Process Piping Materials – Module 2 – M06-036

1.2.9. Hardness

Hardness is the property of being rigid and resistant to pressure; not easily scratched. It’s presence in metals can be an advantage for high pressure systems but can be a disadvantage, as it can increase machining, cutting and fabrication times. It is measured on Mohs scale.

1.2.10. Brittleness

The tendency for a metal to crack or break with deformation. Metals displaying this property are not readily used for pipe or tube as this is a disadvantage to a material.

1.2.11. Toughness

Toughness is the ability of the metal to deform plastically and absorb energy in the process before fracturing. It is the indicator of how the given metal would fail at the application of stress beyond the capacity of the metal, and whether that failure will be ductile or brittle.

Steel is normally considered to be a ductile material, but under certain low temperature conditions, steel piping may shatter just as glass. Two common ASTM test methods used to measure toughness are the Charpy Impact and Drop-Weight tests.

1.2.12. Creep

Creep is a progressive permanent deformation of material subjected to constant stress; aka, time dependent behavior. Creep is of concern for:

•Carbon steels above ~700ºF (~370ºC)

•Stainless steels above ~950ºF (~510ºC)

•Aluminum alloys above ~300ºF (~150ºC)

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