Effect of High Pressure Gaseous Hydrogen on the Tensile Properties of Four Types of Stainless Steels: Investigation of Materials Properties in High Pressure Gaseous Hydrogen—1

Practical application of fuel cell vehicle has started in the world, and high-pressure hydrogen tanks are currently considered to be the mainstream hydrogen storage system for commercially implemented fuel cell vehicle. Application of metallic materials to the components of high-pressure hydrogen storage system: hydrogen tanks, valves, measuring instructions and so on, have been discussed. In this work, tensile properties of four types of stainless steels were evaluated in 45MPa (6527psig) and 75MPa (10878psig) high-pressure gaseous hydrogen at a slow strain rate of 3×10−6 s−1 at ambient temperature. Type 316L (UNS S31603) stainless steel hardly showed ductility loss in gaseous hydrogen, since it had stable austenitic structure. On the other hand, Type 304 (UNS S30400) metastable austenitic stainless steel showed remarkable ductility loss in gaseous hydrogen, which was caused by the hydrogen embrittlement of strain induced martensitic phase. Likewise, Type 205 (UNS S20500) nitrogen-strengthened austenitic stainless steel showed remarkable ductility loss in gaseous hydrogen, though it had stable austenitic structure in the same manner as Type 316L. The ductility loss of Type 205 was due to the hydrogen embrittlement of austenitic phase resulting from the formation of planar dislocation array. Furthermore, Type 329J4L (UNS S31260) duplex stainless steel showed extreme ductility loss in gaseous hydrogen, which was caused by the hydrogen embrittlement of ferritic phase.

A Review of K-Solutions for Through-Thickness Flaws in Cylinders and Spheres

This paper reviews different stress intensity factor solutions for a wide range of configurations and loading conditions for a cylinder with axial and circumferential through thickness cracks and a sphere with through thickness meridional (equatorial) cracks. The most appropriate solutions to use are identified.

Weld Residual Stresses and Primary Water Stress Corrosion Cracking in Bimetal Nuclear Pipe Welds

There have been incidents recently where cracking has been observed in the bi-metallic welds that join the hot leg to the reactor pressure vessel nozzle. The hot leg pipes are typically large diameter, thick wall pipes. Typically, an inconel weld metal is used to join the ferritic pressure vessel steel to the stainless steel pipe. The cracking, mainly confined to the inconel weld metal, is caused by corrosion mechanisms. Tensile weld residual stresses, in addition to service loads, contribute to PWSCC (Primary Water Stress Corrosion Cracking) crack growth. In addition to the large diameter hot leg pipe, cracking in other piping components of different sizes has been observed. For instance, surge lines and spray line cracking has been observed that has been attributed to this degradation mechanism. Here we present some models which are used to predict the PWSCC behavior in nuclear piping. This includes weld model solutions of bimetal pipe welds along with an example calculation of PWSCC crack growth in a hot leg. Risk based considerations are also discussed.

Effect of Surface Machining on the Fatigue Life of Low Alloy Steel for Hydrogen Pressure Vessels

The effect of surface machining on fatigue life in high pressure hydrogen gas was investigated. The test was conducted under the elastic range under 45MPa gaseous hydrogen environment by the ground specimen which were machined so that the surface roughness to be Rmax = 19μm(Mark: 19s), 26μm(26s) and 93μm(93s) and by the polished specimen which are prepared so that the surface roughness to be Rmax = 1μm(1s), 3.6μm(3.6s) and 10μm(10s). The hydrogen fatigue life of ground specimens was considerably reduced with increasing surface roughness as compared to the fatigue life in air at the same surface condition. On the other hand, for the annealed conditions of the ground specimen, the reduction by hydrogen effect was fairly small. The residual stress for the ground specimen at the surface rises sharply in tension while the residual stress for the annealed specimen was nearly equal to zero. We have shown that the hydrogen fatigue damage can be evaluated by obtaining the information about residual stress on surface, stress concentration by maximum surface roughness and the threshold stress intensity SH above which hydrogen fatigue damage occurs.

Elastic-Plastic Finite Element Simulation and Fatigue Life Prediction for Beam and Elbow Under Cyclic Loading

Work hardening and Bauschinger effects on plastic deformation and fatigue life for a beam and an elbow under cyclic loading are examined using finite element analysis (FEA). Three typical material plastic hardening models, i.e. isotropic, kinematic and combined isotropic/kinematic hardening models are adopted in the FEA calculations. Based on the FEA results of cyclic stress and strain at a critical location and using an energy-based fatigue damage parameter, the fatigue lives are predicted for the beam and elbow. The results show that (1) the three material hardening models determine similar stress at the critical location with small differences during the cyclic loading, (2) the isotropic model underestimates the cyclic plastic strain and overestimates the fatigue life, (3) the kinematic model overestimates the cyclic plastic strain and underestimates the fatigue life, and (4) the combined model predicts the intermediate cyclic plastic strain and reasonable fatigue life.

