INFLUENCE OF RETAINED AUSTENITE ON FATIGUE CRACK-PROPAGATION IN HP9-4-20 HIGH-STRENGTH ALLOY-STEEL

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 HP 9‐4‐20 steel, fatigue crack propagation behavior was examined over six orders of magnitude in growth rate, in commercially heat‐treated 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 10−6 mm/cycle, structures containing 14% austenite showed somewhat superior resistance to near‐threshold crack propagation at growth rates less than 10 −6 mm/cycle, the threshold for crack growth (ΔK0) 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 9‐4‐20 steel, and may indeed have some beneficial effect at very low, near‐threshold growth rates by increasing resistance to environmentally‐assisted cracking. Copyright © 1979, Wiley Blackwell. All rights reserved