NR 6/2017 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 259

Laser alloying of 316L steel with boron and Stellite-6Daria Mikołajczak*, Michał Kulka, Natalia Makuch, Piotr Dziarski

Instytut Inżynierii Materiałowej, Politechnika Poznańska; *daria.h.mikolajczak@doctorate.put.poznan.pl.

Austenitic 316L steel belongs to one of the most numerous groups of alloys with special properties. It is well-known for its most effective balance of carbon, chromium, nickel and molybdenum concentrations for corrosion resistance. However, under conditions of appreciable mechanical wear (adhesive or abra-sive), this steel should be characterized by suitable wear protection. Diffusion boronizing and laser alloying with boron were often used in order to improve tribological properties of 316L steel. In this study, the method of laser alloying was modified in this way that alloying material contained the mixture of amorphous boron and Stellite-6 powders. The coated surface was remelted by the laser beam using TRUMPF TLF 2600 Turbo CO2 laser. After the laser alloying process, the composite surface layer was produced. Only two zones occurred in the laser-alloyed 316L steel: remelted zone and the substrate (base material). Heat-affected zone was invisible because the austenitic steel could not be hardened by typical heat treatment. The remelted zone consisted of hard ceramic phases (iron, chromium and nickel borides) in the soft austenitic matrix with the increased concentration of cobalt. Some properties of this layer were investigated and compared to the laser-alloyed layer with boron only. The produced layer was characterized by a compact microstructure which was free of cracks and gas pores. The layer was also uniform in respect of the thickness because of the high overlapping used during the laser treatment (86%). The obtained thickness was significantly higher than that obtained in case of diffusion boriding. In spite of the lower hardness of remelted zone, the increase in wear resistance of the proposed surface layer was observed in comparison with laser-alloyed 316L austenitic steel with boron only.

Key words: laser alloying, 316L steel, microstructure, hardness, wear resistance.

Inżynieria Materiałowa 6 (220) (2017) 259÷265DOI 10.15199/28.2017.6.2© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTION

AISI316L stainless steel is a commonly used corrosion-resistant and heat resistant material. Single phase austenitic microstructure as well as an effective balance of carbon, chromium, nickel and molybdenum content is a reason for such advantageous proper-ties. Therefore, this steel is often used wherever a high temperature or aggressive corrosive media occur. 316L steel is also character-ized by paramagnetic properties, a substantial ductility, low yield strength, high ability to strengthen by cold working as well as no ability to remove possibly existing coarse-grained microstructure by heat treatment. Unfortunately, the relative low hardness of this material (about 200 HV) and its poor wear resistance causes its lim-ited applying, especially, under conditions of appreciable mechani-cal wear (adhesive or abrasive) [1].

Many methods were developed in order to improve tribological properties of austenitic steels. Some of them consisted in diffusion treatment such as carburizing or nitriding. In paper [2], the process of glow discharge-assisted low-temperature nitriding was reported. It was carried out at 440°C (713 K) for 6 h resulting in the obtained layer thickness of about 4 µm. Microstructure consisted of relative-ly expanded nitrogen austenite and CrN nitrides. The increase in the temperature up to 550°C (823 K) caused a significant increase in the thickness of the layer to 30 μm and the appearance of iron ni-trides (Fe4N) in the microstructure [3, 4]. Cr2N was also often iden-tified in the nitrided layer [5]. The process of low-temperature plas-ma carburizing at the temperature below 520°C (793 K) resulted in a microstructure consisting of expanded austenite [6÷9]. The low temperature carburized layer was precipitation-free and consisted of a single expanded austenite phase with an expanded fcc lattice due to the supersaturation [9]. At higher process temperature, i.e. 550÷600°C (823÷873 K), the thickness of the layer obtained 50 µm, and the microstructure consisted of expanded austenite, martensite and chromium carbides Cr7C3 [6]. The multiphase microstructure could result in worsening the corrosion resistance. The process of pack-boronizing [10÷12] in the range of temperature 800÷950°C (1073÷1223 K) improved tribological behaviour of austenitic steels without sacrificing their corrosion resistance. Due to the chemical composition of 316L, the boriding process was much longer com-

pared to the typical structural steels. Pack-boronizing of 316L steel at 950°C (1233 K) for 8 h resulted in producing the layer of the thickness up to 87 μm [10]. Pack-borided layer, produced on 316L steel, didn’t worsen its corrosion resistance [11, 12]. PVD processes were also carried out on austenitic steels in order to produce the TiN coatings. Unfortunately, the thickness of these coatings was rela-tively small, obtaining 1.4 µm [13] or 1.6 to 2.4 μm [14].

