Overheating and crumbling steel

[huntmaster99]

So I have a question brewing that I cant seem to google-foo my way into an answer and youtube isnt helping here.

How is it possible to overheat steel such that it crumbles as soon as you give it a decent blow with a hammer?
I had a piece of H13 crumble in a similar manner, and after some asked questions I came to the probable conclusion that I overheated it and let it soak in the heat too long.

Why can some steels get heated such that they sparkle and not have problems under the hammer, while some, if you THINK about getting it too hot will crumble.

Another thing Im curious about is why people deliberately will heat soak a puck of Wootz steel (future project , need to crawl before flying spaceship though) for hours on end at forge-welding heats and then its perfectly fine to go straight to forging and consolidating the ingot?

I originally thought it might be the carbon and carbide structures within that get too big in an over-heat soaked ingot... but yet this seems to be the EXACT process for forging Wootz/crucible steel.

[Latticino]

Get yourself a free copy of the Heat Treating app and look up forging temperature ranges for the steels in question.  There are a lot of different steel alloys and each has a characteristic "working" range.  Wrought iron can be effectively forged almost white hot, but splits at lower temperatures you can still work mild steel.  Simple 10XX series Medium and high carbon steels have general working ranges that typically work for those types, but once you get into exotic alloys you need to be more careful.

Unfortunately I've not played with wootz, so don't have an answer there.

[Frazer]

Overheating can cause some of the lower temperature alloying elements (phosphorus, sulphur, and manganese for example) to begin to melt while others stay solid. The parts that melted can:  form oxides and/or sulphides, thus becoming more brittle; reform and result in porosity; and/or consolidate as long disruptions in the grain structure. Essentially, you end up with very large grains from the excessive heat and/or a bar where the alloying elements are no longer well mixed. Said another way, there are veins or nodes of dissimilar materials with different mechanical properties. You've weakened the material as a whole by creating faults that, when hit it with a hammer, will crumble.

The spectacular-ness (that's not a word, but let's roll with it) of the failure depends on a whole bunch of variables. The temperatures it reached when it burned, how long it was burning, the forge conditions, the alloying elements, the fuel used  etc., etc..

This is a very basic description of something that I'm really not qualified to describe and the "why" only becomes more complicated with the high alloy steels.

However, Latticino brings up a great example with wrought which has veins of silica inclusions in it that will split apart if worked too cold.

Another example would be cast iron which has so much carbon in it that not all of it stays in solution; nodes or flakes, etc. (the structure depends on the temperature and cooling rate) of carbon precipitate out as it cools after being cast. If you try to forge it then it will crumble, more or less, immediately in a shower of sparks. There are all of these boundaries in the material that don't want to move across each other and will come apart if you try to form it. It's a very pretty, albeit a useless, endeavor.

Wootz is an interesting material that I'm even less qualified to talk about. It's generally right on that boundary between cast iron and steel in terms of carbon content. There is a member here named Daniel C who has shared some of his work and it's very impressive. What I've gathered is it's very temperature and time sensitive to maintain the pattern, but also avoid crumbling your puck into 1000 pieces..

Maybe my description of the why helps? But take it with a grain of salt. I'm no metallurgist. Just a guy with a computer.

Descriptions are all fine and dandy, but hopefully you learned a little about why burning your steel is bad by watching it crumble before your very eyes. While every alloy isn't going to turn into a bunch of cottage cheese after you burn it, the more extreme cases are a good reminder that every time you burn your steel you are not only affecting the aesthetics and wasting it (to the pretty sparkly oxidation), you are also degrading it in ways you can't see without a microscope.

[ThomasPowers]

Part of the thermo cycling of wootz is to make a decarb layer on the outside of the puck to "hold" the puck together while doing lower temp forging on it.   A weird "too high C to forge reliably"  enclosed in a layer low enough to forge.   Or to put it a different way: "Wootz is a weirdness!" 

[huntmaster99]

So I went back to a youtube chanel I had been watching on wootz recently by Peter Burt and Ive managed to answer my own question. Seems like you seem to be barking up the correct tree Frazer and that there is a point where the steel partially liquefies before it melts kinda like a slush (I assume its the other non-iron elements doing this) and if you strike it during that time you get crumbling quickly which is exactly what I experienced with H13. If you let it cool after you realize you got it above the line then you can let it cool and you "should" be ok.

Once again im referencing Peter Burt on youtube and the video is titled as follows:
Wootz: ForgingTemps, 1st edition

He has a series on Wootz from a couple years back that seems to me, a relatively new smith and never having made wootz, to be useful or at least a good stride in the right direction. Ill pitch it up to Daniel C on that thread and see if he can get some use out of it.

