Component Design

Fracture mechanics design methods permit the use of materials that have low to intermediate hydrogen embrittlement (HE) resistance. These methods are used for pressure containing storage tanks and piping components.

Fracture mechanics provides for the evaluation of crack propagation. The essential quantity in fracture mechanics is the stress-intensity factor, K.

• The local tensile and shear stresses ahead of cracks are proportional to the stress-intensity factor
• Crack propagation is related to the magnitude of the stress-intensity factor

The objective of the fracture mechanics design approach is to calculate the number of pressure cycles, N_c, to grow crack to critical depth, a_c. The calculation requires material property measurements in hydrogen gas: threshold stress-intensity factor (KTH) and fatigue crack growth rate (da/dN) vs. stress-intensity factor range (ΔK). The calculation method is described in the ASME Boiler and Pressure Vessel Code.

Austenitic stainless steels and aluminum alloys are the best candidates for hydrogen service.

• Based on previous service experience and materials testing
• For austenitic stainless steels, this previous service experience and materials testing applies to defined ranges of alloy composition and temperature
• Do not extrapolate component service or test data
• Design analysis can quantify safety margins

Ferritic steels (e.g., C-Mn and low-alloy steels) are susceptible to hydrogen embrittlement under wide range of conditions.

• Previous service experience may be adequate, but design analysis is needed in many cases
• Materials testing to support design analysis must be performed for steel characteristics and environmental conditions that apply to component under consideration
  • Important steel characteristics include strength and fabrication method (e.g., welding and forming)
  • Important environmental conditions: hydrogen pressure, temperature

Nickel and titanium alloys are generally not recommended for hydrogen service.

• Materials testing in hydrogen gas and design analysis are required
• May require other measures such as hydrogen permeation barrier