Determining Cast Iron Grade/Quality before welding

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lindse34

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Greetings,

Does anyone have any secrets they would like to share on how to determine what grade of cast iron is sitting in front of you before attempting to weld it? This is one area that continues to frustrate me. One piece will weld so smoothly and the filler will wet nicely and flow easily. The next piece the filler has no wetting action and just pools on the surface.

I usually TIG weld cast iron and like Silicon Bronze (OK so its more like Brazing) or Nickel for the stuff actually needs to be strong.
Just got done fighting an exhaust manifold that will probably not stand the first two thermal cycles it sees, but I tried.

Thanks and Regards,

Matt
 
To successfully weld cast iron it is necessary to consider which type of cast iron you are dealing with and the welding technique required for that particular type. Some types of CI are not weldable such as white aka chilled iron. cast iron. It is also necessary to investigate the reason for the damage and to take into account the requirements of the finished piece.
Types of Cast Iron
Gray cast iron,the most common of cast iron types, is usually identified according to its tensile strength. 30 grade cast iron has a tensile strength of 30,000 psi; 40 grade cast iron has a tensile strength of 40,000 psi; and 60 grade cast iron has a tensile strength of 60,000 psi. The most common grades of cast irons are the 20 and 40 grades. Gray cast iron is a ferrous material containing more than 1.75% of carbon, the greater part of which is present in the structure in the form of graphite. This graphite gives to the fracture a color varying from light to dark gray. Compared with steel, gray cast iron has low malleability or ductility caused by the separation of the major portion of the elementary carbon in the form of graphite, which form a network throughout the metallic matrix and separate from it.
The chemical composition of a common machine casting is as follows:
Carbon 3.0% to 4.2% any higher than this as a carbon content and it get real hard to weld.
Silicon 1.5% to 2.5%
Manganese 0.6% to 1.2%
Phosphorus 0.4% to 0.6%
Sulphur 0.05% to 0.18%
Basically, cast iron is steel with a lot more carbon. Steel contains carbon up to 1.75%. If steel has more than 1.75% carbon, it is called cast iron. Steel can dissolve up to 1.75% carbon. If more carbon is added, it won’t dissolve and remains a free carbon. When you look at cast iron under a microscope the free carbon appears as black flakes, hence the name “gray cast iron.” With the use of proper techniques and the correct filler alloy, most gray cast irons can be successfully welded.Be aware that there is another type of cast iron, called “white” or “chilled” cast iron. It is generally considered not weldable. White cast iron does not have the distinctive gray color of most cast iron and is generally found in equipment where extreme hardness is needed (e.g. rock crushers, steel mill rollers, etc.).

Generally id is usually by its application .It its off a machined part you can usually say its grey cast iron,though the exception is nodular cast iron found in cranks and camshafts. Even preheating is called for especially in castings with thin to thick sections .Braze welding is the easiest, followed by cast iron filler rods with oxy and lastly arc welding with nickel based electrodes. Cast iron does tend to absorb materials that interfere with a sound welding process. Some exhaust manifolds can be difficult to weld as the carbon in the exhaust material mixes with the carbon in compound and raises the overall carbon content too high to do a reasonable job.Diesel oil (a hydracarbon) soaked into a crack ain't a whole lot of fun either.
The Lincoln Electric book "Metals and how to weld them" gives some pretty good info on the types and process .

Personally I like to weld with Oxy acetylene and cast iron filler with borax based flux.Its possible to float the contaminants out on top of the molten weld puddle. Its time consuming but gives the best results ,but the process takes time to learn and is hot and uncomfortable for non welders.

Thats the quick and dirty on it.

Oz
 
Thanks Oz,

I'll have to give oxy welding a shot sometime. Never have done much of it, might be interesting.

After you mention it I would bet that an exhaust manifold would have a ridiculous amount of carbon dissolved in it after 60 years. In this case the fellow trying to extract broken bolts was too agressive with heating and cracked two threaded bosses. I may need to search for different brazing fluxes to try in cases like this.

Regards,

Matt
 
All types of cast iron are machined, I don't know why people keep perpetuating the myth that you can't machine them. Most of your cast iron car parts are finish machined in one pass from the rough casting on dedicated machines. It was cool watching the cover faces for the axels get faced, then drilled and tapped in one shot (all of the holes simultaneously).

Welding cast iron normally makes a "cosmetic" bond, as the carbon contents that cast irons have really interfere with making a structural bond. Not that it can't be done, but if you are welding a piece of cast iron, you didn't know how to design the part to begin with, repurposing material is exempted for us hobby machinists. Please don't weld together a cast iron part that you need to have high structural integrity or involves a safety part.

Exhaust manifolds are made from a type of cast iron referred to as "SiMo" due to the very high silicon and molybdenum contents that push the melting temperatures up but increase flowability of the molten metal so you can cast the extremely thin sections.
 
