A wellhead assembly has a casing hanger for supporting a string of casing, the casing hanger having an external upward-facing shoulder. A shoulder ring is mounted on the shoulder. A radially movable annular lockdown member is carried on the shoulder ring for movement between a refracted position while the casing hanger is being run and an expanded position for engaging an interior surface of a wellhead housing. A casing hanger seal is carried by the casing hanger above the lockdown member. The casing hanger seal has a nose on a lower end that engages and moves the lockdown member to the expanded position while the casing hanger seal is being lowered into a set position. The shoulder ring is formed of a material of greater yield strength than the shoulder. Flow channels between the shoulder and the shoulder ring assist in flowing debris from above the shoulder to below.