This part of IS0 6743 establishes the detailed classification of hardening fluids of family U for use in the field of heat treatment. All the fluids listed belong to class L (lubricants, industrial oils and related products) rn This part of IS0 6743 is intended to provide a standardized code for the identification of lubricants and related products used in the process of hardening metallic materials. It should be read in conjunction with IS0 6743-O. This family is divided into several large product categories according to the different applications involved.