Describe the effects of cryogenic temperatures on the ductile-to-brittle transition of wave spring materials.

For subsea or cryogenic fuel systems, material selection must account for the Ductile-to-Brittle Transition Temperature (DBTT). Carbon steels become extremely brittle at temperatures below $-30^{\circ}C$, making them unsuitable. Austenitic stainless steels like 302 and 316, and precipitation-hardening alloys like 17-7PH, remain ductile at cryogenic temperatures (down to $-196^{\circ}C$ or $77K$). While the Modulus of Elasticity $E$ increases slightly (by roughly $5-10\%$) at low temperatures, causing a proportional increase in the spring rate, the primary concern is the toughness. Materials like Elgiloy or Inconel 718 are often specified for their superior toughness and lack of DBTT in liquid nitrogen or liquid oxygen environments.