Abstract:
The microstructure of high strength steel is
susceptible to delayed failure caused by the absorption of
hydrogen produced either during cathodic charging or
electroplating. When 0.8%C and AISI 4340 steel are
subjected to constant load testing, a wide range of failure
times is observed. By applying Weibull statistics small
changes in experimental parameters such as heat-treatment
are detected readily and are explained in the terms of
hydrogen trapping at microstructural defects.
During the electroplating of steel in a double-cell,
quantitative measurements are made of the amount of
hydrogen permeated. Current densities are measured in the
range 2- 40 mAcm2 and it is shown that, although the
lowest current density produces the most mechanically sound
plate, it also causes the largest amount of hydrogen
absorption. The nickel deposit is found to act as a
reservoir for reversibly trapped hydrogen allowing
diffusion to continue into the steel after the cessation of
plating.
Permeation measurements were taken on AISI 4340 steel
using an electrochemical probe developed from the Barnacle
Electrode. The effects of cathodically charging and
electroplating with nickel are compared. Exposure of the
steel to the atmosphere is shown to have an important
influence on the hydrogen content after a period of time
due to a limited occurrence of corrosion. Various post-
plating treatments are commonly used to remove a damaging
concentration of hydrogen and the quantitative effects of
such treatments are described.
Finally, a mathematical model is proposed which
explains the reason for the wide spread of delayed failure
times. It is found that if the stress intensity necessary
to initiate a crack is known and, provided either the
nominal stress or the crack size is known, it is possible
to calculate either the allowable defect size or the
allowable stress below which cracking is not expected to
occur.
