Patent ID: 7374622

Claim:
A method for making a structural member, consisting of the steps of: alloying iron with carbon in the range of approximately 0.15-0.20 wt. %, manganese in the range of approximately 1.00-1.80 wt. %, silicon in the range of approximately 1.00-1.60 wt. %, chromium in the range of approximately 1.50-2.50 wt. %, nickel in the range of approximately 2.50-3.50 wt. %, molybdenum in the range of approximately 0.40-0.70 wt. % and boron in the range of approximately 0.0025-0.005 wt. %; casting said structural member; and austempering said structural member to impart a bainitic structure in said structural member, said austempering step consisting of heating said casting to a first temperature, whereby said casting exhibits an austenite phase, rapidly quenching said casting to a second temperature below the pearlite formation range and above the martensite formation range, maintaining said casting at said second temperature until said casting exhibits said bainitic structure, and subsequently quenching said casting to room temperature; wherein said structural member has a resulting fracture toughness of at least approximately 100 MPa√m.