1 Copyright © 2013 by ASME Predictive modeling of surface microstructure of hardened steel subject to drilling Ninggang Shen, Hongtao Ding* and Wei Li Department of Mechanical & Industrial Engineering The University of Iowa Iowa City, IA 52242, USA KEYWORDS Microstructure, Drilling, Modeling, Grain refinement; Phase transformation. ABSTRACT Hole surface microstructures are very critical to the mechanical performance and fatigue life of metallic products from drilling processes. When steel material is drilled at a fully hardened condition, hole surface microstructures are often subject to transition because of the intense thermo-mechanical loading in the drilling process. A white layer can be formed on the surface of a drilled hole of carbon steels with high matrix hardness. The formation of the white layer mainly results from two reasons: thermally driven phase transformation and mechanical grain refinement due to severe plastic deformation on the machined surface. In this study, a multi-step numerical analysis is conducted to investigate the potential mechanism of surface microstructure alterations in the drilling process of hardened steels. First, three-dimensional (3D) Finite Element (FE) simulations are performed using a relative coarse mesh with AdvantEdge for hard drilling of AISI 1060 steel to achieve the steady-state solution for thermal and deformation fields. Defining the initial condition of the cutting zone using the previous 3D simulation results, a multi-physics model is then implemented in two- dimensional (2D) coupled Eulerian-Lagrangian (CEL) finite element analysis in ABAQUS to model both phase transformation and grain refinement at a fine mesh to comprehend the surface microstructure alteration. The interaction among surface microstructures, drilling parameters and the hardness of the workpiece material are studied simultaneously. With the comparison to related experimental results, the capabilities of the multi-physics model to accurately predict critical surface microstructural attributes such as phase compositions, grain size, and microhardness during the drilling of carbon steel are demonstrated. 1. INTRODUCTION Drilling of steels is important in modern manufacture, and is often planned as the last operation in aerospace and automobile industries. Undesirable microstructural changes can be induced in the surface layer of the drilled hole for steel materials at a fully hardened condition. During the drilling process, the workpiece material often performs in a complicated manner involving dynamic phase transformation, fracture, severe plastic deformation (SPD), and grain size change, etc. A common detrimental microstructural alteration is referred to “white layer”, because it appears featureless and white when viewed under an optical microscope. The formation of white layer has been a great interest in the past decades. Griffiths [1] attributes white layer formation to one or more of the following possible mechanisms: (1) rapid heating and quenching, which results in phase transformation, (2) severe plastic deformation, which produces a refined homogeneous structure, and (3) surface reaction with the environment, e.g., nitriding. Jawahir et al. [2] argued that metallurgical transformation occurs in the chip or on the workpiece machined surface due to intense, localized and rapid thermo-mechanical working during hard turning. Ramesh et al. [3] suggested that white layers produced by hard turning of bearing steel at low-to-moderate cutting speeds are largely due to the grain refinement induced by SPD, whereas white layer formation at high cutting speeds is mainly due to thermally driven phase transformation. Studies of the drilling of a Proceedings of the ASME 2013 International Mechanical Engineering Congress and Exposition IMECE2013 November 15-21, 2013, San Diego, California, USA IMECE2013-64499 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 02/11/2015 Terms of Use: http://asme.org/terms