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Understanding "Cast Iron" For Welding
03-30-2015, 01:07 AM
Post: #1
Understanding "Cast Iron" For Welding
So while searching to find out whether a cast part I had was cast steel or iron I decided to use the information I was finding and create an information thread here on our forum. Feel free to add to this with factual information. Try to avoid the, "I did this and it worked" unless you know the facts about why it worked. I would rather not lead someone into believing that they can weld some sort of casted iron and have it completely fail on them. Especially when it comes in the forum of axles.

*** And by the way. Custom fabrication is always a do it at your own risk type of thing. Be safe be understand that things can go horribly wrong if you do not now what you are doing.***

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Cast Irons

Ahhh - yes, the hugely misunderstood cast iron. The term CAST IRON does not refer to a specific material with properties that we can discuss. It is a GENERIC TERM for a whole group of ferrous metals that are made up of iron, silicon, and carbon. The name has 2 parts - CAST referring to the fact that these metals can be readily poured into moulds (cast) when molten, to make parts, and IRON for the chief element that makes them up.

Now, there are several different types of cast irons that we should know about, and within each type there are often several different "grades." The different types of cast iron are distinguished by the form which the carbon takes - be it carbides, graphite, flakes, nodules, etc. The different types are:

Gray Iron - **Can Be Welded but is prone to cracking**
Composed of iron and silicon and carbon, with it's carbon content in the form of very thin interconnected flakes of graphite Gray iron possesses excellent castability and machinability so that complex parts can be readily cast and economically finish machined. The material has modest tensile strength values, good wear resistance, and good resistance to galling. It is economical to produce, cast and finish. Some examples of its use include: machine tool bases, ways and housings, disc brake rotors, cylinder blocks and heads.

Grey cast irons contain 2.0 - 4.5% carbon and 1 - 3% silicon. Their structure consists of branched and interconnected graphite flakes in a matrix which is pearlite, ferrite or a mixture of the two (Fig. a). The graphite flakes form planes of weakness and so strength and toughness are inferior to those of structural steels.


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White Iron - **Welding not recommended**
Having it's carbon content in the form of granules of iron carbide (due to low silicon content and rapid cooling when cast); white iron is very hard and brittle and virtually unmachinable. It is therefore of little practical use to us, despite having high compressive strength and good resistance to wear and abrasion. It is, however, often the starting point for malleable iron.

Malleable Iron - **Can Be Welded**
Malleable Iron has most of its carbon content in the form of irregularly shaped lumps or nodules of graphite mixed in the matrix (lattice) of iron. These nodules of graphite are not connected to one another though. Malleable iron is created by careful and precise heat treatment of solid white iron castings. The result is a cast iron that is extremely tough (toughest of all cast irons) and that can have (depending on the exact heat treatment) tensile strength and/or ductility similar to a mild carbon steel. It is not as easy to cast or machine as gray iron and therefore is usually only used to cast relatively simple parts. It is, however, often used for: hand tools (e.g. C-clamp, pipe wrench), brackets, hangars, axle housings, drive yokes, connecting rods, brake callipers, etc.

Melleable Iron is most commonly seen the the forum of cast pipe fittings.

Nodular (Ductile) Iron - **Readily weldable**
Named "Nodular Iron" because all of it's carbon content appears as tiny spherical nodules of graphite, and with carbon content of up to 10%, nodular iron combines the best properties of gray iron and malleable irons. It has both excellent castability as well as very good machinability while being the most ductile of all cast irons and having very high tensile strength (for a castable metal). With carefully controlled changes in chemical composition (it can be alloyed with other elements such as nickel, molybdenum, vanadium, etc - much the same as steel) and/or heat treatment it can be used to manufacture very strong, stiff, tough components. The trucking and transportation industries have been quick to appreciate nodular castings as lighter and stronger replacements for complex steel weldments and for their ability to produce complex structural shapes - both cored and solid - that are strong, light, and cheap. Examples include such critical components as crankshafts, gears (including ring and pinion sets), and front suspension steering knuckles. [4]

The mechanical properties of grey irons can be greatly improved if the graphite shape is modified to eliminate planes of weakness. Such modification is possible if molten iron, having a composition in the range 3.2 - 4.5% carbon and 1.8 - 2.8% silicon, is treated with magnesium or cerium additions before casting. This produces castings with graphite in spheroidal form instead of flakes, known as nodular, spheroidal graphite (SG) or ductile irons (Fig. b). Nodular irons are available with pearlite, ferrite or pearlite-ferrite matrices which offer a combination of greater ductility and higher tensile strength than grey cast irons.


