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dsymes
07-22-2015, 10:07 PM
I have a gray iron casting for a 30-60-90 angle that is 12" on the long side of the right angle and 1" wide. Usual design with a 1/4" thick web with four holes to minimize weight and about 5/16" thick foot around the perimeter. It was fully annealed after casting and I have machined it to about +3 to 5 thousandths over finished dimensions. I will be stress relieving it in the heat treating ovens at the local community college (1100 degrees soak for about 90 min and then taper down 100 degrees per hour to 300 degrees and turn off the oven).

My question is whether or not I need to wrap the casting in stainless steel foil to prevent carbon loss. DuraBar says that carburization is not necessary when heat treating their gray iron, but I'm not sure that translates as not needing either neutral atmosphere or foil wrap. Any insight appreciated. Forrest??

Thanks,
dsymes

wierdscience
07-22-2015, 10:14 PM
Autoignition temperature for carbon in atmosphere is approximately 1300*F IIRC.

If you are only going to 1100*F there won't be any carbon loss because it won't burn out at that low temp.

This is assuming an electric oven and an accurate control.

Lee Cordochorea
07-25-2015, 10:28 AM
Autoignition temperature is not relevant. Carbon does not "burn out." Carbon diffuses. Fick's second law of diffusion describes the approximate behavior. Still, there's no worry about loss of much carbon at 1100F for a couple hours. Three mils is as good as a mile.

Loss of carbon is not the only issue, though. Oxidization is a problem at higher temperatures. Degree of oxidation at higher temperatures of gray cast iron is dependent on the exact recipe of said iron. Maybe three mils will be enough and maybe it won't.

I'd use the foil if it were my project.

Rosco-P
07-25-2015, 02:20 PM
I'd contact DuraBar if it were my project.

J Tiers
07-25-2015, 04:41 PM
That, AND this does not usually seem to be an issue. Most folks seem to toss it in the woodstove, and let it cool in the ashes.....

What is the concern here? It may be good to stress relieve it, but maybe the biggest issue is just getting rid of any possible hard spots, i.e. "annealing" it. With Durabar, not likely a big concern.

The final dims are not particularly critical, as far as I can see. And neither is the exact carbon content.

Further treatment by scraping is likely to put about as much trapped stress in the surface as the machining did, so unless you can see it twisted now from *released* stress, it won't likely move more due to trapped stress. I'd be more inclined to have it cryogenically treated to stabilize it than annealed, anyhow. The durabar is good stuff, cast nicely.

Mark Rand
07-25-2015, 08:01 PM
Having spent this afternoon stress relieving, then cutting in half a 16" length of 1.5" square durabar, I'd say wrap it in a stainless foil blanket if you've got the stuff. Otherwise, you may well lose more than 3-5 thou in oxidation.

If the part were properly stress relieved before the original machining, further stress relieving is unlikely to make any appreciable difference.

wierdscience
07-25-2015, 09:27 PM
Autoignition temperature is not relevant. Carbon does not "burn out." Carbon diffuses. Fick's second law of diffusion describes the approximate behavior. Still, there's no worry about loss of much carbon at 1100F for a couple hours. Three mils is as good as a mile.

Loss of carbon is not the only issue, though. Oxidization is a problem at higher temperatures. Degree of oxidation at higher temperatures of gray cast iron is dependent on the exact recipe of said iron. Maybe three mils will be enough and maybe it won't.

I'd use the foil if it were my project.

Carbon DOES burn out of a cast surface and he's way below diffusion temperature so that point has no application.

dsymes
07-26-2015, 02:22 AM
Thanks all for replies, although I don't see much of a consensus. I did use SS wrap because some 12" foil on a roll was available at the Community college. However, they only have a few feet left and I would like to make some other precision cast iron tooling to be precision scraped, so the question of whether or not I will need to foil-wrap future pieces of precision tooling before scraping is relevant. Also, I will find out on Monday how well I did the wrapping (when I remove it cold from the oven) - getting a twice folded seal was difficult (particularly in the corners), and the oven was only just big enough for the piece.

I think J Tiers misunderstood my question. My casting is not DuraBar - it is gray iron cast by the Silverton Foundry in Silverton, Oregon. I was quoting the DuraBar manual on stress relieving because it is the only information I found about stress relieving Gray Iron. My understanding from posts by Richard King, Forrest Abby, and Stephen Thomas is that if one is making a camelback straight edge or similar precision tooling that will be scraped to two tenths/foot precision, it should be stress relieved or annealed, then machined to within a few thousandths of final dimensions, stress relieved again, and then scraped. That seems a far cry from tossing into the wood stove.

