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Hard spots in cast iron


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  #1  
Old 09-27-2008, 12:12:32 PM
Ronald E. McClellan Ronald E. McClellan is offline
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Default Hard spots in cast iron

I'm making several small cast iron mufflers for the Sieverkropp horizontal engine. I now have a couple back from the foundry and have to put 1/2 NPT male on one end and female on the other. I started on the female end and trying to enlarge the opening to tap the threads. I have it in my lathe and using a carbide boring bar. The first bar got chipped , now using the second bar and I am hitting hard spots that are pushing the bar out of the way and leaving hard shinney lumps on the inside. I am afraid if I keep going I'm going to break something. Even if I do get the opening large enough I'm afraid of breaking the tap. What do I do? Will heating soften the hard spot. Are there tools , cutters , taps , dies that will cut through the hard spots. I'll appreciate any help I can get. Thanks , Ron
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Old 09-27-2008, 04:48:01 PM
Rodrigue Caron Rodrigue Caron is offline
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Default Re: Hard spots in cast iron

Ronald,
A method consiste in grinding the hard spot, and if necessary, weld with machinable electrode.
Rodrigue
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  #3  
Old 09-27-2008, 05:22:27 PM
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John Newman, Jr. John Newman, Jr. is offline
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Default Re: Hard spots in cast iron

Sounds like low grade iron. I once tried to make piston rings from cast iron drain pipe (definately low grade...) and ran into glass, sand, rocks and other debris. Finding un-identifiable hard spots would not be unheard of.
As noted above, grind away what you can and build up any voids with weld.
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Old 09-27-2008, 06:55:41 PM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Hard spots in cast iron

Poor quality iron, or a cold pour. You are probably encountering crystalization in the iron, which hardens it, also making it very brittle. repair as mentioned in above posts. In working with cast iron pipe, I have seen sections that have been crystalized nearly all the entire cross section. The exposed iron when cut is nearly white, with the crystalized graining plainly visible. This stuff will shatter like glass when struck. Other pipe, is extremely fine grained, nearly black in appearance, when cut, and soft enough to actually dent, without breaking - a machinists dream to work with! As for your castings, if the hard spots appear in all of them, you might complain to the foundry - they might re-cast for you.
Andrew
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Old 09-27-2008, 07:25:08 PM
K D Redd K D Redd is offline
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Default Re: Hard spots in cast iron

It is sometime called "CHILLED" cast iron. Very hard. I have an engine inwhich tje cam is a crome-moly billet. The lifters used on it are Chilled ron lifter. When I ran into this term I did not know the meaning.

Kent
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Old 09-27-2008, 11:45:28 PM
Kelly Tytlandsvik Kelly Tytlandsvik is offline
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Default Re: Hard spots in cast iron

I had some flywheels for model engines that were the same thing.......hard spots. An old friend of mine told me to put them in a fire and let them cool slowly in the ashes until morning. I put them in my fireplace and built a good fire and kept it going good and took them out in the morning and they were fine.

Kelly T
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Old 09-28-2008, 02:26:13 AM
Beanscoot Beanscoot is offline
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Default Re: Hard spots in cast iron

Kelly has an excellent suggestion. If you have a woodstove that would be even better.

Unlike John Newman's experience, I have had excellent luck with cast iron sewer pipe in making rings and tapered ring compressors. I've found that the really old sewer pipe has very thick bells so there is a lot more material in there.

Modern sewer pipe that is made to be "snap cut" and joined with rubber connectors is "chilled" and virtually impossible to machine. I had a friend at a tool and die shop put a chunk in a heat treat furnace for me to treat it and it came out like a beautiful malleable cast iron. But the ordeal by fire technique is a lot easier than getting hold of a heat treat oven.

Maybe the foundry that made it has some kind of oven that they can put the castings in?

They could be using feedstuff like modern cylinder heads with the stainless steel exhaust valves still in, resulting in weird alloy hard spots if the molten metal is not stirred sufficiently. Another machinist has told me of having grief machining a casting, then seeing a piece of a Timken roller bearing race in it. If the molten metal was heated long enough and mixed thoroughly "impurities" like this shouldn't be a problem though, and the casting should be uniform in hardness.
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Old 09-28-2008, 02:31:13 AM
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Thumbs down Re: Hard spots in cast iron

Sounds like the caster knocked them out of the mold too soon.
We've experienced that.......and the foundry heard about it too.
As said above......the casting likely got chilled and chilled cast is not to be reckoned with.
I didn't have any luck attempting to anneal it.......your mileage may vary.......
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Old 09-28-2008, 09:08:55 AM
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Default Re: Hard spots in cast iron

My cousin works in the valve making business. They started using a Chineese supplier for cast iron valve bodies. They were buying the raw castings and machining them here. They had a real problem with hard spots for a while. What they did was anneal them before machining. Heat to a specific heat then cool slowly. A little more refined form of "throw it in the camp fire". My thought would be to heat it to ? temp and then bury in ashes or dry sand and allow to cool over night.

