SMLE bolt will not unlock.

[Bruce_in_Oz]

ALL components of ALL Lee Enfields prior to the introduction of the No4 were made of CARBON steels and "Malleable iron" of various types. The specification explictly excludes the use of "alloy" steels unless specifically approved by the folk at the top of the Ordnance tree. I have the recipes for most of these carbon steels and malleable irons, but have NEVER seen the exact recipes for the heat-treatment regimes. "Heat to cherry-red, hold for X minutes and quench in oil" leaves a LOT of room for variable results!! (Sunny or overcast, well-lit or gloomy foundry/workshop, fresh or tired workers, slow day or mad rush.................)

I would suspect that a "performance" spec was set in the contract documentation and then the makers had to "do their own thing" to achieve it. That still happens in a lot of fields today. The difference is that these days we have pyrometers to measure the workpiece temperature fairly precisely and a huge variety of alloy steels that can be made VERY hard and still retain a tough core that is not prone to shattering.

The other thing that is creeping up on EVERY piece of iron-based equipment from the Industrial Revolution onwards, is "age embrittlement". The molecular structure is NOT set in concrete, thus the Carbon in these early materials tends to "move around" and form new structures that tend to increase the brittleness of the components. Furthermore, it didn't much matter what the "recipe" called for, there was pretty much no way of accurately analysing materials until the early twentieth century, and we have come a LONG way since then. Thus, folks should be wary of firing genuine antique muzzle-loaders; the barrel probably won't burst, but forged parts like cocks and frizzens will start to fracture.

[Mk VII]

Another source of these symptoms which I saw recently, was that a No.4 firing pin had put into a SMLE bolt (de-act, as it happens). Every time you pulled the trigger the pin was driven hard into the bolthead and wedged there, stopping you opening the bolt.
I made a brass collar to go on the front of the pin's collar and stop it moving so far.

[Surpmil]

Anyone have knowledge or experience of cryogenic stress relief and its value if any in "treating" older firearms or other steel or iron objects?

It's a pity we don't have access to a x-ray spectrograph to analyze the steels used, particularly where there have been failures.