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1

Microstructure and wear resistance of gas-nitrided steel after laser modification

Panfil D., Kulka M., Wach P., Michalski J.

Journal of Achievements in Materials and Manufacturing Engineering

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2017

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Vol. 85, nr 1

12--20

EN

Purpose: The aim of this work was to study the microstructure and wear resistance of hybrid surface layers, produced by a controlled gas nitriding and laser modification. Design/methodology/approach: Nitriding is well-known method of thermo-chemical treatment, applied in order to produce surface layers of improved hardness and wear resistance. The phase composition and growth kinetics of the diffusion layer can be controlled using a gas nitriding with changeable nitriding potential. In this study, gas nitriding was carried out on 42CrMo4 steel at 570°C (843 K) for 4 hours using changeable nitriding potential in order to limit the thickness of porous e zone. Next, the nitrided layer was laser-modified using TRUMPF TLF 2600 Turbo CO2 laser. Laser tracks were arranged as the multiple tracks with scanning rate vl =2.88 m/min and overlapping of about 86% using the two laser beam powers ( P ): 0.21 kW and 0.26 kW. Microstructure was observed by an optical microscope. Phase composition was studied using XRD. Hardness profiles in the produced hybrid layers was determined using a Vickers method. Wera resistance tests were performed using MBT-01 tester. Findings: Gas nitriding resulted in formation of compound zone, consisting of e nitrides close to the surface and a zone, composed of e + g' nitrides. Below the white compound zone, the diffusion zone occurred with nitric sorbite and precipitates of g' nitrides. In the microstructure after laser heat treatment (LHT) of nitrided layer, the zones were observed as follows: the re-melted zone (MZ) with e nitrides, nitric martensite and non-equilibrium FeN0.056 phase, the heat-affected zone (HAZ) with nitric martensite and precipitates of g' phase and the diffusion zone (DZ) without visible effect of laser treatment. Laser beam power influenced the depth of MZ and HAZ, so the thickness of hardened zone. The hardness of MZ was slightly decreased compared to the hardness of compound zone after gas nitriding. However, the significant increase in hardness was observed in HAZ. The formation of hybrid layers advantageously influenced the tribological properties. The laser-heat treated nitrided layers were characterized by improved wear resistance compared to the only gas-nitrided layer. Research limitations/implications: The effect of LHT on the microstructure and properties of gas-nitrided layer was limited to the two laser beam powers. In the future research, this range should be exceeded, especially, taking into account the lower values of laser beam power. It will result in laser modification without re-melting. Practical implications: The selection of suitable LHT parameters could provide the hybrid layers of modified microstructure and improved wear resistance. Originality/value: This work was related to the new concept of modification of nitrided layer by laser heat treatment.

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Modelling of the effects of laser modification of gas-nitrided layer

Kulka M., Panfil D., Michalski J., Wach P.

Archives of Materials Science and Engineering

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2017

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Vol. 88, nr 2

59--67

EN

Purpose: The effects of laser processing parameters on the dimensions of simple laser tracks, produced on the previously nitrided layer, were analysed. Design/methodology/approach: Gas nitriding is one of the most commonly used thermochemical treatment, resulting in many advantageous properties: high hardness, enhanced corrosion resistance, improved wear resistance and fatigue strength. However, an unfavourable increase in the thickness of compound zone (e + ɣ’) close to the surface was observed after conventional gas nitriding. This was the reason for undesirable embrittlement and flaking of the layer. Therefore, a controlled gas nitriding was intensively developed, reducing the percentage of the most brittle e (Fe2-3N) iron nitrides. In this study, the hybrid surface layer was produced. The controlled gas-nitriding was followed by laser heat treatment (LHT). Laser modification was carried out using various laser beam powers and scanning rates. The dimensions of laser tracks (i.e. depths and widths of re-melted zone and heat-affected zone) were measured. Numerical methods were used in order to formulate a mathematical model. Findings: Laser processing parameters (laser beam power and scanning rate) influenced the microstructure obtained. The microstructure of laser modified nitrided steel with re-melting consisted of re-melted zone (MZ), heat-affected zone (HAZ), nitrided layer without visible effects of laser treatment and the substrate. The use of laser beam power of 0.26 kW resulted in only a partial re-melting of the compound zone. The two characteristic values of laser beam power were estimated. P0MZ corresponded to the laser beam power at which the re-melted zone disappeared (i.e. width and depth of MZ were equal to 0). P0HAZ was a value of laser beam power at which the effects of laser irradiation were invisible in microstructure (i.e. width and depth of HAZ were equal to 0). The model was proposed in order to predict the effects of LHT on microstructure. Research limitations/implications: The presented model was limited to the scanning rates in the range of 2.24-3.84 m/min. In the future research, this range should be exceeded, especially, taking into account the lower values of scanning rate. Practical implications: The presented model could be used in order to control the microstructure and properties of hybrid surface layers, obtained as a consequence of the controlled gas-nitriding and LHT. Originality/value: his work is related to the new conception of laser modification of nitrided layers. Such a treatment provided the hybrid layers of new advantageous properties.

3

Effect of laser heat treatment parameters on the microstructure and hardness of gas-nitrided layers

Kulka M., Panfil D., Michalski J., Wach P.

