May/June_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 1 8 4 1 Because the high temperature gradients are observed near the intake port in all quenching cases (air and water), it is very likely a design issue. COMPARISON OF AIR AND WATER QUENCHING PROCESSES Heat extraction for air and water quenching are very different. Air quenching relies on convection heat transfer to cool the metal, while water quenching relies on water-to-va- por phase change to remove heat. Therefore, metal cools much faster inwater quenching. Themaximum temperature Fig. 7 — Vapor patterns and vapor pockets entrapped during water quenching of cylinder heads in six different quench orienta- tions, 20 seconds into quenching. RE = rear face up; FR = front face up; CC = cam cover face up; JF = joint face up; IN = intake port face up; and EX = exhaust port face up. Fig. 8 — Cooling curves andmaximum temperature gradients for all water quenching configurations. gradient for water quenching is also much larger than for air quenching. Becausewater only vaporizes at locations in con- tact with the hot surface, heat loss is a local phenomenon subject to a vapor escape route and supply of fresh water. In other words, heat transfer might not be as even as with air quenching, which is reflected in the fluctuation of the high temperature gradient plot (Fig. 10). A much higher temperature gradient in water quench- ing does not necessarily generate much higher residual stress. Figure 10 also shows that the duration of peak tem- perature gradient only lasts about 15 seconds, during which the structural stress in metal can exceed yielding stress and plastic deformation begins. However, final deformation also depends on how long the state of stress remains in the plas- tic deformation zone. CONCLUSION Cooling curves and temperature gradients generated by air and water quench modeling methods were used to evaluate quenching performance for various quenching con- figurations. For air quenching, cylinder heads cool faster in basket quenching than in conveyer quenching because air 9