[Frosty]

Pretty good description Frazer, covers more than I know about wootz. All I can speak for is what I've read. Seems nobody forges wootz above red till the puck has been forged into a long-ish billet which refines the crystal structure. It breaks up the crystal boundaries and alleviates the issues described by Frazer so it CAN be folded and welded for use.

That's it, about all I THINK I know about the stuff. One of the guys in our club makes wootz, forges some spectacularly beautiful blades and offers classes.

It's just not my thing though.

Frosty The Lucky.

[patrick]

I'll add a bit more to this topic as it is one that I have personal experience with. 

Pure iron melts at about 2800 F. Adding carbon to the iron to make steel lowers the melting point in the same way that adding salt to water lowers its melting point. When we make steel by modern methods, the carbon in the liquid steel is uniformly distributed. However, as that liquid steel is allowed to cool, the bits that solidify first reject some carbon into the remaining liquid. This happens over and over again resulting in a sold product which no longer has a uniform carbon content (at the microscope level) but instead has areas of higher and lower carbon. The lower carbon regions can be heated to higher temperatures than the high carbon areas before they start to melt. When the carbon content is about 2%, the melting temperature is reduced to about 2100 F. Now, there are all kinds of steel alloys with widely varying carbon content, so not every alloy will have such a wide range between the start and completion of melting. When we forge, we are always using a temperature which is lower than the lowest melting temperature region of the steel. Usually that limit is set a couple of hundred degrees below this temperature because the metal will get hotter as you forge it. When choosing a forging temperature, you have to build in this safety factor.

In general, it is the carbon content that primarily drives the choice of forging temperature. Alloys with 1% of carbon are usually forged at a maximum temperature of 2100 F, while those with less than 0.5% of carbon are forged at about 2300 F. These are rules of thumb and not absolutes because alloying elements other than carbon will affect the melting temperature.

It is important to know that some unexpected "alloying" elements will significantly influence and reduce the maximum forging temperature of steels. The first is sulfur. When sulfur is present, even in amount as little as 0.03%, (and probably less, but my personal experience was with a bar that had this sulfur content) the forging temperature is dramatically reduced making the steel (or iron) "hot short". In such cases the forging temperature may have to be limited to less than 2000 F or the bar will crack and break apart. This effect of sulfur occurs because iron and sulfur react to form iron sulfide (FeS) which collects at grain boundaries. FeS has a lower melting temp than iron. If forging is done at a temp where the FeS is liquid, it will melt and the grains of iron will fall apart. If there is iron oxide mixed with the FeS, the melting temperature of that combination of materials is reduced further, to less than 1800 F.

Since the middle 1800s the element manganese has been added to steel to react with sulfur. This compound, Mangenese Sulfide (MnS) has a melting temperature higher than that of iron so even if all the sulfur is not removed from the steel, the steel with Mn is not hot short. This is why almost all modern steel contains some Mn. Typically the Mn to sulfur ratio must be at least 8:1 for successful prevention of FeS. Manganese has the benefit of also improving depth of hardening during quenching (hardenability) but is a very low cost alloying addition. So we get a two for one benefit with manganese. For industrial applications this is a great benefit. For some knife applications where you are trying to get a hamon using clay coatings on the blade this manganese can make that difficult because the depth of hardening is actually too much for this process. In more recent years there have been suppliers of steel to the knife making community who have special ordered low Mn content carbon steels for this application.

If the steel is contaminated by copper, it too will make the steel hot short. This can happen if you forge copper or bronze, have some of that melt and get on the forge floor and then that copper comes in contact with steel later. There are some steel alloys that do contain copper and in low levels it can be a useful alloying element.

Wootz is a quite different material from modern steels. Most of what we think of as wootz today was very high in carbon content. Historical examples show carbon ranging from 1.3-1.8%. Due to the way in which the ingots were allowed to cool and due to the presence of phosphorus and carbide forming elements such as vanadium or manganese, the separation of iron and carbon was exceptionally dramatic in this material. In at least one case that was investigated in 2018, the researcher found that the high carbon regions contained carbon at near 2% as well as phosphorus. This combination of elements resulted in a region with a composition and melting temperature similar to that of cast iron rather than steel. The result of this is that, in at least some cases, wootz ingots have structure that is alternating regions of high carbon steel and cast iron. In addition to this, the high carbon and other trace additions of carbide forming elements resulted in the formation of large carbides or collections of carbides. These also influence the forging characteristics of the wootz, contributing to the difficulty of forging this material in relation lower carbon steels of more homogenous structure.

 

[Frazer]

Hi Patrick, I appreciate your input and found it to be very informative.