Thanks again for all comments.

dsymes

Rosco-P
07-26-2015, 08:11 AM
I thought it was made from DuraBar as well. Why wouldn't you just contact the Silverton Foundry?

Lee Cordochorea
07-26-2015, 11:53 AM
Carbon DOES burn out of a cast surface and he's way below diffusion temperature so that point has no application.
Easy to look up. No point in debating. Bye.

J Tiers
07-26-2015, 03:41 PM
OK, with unknown stuff, then I definitely agree.

But I think the others were talking about camelbacks etc, where the shape tends to get stresses in it from uneven cooling, etc. With a bar, I'd expect a lot less of that, but re-reading, it looks like it IS a form of camelback-like item , so.........

But it was also already annealed.... which means it really should have essentially nothing left even after machining. Like hot-rolled, vs CRS. One warps, the other really just lays there.

Anyhow, it probably cannot hurt, aside from the surface issues, no harm in wrapping with a bit of paper in there to eat the oxygen.

Mark Rand
07-27-2015, 04:21 AM
My point was more to do with oxidation of the iron surface in a gas fired furnace. A couple of thou might not be enough to compensate for loss due to scaling in places. I'd rather leave 10-20 thou and then take a final cut before scraping.

oldtiffie
07-27-2015, 07:54 AM
A "5 thou" "bow" will need a minimum of 5 thou (0.005") to be removed from each side - possibly 0.010" from each side.

Richard King
07-27-2015, 09:11 AM
Over the years we (my dad and I) cast triangle straight-edges too along with camel-backs. I now buy Dura-Bar and ask them to cut it diagonally to make the triangle SE's. I still high temp stress relieve the Dura-Bar even though they say extruded iron doesn't move as much as a casting. High Stress to me means . Cook the iron for at least 8 to 12 hours at top temp of 1100 F. Then gradually lowering the temp in steps of 200 degree's every hour.

I now use a new Heat treating company who works for many aerospace companies who says he cooks the iron from 24 to 30 hours at 900 to 1000 F and shuts of the furnace after 20 hours and his super insulated furnace slowly cools. It's a big one with the inside dims. are 6' x 6' x 6'

I have not seen any issues so far (knock on wood) with this one step method. Before meeting this new company I used to send to another company and they cooked them for overnight going up to 1100 degree's and again gradually down at 200 F for an hour each step. Them machine it, cook it again, machine it again and scrape it.

Over the years I have found if they hang the SE's inside the furnace instead of laying them on fire brick I get less warpage or bending from the heat. I used to enneal hardened steel balls in a simple charcoal grill. So annealing in a wood stove, I can see that but not stress relieving a straight-edge. I store finished SE's buy drilling and tapping a hole in the end and inserting an eye bolt. Also I like to wring the SE while I scrape it. Hitting it with a softblow hammer or chuck of 2 x 4. Rich

J Tiers
07-27-2015, 10:55 AM
.... Also I like to wring the SE while I scrape it. Hitting it with a softblow hammer or chuck of 2 x 4. Rich

I always do that, not so much to relieve anything, but to knock off any dust I missed brushing or wiping off. Maybe it relieves something also. I tend to use a plastic face hammer for small things, wood chunk for bigger, either while hanging the part.

Do you reckon that machining the annealed part puts stress back in? I wouldn't have figured it did much if anything, but I'll take your word for it.

I assume you anneal, rough machine, anneal, skim it, and scrape, so the last machining is pretty minimal.

dsymes
07-27-2015, 11:42 AM
Hi Richard,

Here's a photo of my casting after taking it out of the oven and unwrapping the stainless steel foil. There is a little rust-like stuff around some of the edge which I assume is from an imperfect foil seal, but it is very light and doesn't look like a problem. As you can see, it's not a triangle straight edge but a 30-60-90 square. It is 12 inches along the bottom side in the photo. I tried to describe it in my initial post but obviously didn't do a very good job.

The thread has gotten a little off track. My question was whether or not I needed wrap the casting in stainless steel foil or heat treat in an inert gas atmosphere in order to avoid decarburization. I don't recall that you addressed this in any of your posts on stress relief over on PM. Do you have your stuff done in a furnace with inert gas or just in air?

http://i863.photobucket.com/albums/ab191/dsymes/30-60-90.jpg (http://s863.photobucket.com/user/dsymes/media/30-60-90.jpg.html)

Douglas

Richard King
07-27-2015, 01:37 PM
You should email Dennis Danich as he used to make squares that look similar. His handle in PM is Dresden....I missed that it's a square in the thread. oops.... Yes J...rough machine first and Stress/Relief and then finish cut and scrape. i was talking to Jeff at Northfield (wood) Machinery Builders last week and he told me he does the 2 step S/R and machine too. To be honest if I did measure after stress relief I don't recall how much it moved before the finish scrape. I do remember, I once used another heat treat company and they didn't cook it as long the SE's would move a lot. I will be scraping 2 camel backs this week for a customer and i will find out if the new method works as good as the new Heat Treat Company says. Rich

dsymes
07-27-2015, 03:07 PM
Richard,

Do you heat treat in air or in an inert gas to prevent scale or decarburization? If in air, do you wind up sand or shell blasting to remove scale?

