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View Full Version : Pores in steel - are they real?



k2man
09-13-2014, 09:47 PM
I've seen a few post that mention pores in steel that hold oil. I had heard of this in years past, and that the oil couldn't easily be removed with detergent and water or even solvents. How much truth is there to this? How big are these pores? Are they different for different alloys? How about hot rolled vs cold rolled?


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Edwin Dirnbeck
09-13-2014, 10:43 PM
Yes indeed. I worked at a factory that received coils of 3/4 inch diameter hot rolled steel (wire) ,yes they called it wire.The ruff wire was run thru a shot blaster to remove the outer scale ,but the surface was still ruff. The wire was pulled thru a grease packed wire draw operation.The wire diameter was reduced to about 5/8 diameter . The wire was very slick and smooth , but when the wire was run thru a cleaning and plating operation ,the trapped grease from the drawing operation often was very visible and caused rejects.Most steel starts out at the mill as a ruff hot rolled product and is often cold rolled into thinner shapes and oil can be trapped in these millions of small chambers.Edwin

J Tiers
09-13-2014, 10:46 PM
Have not really seen it in steel, but it sure is true of cast iron.

Hot and cold rolled are about the same stuff.... everything is same in the processing down to the last sizing roll passes, so teh steel is essentially the same stuff. Hot rolled is just that, comes out more stress-free. Cold rolled is rolled "cold" (and usually cleaned of scale etc first), which adds-in stress but offers other good things to compensate.

My experience is that you can clean oil off steel, and it is then clean, stays clean. Pores don't seem to be a big issue. Cast iron you can clean, but if it was oily to begin with, it may be oily again a while after cleaning, but less so than the first time. Not so oily that you notice, but it will be oily enough to mess up solder etc.

Makes sense.... after being rolled at fairly high pressure, pores in steel are likely to be squeezed shut and probably welded, since the rolling process is done hot from ingot to before final passes. With no further processing, any holes in CI (from carbon, etc) are not modified.

JoeFin
09-13-2014, 11:23 PM
Ever see a welder hold a weed burner to steel to prep it for welding ?

You can see the moisture seeping out of the steel

Fasttrack
09-13-2014, 11:39 PM
Ever see a welder hold a weed burner to steel to prep it for welding ?

You can see the moisture seeping out of the steel

This is a common misconception. What you are seeing is water vapor from the combustion process being condensed on the cooler surface.

Most steels are non-porous. The method in which they are produced and the alloy plays a role so, for instance, sintered stainless steel can be very porous with typical pores ranging from a couple of microns to hundreds of microns. On the other hand, typical stainless steel products (i.e. hot rolled bar, etc) are very non-porous. Cast iron tends to be rather porous and will "bleed" oil for a long time if it has been oily for most of its life. This is an issue with restoring machine tools; brazing, welding and painting old castings can be challenging.

Edit to add:

I should point out that the porosity of cast iron is again a function of how the part was cast. Liquid metal can hold a great deal of dissolved gasses (like a carbonated beverage) but as it cools, the gases are forced from the crystal structure of the metal. Depending upon the details of the process (fluxes used, how it was cast, size of casting, how quickly it cools, etc.), the result can be little pockets of gas that become trapped in the solid. (Imagine the bubbles in your beverage forming but, before they can reach the surface, the beverage is flash frozen, trapping them in place). When the part is machined, it exposes these voids and results in a "porosity". Basically, a surface that appears to be flat and smooth has many small features that increases the effective surface area and tend to trap grease and oil. It can be cleaned, but it takes some extra effort.

J Tiers
09-13-2014, 11:40 PM
I'm gonna suggest that the wire drawing operation trapped grease under the higher points of the roughness, after they were bent over and smeared out by the drawing operation.

I've had to heat and clean CI several times before it solder would "wet" it and allow it to be soldered to, but have never had to do that to steel. I could see the oil being driven out of the pores of the CI.