Laser materials processing was being used for a wide range of applications in order to modify the microstructure and properties of the metals and their alloys [15, 16]. Laser alloying of materials with different elements was an important alternative for the processes, mentioned above. Using these methods, it was also possible to ob-tain hard, wear resistant as well as corrosion resistant layers. Laser alloying with boron had a special importance due to the formation of hard ceramic phases (borides) in remelted zone. Laser boriding of typical structural steel [17], nodular cast iron [18], titanium and its alloys [19÷21] or Ni-based alloys [22, 23] was intensively de-veloped. Austenitic steel was also laser-alloyed with hard particles, e.g. carbides [24, 25], with NiCoCrB powder [26] as well as with boron [27, 28]. Laser alloying of 316L steel with boron resulted in producing the thick, hard and wear resistant composite surface layer, consisting of iron, chromium and nickel borides in austen-itic matrix [28]. However, the borocarbides M23(C, B)6 were also observed in the remelted zone. It could worsen the corrosion resist-ance of this steel.

In this study, the austenitic 316L steel was laser-alloyed with the alloying material, consisting of the mixture of boron and Stellite-6. Stellite-6 was used in order to diminish the percentage of boron in the alloying material and, as a consequence, to improve the corro-sion behaviour of the produced layer as it was observed after laser alloying with NiCrSiB powders [29, 30]. The microstructure and some mechanical properties were compared to the effects of laser alloying with boron only [27].

2. EXPERIMENTAL PROCEDURE

AISI 316L austenitic steel was investigated. Its chemical composi-tion was shown in Table 1. The ring-shaped specimens (external diameter ca. 20 mm, internal diameter 12 mm and height 12 mm)

260 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

were used for the study. The chemical composition of the powder Stellite-6 was presented in Table 2.

Laser boriding process was carried out by two-step method. First, the outer surface of the samples was coated with the paste, consisting of amorphous boron and Stellite-6 powders, blended with a diluted polyvinyl alcohol solution. Chemical composition of Stellite-6 powder was shown in Table 2. Boron and Stellite-6 were blended with a mass ratio of 10:1. The thickness of the ap-plied coating was qual to 200 µm. The next step of the treatment consisted in laser remelting of the alloying material together with the base material. It was schematically illustrated in Figure 1. Laser alloying was carried out using the TRUMPF TLF 2600 Turbo CO2 laser of the nominal power 2.6 kW. Laser processing parameters were as follows: laser beam power P = 1.56 kW and scanning rate vl = 2.88 m/min. The diameter of the laser beam (d) was equal to 2 mm. TEM01* multiple mode of the laser beam was applied. The averaging irradiance (E) of about 49.66 kW/cm2 was calculated. The focusing mirror was characterized by: curvature 250 mm, diameter 48 mm and focal length 125 mm. The multiple laser tracks were ar-ranged with the distance f = 0.28 mm between the axes of adjacent tracks (Fig. 2). The obtained scanning rate vl (2.88 m/min) resulted from the rotational speed n (45.85 min–1) and feed rate vf (0.28 mm per revolution). Shielding gas (argon at a pressure of 0.2 Pa) was used in order to protect the surface against oxidation.

The relatively high overlapping of the laser tracks (86%) was ap-plied during laser alloying. Overlapping (O) was calculated as fol-lows:

O d f

d

100%

(1)

where: d is a laser beam diameter, mm, and f is the distance between the axes of adjacent tracks, mm.

In order to study the microstructure, the laser-alloyed specimen was cut in the plane perpendicular to the scanning direction and was mounted in a conductive resin. The metallographic specimen was ground and polished using the abrasive paper of the different granularity, and, finally, with applying Al2O3. The etching solution, consisting of anhydrous glycerin, HCl and HNO3, was used with a volume ratio of 2:3:1 in order to reveal the microstructure. The microstructure was observed with the use of an light microscope (LM) and scanning electron microscope (SEM) Tescan Vega 5135.

The concentrations of elements in the laser-alloyed layer were measured with PGT Avalon X-ray microanalyser equipped with EDS, using 55° take-off angle. The accelerating voltage was rela-tively low (12 kV) to better reveal boron as a light element. Si (Li) detector with ultra-thin window, standardless quantitative analy-sis, matrix correction algorithms ZAF for SEM bulk analysis were applied. The measured elements were as follows: iron, chromium, nickel, cobalt and boron. The phase analysis was carried out by PANalytical EMPYREAN X-ray diffractometer using Cu Kα ra-diation.

