Influence of the Thermal Cutting Process on Cracking of Pearlitic

materials

Article Inﬂuence of the Thermal Cutting Process on Cracking of Pearlitic Steels

Lechosław Tuz * , Aneta Ziewiec and Krzysztof Pa ´ncikiewicz

Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, 30-059 Kraków, Poland; [email protected] (A.Z.); [email protected] (K.P.) * Correspondence: [email protected]

Abstract: The paper presents research results of the inﬂuence of heat input into high carbon rail steel during cutting processes on microstructure transformation and cracking. The massive block of steel prepared for rail rolling processes was cut and examined by nondestructive magnetic testing and destructive testing by microscopic examination and hardness measurements. The results show unfa- vorable microstructure changes where pearlite and transformed ledeburite were obtained. The effects of the presence of such microstructures are high hardness near to cutting surfaces (above 800 HV) and microcracks which grow into low hardness block cores during rolling and rail shaping.

Keywords: thermal cutting; pearlitic steels; rail ways; cracking; microstructure; magnetic testing

1. Introduction Citation: Tuz, L.; Ziewiec, A.; The railway system plays a vital role in world transport and is therefore an area of Pańcikiewicz,K. Influence of the continuous development in terms of railway infrastructure [1–4], vehicles [5,6], manage- Thermal Cutting Process on Cracking ment [7–10] and services [11–13]. The key elements of the railway infrastructure are the of Pearlitic Steels. Materials 2021, 14, rails enabling trains to move, as well as frogs and turnouts enabling the change of their 1284. https://doi.org/10.3390/ track. The basic materials used for these elements are hypoeutectoid or peri-eutectoid ma14051284 pearlitic steels [14]. To obtain the appropriate shape, it is necessary to use casting, plastic working or heat treatment processes, enabling the profiling of the desired shape and ob- Academic Editors: Dariusz Fydrych, taining the required mechanical properties. European standard EN 13674-1 distinguishes Andrzej Kubit, Ján Slota nine grades of steel—five grades are used in the raw state after rolling (R200, R220, R260, and Agnieszka Kowalczyk R260Mn, R320Cr) and four grades of rails which have been heat-treated (R350HT, R350LHT, R370CrHT and R400HT). Their hardness can vary widely from 200 to 400 HBW [14]. Received: 5 February 2021 The charge material for frogs or turnouts are blocks that are prepared by cutting to the Accepted: 4 March 2021 Published: 8 March 2021 technologically required size. One of the cutting processes used for this purpose is thermal cutting with an acetylene-oxygen or a propane-oxygen method. This process is used for

Publisher’s Note: MDPI stays neutral cutting structural steels, but most of the alloying elements found in steels make cutting dif- with regard to jurisdictional claims in ficult and even impossible from a certain level of their content. These difficulties are mainly published maps and institutional affil- the result of an increase in the flowability of the liquid metal and slag, reduced heat transfer iations. in the material and a decrease in oxidation activity. The negative effect of the alloying elements is combined, which means that in practice the carbon content must be significantly lower than the allowable one, as steels always contain other alloying elements [15]. In the thermal cutting process, a change in chemical composition occurs near the cutting surface. The surface of the metal being cut is enriched with carbon, nickel and Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. molybdenum, while it is depleted in silicon, chromium and manganese [15–18]. A heat This article is an open access article affected zone (HAZ) appears during cutting, but its importance is less than during welding, distributed under the terms and as the separation of the material helps to relieve some of the stresses. Structural changes conditions of the Creative Commons in HAZ can be a problem as hardening of the edges can occur with hardenable steels, Attribution (CC BY) license (https:// making it difficult to machine afterwards. These structural changes can influence the creativecommons.org/licenses/by/ corrosive properties of the steels. Microcracks, similar to quenching cracks, also occur in 4.0/). HAZ, which can become corrosion centers and stress concentration notches [19]. Due to the

