Howard G. Nelson

Embrittlement of a ferrous alloy in a partially dissociated hydrogen environment

Gaseous hydrogen embrittlement of quenched and tempered 4130 steel was studied as a function of temperature from −42° to 164°C in a partially dissociated hydrogen environment at low molecular hydrogen pressures (≈8 × 10−3 torr). Atomic hydrogen was created by dissociation of molecular hydrogen on a hot tungsten filament located near a crack opening. The presence of atomic hydrogen was found to increase the rate of hydrogen-induced, slow crack growth by several orders of magnitude and to significantly alter the temperature dependence of embrittlement from what is observed in the presence of molecular hydrogen alone. Based on a previous study, these observations are interpreted in terms of a difference between the hydrogen-transport reaction step controlling hydrogen-induced, slow crack growth in the molecular hydrogen and the atomic-molecular hydrogen environments. Finally, a comparison is made between the kinetics of hydrogen-induced, slow crack growth observed in the presence of atomic-molecular hydrogen and the kinetics of known, possible hydrogen-transport reactions in an effort to identify the reaction step controlling hydrogen embrittlement in the presence of atomic hydrogen.

Environmental hydrogen embrittlement of an α-β titanium alloy: Effect of microstructure

Environmental hydrogen embrittlement of a Ti-6 Al-4 V alloy has been studied as a function of test displacement rate and of variations inα- β microstructure. Embrittlement in low pres sure (∼1 atm) gaseous hydrogen was inversely dependent on test displacement rate and strongly dependent on microstructure. At a given displacement rate, microstructures having a continuous α-phase matrix were less severely embrittled than those having a continuous β-phase matrix. Further, brittle fracture occurred in the former microstructures by transgranular cleavage and in the latter microstructures by intergranular separation. These observations are consistent with previous studies made on slow strain-rate embrittlement of hydrogen-charged titanium alloys and are explained in terms of relative hydrogen transport rates within the α-phase and β-phase titanium.

Gaseous hydrogen-induced cracking of Ti-5Al-2.5Sn.

The kinetics of hydrogen-induced cracking have been studied in the Ti-5Al-2.5Sn titanium alloy having a structure of acicular α platelets in a β matrix. It was observed that the relationship between hydrogen-induced crack growth rate and applied stress intensity can be described by three separable regions of behavior. The crack-growth rate at low stress-intensity levels was found to be exponentially dependent on stress intensity but essentially independent of temperature. The crack-growth rate at intermediate stress-intensity levels was found to be independent of stress intensity but dependent on temperature in such a way that crack-growth rate was controlled by a thermally activated mechanism having an activation energy of 5500 cal per mole and varied as the square root of the hydrogen pressure. The crack-growth rate at stress-intensity levels very near the fracture toughness is presumed to be independent of environment. The results are interpreted to suggest that crack growth at high stress intensities is controlled by normal, bulk failure mechanisms such as void coalescence and the like. At intermediate stress-intensity levels the transport of hydrogen to some interaction site along the α-β boundary is the rate-controlling mechanism. The crack-growth behavior at low stress intensities suggests that the hydrogen interacts at this site to produce a strain-induced hydride which, in turn, induces crack growth by restricting plastic flow at the crack tip.

Phase relations in the Fe Ni Cr S system and the sulfidation of an austenitic stainless steel

AbstractThe stability fields of various sulfide phases that form on Fe-Cr, Fe-Ni, Ni-Cr, and Fe-Cr-Ni alloys have been developed as a function of temperature and the partial pressure of sulfur. The calculated stability fields in the ternary A-B-S system are displayed on plots of log

Influence of temperature and the role of chromium on the kinetics of sulfidation of 310 stainless steel

{\text{p}}_{S_2 }

Embrittlement of 4130 steel by low-pressure gaseous hydrogen

vs. the conjugate extensive variable (nA/nA−nB), which provides a better framework for following the sulfidation of Fe-Cr-Ni alloys at high temperatures. Experimental and estimated thermodynamic data were used in developing the sulfur potential diagrams. Current models and correlations were employed to estimate the unknown thermodynamic behavior of solid solutions of sulfides and to supplement the incomplete phase-diagram data of geophysical literature. These constructed stability field diagrams are in excellent agreement with the sulfide phases and compositions determined experimentally during the sulfidation of SAE 310 stainless steel. The sulfur potential plots appear to be very useful in predicting and correlating the sulfidation of commercial alloys.

