Abstract: A weldable alloy steel having improved fracture toughness and stress corrosion resistance at very high strength levels in which the essential composition, according to percent by weight is in the range of 0.12 - 0.17% carbon, 1.8 - 3.2% chromium, 0.9 - 1.35% molybdenum, 11.5 - 14.5% cobalt, and 9.5 - 10.5% nickel, the remainder being substantially iron, i.e., with minor amounts of certain impurities and residual elements. Very good stress corrosion resistance and fracture toughness at high strength levels is produced when these elements are alloyed in the percent by weight ranges of 0.15 - 0.17% carbon, 1.8 - 2.2% chromium, 0.9 - 1.1% molybdenum, 13.5 - 14.5% cobalt, and 9.5 - 10.5% nickel. Good fatigue endurance is achieved. Minor amounts of manganese also may be present.