The Properties Of Cast Iron
Cast iron can be repaired with complete confidence and success. But first you need to become more familiar with its chemistry, properties and how it responds to heat.
By Gary Reed
The reality is that much of what you understand and have been taught about welding cast iron may be incorrect. Cast iron is not used because it’s cheap, but because it has very good properties and works extremely well as the metal of choice for many engine blocks and cylinder heads.
Cast iron has gotten a bad rap because uninformed people have tried to weld it unsuccessfully. It is not the fault of the casting but rather the misinformed welder that is to blame. The sad part is that most welders have been duped by the welding rod and equipment manufacturers into believing that they can weld on cast iron with very low preheat temperatures.
In order to properly weld any type of metal, it is necessary to have a very good idea of how the metal will respond to the heat involved in the welding process. In other words, the outcome must be predictable. I assure you that cast iron is as understandable and predictable as steel or aluminum.
To begin with, you need to recognize that when metals are heated they expand, and when they cool they contract. Sounds simple enough, but is it really? The following examples will help you understand and remember what actually happens when heat is applied to cast iron.
Free Expansion Followed By Free Contraction
With free expansion followed by free contraction, there is no change in dimensions to the cube as shown in the above illustrations.
The next example shows what happens when restricted expansion is introduced.
At this point, it is very important to realize that when restricted expansion takes place, a permanent change in shape has taken place and is irreversible. This is the exact cause of heat related cracks. I will explain this later in more detail. However, if the cast iron cube is not confined it shrinks equally in all directions because it is not connected to the vise so it is still considered free contraction.
The next example, shown below, enters into the most complicated and serious problem of all, wherein an area in the center of the casting is heated and the surrounding iron is left colder.
This is where it gets complicated and most welding problems occur. There is a very big difference between welding in the middle of the casting and welding on a corner or an ear of the casting.
Just like heating the cube in the vise, the heated area in the middle of the casting is contained in the vise of the colder iron surrounding the heated area. Unnatural growth occurs, permanently changing the thickness of the confined metal. What happens next is that strain is caused when the thicker iron cools and shrinks. It is impossible for the changed iron to return to its original thickness. The strain usually overpowers the tensile strength of the surrounding iron and cracks to relieve the stress. If cracking isn’t immediate, there will be residual stress left over that can, and most often will, lead to future cracking.
A good example of changing the shape of metal by using the restricted expansion/contraction principle is to remove a pipe plug by heating only the plug and not the surrounding iron. This will cause the plug to tighten against the sidewall of the tapped hole until it becomes confined, and then it will continue expanding in the only direction that is not confined. This means that it is forced to grow in an unnatural direction. The plug gets longer, just like it is being stretched.
Allow the plug to cool completely and you will find that it is now loose because it is now smaller in diameter than before you heated it. As it cools, it shrinks equally in all directions (Cubic contraction).
Corners and ears are not subject to restricted expansion/contraction because the heat-affected area is free to expand and contract without confinement. Therefore, you are free to heat, braze, powder weld or even arc weld without causing stress and cracking. It is impossible to cause a crack even if the casting is cold in this situation.
So, what actually causes cast iron to crack and not other metals that are so easily welded? It is the very thing that gives cast iron so many of its desirable attributes. Carbon. The quantity of carbon found in gray iron ranges from 2.5 to 3.5 percent. This is a high enough percentage to oversaturate. A simple example would be to take a glass of water and add salt until the water becomes saturated.
If you continue to add salt, you will notice that it begins to accumulate on the bottom of the glass. This is oversaturation. Stir the water, and the undissolved salt will become temporarily suspended. Cast iron has undissolved carbon in suspension. The undissolved carbon found in gray iron is in flake-form whereas in ductile or nodular iron it’s found in small spheres or nodules.
The significance of the extra carbon is that it denies cast iron the advantage of having a yield point lower than its tensile point. In other words, it doesn’t yield to the contraction strain (stretch) before it breaks. When compared to steel in the chart below left, you can easily see the advantage that steel has over cast iron. The numbers on the left represent strength in thousands of pounds per square inch.
Yield is the point at which the metal is strained and begins to stretch. The stretching continues until it reaches its tensile or ultimate strength and breaks into two pieces. Notice that cast iron’s yield is equal to its tensile point whereas steel has a yield point significantly lower than its tensile point.
This means that when steel is heated or welded in a confined condition it can stretch when contraction occurs next to the heat affected zone. This is why steel does not crack and cast iron does when it is welded. Cast iron doesn’t stretch or bend.
However, if cast iron is heated to 1,200° F, it will take on an artificial yield point allowing it to stretch and relieve confined contraction stress. This is why cast iron must be preheated to 900° to 1,500° F prior to welding depending on the process.
There is one more issue that you have to deal with. The carbon that is dissolved is attached to iron molecules and is responsible for the iron’s ability to be hardened. Just as carbon steel can be hardened, cast iron can be hardened in exactly the same way. Cast iron will get so hard that you cannot drill, tap or machine it. A good example is induction hardened exhaust seats in late model cast iron cylinder heads.
However, hardening destroys many of the positive attributes of cast iron. Induction hardening of valve seats actually causes stress that leads to cracking because it is impossible to heat cast iron to its critical temperature (1,200° F) in a restricted expansion/contraction environment without causing stress. Cracks in many late model heads with induction-hardened seats are actually caused by the hardening process.
Hardened cast iron can be annealed (softened) by heating to 1,500° F for two hours and slow cooled over 24 hours. Hardening occurs when the iron is heated past 1,200° F and cooled quickly.
It does not matter what welding process or procedure is used, not the filler material nor the welding machine. It’s the heat that causes restricted expansion/contraction cracking and hardening. There is simply no way around it. There has never been nor will there ever be a magic welding rod or process that can ignore these truths.
So, remember if you weld on cast iron and it cracked, it was not hot enough and if it got hard, it cooled too fast.
If you have cold welded on cast iron and it did not crack immediately, don’t be fooled into thinking it will not crack in the future. Cast iron can store stress for a very long time before a crack will appear.
We follow these rules strictly at LOCK-N-STITCH Inc. In the thousands of castings we’ve welded, we’ve never had cracks caused by our welding. About 50 percent of the damaged castings that come in our door have been previously welded and failed. Of these, nearly all have been nickel welded with an electrode by someone who was following the rod manufacturer’s guidelines.
The big problem is that it takes a long time to learn to braze and fusion weld to produce strong pressure-tight welds free from pinholes. It takes a good welder about three years to become a good cast iron welder. We build the oven around each individual casting because they are all different and need to be heated differently.
For structural repairs, we normally braze using a 70,000 psi tensile bronze rod with a minimum 900° F preheat. For cylinder heads and other small dense casting that can be totally remachined, we fusion weld with gray cast iron rod using an oxy-acetylene torch with a minimum 1300° F preheat.
If the damage is near the end of the casting or on a corner we don’t heat the entire casting. This helps reduce distortion. But if the damage is near the center of the casting, the entire piece is heated to the desired temperature.
We follow the same rules for powder welding if we intend to fuse the powder to the iron. And we use the Metco oxy-acetylene wire metal spray process, which establishes a mechanical bond at only 400° F pre-heat. It’s hard to argue with a 100 percent success rate.
Gary J. Reed is president of LOCK-N-STITCH Inc., Turlock, CA.
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