The Possibilities for Reuse of Steel Scrap to Obtain Blades for Knives (2017) - PDF

Revista de Metalurgia Vol. 53, Issue 1, January–March 2017, e086 ISSN-L: 0034-8570...

Revista de Metalurgia Vol. 53, Issue 1, January–March 2017, e086 ISSN-L: 0034-8570 http://dx.doi.org/10.3989/revmetalm.086 The possibilities for reuse of steel scrap in order to obtain blades for knives Nada Štrbaca, Ivana Markovića,*, Aleksandra Mitovskia, Ljubiša Balanovića, Dragana Živkovića, †, Vesna Grekulovića a University of Belgrade, Technical Faculty in Bor, VJ 12, 19210 Bor, Serbia *Corresponding author: [email protected] (†Deceased 26th November 2016) Submitted: 4 February 2016; Accepted: 22 February 2017; Available On-line: 23 March 2017 ABSTRACT: The paper presents the characterization results of various types of steel component at the end of product life with the unknown chemical composition, mechanical properties and previously implemented thermo–mechanical treatment. This study was done aiming to examine the possibilities for reuse of some end– of–life agricultural and industrial steel products in order to obtain blades for knives in non–industrial conditions with appropriate and acceptable properties. Demanded shapes of the blades were obtained by applying vari- ous types of thermo–mechanical treatment. Chemical analysis of the investigated steel components was done using the energy–dispersive spectrometer. The microstructure was analyzed using optical and scanning electron microscopy. Hardness of analyzed steel scrap and obtained blades was measured using Rockwell C scale. The hardness values of the obtained blades (with optional quenching or not) indicate to a good selection of the steel end–of–life products for this purpose. KEYWORDS: Blade for knife; Hardness; Microstructure; Steel scrap Citation / Citar como: Štrbac, N.; Marković, I.; Mitovski, A.; Balanović, L.; Živković, D.; Grekulović, V. (2017) “The possibilities for reuse of steel scrap in order to obtain blades for knives”. Rev. Metal. 53(1): e086. http://dx.doi. org/10.3989/revmetalm.086 RESUMEN: Posibilidades de reutilización de la chatarra de acero para la obtención de cuchillas para cortar. El trabajo presenta los resultados de la caracterización de diversos tipos de aceros que han llegado al final de su ciclo de vida útil, y de los que se desconocía su composición química, propiedades mecánicas y tratamiento termomecánico aplicado previamente. El estudio se realizó con el objetivo de analizar las posibilidades de reuti- lización de algunos de estos materiales en aplicaciones agrícolas e industriales, obteniendo hojas de corte. Las formas exigidas a las hojas de corte se consiguieron aplicando diversos tipos de tratamientos termomecánicos. El análisis químico de la chatarra de acero de acero se realizó utilizando Energías Dispersivas de Rayos X. La microestructura se estudió utilizando Microscopía Óptica y Microscopía Electrónica de Barrido. La dureza de la chatarra de acero y de las cuchillas obtenidas se midió utilizando la escala Rockwell C. Los valores de dureza de las cuchillas obtenidas indican una buena selección de los productos finales de acero. PALABRAS CLAVE: Chatarra de acero; Cuchilla; Dureza; Microestructura ORCID ID: Nada Štrbac (http://orcid.org/0000-0003-4836-1350); Ivana Marković (http://orcid.org/0000-0003- 4431-9921); Aleksandra Mitovski (http://orcid.org/0000-0002-9130-2076); Ljubiša Balanović (http://orcid.org/0000- 0002-3551-6731); Dragana Živković (http://orcid.org/0000-0002-2745-5676); Vesna Grekulović (http://orcid. org/0000-0001-6871-4016) Copyright: © 2017 CSIC. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) Spain 3.0. 