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 Pa´ncikiewicz,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 10in 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