Final answer:
Transport chains are the type of chains that are heat-treated to improve strength for securing cargo. Grades like 70 for transport and 43 for high test indicate heat treatment, whereas proof coil and low test chains are not heat-treated.
Step-by-step explanation:
The type of chain that is heat treated among the options provided is the transport chain. Heat treatment is a process applied to metal to increase its strength and load-bearing capacity. This particular chain is often used to secure cargo during transport, which is why it is designed to be stronger than proof coil and low test chains. Transport chains typically fall under Grade 70, indicating a heat-treated carbon steel with a high strength-to-weight ratio, suitable for binding and tie-down applications. The grade number associated with chains can often be a guide to whether they have been heat-treated. For example, high test chains, also known as Grade 43, are also heat treated, while proof coil (Grade 30) and low test chains are made from lower strength steel and are not heat treated.