Microhardness profile, through the investigated layer, was in-vestigated in the polished cross-section of specimen. The Vickers method was applied for microhardness measurements using the ap-paratus ZWICK 3212 B. The tests were performed under the inden-tation load of 0.1 kG (about 0.981 N).

Tribological properties of laser-alloyed layer were tested us-ing MBT-01 device. The frictional pair consisted of a ring-shaped specimen with laser-alloyed layer and of a plate made of sintered carbide S20S as counter-specimen. The scheme of wear was shown in Figure 3.

The sintered carbide was composed of: 58 wt % of WC, 31.5 wt % of (TiC + TaC + NbC), and 10.5 wt % of Co. It was char-acterized by a density of 10.7 g/cm3 and hardness 1430 HV. The wear test was carried out under conditions of dry friction (unlu-bricated sliding contact) using the load P = 49 N and the specimen speed of 0.26 m/s, resulting from the rotational speed n = 250 min–1

Table 1. Chemical composition of material used, wt %Tabela 1. Skład chemiczny stali 316L, % mas.

Material C Cr Ni Mo Mn Si Fe

316L 0.023 17.45 12.92 2.88 0.56 0.45 balance

Table 2. Chemical composition of Stellite-6 powder [wt %]Tablica 2. Skład chemiczny proszku Stellite-6 [mas. %]

Material C Cr Ni W Mn Si Fe Co

Stellite-6 1.2 28.0 < 3.0 4.5 0.56 1.1 < 3.0 balance

Fig. 1. Two-step method of laser alloyingRys. 1. Laserowe stopowanie dwustopniową metodą przetapiania

Fig. 2. Method of multiple tracks producing; d – laser beam diameter (d = 2 mm), vf – rate of feed, vl – scanning rate, n – rotational speed, vt – tangential rate, f – distance from track to trackRys. 2. Metoda wytwarzania ścieżek wielokrotnych; d – średnica wiąz-ki laserowej (d = 2 mm), vf – posuw, vl – prędkość skanowania wiązką, n – prędkość obrotowa, f – odległości między ścieżkami

Fig. 3. Scheme of wear test; P = 49 N, rotational speed n = 250 min–1

Rys. 3. Schemat próby zużycia; P = 49 N, prędkość obrotowa n = 250 min–1

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and the external diameter of the specimen (20 mm). Although the surface roughness was changed after laser alloying, the surface sample was not specially prepared before the test.

Wear resistance was evaluated by relative mass loss of specimen and counter-specimen (Dm/mi) according to the equation:

mm

m mmi

i f

i

(2)

where: Dm is mass loss, mg, mi is initial mass of specimen or coun-ter-specimen, mg, mf is final mass of specimen or counter-speci-men, mg.

3. RESULTS AND DISCUSSION

Microstructure of the laser-alloyed 316L steel with boron and Stel-lite-6 was shown in Figure 4. The produced layer was uniform and continuous and free of cracks and gas pores. The only two zones were observed in the microstructure: MZ – laser remelted zone (1) and the substrate (2). Heat affected zone (HAZ) was invisible. The reason for such a situation was that an austenitic structure of 316L steel could not be quenched, irrespective of cooling rate obtained during laser heat treatment. Therefore, there were no changes in the microstructure below MZ, including the unchangeable grain size.

The MZ was compact and uniform regarding the thickness due to the relatively high overlapping used (86%). There were only small differences in the thickness of the remelted zone between the axes of multiple tracks and contact of tracks. The higher thickness was observed in the axes of tracks. The average thickness was equal to 343 μm.

After diffusion boriding of austenitic steel, literature data [10÷12] reported the presence of iron borides (FeB and Fe2B). Ad-ditionally, the zonation of iron borides was observed. FeB phase occurred close to the surface, and the second phase (Fe2B) — below the first one. After laser alloying with boron and Stellite-6, Fe2B, Cr2B and Ni2B borides were identified in MZ based on the obtained XRD patterns (Fig. 5). Additionally, the γ-FeCrNiCoC phase (aus-tenite) was observed in this zone. Hence, the laser alloying with boron caused the formation of a composite layer. SE image of MZ indicated the presence of composite microstructure, consisting of hard ceramic phases (iron, chromium and nickel borides) and soft austenitic matrix (Fig. 6). The similar microstructure was previous-ly observed for laser-fabricated Fe–Ni–Co–Cr–B austenitic alloy on 316L steel [26].