You sure that was "moisture in the steel"? A weedburner would pre-heat the steel, and might condense some of the products of combustion ON the steel.... I've seen that with a propane torch, and not always on steel. But I sure have never seen anything being driven out from inside. If there is moisture inside, then it should rust.

edit: Looks like Fasttrack and I were typing at the same time

Old Hat
09-14-2014, 01:43 AM
This is a common misconception. What you are seeing is water vapor from the combustion process being condensed on the cooler surface.

................................
Liquid metal can hold a great deal of dissolved gasses (like a carbonated beverage) but as it cools, the gases are forced from the crystal structure of the metal. Depending upon the details of the process (fluxes used, how it was cast, size of casting, how quickly it cools, etc.), the result can be little pockets of gas that become trapped in the solid.

+1
Great post.
I'm finding that many don't understand anymore that forged steels similarly have a bark of metal
that has lost it's matalurgic integrity, and can also have trapped gasses and or debriss trapped in it.
As well as having it's carbon and other elements burned away near the surface.
This bark needs to be sacrificed, leaving only good metal for manufacturing a thing out of it.

Peter.
09-14-2014, 03:46 AM
I'm sure that steel does have a certain amount of porosity because my motorbike fuel tank is made from steel and I have to re-fill it every third day.

J Tiers
09-14-2014, 09:17 AM
I'm sure that steel does have a certain amount of porosity because my motorbike fuel tank is made from steel and I have to re-fill it every third day.

Those typically have two holes in them. For some reason one is always quite a bit larger than the other, and they are on opposite sides so you never can just tip the thing until the hole is on top and nothing runs out. Very annoying.

You need to plug them both.

vpt
09-14-2014, 10:16 AM
While welding I have come across pockets of crap in every type of hard cold stuff that has come across the table.

It is amazing to see how much "stuff" different metals can absorb or trap after being used especially. Cast especially soaks up oil like a sponge. I normally just tell people that used cast parts can't be welded. If it is a good friend or if I am real bored I will take on a used cast job. Most times having to bake out the oils first in the oven, then running over the joint to be welded with the tig first to bring more oil and crap tot he surface to be ground out a few times. Then it can finally be gone after with the stick and still there will be contaminants.

ahidley
09-14-2014, 01:35 PM
"It all makes sense now" said the blind man, as he spit into the wind. "It all comes back to me now"!

Optics Curmudgeon
09-14-2014, 02:00 PM
It's possible for inclusions (slag, scale, etc.) to be rolled into bar or plate at the mill and be stretched out into lengthwise oriented porous areas. This is why vacuum flanges are cut out of plate rather than sliced off of round bars. Porous areas invisible to the naked eye will cause leaks in the bar cut flanges.

radkins
09-14-2014, 03:00 PM
Ever see a welder hold a weed burner to steel to prep it for welding ?

You can see the moisture seeping out of the steel


To add a bit to what was already said, if you have done this in the past you may have noticed a significant difference in the amount of moisture formed on cold steel vs warm steel, this is because the colder steel surface will tend to condense more water vapor from the flame. The colder the steel is the more condensation that will form but this is a by-product of combustion rather than coming from the steel itself, you will also get far more moisture on the steel using a propane heater than you will from an acetylene torch due to the differences in fuel chemistry. Hard as it may be to realize that this moisture that forms is actually coming from the flame it's fact.

Fasttrack
09-14-2014, 07:05 PM
It's possible for inclusions (slag, scale, etc.) to be rolled into bar or plate at the mill and be stretched out into lengthwise oriented porous areas. This is why vacuum flanges are cut out of plate rather than sliced off of round bars. Porous areas invisible to the naked eye will cause leaks in the bar cut flanges.

Hmm... doesn't it make more sense to make the flanges from round bar, then? The outer diameter is likely to have some "bark" on the outside but that doesn't matter.

Optics Curmudgeon
09-14-2014, 07:20 PM
They are on the inside, that's the problem. If there are any inclusions in the ingot they get stretched out into long "straws", microscopic but big enough to cause leaks. If the flange is made from plate these straws are in the wrong direction (across the plate instead of through it) to cause problems (most of the time).

