Structural Metals

DOE-HDBK-1129-99

faster. This type of surface passivation can be expensive, so many parts of tritium systems are not

passivated; normally only parts requiring the special properties of passivated surfaces are treated.

5.1.2 Structural Metals

Exposure of metals to high pressure (> 2,000 psia) hydrogen, deuterium, or tritium will result in

hydrogen embrittlement of the material. This could eventually result in material failure. The time

until failure is a function of the container material, the pressure, and temperature. Additionally,

materials exposed to high-pressure tritium are also subject to helium embrittlement. Tritium at high

pressure enters the metal and decays to 3He. The buildup of helium in the metal results in helium

embrittlement, which, depending upon the pressure, temperature, and type of material, will

eventually result in failure of the material. Exposure of metals to low to medium pressure tritium at

normal temperatures does not generally result in material failure within any reasonable period of

time.

Some metals are more resistant to embrittlement than others and, therefore, are more compatible

with tritium. Depending upon the specific application, 304L and 316L stainless steel are generally

considered to be the most hydrogen compatible and readily available stainless steels for tritium

service. High-pressure vessels, valves, and tubing designed of these materials when used at their

rated pressure and temperature will provide many years of service without material failure. When

equipment is designed for tritium operations, a materials expert should be consulted to ensure that

the materials selected are compatible with their intended service.

5.1.2.a Austenitic Stainless Steels

The recommended materials of construction for tritium-handling systems are from the class of

wrought 3XX series of austenitic (face-centered-cubic) stainless steels, including Types 304L,

316L, and 347. Types 304L and 316L are most often used in tritium processing systems. These

steels provide good strength, weldability and resistance to hydrogen embrittlement. Components

fabricated from these materials are procured routinely. Many commercially available vacuum

system components that are used in tritium systems, such as valves, piping, pumps, and analytical

instrument sensors, are fabricated from these types of austenitic stainless steel. Wrought

materials are preferred to cast because wrought materials normally have a more homogenous

microstructure. In the past, tritium has leaked through parts having poorly oriented stringers and

inclusions. The forging direction of some wrought components has been specified so that the

orientation of inclusions is not in a direction that could result in a tritium leak path. Low carbon

grades (such as 304L and 316L) are preferred to avoid weld sensitization and to reduce the

number of inclusions (impurity particles such as oxides and carbides). Modern vacuum-arc-

remelted steels are a good choice because they have lower impurity levels, thereby resulting in

fewer inclusions that could aid hydrogen-induced cracking or provide leak paths. Typically, tritium

system components employ seamless pipe and tube where practical.

Stabilized grades, such as Type 347, have been employed in applications where post-weld heat

treatment is not possible. This usually occurs when a process vessel contains a working material

(such as hydride or getter) that will degrade when exposed to the post-weld heat treatment, which

is typically performed at about 1,100C for austenitic stainless steels.

High carbon grades, such as Type 347H or Type 316H (having 0.04 percent carbon minimum),

have been successfully employed for tritium service if high temperature strength is required. Type

347H is employed in the Hydride Transport Vessel (see section 6.2.2), and Type 316H was