Recently, a friend was over while I was casting some parts, about 10-12 minutes after the pour I pulled the part from the sand (solid, but still extremely hot) and pitched it into the closest snow bank.
My chum, who has never casted anything in his life, but whos opinion I do respect, was freaked that I was ruining the piece by cooling it like this...
I've always done this, and if there is no snow, I run them under cold water.
Is it possible that I'm causing internal stresses by this forced cooling process, or (as I believe) once the melt has solidified, I'm just disipating the stored energy...
Any thoughts appreciated
Frank
http://www.theworkshop.ca
Cooling Stress...
Moderator: Harold_V
Re: Cooling Stress...
Its sure a possibility. Its never a good thing to subject anything to temp extremes in a short or drastic manner. Hot to cold or cold to hot. Unless the specifics of the metal your dealing with calls for it to be rapidly cooled, I would still be inclined to let it cool down slowly without any additonal outside help. I know its hard to resist, after making a pour the anticipation is killing you to see how it turned out, and I do it myself from time to time. I pulled an extra side pklate for the Gingery shaper out of the mold pretty quick and could not wait to "fondle" it, and imediately put under a water hose. I did nt appear fromwhat I could see, to suffer any stress or thermal cycling damage, but the possiblity is still there.
I have been told that the longer its left in the mold the shinier the castings finish turns out, and less apt to discolor, and also distort. So far distortion or cracking has not been a problem for me.
I have been told that the longer its left in the mold the shinier the castings finish turns out, and less apt to discolor, and also distort. So far distortion or cracking has not been a problem for me.
Re: Cooling Stress...
Frank,
While I'm far from an expert, your friend is likely to be right, especially if you are working with metals that are hot short. Had you pulled a piece of brass that soon, it's entirely possible it would crumble. On the other hand, I'm sure there are instances where pulling the part form sand early may prevent hot tearing. Wish I knew more.
The other issue that would be desirable to understand is grain growth. It would be nice if someone with a degree in metallurgy would talk to us about that subject. How long things remain hot effects grain growth and I don't know if that's a good or bad thing. So much to learn, so little time! [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]
Harold
While I'm far from an expert, your friend is likely to be right, especially if you are working with metals that are hot short. Had you pulled a piece of brass that soon, it's entirely possible it would crumble. On the other hand, I'm sure there are instances where pulling the part form sand early may prevent hot tearing. Wish I knew more.
The other issue that would be desirable to understand is grain growth. It would be nice if someone with a degree in metallurgy would talk to us about that subject. How long things remain hot effects grain growth and I don't know if that's a good or bad thing. So much to learn, so little time! [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]
Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
Re: Cooling Stress...
A few insights on cooling rates and hot short materials.
Several issues cross over with casting cooling rates. These include:
hardening in the alloy
ductility and grain size (they're related)
cracking in the cast alloy
hot tearing
For hardening, carbon steels and irons in particular harden when quenched and become brittle. The phase changes in the material cause expansion and contraction independent of the temperature effect. The stresses from the phase change can make parts literally explosive. This is pretty much limited to carbon steels and cast irons. On these products you don't put a chill in the mold if you can avoid it unless that is your intent. A chill in a carbon steel mold will immediately form martensite, which is very hard. In cast iron, the martensitic form is called white cast iron. It is extremely hard and brittle, which is good if you want it. White cast iron is also the first step in making maleable iron. Maleable iron is made by tempering white cast iron until the graphite comesout of solution. The graphite grows in spherical nodules in a very ductile, low carbon iron. The resulting part is ductile, like low carbon steel, but is cheaper to produce than steel.
The faster metal cools, the smaller the grain size typically. To picture it think of sugar crystals growing in sugar water. If you evaporate the water slowly, the sugar molecules have time to line up with existing crystals and the result is really big crystals. The same happens in solidifying metal. I have been making castings from a zinc-aluminum alloy that is intended to be quickly cooled in a metal mold. If I chill it, the grains are a few thousandths of an inch across and the material is fairly tough. I cast some into 4" diameter tin cans and cut some pieces out for a project. The pieces cracked if you looked at them wrong and the grains were over 1/2 inch across. The rule of thumb is to cool as fast as you can without getting cracks or hardening the material. In some industries, submerged arc welding is prohibited because the weld is huge, cools slowly and is much lower in toughness and much larger grain size than a weld built up of small, quickly cooled weld beads.
