This paper presents the results of research on the determination of the coefficient of thermal conductivity and hot hardness of cutters made of selected grades of low-alloy high-speed steels, HS 6-5-2 and HS3-1-2. The investigations of hot hardness and yield stress values of HS6-5-2 steel at elevated temperatures have shown that the hot hardness value decreased to 650 – 700 HV (59 – 60 HRC) in the temperature range of 500 – 550 °C. However, the hardness of the samples preheated to the temperature of 500 – 550 °C and measured at room temperature does not change. A decrease of the hot hardness of the steel is correlated with decreasing yield stress at elevated temperature

J. Jaworski

T. Trzepieciński

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Properties of low-alloy high-speed steel at elevated temperature

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75METALURGIJA 56 (2017) 1-2, 75-78J. JAWORSKI, T. TRZEPIECIŃSKIPROPERTIES OF LOW-ALLOY HIGH-SPEED STEEL AT ELEVATED TEMPERATUREReceived – Prispjelo: 2016-04-11Accepted – Prihvaćeno: 2016-08-15Original Scientific Paper – Izvorni znanstveni radJ. Jaworski, T. Trzepieciński: Faculty of Mechanical Engineering and Aeronautics, Rzeszow University of Technology, Rzeszów, PolandThis paper presents the results of research on the determination of the coefficient of thermal conductivity and hot hardness of cutters made of selected grades of low-alloy high-speed steels, HS 6-5-2 and HS3-1-2. The investigations of hot hardness and yield stress values of HS6-5-2 steel at elevated temperatures have shown that the hot hardness value decreased to 650 – 700 HV (59 – 60 HRC) in the temperature range of 500 – 550 °C. However, the hardness of the samples preheated to the temperature of 500 – 550 °C and measured at room temperature does not change. A decrease of the hot hardness of the steel is correlated with decreasing yield stress at elevated temperature.Key words: low-alloy steel, high-speed steel, temperature, hardness, thermal conductivityINTRODUCTIONThe selection of suitable material for a tool is de-pendent on the method of its wear or destruction. De-tailed reviews of tool wear mechanisms in machining have been undertaken by, for example, Akhtar et al. [1] and Dolinšek et al. [2]. This selection is facilitated by characteristics describing the relations between the tool material structure and properties and the work condi-tions of the tool and method of its wear [3]. It is very important to establish suitable physical and mechanical properties of selected tool attributes such as hardness, thermal conductivity, and yield stress of the tool mate-rial [4, 5].To ensure applicable quality requirements during the production of tools made of low-alloy high-speed steel, it is very important that their cutting ability will not be lower than analogical tools made of classical high-speed steels [6, 7].However, if the operating temperature of the tool is close to the tempering temperature of the individual steel grade, reversible processes may affect the stability of the cutting edge of the tool, leading to intensification of the tool wear [8].One of the methods of measurement of thermal con-ductivity λ of metal is a quasi-stationary technique that makes use of phase transition. This method is used to absolutely non-stationary measurements of thermal conductivity [9]. The temperature of a blade during the cutting process can reach significant values [10, 11]. Hot hardness is one of the main parameters that affect the tool durability at elevated temperatures [12, 13].In this paper, the results of research on the determi-nation of the coefficient of thermal conductivity and ISSN 0543-5846METABK 56(1-2) 75-78 (2017)UDC – UDK 669.15:669.14.018:621.185:536.45:620.178.1:536.2=111hot hardness of cutters made of selected grades of low-alloy high-speed steel are presented.EXPERIMENTAL DETAILSMaterials. The investigations of thermal conduc-tivity and hot hardness were carried out for selected grades of low-alloy high-speed steel. The hardness and chemical composition of the tested steel are presented in Table 1.Table 1  Hardness and chemical composition of low-alloy high-speed steelSteelgradeChemical constitution / mas. %Hardness/ HRCC W Mo V Cr Si W+1.5 MoHS3-1-2 1,11 3,35 1,15 1,75 4,65 2,05 5,075 44HS6-5-2 0,82 5,55 5,13 2,06 4,18 – 13,245 40Hardness measurement. In the tests, the measur-ing head is equipped with a spherical indenter with a diameter of 3 mm. The indenter is made of cemented carbide and is attached in a movable flume that addi-tionally contains a permanent magnet (Figure 1).The hard indenter tip deforms the test material elas-tically and plastically and is