I am attempting to determine which material has been used in a spring. I know that the spring is ferrous. However, I would like to know whether the spring would have been heat treated.
Spring stock is not extruded- it is cold rolled which greatly increases its yield point and severely elongates the grains. It is not low-carbon; it is low-alloy manganese or medium or high-carbon steel.
Had it been heat-treated after rolling, it would have recrystallized into bulky, ductile grains.
High-quality battery terminals/springs are subject to a very thin copper flash coating which serves as an adhesion layer for a topcoat which is either intended to increase corrosion resistance (nickel) or provide low contact resistance (gold; usually selective-area plated to minimize costs).
Since you apparently have a metallurgical lab to make micrographs for you, find our what chemical analysis kit they have. Maybe they have an SEM in a back room that can give you elemental compositions of both materials.
Even if they don't have full wet chemical analysis or SEM, they will have chemical and electroetch kit, which combined with an etch cookbook and some more microscope work should let you identify whether the steel is stainless, and what the plating is.
Have a look at flat spring and electrical contact springs for sale online. You will find what the common steels and platings are.
Springs are steel because it is : 1- cheap, 2- available and, 3 - it makes excellent springs. It may contain various alloys and carbon levels. It may be cold drawn or heat-treated ( quench and tempered ) . Very likely the spring is cold drawn carbon steel because that is the cheapest. The main concern for most springs is hydrogen stress cracking . Very little hydrogen will cause this because of the high strength and high stress. So some protective coating may be applied , as simple as paint. Recently, hydrogen stress cracking caused a torsion spring in my garage door to fail . The present weather of cold nights and warm days caused moisture to condense on the cold spring , that introduced enough hydrogen ( few ppm ) from corrosion to fracture the spring . The spring was 25 years old and the original coating of oil/ grease was gone. I suggest oiling garage door springs as I have now done , to resist this corrosion / cracking.