Fasttrack
09-14-2014, 08:19 PM
They are on the inside, that's the problem. If there are any inclusions in the ingot they get stretched out into long "straws", microscopic but big enough to cause leaks. If the flange is made from plate these straws are in the wrong direction (across the plate instead of through it) to cause problems (most of the time).


Got it. Is this a well documented problem or more of a "best practice" type of thing? I have made a number of apparatus for UHV applications and never run into this before. I have cut from plate and sliced from bar as was convenient. I have at least one 8" diameter flange on a 10^-10 torr system with no problems. The flange is in a target area that is actually some distance from the ion pump but we have a magnetron gauge at that location.

J Tiers
09-14-2014, 09:00 PM
Sheet has "grain", more pronounced in aluminum, but somewhat present in steel. It runs along the rolling direction.

Sometimes, bends that align with the "grain" will cause fractures to start. Generally across the grain they do not, although it can cause a split on the end of a bend.

boslab
09-14-2014, 09:10 PM
There is what is loosely called porosity in steel, it arrives in the continuous casting process as argon is fed into the ladle shroud, tun dish stoppers and submerged entry nozzle, in an effort to stop the alumina build up inside the tundish gate SEN nozzle, a common gate mechanism is called a NKK gate, it has argon fed in too, it's a compromise, if air gets in when you cast it makes Al2O3, alumina, aka dirty steel
The argon bubbles are plainly visible in the steel slab when sectioned and etched, when rolling the steel the bubbles if bad can cause laminations in the steel, on the surface it's a skin lamination, really bad for surface critical stuff like cars and fridges.
The can industry stuff is critical too, the porosity causes seams to fail and cans loosing pressure, the steel can be reduced in thickness to thicknesses less than the diameter or thickness of the stretched out pore or lamination, that can bugger a set of draw re draw or drawn wall ironed tools, DRD and DWI for short, lacquering provides a seal to the can, it's rare for an unlaquered can to hold pressure, tin plating also solves the problem
It's common to cut pressure plating at 90 degrees from rolling direction to avoid the laminar defects too
Additionally there are bits of alumina in the steel as a residjual from making it too, generally steels off the shelf are called Aluminium Killed or AK (there are other ways to "kill"), when the iron is "converted" to steel it's done with an oxygen lance in a big refractory lined vessel aka converter, or bos converter
Lots of oxygen say about 70,000 cu m min at Mach 2 for 10 mins, quite a blast!
This leaves dissolved oxy in the steel, it has to be killed or removed so coal is added in dust form to react with the oxy, gets most of it but not all, Ali is then added to kill off the rest, but the reaction leaves clumps of the stuff in the steel, most floats up to the slag layer if there's enough time, usually the stuff is casting before this happens completely as the steel is cooling off, at 1450 degrees for low carbon steel, if it's in there it's staying, crumbly little voids in the steel.
Fascinating stuff to me
http://www.alcarinternational.com/products/
Mark

Mcostello
09-14-2014, 09:33 PM
Realized this might be easy to prove if the moisture comes from the steel. Just light up a carbon electrode for a second on a piece of steel and see if any water weeps out. Then again how would welding ever progress if when striking an arc We had to wait for the water to burn off?

boslab
09-14-2014, 09:36 PM
There is hydrogen in there! It helps not to add any more though!
Mark

J Tiers
09-15-2014, 08:08 AM
I don't know if bubbles would be counted as "pores" the same as in CI. They should be closed bubbles, meaning not extending back into the steel. I suppose they might be smeared-over and therefore be hard to wash out, but then they would probably be small as well. Not really hard to clean, and unlikely to weep oil through multiple cleaning cycles.

CI you can clean 6 times and it is still dirty and oily. And not just a little bit. The carbon that is not dissolved forms cracks etc which might function as "pores". In any case, I have had to burn off the oil repeatedly, and clean between-times, before the CI would take solder or silver-brazing.

The worst was a sliding block for a shaper. I needed to build it up (it was sloppy in its track), and it had been obviously covered in oil for 50 years. I think I did the burn and clean a dozen times before it was clean enough.

Steel normally needs no more than a rinse or two and it's good to go.

