Hardness control of grind-hardening and finishing grinding by means of area-based specific energy U. Alonso a,n , N. Ortega a , J.A. Sanchez a , I. Pombo b , B. Izquierdo b , S. Plaza a a Faculty of Engineering of Bilbao, University of the Basque Country, Alameda de Urkijo s/n, 48013 Bilbao, Spain b Faculty of Technical Engineering of Bilbao, University of the Basque Country, Paseo Rafael Moreno “Pitxitxi” 3, 48013 Bilbao, Spain article info Article history: Received 11 April 2014 Received in revised form 1 September 2014 Accepted 3 September 2014 Available online 16 September 2014 Keywords: Grind-hardening Area based specific energy Hardness penetration depth Surface layer softening abstract The grind-hardening process uses the heat generated within the grinding zone in order to produce surface hardening of the workpiece. However, after the process, workpieces present dimensional inaccuracies and poor surface roughness. Thus, a final grinding operation has to be performed. For an industrial implementation of the whole process, two problems need to be solved. On the one hand, on- line control of the hardness penetration depth (HPD) should be achieved. On the other hand, excessive softening of the workpiece has to be avoided during the finishing grinding. This paper, firstly, investigates the feasibility of using the area based grinding energy (E ″ c ) for the prediction of the HPD. Surface grind-hardening tests carried out on 100Cr6, 42CrMo4 and AISI 1045 steels have shown that, for all the tested parameter sets, a linear correlation exists between E ″ c and HPD. Furthermore, the slope of this linear relationship can be estimated from the chemical composition of the hardened steel based on the equivalent carbon number. On the other hand, the influence of varying wheel dressing conditions on the E ″ c –HPD relationship is analysed. Secondly, it has been found that a relationship exists between E ″ c and the surface softening during the finishing grinding operation. This relationship is independent of the grinding parameter combination when the maximum undeformed chip thickness is over a threshold value. Thus, E ″ c is a very appropriate parameter to control both the hardening and the finishing process of grind-hardened workpieces. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction 1.1. Motivation In industry, machine steel components working under high loads and frequent sliding contact have to be subjected to a surface strengthening heat treatment. This process improves fatigue strength and wear resistance while maintaining the toughness of the workpiece. In order to achieve such requirements, an ineffective manufacturing process is usually followed. After soft machining, workpieces have to be cleaned and transported to the heat treatment facility where a thermal or a thermo-mechanical process, e.g. case hardening or induction hardening, is applied. Then, workpieces are discharged and brought back to the produc- tion line for a final finishing operation that gives them accuracy and a good surface finish. However, this process cannot simply be introduced into the production line, which leads to high costs and longer production times. Moreover, it involves serious health problems associated with the use of toxic coolants [1]. Abrasive machining processes with undefined cutting edges, such as grinding, require a high energy input. In fact, almost all the mechanical energy from the grinding wheel is transformed into heat within the grinding zone, causing the wheel and workpiece temperatures to rise. Grind-hardening is an innovative technology that uses the heat generated within the contact area so as to achieve surface hard- ening. In order to do so, during the grinding pass, a short time austenitization is enabled in the surface layer of the machined workpiece [2]. Subsequently, self-quenching effect and the usage of coolant induces a martensitic phase transformation of the generated austenite and the desired hardened layer is obtained [2,3]. This technology can be fully integrated into the production line, and therefore, it offers an outstanding potential to reduce costs, energy and time. At first, the viability of the process was questioned due to various issues such as the control of the sparks generated, low repeatability of the process [2], or the workpiece geometry. In recent years, this process has been subject of several investiga- tions that have proven its feasibility for surface grinding [2,4–6] and cylindrical grinding [3,7–9]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ijmactool International Journal of Machine Tools & Manufacture http://dx.doi.org/10.1016/j.ijmachtools.2014.09.001 0890-6955/& 2014 Elsevier Ltd. All rights reserved. Abbreviations: CE, Equivalent carbon content; HPD, Hardness penetration depth n Corresponding author. Tel.: þ34 946017347. E-mail address: unai.alonso@ehu.es (U. Alonso). International Journal of Machine Tools & Manufacture 88 (2015) 24–33