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What is “Soft Iron” in Japanese bladesmithing?


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Japanese tools and cutlery are often a composite of a piece of edge steel and a material usually referred to as “soft iron.” I vaguely remember reading somewhere here that this isn’t just mild steel (that it was lower carbon I believe) but I can’t find the thread. I would be glad to hear any insight into what alloy is used. 
 

I personally switched from A36 to the nominally less “soft” 1018 for greater certainty of the chemistry, but on thick knives like deba, I could see the benefits of a super low yield strength cladding (like pure-ish iron) in the inevitable post heat treat tweaking to get a straight blade. There also would presumably be some reduced abrasion resistance to ease sharpening and maybe a bit more corrosion resistance. 
 

Again, I would be very interested in thoughts/sources on this, and thank you for looking. 

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I do know I've watched at least a part of a documentary that covered this a bit and from what I remember it is just "mild" steel. Hopefully someone has direct sources as I take most documentaries with a grain of salt when mixing historical tradition in modern times especially when there is a lot of mythical fantasy about the topic. What I mean by "mild" is that there are different standard of what mild steel are and I know US vs Europe has some differences let alone Asia. That is without each knife maker or steel maker having their own blends as mild is just a generalized category as you know. I do know that 1018 is way better than A36 for the same reason people suggest you avoid rebar as its based on yield standards more than composition. 

 

So to start its not pure iron for many reasons and classically if it wasn't the cheaper/est steel it was closer to wrought same as Europe. I would say 1018 or if you can find it worth while something even lower but realistically it wouldn't be that worthwhile as 1018 seems to be pretty effective as a alternative. I believe in reality its what is considered low carbon steel so less than .12 I believe I don't remember since most things below 1035 is just called mild steel all the way to not steel in common speak.

 

So if you could find 1008 I believe it would be closer but in the end 1018 is close enough unless you are going for perfect reproductions then it might just be easier to buy from Japan than find a local alternative.

 

Sorry for the rambling nonsense TL:DR lower than 1018 is correct more than likely but not necessary if its still working. As long as its a minimal alloy steel like the xx00 series and low carbon it should be similar enough. If going for 1:1 recreation than more research is required.

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Thank you for the reply, that makes sense. I have used 1018 on a number of knives to good effect, but figured I would see if there is another option with properties closer to what is normally used on these knives and tools, especially as far as yield strength and ductility because of how cold forging/straightening is a significant part of the process. I do all of my forging by hand, so softer metal would also make that a bit easier. I may try and get ahold of some 1008, it seems like it's actually quite different than 1018 in composition (~half the C and Mn) and properties (more elongation at break and ~2/3 the yield strength). From a preliminary look, it does seem a bit hard to find though, especially as bar rather than sheet. It seems pretty similar to the Japanese grade SGD1.

 

I was able to find the term "gokunantetsu" (極軟鉄, "Extremely soft iron" according to google translate)to refer to the cladding material, but my feeling is that it would be very hard to source. From what I can tell, it's so very low C/Mn steel. I've been able get odd materials from Japan before, but I'm not sure if this is worth the effort in this case.

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I know there is options below 1018 just hard to find 1008 was just one I was pretty sure a real thing as I didn't want to go down the sub .18 rabbit hole at 2am. I also saw the gokunantetsu and since it really doesn't turn up anything in a western based google search I have the feeling its their version of low carbon steel since the differences in language makes some things more specific than how we think of them in English. My feeling is its either a very low carbon steel or a cleaner wrought iron since "pure" iron .02 or less is its own word I think it just might be one of those reginal specifics that just doesn't translate to a real meaning and requires a lived context.

 

Basically you want enough carbon for it to be resistant to rust like mild steel but as close to pure iron without being iron. Downside is it will as you know make welding it harder as it requires very high heat compared to the edge steel. I'm fairly certain that nowadays they use a 1018 equivalent if its not the $600 knives for ease of access and price since most non industrialized makers don't put out enough to custom order alloys I believe.

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