Inżynieria Materiałowa

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2016

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Vol. 37, nr 6

328--334

EN

Nitriding is commonly used method of thermochemical treatment in order to produce surface layers of improved hardness and wear resistance. Using a gas nitriding with changeable nitriding potential, a nitrogen concentration at the surface could be controlled, influencing the phase composition and the growth kinetics of the layer. In this study, the hybrid surface treatment was applied. It consisted in gas nitriding and laser heat treatment (LHT) of 42CrMo4 steel. Two nitriding processes were carried out using changeable nitriding potential. Parameters on first process were as follows: temperature 570°C (843 K), time 4 h. The second process was performed at lower temperature 520°C (793 K) and longer duration 10 h. This resulted in various depths of the compound zone at the surface (20 and 8 μm, respectively). Next, the nitrided layers were laser heat-treated using TRUMPF TLF 2600 Turbo CO2 laser. Laser tracks were arranged as the single tracks with various scanning rates (vl = 2.88 m/min and vl = 3.84 m/min). The laser beam power (P) ranged from 0.26 to 0.91 kW. The effects of the depth of compound zone as well as LHT parameters on the microstructure, dimensions and microhardness of laser tracks were analysed. In the majority of the produced laser tracks, remelted (MZ) and heat-affected (HAZ) zones were easily identified. Different microstructure was visible at low laser beam power (0.26 kW). The dimensions of MZ were limited, whereas the HAZ was clearly observed. The compound zone was still visible at the surface. Only the porous ε nitrides were slightly melted. Hardness increased significantly after LHT with complete and partial remelting of compound zone. Laser beam power and scanning rate influenced the depth and width of MZ and HAZ, so the thickness of hardened zone. The greater laser beam power or the smaller scanning rate, the larger hardened zone was observed.

PL

Azotowanie jest jedną z najpopularniejszych metod obróbki cieplno- chemicznej prowadzącą do wytwarzania warstw powierzchniowych o dużej twardości i odporności na zużycie. Stosując odpowiedni proces azotowania gazowego (ze zmiennym potencjałem azotowym), można kontrolować stężenie azotu na powierzchni, wpływając na kinetykę wzrostu strefy związków (azotków) oraz strefy dyfuzyjnej. Celem pracy była modyfikacja warstwy azotowanej za pomocą laserowej obróbki cieplnej. Badano wpływ parametrów laserowej obróbki cieplnej (mocy wiązki laserowej i szybkości skanowania) oraz grubości wytworzonej w wyniku azotowania strefy związków (azotków żelaza) na mikrostrukturę i twardość.

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The influence of laser re-melting on micro-structure and hardness of gas-nitrided steel

Panfil D., Wach P., Kulka M., Michalski J.

Archives of Mechanical Technology and Materials

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2016

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Vol. 36

18--22

EN

In this paper, modification of nitrided layer by laser re-melting was presented. The nitriding process has many advantageous properties. Controlled gas nitriding was carried out on 42CrMo4 steel. As a consequence of this process, ɛ+ɣ’ compound zone and diffusion zone were produced at the surface. Next, the nitrided layer was laser re-melted using TRUMPF TLF 2600 Turbo CO2 laser. Laser tracks were arranged as single tracks with the use of various laser beam powers (P), ranging from 0.39 to 1.04 kW. The effects of laser beam power on the microstructure, dimensions of laser tracks and hardness profiles were analyzed. Laser treatment caused the decomposition of continuous compound zone at the surface and an increase in hardness of previously nitrided layer because of the appearance of martensite in re-melted and heat-affected zones.

5

Laser heat treatment of gas-nitrided layer produced on 42CrMo4 steel

Kulka M., Michalski J., Panfil D., Wach P.

Inżynieria Materiałowa

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2015

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Vol. 36, nr 5

301--305

EN

Gas nitriding, together with gas carburizing and gas carbonitriding, is the most commonly used thermochemical treatment. This process resulted in many advantageous properties: high hardness, enhanced corrosion resistance, and considerably improved wear resistance and fatigue strength. A wide range of steels, including special nitriding steels (41CrAlMo7, 33CrMoV12-9), low alloy steels, tool steels as well as austenitic steels, can be nitrided. Special attention requires the nitride layer at the surface that is mainly critical to such properties as corrosion resistance or resistance to friction wear. In this study, gas nitriding was carried out on 42CrMo4 steel resulting in ε + γ′ compound layer at the surface. Next, the nitrided layer was laser heat treated (LHT) using TRUMPF TLF 2600 Turbo CO2 laser. Laser tracks were arranged as a single tracks and as multiple tracks with overlapping of about 86%. LHT caused the decomposition of continuous compound layer and an increase in hardness of previously nitrided layer, enlarging the hardened zone. The results showed an advantageous influence of laser heat treatment on the wear resistance.

PL

Azotowanie gazowe, obok gazowych procesów nawęglania i azotonawęglania, jest najpowszechniej stosowaną obróbką cieplno-chemiczną. Proces ten skutkuje wieloma korzystnymi właściwościami: dużą twardością, poprawą odporności korozyjnej oraz znacznie zwiększoną odpornością na zużycie i wytrzymałością zmęczeniową. Proces jest szeroko stosowany w odniesieniu do stali, w tym specjalnych stali do azotowania (41CrAlMo7, 33CrMoV12-9), stali niskostopowych, narzędziowych, jak również stali austenitycznych. Szczególnej uwagi wymaga warstwa azotków przy powierzchni, która ma istotne znaczenie dla takich właściwości jak odporność na korozję czy na zużycie przez tarcie. W pracy przeprowadzono azotowanie gazowe stali 42CrMo4, wytwarzając przy powierzchni strefę związków ε + γ′. Następnie, warstwę azotowaną poddano laserowej obróbce cieplnej (LOC) za pomocą lasera CO2 TRUMPF TLF 2600 Turbo. Wykonano pojedyncze ścieżki laserowe, a także ścieżki wielokrotne ze stopniem zachodzenia około 86%. LOC powodowała rozpad ciągłej strefy związków i zwiększenie twardości wcześniej azotowanej warstwy, rozszerzając strefę utwardzoną. Wyniki wykazały korzystny wpływ obróbki laserowej na odporność na zużycie.