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The hydrogen trapping characteristics of 13CrNiMo martensitic steel weld metals, with different austenite contents resulting from different post-weld heat treatments, have been analysed. Scanning electron microscopy and X-ray diffraction have been used to study stable austenite resulting from intercritical tempering of these soft martensitic stainless steels weld metals. Austenite contents up to 25 vol.% have been obtained. Hydrogen diffusion and permeation coefficients have been obtained from an analysis of the permeation rate of hydrogen through these materials. A decrease of the hydrogen apparent diffusion coefficient is observed when the tempering temperature is increased in the range 400 ° C ⩽T⩽670 ° C ; this decrease is attributed to changes in the martensitic matrix as well as to the increase of austenite content. The role of the austenite phase on trapping is discussed.

In previous works, extensive desulphurisation, including shape control and low hardness in segregation zones, were pointed out as effective ways for increasing resistance to hydrogen induced cracking (HIC) in steels exposed to wet H2S environment. In this study, the behaviour of eight plates was evaluated by immersing samples in H2S-saturated NACE solution. HIC susceptibility was assessed by an ultrasonic method and correlated with the elongated MnS inclusion distribution. The results allow us to remark that an extremely low sulphur level is not necessary to reach good HIC resistance, provided that no hard bands are present in the steel. Under this condition, a relationship to calculate the minimum average sulphide length which provides low HIC susceptibility was found.

The effects on the hydrogen embrittlement (HE) due to variation of the delta ferrite contents were investigated on austenitic stainless steel welds. The hydrogen distriburion in the microstructure was studied by means of the hydrogen microprint technique. The damage caused by the hydrogen was evaluated through bending and tensile tests. The ferrite-austenite interfaces act as hydrogen traps. Ferrite increases the susceptibility to hydrogen, the greater the quantity of ferrite, the more HE, which basically modifies the form in which the hydrogen damages the material: it causes the austenite to change from a normal fragile fracture to a ductile fracture, accompanying the ferrite fragile fracture.

The kinetics of carbide precipitation during the tempering of low-carbon martensite at 453K was studied by means of the hydrogen-permeation method. The hydrogen diffusivity obtained by this technique attained a minimum value in the quenched specimens and increased with increasing tempering time to reach a constant value between 3 and 6h. This change in the hydrogen diffusivity is attributed to modifications in hydrogen trapping due to carbide precipitation, and the kinetics of precipitation are described by a Johnson-Mehl or Avrami equation such that the precipitated carbide fraction is given by f(t) = 1 − exp(−0.06t0.36). Hydrogen-trapping modifications are explained in terms of the generation of a plastic zone when a local volume change occurs.

The surface carbide layer, interior chain-like carbide arrangements and the carbide/matrix interfaces present in an initial Zr-2.5Nb alloy were studied in hydriding and oxidation experiments. Hydriding performed by cathodic charge in 0.1 M H 2 SO 4 solution showed that neither the inner carbide chains nor the corresponding interfaces offered short circuit paths for hydrogen diffusion, while the surface carbide layer acted as a protective layer. The oxidation tests performed in autoclaves during 24 h in deaerated steam at 400°C and 105 bar showed weight gains higher than one order of magnitude with respect to the samples free of carbides. The surface carbides promoted the accelerated oxidation of the surrounding matrix and were stripped during the experiments. Evidence was found of the oxidation of the inner carbides, probably due to the transformation strain energy produced by the former oxidation of the surrounding matrix. Moreover, the carbide/ matrix interfaces acted as short circuit paths for oxygen diffusion.

The effect of the microstructure on the hydrogen embrittlement susceptibility, hydrogen mobility and hydrogen distribution in a low carbon steel was studied by means of the electrochemical permeation method (electrochemical and gas charging), mechanical tests and hydrogen microprint technique. The hydrogen diffusivity attains a minimum value in a fresh martensite and diffusivity increases with increasing tempering temperature. The quantity of desorbed hydrogen is at a maximum for a fresh martensite and decreases with increasing tempering temperature. The trapping energy was estimated to be ∼ 46 kJ mol −1 . There is a direct relationship between hydrogen embrittlement susceptibility and the quantity of desorbed hydrogen as well as between the hydrogen segregation and crack location. The steady state flux is maximum for a normalized state. These facts can be explained in terms of hydrogen trapping process.

Young's moduli of AISI 1005 and 1070 steels, in different metallurgical states, were measured before and after hydrogen charging by means of the classical resonant bar method. Different decreases in Young's modulus were found which could be correlated with the microstructural state of the steel. The maximum diminution of elastic modulus was observed for the martensitic structure of 1070 steel (−1.7%) and the minimum in 1005 steel in the normalized state (−0.4%). Moreover a linear relationship was established between the change of Young's modulus from the effect of hydrogen and the hydrogen embrittlement susceptibility of these steels.

Shot-peening surface treatments were performed on a low-carbon steel and a 304 stainless steel. The influence of residual stresses on hydrogen permeation and distribution was investigated. The results were correlated with the tensile properties measured in air after cathodic charging and in a NACE medium under slow load-rate straining. The role of shotpeening on the hydrogen embrittlement of the low-carbon steel is strongly dependent on the severity of the hydrogen environment and testing conditions. The detrimental effect of shotpeening on the hydrogen embrittlement of the stainless steel is related to phase transformations induced by cold work.

The effect of the microstructure on the hydrogen embrittlement susceptibility, hydrogen mobility and hydrogen distribution in a low carbon steel was studied by means of the electrochemical permeation method (electrochemical and gas charging), mechanical tests and hydrogen microprint technique. The hydrogen diffusivity attains a minimum value in a fresh martensite and diffusivity increases with increasing tempering temperature. The quantity of desorbed hydrogen is at a maximum for a fresh martensite and decreases with increasing tempering temperature. The trapping energy was estimated to be ∼ 46 kJ mol −1 . There is a direct relationship between hydrogen embrittlement susceptibility and the quantity of desorbed hydrogen as well as between the hydrogen segregation and crack location. The steady state flux is maximum for a normalized state. These facts can be explained in terms of hydrogen trapping process.

In the present work the hydrogen embrittlement susceptibility in austenitic stainless steel weld metals is investigated. The role of δ ferrite and sulphur segregation in hydrogen cracking is pointed out. The influence of austenitic stability is also analyzed.