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  #1  
Old 04-05-2013, 09:06 AM
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Eli Jensen Eli Jensen is offline
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Iron vs Steel

So I always thought this one was a no brainer. Iron is iron, and steel is iron plus carbon. But I was reading last night and wrought iron has less carbon than steel, but cast iron and pig iron have more carbon than steel. What makes steel, steel and what makes iron, iron?
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Old 04-05-2013, 09:32 AM
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Ray Rogers Ray Rogers is offline
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Iron is an element, steel is an alloy of iron+carbon+other stuff. Iron mixed with less than 2% carbon is steel, iron+2% or more carbon is pig iron...


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Old 04-05-2013, 11:13 AM
Doug Lester Doug Lester is offline
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To make it even more fun, wrought iron can have up to about 0.30% carbon in it plus the strings of silicate slag running through it. It's the slag that gives it it's pattern when etched and can make it difficult to work if there is a lot of it. There are also different grades of wrought iron depending on how much slag is in it.

Pig iron is high carbon iron with slag inclusions just the way it comes out of the smelter just as Ray said. Cast iron is a little more refined but with still >2% carbon.

Doug


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Old 04-05-2013, 01:17 PM
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So nothing other than an arbitrary carbon range?
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Old 04-05-2013, 01:55 PM
Doug Lester Doug Lester is offline
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No, it's not exactly arbitrary or just the carbon. When the carbon gets above about 2% it's behavior is really different than what is considered steel. It becomes brittle and very difficult to impossible to forge and welding it is a real problem. The slag inclusions in the cast iron also helps it dampen vibrations. That's what makes it handy to make machines bodies with.

With wrought iron, there are the slag inclusions that separate it from structural steel. It is those slag inclusions that can make the wrought iron difficult to forge without crumbling if it is in great enough amount. The slag inclusions also effect toughness of products made from it. It is felt that it was low quality wrought iron rivets that was responsible for the loss of the Titanic. They think that it was the heads of these low quality rivets that sheared off and allowed to hull plates to open up when the ship brushed the iceberg.

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Old 04-05-2013, 02:26 PM
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Ok cool. Makes a little more sense now. I just always thought iron was only iron, and carbon was what made steel. Mind was a little blown.
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Old 04-05-2013, 05:10 PM
Doug Lester Doug Lester is offline
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If you really want to set your mind spinning try reading Steel Metallurgy for the Non-Metallurgist. I can identify with being a little mystified when I found out that wrought iron can have a little carbon in it. Sometimes what we read and hear about steel and iron are only the short version of the story and reality is conditional.

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Old 04-05-2013, 11:59 PM
Imakethings Imakethings is offline
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Eli, it's kinda like the difference between copper and bronze.
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Old 04-06-2013, 08:58 AM
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I don't think it is. Copper is just copper, pig and cast iron is iron and friends
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Old 04-06-2013, 02:39 PM
Doug Lester Doug Lester is offline
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I think that some of the confusion goes back to when people thought that iron was just iron. You put iron ore into a furnace and you pulled out metallic iron. Even steel was thought to be the most purified form of iron instead of an iron alloy with carbon. Remember, when you make bronze you add tin to the copper and you know that you are adding tin to the copper. With steel no one saw the carbon that was added it. Heck, they didn't even know what carbon was until after sometime in the late 1700's. All the smelters saw was that they were burning wood or charcoal or adding something like leather or bone to a crucible to make steel.

Another thing that happens is people, even authors of text books, try to keep things simple. Like they illustrate the iron-carbon phase diagram to what the carbon does to an iron alloy as the carbon level increases. What they don't tell you is that once you add another element, like manganese, to the mixture everything changes. I don't know how may time I've read things like don't soak steels with high carbon content, like 1095 or 52100, much above 1450-1475?. That's fine for such steels that would most often be used by smiths but when you get to the more complex tool steel alloys and stainless steels the advice to totally wrong. With them you may have to soak at around 1800-1900? to dissolve the carbides get the carbon into solution and you also need a preheat soak the steel at around 1400? to prevent cracking as the steel changes phase.

That's why you have to know the steel that you are using.

Doug


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Old 04-06-2013, 07:29 PM
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I didn't know any of this. Wouldn't these types of "iron" technically not iron. As was said iron is an element. A lot of confusion about a lot of things is caused be the intermittent use of common and proper terms. For example, I could be wrong, but isn't brass technically a bronze? I say bronze or brass, but I read somewhere that bronze was a general term for different alloys of copper which includes brass. I looked up some different dictionary definitions of "steel" and they vary wildly. Some define it as a term for basically any iron+carbon+x alloy, while some seem to indicate a specific range or carbon content. I'm not sure it matters much in practice, but it's interesting to think about and I'm glad I could hear some different takes on the subject.
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Old 04-06-2013, 10:03 PM
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Actually the lack of distinction between bronze and brass is one of my biggest pet peeves. Brass is copper and zinc, bronze is copper and tin. That's how it is in my book. Is that 100% accurate? No. I believe the first "bronze" was copper and arsenic? But nowadays, the terms are both so generalized, they both sort of only mean copper based alloy.
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Old 04-07-2013, 02:47 PM
Doug Lester Doug Lester is offline
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The problem that has arose with brass and bronze it that there are copper alloys that have both tin and zinc in them. Then there are some aluminum "bronzes" that don't have either

With iron, you had pig iron right from the furnace which was refined a bit but still had to be cast into but then you could take some of that and refine it in another furnace and forge slag out of it and make wrought iron. Steel was make by adding more carbon to the wrought iron, or subtracting it from the cast iron, or just choosing sections of a bloom that seemed to be the forgible hard stuff that people called steel. Some of this was done back before people understood what carbon was and no one knew about the trace elements that could make iron/steel from certain ores better than other ores.

Some of it also has to do with how steels are named. You have the American Iron and Steel Institute (AISI) that gives us designaters such as 1070, 1080, and 1095, which are often referred to as simple steels or simple spring steels. However you have the Society of Automotive Engineers (SAE) that would lump them all together under the designation of W1. And that just deals with the situation in North America.

Ain't it fun?

Doug


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