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Abstract-Industrial multi-pass TIG weldments of HP 9-4-20 high strength alloy steel have been found to contain significant volume fractions (around 10%) of retained austenite which are not readily transformed after stress relieving and subsequent refrigeration procedures. To determine whether the presence of such retained austenite in tempered martensitic structures could be detrimental to fatigue resistance in HP9-4-20 steel, fatigue crack propagation behavior was examined over six orders of magnitude in growth rate, in commercially heattreated material (containing less than 3% austenite) and in intercritically heat-treated and tempered material (containing approx. 14% austenite) in an environment of moist, ambient temperature air. Whereas crack propagation rates were unchanged at growth rates exceeding mm/cycle, structures containing 14% austenite showed somewhat superior resistance to near-threshold crack propagation at growth rates less than mm/cycle, the threshold for crack growth (AK,) being over 20% higher than in commercially heat-treated material. The presence of retained austenite further appeared to inhibit the occurrence of intergranular fracture at near-threshold levels. It was concluded that significant proportions of retained austenite are not detrimental to fatigue crack propagation resistance in HP 9 -4 2 0 steel, and may indeed have some beneficial effect at very low, near-threshold growth rates by increasing resistance to environmentally-assisted cracking. INTRODUCllONHP 9-4-20 ALLOY STEEL is a high strength, high toughness, secondary hardening steel capable of being welded in the heat-treated condition without loss in toughness [ 13. The steel contains 9% Ni, 4%co and 0.2% C, and is generally heat-treated to a tensile strength of between 1300 and 1500 MPa by double austenitizing and tempering procedures. Due to its excellent toughness/strength characteristics, the steel has found many applications in military aircraft structural components [2].As-quenched martensitic structures of this alloy generally contain less than 3% austenite, retained after austenitizing, which is readily removed commercially by refrigeration procedures prior to tempering. However, microstructural analysis of industrial TIG weldments of HP 9-4-20 has revealed unusually large amounts of austenite, as high as lo%, in stress-relieved weld and heat-affected zone areas, which are only partially removed by subsequent refrigeration. This increased volume fraction of austenite is thought to arise from austenite reversion in the intercritical temperature range when the weld is cycled through the two-phase ( y + a ) region during multi-pass welding, and also during subsequent stress-relieving heat-treatments above 500°C.In view of the fact that such large volume fractions of austenite may persist in HP 9 -4 20 weldments, investigations were undertaken to examine such weldments and to 107