Rockwell 56 unless specified otherwise. 

Rockwell hardness refers to a standard measure of the hardness of a material, typically metals. The Rockwell hardness scale uses an indenter to penetrate the material's surface and measures the depth or extent of the indentation. 

A Rockwell hardness of 56 HRC indicates a relatively high level of hardness for a metal. 

Tire chains are designed to strike a balance between hardness and flexibility. They need to be hard enough to provide traction and resist wear but also flexible enough to accommodate tire movement and road conditions.