Which expands more when heated to the boiling point of water, aluminum or steel?

The “CTE” or Coefficient of Thermal Expansion is expressed as an AVERAGE rate over a
specific range of temperature…. In common English units it will be …micro inches per inch per
degree Fahrenheit of temperature change.

The typical CTE for 1018 steel is 6.7 microinches per inch and for 1100 aluminum it is 13.2
….almost twice as much over our range of temperature from 70 deg F to 212 deg F.

CTE is dependent on the metal / alloy and is not constant with temperature so be sure you are
using a value that represents both the alloy and the temperatures you are interested in.
general, CTE increases with increasing temperature.