The linear X-ray microanalysis was carried out considering the concentration of: boron, iron, chromium, nickel and cobalt. In spite of the dominating content of cobalt in Stellite-6 powder, a presence of cobalt borides wasn’t confirmed by XRD. However, the results of X-ray microanalysis showed the increase in cobalt concentration in the remelted zone (Fig. 7). It indicated that cobalt only dissolved in austenite because of its relatively low content in MZ. The same effect, consisting in higher concentration in MZ, was observed in case of the chromium due to the relatively high amount of this ele-ment in Stellite-6 powder. However, the increased chromium con-centration resulted in presence of chromium borides.

Taking into account the results of XRD and X-ray microanalysis, the obtained concentration profiles enabled to formulate the conclu-sions concerning the probable percentage of some phases in MZ. The reduced average concentrations of iron and nickel were clearly visible in the MZ compared to the substrate (base material). It was caused by the increase in average contents of boron in this area (Fig. 7). Iron and nickel were bonded to boron, producing the bo-rides. This bonding had to result in diminished concentration of iron and nickel. It also indicated the predominant percentage of iron and nickel borides (Fe2B and Ni2B) in the laser-alloyed layer.

The microhardness profile of laser-alloyed austenitic 316L steel with boron and Stellite-6 was shown in Figure 8. The results were

Fig. 4. Microstructure of laser-alloyed 316L steel with boron and Stellite-6 at laser beam power of 1.56 kW; 1 – remelted zone (MZ), 2 – substrateRys. 4. Mikrostruktura stali 316L laserowo stopowanej borem i prosz-kiem Stellite-6 przy mocy wiązki laserowej 1,56 kW; 1 – strefa przetopio-na, 2 – podłoże

compared to the profile which was reported for the same steel after laser alloying with boron only [27]. The measurements were per-formed perpendicular to the laser-alloyed surface along the axis of laser track. The hardness of remelted zone ranged from 462 to 771 HV and was lower than that measured for the laser-alloyed layer with boron only [27]. It was obvious that the lower amount of boron in alloying material had to result in diminished hardness as a consequence of the reduced percentage of borides in MZ. In the base material, the comparable hardness was observed in both cases (170÷205 HV). Besides, the compared profiles didn’t differ from each other regarding the thickness of the re-melted zone. In reality, the average thickness of MZ for laser-alloyed layer with only boron

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was slightly higher, obtaining 365 mm. It required an additional ex-planation. The considered laser-alloyed layers were produced using different laser processing parameters. These differences concerned the laser beam power and the thickness of the paste with alloying material. These parameters influenced the dilution ratio (DR) which could be calculated based on the paste thickness (tC) and the depth of MZ (dMZ):

DR t

d 1 C

MZ (3)

where: tC is the thickness of preplaced coating, μm, and dMZ is the average depth of remelted zone, μm.

The obtained dilution ratios were compared in Table 3. It was ob-vious that the thicker paste coating as well as the lower laser beam power had to result in diminished depth of MZ. The laser-alloyed layer with boron and Stellite-6 was produced using the lower laser beam power. However, the thinner paste coating was used. Addi-tionally, the melting point of Stellite-6 was lower than that char-

Fig. 5. XRD patterns of laser-alloyed 316L steel with boron and Stellite-6 at laser beam power of 1.56 kW (multiple tracks)Rys. 5. Dyfraktogram rentgenowski stali 316L stopowanej laserowo bo-rem i proszkiem Stellite-6 przy mocy wiązki lasera 1,56 kW (ścieżki wie-lokrotne)

Fig. 6. SE images of laser remelted zone in laser-alloyed layer; com-posite microstructure consisting of austenite and iron, nickel and chro-mium borides Rys. 6. Obraz SE strefy przetopionej w warstwie stopowanej laserowo; kompozytowa mikrostruktura składająca się z austenitu i borków żelaza, niklu i chromu

Fig. 7. Elements concentration profiles of laser-alloyed 316L steel with boron and Stellite-6 Rys. 7. Profile stężenia pierwiastków stali 316L stopowanej laserowo bo-rem i proszkiem Stellite-6

Fig. 8. Hardness profiles of laser-alloyed 316L steel with boron and Stellite-6 and with boron only [27] Rys. 8. Profile twardości stali 316L laserowo stopowanej borem i prosz-kiem Stellite-6 oraz wyłącznie borem [27]

acteristic of boron. That was why the difference in depth of MZ between the compared layers was negligible.

Wear resistance tests were performed for laser-alloyed layer with boron and Stellite-6 as well as with boron only. Specimens were in-vestigated for 1.5 hour with a change in the counter-specimen (sin-tered carbide S20S) every half an hour. The mass loss obtained during the first stage of wear (grinding-in) were omitted. It was assumed that grinding-in lasted 0.5 hour. Therefore, the presented results took into account the mass loss measured during the last hour of the tests. The results were compared in Table 3 and were shown in Figure 9.

