Abstract: The invention relates to an alloy steel with 0.3 to 1.0% carbon, 0.2 to 2.5% silicon, up to 0.8% manganese, 30.0 to 48.0% nickel, 16.0 to 22.0% chromium, 0.5 to 18.0% cobalt, 1.5 to 4% molybdenum, 0.2 to 0.6% niobium, 0.1 to 0.5% titanium, 0.1 to 0.6% zirconium, 0.1 to 1.5% tantalum and 0.1 to 1.5% hafnium, balance more than 20% iron when the cobalt content is at least 10% and more than 30% iron when the cobalt content is less than 10%. The steel is particularly suitable for use as a heat resistant and high hot strength material for parts, in particular pipes, of petrochemical cracking furnaces for the production of ethylene or synthesis gases.