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MT04Life, " When people make a breech from raw steel stock, do they harden it once done?"
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"Raw steel" you mention, needs a lot of explanation, because it isn't one particular type steel used for this purpose:

Old classic double rifle actions were made from only mild steel, low carbon common steel. They were color case hardened, meaning, in a nutshell, that they were packed in wood charcoal, annealed in furnace, to remove work stresses in the steel, to make it its original soft state, and for other reasons, packed again in charcoal, but this time in a mix of wood and bone charcoal, heated again to specific temperature in furnace, removed and quinced in water, giving the colors on exterior surfaces we are so familiar with. This is done mostly to impart a hard surface on that steel, to protect it somewhat from wear, scratching, etc.; this hard surface is only a very thin surface hardness, leaving the core its original soft state, so it isn't brittle. The colors are secondary in importance; are simply a part of the overall case hardening process. The whole process has little or no effect on the overall tensile strength of the piece; again, it is only to impart a thin hard surface. Such steel exhibits a characteristic to flex, slightly, under stress from forces involved in firing cartridges, but to also quickly return, spring back, to their original unflexed state. It does so without yielding in its tensile strength, if designed properly and is of proper size for cartridges used in it, so the steel will hold up to stresses involved, without fatiguing. I have done a lot of color case hardening of these type actions, having learned the process in gunsmithing school, so I understand the process.

Newer, modern double rifle actions are usually made from modern high quality alloy steels that exhibit characteristics of superior strength, superior hardness, etc., over mild steel, with tensile strengths many timed over the earlier mild steel, low carbon steels, used for this purpose. In earlier days, when the low carbon steels were used, the gunmakers had no clue about such vastly superior steels, for alloy steels were a thing of the future; eventually coming into common use. such as we see today. Many of these alloy steels are so good, that they need no heat treating to impart adequate strength, for purpose, but many are, indeed, heat treated, to impart a very hard surface along with harder interior, and superior strength overall as well. Such alloy steels are able to withstand many times the stresses that mild steel can.