Enhanced stress corrosion resistance from steels having a dual-phase austenite-martensite microstructure
- PDF / 1,806,323 Bytes
- 8 Pages / 603.28 x 788 pts Page_size
- 102 Downloads / 154 Views
I.
INTRODUCTION
IN many engineering applications the use of ultra high strength steels is limited by their high susceptibility to stress corrosion cracking in aqueous environments, this being reflected in very low values of the threshold stress intensity factor, KrH. In recent years, considerable progress has been made in increasing the air fracture toughness, Ktc, by a combination of improved steel making techniques and compositional modifications. Unfortunately, these improvements in fracture toughness have not been reflected in a corresponding improvement in KTH. The basic problem is that the martensitic microstructure that is essential for high strength is inherently susceptible to embrittlement by hydrogen which arises from the corrosive action of the environment. One of the potential solutions to this problem is to develop steels having dual-phase microstructures in which the separate constituents are responsible for the different property requirements. A tempered martensite phase must remain as the source of strength. However, an obvious choice of constituent for conferring fracture resistance is austenite. Being a low strength material with a high work hardening capacity, austenite is inherently tough. More importantly, hydrogen has a high solubility in austenite, but does not give rise to significant embrittlement. Consequently, an appropriate dispersion of austenite has the potential to act as both a mechanical crack arrestor and a trap for hydrogen. Previous investigations of the effect of austenite on fracture resistance have been concerned mainly with the air fracture toughness. For example, investigations of ferriteaustenite mici'o-duplex structures in stainless steels demonstrated a beneficial influence of austenite. L2 Similarly, in a high strength martensitic steel, a direct correlation was established between Kzc and the volume fraction of austenite. 3 The latter study included measurements of resistance to stress corrosion cracking, and it is interesting to note that the KrH in NaCI solution also increased with austenite content. Both Kzc and Krn were observed to decrease as tempering temperature increased, this being attributed to the decomposition of the metastable austenite. A1T. V. VENKATASUBRAMANIAN, Research Assistant, and T. J. BAKER, Lecturer, are both with the Department of Metallurgy and Materials Science, Imperial College of Science and Technology, Prince Consort Road, London SW7 2BP, United Kingdom. Manuscript submitted October 25, 1982.
METALLURGICALTRANSACTIONS A
though no correlations were established between austenite content and impact transition temperature, it was observed that the extent of cleavage fracture decreased as the austenite content increased. Similar duplex austenitemartensite microstructures have been produced by thermal cycling in a maraging steel and again increased Klc values were obtained. 4 The use of high austenitizing temperatures in steels of the AISI 4340 type can also lead to improved Kzc in the as-quenched and lightly tempered condition. 5 Such treat
Data Loading...