2 Nada Štrbac et al. 1. INTRODUCTION possibilities for reuse of some end–of–life agricul- tural and industrial steel products in order to obtain Steels refer to alloys of iron with up to approxi- blades for knives (hereinafter termed as blades) in mately 2 wt. % of carbon, very complex by structure, non–industrial conditions with appropriate qual- and widely used as engineering materials because of ity. The article presents the characterization results high iron content in the Earth’s crust, very good fea- of four different steel scrap materials with various tures and low price (Zeng et al., 2009; Kumar and structural, chemical, mechanical properties and Bhushan, 2015). They are characterized by a wide previously implemented thermo–mechanical treat- range of mechanical properties, due to correspond- ment in order to study the possibility for blades ing microstructures, generated during phase trans- production. formations. The special significance of these alloys represents a possibility for a very precise properties 2. EXPERIMENTAL modification during thermal treatment under con- trolled conditions (Goune et al., 2015). For the experimental research, four differ- Any steel product has its own life cycle consisting ent steel components were selected, which repre- of ore extraction, production, processing and fin- sent consumable parts (at the end of their life) of ishing, product use, recycling or withdrawal at the the agricultural and industrial machineries with end of its life cycle (Morfeldt et al., 2015). All steel the unknown chemical composition, previously products have limited usable life cycle after which implemented thermo–mechanical treatment and they lose their properties and become scrap that can mechanical properties. The labels of the steel scrap be returned to the reproductive cycle (Bramfitt and (SS1, SS2, SS3 and SS4) and a pre–given purpose Brenscoter, 2002). of these components at the end of their life are Steel production can be divided into two tech- listed in Table 1. niques: primary and secondary production tech- Figure 1 shows the macrophotographs of the niques which use iron ore or steel scrap, respectively, investigated steel components at end of products as a ferrous resource. Steel primary production life. Experimental procedure included production requires high process energy and large amounts of of blades from the steel scrap by applying different coal, resulting in high CO2 emissions (steel produc- thermo–mechanical treatment. End–of–life prod- tion is the major source of carbon dioxide emis- ucts marked with SS1 and SS4 were mechanically sions). Secondary production technique has a lower treated by cutting, grinding, and polishing in order energy requirement (about one–third of the energy to obtain the desired shape of the blades. The steel for primary production) and CO2 emissions (less scrap marked as SS2 was heated in a forge fire at than one–quarter of the emissions during the pri- temperature 800 – 850 °C for scrap straightening mary production) (Oda et al., 2013; Morfeldt et al., and reshaping it to the straight product. Further 2015). Recycling and reuse of steel component shaping to the final blade was done in the same influence to the decrease in CO2 emissions, decrease route as in samples SS1 and SS4. The steel scrap in use of iron ore as ferrous resource, reduction of sample marked as SS3 was heated to 400 °C and waste and used energy. Steel recycling includes melt- further mechanically treated in the same way as ing of steel components, their recasting and reshap- samples SS1 and SS4. After processing and obtain- ing into new products. Recycling is always more ing the desired blade shapes, the blades made from acceptable option wherever the primary produc- SS2 and SS3 were further annealed in a forge fire to tion can be avoided. Steel reuse is a nondestructive red–heat and quenched in oil. Figure 2 shows the method of reshaping steel components at the end obtained knives as the final products made from the of product life without melting. It can be very effec- initial steel scrap defined by Table 1. tive due to the avoiding of the high costs of energy The samples of the initial steel scrap, obtained required for recycling and conserving the micro- blades and blades made of SS2 and SS3 after oil structure and properties of the initial components quenching were subjected to hardness measurement (Cooper and Allwood, 2012). using the WPM Leipzig Rockwell C hardness tester. During the exploitation and maintenance of agri- cultural and industrial machineries, certain amount of steel scrap is being produced. The maintenance Table 1. Labels and pre–given purpose of the investigated of these machineries, among other