The quenching characteristics of sodium polyacrylate solutions.

The quenching characteristics of a range of concentrations of sodium polyacrylate, a commercially available polymer quenchant, have been studied. These solutions showed a stable film boiling stage the duration of which increased with increasing concentration. The maximum surface heat transfer coefficients were significantly below those recorded in water or polyalkylene glycol solutions and decreased with increasing concentration. Just after the passage of this maximum the surface heat transfer coefficient declined rapidly to reach values, at a surface temperature of about 300 C, equivalent to those recorded in the film boiling stage. Photography showed that this was associated with a decline in the mobility of the vapour bubbles formed in this stage. The surface heat transfer coefficients were used to calculate the stress and strain generated during quenching using a visco-elasticplastic model of an infinite plate of a low alloy steel. Comparisons of the predicted residual stresses in the case of the sodium polyacrylate solutions with residual stresses predicted in the case of other quenchants indicated that sodium polyacrylate solutions were capable of producing residual stress distributions similar to that produced by a medium speed quenching oil and greatly below those produced in the case of polyalkylene glycol solutions. This was achieved by a decline in the temperature gradient in the specimen before transformation to martensite began associated with the rapid reduction in surface heat transfer coefficient caused by the loss of mobility of the vapour at these surface temperatures. The predicted residual stresses and strains were also compared to experimentally measured residual stresses and strains to validate the model used. Three boundary layer theory models of film boiling were evaluated in the case of quenching in both water and a sodium polyacrylate solution and the predicted surface heat transfer coefficients compared to experimentally obtained values. None of the models produced a close agreement therefore a modification has been proposed to allow the inclusion of a turbulent interface in the models.