Thanks,
Douglas

Mark Rand
07-28-2015, 05:30 AM
As a datum, the length of durabar I cooked on Sunday was done at a medium orange heat (900C or so) for 45 minutes after the temperature stabilized, then cooled over 4 hours. This was done in air with a propane torch in an insulating firebrick tunnel. There was about 20-30 thou of loose scale where the flame had impinged and less in other areas. For this job it didn't matter since I've taken a minimum of a quarter of an inch off each side since then. The distortion when I cut the 15 inch length in half lengthways was of the order of 5 thou.

J Tiers
07-28-2015, 10:28 AM
As a datum, the length of durabar I cooked on Sunday was done at a medium orange heat (900C or so) for 45 minutes after the temperature stabilized, then cooled over 4 hours. This was done in air with a propane torch in an insulating firebrick tunnel. There was about 20-30 thou of loose scale where the flame had impinged and less in other areas. For this job it didn't matter since I've taken a minimum of a quarter of an inch off each side since then. The distortion when I cut the 15 inch length in half lengthways was of the order of 5 thou.

That is pretty minimal for radical machining like that. Slicing a piece of CRS would not have been the same.....

Richard King
07-28-2015, 10:48 PM
Douglas here is the name of the company I use. Give him a call as I am not real familiar with the technical side of it. Tell him i told abot his company and your on the east coast.
Nate Beyerstedt Engineered Foundry Solutions / 1272 Breezy Lane, Winona MN 55987 W 507.457.0718

oldtiffie
07-29-2015, 07:13 AM
Hi Richard,

Here's a photo of my casting after taking it out of the oven and unwrapping the stainless steel foil. There is a little rust-like stuff around some of the edge which I assume is from an imperfect foil seal, but it is very light and doesn't look like a problem. As you can see, it's not a triangle straight edge but a 30-60-90 square. It is 12 inches along the bottom side in the photo. I tried to describe it in my initial post but obviously didn't do a very good job.

The thread has gotten a little off track. My question was whether or not I needed wrap the casting in stainless steel foil or heat treat in an inert gas atmosphere in order to avoid decarburization. I don't recall that you addressed this in any of your posts on stress relief over on PM. Do you have your stuff done in a furnace with inert gas or just in air?

http://i863.photobucket.com/albums/ab191/dsymes/30-60-90.jpg (http://s863.photobucket.com/user/dsymes/media/30-60-90.jpg.html)

Douglas

That really is a first class casting of a 30-60-90 set/try square.

It would really be a first class effort and level of manual and planning and accuracy skills to scrape that to match a Starrett or similar Grade A set square.

I really do admire those who can and will manufacture squares and flats from castings etc. as I just simply do not have that level of patience even though I have the skills.

Perhaps like this Shars square?

http://www.shars.com/15-x-10-x-1-5-precision-granite-square

http://www.judgetool.com/cylindricalsquare-2.aspx

http://i200.photobucket.com/albums/aa294/oldtiffie/Surface%20plate/Precgransqsheet1.jpg

boslab
07-29-2015, 07:22 AM
Over the years we (my dad and I) cast triangle straight-edges too along with camel-backs. I now buy Dura-Bar and ask them to cut it diagonally to make the triangle SE's. I still high temp stress relieve the Dura-Bar even though they say extruded iron doesn't move as much as a casting. High Stress to me means . Cook the iron for at least 8 to 12 hours at top temp of 1100 F. Then gradually lowering the temp in steps of 200 degree's every hour.

I now use a new Heat treating company who works for many aerospace companies who says he cooks the iron from 24 to 30 hours at 900 to 1000 F and shuts of the furnace after 20 hours and his super insulated furnace slowly cools. It's a big one with the inside dims. are 6' x 6' x 6'

I have not seen any issues so far (knock on wood) with this one step method. Before meeting this new company I used to send to another company and they cooked them for overnight going up to 1100 degree's and again gradually down at 200 F for an hour each step. Them machine it, cook it again, machine it again and scrape it.

