Date of Award

11-7-2015

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

First Advisor

Northwood, Derek

Second Advisor

Stoilov, Vesselin

Keywords

composition-depth profiles, distortion, ferritic nitrocarburizing, Navy C-rings, plain carbon steel, residual stress

Rights

CC BY-NC-ND 4.0

Abstract

Nitrocarburizing is a case hardening process which improves the hardness and wear resistance of a component, but results in geometric distortions. Torque converter pistons (automotive component) and Navy C-ring specimens (measuring tool) made from SAE 1010 plain carbon steel were used for the distortion analysis. Navy C-rings are generally used for studying the dimensional changes of a material before analyzing the dimensional changes of the actual component. Navy C-rings of varying thicknesses were used to analyze the effect of distortions with the change in thickness. Finite element simulations of Navy C-rings and torque converter pistons were developed to study the effect of nitrocarburizing process on distortions. For thinner specimens, the predicted distortions compare favorably with the experimental values. However, the thicker C-ring specimens showed high prediction error. To better understand the prediction difference of the different C-rings and TC pistons, an empirical ratio (bulk volume to nitrocarburized volume (V/VN)) was introduced. The V/VN ratio will not only help to separate the nitrocarburized surface dominant and bulk dominant specimens, but also provide a better comparison of the C-ring size with the actual component. The reduction of bulk volume to nitrocarburized volume (V/VN) ratio led to a decrease in both the C-ring’s inner diameter (ID) and gap width (GW) distortion, and a small increase in outer diameter (OD) distortion. A composition-depth profile simulation model was also developed to predict the local distortion due to nitrocarburized phases. The local distortion results showed that the γ′-phase (Fe4N) in the diffusion zone dominates the overall magnitude of distortion. The residual stress distribution for 1-step nitrocarburizing treatment was successfully modeled and validated against the experimental stress. The surface stress for one-step nitrocarburizing treatment was found to be tensile in nature. This tensile (surface) residual stress was further reduced by introducing 2-step nitrocarburizing treatments. Using the nitrogen profile model for 2-step nitrocarburizing, it was found that the additional γ′-Fe4N phase at the surface resulted in a notable residual (tensile) stress reduction. Also, the simulated 2-step nitrocarburizing treatments produced a same level of distortion as 1-step nitrocarburizing treatments. Therefore, the proposed 2-step nitrocarburizing treatments could potentially improve both the surface quality and fatigue resistance.

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