Hydrogen in metals: a nondestructive test
In many manufacturing and service industries, a need exists for a nondestructive test to determine the presence of hydrogen in a material system. The feasibility of such a system is examined here.
Acoustic emission activity resulting from a microhardness indentation is employed to detect hydrogen in A106 and 4340 steel bars following cathodic, gaseous, and chemical charging. These tests show a large increase in emission energy after charging followed by a drop to precharge levels with time. These activity levels are used to calculate hydrogen diffusivity and binding energy of hydrogen to traps in the steel. A mechanism of acoustic emission generation is proposed involving the breakaway of dislocations from Cottrell-like hydrogen atmospheres.
The effects of surface roughness and microstructure are also evaluated. Testing of various surfaces indicates that limited surface preparation is necessary prior to implementing the test procedure. Low activity levels before and after charging in 4340, and in martensitic and bainitic A106 indicate possible difficulties in applying the test to harder, more dispersed structures.
Despite this limitation and a large amount of scatter in the acquired data, the results indicate that acoustic emission monitoring of microhardness indentations may be of value in detecting the presence of hydrogen in metals and as a research tool in the study of hydrogen transport and embrittlement mechanisms.