Porous Casting

[John Andrews 2Participant @johnandrews2]

[John Andrews 2Participant @johnandrews2]

G'day All,

I am building a Baldwin C15, Queensland Railways 1879, virtually same as New Zealand "T" Class of the same vintage.

The bronze cylinder block was cast by a friend in NZ, and has steam passages and exhaust passages cast into the block, which is also a major part of the frame and has the smokebox support cast in also.

Cast Iron cylinder liners are fitted and all machining on the block is completed. The wheels and valve gear is complete, and it will run on air.

HOWEVER, there is leakage between the steam inlet and the exhaust outlet inside the casting. I have eliminated all other possible sources of leakage, and I can only conclude that there is either a small hole or porous bronze between the inlet and outlet passages.

I have canvassed club members for a solution, and two have been suggested.

I would particularly like to get advice from anyone who has actually had this problem (and fixed it) or who knows someone who has), with whom I can get in touch.

Of course, advice is always welcome from anyone else too, but I need a solution.

Many Thanks

John Andrews2

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi John ,

I've no doubt that people will suggest solder and modern resins but the only real solution is to cut the bad material out of the casting and replace .

(1) Basically a controlled boring or milling operation from the outside and insertion of a preshaped block of bronze .

With proper planning most of the geometric features of the lost piece can be machined into the new block before insertion .

Mechanical fixing with screws . A press fit may be possible if shape is simple . With a good engineered fit for the piece in its hole a little high temperature sealant will be adequate .

A litle remedial machining of completed cylinder block and it will be almost good as new .

(2) New piece could be silver soldered in , some risk of distortion but definately strongest fixing .

nb : Not all bronzes can be silver soldered – best check first .

(3) Specialist firms can weld bronze . Cut out bad metal and fill with weld or weld in a block . I wouldn't recommend this at all because of risk of likely bad distortion but if it comes down to desperate measures it could be worth a try .

Regards ,

Michael Williams .

[Ian S CParticipant @iansc]

Michael's way is the propper way, but if that is too much, I'v used Loctite on cracked engine blocks, where some would repair with the Metal Lock system (vintage vehicle). Larger voids may be filled with the likes of JB Weld, get the slow curing version, it has a higher temperature rating, and I think it's harder. Ian S C

[simondavies3Participant @simondavies3]

John,

i had an aluminium head from a road car that developed holes which were subsequently welded back together- mostly around one of the combustion chambers. Whilst the welding was successful in the combustion chamber, it was not so successful in the water jacket which continued to cause problems. I subsequently found a company that pressure injected a resin into the head cavities which dealt with the problem areas. When I was discussing the options with them, it turned out that several F1 teams used the company to pressure fill their engine and gearbox castings to avoid any porosity.

The head has never leaked again despite some reasonably serious exercise since the fix and certainly is in a highly stressed environment.

If you don't want to go down the route of replacing the problem area, this might be an alternative method.

Simon

[Jens Eirik Skogstad 1Participant @jenseirikskogstad1]

Before pouring the molten metal, the metal must be degassed first and the slag is removed from the floating metal surface. Then pour the molten metal until it is complete, do not stop to pouring until the metal is not longer to shrinks due the metal is cooled down. If you are pouring the molten metal until it is full of metal then stop to pour, the consequence is: hollowed and porous cast metal part.

[John Andrews 2Participant @johnandrews2]

Thank you all for your advice.

Michael, I received the same advice here. Trouble is I cannot tell where is the porosity, except that it is hidden deeply inside the casting. What you suggest would surely be foolproof, but extremely difficult.

Simon, your reply got the old brain cells going. I had been told that Loctite 209 (or 290?), the "wick-in" variety, did the job for the other loco built in NZ with the same castings. The chemist at Loctite in Melbourne recommended the same, but with the twist that the casting should be put in a tin, and a vacuum applied with a vacuum cleaner, and then heated to about 50 -70 deg C for about 30 minutes.

An internet search turned up two firms here which did the job commercially. I engaged the services of one, and the job was done in two days – I picked up the castings today. For $55.00.

They use the same idea, but with a bit more elaborate equipment. If it fails, then I guess major surgery is the only answer. The chap said it is used on turbine blades for turbochargrs, and he was confident it would stand up to the steam. The temp of saturated steam at 100 psi is about 170 deg C. Everybody claims that superheating does not increase the temp significantly at our sizes. I guess I will find out. The stuff used is rated to 207 deg C.

Thank you all for your advice

John A

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hello again ,

I'm glad that you found a relatively painless solution to your casting problem .

For future reference :

It is possible to X-Ray castings and find out exactly where porosity is located . There are several firms in the UK which can do this and possibly there are some where you are . Charges are usually reasonable and I've known odd model engineering jobs to be done for nothing ..

There are poor man's versions of the X-Ray system using ultrasound and a completely alternative method using dye penetrant . Dye penetrant only really works where porosity is in surface layers of component but nevertheless it is very useful and extensively used .

Other matters :

Steam does get superheated in models in same way as full size engines . Only problem is that volume flow is very much less and there is possibility of steam cooling down again before it does much in cylinders .

Bulk of cylinder block does not get anywhere near steam temperature .

Regards ,

Michael Williams .

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