In this paper is given the research conducted at the specified quality screws which had different mechanical properties after completion of the same heat treatment. Results of chemical and metallographic analysis and hardness tests indicated the deficiencies that can be corrected in the course of production and thermal processing of finished products - screws

B. Fakić

J. Duraković

M. Oruč

M. Rimac

S. Muhamedagić

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Effects of heat treatment on the properties of low carbon steel 19MnB4 for screws

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193METALURGIJA 55 (2016) 2, 193-196M. ORUČ, J. DURAKOVIĆ, S. MUHAMEDAGIĆ, B. FAKIĆ, M. RIMACEFFECTS OF HEAT TREATMENT ON THE PROPERTIES OF LOW CARBON STEEL 19MnB4 FOR SCREWSReceived – Primljeno: 2015-04-07Accepted – Prihvaćeno: 2015-10-05Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 55(2) 193-196 (2016)UDC – UDK 4DC 669.14:536.4:620.18:620.178.1=111M. Oruč, University  of  Zenica,  Zenica, B and H; J. Duraković, S. Muhamedagić, University of Zenica, Faculty of Metallurgy and Ma-terials Science, Zenica, B and H; B. Fakić, M. Rimac, University  of Zenica, Metallurgical Institute „Kemal Kapetanović“, Zenica, B and HIn this paper is given the research conducted at the specified quality screws which had different mechanical proper-ties after completion of the same heat treatment. Results of chemical and metallographic analysis and hardness tests indicated the deficiencies that can be corrected in the course of production and thermal processing of finished products - screws.Key words: low carbon steel, thermal processing, microstructure, hardnessINTRODUCTIONSteels for making screws, nuts and rivets can be low carbon steels with up to 0,20 % C, carbon steels with 0,20 to 0,50 % C and low alloyed (Cr, V, Mo) steels for quenching and tempering [1, 2].Guaranteed properties of carbon steels for screws, nuts and rivets are prescribed by standards (such as DIN 17111). Depending on the method of production these steels are divided by the level of quality in three groups with precisely defined types of steel and quality [1].The proper selection of the type of steel for screws can be made if you know the approximate value of strength features by using the so-called strength class.This paper investigated the characteristics of screws obtained from steel tagged 19MnB5 after heat treatment - hardening. From this steel were made screws labeled as M 16x165 in accordance with DIN or ISO 931/8.8 4014. These are hexagons screws for significant parts of machine constructions. Steel with the mentioned tag is a fine-grain steel micro-alloyed with boron. Boron is added to unalloyed and low-alloy steels to increase hardness and hardenability [3].Boron-containing steels are used as high-quality structural steels intended for heat treatment, hardening steels and steels for cold working such as steels for screws.The basic effect of boron on the properties of the steel is to increase hardenability, which is evident al-ready at a very low concentration, of the degree of 0,0010 % B. Even a relatively small amount of boron gives the same effect of hardenability enhancement as other more expensive elements which must be added in much bigger quantities [4].According to research, there is no difference in the hardness on the surface between the boron-containing and the boron-free steel. The hardness of the surface of hardened steel is not determined by boron, but by the martensitic structural state influenced by the carbon content. The effect of boron on the increase of hardness is essential just below the surface [3].The operative mechanism which is decisive for the enhancement of hardenability by boron derives from a delay in the transformation to the bainite, ferrite and pearlite structures, which are softer than martensite, taking place over cooling from the austenitisation tem-perature after annealing or from the hot working [3].EXPERIMENTAL WORKThis paper presents testing of the three screws made according to DIN 931 Class 8.8 (Figure1) from steel tagged 19MnB4. Screws, after quenching in oil with a temperature of 900 °C had partly unsatisfactory results of tensile strength, ie two screws did not satisfy pre-scribed values (screws labeled 1 and 2) and screw num-ber 3 had satisfactory strength