On 12/28/2022 at 12:56 PM, patrick said:

If the steel is contaminated by copper, it too will make the steel hot short.

I have heard that when working in a coal forge the material can pick up sulfur from the coal. I believe this came up during a conversation about forging copper... maybe it was stainless? But I think it was copper. I am, to some extent, combining ideas in the sulfur section with your comment above on tramp copper in steel. Regardless, I'm curious if you think the material (be it copper or steel) can pick up elemental sulfur from the coal under the right (or depending on one's point of view, the wrong) conditions.

Even if it does, it doesn't appear to cause problems in most cases. Otherwise I expect it would be discussed more. It's not often I can pick a metallurgist's brain so I figured I would ask.

I'm not suggesting that this is what happened to huntmaster. This is more of a conversational tangent to satisfy my curiosity.

[anvil]

One of the characteristics of metallurgical coal(blacksmith coal) is that it is low in sulfur. The lower the better. Also, when it goes thru the coking process, sulfur and other impurities are removed. I'm sure you know this, thats the reason not to have any green coal in your firepot. The sulfur and other impurities in the coal come into direct contact with your steel. This is also the reason that using anthracite, even when the sulfur content is lower than in bituminous, is a poor quality smithing coal. The sulfur and impurities are in direct contact with your work.

[Frazer]

I agree with all of your points. The conversation was centered on the "increased importance" (it's always important, hence the scare quotes) of fire maintenance when working with certain materials due to the potential for sulfur contamination. I expect that when most people think about fire maintenance it's oxygen that comes to mind.

I have no reason to doubt the source of the information. Especially given that we know sulfur -- unless it's purposefully added to improve other characteristics (i.e. machinability) --  is generally not desirable. We also have the potential for a source of sulfur. I figured Patrick might have some insights on this given his experience.

As an example, if there is sulfur present I expect that it would react with the surface layer of the material given it's in the same group as oxygen. However would those sulfides just flake off like scale or would they actually migrate into the material and collect at the grain boundaries as noted above? If it does make its way in, how quickly does sulfur migrate? Carbon diffuses relatively quickly due to its size. While chromium, for example, moves relatively slowly (again do to its size). Sulfur is somewhere in between, but I don't expect diffusion rates to be linear as you increase the atomic number.

[anvil]

I understand what your question is, and its a good one. Sticking to sulfur in met coal, I believe it is burn't off during the coking process. Perhaps not all, I don't know. When discussed around a few blacksmith campfires, beer in hand, We have discussed why its called green coal. When sulfur burns, it puts off a green smoke and we all prolly have seen this in our forge fires. Does it all burn off? I don't know. Always interested in hearing more on this topic. 

Actually what I was doing was supporting your question. Many people believe Anthracite is as good of a fuel as met coal (coking coal). Its not because it burns off the sulfur in the firepot in contact with your steel. Thus your question always applies with anthracite. Hope this is a bit clearer.

[ThomasPowers]

The prevalent use of Manganese in modern steels is to counteract the sulfur they pick up during smelting the ore with coke so it seems some must be present.

[anvil]

True, I've also read that for tool smithing in our situation that charcoal is a cleaner fuel, which supports what you say, Thomas. 

The question is forging is different than smelting, obviously, so is there any significant damage done especially with the added magnesium. My belief is that it is insignificant. 

[patrick]

Sorry it has taken me a while to get back to this question. Historically sulfur was found in the iron ore and also in coal. The use of coal in the conversion of iron ore to iron has only been going on for a few hundred years or so. Prior to that it was done using charcoal which is free of sulfur, so in those older iron pieces the sulfur was coming along from the ore. This is a big reason why irons made in different areas had different characterisitcs and why iron from some areas, such as Sweden, were highly prized. When dealing with iron and steel made by converting iron ore to pig iron and then converting that to pure iron or steel, the points in the process where the metal is liquid are the times it is most likely and able to absorb sulfur from the fuel. When solid, as in the case of general forging, there will be much less pick up of sulfur by the iron or steel. As Thomas noted, most modern steel contain additions of manganeese to counteract the red shortness caused by sulfur, so it is extremely unlikely that you will find sulfur in the coal to be a problem. I'd be more concerned about just getting the work piece too hot and having it crumble due to overheating rather. I hope that helps.

[anvil]

I agree that the way sulfur is added to modern steel is as you stated. And for the reasons given. However its a pretty ingrained idea from early in the coal era, that the lower the sulfur, the better the smithing coal and charcoal, due to its having no sulfur is a better fuel especially when doing work with high carbon steels. I'm pretty sure I have some smithing books from the 18th and late 19th century that bring this up. There must be a reason.  I haven't a clue as to what harm it may or may not cause.