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 2 stressed area ahead of another allows the residual stresses to distort the part. The table shown here specifies modulus of elasticity and thermal conductivity for several metals. Thermal stresses increase as thermal conductivity decreases and modulus of elasticity increases. To avoid distortion due to residual stresses, it is critical to uniformly heat parts to the stress relieving temperature. As a general rule, the tempera- ture should be kept uniformwithin 20°F until the stress relief temperature is reached. DISTORTION DUE TO POOR MACRO AND MICRO STRUCTURE Poor metallurgical quality of the material may cause internal stresses during heating. Distortion caused by poor material quality is very difficult to control or minimize. DISTORTION DUE TO IMPROPER FIXTURING To minimize distortion, parts should be evenly posi- tioned in the fixture. There should be an unhindered flow of oil through the load and parts spacing should be 0.13 to 0.25 in. apart, and vertically between grids about 2 to 4 in. apart. For oil quenching fixtures, the ability to submerge the part is required to reduce fumes and flashing. CONCLUSION In order to control distortion and minimize operating costs, the following steps are recommended: • Control temperature uniformity during phase transformation. • Heat parts uniformly to stress relieving temperature within +/- 20°F until the stress relief temperature is reached. • Use properly designed fixtures with tolerance for thermal expansion. • Use smart loading dummy parts, shields, low gauge fixtures, baskets, and grids made from low expansion materials. ~HTPro For more information: Janusz Kowalewski, Ipsen USA, 984 Ipsen Rd., Cherry Valley, IL, 61016, 815.298.6367, janusz. kowalewski@ipsenusa.com, www. ipsenusa.com. Selected References 1. I. Elkatatany, et al., Performance and Computer Simulation of Large H13 Dies in Vacuum Heat Treatment, Proceedings, 17th annual ASM Heat Treating Society Con- ference, Indianapolis, 1997. 2. H. Walton, Dimensional Changes duringHardening and Tempering of Trough-Hardened Bearing Steels, Quenching and Distortion Control, Conference Proceedings, ASM International, 1992. 3. C.C. Tennenhouse, Control of Distortion During the Furnace Cycle, Welding Journal, October 1971. 4. Aerospace Material Specification AMS2759/7B, section 3.6.2, 2010. 5. A.K. Sinha, Defects and Distortion in Heat-Treated Parts, ASMHandbook, Vol 4, Heat Treating, p 601-619, 1991. 10