keithw
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Old 09-28-2008, 10:33:45 AM
Ronald E. McClellan Ronald E. McClellan is offline
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Default Re: Hard spots in cast iron

Hey guys; Thanks for the feedback. I'm going to try the heat and sand box as I can do that myself. If I have to grind and weld I will have to take it to a weld shop and that will be time , gas , and $$. Thanks , Ron
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Old 09-28-2008, 01:51:06 PM
J.B. Castagnos J.B. Castagnos is offline
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Default Re: Hard spots in cast iron

The same thing happens when you arc weld cast iron, it heats up and chills, not machinable.
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Old 09-29-2008, 01:14:33 AM
Ray Freeman Ray Freeman is offline
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Default Re: Hard spots in cast iron

A dull red should be hot enough and keep it hot for a couple of hours then a slow cool down. I have some fire bricks and an LP torch and its got me out of trouble many times.
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Old 09-29-2008, 06:14:29 PM
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Default Re: Hard spots in cast iron

Sounds like the hard spots are chilled iron(white iron) from too rapidly cooling the casting. The transition temperature from white iron to gray iron is ~1550F so heat the casting to a red heat and hold for an extended time and then slowly cool.
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Old 09-30-2008, 10:24:45 PM
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Default Re: Hard spots in cast iron

from time to time i get an order of horse-head castings for John Deere rippers that i bore for mounting pins, recently we had a small issue with the supplier arc-welding to fill holes and bubbles, rendering the casting an all out bit-breaker. if there is a local heating and a/c shop near you check and see if they have any high temp fiberglass insulation, i got mine from a salvaged natural gas forge, make sure you check the heat rating, mine will keep a part glowing red for several hours heat the ENTIRE casting whether in a wood stove, camp fire or what have you. annealing a casting in places rather than as a whole will cause it to be unstable, and very susceptible to bending (yes you can bend cast) when the part is a dull red, with flecks of grey visible, usually in the form of "Scale" pull it out of the fire and surround it with the fiberglass insulation, you will have to wait almost a day for it to cool depending on how big that part is, but when its all nice and cool you will have a uniform, and very machineable part. a tip on boring castings with smaller holes, carbide tipped tooling isnt always youre best friend as they tend to hang up in the soft metal of the casting, a good sharp drill bit followed by a reamer does wonders.
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Old 10-01-2008, 10:55:28 AM
gpkull gpkull is offline
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Default Re: Hard spots in cast iron

a freind of mine uses kitty litter to let his cast cool slowly. just covers part up and says leave it for 24 hrs
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Old 10-03-2008, 02:36:08 PM
Wm. Galloway Wm. Galloway is offline
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Default Re: Hard spots in cast iron

Everyone is pretty much right on. Is the area you are trying to tap thinner than other areas of the casting? Thin wall sections result in carbides forming in the cast iron. This is the "brittle" everyone mentions. Some times it's impossible to prevent. even if the foundry didn't "shake out" too early it does happen.

What you need to do is anneal the iron.

Annealing
The heat treatment most frequently applied to gray iron, with the possible exception of stress relieving, is annealing. The annealing of gray iron consists of heating the iron to a temperature high enough to soften it and/or to minimize or eliminate massive eutectic carbides, thereby improving its machinability. This heat treatment reduces mechanical properties substantially. It reduces the grade level approximately to the next lower grade: for example, the properties of a class 40 gray iron will be diminished to those of a class 30 gray iron. The degree of reduction of properties depends on the annealing temperature, the time at temperature, and the alloy composition of the iron.

Gray iron is commonly subjected to one of three annealing treatments, each of which involves heating to a different temperature range. These treatments are ferritizing annealing, medium (or full) annealing, and graphitizing annealing.

Ferritizing Annealing. For an unalloyed or low-alloy cast iron of normal composition, when the only result desired is the conversion of pearlitic carbide to ferrite and graphite for improved machinability, it is generally unnecessary to heat the casting to a temperature above the transformation range. Up to approximately 595°C (1100°F), the effect of short times at temperature on the structure of gray iron is insignificant. For most gray irons, a ferritizing annealing temperature between 700 and 760°C (1300 and 1400°F) is recommended.
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Old 10-03-2008, 07:41:22 PM
Beanscoot Beanscoot is offline
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Default Annealing cast iron

"For most gray irons, a ferritizing annealing temperature between 700 and 760°C (1300 and 1400°F) is recommended"

How long should the parts be held at that temperature?

And how slowly need it be cooled?
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  #18  
Old 10-03-2008, 07:50:56 PM
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Default Re: Hard spots in cast iron

There's much talk about trying to "fix" lousy castings.......send the darn things back and tell 'em you want decent castings.......
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