things, includes steel scrap consumable machinery components replacement at Label Pre–given purpose of steel scrap the end of their useful life. Therefore, agricultural and industrial steel scrap is of a great environmental SS1 Hacksaw and economic potential (Pacelli et al., 2015). Reuse SS2 Steam turbine moving blade of steel components at the end of product life offer SS3 Sword of a chainsaw even greater advantages for the environment than SS4 Rototiller hoe recycling. Because of that, this article studies the Revista de Metalurgia 53(1), January–March 2017, e086. ISSN-L: 0034-8570 doi: http://dx.doi.org/10.3989/revmetalm.086  The possibilities for reuse of steel scrap in order to obtain blades for knives 3 Figure 1. Macrophotographs of the investigated steel scrap. (a) (b) (c) (d) Figure 2. Knives as the final products made from different steel scrap: (a) SS1 – hacksaw; (b) SS2 – steam turbine moving blade; (c) SS3 – sword of a chainsaw; and (d) SS4 – rototiller hoe. The hardness of the blades was measured at five etched using a solution prepared by mixing 15 ml points, depending on the distance from the cutting HCl, 5 ml HNO3, and 80 ml H2O. Microstructure edge. The average value was taken for further analy- was analyzed using Carl Zeiss Jena Epityp 2 optical sis. The microstructure of the initial steel scrap was microscope (OM) and Tescan Vega 3LMU scanning analyzed after standard procedure for microstruc- electron microscope (SEM). Oxford Instruments tural preparation (grinding, polishing and etching X – Act energy–dispersive spectrometer (EDS) was with 2% solution of nital). Only the sample SS2 was used to determine the chemical composition of the Revista de Metalurgia 53(1), January–March 2017, e086. ISSN-L: 0034-8570 doi: http://dx.doi.org/10.3989/revmetalm.086 4 Nada Štrbac et al. initial steel scrap samples and the element distribu- Figure 4 shows the results of SEM – EDS analy- tion maps of the sample SS4. sis for the steel scrap SS2. Chemical analysis of the area in Fig. 4a is given by EDS spectrum in Fig. 4b. 3. RESULTS AND DISCUSSION It shows that the investigated material is a chrome– alloyed steel with medium carbon content with In Fig. 3, the results of SEM – EDS analysis for grade X20Cr13, according to EN 10250 standard the steel scrap SS1 are given. Chemical analysis of (EN 10250–2, 2000). This was expected, because the the area in the Fig. 3a was given by EDS spectrum steel scrap SS2 was the end–of–life steam turbine in Fig. 3b. It can be observed that the investigated moving blade. Due to its high chrome content, this material is a tool steel with tungsten, molybdenum, steel has got good corrosion resistance, plasticity chromium and vanadium, corresponding to the and high shock resistance (Xi et al., 2008). high speed tool steel with grade HS 6–5–2 according SEM microstructure of the steel scrap SS3 is to EN ISO 4957 (1999). The numerical designation shown in Fig. 5a. Fig. 5b shows the EDS spectrum in the steel grade refers to the content of W, Mo and of the area in the microphotograph in Fig. 5a. The V in wt.%, respectively. This steel has got improved EDS analysis shows that the investigated end–of– cutting performance due to a high content of tung- life product SS3 is a spring steel of grade 38Si7, sten, and it is often used in the manufacturing of according to EN 10089 standard (EN 10089, 2002). a numerous cutting tools (Da Silva Rocha et al., Its chemical composition contains 0.4 – 0.45 wt.% 1999), hacksaws among others. Results of EDS C, 1.8 – 1.9 wt.% Si, 0.8 – 1 wt.% Mn. analysis of white phase (carbides) at points 1, 2 and Figure 6a represents the SEM microstructure of 3 in Fig. 3c were given in Fig. 3d. It is shown that the steel scrap SS4. In Fig. 6b, the EDS spectrum of this phase was the complex carbide phase with W, the area in the microphotograph in Fig. 6a is shown. Mo, Fe, V and Cr. Having into account the chemical composition, (a) (b) 14000 Spectrum 1 12000 Fe 10000 8000 6000 Fe 4000 Mn Cr 2000 V W Mo Cr Fe V Mn W W C W W 0 0 2 4 6 8 10 12 14 16 18 Full Scale 14430 cts Cursor: 19.794 (3 cts) keV (c) (d) Point Elements’ content, at. % C Ti V Cr Fe Mo W 1 34.27 12.10 6.40 15.67 19.76 11.81 2 26.93 