deformed itself elastically. After the impact body is fully stopped, elastic recovery of the test material and the impact body takes place and causes rebound of the impact body. Both bounce and impact velocities are measured using the coil, in which a voltage proportional to the velocity is induced.Elastic strain energy is calculated based on the measurement of the speed of impact and rebound of the indenter. The Leeb hardness HL is evaluated according to the formula:76J. JAWORSKI et al.: PROPERTIES OF LOW-ALLOY HIGH-SPEED STEEL AT ELEVATED TEMPERATURE METALURGIJA 56 (2017) 1-2, 75-78 HL = v2v1 · 1000 (1)where v1 is the impact velocity and v2 is the velocity of indenter bounce.The test samples are heated in a resistance furnace. The temperature of the sample is recorded by a thermo-couple placed on its surface. The measuring head is placed in the hole in the furnace housing and the meas-urement of hardness HL is carried out. The head is then moved in the x or y direction (Figure 1) and the meas-urement is repeated. The research was carried out at the Materials Center Leoben (Austria).Thermal conductivity measurement. The meas-urement of thermal conductivity based on the quasi-stationary method was carried out on two samples. The first cylindrical sample with a diameter of 6 mm and height of 21 mm was made of HS3-1-2 steel. The sec-ond was made of HS6-5-2 steel with a diameter of 6 mm and height of 20,5 mm. In the measurement meth-od, the sample is located vertically between heat sourc-es. The stream of water flows in from the upper source, generating an increase in the temperature of the sample and the lower source.Four measurements were performed on each sam-ple. Two measurements were also made for average sample temperatures of 50 and 300 °C.RESULTS AND DISCUSSIONThe hardness of HS6-5-2 steel measured after heat-ing the sample to a temperature of 550 °C, holding at this temperature, and cooling to ambient temperature shows almost no change (Figure 2). This suggests that there are no irreversible processes in the steel at this temperature. However, the measurement of hardness at elevated temperature indicates a decrease in hardness of about HV 30 – 40 for every 100 °C, wherein the hardness at 500 °C is about HV 700, which corresponds to a hardness of about 60 HRC. The second area of hardness reduction occurs in the temperature range of 550 – 600 °C and is characterized by a decrease of hardness at elevated temperature. The decrease in hard-ness value is equal to 300 – 400 HV for every 100 °C.Figure 1  Schematic of the Leeb hardness tester for measuring hardness at elevated temperatureFigure 2  Effect of temperature on the hardness of HS6-5-2 steelThe results of measurement of the yield stress (Fig-ure 3) at temperatures corresponding to Figure 2 are comparable to the results of hardness measurements at elevated temperature. The areas of the slight decrease in hardness and an intensive decrease in hardness cor-respond to the areas of intense and slight declines in the value of the yield stress.To assess hardness at elevated temperatures, an in-direct method can be used. It is known that the depend-ence between the yield stress and the temperature is close to the curve describing the hardness–temperature relationship. Knowing the value of the yield stress at a given temperature, we can estimate the changes of hardness with increasing temperature [14].For each measurement of thermal conductivity, the temperature between the upper source Tu and the lower source Tl and the average sample temperature Ta are registered (Table 2).The thermal conductivity decreases for HS3-1-2 steel and increases for the HS6-5-2 in the measured range of temperatures (Figure 4). In the case of a tem-perature of 50 °C, the thermal conductivity differs by only about 20 % for both steel grades. A decrease of the λ coefficient for HS3-1-2 along with an increase in temperature (Figure 4) causes a decreasing amount of conducted heat and, at the same time, an increase of temperature.77J. JAWORSKI et al.: PROPERTIES OF LOW-ALLOY HIGH-SPEED STEEL AT ELEVATED TEMPERATUREMETALURGIJA 56 (2017) 1-2, 75-78The obtained results show that the increased content of alloy additions has not affected the change of the thermal-physical properties of HS3-1-2 steel. For the temperature of 590 °C, the hardness value decreases to about HV 600 and a further reduction in hardness value with increasing sample hold time is observed.After a hold time of approximately 1000 minutes, the sample hardness reaches a value of about HV 500. This means that irreversible processes were proceeding in the sample material. The results of the research show that the decrease in