I was kind of wondering about that... I happened to tour the Savage factory about a year and a half ago, and saw the stage where the actions get heat-treated... pretty much the whole thing is done as one. I didn't see how there would be 'surface' hardening given the way it looked like the whole action came out cherry red... but not being a metallurgical type I wasn't sure enough to argue with them.
Heat treating is, or at least can be, a complicated process. I have no idea how Savage, or any other gun maker, heat treats their action. But the fact that the pieces come out of the oven glowing red in no way indicates that there can't be surface hardening.
In it's most simple form, steel is iron alloyed with a little bit of carbon, usually not very much; around 1/4% but it can be more. Of course, there are all sorts of other metals that can be added to give just the desired properties like corrosion resistance to mention one example. Often times a steel product needs to be "tough", which is to say it needs to be able to bend with out cracking. In other words it shouldn't be too brittle. Low carbon steel with about 1/4% carbon is perfect for lots of structural applications. However, it isn't very hard so it may not be appropriate where sliding wear is of concern.
That's where surface hardening comes in. By heating steel in the presence of carbon, the carbon can diffuse into the steel and make it harder. Old gun makers knew this; therefore, they were able to fashion pieces out of easily worked low carbon steel which was easily bent and even engraved since the metal was soft. Then they would surround the metal in ground charcoal (and sometimes other materials like bone, hoofs, and even urine) and heat it. The carbon would diffuse into the surface making it hard and wear resistant while the rest of the piece retained the toughness and lack of brittleness of the lower carbon steel.
This process is still used today, but adjusting the carbon of the surface of a piece of steel can also be done with certain gasses. For example, if you heat a piece of steel in an atmosphere of carbon dioxide the gas can react with the surface carbon of the steel and make carbon monoxide, reducing the carbon on the surface of the steel. Or, you can use hydrocarbons like methane in which instance carbon is added to the surface of the hot steel achieving the same result old time gun smiths got by using charcoal, bone, and urine.
So, it is certainly possible that a gun maker could heat his action in an atmosphere of something like methane to increase the surface hardness of an action or a bolt (to reduce wear) without making the entire action so hard that might become brittle. I have no idea if Savage surface hardens their actions but I do know that Buck knives uses a process just like this as part of the heat treatment of knives.