9.86 4.81 30.00 16.86 11.54 3 42.77 1.53 16.31 3.00 28.72 4.29 3.38 Figure 3. SEM – EDS results of the steel scrap SS1: (a) SEM microphotograph; (b) EDS spectrum of the area in the microphotograph in Fig. 3a; (c) SEM microphotograph with higher concentration of the white phase; and (d) Results of EDS analysis of white phase (carbides) at points 1, 2 and 3 in Fig. 3c. Revista de Metalurgia 53(1), January–March 2017, e086. ISSN-L: 0034-8570 doi: http://dx.doi.org/10.3989/revmetalm.086  The possibilities for reuse of steel scrap in order to obtain blades for knives 5 (a) (b) 15000 Spectrum 1 Fe Mn 10000 5000 Fe Mn Cr Cr Cr Fe C Si Mn 0 0 2 4 6 8 10 12 14 16 18 20 Full Scale 15362 cts Cursor: 20.248 (4 cts) keV Figure 4. SEM – EDS results of the steel scrap SS2: (a) SEM microphotograph; and (b) EDS spectrum of the area in the microphotograph in Fig. 4a. (a) (b) 35000 Spectrum 1 Fe 30000 25000 20000 15000 Fe 10000 Mn 5000 Fe C Si Mn 0 0 2 4 6 8 10 12 14 16 18 20 Full Scale 35944 cts Cursor: 20.118 (7 cts) keV Figure 5. SEM – EDS results of the steel scrap SS3: (a) SEM microphotograph; and (b) EDS spectrum of the area in the microphotograph in Fig. 5a. it is concluded that the material used in the produc- of production and thermo–mechanical treatment tion of this component is a low–alloy steel of grade in order to product a hacksaw. A 2% nital solution 59Si7 according to ISO 683 – 14 standard (ISO 683– was used for etching steel scrap SS1 because it is the 14, 2004). Its chemical composition contains 0.58 – most commonly used etchant for tool steels. It is a 0.61 wt.% C, 1.8 – 2 wt.% Si, 0.8 – 0.9 wt.% Mn, 0.2 suitable etchant for showing structure of carbides – 0.37 wt.% Cr. The distribution maps of elements (Small et al., 2008). The microstructure is consisted Fe, Cr, Mn, Si and C in the scanning region in Fig. of tempered martensitic structure and un–dissolved 6a are shown in Fig. 7. From the distribution maps, eutectic carbide particles (Leskovsek and Ule, the even distribution of all elements on the investi- 1998). In this kind of steel, directed carbide par- gated region can be observed. ticles of the following types can occur: M6C, MC, The optical microphotographs of the initial steel M2C and M7C3, indicating previous deformation scrap are given in Fig. 8. SEM and optical micro- (Dziedzic, 2007). The chemical composition of the photographs of the steel scrap SS1, given in Figs. 3a carbides done by EDS (Figs. 3c and 3d) shows that and 8a, respectively, show a fine–grained structure the carbides are mainly of the type M2C or M7C3. with directed distribution of complex carbides, Additionally, some primary carbides of the type which has been achieved by the specific methods MC are visible. Revista de Metalurgia 53(1), January–March 2017, e086. ISSN-L: 0034-8570 doi: http://dx.doi.org/10.3989/revmetalm.086 6 Nada Štrbac et al. (a) (b) 30000 Spectrum 1 Fe 25000 20000 15000 10000 Fe 5000 Fe Cr C Si Mn 0 2 4 6 8 10 12 14 16 18 20 Full Scale 30810 cts Cursor: 20.474 (0 cts) keV Figure 6. SEM – EDS results of the steel scrap SS4: (a) SEM microphotograph; and (b) EDS spectrum of the area in the microphotograph in Fig. 6a. (a) (b) (c) 60 µm 60 µm 60 µm (d) (e) 60 µm 60 µm Figure 7. Distribution map of elements in the scanning region in Fig. 6a: (a) Fe; (b) Cr; (c) Mn; (d) Si; and (e) C. Revista de Metalurgia 53(1), January–March 2017, e086. ISSN-L: 0034-8570 doi: http://dx.doi.org/10.3989/revmetalm.086  The possibilities for reuse of steel scrap in order to obtain blades for knives 7 (a) (b) (c) (d) Figure 8. Optical microphotographs of the starting steel scrap: (a) SS1; (b) SS2; (c) SS3; and (d) SS4. The microstructure of the steel scrap SS2 is It is known that in the steel which contains 2% sili- shown in Figs. 4a and 8b. For steel of this composi- con, a large amount of austenite is retained during tion, homogene martensitic structure is expected to cooling to room temperature. With increasing the be formed after air cooling from austenitiziting tem- amount of retained austenite, both ductility and perature (Mann, 2013; Gupta, 2015). Depending strength increase. Silicon is an inhibitor of carbide on a heat treatment conditions, various phases