The metal at the break looks crystallised and it should not.
I would remove the bolt from the rifle, strip it down and take it to a place that works on aircraft engines or such, ask about the hardness.
There used to be old-fashioned shops with real Machinists who could take one look at it, touch it to a grinder for half a second, tell you the composition and hardness and what should be done, but these places seem to have been supplanted by people with computers and CAD programs and not much feeling for STEEL.
Read the chapter in "Hatcher's Notebook" on "Receiver Steels and Heat Treatment". It's in the 2nd and 3rd Editions, but I don't believe it is in the 1st. Mandatory read.
Get it checked out, whatever.
Ganderite is about 200% right on this.
That said, at the last gun-show that my Mom attended, I did pick up a 1906 Moisin-Nagant with the right lug cracked right through and hanging onto the bolt-head by a thin bit of iron. Evidently it had been broken and the rifle continued in Service. Head showed evidence of having been fired..... a lot..... since the break. Rifle was greasy and filthy and the light was terrible, of course. I wasn't going to buy the thing, really, (short of cash, as usual) but Mom insisted and actually paid for the ugly thing: $40 cash, no tax, marked down end-of-show special from $60. The damage was discovered only after the ugly old thing (Finned 1906 Tula, counterbored but with a lovely bore) got home. I didn't want to trash the rifle or part it out because Mom fell very ill shortly afterwards. As it turned out, the rifle was her final lifetime gift to me.
Being rather much of a wimp myself, I replaced the bolt-head with a brand-new NOS one with Tula markings. Headspacing was within limits, so I tried it out and.... and the old girl shoots beautifully! Ugly old thing.... and it hammers them in at 100 yards at just over an inch.
Sometimes, I swear, we have Someone looking over our shoulder......