Over the years I have found if they hang the SE's inside the furnace instead of laying them on fire brick I get less warpage or bending from the heat. I used to enneal hardened steel balls in a simple charcoal grill. So annealing in a wood stove, I can see that but not stress relieving a straight-edge. I store finished SE's buy drilling and tapping a hole in the end and inserting an eye bolt. Also I like to wring the SE while I scrape it. Hitting it with a softblow hammer or chuck of 2 x 4. Rich
What is the purpose of the ringing?, all I can think of are crack detection and magnetic, but I have no clue as to why, eye bolts are the best way to store, don't have to worry about airy points and all that I suppose, a beautiful square, something so simple but deceptive in making, well done that man
Mark

TGTool
07-29-2015, 08:55 AM
What is the purpose of the ringing?, all I can think of are crack detection and magnetic, but I have no clue as to why, eye bolts are the best way to store, don't have to worry about airy points and all that I suppose, a beautiful square, something so simple but deceptive in making, well done that man
Mark

There's a whole field of vibratory stress relieving. Some companies make the devices to vibrate a part along with software and perhaps sensors to zero in on the best frequency for maximum effect. I don't have a firm enough grip on the engineering behind it to know whether it falls closer to the art or the science end. I'm presuming that whacking castings to get them to vibrate or ring falls into this field but I haven't seen anything on the effectiveness of this ad hoc relief process.

Richard King
07-29-2015, 10:06 AM
TG pretty much said it all, but I had an Engineer tell me it's spelled wringing..lol.....My Dad used to tell me it moves the molecules around. Much like a tuning fork vibrates. When we are scraping a surface (ie) straight-edge (SE), lathe saddle, etc. and after we wring it the airy or hinge point changes 9 out of 10 times. You need to do this especially after dropping a SE. I had a student who worked for a new machine dealer in Nebraska tell me he had a CNC lathe crash into the chuck and they had to send it out to have it high temp stress relieved before it settled down.

Anyone who has a straight-edge scraping master and uses it a lot and they tell you they never dropped it is a liar....lol... But it is an easy way to make sure it doesn't move when your using it when scraping. This is why I always tell people to re-check the SE's before using it. More so when you work in a shop as one of the other guys may have dropped it and was afraid to tell anyone. Rich

TGTool
07-29-2015, 12:18 PM
Wikipedia has this write up (https://en.wikipedia.org/wiki/Vibratory_stress_relief) on vibratory stress relieving. Since wiki entries often have multiple contributors there can be discrepancies and even contradiction as this article shows. It gives a theoretical foundation for how and why it works but also includes the comments that it's not universally accepted as having any efficacy. We looked into it at one place I worked but weren't persuaded enough to invest. We dealt with large weldments and had them thermally stress relieved as a matter of course.

As to "ringing" versus "wringing" it might be a matter of preference or just uncertainty given that the whole field isn't well established. To wring something in common usage is to squeeze something out as in wringing a dishrag or the old washing machines with separate wringers to extract water. The old machines live on in language. What would we have used for an idiom in place of "getting your tit in a wringer"? So, in terms of stress relief (having extracted that private and tender part) would "ring" be more representative of utilizing resonance or "wring" in respect to removing stresses. I tend to go with ring but I certainly know I'm not always wright.

dsymes
07-29-2015, 01:03 PM
oldtiffie- Thanks for your kind words. My intention is to aim for two tenths/ft accuracy on both the 90 and 30/60 angles. I wish I could afford the square you mention or the cylinder square, but that's over my budget for now. I have a Chinese precision ground angle plate (4x5x4) that I checked against a tired-looking cylindrical square at the community college yesterday - it's about three tenths off over 5 inches, so I can use that as a target. The college also has one of those measuring machines with a granite plate and a probe that can measure down to one micron, so I will use that to check my angles and then step scrape to correct. Also, I recently had my Shars 24x18 granite surface plate lapped and certified as "A" (for a total of $106) so I have confidence in my spotting.

oldtiffie
07-29-2015, 08:12 PM
I've found that my (Chinese) "Frame Level" square is - so far as I can determine - as accurate as stated and intended - ie 0.02mm per metre - which matches Starrett accuracy.

It is flat on all edges and is square at all corners and is really a "multi-purpose" tool which gets an occasional use.

http://i200.photobucket.com/albums/aa294/oldtiffie/measuring/Machinist_Square1.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/measuring/Machinist_Square2.jpg

http://www.shars.com/12-x-1-3-4-x-12-precision-frame-spirit-levels

http://www.machineryhouse.com.au/Q208