The 4450 lb weight BLU-122 deep penetrating bomb was developed by the U.S. Air Force in 2003 making improvements upon the BLU-113 penetrator which would enable it to have significant penetration through hard targets. The weapon incorporates ~ 780 lb of energetic explosive filling, a higher strength case made from Eglin steel with an ultimate tensile strength of 245-255 ksi, impact toughness of 11.5-15.5 ft-lb @ -40°F, and a modified nose shape for increased penetration. Despite the improved case, the BLU-122 possesses only 18-20 ft penetrability in 5000 psi strength reinforced concrete.
The U.S. Air Force is moving ahead to get a new 2000-lb class bunker-buster bomb filled with 530 lb of explosive material, built around an improved warhead called the BLU-137, which has a steel case made from the USAF-96 steel. This bomb will ultimately replace existing weapons in this category, which have already been a key part of recent engagements. The BLU-137 is intended to provide improvements in capability and survivability over the BLU-109 bomb. The BLU-109 has a steel case made from 4330 modified steel with an ultimate tensile strength of 245-255 ksi and Charpy v-notch impact toughness energy of 20 ft-lb @ -40°F.

New developed high strength, high impact toughness steel (Military-Steel) is a new material for cases of deep penetrating bombs (penetrators).

The table shows the typical room temperature ASTM standard tensile test results and Charpy v-notch impact test results of the air melted quenched and low tempered Military-Steel, Eglin-steel, and USAF-96 steel.

Military-Steel is superior to Eglin steel and USAF-96 steel due to:

• Higher hardness and strength at the same ductility and toughness
• Better formability at hot working and better machinability
• Significantly lower concentrations of Cr, Mo, and elimination of W.

Also, Military-Steel is applicable for helicopter and automotive gears, shafts, and powertrain components due to its high strength and high impact toughness.