Can anyone diagnose what the problem here is?  

I use EN9 for punches but I found the smaller ones bend after a few uses. So I have tried 01 steel, I forged, brought up to non magnetic, quenched half in linseed oil, allowed temper colours to reach end, re quenched half and left to cool under the forge ash. The first time i tapped it it snapped. 

What did i do wrong?

thanks!

 

Large grain indicates overheating.  Is that dark line on the right piece a pre-existing crack? (scaled interior)

The large grain structure says it was above critical temperature too long without being annealed. The black line if it's a crack says it was doomed. Being black says it's been there and isn't an artifact of the break. Being there it was PROBABLY the failure initiation point but without being able to use a loupe to determine if there are directional indicators in the fracture it's just a probability.

If that had happened to a punch I'd made I'd forge it faster and stop sooner. Anneal and grind to the final shape.

What I just said doesn't mean I disagree with Thomas, overheating is at least as likely.

Frosty The Lucky. 

It looks like it has cracked at the end you would not have quenched? (I'm just asking, no idea if that is a factor!).

 

I assumed the crack was formed at the same time as the break as i didn't notice it before. 

Thinking back I did get the top half hotter than I liked (white heat), i was taught to forge EN9 at a red heat, is this true of 01?

Can I save the bottom of the punch by annealing and re forging?

Does annexing require a long cool down or is it sufficient to bring the pice up to non magnetic and quench straight away?

Thanks. 

I don't know anything about the steel so I'd just be speculating. You shouldn't have any trouble finding heat treat information and specs for the alloy, the manufacturer should have it available on their web site. 

Heat treatment can be a highly precise process requiring ramping oven and timed sequences. That's just a FYI I'm not saying anything specific about the alloy you're dealing with.

And yes, if that is a crack it was there when you heat treated or it'd be the same color as the faces of the break.

Frosty The Lucky.

that surgary looking grain indicates to me that your metal was way way to hot when you quenched...... bright orange to white hot is to hot. I quench at mild orange heat, just a little bit hotter the the point the steel loses it magnetic qualities. I also use a slow quenching oil. Also when your using the tool if it get red hot don't quench it, let it cool to black then you can quench it in water to keep it cool. It took me a few broken punches and slitters before I got clued in on my tool getting red from use.... It gets stuck or I can't get the steel to release quick enough. But since I stopped doing the 2 things I mentioned above...... no more broken tools, which is great because snapping off a hammer eye punch after only 2 uses was a real bummer

 

I think you just left it to hard.

should  temper  it some more.

Look up "normalising steel" on the smithing and bladesmithing forums.

The grain will have grown at forging temperature and it sounds like you did nothing to get the grain size down before quenching it.

Normalising involves heating to JUST above critical (look for decalescence), then air cooling to black. It starts new grain formation but doesn't allow the time at temperature for the new grains to grow big, thereby setting things up for the Austenitization and quenching with nice fine grain.

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O1 is a deep hardening alloy steel. It is not normalized, because you can't!  It will harden in air from critical, but it will be an unstable hardness. Therefore, after forging, it is annealed in lime, wood ashes, or vermiculite, not air cooled. You'll need to experiment with hardening and tempering. I would be tempted to oil harden the business end by figure-eight agitating it vertically in the oil, and immediately baking it at between 400 and 500 Fahrenheit.