International Research Journal of Engineering and Technology (IRJET)
e-ISSN: 2395-0056
Volume: 13 Issue: 01 | Jan 2026
p-ISSN: 2395-0072
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Modeling the Vickers Hardness Response of HSS after Hardening– Tempering Heat Treatment Using Respon Surface Methodology Afif Prasetya Nugraha Putra1, Agus Suprihanto1, Sri Nugroho1 1 Department of Mechanical Engineering, Faculty of Engineering, Diponegoro University, Semarang 50275,
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Abstract - This study examines the Vickers hardness
Among the various property metrics used to qualify heattreated HSS, hardness is one of the most widely adopted because it is closely associated with resistance to plastic deformation and edge retention in service. Vickers hardness (HV) is particularly suitable for laboratory-scale process development and model building due to its repeatability on small specimens and sensitivity to microstructural variations induced by thermal processing [8], [9], [10], [11]. However, optimizing hardness in HSS is not a simple monotonic “increase temperature to increase hardness” problem. During hardening (austenitizing), increasing temperature generally promotes dissolution of alloy carbides and enriches austenite with carbon and alloying elements, which can increase the hardness of martensite after quenching. Beyond an optimum window, excessive hardening temperature may produce adverse effects such as austenite grain coarsening and increased retained austenite, leading to reduced hardness or higher scatter [12], [13], [14], [15]. Tempering introduces additional complexity because it can either increase or decrease hardness depending on the competition between martensite decomposition and precipitation strengthening. Many HSS grades exhibit secondary hardening at appropriate tempering temperatures due to precipitation of fine alloy carbides; at higher temperatures and/or prolonged exposure, overtempering and precipitate coarsening reduce strengthening and hardness [16], [17], [18], [19].
response of SKH9 high-speed steel (HSS) after hardening– tempering heat treatment using Response Surface Methodology (RSM) with a two-factor interaction (2FI) model. Hardening temperature (850–950 °C) and tempering temperature (450–550 °C) were selected as the primary process variables, while Vickers hardness (HVN) was used as the response. All specimens were hardened for 15 minutes, quenched in water, and tempered for 60 minutes to ensure consistent processing conditions. The experimental results showed that the hardness varied from 448 to 505 HVN within the investigated parameter range, demonstrating a strong dependence on the applied heat treatment. Statistical analysis confirmed that the 2FI model is significant at the 95% confidence level, with tempering temperature identified as the dominant factor controlling hardness reduction. The response surface analysis indicated that high hardness is achieved by combining moderate-to-high hardening temperature with low tempering temperature, whereas lower hardness can be obtained by using high hardening temperature together with high tempering temperature. The developed response surface model provides an effective and practical framework for selecting hardening–tempering conditions to tailor the hardness of SKH9 HSS for specific application requirements. Key Words: High-Speed Steel (HSS); SKH9; Hardening– Tempering; Vickers Hardness; Response Surface Methodology
The current state of the art reflects three complementary streams of work. First, metallurgical investigations have established the mechanisms linking heat-treatment parameters to microstructure evolution particularly carbide precipitation behavior, retained austenite stability, and matrix transformations and have related these changes to mechanical properties, including hardness. These studies provide essential physical understanding of why hardness peaks can occur at intermediate conditions and why secondary hardening may be followed by softening at higher tempering temperatures. Second, industrial and handbookbased guidance offers recommended hardening and tempering practices for specific grades and applications, providing baseline parameter windows and emphasizing the need for strict thermal control to reach target hardness levels. Third, within the broader materials-processing literature, statistical design-of-experiments approaches especially Response Surface Methodology (RSM) implemented using structured designs are widely used to
1. INTRODUCTION High-Speed Steel (HSS) remains a principal engineering material for cutting tools and wear-critical components because it can sustain high hardness while retaining adequate toughness under severe thermo-mechanical loading [1], [2]. In practical manufacturing, the performance of HSS is not determined by chemical composition alone; it is strongly governed by the result of the applied hardening– tempering heat treatment, which establishes a hardened matrix together with a characteristic distribution of alloy carbides and a controlled fraction of retained austenite [3], [4], [5]. Consequently, parameter selection in the heattreatment cycle is a decisive step in achieving the required hardness level and ensuring consistent product quality [6], [7].
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