Materials 2021, 14, 1284. https://doi.org/10.3390/ma14051284 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 10

making it difficult to machine afterwards. These structural changes can influence the cor- Materials 2021, 14, 1284 rosive properties of the steels. Microcracks, similar to quenching cracks, also occur2 of 10 in HAZ, which can become corrosion centers and stress concentration notches [19]. Due to the presence of HAZ and geometric deviations on the edges of the cut, it is often recom- presencemended ofto HAZfinish and them geometric mechanically deviations by cutting on the or edges grinding. of the Such cut, it operations is often recommended increase the tocost finish of execution, them mechanically but even the by total cutting cost or of grinding. the thermal Such cutting operations and finishing increase mechanical the cost of execution,treatment butis almost even thealways total much cost of lower the thermal than mechanical cutting and cutting, finishing which mechanical also often treatment requires isfinishing almost always. much lower than mechanical cutting, which also often requires finishing. TheThe paperpaper presentspresents thethe resultsresults ofof researchresearch onon crackingcracking R260R260 steelsteel usedused forfor railsrails andand turnouts.turnouts. In thethe productionproduction processesprocesses ofof shapingshaping products,products, thethe presencepresence ofof thethe surfacesurface microcracks,microcracks, asas wellwell asas macrocracksmacrocracks leadingleading toto fragmentationfragmentation ofof surfacesurface fragments,fragments, waswas observed.observed. BasedBased onon thethe analysisanalysis ofof manufacturingmanufacturing processes,processes, itit waswas revealedrevealed thatthat crackscracks appearappear already at the the stage stage of of the the thermal thermal cutting cutting process process of billets of billets into into smaller smaller parts. parts. The Thepaper paper presents presents the theresults results of the of thetests tests carried carried out outfor forthe theR260 R260 stee steel,l, made made to determine to determine the theinfluence influence of the of thethermal thermal flame flame cutting cutting process process on the on microstructural the microstructural changes changes of the of HAZ the HAZand cracking and cracking of the of cut theting cutting surfaces. surfaces.

2.2. MaterialsMaterials andand MethodsMethods ForFor thethe tests,tests, aa rodrod with with dimensions dimensions of of 260 260 mm mm× ×200 200mm mm× × 5555 mmmm afterafter thermalthermal cuttingcutting withwith anan acetylene-oxygenacetylene‑oxygen burnerburnerwas was used.used. TheThe automaticautomatic EckertEckert cuttingcutting machinemachine withwith gasgas torchtorch withwith A-MDA‑MD nozzlenozzle (Eckert(Eckert AS,AS, Legnica,Legnica, Poland)Poland) waswas usedused toto cutcut thethe work-work- piece.piece. CuttingCutting process was carried carried out out with with the the following following process process parameters: parameters: diameter diameter of 3 ofa gas a gas nozzle nozzle 6 mm, 6 mm, gas gas flow ﬂow 600 600 dm dm3/min,/min, oxygen oxygen pressure pressure 300 300kPa, kPa, cutting cutting speed speed 140 140mm/ mm/ min. min A general. A general view viewof the ofcut the rod cut is presented rod is presented in Figure in 1a. Figure The1 chemicala. The chemical composi- compositiontion of the tested of the steel tested is given steel is in given Table in 1. Table Magnetic1. Magnetic particle particle tests were tests performed were performed on the oncut the rod cut. The rod. results The results of the magnetic of the magnetic particle particle tests showed tests showed the cut theting cutting surfaces surfaces with cracks with cracks(Figure (Figure 1b). 1b).

(a) (b)

FigureFigure 1.1.General General viewview ofof thethe cutcut rodrod after:after: ((aa)) cutting,cutting, ((bb)) paintingpainting withwith primer primer paint paint and and magnetic magnetic particle particle test. test.

TableBending 1. Chemical a sample composition with of an tested existing steel (datacrack from was the carried manufacturer out on an inspection Instron certiﬁcate MTS testing 3.1). machine (Instron, Norwood, MA, USA) at a speed of 20 mm/min, in order to open of a crackElement surface. The C depth of Mn the cracks Si was measured Cr on the Ni fracture using Cu a stereoscopic P microscopeWt. % and 0.79 it does not 1.17 exceed 10 0.48 mm. 0.10 0.09 0.18 0.018 A characteristic feature of the observed cracks is the fact that the transverse and lon- gitudinal cracks run in the same planes. Macroscopic and microscopic tests (light and Bending a sample with an existing crack was carried out on an Instron MTS testing scanning microscopes) and hardness measurements were performed on the samples. One machine (Instron, Norwood, MA, USA) at a speed of 20 mm/min, in order to open of a sample was cut transversely to the bar axis, the other was cut along the bar length. crack surface. The depth of the cracks was measured on the fracture using a stereoscopic microscope and it does not exceed 10 mm. A characteristic feature of the observed cracks is the fact that the transverse and longitu-

