atoms react in forming nodular graphite We know from our studies that nature in its wisdom will always forms components in a minimum surface area per volume if unhindered!  For example, take your dirty automobile hood. When it rains, the dirt offers resistance to the water doing its thing, however whenever the car is waxed the resistance is lessened and the water is able to ball up offering a minimum area per volume. We can offer many other examples, however we're quite sure you get the point! In ductile iron the graphite acts exactly the same way.

The student will remember in gray irons that the metal solidifies at the same temperature the graphite precipitates and is called the eutectic. In ductile irons there is no true eutectic. There is a great deal of research still going on as to how the nodule forms but let's use a little common sense to figure out what happens according to the laws of nature.

Let the Egghead's continue their research, we will describe it our way! (Wasn't there a song by that title?) Let's make some dots to connect!

1. We know the graphite forms in a perfect round ball.
2. We know that the metal forms dendrites when solidifying.
3. We know that the if the graphite came out at the same time, they would be squashed between the branches.
4. The conclusion must be, that the graphite had to precipitate while the metal was liquid and offered no resistance to the graphite following natures law and forming a minimum surface area per volume, which is a round ball!

The author has proved this point by pouring ductile iron into water and examining the metal under a microscope. To no surprise, at high magnification tiny round balls were seen. In follow-up research, we isolated normal nodules by cold leaching in acid and thought that we had isolated pure graphite. We were wrong! A stupid very pretty little girl, working in the lab screwed up the experiment by picking up the whole mess with a magnet! Eventually we came to a conclusion that without any interference from dendrites, the open roses we speak of closed into the bud, capturing iron atoms between its petals! Cool! I fired her. (Just kidding!)

Because graphite is present in ductile iron and because nature favors the formation of graphite, the reaction of the carbon atom is the same as in all gray irons.

The metal is subject to time and temperature. If the metal is cooled slowly from austenite the iron carbide close to the graphite nodule will break down and a carbon atom will migrate to," mother graphite", because it now has a home to go to! The slower the cooling the more pearlite will be broken down to its components of iron and carbon.

The carbon atom will then migrate to the nearest nodule as secondary graphite on to the prime. This is why in the photomicrograph of, "as cast", ductile iron, it shows a bulls eye structure of ferrite around the nodule of graphite.

There are many ways that cast irons can cool slower from austenite. One way involves the method of heat-treating. However, more important to be understood now, is that the section of the casting will dictate the rate of cooling.

The heavier the part, the longer it will take to cool and the more ferrite will form. This applies to any ferrous casting that contains an appreciable amount of silicon. The student may think we're being redundant, but this is necessary if you are going to understand heat treating, especially what may happen during normalizing. So stay with us! This is a cram course! We must do it our way.

We will discuss what happens in the heat treating of cast irons later, For now we want the student to relate the action of the atoms, especially the action of the carbon. Please take some time to think deeply of the action, always thinking that the atoms are going to obey natures law, given the time and temperature!

We are now going to prepare the student to the introduction of compacted graphite iron, which is part of the ductile iron group. We will explain why. Research indicated to us, right or wrong, that graphite nodules formed in the liquid, because when treated with magnesium the metal was out of equilibrium with oxygen.

What seems to prove this point is that, if the ductile iron after treatment, is not poured within a certain time the whole mess reverts back to gray iron containing flake graphite. The student is asked to remember this fact because it will help to understand the next grade of metal, which we discuss. Is called...

Compacted graphite iron
This grade of cast iron is also referred to as vermicular graphite or semi ductile cast iron. It's kind of funny how this grade was developed. It's the first time we have ever heard of doing something wrong, to do it right! This grade came about by not treating the metal properly to make ductile iron. There was either not enough magnesium added or waited too long to pour the metal. In any event the resulting metal contains graphite that appears as clusters that are interconnected between the dendritic branches along with a few nodules here and there but no classical flake graphite. Let's see the difference.

Compacted Graphite
This is the photomicrograph of compacted graphite. Notice it is neither a round nodule nor flake. It is between the two!

This metal is difficult to control because what has to occur is akin to making a deliberate mistake in producing ductile iron. Eventually it was learned that the control of this unconventional type of graphite was to deliberately add impurities to retard or hold back the formation of nodular graphite! So you see why we say that this metal was developed by doing something wrong, to make something right!

What resulted was a grade of metal spawned from ductile iron that became a very useful metal that has taken its place among the rest of the cast iron family. The student will remember that the only difference between properties of cast iron with compacted, nodular, or flake graphite is the shape and distribution of graphite.

The production of compacted graphite requires very careful control because it is sensitive to many factors! If the student has absorbed the information so far in our studies, the whole process of the production of cast irons will become very clear, but only if the change in relationship of the carbon atom with the element iron, in the presence of silicon, is understood.
Remember, with graphite present the carbon atom has a home to go to and the choice is dependent upon time and temperature! Please slow down and connect the dots!

We know that the matrix of cast irons is the same as that of steels. We know how to strengthen the matrix by controlling the amount of Pearlite present and we will learn how to enhance the strength further by heat treatment. That is the positive part of our story in as much as strength is concerned.

The negative end is to what level the shape and distribution of graphite affects the matrix. If the student recognizes the distinction between the occurrence of the three types of graphite, it will be apparent that the properties of a compacted graphite cast iron is in between that of flake iron and nodular iron!
All of the literature indicates that the biggest advantage of compact graphite is in its heat conductivity. The metal is relatively new and to our mind has not been fully developed as yet.

We have covered, very superficially, malleable irons, gray cast irons, ductile irons, and compacted graphite irons. We will address these metals later in the section on heat treating, but right now it's time for a review of the properties and the possible uses for each category of cast irons.

1. Gray iron is the most economical of the cast irons and should be used wherever the superior properties of the other cast irons are not required. We report, you decide!
2. While ductile iron has replaced malleable iron in many cases, malleable iron is still preferred in very thin castings requiring impact resistance combined with strength.
3. Ductile (nodular iron) can, and has replaced steel in many cases. It is up to the design engineer to evaluate the properties of ductile iron to see if it fits a particular application. Ductile iron, because of its castability combined with superior properties can be used to replace gray iron enabling the design engineer to reduce the section and hence weight. We report you decide
4. Compacted graphite is used mostly to produce ingot molds because of the high heat conductivity. Design engineers discovering this superior property have started to use it in automotive parts that are subjected to heat. This is a relatively new material and remains for the imagination of the design engineer to find other applications. If heat is not a problem we see no particular use for compacted graphite, but we could be wrong! Wow! What am I saying! I better go see a shrink!

Well, that's it. Let's start on basic heat treatment.