Hello Marion;
Short of going to the web page referenced by Gunther, ASTM uses the prefix letter A to indicate ferrous metals and the letter B to indicate nonferrous metals. ASME uses the prefix SA fopr ferrous metals and SB for nonferrous metals.
As stated by the other respondent, in general the numbers have no meaning. It is just a coincident that A36 carbon steel has a yield strength of 36,000 psi. You have to refer to the ASTM standards to find out what the mechanical properties are of the material in question. Many base metal specifications contain several alloys that have different strength levels that are indicated by grades or classes. Two examples would be ASTM A572 Gr 50 and A53 Grade B. In the case of ASTM A572 grade 50, the grade 50 indicates the yield strength is 50,000 psi and the tensile strength is 65,000 psi. ASTM A53 Grade B is carbon steel pipe that has a yield strenght of 35,000 psi and a tensile strenght of 60,000 psi. As you can see, in the examples listed, the number of the material specification doesn't tell you much. You have to read the material specification contained in the ASTM standard to find out all the information about the materil.
Another good example is ASTM A312. That specification covers stainless steel pipe and contains the chemistry and properties of the common austenitic stainless steel alloys such as 304, 316, 321, etc. All of the alloy grades have different chemistries, i.e., % of chrome, % of nickel, % of carbon, etc, but they are all contained in specification A312.
Best regards - Al
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