properties. In order to solve the problem, apart from control of parameters in Heating Furnaces we approached the de-tailed analysis such as examination of the chemical composition, metallographic examination, testing the hardness and layer decarburisation.Figure 1 Appearance of hexagons screws [5]194M. ORUČ et al.: EFFECTS OF HEAT TREATMENT ON THE PROPERTIES OF LOW CARBON STEEL 19MnB4 FOR SCREWS METALURGIJA 55 (2016) 2, 193-196Chemical analysisResults of the chemical composition of screw mate-rials are given in Table 1. We studied the three screws, two which did not satisfy after the tensile test and the one which had the required value of tensile strength (code 3).In Table 1 is given the chemical analysis of steel 19MnB4 prescribed by standard EN10269 for compari-son [6]. The chemical analysis of all three screw samples in-dicates that all elements except boron are within the prescribed limits. Chemical analysis also indicates that microalloying with boron has not been achieved, and its influence could not come to the fore. Also, increased aluminum content can affect the appearance of inclu-sions (alumina) and also the grain size, hardenability and thus properties of the steel [7].Hardness testingHardness testing was conducted on the surface of the body of the screw, on the cross-section, and on the head of the screw. Results are given in Table 2. Testing was performed on universal hardness tester at the Institute of Metallurgy „Kemal Kapetanović“, University of Zenica.From the Table 2 it can be seen that the results of the hardness after heating at 900 °C and then quenching in the hot oil, at the temperature of 70 °C, are not uniformed.All three screws have uniformed hardness along the rim and on the surface of the screw heads. Hardness section and on the surface of the screw is considerably smaller for the screw 1 and 2 in relation to the screw 3 which had a satisfactory values of tensile strength.Metallographic examinationAfter the hardness tests, were carried out micro-structural examination and testing of the presence of non-metallic inclusions and layers of decarburisation. Test results of microstructure are given in Figure 2 for all three samples. In the first sample microstructure is: ferrite, finela-mellar pearlite, bainite, martensite,In the second sample microstructure is: ferrite, finelamellar pearlite, bainite, martensite,In the third sample microstructure is: bainite, mar-tensite. Based on the testing results of the microstruc-ture was found that the obtained microstructure corre-sponds to the results of the hardness.By testing of non-metallic inclusions it was found presence of globular oxides in all screw samples while in the second sample was noted and a series of thin alu-minate inclusions Table 3. On screw samples - the body of the screw and the part of the screw with thread was noted partial decar-burisation. Testing was performed according to ASTM E 1077-01 (R 2005) and by measuring microhardness according to standards EN ISO 898-1:2010.The test results of samples of screws show partial decarburisation at the body of screws of 0,15 mm which was confirmed by testing of micro-hardness Figure 3.Table 1 The chemical compositon / wt. %Steel C Si Mn P max S max Al min BSample screws 1 0,24 0,08 0,99 0,010 0,011 0,030 < 0,00012 0,22 0,08 0,99 0,010 0,012 0,027 < 0,00013 0,20 0,08 0,99 0,010 0,013 0,032 < 0,0001Steel 19MnB4(according to EN 10269)0,17-0,24± 0,010,40± 0,030,80-1,15± 0,040,030+ 0,0050,035+ 0,0050,020± 0,050,0008-0,005± 0,0005Table 2 Results of hardness testing Mark ofscrewHardness, average valuesThe body of the screw Head of the screwCross Body Surface AreaScrew 1 200/HV10 229/HV10 35/HRC 35/HRCScrew 2 25/HRC 29/HRC 36/HRC 36/HRCScrew 3 30/HRC 36/HRC 37/HRC 37/HRCTable 3 Test results of inclusionsASTM E 45, method A, the worst fi eldSampleSulfi des (A)Aluminates (B)Silicates (C)Oxides(D)Globular oxides (DS) d >13 μmT D T D T D T D -1 1,5 - - - - - 2,5 1,0 -2 1,0 - 1,0 - - - 2,5 1,0 -3 1,0 - - - - - 2,5 1,0 -T – thin; D – heavy; DS – single globular type195M. ORUČ et al.: EFFECTS OF HEAT TREATMENT ON THE PROPERTIES OF LOW CARBON STEEL 19MnB4 FOR SCREWSMETALURGIJA 55 (2016) 2, 193-196Microhardness was tested on a Zwick apparatus for measuring the microhardness at the Institute of Metal-lurgy of the University of Zenica.ANALYSIS OF RESULTSAccording