Effect on Hardness and Distortion by Replacing Quenching Oils with Aqueous Polymer Quenchants

Abstract

In recent years, it is required to improve productivity and reduce environmental load in the heat treatment process. Since aqueous polymer quenchants pose little danger of fire, it is possible to unify production lines with pre- and post-processes, which is expected to improve productivity. In addition, it is expected to reduce CO₂ emissions. On the other hand, aqueous polymer quenchants have a longer vapor blanket stage length and a faster cooling rate near the martensite transformation point than quenching oils, so there are concerns about changes in hardness, quenching crack, and increased distortion. In this study, we developed aqueous polymer quenchants with oil-like cooling properties and investigated the effect on the quenching quality with conventional aqueous polymer quenchants. The hardness and distortion of conventional aqueous polymer quenchants, developed aqueous polymer quenchants, water and quench oils were investigated, and examined whether the developed aqueous polymer quenchants could improve the hardness and distortion. As a result of quenching round bars of different materials, the developed aqueous polymer quenchants has the same hardness as oil regardless of the material, while the conventional aqueous polymer quenchants have the same hardness for chromium molybdenum steel , but lower hardness for carbon steel. It is considered that the developed aqueous polymer quenchants did not decrease in hardness because the vapor blanket stage length was shortened, so cooling proceeded without passing through the pearlite nose. In the quenching of C-type chromium molybdenum steel test pieces, there was no crack with the developed polymer quenchants, while crack occurred when quenching with the conventional polymer quenchants and water. When a heat treatment simulation was performed, there was a correlation between the actually measured residual stress and the maximum principal stress, which could be used to predict quenching crack. The distortion change of the developed aqueous polymer quenchants were as small as that of the quenching oils compared to the conventional aqueous polymer quenchants. Visualization and multi-point cooling performance evaluation experiments confirmed that the aqueous polymer quenchants had less uneven cooling than the conventional one. It is considered that the shortening of the vapor blanket stage length suppressed uneven cooling and reduced distortion.