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One of the most popular examples of nodular iron applicable to us must be the famous Ford 9 inch rear axle with the highly sought after nodular housing. Note also that some popular Dana axles (center sections) are cast from malleable iron (e.g. Dana 30) and some from Nodular Iron (e.g. Dana 44, 60)

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So now take this information and do some research on why type of cast iron you're dealing and with and what is the best process to weld it. I will add to this as I gather more information.

It would also be nice to gather a list of axle parts and list what type of cast metal they are. Whether it is cast iron, steel, or aluminum. There are different grades of all of these types of metals. It get's really complicated to be honest. Sometimes when you are dealing with the unknown you just have to try it and see what happens...

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03-30-2015, 07:39 AM (This post was last modified: 03-30-2015 08:19 AM by Smithy.)
Post: #2
RE: Understanding "Cast Iron" For Welding
A quick way to tell what you are working with is to touch a grinding wheel to it and watch the sparks. For example grey iron will not produce short sparks with many offshoots, and they will have a reddish yellow color. While mild steel will have long whitish sparks with few offshoots.

   

The biggest issue with repairing cast iron is its ability to dissipate heat quickly, much faster than steel. This leads to the iron around the weld becoming brittle and causes cracking.

Cast iron can be welded using a process similar to forge welding in blacksmithing, where the material is evenly heated almost to the point of melting, then is hammered to weld the metal at a molecular level. The repair will have the same characteristics as a newly cast piece. The whole process is time consuming and requires tools that most people dont have access to.

Or it can be electrically welded with a minimum preheat of 1500°F and allowed to slowly cool afterwards for up to several days. Again not practical.

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03-31-2015, 07:23 PM
Post: #3
RE: Understanding "Cast Iron" For Welding
Sorry for the hijack. . .

A good rule of thumb is that if you are welding link mounts or a truss to a cast housing you should have a minimum 4" of contact between the two pieces.

So for example if you were to weld this on as is, most likely it will come off.


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Most link mounts are actually welded to a truss of some sort that attaches to the steel axle tubes but not the cast portion of the housing. But if you must, have as much surface contact between the two pieces as possible.

   

Another thing when welding to cast, everyone always talks about a pre heat, but almost no one mentions post heat. A post heat is bringing the material to a higher temperature than the pre heat. Holding it there for a length of time. The larger the piece, the longer the time. Then wrapped in welding blankets or buried in hot sand and allowed to cool slowly. This will help to even out any stress risers created during welding, and reducing the chances of cracks forming.

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03-31-2015, 10:13 PM
Post: #4
RE: Understanding "Cast Iron" For Welding
(03-31-2015 07:23 PM)Smithy Wrote:  Another thing when welding to cast, everyone always talks about a pre heat, but almost no one mentions post heat. A post heat is bringing the material to a higher temperature than the pre heat. Holding it there for a length of time. The larger the piece, the longer the time. Then wrapped in welding blankets or buried in hot sand and allowed to cool slowly.

This is 100% true. The cracks typically form from the heating and cooling process. Heating to fast will crack. Cooling to fast will also crack. I was welding a table base for a customer that cracked on me. without any preheat or post heat. It cracked right away. I was welding gray cast iron to a steel tube with standard ER70S-6 mig wire. This should not work and I told the customer that it could fail. After it failed the first time right away I preheated both metals and then welded them together. Then took the propane torch that I used to preheat them and applied more head after I finished the weld. The reason I did this was to let the metal cool down from the molten state while not letting it cool past the preheat temp. After keeping heat applied to it for about 20 min or so I quickly buried it in sand and left it there for 24 hours. This process kept it from cracking during the welding and cool down process. The customer was informed that it could still crack and fail down the road but I haven't heard anything from him about the weld failing. The reason bring up this example is to hopefully explain how much of a difference pre and post heat can make when welding cast iron.

No keep in mind gray cast iron is very prone to cracking. The Nodular (Ductile) Iron is much more forgiving and easier to work with. However pre and post heat should always be used when ever possible when welding any type of cast iron. Just because it didn't crack during the cooling process does not mean it won't crack down the road.

Also I can't not express this enough. Just because it's a casted metal does not mean it's cast iron. Figure out what you have beforehand.

As I said in my OP "It would also be nice to gather a list of axle parts and list what type of cast metal they are". So if any of you can verify what metal these parts are on your rides we can start this list on this page. Once I dig out my bronco d44 I'll be grind testing the casted parts to see what type of metals they are.

If I were to go out on a limb I'd have to assume that any cast metal parts on your axles that are welded from the factory will more than likely be a cast steel or a Nodular (Ductile) Iron. But don't take my word for it.

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04-01-2015, 07:00 AM
Post: #5
RE: Understanding "Cast Iron" For Welding
As far as I know, all Dana housings are nodular iron, or steel.

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