can precipitation and ferrite stabilizer (Matsumura occur in the microstructure. The most appropriate et al., 1987; Chen et al., 1989). The microstructure procedure for heat treatment of this steel involves of steel with silicon may contain retained austenite, quenching from the temperature of about 1000 °C ferrite and martensite or bainite, depending on heat in oil or in the air, following tempering at 700 °C. treatment (Chen et al., 1989). Microstructural con- The resulting microstructure which consists of uni- stituents vary with annealing temperature and time. formly distributed globular carbides in the ferrite For a shorter annealing time during tempering, a matrix can be expected in the structure (Masters, martensite–ferrite structure is a predominant, with 1989), which corresponds to the microstructures a small fraction of bainite and retained austenite, shown in Figs. 4a and 8b. Gooch (Gooch, 1982) similar to the microstructures shown in the Figs. 6a identified these precipitates as M23C6 carbides. and 8d. Longer holding time at annealing tempera- SEM and optical microphotographs of the steel ture results in removal of retained austenite and scrap SS3 are shown in Figs. 5a and 8c. The micro- formation of ferrite–bainite structure (Matsumura structure of the steel scrap SS4 is shown in Figs. et al., 1987). 6a and 8d. According to their composition, both Figure 9 shows the hardness values of the initial steel scrap, SS3 and SS4, belong to spring steel scrap (first column), obtained blades (second col- of type Si2Mn, which have a good combination umn) and hardness values of the blades made of of high strength, good ductility and high shock–­ end–of–life products SS2 and SS3 after oil quench- resistance in the quenched condition as well as in ing (third column). the tempered condition. These types of steel belong The hardness value of the scrap SS1 is 63 HRC, to the hypo–eutectoid class (Qinghua et al., 2003). while the blade made of this steel scrap only by Revista de Metalurgia 53(1), January–March 2017, e086. ISSN-L: 0034-8570 doi: http://dx.doi.org/10.3989/revmetalm.086 8 Nada Štrbac et al. 70 Steel scrap the starting steel scrap. The same hardness value Blades was obtained on the blade made of steel scrap SS3 60 Blades after oil quenching after thermo–mechanical treatment followed by quenching. The hardness values of blades made of 50 steel scrap SS2 and SS4 were lower, 34 HRC and 43 HRC, respectively. By reusing steel scrap for dif- Hardness, HRC 40 ferent purposes various benefits can be achieved 30 (reduction costs, saving energy, less raw material usage and decrease in waste disposal costs), which 20 is in accordance to the basic requirements of the European Union waste management strategy 10 (Vehlow et al., 2007). 0 ACKNOWLEDGMENT SS1 SS2 SS3 SS4 Samples The research results were developed under the Figure 9. Hardness values (HRC) of steel scrap SS1, SS2, projects TR34023, TR34003 and OI172037 for SS3, and SS4; blades made of SS1, SS2, SS3, and SS4; and blades made of SS3 and SS4 after oil quenching. which the funds were provided by the Ministry of Education, Science and Technological Development of the Republic of Serbia. The authors would like using mechanical treatment (cutting, grinding and to thank Prof. Dr. Svetlana Nestorović (deceased in polishing) shows the same hardness value. 2015) and Igor Kalinović for their help. The steel scrap SS2 has a significantly lower value of hardness of about 19 HRC. The blade REFERENCES made of this type of steel scrap has a slightly higher hardness value of about 22 HRC, as a result of the Bramfitt, L., Brenscoter, A.O. (2002). Metallographer’s Guide. applied thermo–mechanical treatment in order to Practices and Procedures for Irons and Steels, ASM Inter- national, Materials Park, Ohio, USA. shape scrap to final blade. This blade was further Chen, H.C., Era, H., Shimizu, M. (1989). Effect of phospho- quenched from the austenitization temperature, rus on the formation of retained austenite and mechani- which additionally increased its hardness value to cal properties in Si–containing low–carbon steel sheet. Metall. Trans. 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