Mechanics, Materials Science & Engineering, April 2017 – ISSN 2412-5954

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Laser Hardening and Pack Boriding of EN 8D Steel43

K. Monisha1, P. Selvamuthumari1, D. Narendran1, Rasik Ahmad Parray1, J. Senthilselvan1,a

1 – Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai-25, Tamil Nadu, India

a – jsselvan@hotmail.com

DOI 10.2412/mmse.13.65.82 provided by Seo4U.link

Keywords: pack boriding, laser hardening, surface modification, hardness.

ABSTRACT. This work is composed of two different techniques, laser hardening using HPDL and conventional pack boriding of EN 8D steel to improve the mechanical properties. The α-Fe phase observed in the XRD pattern of untreated EN 8D transformed to martensite and cementite phases due to the effect of laser irradiation, which is evidenced from the microstructure analysis. The martensite phase in the laser hardened sample results in improved hardness with maximum of about 825 HV0.2. In case of pack boriding, the sample was packed with boriding agent and sealed in a stainless steel container at 950°C for 6 hours. XRD pattern reveals the Fe2B peaks, which confirms the formation of borided layer at the sample surface. The hardness value in the tooth like borided layer is increased from 261 HV to 1193 HV due to this Fe2B phase.

Introduction. Iron-Carbon alloys are the most versatile, economical and globally utilized materials for industrial constructions and engineering systems. Depending on iron and carbon distribution, the microstructures get modified leading to miscellaneous mechanical properties [1]. Hypo-eutectoid steels with carbon concentration less than 0.8 wt% is composed of pearlite microstructure in untreated condition. The lamellar cementite distributed in the ferrite matrix does not possess superior surface properties for industrial and engineering applications [2]. Redistribution of the carbon and iron accordingly modifies the surface properties. Subjecting to heat treatment and customizing the process parameters favors the surface property modification.

In Iron-Carbon steels, the martensite microstructures with fine needles exhibits high hardness and high wear resistance property. Fe-C phase diagram [3] gives the relationship between carbon composition and the processing temperature. Hypo-eutectoid steels [2, 4] when heated above 800°C (A3 transformation temperature), it completely transforms into γ-austenite. The fcc-austenite favors the solubility of carbon atoms into ferrite matrix than bcc-ferrite. On subsequent cooling, the carbon do not have sufficient time to form cementite and hence thin carbon needle starts to grow from the grain boundaries of austenite phase resulting in highly strained bct lath martensite which is responsible for high hardness [5].

Laser hardening [2] rapidly heats the material to several thousand degrees followed by rapid quenching in a fraction of seconds. During laser irradiation, the surface temperature shoots above the austenizing temperature and melts the substrate. Resolidification results in extremely high hardness as the cooling rate of laser hardening is more rapid than conventional hardening due to self quenching by the substrate. Boriding is a thermo-chemical process [6] where the substrate pre-coated with boriding agent and heated above the austeninzing temperature in conventional furnace. Boron being the hardest material, when diffused in ferrite matrix incorporates its high hardness property with the ductility of iron by

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forming Fe-B composite. From Fe-B phase diagram [7] it can be interpreted that, above 700 °C the boron species in the boriding agent diffuses into the γ- ferrite matrix with the formation of Fe2B composite. Harder FeB composite can be obtained when 50% of the ferrite matrix is occupied by Boron [6, 7]. The composite layer formed depends on the percentage of boron species available in the boriding agent. Though FeB is harder than Fe2B composite, it is not widely preferred due to its brittleness. In this report, EN-8D steel discs were chosen for laser hardening and conventional pack boriding. Laser hardening was carried out using high power diode laser with 4 mm beam width at three laser powers 1.5, 1.7 and 2 kW with various scanning rate. In case of pack boriding, the sample was packed with boriding agent composed of Boropak powder at 50 wt% (source), KBF4 at 20 wt% (activator) and SiC at 30 wt% (filler) and sealed in a stainless steel container at 950 °C for 6 hours.

Experimental Technique. EN 8D steels were hardened using high power diode laser with Gaussian beam mode. The case depth and hardness was controlled by optimizing the processing parameters such as laser beam power, scan rate and beam size. The laser beam was fixed at 1.5, 1.7 and 2kW with the scanning speeds of 400, 600, 800 mm/min and sufficient number of experimental trials were done. For pack boriding, the steel disc was packed inside the indigenously designed boriding chamber. The mentioned composition of boriding agents on all the sides followed by a layer of glass wool was packed in a stainless steel container. The container was placed in conventional box furnace directly at 950 °C with soaking time of 6 hours. The sample is then water quenched to restrict the diffusion of boron through the substrate resulting in localized high hardness on the surface.