Corrosion of 310 stainless steel in H2-H2O-H2S gas mixtures: Studies at constant temperature and fixed oxygen potential

A study has been conducted of the terrace-like fracture morphology of gaseous hydrogen-induced crack growth in acicular alpha-beta titanium alloys in terms of specimen configuration, magnitude of applied stress intensity, test temperature, and hydrogen pressure. Although the overall appearance of the terrace structure remained essentially unchanged, a distinguishable variation is found in the size of the individual terrace steps, and step size is found to be inversely dependent upon the rate of hydrogen-induced slow crack growth. Additionally, this inverse relationship is independent of all the variables investigated. These observations are quantitatively discussed in terms of the formation and growth of a thin hydride film along the alpha-beta boundaries and a qualitative model for hydrogen-induced slow crack growth is presented, based on the film-rupture model of stress corrosion cracking.

Stability of chromium (III) sulfate in atmospheres containing oxygen and sulfur

The sulfidation of 310 stainless steel was studied over the temperature range from 910 to 1285° K. By adjusting the ratio of hydrogen to hydrogen sulfide, variations in sulfur potential were obtained. The effect of temperature on sulfidation was determined at three different sulfur potentials: 39 N·m−2, 1.4×10−2 N·m−2, and 1.5×10−4 N·m−2. All sulfide scales contained one or two surface layers in addition to a subscale. The second outer layer (OL-II), furthest from the alloy, contained primarily Fe-Ni-S. The first outer layer0 (OL-I), nearest the subscale, contained Fe-Cr-S. The subscale consisted of sulfide inclusions in the metal matrix. Two different phases were observed in OL-II depending on the temperature and sulfur potential. Below 1065°K OL-II is composed of a mixture of monosulfides of iron and nickel (Fe Ni)1−xS and pentlandite (Fe4.5Ni4.5S8) with the pentlandite phase exsolved as lamellae upon cooling. At temperatures higher than 1065°K only the pentlandite phase was formed, which melted above 1145°K at sulfur potentials greater than 10−2 N·m−2, yielding metal-rich iron-nickel-sulfur. Above 1145°K, and at sulfur potentials less than 10−2 N·m−2, OL-II ceased to exist (this temperature is termed transition temperature). Below the transition temperature, where OL-II exists, OL-I could be represented by the general composition (Fe, Cr)1−xS. This phase on cooling transformed into an array of structures differing in Fe∶Cr ratio. These substructures, however, were not observed in quenched samples. Above the transition temperature OL-I changed to a chromium-rich sulfide composition and was associated with a sudden decrease in reaction rate. Subscale formation was found to be due to the dissociation of OL-I at the scale-metal interface, and the extent of subscale growth was found to depend on the temperature and the sulfur potential, as well as the composition of OL-I. At a given temperature and sulfur potential the weight-gain data obeyed the parabolic rate law after an initial transient period. The parabolic rate constants obtained at the sulfur potential of 39 N·m−2 did not show a break when the logarithm of the rate constant was plotted as a function of the inverse of absolute temperature. Sulfidation carried out at a sulfur potential below 2 × 10−2 N·m−2, however, did show a break at 1145°K. This break was found to be associated with the changes which had occurred in the Fe∶Cr ratio of OL-I. Below the transition temperature the activation energy was found to be approximately 125 kJ · mole−1. Above the transition temperature the rate of sulfidation decreased with temperature but depended on the Fe∶Cr ratio in the ironchromium-sulfide layers of the OL-I. A reaction mechanism consistent with the experimental results has been proposed in which the diffusion of cations through OL-I is the rate-controlling step. Below the transition temperature the diffusion of Fe and Ni through OL-I contributes to the scale formation, whereas above the transition temperature the diffusion of Cr through OL-I controls the scale formation. Existing literature on the Fe-Ni-S system is compared with the present results.