MICROSTRUCTURE STUDIES ON M250 MARAGING STEEL WELDMENTS IN RELATION TO ACOUSTIC EMISSION
T. Chelladurai, R. Krishnamurthy*, P. Ramesh Narayanan, and A. R. Acharya Vikram Sarabhai Space Centre, Trivandrum - 695 022, India *Indian Institute of Technology, Madras - 600 036, India
The use of M250 maraging steel for aerospace pressure chambers has led to an increased emphasis on the study of brittle fracture. This requirement has led to the need for a more stringent and efficient NDT method of inspection which shall detect flaws associated with brittle fracture smaller than the size detectable by radiography or ultrasonics. Acoustic Emission (AE) technique satisfactorily meets the above requirement in view of its unique potential for detecting and characterising micro level active discontinuities. Comprehensive studies have been carried out on a large number of M250 maraging steel tensile specimens bearing inserted part through cracks [1], also called precracks. Although the AE response has been significant on the welded specimens, there has been quite a perceivable extent of scatter int the AE response data. An attempt has been made to investigate these variations. Scanning electron micro graphs aided the investigation indicating microstructure changes in the first (transition zone) and second extended zones. Presence of spherical particles containing oxides of iron, nickel and titanium seems to be one of the sources for the variations. Presence of brittle patches and dimples and their orientation represent brittle or ductile mode of failure as the case may be with the pertinent variations in AE response. The sizes of dimples also largely influence the AE response. The study covers specimens bearing almost same size of inserted surface cracks with d/t (depth of crack/specimen thickness) ratio around 11/80. A very good correlation is found between the microstructure variations exhibited by the crack surface and Acoustic Emission response. The details of the aforesaid studies along with the relevant microstructure are presented concisely in the paper.
REFERENCE
T. Chelladurai, "Studies on Structural Integrity of M250 Maraging Steel Pressure Chambers Through Acoustic Emission", Ph. D Thesis, IIT Madras, 1992
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 4, pages 2399 - 2404 Publisher:Ashgate Publishing Company
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