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ARP2000 is a heavily alloyed martensitic quench and temper steel. It has excellent stability at high temperatures. But most important, ARP research discovered that in addition to temperature stability it has excellent notch toughness in the higher strength ranges and is alloyed to be tempered to Rockwell C45/47. 8740 and 4340 can be tempered to the same hardness. But, the tempering temperature would yield material in the "temper brittle zone" (between 500°F and 700°F), producing significant notch sensitivity. ARP2000 is tempered above that temperature range and has a strength between 200,000 and 220,000 psi.