Heat treatment of 34CrNiMo6 steel used for mooring shackles

This study presents an experimental investigation of the heat treatment of 34CrNiMo6 steel used for shackles. Indeed, industrial requirements impose specific mechanical properties to the materials which constitute shackles. The aim of this work is to propose a method to relate heat treatment parameters and the resulting mechanical properties. Instrumented cylindrical specimen are submitted to successive heat treatments under conditions representative of the industrial process. Core and surface temperatures are recorded during cooling. Metallographic observations and hardness measurements are performed after each heat treatment, in order to characterise the final phase distribution and mechanical properties. A model based on the one-dimensional heat conduction equation in cylindrical coordinates is developed within an implicit scheme, taking into account the temperature-dependent phase transformations. The results firstly provide a validation of the input data using specific dilatometric experiments, which then leads to a comparison between the experimental and numerical evolution of the core temperature under each cooling condition, as well as the prediction of the phase volume fractions and the resulting hardness. It is shown that the proposed approach yields a very good representation of the material properties.