Some metallurgy questions

Alex Topfer

Alex Topfer

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Hi
I have a few metallurgy questions I haven't been able to find answers to.

1) where does the pattern in cable Damascus come from?
With, say, 1095/15n20 it due to the different alloying compounds giving different finishes with etching and polishing, but I would expect the cable to be roughly uniform. is it due to decarbonisation in the outside of the individual strands?

2) what exactly happens during normalisation to reduce grain size? the steel doesn't melt, so why would the crystal structure change in that way?

3) a dodgy heat treat results in less hardness, but can it still be just as brittle?
I heat treated a 1095 steel blade a while ago, which then broke when i was trying to bend out a slight bend in the tang/handle (yeah, bad move on that). Given 1095 with a backyard heat treat and ~200c (~400f) temper I had expected it to be more flexible. Grain at the fracture point looked ok, not too large. (Could be the temper being bad, lots of variables here)

Thank you
 
Question 3 won't be directly answered in stickies
A backyard HT can still be a good HT
And usually if you have a warp or bend after you really know it hardened
I would temper twice for 2 hours before trying to fix bend
 
1- the patterns shows the lines of surface between the strands. the surface will have oxidation and some amount of decarb, which creates the pattern. The weld between the surfaces creates a barrier that grain growth cannot cross. That also shows ( and is part of why a weld will break at the joint more easily).
2 - The things we do in HT are solid state transformations. Even as a solid, things move … the hotter iyt is the faster they move. In normalization you are returning the structures to a more unified and desirable structure and size.
3 - A poor HT gives a piece of steel with mixed structures. Done properly, the steel is mostly martensite. As-quenched, martensite is brittle. In a poor HT there may be some pearlite and other structures that you don't want. Since only the martensite is hard, the blade will test as softer. The martensite that was formed is brittle, though, and will break if not tempered. Tempering will lower the hardness some more. The temptation to use the blade as-quenched or under temper to retain as much hardness as possible will produce a blade that may have a chippy edge or will break more easily. Very few blades should be tempered below 400F if the HT was right.
 
Thanks! That makes sense
I regards to the blade I broke, I'm wondering if my oven runs cold and I tempered closer to 180c. Guess I need a thermometer for it
 
180C is a bit low and would not have hardness back as much as would be desirable ( except on a kitchen slicer), but the blade probably broke for multiple reasons.

I just noticed you live in Australia - You have to deal with the inverse gravitational effect. It is a complex math system to get exact, but the simple solution is to turn the tempering and hardening charts upside down (since most are made in the US and Europe). Basically, as the tempering temperature goes up the blades get harder, and as the austenitization temps goes up the grain gets finer. Also don't forget to convert those charts from F to C or you may crash a blade into Mars (figuratively).
Just kidding, of course.
 
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Yeah, I think I'm getting the hang of the conversions, but the first couple I screwed up inch to mm and they came out wider than they were long :P
Ironically it was meant to be a kitchen slicer (a cai dao/chinese cleaver). I've ordered a thermometer for my oven so that should help.

I'm currently reading through @Larrin's blog. A lot of it is a bit over my head, i'm more a biochemisty geek
 
I thought of another one:
Wouldn't modern damascus steel be weaker than modern mono-steel, in general? In most cases the pattern welding process isn't perfect, so wouldn't it introduce some small flaws into the steel?
Obviously in a perfect case the damascus won't have flaws, and historic processes are completely different.
I'm also not sure you would ever even notice this, outside very careful lab tests. It's obvious that damascus blades work for what we need in a blade.
 
Damascus is prettier than mono-steel. That is it's only advantage. It isn't harder, stronger, better cutting, etc. .
Properly made, the difference is small, but as you suspected, technically, it is weaker.
 
Alex Topfer said:
I thought of another one:
Wouldn't modern damascus steel be weaker than modern mono-steel, in general? In most cases the pattern welding process isn't perfect, so wouldn't it introduce some small flaws into the steel?
Obviously in a perfect case the damascus won't have flaws, and historic processes are completely different.
I'm also not sure you would ever even notice this, outside very careful lab tests. It's obvious that damascus blades work for what we need in a blade.
Click to expand...
My understanding is that this is true but it's weaker by such a small margin that it really doesn't affect anything.
 
Don't overlook the possibility of quench cracks, particularly in 1095. This could be the cause of the break, but might be hard to determine. They can be small, but still cause problems. Also, hardened steel, particularly with a low temper, won't take well to trying to bend out a warp. Its possible but may be higher than 50/50 that it breaks. I had a blade of A2 do the same thing. Only 1/16" thick (1.5 mm), and I tried to bend out just a tiny bit of warp, and off it went.
 
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