dinal cracks run in the same planes. Macroscopic and microscopic tests (light and scanning Materials 2021, 14, 1284 3 of 10

microscopes) and hardness measurements were performed on the samples. One sample was cut transversely to the bar axis, the other was cut along the bar length. Macroscopic examinations were carried out using a Delta Optical XTL-IV PRO T (Delta Optical, Mi´nskMazowiecki, Poland) stereoscopic microscope. Observation of the microstructure was performed with a Leica DM/LM (Leica, Wetzlar, Germany) light microscope. Pearlite and cementite percentages were assessed using the image analysis software Sigma Scan Pro (Version 5, 2007, Systat Software, San Jose, CA, USA). Carbon content was determined using the function (Equation (1)) based on lever rule [20].

0.77% × xP 6.67% × xCm %C = + (1) 100 100 where: xP is pearlite percentage, xCm is cementite percentage. Fractographic studies were carried out with the use of a scanning electron microscope Phenom XL (Thermo Fisher Scientiﬁc, Waltham, MA, USA). Energy Dispersive Spectroscopy (EDS) analysis was performed at a distance of approx. 0.2, 0.3, 0.4, 1.0 and 5.0 mm from the cut surface with 20 kV and 10 nA beam for 60 s. For microscopic examination, the samples were etched in a 4% alcoholic nitric acid solution. X-Ray Diffraction (XRD) was performed on a D8 Advance Diffractometer (Bruker, Billerica, MA, USA) using CuKα radiation. Hardness measurements were made using the Vickers method with an intender load of 10 kG (98.07 N), using a Zwick/Roell ZHU 187.5 (Zwick Roell Group, Ulm, Germany) hardness tester and with an intender load of 0.01 kG (0.09807N), using a Innovtest (Bowers Group, Bradford, UK) hardness tester.

3. Results and Discussion The microstructure analysis revealed the presence of a heat affected zone (HAZ) under the cutting surface. Figure2a shows the HAZ microstructure with marked areas of hardening, normalization and the pearlitic microstructure of the base material that was not transformed during cutting. A characteristic feature of the structure after thermal flame cutting is the presence of a layer of liquid metal with a structure containing transformed ledeburite, pearlite and secondary cementite (Figure2b). Under the solidified metal layer, there is a dark unhardened zone with the structure of fine pearlite and secondary cementite, and then the hardened area. The presence of pearlite under the solidified metal layer is the result of a lower cooling rate of the surface layer compared to the deeper layers. The influence of heat release on the cooling rate was observed in several experiments [15,21]. The lower cooling rate is caused by the heat release in the process of solidifying the layer of nonremoved liquid and thus lowering the cooling rate of the steel under the solidified layer. The presence of cementite is the result of carbon diffusion into the steel surface layer. Further away, the cooling rate is high due to the high rate of heat dissipation to the cool material. A similar tendency was observed after the thermal cutting of S355 steel [15,21]. The further area of the material heated in the cutting process to the temperature lower than Ac1 has a pearlitic microstructure (Figure2g,h). The base material has a pearlitic microstructure. There is no difference between the base metal microstructure and the pearlitic microstructure of the material heated in the cutting process to the temperature lower than Ac1. The microstructure of the hardened zone consists of martensite and a large amount of residual austenite (Figure2e,f). The presence of austenite was confirmed by XRD (top right inset in Figure2e). In the segregation bands with a reduced carbon and manganese content, pearlite appears at the grain boundaries. The hardened area is approx. 3 mm thick. Hardness measurements by the Vickers method showed that the hardness of the hardened layer is very high and exceeds 800 HV10 (Figure3). It lowers as it moves away from the cut surface. Similar hardness profiles were obtained when cutting S345, S390, S700MC, S690QL steels [22,23]. A significant problem in the thermal cutting process is the change in the chemical composition that occurs near the cut surface. The carbon content in the liquid layer was Materials 2021, 14, 1284 4 of 10

