S. Waldeck, M. Castens, N. Riefler, F. Frerichs, Th. Lübben, U. Fritsching
August 8, 2019
Beginning in the middle of the last century aqueous polymer solutions like polyvinylpyrrolidone (PVP) and polyalkylene glycol (PAG) have been used for immersion quenching because of their unique features and quenching characteristics. Known problems of quenching in polymer solutions are process reproducibility and work safety due to spontaneous reforming of polymer films and almost explosive vapor layers collapse. The boiling and quenching process within polymer solutions and their mechanisms are investigated here. In quasi-steady experiments a newly developed in-line sensor measures the local concentration in the vicinity of a fixed vapor bubble depending on the distance to the phase boundary. These experiments show that the polymer concentration increases with decreasing distance to the vapor bubble surface. Rheological investigations on polymer solutions show that a higher polymer concentration especially increases the solution viscosity considerably. In a typical immersion quenching process with polymer solutions, in the initial phase of the quenching process individual vapor bubbles are formed on the surface of the immersed hot specimen. The high solution viscosity keeps the vapor bubbles on the surface leading to bubble growth and coagulation. Thus, the formation of a vapor layer is promoted. Along with the stabilization of the vapor phase by a concentrated polymer skin a stable vapor layer may emerge. Within the progress of the quenching process, this vapor layer may repeatedly collapse in an explosive like manner. A major influence of this process is the type of polymer (chain length), the polymer concentration and the liquid solution temperature. These effects are also investigated on a larger application-oriented scale setup. Cooling curves and global electric conductance measurements as well as sound and video recordings during specimen quenching in polymer solutions are used to locate vapor layers and their collapses. The experiments show that these effects can be influenced by suitable flow conditions around the specimen.