cast iron welding

By Niekie3 (***)

Date 10-01-2002 20:00

It is not easy to tell different cast iron types appart. If the casting has markings on it, this may well reference a specification (e.g. ASTM spec.) that will give you this information.

I have been involved in the repairing of a number of cast iron motor housings. These were generally made of grey cast iron. I would think that your motor base would also be made from grey cast iron. (This is however a guess!)

As long as your grey cast iron is not contaminated with low melting point constituents, it is relatively easily welded. Use a Ni based or Ni-Fe based electrode specially formulated for welding cast irons. (e.g. ENi-CI) Using these elctrodes, pre-heating is minimized and the deposit is generally abe to be machined. The HAZ of your weld will however be very hard and brittle and will not be able to take much of a stress.

Be advised that some cast irons are basically unweldable. If your motor base is one of these, you will have to throw it away, unless you try repairing by way of brazing. Sometimes this is the best alternative.

Hope this helps

Regards
Niekie Jooste

By - Date 10-02-2002 16:48

to: Niekie3
yes i thing the type what i have is grey castiron , i repair one bed
but the other coming some crack near weld joint , i want to know the
preheating must done for all bed or only near the crack ,also i used
same electrod you mention a bove and i put the bed in dry sand
after repairing .
can you tell me what test can i do after repairing to insure my jop strong
and ok.
thanks
ali

By Niekie3 (***)

Date 10-03-2002 17:57

Hi Ali

In general there are two broad approaches to welding cast iron. They are:

1) Make the weld and heat affected zone as ductile as possible to withstand stresses. In other words, try to get the properties of the HAZ as close to the properties of the base metal as possible.
2) Try to deposit a ductile weld that can "give" to allow for stresses and make the brittle HAZ as small as possible.

The first option is the best, but often not practical. To achieve a relatively ductile HAZ, you need extremely slow cooling. This means giving your part an extremely high pre-heat. Typically here you are talking about 550°C or so. This is very high, and is basically only practical if you can place the entire part in an oven and maintain the temperature while welding, followed by slow cooling under insulation or warm sand. This means that high heat inputs are also recommended. Typically the "block welding" technique can be used to ensure the maximum heat is retained in the area you are welding. In the block technique, you weld a section of your first run, then immediately weld the second run on top of the first, then the third etc. till you have finished that "block". Now move onto the next block and do the same.

The second option is mostly the only practical option because comonents are too large to place in an oven, or the facilities are not available, or heating it to such a temperature may destroy other parts connected to the component. When using this second option, we know that we will have a brittle HAZ. The whole trick is to maintain this HAZ as small as possible. With a small HAZ, the weld will be relatively crack resistant. Here we use a Ni based filler such as ENi-CI and weld the cast iron cold. We also weld with low heat inputs and only weld a small section (e.g. 30mm) at a time, allowing the weld area to cool down to around 100°C before placing the next weld deposit in the same area. If your weld is relatively long, you can use the "controlled wandering" technique. Here you place a small weld in one position and then move to a "cold" area and again place a small weld. Continue this untill finished.

If your casting is rather thick in the area you are welding (30mm or thicker) then it is required that you place a pre-heat, even when using this technique. This can be up to 300°C.

There is obviously much more to welding cast irons than we can describe here, but here are a couple of things to remember when welding cast irons:

1) The shrinkage stresses on most cast iron welds can easily result in cracking because of the low ductility of grey cast iron. If you have a weld that is highly constrained, then this will be your biggest problem. Try to alleviate this by using the "back step" welding technique or by peening.
2) Many cast iron components that have been in service have been contaminated with hydrocarbons and the like. You need to get this out of the component as best as possible, or you will have serious porosity problems. Heating to 300°C or so will drive off most of these contaminants.
3) Many cast iron components have been cast with a great deal of impurities in the metal. Some of these will result in extremely poor quality welds and may even be unweldable. The best solution that I have found for this problem is to weld a "buttering" layer onto the cast iron using the Ni welding consumable. Then use a chipping hammer and try to hammer it off. Chunks of the buttering layer will come off if you have these contamination problems. Then weld another butter layer in these areas and again go through the chipping routine. Keep repeating this untill the butter layer stays on. You can now weld onto this butter layer without too much problems. (This process of repeatedly welding on the same spot and chipping, can draw out enough of the contamination to render the cast iron weldable.)

Regarding your test for a sound weld, I can not really give you a good answer. We no not place repaired cast iron into any safety critical applications, so we do not do too much testing other than dye penetrant testing. Keep in mind that with any cast iron repair that needs to be able to take a load, you need to supplement the welding with some other mechanical jointing techniques such as metal stitching.

Hope this helps

Regards
Niekie Jooste

By sparkycanuck (*)

Date 10-03-2002 02:45

Some good information on cast at Dave Wright Welding site http://pw1.netcom.com/~dwelding/ under the "cast iron info" line - WS

By billvanderhoof (****)

Date 10-03-2002 05:02

If you can preheat the whole thing that is helpful.

Weld a little at a time say about 2 cm then hammer with a blunt chipping hammer or an air chisel with a blunt point (turn the air down some). The hammering expands the weld metal into the groove and gets rid of the tension that is causing the cracks. Keep hammering until the weld is more or less the same temperature as the surrounding metal. If you use the air chisel give it a blast then wait a minute and so on. Then do another little bit the same way. A large repair requires patience. As stated above the metal near the weld will have been effected. To return it to its original state may require heat treatment, a subject I leave to others.

If this thing (I don't really know what it is) is subject to substantial stress it may be easier to fabricate a replacement from steel.

Slow cooling is best you are doing the right thing with the sand. If the sand starts out hot that is even better.

Careful inspection when finished will show up most problems with cast iron. Tap with a hammer should ring, dead sound indicates crack. Dye check is helpful.

I hope this helps. It has worked for me.

Bill

By - Date 10-03-2002 10:34

ok, it is motor bed (base) and this motor conected by flixible coubling
with centrifugal pump use to pumping petroluem product.

By RonG

Date 10-03-2002 11:50

A Cast Iron Petroluem pump base (bed pan I guess), I would have to say thats pretty much a lost cause for welding, unless you want spend one heck a lot of time and money trying to get it clean.

It can be done but is it worth it. You can use pre-fluxed Brass and an Oxy-fuel tourch or you can (again if its worth it) use Metal Locking.