OCR Text |
Show and therefore is assumed to affect more uniform load temperature distributions. As stated above, improvements in temperature uniformity may be related to improvements in product quality and a reduction in rejected parts. Economic quantification of these results would require additional testing followed by material analysis specific to particular heat treating cycles. The increase in the rate of heating as a result of FM temperature control is indicated in Figure 8. As shown, the heating rate for the FM control scheme is greater than the rate of the AM control scheme. By observation of these curves, it may be seen that as the rate of heat input is reduced, the ratio of heating rates for FM control to AM control increases. This observation appears to support hypotheses which relate the convective heat transfer coefficient to momentum flux. Although increases in the load heating rate are supported by the data, these increases are not dramatic and only slight decreases in total heating time were observed. In addition, the relative decrease in heating time for the FM control scheme relative to the AM control scheme appears to increase as setpoint temperature decreases. Several arguments may be presented to explain occurrances. Increases in the convective heat transfer coefficient evidenced by increases in load heating rate are difficult to detect since even substantial increases in the convective coefficient are rendered insignificant by the large radiative heat transfe coefficient which prevails at the elevated temperatures under which the experiments are performed. 7-11 |