Abstract
A series of four 4130 base steels with various phosphorus concentrations was subjected to cathodic charging to determine the effect of P on hydrogen stress cracking resistance. Static fatigue curves for several different yield strengths were obtained for each alloy. At high yield strengths under applied loads of 60 to 80 pct of the yield, 50 ppm P (bulk concentration) was enough to provide sufficient grain boundary P for an impurity-hydrogen interaction which produced intergranular fracture along prior austenite grain boundaries. Decreasing yield strength and applied stress caused a transition in fracture mode to transgranular while the resistance to hydrogen stress cracking increased with decreasing P. Microhardness measurements of prior austenite grain boundaries were made to establish the role of P. The role of P is not apparently related to its capacity as a strengthening element but more probably as a hydrogen recombination poison. Grain boundary hardness measurements for low temperature tempers (200 °C) appear to be valid while those at 500 °C were not.
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