approx. 3.2% (area A in Figure4) calculated on the basis of equation (1) [ 20]. The data of the volume fraction of the phases used during calculations was: 59% perlite and 41% cementite. In the nonmolten metal layer under the solidiﬁed liquid layer, the carbon content is about 1.2% (93% of pearlite and 7% of cementite) (area B in Figure4). In the base material, the calculated carbon content was 0.77 and was close to the average carbon content in steel. The quantitative evaluation of the cementite content showed that the solidiﬁed liquid is highly carburized. The authors [15–18] show that carburization is the result of the selective oxidation Materials 2021, 14, x FOR PEER REVIEWof the steel components. The surface layer of the cutting surface is depleted in silicon,4 of 10 chromium, and manganese and enriched with carbon, nitrogen, nickel and molybdenum.

(b) (c)

(a) (d) (e)

(f) (g) (h)

Figure 2. MicrostructureMicrostructure atatthe the cut cut surface: surface: (a ()a general) general view view (H—hardened (H—hardened zone, zone, N—normalization N—normalization zone, zone, BM—base BM—base material, material,L— transformed L— transformed ledeburite ledeburite + pearlite + pearlite + secondary + secondary cementite, cementite, P—pearlite P—pearlite with secondarywith secondary cementite, cementite, M—martensite M—martensite with with austenite); (b) detail view near cut surface; (c) transformed ledeburite (486 HV0.01), pearlite (222 HV0.01) and sec- austenite); (b) detail view near cut surface; (c) transformed ledeburite (486 HV0.01), pearlite (222 HV0.01) and secondary ondary cementite (605 HV0.01); (d) pearlite with secondary cementite (375 HV0.01), (e,f) martensite with austenite (820 cementite (605 HV0.01); (d) pearlite with secondary cementite (375 HV0.01), (e,f) martensite with austenite (820 HV0.01) and HV0.01) and XRD pattern in top right inset; (g–h) pearlitic microstructure of the material heated in the cutting process to XRD pattern in top right inset; (g,h) pearlitic microstructure of the material heated in the cutting process to a temperature a temperature lower than Ac1—base material (227 HV0.01). lower than Ac1—base material (227 HV0.01). The microstructure of the hardened zone consists of martensite and a large amount of residual austenite (Figure 2e–f). The presence of austenite was confirmed by XRD (top right inset in Figure 2e). In the segregation bands with a reduced carbon and manganese content, pearlite appears at the grain boundaries. The hardened area is approx. 3 mm thick. Hardness measurements by the Vickers method showed that the hardness of the hardened layer is very high and exceeds 800 HV10 (Figure 3). It lowers as it moves away from the cut surface. Similar hardness profiles were obtained when cutting S345, S390, S700MC, S690QL steels [22,23].

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FigureFigure 3. 3. HardnessHardness distribution distribution from from the the cutting cutting surface. surface.