Results. Microstructure of the untreated, laser hardened and borided teeths are shown in fig. 2. The untreated substrate exhibits larger alternate ferrite and cementite grains representing the pearlitic microstructure. During laser irradiation, the carbon dissolves into the ferrite melt pool. Rapid solidification due to self quenching restricts the diffusion of carbon on cooling[8]. Due to this constrain, the carbide diffuses into austenite from the lateral side of the pearlite, resulting in needle shaped martensite structure [2].

Fig. 1. Microstructure of the (a) untreated, (b) laser hardened and (c) borided teeths.

The pre-placed boriding agent, when heated above austenite transformation temperature, the boriding agent dissociates and diffuses the boron species into the substrate. The presence of ferrite and carbide atoms opposes the diffusion of boron through the substrate [3] resulting in tooth like microstructure corresponding to the growth of single Fe2B layer on the surface (Fig.1c) [6]. Further growth of FeB layer is not favored by the processing conditions, which may be due to the lack of boron species in the agent.

XRD pattern of untreated, laser hardened and borided EN 8D steel is shown in Fig. 2. The XRD of untreated substrate confirms the presence of α-Fe phase. On laser hardened surface the formation of laser transformed martensite and cementite phases are confirmed which are in the body centered

(a) (b) (c)

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tertragonal and orthorhombic structures, respectively [9]. During laser heating, the surface of the steel attains austenite temperature and on self-quenching it gets transformed into martensite. The formation of cementite phase may be due to delayed quenching by the effect of slow scan rate. The XRD pattern for borided EN 8D steel confirms the formation of the Fe2B layer (JCPDS number 75-1062) which is also evidenced by the tooth like microstructure observation.

Fig. 2. XRD pattern of (a) untreated and laser hardened and (b) Borided EN 8D steel.

Fig.3 shows the hardness measured at different depths of the laser hardened cross sections using vicker’s microhardness tester at 200g load with dwell time of 3 secs. After laser hardening, hardness value increases from 178 HV to 825 HV, which is 5 times harder than the base metal. Higher hardness is achieved due to martensite phase formation by rapid heating and cooling [8]. The maximum case depth of 1200 µm was achieved. The hardness of the borided sample is measured to be 1193 HV due to the formation of Fe2B phase on the surface [6], evidenced from microstructure and XRD analysis.

Fig. 3. (a) Hardness profile of laser hardened sample and (b) indentation mark on borided surface.

Summary. The effect of high power diode laser surface hardening and pack boriding of EN 8D steel were studied. The laser heating process transformed the base metal α-Fe ferrite microstructure to high hardened martensite phase in a fraction of second, where as boriding heat treatment take a quite longer processing duration of 5 to 10 hours. The surface hardness of laser treated EN 8D steel increases from 178 HV to about 825 HV, with increased case depth of 1 mm. Five-fold increase in hardness was obtained by laser hardening technique. Pack boriding results in extreme high hardness of about 1193

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HV with the borided layer thickness of 25 µm. Hence laser treatment gives better result compared to conventional boriding treatment.

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[4] M.F. Ashby, K.E. Easterling, The transformation hardening of steel surfaces by laser beams—I. Hypo-eutectoid steels, Acta Metallurgica, 1984. http://dx.doi.org/10.1016/0001-6160(84)90175-5

[5] P. Zhang, Y. Chen, W. Xiao, D. Ping, X. Zhao, Twin structure of the lath martensite in low carbon steel, Progress in Natural Science: Materials International, 2016. http://dx.doi.org/10.1016/j.pnsc.2016.03.004 [6] I.E. Campos-Silva, G.A. Rodríguez-Castro, 18 - Boriding to improve the mechanical properties and corrosion resistance of steels, in: Thermochemical Surface Engineering of Steels, Woodhead Publishing, Oxford, 2015, pp. 651-702.

[7] M.-A. Van Ende, I.-H. Jung, Critical thermodynamic evaluation and optimization of the Fe–B, Fe–Nd, B–Nd and Nd–Fe–B systems, Journal of Alloys and Compounds, 2013. http://dx.doi.org/10.1016/j.jallcom.2012.08.127 [8] Y.K. Chuang, D. Reinisch, K. Schwerdtfeger, Kinetics of the diffusion controlled peritectic reaction during solidification of iron-carbon-alloys, Metallurgical Transactions A, 1975. 10.1007/BF02673703

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Cite the paper

K. Monisha, P. Selvamuthumari, D. Narendran, Rasik Ahmad Parray, J. Senthilselvan (2017). Laser Hardening and Pack Boriding of EN 8D Steel, Vol 9. doi 10.2412/mmse.13.65.82

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