Development of New Material Testing Apparatus in High-Pressure Hydrogen and Evaluation of Hydrogen Gas Embrittlement of Metals

A new type of apparatus for material testing in high-pressure gas of up to 100 MPa was developed. The apparatus consists of a pressure vessel and a high-pressure control system that applies the controlled pressure to the pressure vessel. A piston is installed inside a cylinder in the pressure vessel, and a specimen is connected to the lower part of the piston. The load is caused by the pressure difference between the upper room and the lower room separated by the piston, which can be controlled to a loading mode by the pressure valves of the high-pressure system supplying gas to the vessel. Hydrogen gas embrittlement (HGE) and internal reversible hydrogen embrittlement (IRHE) of austenitic stainless steels and iron- and nickel-based superalloys used for high-pressure hydrogen storage of fuel cell vehicle were evaluated by conducting tensile tests in 70 MPa hydrogen. Although the HGE of these metals depended on modified Ni equivalent, the IRHE did not. The HGE of austenitic stainless steels was larger than their IRHE; however, the HGE of superalloys was not always larger than their IRHE. The effects of the chemical composition and metallic structure of these materials on the HGE and IRHE were discussed. The HGE of austenitic stainless steels was examined in 105 MPa hydrogen. The following were identified; SUS304: HGE in stage II, solution-annealed SUS316: HGE in stage III, sensitized SUS316: HGE in stage II, SUS316L: HGE in FS, SUS316LN: HGE in stage III and SUS310S: no HGE.

Failure Analysis of a Ruptured Final Reheat Tube

Two damaged final reheat tubes from a 30 year old supercritical unit were submitted to the laboratory for evaluation following the discovering of a failure of one of the tubes after deslagging operations; a third, dented tube was left in service. The 304H stainless steel tubes were installed in 1990 when the reheater was replaced. The bulk microstructure of both tubes shows evidence of sensitization, which is not unusual given this application (reheater). The failed tube appears to be an intergranular separation that started either subsurface or at the ID, propagating to the OD surface. The sensitization of the steel apparently made the material susceptible to corrosion as well as significantly reduced the impact strength of the material to 10–15% of its estimated original level (verified by Charpy impact test). Examination of the dented tube (#101A) showed a subsurface plane of damage some 30 mils from the ID surface, running parallel to the surface. The damage consisted of intergranular separation, caused by the impact loading event, and referred to in the literature as an “attached spalling failure”. Spalling failures occur when the shock wave is reflected from the back surface (the ID surface of the tube), interacting with the incident shock wave as a stress wave. When the magnitude of this tensile stress exceeds the inherent strength of the material, failure occurs. The overall area of the attached spalling failure is being investigated; the concern there is if it is exceptionally large, it may provide a thermal barrier to heat transfer from the OD to the ID and result in a local overheating failure. Within the metallographic sample, however, the damage area was quite small and therefore did not appear to be an immediate issue. The long-term suitability of tube 105A, which remains in service with a dent induced by the same deslagging process that damaged tubes 101A and 103A, is doubtful and should be addressed during the Fall 2006 boiler overhaul. For the shortterm, the assumption was made that cracking due to the deslagging impact would be oriented similar to non-failed tube and extension of these fissures to failure between Spring 2006 and the Fall outage is not expected.

Crack Growth Analyses of SCC Under Various Residual Stress Distributions Near the Piping Butt-Welding

Stress corrosion cracking (SCC) of core internals and/or recirculation pipes of austenite stainless steel (Type 316L) has been observed. When a SCC is detected at the reactor internals or pipes, it is necessary to calculate crack growth behavior of the crack for a certain operational period. The SCC initiates and grows near the welding zone because of high tensile residual stress by welding relative to the other contributing factors of material and environment. Therefore, the residual stress analysis due to welds of austenitic stainless piping is becoming important and has been already conducted by many researchers. In present work, the through-thickness residual stress distributions near multi-pass butt-welds of Type 316L pipes have been calculated by thermo-elastic-plastic analyses with the geometric and welding conditions changed and collected from literatures. Then crack growth simulations were performed using calculated and collected residual stress distributions. The effects of geometric and welding conditions on crack growth behavior were also discussed.

Measurement of Residual Stresses in a Nozzle-to-Cylinder Weld After Thermal Ageing at 550°C

This paper presents results from a programme of experimental measurements of residual stresses in a type 316H stainless steel component consisting of a nozzle welded to a cylinder. The residual stresses were measured using the deep-hole drilling (DHD) technique. The welded component had been thermally aged in a furnace at 550°C for 19,644 hours prior to the residual stress measurements. Measurements were obtained in the through-thickness section of the component at two locations: (i) in the cylinder heat affected zone (HAZ) at the flank of the nozzle-to-cylinder weld intersection and (ii) in the cylinder HAZ near the crown of the nozzle-to-cylinder weld intersection. The stress measurements made after the furnace heat soak treatment are compared with the earlier as-welded stress measurements. In comparison with as-welded residual stress measurements on the same component and with residual stresses in a service-aged (55,000 hours at 525°C) component, it was evident that the thermal soak test treatment had significantly relaxed the weld residual stresses. In particular the soak test hoop stress profile was almost identical to the service-aged condition, whereas the transverse stress distribution had only been partially relaxed by the thermal soak test.

A Tribute to Dr Spencer Bush (1920-2005): On Behalf of the Institution of Mechanical Engineers, UK

This is not a technical paper but simply a tribute to Dr Spencer Bush. Through recollections in various areas of activity, some of the contributions made by him are identified.