A significant problem in the thermal cutting process is the change in the chemical composition that occurs near the cut surface. The carbon content in the liquid layer was approx. 3.2% (area A in Figure 4) calculated on the basis of equation (1) [20]. The data of the volume fraction of the phases used during calculations was: 59% perlite and 41% cementite. In the nonmolten metal layer under the solidified liquid layer, the carbon content is about 1.2% (93% of pearlite and 7% of cementite) (area B in Figure 4). In the base material, the calculated carbon content was 0.77 and was close to the average carbon content in steel. The quantitative evaluation of the cementite content showed that the solidified liquid is highly carburized. The authors [15–18] show that carburization is the result of the selective oxidation of the steel components. The surface layer of the cutting surface is depleted in silicon, chromium, and manganese and enriched with carbon, nitrogen, nickel and molybdenum. Surface carburization during cutting with oxygen is associated with the selective oxidation of iron. The iron from the liquid layer can easily diffuse through the liquid FeO layer to reach the cutting oxygen. Carbon does not dissolve in FeO. The direct oxidation of carbon with oxygen from the cutting stream does not take place because the rate of oxygen( adiffusion) through the FeO layer is too slow (comparedb) to the speed of the cutting Figure 4. TheFigure results 4. ofThe the resultsprocess. microanalysis of theMuch microanalysis of less the carburization chemical of the composition chemical or slight composition in thedecarburization areas A,in B,the and areas of C, the A, performed B,upper and C part on, a of SEM-EDS: the cutting (a) microstructure;performed (b) SEM-EDS on surfacea SEM results‑EDS is related: (a) microstructure to the direct; (b )action SEM-EDS of theresults cutting oxygen on the metal near the torch nozzle. Edge carburization is possible only when a liquid FeO film is formed and carbon The Si contentcombustionSurface at the is carburizationcuttinginhibited. surface The during presentedis 0.14 cutting% and processes Mn with 0.3 oxygencause% (Table an is intensive associated2). This areachange with is thein the selective con- depleted in Sicentrationoxidation and Mn inof of relationcomponents iron. The to the iron from average from the thecutsteelting liquid composition. surface layer to can the A easily reductionbase material diffuse in the through in Sithe area the of liquid the content by aboutmeltedFeO 70% layer layer and to [ 15Mn reach–18,2 by 3about the–25 cutting]. 74%The presentwas oxygen. observed. research Carbon As showed you does move notthat away dissolvethe surface from in the FeO.layer Theof the direct cut cutting surface,surfaceoxidation the content is depleted of carbon of Mn of withand silicon Si oxygen increases. and frommanganese At the a cuttingdistance in relation stream of 0.3 to mm does the from notaverage take the sur- placecontent because of these the face, it is 0.18elementsrate% Si of and oxygen in 0.6steel.5% diffusion Fig Mn.ure In 4 throughthe shows area the the of martensitephotomicrograph FeO layer is with too slowretained of the compared cutting austenite, tosurface the it speed with ofEDS the amounts to 0.4analysiscutting5% Si sites. process.and 1.28 Much% Mn lesson carburizationaverage, and oris close slight to decarburization the average Si of and the Mn upper part of the content in steel.cutting surface is related to the direct action of the cutting oxygen on the metal near the torch nozzle. Edge carburization is possible only when a liquid FeO ﬁlm is formed and Table 2. Averagecarbon values combustion of elements content is inhibited. in EDS The analysis presented for theprocesses base material cause and an areas intensive A, B, change in the C shown in Figureconcentration 3. of components from the cutting surface to the base material in the area of the melted layer [15–18,23–25]. The present research showed that the surface layer of the Chemical Composition, wt. % Elementcut surface is depleted of silicon and manganese in relation to the average content of these Area A Area B Area C Base material elements in steel. Figure4 shows the photomicrograph of the cutting surface with EDS Fe analysis sites. 99.56 99.17 98.28 98.04 Mn The Si content0.30 at the cutting0.65 surface is 0.14%1.28and Mn 0.3%1. (Table34 2). This area is Si depleted in Si0.1 and4 Mn in relation0.18 to the average0.4 steel4 composition.0.62 A reduction in the Si content by about 70% and Mn by about 74% was observed. As you move away from To determine the type of the cracks, the sample was broken from the side of the transverse cut. The analysis of the fracture surface after the sample fracture (Figure 5a, R side) showed that in the area near the surface there is a corroded intercrystalline fracture layer (area b). Under this layer, there is a transcrystalline corroded fracture area (area d). The area of breaking is brittle, transcrystalline and shows no signs of corrosion (area e). A small area of the brittle transcrystalline fracture (area c) was observed near the sheet surface. Fractographic analysis showed no signs of corrosion in this area, indicating that there was no crack (prior to the intended breaking). This area was created in the process of breaking the sample. The atypical appearance of the fracture surface (Figure 5a, L side) indicates that the fracture did not start on the surface in the hardened zone but under the hardened zone in the pearlitic structure. The resulting crack developed towards the inside of the bar and towards the hardened surface. The presence of an unoxidized atypical area (area c) near the sheet surface indicates that a crack could not have formed on the cut surface in the hardened area. Analyzing the L side in Figure 5a, it can be observed that it is not possible for a crack to arise in the hardened layer, stopped its developing and continued to nucleate at a distance from the cut surface.

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the cutting surface, the content of Mn and Si increases. At a distance of 0.3 mm from the surface, it is 0.18% Si and 0.65% Mn. In the area of martensite with retained austenite, it amounts to 0.45% Si and 1.28% Mn on average, and is close to the average Si and Mn content in steel.

Table 2. Average values of elements content in EDS analysis for the base material and areas A, B, C shown in Figure3.