Concerning hot tearing. Some alloys are horrible for this. Stainless steel is the worst with cast iron and aluminum and brass close seconds. The curved spokes on old cast iron hand wheels were used specifically to minimize hot tearing of the spokes in cast iron with phosphorus content. Phosphorus was added to improve flowability of cast iron but caused hot tearing and brittleness. For sand casting, the mold is collapseable enough to prevent the worst problems, but hard cores that won't collapse are a problem. Cores used to be bound with wood flour and linseed oil and were baked like cookies. The metal would burn out the linseed oil and the core would collapse in the part. CO2 and no-bake cores don't burn out and don't collapse nearly as well, are stronger in compression and as a result have worse problems with hot tearing. I used to work in a stainless steel foundry and saw large 60 ton pump castings with up to 60% weld repair by weight due to hot tearing on no-bake cores.
As far a shaking out goes, let the mold cool a while to avoid breaking hot short alloy parts.
Several issues cross over with casting cooling rates. These include:
hardening in the alloy
ductility and grain size (they're related)
cracking in the cast alloy
hot tearing
For hardening, carbon steels and irons in particular harden when quenched and become brittle. The phase changes in the material cause expansion and contraction independent of the temperature effect. The stresses from the phase change can make parts literally explosive. This is pretty much limited to carbon steels and cast irons. On these products you don't put a chill in the mold if you can avoid it unless that is your intent. A chill in a carbon steel mold will immediately form martensite, which is very hard. In cast iron, the martensitic form is called white cast iron. It is extremely hard and brittle, which is good if you want it. White cast iron is also the first step in making maleable iron. Maleable iron is made by tempering white cast iron until the graphite comesout of solution. The graphite grows in spherical nodules in a very ductile, low carbon iron. The resulting part is ductile, like low carbon steel, but is cheaper to produce than steel.
The faster metal cools, the smaller the grain size typically. To picture it think of sugar crystals growing in sugar water. If you evaporate the water slowly, the sugar molecules have time to line up with existing crystals and the result is really big crystals. The same happens in solidifying metal. I have been making castings from a zinc-aluminum alloy that is intended to be quickly cooled in a metal mold. If I chill it, the grains are a few thousandths of an inch across and the material is fairly tough. I cast some into 4" diameter tin cans and cut some pieces out for a project. The pieces cracked if you looked at them wrong and the grains were over 1/2 inch across. The rule of thumb is to cool as fast as you can without getting cracks or hardening the material. In some industries, submerged arc welding is prohibited because the weld is huge, cools slowly and is much lower in toughness and much larger grain size than a weld built up of small, quickly cooled weld beads.
Concerning hot tearing. Some alloys are horrible for this. Stainless steel is the worst with cast iron and aluminum and brass close seconds. The curved spokes on old cast iron hand wheels were used specifically to minimize hot tearing of the spokes in cast iron with phosphorus content. Phosphorus was added to improve flowability of cast iron but caused hot tearing and brittleness. For sand casting, the mold is collapseable enough to prevent the worst problems, but hard cores that won't collapse are a problem. Cores used to be bound with wood flour and linseed oil and were baked like cookies. The metal would burn out the linseed oil and the core would collapse in the part. CO2 and no-bake cores don't burn out and don't collapse nearly as well, are stronger in compression and as a result have worse problems with hot tearing. I used to work in a stainless steel foundry and saw large 60 ton pump castings with up to 60% weld repair by weight due to hot tearing on no-bake cores.
As far a shaking out goes, let the mold cool a while to avoid breaking hot short alloy parts.
Re: Cooling Stress...
Thanks to all, the advice points to a bit more patience, on my part...
As to the grain, I have noticed that commercial castings that I've busted (cold with a sledge hammer) all have a uniform dull break-line where the fracture(s) occurred. But almost all of my castings when broken in the exact same fashion have a distinctive crystalline structure. Some of the small facets appear to have a chrome sheen to them...
I've always chocked this up to the mixed bag of Hard Discs, transmissions, and any other grey alloy I can find. Could this be the extreme cooling...
Frank
http://www.theworkshop.ca
As to the grain, I have noticed that commercial castings that I've busted (cold with a sledge hammer) all have a uniform dull break-line where the fracture(s) occurred. But almost all of my castings when broken in the exact same fashion have a distinctive crystalline structure. Some of the small facets appear to have a chrome sheen to them...
