Influence of deterioration of polymer solution on quenching crack in polymer quenching process for large forged products

One of the advantages of polymer quenching is that it has slower cooling characteristics than water quenching. On the other hand, in polymer quenching, unlike oil quenching and water quenching, cooling effect changes with an increase of the number of quenching times. When slow cooling performance in polymer quenching deteriorates, quenching cracks sometimes occur. One of the causes of quenching cracks is regarded as the stress that occurs due to quenching. Quenching stress is affected by a cooling rate. Therefore, in order to prevent the quenching crack in polymer quenching process, it is important to understand the relationship between the change of cooling performance with deterioration of polymer solution and the quenching stress. The change of slow cooling performance of polymer solution with the number of quenching times was investigated. The cooling performance was evaluated by heat transfer coefficient which was obtained from quenching experimentation. The slow cooling performance of deteriorated polymer solution decreases below the solution temperature of 350°C comparing to the fresh polymer solution at the same polymer concentration. As a result of quenching stress analysis, it was found that the largest quenching stress was obtained in deteriorated polymer quenching. In addition, in order to investigate the cause of the decrease of slow cooling performance, molecular weight of polymer solution was measured. The percentage of low molecular weight contents increased in deteriorated polymer solution. In order to confirm the effects of molecular weight, the cooling performance was investigated using deterioration-simulated polymer solution, which was prepared by mixing polymers having different average molecular weight. As a result, it was confirmed that the slow cooling performance of simulated polymer solution also decreased as well as actual deteriorated polymer solution.