Ok,i see the photo of the break,you've done a great job photographing it.
No,grain-size that large is not acceptable for a tool such as an axe(at least not Desirable that's for sure).
I realize it's magnified but the individual grains Are visible,by the naked eye,and they oughtn't be.
What it may mean is not that certain.I know nearly nothing of HSS,nor of many other alloys it may be,or you may even be right about it being a casting.
If it was me,and i was up against having to use this alloy for a tool,next step would be to see if it's susceptable to grain-reduction by Normalizing.
That involves heating it 3 times,first to just over-critical,then to just crit.,and third time to just below crit.
After each heat letting it cool in still air till black(lack of any invisible)heat.
Then quench again,and make another break.
Now here's the thing about grain-growth:It is a function of being held for long periods at high heat.So after repeated forging heats et c.,right before HT it gets normalized,and all's cool.
But here's the rub.One very important aspect of quenching is Soak-time(at critical T).Each alloy has it's correct time,say plain-C alloys it's 1-1.5 minutes.But for say O1 it's about 30 minutes...
Exceeding this time causes the grain to enlarge(not soaking it enough does not convert enough of the structure to what you need it to become).
Excessive T also causes grain to grow.
So sometimes after normalizing properloy one can screw it all back up by incorrect T and/or soak-time at quench.
(junk alloys are rough to play with,after a while you'll just want to buy a legit,Known alloy,it saves So much hassle!
The photos really do make it look werse then it is. To the naked eye it looks like a mat grayish white surface. The grains are not visible
When breaking this piece it was surprisingly tough. I had to hit it quite hard for it to break
With plain-carbon(10xx series)and other "simpler" alloys the loss of existing HT begins at close to 300F
(look up Oxidation colors,they start at Light Straw and go on from there forget my temps in degrees).But that is only as seen on Shined-up steel,otherwise you may miss it.
Cold enough to hold your bare hand on is safe,too hot to touch is beginning of danger zone.
Yeah, I am quite familiar with plain carbon steels. I was hoping this axe to be at least that but it appears to be a much more complex steel.
With plain carbon steels heating to gray (just passed blue) noticable softening will occur. With this steel it didn't. That is why I'm thinking it may be M2 or some other variety of HSS which doesn't really soften until about 1200* f or so