I've always chocked this up to the mixed bag of Hard Discs, transmissions, and any other grey alloy I can find. Could this be the extreme cooling...
Frank
http://www.theworkshop.ca
Re: Cooling Stress...
Great information, which I have saved to a file. Thanks for your efforts. [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]
The thing I most appreciate at this point is your input on cores. I've seen them done by the linseed oil method and had thought that might be a good way to go, but knowing that they collapse easier in the casting cinches it for me. I'll definitely use that process in place of the no bake core method. I've been told they also shake out much easier, which makes sense.
I understand fairly well the carbon cycles, though I never remember which state (by name) is hard or soft. I also understand about chilling. The one place I would appreciate a little more input is in ductile iron casting. I am aware of how it is prepared and have the inoculant and ferrosilicon on hand at this point.
I guess my one question is slightly redundant. Would you be inclined to break up molds and quickly cool items that were cast in ductile, or would you suggest allowing the molds to cool naturally, breaking them up the following day, for example. One has to consider thermal shock, and certainly allow the castings to cool below a heat treatable temperature if rapid cooling were to be desirable. What is the typical procedure in commercial foundries? Is grain growth a consideration in this case?. TIA! [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]
Harold
The thing I most appreciate at this point is your input on cores. I've seen them done by the linseed oil method and had thought that might be a good way to go, but knowing that they collapse easier in the casting cinches it for me. I'll definitely use that process in place of the no bake core method. I've been told they also shake out much easier, which makes sense.
I understand fairly well the carbon cycles, though I never remember which state (by name) is hard or soft. I also understand about chilling. The one place I would appreciate a little more input is in ductile iron casting. I am aware of how it is prepared and have the inoculant and ferrosilicon on hand at this point.
I guess my one question is slightly redundant. Would you be inclined to break up molds and quickly cool items that were cast in ductile, or would you suggest allowing the molds to cool naturally, breaking them up the following day, for example. One has to consider thermal shock, and certainly allow the castings to cool below a heat treatable temperature if rapid cooling were to be desirable. What is the typical procedure in commercial foundries? Is grain growth a consideration in this case?. TIA! [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]
Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
Re: Cooling Stress...
Frank,
You commented:
I've always chocked this up to the mixed bag of Hard Discs, transmissions, and any other grey alloy I can find. Could this be the extreme cooling...
Sort of makes me wonder if it's more a case of iron contamination and other factors, considering how and what you're melting. It's entirely possible that aluminum used in permanent mold castings does not cast well in sand. The cooling period may be too long and you experience excessive grain growth. That seems to go along with the information posted by jpfalt.
I checked out your web site and was duly impressed. The one thing I do wonder about is the metal crucible. It would be interesting to know if that is a part of the difference in your castings as opposed to those done by commercial outfits. One thing for sure, permanent molded castings (hard drives, transmissions) cool very quickly, so grain growth is limited, which may account for the major difference. The very informative post by jpfalt covers that subject nicely.
If we could buy this guy a box of donuts and keep him talking it's likely we'd be a lot smarter at the end of the week! [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]
Lets see if he has an opinion on your situation.
Harold
You commented:
I've always chocked this up to the mixed bag of Hard Discs, transmissions, and any other grey alloy I can find. Could this be the extreme cooling...
Sort of makes me wonder if it's more a case of iron contamination and other factors, considering how and what you're melting. It's entirely possible that aluminum used in permanent mold castings does not cast well in sand. The cooling period may be too long and you experience excessive grain growth. That seems to go along with the information posted by jpfalt.
I checked out your web site and was duly impressed. The one thing I do wonder about is the metal crucible. It would be interesting to know if that is a part of the difference in your castings as opposed to those done by commercial outfits. One thing for sure, permanent molded castings (hard drives, transmissions) cool very quickly, so grain growth is limited, which may account for the major difference. The very informative post by jpfalt covers that subject nicely.
If we could buy this guy a box of donuts and keep him talking it's likely we'd be a lot smarter at the end of the week! [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]
Lets see if he has an opinion on your situation.
Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
Re: Cooling Stress...