Optics Curmudgeon
09-15-2014, 09:08 AM
The worst was a sliding block for a shaper. I needed to build it up (it was sloppy in its track), and it had been obviously covered in oil for 50 years. I think I did the burn and clean a dozen times before it was clean enough.

Steel normally needs no more than a rinse or two and it's good to go.

If it was an Atlas shaper then it was neither steel nor cast iron, it was Super Oilite, which is sintered iron instead of bronze. It's designed to be porous and hold oil and be more durable than the bronze version.

Optics Curmudgeon
09-15-2014, 09:12 AM
Got it. Is this a well documented problem or more of a "best practice" type of thing? I have made a number of apparatus for UHV applications and never run into this before. I have cut from plate and sliced from bar as was convenient. I have at least one 8" diameter flange on a 10^-10 torr system with no problems. The flange is in a target area that is actually some distance from the ion pump but we have a magnetron gauge at that location.

It's best practice for those that have gotten burned by the porosity problem. NRC used to advertise that they made their flanges from plate. The Princeton Plasma Physics Lab vacuum group had the problem with some 14 inch flanges and went to plate after that.

michigan doug
09-15-2014, 10:40 AM
Porosity in steel is not a yes or no proposition, but how much and what kind, and in what orientation.

If you doubt it, go talk to the people who try to store hydrogen in steel vessels.

So, the presence of small minor porosity is a big deal to the hydrogen storage folk, but that same steel might be perfectly adequate for a gas tank since the octane molecules are so much bigger than the hydrogen molecules.

radkins
09-15-2014, 11:18 AM
Realized this might be easy to prove if the moisture comes from the steel. Just light up a carbon electrode for a second on a piece of steel and see if any water weeps out. Then again how would welding ever progress if when striking an arc We had to wait for the water to burn off?


As has been pointed out steel exposed to the atmosphere will have hydrogen present near the surface and this is one of the reasons for preheating but there is no moisture to "weep" out, this is an old myth perpetuated over the years. If heat caused moisture to "weep" out of cold steel then obviously moisture would "weep out" alongside a weld bead until the metal became hot enough to evaporate it but clearly this does not happen, simply the moisture is condensate from the combustion process of hydrocarbon fuels and oxygen forming water vapor and not from the steel.

J Tiers
09-15-2014, 08:14 PM
Porosity in steel is not a yes or no proposition, but how much and what kind, and in what orientation.

If you doubt it, go talk to the people who try to store hydrogen in steel vessels.



Add "what size" to that list..... and "what you want the material for". To get the discussion out of the theory area and into "meaningful reality" you need that info.

After all, the actual material is nearly all empty space anyway, there is a fairish bit of space between atomic nuclei. But that has essentially no effect on what we consider porosity, which is porosity that affects what is being done with the material.

becksmachine
09-16-2014, 01:19 AM
you will also get far more moisture on the steel using a propane heater than you will from an acetylene torch due to the differences in fuel chemistry. Hard as it may be to realize that this moisture that forms is actually coming from the flame it's fact.

I suggested something like this once over on the welding forum. Someone was asking about brazing with brass or silver solder and wanted to know if a propane or oxy-propane flame/torch would be appropriate. I related my experiences of creating instant rust when playing said flame on otherwise meticulously prepared surfaces. My understanding is that the propane flame creates the by products of carbon dioxide and water, and the water instantly condenses and causes rust, thus creating instant mistrust of the resulting braze.

I was politely informed that the oxy-acetylene flame produces the very same by products, but no one wanted to explain why I had never seen the "instant rust" phenomenon when using it.

