Wu-Yang Chu

The concentration of hydrogen at crack tip of austenitic stainless steel after stress corrosion and polarization

Summary The hydrogen concentration at the crack tip and in the bulk under different experimental conditions were determined as summarized in Table 1. Although the concentration of hydrogen at the crack tip after either 3CC under open circuit conditions or under cathodic polarization is higher than that under anodic polarization, the cracking time of the latter is much shorter than that of the former. Although the hydrogen evolved during 3CC can enter and become enriched in the crack tip region, the amount of enrichment is not sufficient to cause cracking.

Effect of hydrogen on the Young's modulus of iron

A Bordoni type apparatus was used to measure the change of the apparent Youngs modulus ofα-Fe induced by hydrogen. The solution of the flexural vibration equation of a beam under stress indicatesE = C(σ)ω2. If the resonant frequencies of the first and the third tone are measured at about the same time,E andσ can be calculated. The change of the apparent Youngs modulus after charging with hydrogen is defined as ΔE = ΔE1(H) + ΔE2, where ΔE1(H) relates to the change of the perfect crystal interatomic cohesive force and ΔE2 is induced by the change of stress. An artificially partial stress relaxation test was carried out to measure ΔE2. The results show that during aging, after both charging with hydrogen and artificial stress releasing, the change of the apparent modulus is the same,i.e., ΔE = ΔE2. Thus, the ΔE1(H) associated with the interatomic cohesive force does not evidently change during aging with escaped hydrogen of 7 to 8 wppm at room temperature,i.e., this amount of hydrogen does not decrease the interatomic cohesive force ofα-Fe.

Stress corrosion cracking in high strength steel under mode III loading

Stress corrosion cracking (SCC) of high-strength steel in aqueous environment and hydrogen induced cracking (HIC) during dynamic charging under Mode III loading were investigated. The threshold stress intensities for SCC and HIC under Modes III and I were measured and compared. It was found that both SCC and HIC under Mode III loading initiated and propagated on the planes inclined at 45 deg to the notch plane, differing from that under Mode I loading. The fracture surfaces, however, revealed intergranular facets, similar to that under Mode I loading. The addition of thiourea decreased the threshold value for SCC under Mode III and Mode I loading, which was still higher than that for dynamic charging. The threshold values of both SCC and HIC under Mode III were larger than that under Mode I,i.e., KIIIH> KIH, KIIISCC > KISCC. Based upon the fracture mechanics analysis, this difference is attributed to the different equilibrium hydrogen concentration between Modes III and I loading. These results give strong evidence that the SCC mechanism in high strength steel under Mode III loading is also related to hydrogen induced cracking.

The mechanism of hydrogen induced cleavage in Fe-3 pct Si alloy

Hydrogen promotes the cleavage fracture of Fe-3 pct Si alloy. Hydrogen atmospheres moving alone with a dislocation will reduce the strain energy of the dislocation, resulting in decreasing the exte nal stress necessary to operate the Frank-Read source. Thus, hydrogen can promote dislocation mul tiplication, which has been verified using a dislocation decoration technique. Fractography and mechanics analysis indicate that a set of the [001] sessile dislocations on the [001] plane is just a cleavage microcrack. Hydrogen carried by the gliding dislocations will enter into the microcrack and the hydrogen pressure will augment the external stress and reduce the critical numbern necessary to form a stable cleavage crack and the numberm of the piled-up dislocations. This indicates that hydrogen promotes the formation of the stable cleavage crack before the other slip systems operate

Corrosion Fatigue Crack Initiation in a Mode II Notch Specimen

The stress intensityKII of a Mode II specimen was calculated using a finite element methodvia theJ integral. The site, direction, and the threshold value for crack initiation from the notch under cyclic Mode II loading in air, in water, and under dynamic charging with hydrogen were investigated. The results showed that the Mode II fatigue crack in a high strength steel initiated at or close to the site of the maximum principal stress, rather than at the site of the maximum shear stress, and the subsequent crack growth was oriented approximately normal to the direction of the maximum principal stress. The site and direction of crack initiation in water and under dynamic charging with hydrogen were similar but different from that in air. The threshold values for crack initiation in air, in water, and under dynamic charging were 28.8, 12.3, and 10.2 MPa m1/2, respectively. The fracture surface of a corrosion fatigue crack in water and under dynamic charging consisted of intergranular facets at low ΔKII values but of quasi-cleavage at higher ΔKII values and were different from those in air.

