Lubricants 2023, 11, 101. https://doi.org/10.3390/lubricants11030101 www.mdpi.com/journal/lubricants Article Machinability Investigations based on Tool Wear, Surface Roughness, Cutting Temperature, Chip Morphology and Material Removal Rate during Dry and MQL-Assisted Milling of Nimax Mold Steel Rüstem Binali *, Havva Demirpolat, Mustafa Kuntoğlu and Hacı Sağlam Department of Mechanical Engineering, Faculty of Technology, Selcuk University, 42130 Konya, Turkey * Correspondence: rustem.binali@selcuk.edu.tr Abstract: Using cutting fluids is considered in industrial applications and academia due to their increased influence over many aspects such as machinability, sustainability and manufacturing costs. This paper addresses the machinability perspective by examining indicators such as rough- ness, cutting temperature, tool wear and chip morphology during the milling of mold steel. A spe- cial type of steel is Nimaxm which is a difficult-to-cut material because of its high strength, toughness, hardness and wear resistance. Since mold steels have the reverse geometry of the components produced by this technology, their surface quality and dimensional accuracy are highly important. Therefore, two different strategies, i.e., dry and minimum quantity lubrication (MQL), were chosen to conduct an in-depth analysis of the milling performance during cutting at different cutting speeds, feed rates and cutting depths. Without exception, MQL technology showed a better performance than the dry condition in obtaining better surface roughnesses under different cutting parameters. Despite that only a small improvement was achieved in terms of cut- ting temperature, MQL was found to be successful in protecting the cutting tool from excessive amounts of wear and chips. This paper is anticipated to be a guide for manufacturers and re- searchers in the area of mold steels by presenting an analysis of the capabilities of sustainable machining methods. Keywords: Nimax; machinability; milling 1. Introduction Plastic mold steel is an essential material in industry and is generally machined under harsh cutting conditions used for forming plastic materials [1]. Unlike other types, this steel tool has a high corrosion resistance, hardness, polishability and patterning ability [2]. Nowadays, a steel named Nimax is used as an extrusion die, an injection die, a blow mold, a forging mold and a go/no go gauge, and contains alloying elements such as chromium, manganese, molybdenum, nickel, vanadium and aluminum [3]. On the other hand, pre-hardened mold steels can endure harsh working conditions thanks to their high hardness and toughness which are related to their microstructure [4]. In addition, heat treatment processes have beneficial effects on the microstructure and mechanical properties. These properties can be enhanced by controlling the heat treatment condi- tions [5]. The heat treatment process generally consists of quenching and tempering or continuous cooling during working [6]. Quenching and tempering processes have been accepted to improve the combination of strength, mechanical properties, and machina- bility [7,8]. The pre-hardened plastic mold steel Nimax is commercially available at about 40 Rockwell C hardness [9]. Low-carbon Nimax, which is a mold steel, has a high toughness and shock resistance. It is among a new generation of plastic mold steels that Citation: Binali, R.; Demirpolat, H.; Kuntoğlu, M.; Sağlam, H. Machinability Investigations based on Tool Wear, Surface Roughness, Cutting Temperature, Chip Morphology and Material Removal Rate during Dry and MQL-Assisted Milling of Nimax Mold Steel. Lubricants 2023, 11, 101. https:// doi.org/10.3390/lubricants11030101 Received: 2 February 2023 Revised: 21 February 2023 Accepted: 24 February 2023 Published: 26 February 2023 Copyright: © 2023 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/license s/by/4.0/).