Dave

boslab
09-16-2014, 03:48 AM
You can make a oxy fuel flame produce carbon monoxide too, if there isn't enough oxygen for complete combustion aka carburising flame in gas weldors talk, it's this thing caled the stoichiometric ratio, the only way to ensure that you don't make CO is to use excess oxygen, the idea is the perfect flame they call neutral, tight blue cone with a tiny feather, an oxy acetylene flame is hotter than the oxy propane
By about 1000 degrees so the heating process will be quicker, a slow heating cycle gives the surface much more opertunity to end up oxide coated
Steel is porus to H2 as stated but Hydrogen is a bugger to keep in, it goes through ceramics and plastics too, if I remember correctly when a new spectrometer arrived in work getting the vacuum down was a bugger at the start, the guy from thermo reckoned it was gas aka H2 coming out of the surface of the vac chamber, don't know if it's true but it was plausible
On an atomic level there are loads of holes that have a lovely name, dislocations, half planes of atoms wedged between full planes, without which steel couldn't be hardened or annealed, also there are cracks that form during solidification as inter columnar macro segregation it's easy to see them if you use a sulphur print of the steel, inclusions of mould powder, alumina, bits of refractory that inadvertently dissolve too, making clean steel is difficult
Mark

J Tiers
09-16-2014, 08:27 AM
I suggested something like this once over on the welding forum. Someone was asking about brazing with brass or silver solder and wanted to know if a propane or oxy-propane flame/torch would be appropriate. I related my experiences of creating instant rust when playing said flame on otherwise meticulously prepared surfaces. My understanding is that the propane flame creates the by products of carbon dioxide and water, and the water instantly condenses and causes rust, thus creating instant mistrust of the resulting braze.

I was politely informed that the oxy-acetylene flame produces the very same by products, but no one wanted to explain why I had never seen the "instant rust" phenomenon when using it.

Dave

Well, they are part right and largely WRONG. At least as far as it relates to this matter....

The same TYPE of combustion products are formed, but they are different in RATIO, and also in temperature.

Propane is a normal carbon-hydrogen chain.... C3 H8.... so there are 8 hydrogen atoms to 3 carbons. you get 4 water molecules and 3 CO2 molecules when it is burned.

Acetylene is C2H2, with the two carbons joined by a very unstable triple bond. You get TWO CO2 and ONE water when it is burned. Obviously the ratio of CO2 to water is very far different between the two, acetylene produces far less water.

Not only that, but the unstable triple bond means that the "activation" energy is far less total for acetylene. That's the energy necessary to break the existing bonds so that a stronger one (to oxygen) can be formed.

Making the stronger bond releases energy, but breaking the existing uses some of it up. Since acetylene needs less energy to open up bonding sites, it takes away less, and so "burns hotter", the products of combustion are hotter. Therefore it tends less to condensation than a cooler flame.

Also, water vapor has a high heat capacity, so the more water formed, the more heat of combustion is carried away at a lower temperature by the water, and again the cooler the flame, and the more chance of condensation. The metal heats slower, and condenses more on top of that. Acetylene wins all around.... at the price of being extremely unstable.

Then, using pure oxygen means less non-participating gas, that neither burns nor oxidizes the burnables, is around to carry off heat uselessly. Air has a lot of non-participating gas, nitrogen etc, and so if it is in the flame, it carries away heat that is therefore not used to heat the material or the water vapor.

fjk
09-16-2014, 09:11 AM
A long time ago, while in college, I worked in a metallurgical lab. Among the things I did was preparing steel samples and then taking micrographs (100-200x iirc) of them. We worked with two types of samples, "regular" steel (I don't recall any specs on it) and powder metallurgical. Any voids/porosity we could see in the regular steel were considered serious defects. For the powder-met. stuff we were doing research to reduce voids and porosity as much as possible (by varying the alloying elements and the pressing and sintering process) but, the very nature of that technology meant that there always would be some...

Frank

mrriggs
09-16-2014, 12:42 PM
Steel is porus to H2 as stated but Hydrogen is a bugger to keep in, it goes through ceramics and plastics too, if I remember correctly when a new spectrometer arrived in work getting the vacuum down was a bugger at the start, the guy from thermo reckoned it was gas aka H2 coming out of the surface of the vac chamber, don't know if it's true but it was plausible

I work in vacuum heat treat and we call this "out-gassing". The parts always give off gas when they are heated in a vacuum. The chamber pressure must be monitored closely and the heat rate reduced if the parts are out-gassing too much for the vacuum pumps to maintain the required pressure.

k2man
09-19-2014, 12:04 AM
Thanks everyone. Incredibly good information here! Answered my questions and then some. I really appreciate everyone sharing their knowledge here.


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