The distribution of palladium in a Pd-modified 4130 steel
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I.
INTRODUCTION
C A R B O N or low-alloy steels exposed to wet hydrogen sulfide or gaseous hydrogen at elevated temperatures are susceptible to the formation of fine cracks even in the absence of applied external stresses. It was found that the addition of palladium above a critical amount reduces or eliminates this hydrogen induced degradation. ~'2'3 The mechanism responsible for this beneficial effect is not well understood. It has been suggested that the ferritecementite interfaces in the steel are modified by the palladium increasing their capacity to adsorb and trap hydrogen. 1,2 Lumsden, Wilde, and Stocker suggested that this effect is due to the segregation of palladium to manganese sulfide inclusions. 3 These authors proposed that the palladium increases the cohesive strength of the MnS-matrix interface thereby raising the crack initiation stress. To support this hypothesis, a correlation of the spatial distribution of S and Pd was found in the Auger map of a fracture surface of a 4130 steel containing 1 wt pct Pd. Since little is known about the distribution of palladium in Pd-modified 4130 type steels, the present study was undertaken to characterize the microstructure and microchemistry of this steel using the high resolution atom probe field-ion microscope (APFIM) in conjunction with the more conventional transmission electron microscope (TEM). Details of the atom probe and the types of analyses that may be performed with the instrument may be found in Reference 4. It should be noted that the atom probe is uniquely suited to determine the chemistry of unembrittled boundaries with near atomic spatial resolution. The emphasis of the study was to determine the distribution of palladium in the various phases and at internal interfaces rather than on developing a model to explain hydrogen crack inhibition.
II.
EXPERIMENTAL
The materials used in this investigation were laboratory melted AISI 4130 steels, modified with palladium additions of 0.31 at. pct and 0.65 at. pct. The bulk chemical compositions of the two alloys are presented in Table I. All compositions are expressed in atomic percent except where M.K. MILLER is with Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. S. S. BRENNER and M. G. BURKE are with the Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261. Manuscript submitted November 18, 1983. METALLURGICALTRANSACTIONS A
Table I. Chemical Composition of Palladium-Modified 4130 Steels
Palladium Carbon Chromium Manganese Molybdenum Silicon Sulfur Phosphorus Iron
0.65 Pct Pd Alloy At. Pct Wt Pct 0.65 1.25 1.57 0.34 1.25 1.13 0.59 0.59 0.11 0.18 0.61 0.31 0.022 0.013 0.014 0.008 balance balance
0.3 Pct Pd Alloy At. Pct Wt Pct 0.31 0.60 1.56 0.34 1.24 1.17 0.61 0.61 0.11 0.18 0.63 0.32 0.022 0.013 0.014 0.008 balance balance
noted. The steels were tempered for 2 hours at 650 ~ after quenching from the austenite region. All field-ion micrographs were obtained using neon image gas with a specimen temperature of approx
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