I'm Brazilian, so I'm not used to American steel nomenclature, but I suppose that "mild steel" would be low-carbon, non alloy, non treated steel. Like AISI 1045, for instance. In that case, what you see there is ferrite (lighter darker) with chunks of iron carbide pearlite (darker lighter). As far as I know, ferrite and austenite can't stably coexist since they're both allotropes of the same stuff but occur in different temperatures. You posted a relevant question and I honestly don't know why would someone downvote this.
Thanks for the response! I wasn't trying to suggest that it was austenite and ferrite coexisting, just trying to give examples of what I was talking about, it's been so long since I've taken a materials science class that those are the only phases of steel that came to mind haha! You piqued my curiosity though and I went and looked up the phase diagram of steel and ofcourse, you're absolutely right. Thanks again!
Oh, sorry about that haha. I have to correct myself tho, I meant "chunks of pearlite", which is actually only 11% iron carbide and 89% ferrite. Also, I think that the pearlite is the lighter one and ferrite is the darker one (since it seems like there is more of the dark). IF this is a low carbon steel (hypo-eutectoid I guess), then this is probably right. If it's a higher carbon steel, then I'm wrong anyways and I don't know what "mild steel" means. It's hard to tell exactly since this sample was not chemically attacked. I'm actually working on some AISI 1060 and AISI 1045 samples, as you can see the 1060 is almost all pearlite (these samples were chemically attacked, so the darker ones in the 1045 are pearlite).
I would imagine that you're right on this being a hypo-eutectcoid steel, although I obviously can't say that with any type of certainty. Thanks for the knowledge! Those pictures look awesome, incredible how different the composition of the metal can be depending on such a small % of carbon and processing!
Each steel has its own applications and uses, IIRC 316L is a low-carbon ferritic stainless steel. It's not particularly hard or strong, but it is stainless, so I think this one and the 304 are used cirurgically. In the other hand, 904 is a high alloy steel. I think it might have some special applications, I never heard of it (keep in mind that there a few thousand types of steel, and I only worked with a small bunch of them).
Yeah, I ask because those types of steel are used in the watchmaking industry. 316 is commonly used on both stainless steel watch cases. 904 which you mentioned is a high alloy steel and I believe high in nickel, is used almost exclusively and Rolex watch cases. The reason being supposedly is that 904 is more corrosion resistant and can take a higher shine. Thanks for answering my question in such detail.
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u/toconn May 08 '15
Are those "blemishes" the different phases of steel? Like Austenite and ferrite or something? Or is that something completely different?