to the results of tests of screws it can be concluded that the screws tagged as M 16x165 DIN 931/8.8 do not have uniformed quality, ie they do not have uniformed hardness values  nor the microstructure. Conducted tensile tests were eliminatory from the as-pect if the screws satisfied in terms of tensile strength, which has also been uneven.Testing of the chemical composition of steel screws tagged 19MnB4 confirmed the absence of microalloy-ing with boron, with which this steel could not be treat-Screw 12 % HNO3   x 150 a) MicrostructureScrew 22 % HNO3   x 1500 b) Detail of the microstructureScrew 32 % HNO3   x 150 c) Microstructure2 % HNO3   x 1500d) Detail of the microstructure2 % HNO3   x 150 e) Microstructure2 % HNO3   x 1500 f) Detail of the microstructureFigure 2 The microstructure of the screw 1 (a, b), the screw 2 (c, d) and the screw 3 (e, f )196M. ORUČ et al.: EFFECTS OF HEAT TREATMENT ON THE PROPERTIES OF LOW CARBON STEEL 19MnB4 FOR SCREWS METALURGIJA 55 (2016) 2, 193-196ed as indicated. Lack of boron at this dimension of screws and heat treatment which was carried out at the production process could not provide the stability of the quality characteristics that were required.Hardness tests revealed that only screw 3 labeled as good had a hardness value which was required by stand-ard, while the other two screws had lower hardness val-ues  on the screw body noting that the cross-sectional hardness was greater than the hardness of the surface of the screw. This can be interpreted and by decarburisa-tion of the surface.Metallographic testings showed that the microstruc-ture of the screw 3 was martensitic + beinitic and it was possible with thermal annealing treatment with cooling in oil to obtain mechanical properties. Structure of screws 1 and 2 is ferrite, pearlite, mar-tensite and beinitic, ie with components that will not give the required characteristics but which indicate that thermal treatment is not carried out completely [8].The presence of non-metallic inclusions, particulary type of globular oxide contributes to poorer results by tensile testing [9].CONCLUSIONS Steel from which are made  screws according to DIN 931/8.8 and according to the chemical composition does not belong to the group of carbon steel alloyed with boron. This steel according to the percentage of carbon, during the heat treatment of quenching in oil pro-vides martensitic- bainitic structure but must take into account the temperature at which is performed tempering and cooling rate. Lack of boron would affect the microstructure and consequently the maximum hardness and hardness beneath the surface. Increased aluminum content influences the occur-rence of inclusions and the grain size, hardenability and thus the properties of the steel. The presence of non-metallic inclusions particular type oxide also affects the achievement of good me-chanical properties, and from that aspect during pro-duction should take into account the strength of steel, which is intended to create responsible fasten-ers - screws. Controlled warm up in the oven and furnace atmos-phere can affect the layer of decarburisation steel and feature enhancement.REFERENCES[1] BAS EN 10020/2002[2] N. Haračić: Savremeni materijali za mašino-gradnju, MF, Univerzitet u Zenici, 2012, p. 82[3] N. Haračić: Boron and Low Alloy Steels for Carburisation and Direct Quanching, Journal of Mechanical Engineering 4 (2002) 6, 215-226[4] https:/www.scribd.com/doc/69259568/celici-za-vijke [ac-cessed 18 oct. 2011][5] DIN 931/8.8[6] EN 10269/2009[7] B. Baručija, M. Oruč, O. Beganović, M. Rimac, S. Muha-medagić: Influence of Thermomechanical Processing on the Cold Deformability of Low - Carbon Steel Materiali in technologije 47 (2013) 5, 665-668[8] Metals Handbook, Vol. 7, Atlas of Microstructures of In-dustrial Alloys, 8th edition, in 1972[9] J. Duraković i dr.: Termički tretman vijaka od niskouglje-ničnog čelika u protočnoj peći, X Naučno/stručni simpozij s međunarodnim učešćem, “Metalni i nemetalni materija-li”, B and H, Zenica, 2014, p. 87-92Note:  English language lecturer: Dijana Hasanica, Zenica, Bosnia and Herzegovina2 % HNO3   x 36Figure 3 Sample screws 1, hardness values HV0,3 – screw body

Effects of heat treatment on the properties of low carbon steel 19MnB4 for screws

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