Indentation loading studies of acoustic emission from temper and hydrogen embrittled A533B steel
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INTRODUCTION
REVERSIBLE temper embrittlement is the phenomenon whereby a quenched and tempered low alloy steel, upon tempering in (or slowly cooling through) the temperature range 300 to 600 ~ suffers an upward shift in ductile to brittle transition temperature, a reduction in upper shelf toughness, and a transition of low temperature fracture mode from transgranular cleavage to intergranular cracking.~'2 The effects have been linked to the co-segregation, during tempering, of certain trace impurities (P, Sn, Sb, As) with alloying elements such as nickel. It is a potential concern in all low alloy steels that are tempered in the critical temperature range during either fabrication or service. Temper embrittlement can also enhance considerably the susceptibility of a steel to hydrogen embrittlement. Acoustic emission is the name given to the elastic waves generated by certain types of deformation, fracture, and phase transformation processes in solids. The elastic waves are generated by sudden local rearrangement of the internal stress (or strain) field. The properties of the generated elastic wave field (e. g., amplitude and frequency spectrum) are controlled by the magnitude and dynamics of individual deformation or fracture events. In particular, it has been shown that for crack growth sources, crack increments of - 2 to 3/xm at average speeds of - 5 0 0 ms -l are required to generate detectable signals) In A533B in the quenched and
R.B. CLOUGH and H. N. G. WADLEY, the latter currently on leave from Metallurgy Division, Atomic Energy Research Establishment, Harwell, England, are both with the National Bureau of Standards, Washington, DC 20234. Manuscript submitted February 11, 1982. METALLURGICAL TRANSACTIONS A
650 ~ tempered state, it has been found that almost no detectable acoustic emission is associated with the propagation of ductile dimple fracture (some is detected from MnS debonding/cracking). 4 In other low alloy steels, temper and hydrogen embrittlement has been found to generate extremely energetic acoustic emission signals indicative of many quasi-brittle microfractures prior to failure. 5 It would thus seem that monitoring acoustic emission activity should be a very sensitive method of detecting the development of temper embrittlement in A533B. In the majority of acoustic emission experiments, acoustic emission has been observed during simultaneous measurement of mechanical properties on standard tensile or crack growth geometry samples. The advantage of this is that the macroscopic stress state at the moment of emission is fairly well characterized in these samples, making it somewhat easier to interpret the emission data. However, because of the large volume of material undergoing deformation it is very difficult to correlate, using metallographic techniques, a particular acoustic emission with a particular source. A major step in understanding acoustic emission would be independent observation of the emission source. The development of an indentation method of acoustic emission testing 6'7'
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