No, joint replacements don’t melt. After the cremation they put the remains in a tray and allow them to cool. Then they sift out all the large metal objects and set them aside. They send them to a company in the Netherlands called OrthoMetals that specializes in post-cremation recycling and have a purpose-built facility for recycling and separating all of the metals. The service is provided free of charge to crematoriums and a percentage of revenue raised from the recycled metals is given back to the crematoriums, many of whom donate it to charity.
Hip implant after cremation