Surface cracks in sheet metal

[purplepepper8]

Hi, quite often I get these little areas on my sheet metal that are kind of cracks but they dont run deep. While engraving, or in this case doming, they come apart like you see in the image. In this case you can also see the metal cracking along those horizontal lines. This metal is 18k yellow. Anyone know why this is happening? Thank you

 

[silvermon]

Hi, quite often I get these little areas on my sheet metal that are kind of cracks but they dont run deep. While engraving, or in this case doming, they come apart like you see in the image. In this case you can also see the metal cracking along those horizontal lines. This metal is 18k yellow. Anyone know why this is happening? Thank you

Hard to say just from a picture, but it looks like your rolling mill has too high a differential in the roller speeds. Some difference in speed between the top and lower roll is normal, but if your machine doesn't have adequate mesh in the gears, then the upper roll can move to much slower than the lower. Not all manufacturers are equal in this matter. I have heard very good things about Durston. All my machines (small and large powered) are Cavallin, which I like very much, but on wider openings does have this issue a small amount.

Basically, what could be happening is a small surface imperfection when the ingot is rolled the first several passes, is being rolled over, rather than rolled down. It emerges later in exactly the manner pictured. A very, very careful inspection after the first several passes, may reveal the problem. Takes practice seeing it when it is small.

 

[AllenClapp]

Are you annealing the metal after every few passes?

 

[Jeff2t]

Are you melting down and rolling scrap, if so it could be that some of the scrap is of a different alloy mix and it doesn't melt and mix with the rest, it may look like it has all melted together, but if you fish around in the molten metal you can pull out solid bits, then when you put it through the roller it starts to peel away.

 

[mdengraver]

When rolling sheet metal, annealing/softening the metal beforehand, with the torch or kiln, and every few passes thru the mill annealing/softening the metal with torch or kiln, realligns the crystalline structure of the metal, otherwise, the metal gets work hardened, and has a tendency to crack when used or rolled in the mill, and more and more difficult to roll in the mill! Critical, when rolling in the mill, and then flipping it on the next pass, always roll the metal in the same direction, otherwise the crystalline structure of the metal becomes more and more chaotic with each roll in the mill increasing the likelihood of the metal cracking!

 

[purplepepper8]

Hard to say just from a picture, but it looks like your rolling mill has too high a differential in the roller speeds. Some difference in speed between the top and lower roll is normal, but if your machine doesn't have adequate mesh in the gears, then the upper roll can move to much slower than the lower. Not all manufacturers are equal in this matter. I have heard very good things about Durston. All my machines (small and large powered) are Cavallin, which I like very much, but on wider openings does have this issue a small amount.

Basically, what could be happening is a small surface imperfection when the ingot is rolled the first several passes, is being rolled over, rather than rolled down. It emerges later in exactly the manner pictured. A very, very careful inspection after the first several passes, may reveal the problem. Takes practice seeing it when it is small.

Interesting. Maybe one way to test this would be to mark a spot on each roller with a marker and then roll and see if they both move at the same rate.
Sometimes these cracks arent 'U' shaped though, and instead just streaks in one direction. Does that also happen with a differential in roller speeds?

 

[purplepepper8]

Are you annealing the metal after every few passes?

Yes I am, but maybe I need to anneal more often. I’ll try that. Thank you

 

[purplepepper8]

Are you melting down and rolling scrap, if so it could be that some of the scrap is of a different alloy mix and it doesn't melt and mix with the rest, it may look like it has all melted together, but if you fish around in the molten metal you can pull out solid bits, then when you put it through the roller it starts to peel away.

This batch did have metal that was melted 3/4 times but it was all the same alloy

 

[purplepepper8]

When rolling sheet metal, annealing/softening the metal beforehand, with the torch or kiln, and every few passes thru the mill annealing/softening the metal with torch or kiln, realligns the crystalline structure of the metal, otherwise, the metal gets work hardened, and has a tendency to crack when used or rolled in the mill, and more and more difficult to roll in the mill! Critical, when rolling in the mill, and then flipping it on the next pass, always roll the metal in the same direction, otherwise the crystalline structure of the metal becomes more and more chaotic with each roll in the mill increasing the likelihood of the metal cracking!

I am annealing but I think I might have to anneal more often. I’ll try that, thank you. And yes I'm aware to not change the direction in the mill unless the metal is annealed- in any case in this case I didn't change direction at all.

 

[silvermon]

Interesting. Maybe one way to test this would be to mark a spot on each roller with a marker and then roll and see if they both move at the same rate.
Sometimes these cracks arent 'U' shaped though, and instead just streaks in one direction. Does that also happen with a differential in roller speeds?

I don't think a mark from a marker would be fine enough to trace the problem. When I say some metal rolled over as opposed to inward, during the first couple passes; I am talking about an imperfection that could be as small as a few ten thousandths of an inch across, but at least usually more like a couple thousandths, and perhaps and eighth or quarter inch long. You have to remember that when you do the initial pour of the ingot, the imperfects are stratified. Long and narrow, across the face of the ingot. The eventual shape of the crack grows out (an imperfection that starts as .002" by .33", after twenty passes could grow out to a crack/peal covering over a square inch) from the initial imperfection through a combination of micro-cracking and simple forming/stretching.

I don't pour ingots much anymore because it is quite easy to buy most alloys you could want these days. I used to pour and roll 10-12 ingots per week in the eighties and nineties, less after 2000 and eventually none. I am therefore going on recollection that can be faulty. I do remember eventually developing the habit of filing some ingots more and finer than others. Some examples. Green 18K gold alloys I wouldn't do anything extra. Sterling alloys that incorporated palladium or increased amounts of zinc, I would file and sand (remember to clean well before rolling). Sterling alloys with tin I wouldn't do much extra (seldom did these ingots either). Rose gold alloys, 14K yellow or rose, and any 10K would get thoroughly filed and forged on an anvil before first rolling. Maybe about a 10% reduction, and not focused too much on the finish other than thoroughly covering all surfaces of the ingot with a decent amount of hammer impacts. Forging won't help with inclusions, but does sometimes reveal them faster than rolling.

