I only impersonate a metallurgist on DoubleGunBBS, only know what I read (which requires the willingness to do the research), and what I have been told by genuine metallurgical, materials, and mechanical engineers.
Choosing a steel that is suitable for a shotgun barrel (or any application) has multiple factors:
Ultimate tensile strength (when it breaks)
Yield strength and % elongation (measure of ductility)
Brittleness (the opposite of ductility – a brittle metal has a low % elongation before fracture)
Impact resistance/hardness
Corrosion resistance
And some other stuff I don’t understand
Early “steel shot” (which is annealed iron) had a Brinell hardness of 70 – 110
The standard now is 95 DPH or Brinell 79
Lead shot has a Brinell hardness of about 12
A pre-WWI shotgun barrels of AISI 1020 has a (NON HEAT TREATED) tensile strength of about 62,000 psi with a Brinell hardness of about 120
A post-1920s shotgun barrel of AISI 1040 has a tensile strength of about 90,000 psi and Brinell hardness of 197
AISI 2340 Nickel steel has a tensile strength of about 110,000 psi and Brinell hardness of 194
AISI 4140 Chrome Moly has a tensile strength of about 100,000 psi and Brinell hardness of 197
A single specimen of Winchester Nickel Steel was non-standard AISI 2340 with a tensile strength of 124,000 psi (suggesting some form of heat treatment)
A single specimen of Winchester Proof Steel (Chrome Molybdenum alloy) was non-standard AISI 4135 with a tensile strength of 107,000 psi
Steel alloys (the extremely short version)
Carbon – Higher C increase hardness, tensile strength, and hardenability with heat treatment
Manganese - prevents the formation of iron sulfide inclusions, and increases the hardenability
(Manganese sulfide improves machinability but inclusions weaken the steel)
Chromium - increases the hardenability of steel and markedly improves the corrosion resistance
Molybdenum - increases hardenability and elevated temperature strength.
Nickel - increases hardenability, impact resistance, and ductility of the steel.
Chrome lining (done correctly) adds significant hardness and corrosion resistance
There is no doubt that steel shot can score “softer” shotgun barrels
"Wrinkling" and fracture at the choke area
It has also been suggested that steel shot can cause micro-fractures in the forcing cone
Few modern makers are willing to disclose the composition of their shotgun barrels. Beretta Steelium (NOT on the periodic table
) is a "proprietary tri-alloy blend". There is good data however that most (non-stainless steel) "steel shot compatible" barrels are something similar to 4140 or 4340.