Chemical Composition, wt. % Element Area A Area B Area C Base Material Fe 99.56 99.17 98.28 98.04 Mn 0.30 0.65 1.28 1.34 Si 0.14 0.18 0.44 0.62

To determine the type of the cracks, the sample was broken from the side of the transverse cut. The analysis of the fracture surface after the sample fracture (Figure5a, R side) showed that in the area near the surface there is a corroded intercrystalline fracture layer (area b). Under this layer, there is a transcrystalline corroded fracture area (area d). The area of breaking is brittle, transcrystalline and shows no signs of corrosion (area e). A small area of the brittle transcrystalline fracture (area c) was observed near the sheet surface. Fractographic analysis showed no signs of corrosion in this area, indicating that there was no crack (prior to the intended breaking). This area was created in the process of breaking the sample. The atypical appearance of the fracture surface (Figure5a, L side) indicates that the fracture did not start on the surface in the hardened zone but under the hardened zone in the pearlitic structure. The resulting crack developed towards the inside of the bar and towards the hardened surface. The presence of an unoxidized atypical area (area c) near the sheet surface indicates that a crack could not have formed on the cut surface in the hardened area. Analyzing the L side in Figure5a, it can be observed that it is not possible for a crack to arise in the hardened layer, stopped its developing and continued to nucleate at a distance from the cut surface. The cracking mechanism in the thermal cutting process can be explained by analyzing the stress distribution. Heating to high temperature in the cutting process causes the near- surface zone to become hardened during the rapid cooling of the material. Compressive stresses occur in the hardened zone due to the phase transitions (Figure6a) [ 19,26–29]. Based on the conducted research, it is possible to present the mechanism of steel cracking in the thermal cutting process. Under the hardened layer with a martensitic structure, there is an area of pearlite with maximum tensile stresses. Due to the high brittleness of the pearlite, transcrystalline brittle cracks appear in this region. Figure6b shows a possible diagram of the development of cracks after thermal cutting. The cracks appear under the hardened layer in the area of maximum tensile stresses and during further cooling they spread both to the hardened surface and to the deeper areas of the base material. Materials 2021, 14, 1284 7 of 10 Materials 2021, 14, x FOR PEER REVIEW 7 of 10

(a)

(b) (c)

(d) (e)

FigureFigure 5. The 5. The appearance appearance of cracks of cracks in the in sample the sample cut transversely: cut transversely: (a) general (a) general view; (b view;) brittle (b ) brittle intercrystallineintercrystalline fracture fracture in the in the hardened hardened (oxidized) (oxidized) area area;; (c) transcrystalline (c) transcrystalline brittle brittle fracture fracture in the in the pearlitic region (not oxidized); (d) transcrystalline fracture with visible corrosion products; (e) pearlitic region (not oxidized); (d) transcrystalline fracture with visible corrosion products; (e) sample sample fracture area (not oxidized). fracture area (not oxidized). The cracking mechanism in the thermal cutting process can be explained by analyzing the stress distribution. Heating to high temperature in the cutting process causes the near‑surface zone to become hardened during the rapid cooling of the material. Compres- sive stresses occur in the hardened zone due to the phase transitions (Figure 6a) [19,26– 29]. Based on the conducted research, it is possible to present the mechanism of steel cracking in the thermal cutting process. Under the hardened layer with a martensitic structure, there is an area of pearlite with maximum tensile stresses. Due to the high brittleness of the pearlite, transcrystalline brittle cracks appear in this region. Figure 6b shows a possible diagram of the development of cracks after thermal cutting. The cracks appear under the

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Materials 2021, 14, 1284 8 of 10 hardened layer in the area of maximum tensile stresses and during further cooling they spread both to the hardened surface and to the deeper areas of the base material.

(a) (b)

Figure 6. (a)Figure Distributions 6. (a) D ofistributions circumferential of circumferential residual stresses residual in induction stresses hardenedin induction 41Cr4 hardened specimen 41Cr4 to a speci- depth of 2.3 mm (1) based on [men26] and to a in depth cutting of 2.3 edge mm of (1) 40 mmbased wear-resistant on [26] and in steel cutting plate edge (2) based of 40 onmm [19 wear]; (b)‑resistant scheme of steel crack plate development (2) based on [19]; (b) scheme of crack development under the thermally cutting surface. under the thermally cutting surface.