When I went to buy some certified ingots for a project, the dealer gave me a run down on some material I had inquired on. I do not remember the alloy off the top of my head, but he informed me it was just not suitable for sand cast items, and was primarily alloyed for use in a permanent mold that was chilled imediately after casting. He went on to say it would exhibit inproper grain growth, and laso would not flow properly, and the results would be deformed and weak castings. To maintain this alloys intended properties it was imperative that it was cooled at a certain point in the process.
Thats just one problem associated with melting scrap of unknown content. Just because it looks like alum, smells like alum does not make it alum worthy of use. True, if its mixed with other alloys and mixes of scrap a lot of times we still get good results, but its still a chance.
Anymore I try and maintain my scrap into segragated piles, so the odds are more in my favor. The days of just throwing it all in a pot is over. I know what and how the cases on Briggs & Stratton engines perform, and BBQ grills, wheels and pistons etc, its the odds and ends that one usually wind up with that can get you bit. I'm not saying I don't use those items, but I play with them for a bit first before I commit myself to using them for a project.
Thats just one problem associated with melting scrap of unknown content. Just because it looks like alum, smells like alum does not make it alum worthy of use. True, if its mixed with other alloys and mixes of scrap a lot of times we still get good results, but its still a chance.
Anymore I try and maintain my scrap into segragated piles, so the odds are more in my favor. The days of just throwing it all in a pot is over. I know what and how the cases on Briggs & Stratton engines perform, and BBQ grills, wheels and pistons etc, its the odds and ends that one usually wind up with that can get you bit. I'm not saying I don't use those items, but I play with them for a bit first before I commit myself to using them for a project.
Re: Cooling Stress...
Thanks, Roy. It's nice to get a confirmation of my hunch. It's things like this that I like to know. [img]/ubb/images/graemlins/smile.gif"%20alt="[/img] I guess by now you can see I'm a nut for detail and procedures! [img]/ubb/images/graemlins/blush.gif"%20alt="[/img]
Best regards,
Harold
Best regards,
Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
Re: Cooling Stress...
I had eased up on the mixing, when I had a full-sized transmission to plug away at. But since it is done things have been a lot less consistent.
Unfortunately it's been under minus 10 Celsius here for weeks and pulling out a 2.8L V6 motor from an old Jimmy I have for parts is less than appealling.
Frank
http://www.theworkshop.ca
Unfortunately it's been under minus 10 Celsius here for weeks and pulling out a 2.8L V6 motor from an old Jimmy I have for parts is less than appealling.
Frank
http://www.theworkshop.ca
Re: Cooling Stress...
Harold........Really? You a stickler for proceedures and detail ........naw come on not you [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]
Re: Cooling Stress...
Hey Roy,
Well, I thought I hid it well, but nothing seems to get by you--- [img]/ubb/images/graemlins/smirk.gif"%20alt="[/img]
It's hard to change how you think after so many years and so many rewards from the thinking. [img]/ubb/images/graemlins/smile.gif"%20alt="[/img] While my tunnel vision drives most folks crazy, you'd be amazed at the good results that came from my ways when I interacted with industry! You become very popular with the right people. [img]/ubb/images/graemlins/cool.gif"%20alt="[/img] You can believe me when I tell you that if you do good work, the kind that always meets specs, you are busy, and with good quality work as well. I ran my shop for years and was without work only three times, the longest period was less than a day. I never solicited work, nor was I in the yellow pages. My reputation for quality carried me for all my years in the shop. It was truly my signature. [img]/ubb/images/graemlins/cool.gif"%20alt="[/img]
Harold
Well, I thought I hid it well, but nothing seems to get by you--- [img]/ubb/images/graemlins/smirk.gif"%20alt="[/img]
It's hard to change how you think after so many years and so many rewards from the thinking. [img]/ubb/images/graemlins/smile.gif"%20alt="[/img] While my tunnel vision drives most folks crazy, you'd be amazed at the good results that came from my ways when I interacted with industry! You become very popular with the right people. [img]/ubb/images/graemlins/cool.gif"%20alt="[/img] You can believe me when I tell you that if you do good work, the kind that always meets specs, you are busy, and with good quality work as well. I ran my shop for years and was without work only three times, the longest period was less than a day. I never solicited work, nor was I in the yellow pages. My reputation for quality carried me for all my years in the shop. It was truly my signature. [img]/ubb/images/graemlins/cool.gif"%20alt="[/img]
Harold
Wise people talk because they have something to say. Fools talk because they have to say something.