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 8 AISI 8620H steel is one of the most widely used carburizing alloy steels for a variety of medium-strength applications in- cluding gears, camshafts, fasteners, chains, and pins [1] . Flexibility in carburization treat- ments not only enables various case depths, but also refinement of other material proper- ties such as beneficial compressive residual stress and retained austenite content. Due to a gradient in carbon content generated by the carburization process, compressive residual stresses are formed, which can sig- nificantly improve the fatigue properties of a component [2] . The compressive residual stress depth and magnitude needed for op- timal fatigue performance varies depending on the engineering design requirements of the particular application. Retained austenite is also generated by the carburiza- tion process. Often, a low amount of retained austenite in martensitic structures is desirable because the body cen- tered cubic (bcc) ferrite and body centered tetragonal (bct) martensite phases are more stable than the face centered cubic (fcc) austenite phase. High service temperatures and mechanical stress can cause dimensional increases and out- of-bounds tolerances due to isothermal transformation of the austenite. Retained austenite also lowers the compres- sive yield and ultimate tensile strength and decreases hard- ness and resistance to scuffing while increasing susceptibili- ty to heat checking in grinding operations [3,4] . Rolling contact fatigue (RCF), or contact fatigue, a common failure mechanism of carburized components, is a surface pitting-type failure resulting from Hertzian stress present when curved surfaces are in contact under normal load. Contact fatigue is a common cause of failure in com- ponents such as gears, cams, railroad wheels, and bearings; components made of carburized steels are especially prone to contact fatigue-type failures [5,6] . In this investigation, fatigue performance was charac- terized as a function of carburization case depth, hardness, residual stress, and retained austenite to demonstrate that an optimized carburization process could be engineered to meet specific application requirements. EXPERIMENTAL PROCEDURE Test samples (6 in. or 150 mm long) were machined from purchased 0.5 in. (13 mm) diameter AISI 8620H steel and carburized in an integral quench furnace in an exother- mic atmosphere with methane enrichment gas for two, four, eight, 12, and 24 hours (Fig. 1). After carburization, sam- ples were low-stress ground to a final diameter of 0.470 in. (12 mm). The reduced diameter enabled testing samples in rolling contact fatigue. Low-stress grinding was perform- ed in a manner similar to that used in a typical finish op- eration on gears [7] . Examination of test samples included hardness testing and measurement of residual stress and retained austenite. HARDNESS Microindentation hardness measurements were per- formed on samples from each of the five carburization groups using a Knoop style indenter and a 500 g load [8] , and were converted to Rockwell C values (Fig. 2). Hardness distri- butions near the surface are similar for samples carburized OPTIMIZING CARBURIZATION IN 8620H STEEL COMPONENTS Process optimization and optimal component performance can be achieved when multiple variables are measured and studied as a whole. Perry W. Mason, Kyle A. Brandenburg, and Doug J. Hornbach Lambda Technologies Group, Cincinnati 6 Heat-treated gears. Courtesy of Lambda.