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 9 3 9 between eight and 24 hours. Effective case depths, defined as the depthbelow the surface at which a hardness of 50HRC is maintained, were determined from the hardness travers- es. Effective case depth increases with a longer carburization time due to the added time for carbon from themethane gas to diffuse deeper into the steel. RESIDUAL STRESS X-ray diffraction residual stress measurements were performed in accordance with SAE HS-784 [9] . Longitudinal residual stress distributions as a function of depth are shown in Fig. 3. Material was removed electrolytically for subsurface measurement to minimize possible alteration of the subsur- face residual stress distribution resulting from material re- moval. Data obtained as a function of depth were corrected both for effects of penetration of the radiation used to mea- sure residual stress into the subsurface stress gradient [10] and for stress relaxation caused by layer removal [11] . Shallowsurface residual stresses aremore compressive than the carburized case as a result of the low stress-grind- ing operation, typical of proper finish grinding of hardened steels. Carburization processes also produce compressive residual stresses in the hardened case primarily due to the volumetric change associated with the martensite transfor- mation of the case-hardened surface. Within the carburized layer, the depth of compression increases with increasing carburization time. The increase in residual stress is asso- ciated with the increased thickness of the carbon enriched layer formed during the carburization process. These obser- vations are consistent with previously published articles on this topic [12] . VOLUME PERCENT RETAINED AUSTENITE The volume percent retainedaustenitewas determined based on first principles using a Bragg-Brentano focusing geometry and direct comparison method of Averbach and Cohen [13] in accordance with ASTM E975 and SAE SP-453. In- tensity factors, R, were calculated using the unit cell volume and chemical composition of AISI 8620H steel. Subsurface measurements were accomplished by removing material electrolytically. Mean volume percent retained austenite distributions are plotted as a function of depth in Fig. 4. Retained aus- tenite content generally increases with increasing carbu- rization time. Surface retained austenite is as high as 16% in the sample carburized for 24 hours and as low as 6% in samples carburized for two and four hours. Austenite is re- 7 Fig. 2 — Hardness profiles of carburized test samples. Fig. 1 — Carburizing process. Fig. 3 — Residual stress distributions as a function of depth.