November/December 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 | N O V E M B E R / D E C E M B E R 2 0 1 8 6 5 After 2 sec of spray quenching, the surface temperature is drastically reduced to about 210 o C/410 o F. The maximum temperature reaching about 400 o C/752 o F will be located at approximately 10-12 mm beneath the surface. Note: The temperature at the center of the shaft continues rising during the first 6 sec of quenching. After 6 sec of quenching, the surface temperature has decreased below 100 o C/212 o F, however a considerable amount of heat is retained in the interior of the shaft (the temperature at the core exceeds 300 o C/572 o F with the aver- age temperature being about 225 o C/437 o F). If at thismoment the supply of spray quenchant is cut off, the surface of the part will begin to be heated again due to the heat accumulat- ed inside the workpiece. After 5 sec of soaking (heating power and spray quench are not applied), the surface temperature rises to about 215 o C/419 o F and the core temperature will be about 260 o C/500 o F. Therefore, with proper selection of process conditions, this retained heat can be used to temper the workpiece. Inmany cases of using plain carbon and lowalloy steels for automotive applications, the self-tempering tempera- tures (if applied) typically do not exceed 250 o C (480 o F) and are usually in the 180 o C (360 o F) to 220 o C (430 o F) range. Self-tempering provides several recognizable benefits [1] : • It eliminates an additional operation due to incorporation of self-tempering into the hardening operation. Therefore, the capital equipment cost and total cycle time are reduced making it very attractive from a cost-reduction perspective. • The time delay between hardening and tempering stages is virtually eliminated. Too long a time delay can be detrimental due to the potential appearance of delayed cracking particularly when dealing with steels and cast irons that exhibit low toughness. • Since self-tempering utilizes the residual heat that is retained after hardening, there is no need to apply any additional energy for tempering making it highly energy efficient. A reduction in overall needed energy is not only associated with the reheating stage, but also with the fact that less energy is associated with the cooling stage. • In some cases, self-tempering produces more desirable distribution of residual stresses because the heat accumulated in internal regions of the workpiece flows from inside-out toward the surface in contrast to flowing outside-in (from the surface toward internal areas) as it is with conventional induction tempering. • There is obviously a savings in shop floor space, because there is no need for additional space to locate tempering equipment. All of these factors are very attractive and are the reasons for applying self-tempering in some applications. However, several precautions must be taken to ensure that the self-tempering process is performed correctly and de- spite the considerable benefits, self-tempering does have noticeable limitations, which restrict its broader use in in- dustry and make furnace tempering and induction temper- ing more popular processes. Some of those limitations are outlined below [1] : Residual heat must be accurately controlled and uni- formly distributed or distributed in a way that takes into consideration the complex geometry of the workpiece. The energy generated within the part must be monitored close- Fig. 2 — Results of numerical computer modeling of induction surface hardening of a medium carbon steel solid shaft [1] . 13