Hydrogen-Induced Delayed Fracture under Mode II Loading

Hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) of a high-strength steel 34CrNi3Mo (T.S = 1700 MPa) under Mode II loading were investigated using notched specimens. The stress field around the notch tip was analyzed by means of finite element method. The result shows HIC and SCC under Mode II loading initiated at the back of the notch tip,i.e., θ = -110 deg, where hydrostatic stress has maximum value. However, cracking is oriented along the shear stress direction at the site, not normal to the direction of maximum principal stress component. On the contrary, if the specimens are loaded to fracture in air under Mode II loading, cracking at the maximum shear stress site around the notch tip and the cracking direction coincide with the direction of the maximum shear stress. The above facts indicate that hydrogen induced delayed plastic deformation is a necessary condition for HIC, and the nature of SCC for high-strength steel in 3.5 pct NaCl solution is HIC. The results show that HIC and SCC under Mode II loading can occur during dynamic charging with hydrogen and in 3.5 pct NaCl solution, respectively. The normalized threshold stress intensity factors under Mode II loading during dynamic charging in 1 N H2SO4 + 0.25 g As2O3/L solution and in 3.5 pct NaCl solution are KIIH/KIIX = 0.1 and KIISCC/KIIX = 0.45, respectively. The corresponding values under Mode I loading are KIH/KIX = 0.02 and KISCC/KIX = 0.37, where KIIX and K,IX are critical values loaded to failure in air under Mode II and Mode I loading, respectively. Thus, (KIIH/KIIX)/ KIH/KIX) = 5 and (KIISCC/KIIX)/K,(ISCC/KIX) = 1.2. A typical intergranular fracture was observed during HIC and SCC under Modes II and I loading. But the fracture surfaces of specimens failed in air are composed of dimples for both kinds of loading.

Hydrogen Induced Cracking in Fe-3% Si Single Crystals under Mode 2 Loading

Abstract Hydrogen induced cracking (HIC) in Fe-3% Si single crystals under Mode 2 loading was investigated. The results revealed that hydrogen could promote cleavage fracture occurring on a (001) plane, regardless of what the orientation of tensile loading axis was. If it was along a [001] or [110] direction, the normalized threshold stress intensity (KIIH/KIIX) had a minimum value, while the maximum value was obtained when the tensile loading axis was nearly along [111]. Some hydrogen induced secondary cracks were found in which relative displacements normal and/or parallel to the cracking direction were evident on both sides of the secondary crack. This results from the unequal number of screw and/or edge dislocations slipping out or in both sides of the secondary crack; i.e., severe local plastic deformation occurs during hydrogen induced cleavage cracking. The reason that hydrogen promotes cleavage cracking was discussed.

Effect of hydrogen on the Young's modulus of commercial purity iron

Abstract 1. 1. The apparent Youngs modulus and elastic strain of commercial purity iron increased by 0.11% and 12.9%, respectively, during aging after severe charging. 2. 2. The increase of apparent Youngs modulus of commercial purity iron during aging after charging was nearly equal to that after artificial partial stress relaxation. That is to say, hydrogen does not significantly change the Youngs modulus associated with the interatomic cohesive force of commercial purity iron, even during aging after severe charging.

ROLE OF HYDROGEN IN STRESS CORROSION CRACKING OF AUSTENITIC STAINLESS STEEL IN BOILING 42% MgCl2 SOLUTION

ABSTRACT The role of hydrogen in stress corrosion cracking (SCC) of austenitic stainless steels in boiling chloride solution was investigated. It was found that the threshold stress intensities for SCC of both type 321 and type 310 steel were much lower than that for hydrogen induced cracking (HIC) during dynamic charging at high fugacity at room temperature and 160°C. The morphologies of SCC fracture surface also differed from those of HIC. In addition, anodic polarization promoted the SCC in boiling LiCl solution, while cathodic polarization with small current density retarded the SCC. Hydrogen entering into the specimen via precorroding in boiling MgCl 2 solution with pH=1 for 200 hours was about 32 wppm and it could cause elongation reduction but no HIC. However, there existed a critical hydrogen concentration during dynamic charging in 1N H 2 SO 4 solution or in boiling LiCl solution, below which no HIC occurred. This shows that hydrogen can enter into the specimen during SCC but the hydrogen concentration is below the critical value so that HIC could not occur during SCC.