Cast iron exhaust manifolds can be cleaned up by immersing in a mix of molasses and water. Use 10% molasses to 90% water.Immerse for a few days only but don't forget and leave it.
This stuff will eat carbon and leave the cast iron looking like sponge cake after a week or more.
Regards the machinabilty of some cast iron , cast iron weld with some grades of nickel can only be ground and are not suitable for grinding with HSS tools. Maybe some with the new grades of carbide, but hey its been a decade or two or three since I was last involved with machining it.

Eutectic and other electrode suppliers specifically stated on the packets that weldments were either machinable or non machinable dependent on the type of nickel percentage in the electrode.Not having electrode welded much cast iron in recent years it may well have changed.I have personally witnessed some poorly welded cast iron destroy the the stones of a rotary surface grinder as the process used was Oxy acetylene welded with a carburising flame.

Another thing that occurred to me is preparation using a grinding disc especially the ones that have a resinous binding.
Whatever they deposit in the weld prep makes it difficult to arc weld sometimes a green glow can be seen around the arc.I suspect that causes the wetting problems.

Bear in mind that that cast iron when welded has a capacity for distortion, the manifold mating surfaces can easily be pulled out of alignment unless restrained by a very substantial structure.I have had success using a junk cylinder head, but some heavy structural section would suffice.


Back to the processes used:
Oxy acetylene cast iron filler rod with borax based flux or Brazing which is NOT welding in the true sense.It is a mecahical bond as rthe filler never mixes with parent metal as does arc and Oxy welding.Also there is spray powder, Oxy acetylene deposited through the flame but expensive in terms of both equipment and and material. Not gear a home welder is likely to have or buy for one job.

I have also welded small cast iron jobs with tig and used old cast iron piston rings as filler rods but again this must be ultra clean and mechanically prepped =no grinder discs.

I hope this helps you.

Ozwelder
 
Flammable_Solid said:
All types of cast iron are machined, .
Click to expand...

I should have clarified it a little better then.Generally the machined cast iron you will commonly come across will be generally grey and some nodulair cast irons.One of the carbide manufacturers(Sandvik) has stated :
"
Influence of hardness

The influence of hardness related to machinability for cast irons follows the same rules as for any other material.
E.g, ADI (austempered ductile iron) and CGI (compacted graphite iron) as well as NCI (nodular cast iron) have hardnesses up to 300-400
HB. MCI and GCI average 200-250 HB.
White cast iron can achieve a hardness over 500 HB at rapid cooling rates where the carbon reacts with the iron to form a carbide Fe3C (cementite), instead of being present as free carbon. White cast irons are very abrasive and difficult to machine. "

Indeed some of the white cast irons are so hard they are effectively a cermet. Reference Wikipaedia

<White cast iron

It is the cast iron that displays white fractured surface due to the presence of cementite. With a lower silicon content(graphitizing agent) and faster cooling rate, the carbon in white cast iron precipitates out of the melt as the metastable phase cementite, Fe3C, rather than graphite. The cementite which precipitates from the melt forms as relatively large particles, usually in a eutectic mixture, where the other phase is austenite (which on cooling might transform to martensite). These eutectic carbides are much too large to provide precipitation hardening (as in some steels, where cementite precipitates might inhibit plastic deformation by impeding the movement of dislocations through the ferrite matrix). Rather, they increase the bulk hardness of the cast iron simply by virtue of their own very high hardness and their substantial volume fraction, such that the bulk hardness can be approximated by a rule of mixtures. In any case, they offer hardness at the expense of toughness. Since carbide makes up a large fraction of the material, white cast iron could reasonably be classified as a cermet. White iron is too brittle for use in many structural components, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications as the wear surfaces (impeller and volute) of slurry pumps, shell liners and lifter bars in ball mills and autogenous grinding mills, balls and rings in coal pulverisers, and the teeth of a backhoe's digging bucket (although cast medium-carbon martensitic steel is more common for this application).

It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through. However, rapid cooling can be used to solidify a shell of white cast iron, after which the remainder cools more slowly to form a core of grey cast iron. The resulting casting, called a chilled casting, has the benefits of a hard surface and a somewhat tougher interior.

High-chromium white iron alloys allow massive castings (for example, a 10-tonne impeller) to be sand cast, i.e., a high cooling rate is not required, as well as providing impressive abrasion resistance.[citation needed]>

This also explains why white cast iron is unweldable.

Ozwelder
 
If you are able to clean up a bit of the surface with a grinder, put a fine grit wheel on your grinder and make a smooth spot. Using a magnifier, you should be able to see if your surface has flakes or little pockmarks in the surface. If it has flakes, it is a grey iron, if it has round pockmarks, it is ductile iron.

You could "ring" test the pieces, depending on the size. A ductile iron casting will resonate a little bit, a grey iron casting will not.

What starts to confusing these tests is the emergence of compacted graphite irons in the automotive industry, as it combines properties of both grey and ductile iron.

What type of parts are you trying to weld?
 
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