Experimental measurements and numerical simulation of stress and microstructure in carburized 5120 steel disks

A combined experimental and numerical study of residual stress and microstructure was performed on a carburized disk of 5120 steel. The disk-shaped specimen was carburized and quenched in agitated oil at 71°C. X-ray diffraction combined with electropolishing layer removal was used to determine stresses through the case of the disk, within ∼1 mm of the surface. Both martensite and retained austenite volume fractions were determined through one flat surface. Rietveld analysis was used to determine the lattice parameters of the constituents at sequential depths. Microstructure and residual stress profiles were compared to predictions. The numerical predictions were from ABAQUS; a finite element code coupled with a user-defined material subroutine (UMAT), that accounted for microstructure evolution. Measured retained austenite values varied from ∼25 vol.% at the surface to a maximum of ∼30% at 100 μm, then decreased to 4% at a depth of 600 μm. The numerical simulation predicted a maximum of 25 vol.% at the surface that monotonically decreased to 7% at a depth of 600 μm, and reached a minimum of 4% at 1.0 mm. The maximum measured compressive stress was 380 MPa at 550 μm, compared to the predicted value of 450 MPa at 330 μm. In addition, the carbon profile predicted from the numerical simulation was comparable to the profile obtained from the combustion burnout technique.