tool life in the temperature range of 500 – 650 °C, which is equal to approximately HV 40 per 10 minutes, is associated with the occurrence of plastic de-formation of the edge of the cutting blade [15].CONCLUSIONSIn the case of low-alloy high-speed steel, a decrease in hot hardness begins at a lower temperature than in the case of steel with a high tungsten content. Further-more, for low-alloy steel, the coefficient of thermal conductivity decreases with an increase in temperature. The thermal conductivity coefficient of steel with a high tungsten content increases with an increase in temperature. HS3-1-2 steel, which has a reduced con-tent of alloy additions, does not show significant dete-rioration of the thermal-physical properties compared to HS6-5-2 steel. A decrease in the thermal conductiv-ity coefficient value with increasing temperature for HS3-1-2 steel causes a decrease in the amount of con-ducted heat and, at the same time, an increase in tem-perature in the cutting edge, which is disadvantageous.AknowledgementsThe work has received funding from the People Programme (Marie Curie Actions) of the European Un-ion’s Seventh Framework Programme FP7/2007-2013/ under REA grant agreement PIRSES-GA-2013-610547 - TAMERREFERENCES[1] W. Akhtar, J. Sun, P. Sun, W. Chen, Z. Saleem, Tool wear mechanisms in the machining of Nickel based super-al-loys: a review, Frontiers of Mechanical Engineering 9 (2014) 2, 106-119, doi: 10.1007/s11465-014-0301-2[2]  B. Dolinšek, J. Šuštaršič, J. Kopač, Wear mechanisms of cutting tools in high-speed cutting processes, Wear 250 (2001) 1-12, 349-356, doi: 10.1016/S0043-1648(01)00620-2[3]  S. Jozić, D. Bajić, A. Stoić, Flank wear and surface rou-ghness in end milling of hardened steel, Metalurgija 54 (2015) 2, 343-346Figure 4  Effect of temperature on the value of the thermal conductivity coefficientFigure 3  Effect of temperature on the yield stress of HS6-5-2 steelTable 2 Contact sample temperaturesMaterial Sample no. Tu / °C Tl / °C Ta / °CHS3-1-21 37,317 41,463 33,1712 43,659 52,683 34,6343 280,488 317,07 243,904 315,854 375,61 256,09HS6-5-21 52,366 61,317 43,4152 53,415 63,902 42,9283 314,63 356,098 273,174 348,78 404,878 292,68Figure 5  Effect of temperature and hold time on the hardness of the sample made of HS6-5-2 steel78J. JAWORSKI et al.: PROPERTIES OF LOW-ALLOY HIGH-SPEED STEEL AT ELEVATED TEMPERATURE METALURGIJA 56 (2017) 1-2, 75-78[4]  V. P. Astakhov, The assessment of cutting tool wear, Inter-national Journal of Machine Tools and Manufacture 44 (2004) 6, 637-647, doi 10.1016/j.ijmachtools.2003.11.006[5] M. Kowalik, T. Trzepieciński, Examination of the influen-ce of pressing parameters on strength and geometry of joint between aluminum plate and sheet metal, Archives of Civil and Mechanical Engineering 12 (2012) 3, 292-298, doi: 10.1016/j.acme.2012.05.002[6]  J. Jaworski, T. Trzepieciński, Research on durability of tur-ning tools made of low-alloy high-speed steel, Kovove Materialy–Metallic Materials 54 (2016) 1, 17-25, doi: 10.4149/km_2016_1_17[7]  J. Jaworski, R. Kluz, T. Trzepieciński, Operational tests of wear dynamics of drills made of low-alloy high-speed HS2-5-1 steel, Eksploatacja i Niezawodnosc – Maintenan-ce and Reliability 18 (2016) 2, 271-277,  doi: 10.17531/ein.2016.2.15[8] B. Podgornik, V. Leskovšek, Experimental evaluation of tool and high-speed steel properties using multi-functional KIc-test specimen, Steel Research International 84 (2013) 12, 1294-1301, doi: 10.1002/ srin.201300053[9] T. F. Fodemski, Thermal Measurements, WNT, Warszawa, 2001, pp. 145-157[10] M. Pellizzari, D. Cescato, M. G. De Flora, Hot friction and wear behaviour of high speed steel and high chromium iron for rolls, Wear 267 (2009) 1-4, 467-475,  doi: 10.1016/j.wear.2009.01.049[11] M. Sato, T. Aoki, H. Tanaka, S. Takeda, Variation of tem-perature at the bottom surface of a hole during drilling and its effect on tool wear, International Journal of Machine Tools and Manufacture 68 (2013), 40-47, doi: 10.1016/j.ijmachtools.2013.01.007[12] D. Bombač, M. Terčelj, M. Fazarinc, P. Fajfar, Increasing of hot workability of 1.3302high speed steel, Metalurgija 51 (2012) 3, 313-316[13] W. D. Kingrey, Property Measurements at High Tempera-tures, John Wiley & Sons, New York, 1959, pp. 352-362[14] P. Helman, ASP High-Speed Steels, Properties and Possi-bility of Application, Stora Koparberg, Warsaw, 1977, pp. 18-19.[15] J. A. Geller, Tool Steels, Metallurgy, Moscow, 1983, pp. 89-97 (in Russian)Note:  The person responsible for English language is Mrs. Anna Pilch, Rzeszów, Poland

Properties of low-alloy high-speed steel at elevated temperature

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