4. Conclusions4. Conclusions The heat affectedThe zone heat of affected R260 steel zone shows of R260 microstructural steel shows changes microstructural after thermal changes cut- after thermal ting. Under thecutting. cutting Under surface, the there cutting is a surface,layer of solidified there is a metal layer ofwith solidified a transformed metal with lede- a transformed burite microstructureledeburite with microstructure secondary cementite. with secondary Under cementite.this layer there Under is an this unhardened layer there is an unhard- zone with a structureened zone of withfine pe a structurearlite and of secondary fine pearlite cementite, and secondary and then cementite, a hardened and area then a hardened consisting of martensitearea consisting and residual of martensite austenite. and The residual further austenite. region has The the further pearlitic region micro- has the pearlitic structure of themicrostructure base material. of the base material. After thermalAfter cutting thermal in the cutting surface in layer, the surface the chemical layer, the composition chemical composition changes. There changes. There was was a decreasea decreasein the content in the of content Si by ofabout Si by 70%, about and 70%, of Mn and by of about Mn by 74%. about However, 74%. However, the the carbon carbon increasesincreases content content to approx. to approx. 3.2%. In 3.2%. the layer In the of nonmelted layer of nonmelted metal under metal the under solid- the solidified ified liquid layer,liquid the layer, carbon the content carbon contentis about is 1.2%. about As 1.2%. you As move you away move from away the from cutting the cutting surface, surface, the contentthe content of Mn of and Mn andSi increases, Si increases, approaching approaching the theaverage average content content of elements of elements in the steel. in the steel. Based on the research and references [26–29], the mechanism of crack formation during Based onthe the thermal research cutting and references of R260 steel [26 can–29] be, the presented. mechanism The of heating crack formation of the surface dur- during thermal ing the thermalcutting cutting and of subsequent R260 steel can cooling be presented. causes phase The changes heating to of occur. the surface Compressive during stresses arise thermal cuttingat the and surface subsequent in the hardened cooling causes area, and phase tensile changes stresses to occur occur. under Compres it insive the pearlitic area. stresses arise atHigh the tensilesurface stress in the in hardened the pearlite area, region and causestensile brittlestresses transcrystalline occur under it cracks. in the These cracks pearlitic area.spread High towards tensile stress the cutting in the surface pearlite and region the base causes material. brittle The transcrystalline depth of the cracks does not cracks. These exceedcracks spread 10 mm. towards the cutting surface and the base material. The depth of the cracks does notDue exceed to its high10 mm. C content, R260 steel has a high hardness with a surface of over Due to its800 high HV. C Steelscontent, with R260 such steel a high has carbona high contenthardness are with difficult a surface to cut. of over Therefore, 800 the cutting HV. Steels withprocess such a should high carbon be carried content out are on difficult preheated to cut. bars. Therefore, To minimize the cutting the possibility pro- of cracks cess should beappearing carried out after on the preheated cutting process,bars. To veryminimize slow coolingthe possibility should of be cracks used. ap- This will reduce pearing after the cutting temperature process, gradient very slow and cooling stress state should below be used. the surface This will and reduce prevent the the excessive temperature gradienthardening and of stress the surface state below layer. the surface and prevent the excessive hardening of the surface layer. Author Contributions: Conceptualization, A.Z., L.T. and K.P.; methodology, A.Z. and L.T.; inves- Author Contributions:tigation, Conceptualization, A.Z., L.T. and K.P; A writing—original.Z., L.T. and K.P.; methodology, draft preparation, A.Z. and A.Z L.T. and; investi- L.T.; writing—review gation, A.Z., L.T.and and editing, K.P; writing A.Z.,— L.T.original and K.P. draft All preparation, authors have A.Z read and andL.T.; agreedwriting to—review the published and version of the manuscript.

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Funding: This research was supported by the Polish Ministry of Science and Higher Education (Poland), grant number 16.16.110.663 (in AGH University of Science and Technology). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All data is provided in full in the results section of this paper. Acknowledgments: The authors are grateful to Marcin Goły (AGH University of Science and Tech- nology in Krakow, Poland) for the XRD analysis. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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