Today when I do pour the rare ingot, I will forge it on a series of anvils until it is reduced by half, before it goes to rolling. Sometimes I will forge to the finished product. It is possible to forge by hand a pretty nice sheet of sterling. Haven't forged a sheet of gold, because it requires extra material for the tongs to grip, and well, gold is expensive.

 

[silvermon]

This batch did have metal that was melted 3/4 times but it was all the same alloy

Remelts increase the presence of carbides and hydrogen. Both embrittle the metal. You eventually have to refine the metal to restore the alloy.

 

[purplepepper8]

I don't think a mark from a marker would be fine enough to trace the problem. When I say some metal rolled over as opposed to inward, during the first couple passes; I am talking about an imperfection that could be as small as a few ten thousandths of an inch across, but at least usually more like a couple thousandths, and perhaps and eighth or quarter inch long. You have to remember that when you do the initial pour of the ingot, the imperfects are stratified. Long and narrow, across the face of the ingot. The eventual shape of the crack grows out (an imperfection that starts as .002" by .33", after twenty passes could grow out to a crack/peal covering over a square inch) from the initial imperfection through a combination of micro-cracking and simple forming/stretching.

I don't pour ingots much anymore because it is quite easy to buy most alloys you could want these days. I used to pour and roll 10-12 ingots per week in the eighties and nineties, less after 2000 and eventually none. I am therefore going on recollection that can be faulty. I do remember eventually developing the habit of filing some ingots more and finer than others. Some examples. Green 18K gold alloys I wouldn't do anything extra. Sterling alloys that incorporated palladium or increased amounts of zinc, I would file and sand (remember to clean well before rolling). Sterling alloys with tin I wouldn't do much extra (seldom did these ingots either). Rose gold alloys, 14K yellow or rose, and any 10K would get thoroughly filed and forged on an anvil before first rolling. Maybe about a 10% reduction, and not focused too much on the finish other than thoroughly covering all surfaces of the ingot with a decent amount of hammer impacts. Forging won't help with inclusions, but does sometimes reveal them faster than rolling.

Today when I do pour the rare ingot, I will forge it on a series of anvils until it is reduced by half, before it goes to rolling. Sometimes I will forge to the finished product. It is possible to forge by hand a pretty nice sheet of sterling. Haven't forged a sheet of gold, because it requires extra material for the tongs to grip, and well, gold is expensive.

Thanks for your detailed response- its informative and helpful

Unfortunately I don't think these ready alloys are available here in India, where I am based out of. I’ll try doing the hammering like you said. What purpose does forging it to such a great extent have? Doesn't a 10% or so reduction serve the purpose of levelling out the imperfections?

Also thought id mention that sometimes when annealing the sheet there are these 'bubbles' that form and those are sometimes the sites of these cracks.

 

[purplepepper8]

Remelts increase the presence of carbides and hydrogen. Both embrittle the metal. You eventually have to refine the metal to restore the alloy.

I see. I will try with fresh metal and check the results. Thank you.

 

[silvermon]

Thanks for your detailed response- its informative and helpful

Unfortunately I don't think these ready alloys are available here in India, where I am based out of. I’ll try doing the hammering like you said. What purpose does forging it to such a great extent have? Doesn't a 10% or so reduction serve the purpose of levelling out the imperfections?

Also thought id mention that sometimes when annealing the sheet there are these 'bubbles' that form and those are sometimes the sites of these cracks.

Forging is omni-directional in it's deformation. The metal flows in the path of least resistance. Rolling forces the metal in one direction. The result is that forging equalizes the stresses left over from pouring the ingot. The amount of forging you do is a choice. Keeping track by using time (less accurate) or a gauge like a dial caliper.

I think in general, the better your pours (and cleaner the metal) the less forging needed before rolling. More forging won't hurt. Like I said, you could forge an ingot into a usable sheet quite readily, but definitely not quickly. Rolling is far quicker. That is why you have to explore what works best for you.

 

[silvermon]

Thanks for your detailed response- its informative and helpful

Unfortunately I don't think these ready alloys are available here in India, where I am based out of. I’ll try doing the hammering like you said. What purpose does forging it to such a great extent have? Doesn't a 10% or so reduction serve the purpose of levelling out the imperfections?

Also thought id mention that sometimes when annealing the sheet there are these 'bubbles' that form and those are sometimes the sites of these cracks.

Usually an inclusion caused by carbides. That is why you may want to refine your metal more often. You can refine the metal in-house, but strong chemicals are needed.

 

[purplepepper8]

Forging is omni-directional in it's deformation. The metal flows in the path of least resistance. Rolling forces the metal in one direction. The result is that forging equalizes the stresses left over from pouring the ingot. The amount of forging you do is a choice. Keeping track by using time (less accurate) or a gauge like a dial caliper.

I think in general, the better your pours (and cleaner the metal) the less forging needed before rolling. More forging won't hurt. Like I said, you could forge an ingot into a usable sheet quite readily, but definitely not quickly. Rolling is far quicker. That is why you have to explore what works best for you.

I understand, that makes sense. Thanks for all your help silvermon

 

[purplepepper8]

Usually an inclusion caused by carbides. That is why you may want to refine your metal more often. You can refine the metal in-house, but strong chemicals are needed.

I’ll do this more often then. I think Id prefer to not work with strong chemicals but getting it refined outside isn't difficult