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The evaluation using relative mass loss of specimen and counter-specimen Δm/mi indicated the increase in wear resistance of laser-alloyed 316L steel with boron and Stellite-6 in comparison with the same steel which was laser-alloyed with boron only. In spite of lower hardness of the produced layer, the both mating materials (sintered carbide S20S and laser-alloyed layer with boron and Stel-lite-6) were characterized by diminished relative mass loss.

4. CONCLUSIONS

Laser alloying with boron and Stellite-6 was proposed in order to improve the tribological properties of austenitic 316L steel. The mi-crostructure was characterized by only two zones: laser remelted zone (MZ) and the substrate. There were no visible effects of heat treatment below MZ in microstructure. Heat-affected zone was not observed because the austenitic steel could not be hardened by typi-cal heat treatment (austenitizing and quenching). The composite microstructure of MZ consisted of hard ceramic phases (iron, chro-mium and nickel borides) with a soft austenitic matrix. The uniform laser-alloyed layer in respect of the thickness was produced because of the relatively high overlapping of multiple laser tracks (86%). Cracks and gas pores were not observed.

The hardness of remelted zone (462÷771 HV) was lower com-pared to the previously produced laser-alloyed layer using only bo-ron as alloying material. It was caused by the decreased amount of boron in alloying material what resulted in the diminished percent-age of hard ceramic phases (borides).

The proposed laser alloying resulted in increase of wear re-sistance. In spite of lower hardness in MZ, the diminished wear of specimen and counter-specimen was observed if the Stellite-6 powder was added to alloying material. It was confirmed by the diminished mass loss of the both mating materials: laser-alloyed specimen with boron and Stellite-6 and the counter-specimen made of sintered carbide S20S.

The application of proposed layers in industry will require the appropriate corrosion resistance. It should be examined in the fu-ture and should be compared to the properties of 316L steel without surface layer.

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Table 3. Depths of remelted zone and dilution ratios Tabela 3. Głębokość strefy przetopionej i proporcje rozcieńczenia

Material

Laser beam

power P kW

Thickness of coating

tC µm

Average depth of remelted

zone dMZ µm

Dilution ratioDR

Laser-alloyed 316L steel with boron and Stellite-6 1.56 200 343 0.42

Laser-alloyed 316L steel with boron only [27] 1.82 230 365 0.37

Table 4. Relative mass loss of specimen and counter-specimenTabela 4. Względny ubytek masy próbki i przeciwpróbki

MaterialRelative mass loss Δm/mi

specimen counter-specimen

Laser-alloyed 316L steel with boron and Stellite-6 0.000773 0.000022

Laser-alloyed 316L steel with boron only [27] 0.001052 0.000054

Fig. 9. Results of wear resistance testsRys. 9. Wyniki prób odporności na zużycie

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Laserowe stopowanie stali 316L borem i Stellite-6Daria Mikołajczak*, Michał Kulka, Natalia Makuch, Piotr DziarskiInstytut Inżynierii Materiałowej, Politechnika Poznańska; *daria.h.mikolajczak@doctorate.put.poznan.pl

Inżynieria Materiałowa 6 (220) (2017) 259÷265DOI 10.15199/28.2017.6.2© Copyright SIGMA-NOT MATERIALS ENGINEERING

Słowa kluczowe: stopowanie laserowe, stal 316L, mikrostruktura, twardość, odporność na zużycie.

1. CEL PRACY

Stal 316L jest powszechnie stosowanym materiałem odpornym na korozję i żaroodpornym. Te korzystne właściwości zawdzięcza jed-nofazowej, austenitycznej mikrostrukturze i odpowiedniej zawarto-ści węgla, chromu, niklu i molibdenu. To sprawia, że materiał ten jest stosowany często tam, gdzie jest spodziewane agresywne śro-dowisko lub wysoka temperatura.

Celem pracy było przeprowadzenie stopowania laserowego stali 316L z zastosowaniem materiału stopującego w postaci mieszaniny amorficznego boru i proszku Stellite-6. Bor amorficzny miał pro-wadzić do wytworzenia w strefie przetopionej twardych borków żelaza, chromu i niklu, podstawowych pierwiastków występują-cych w stali 316L. Dodatek kobaltu, głównego składnika proszku Stellite-6, miał powodować ograniczenie udziału borków w mikro-strukturze i sprzyjać jego odporności korozyjnej. Spodziewano się znacznego zwiększenia twardości oraz odporności na zużycie przez tarcie wytworzonej warstwy powierzchniowej w porównaniu ze stalą 316L nie poddaną żadnej obróbce.

2. MATERIAŁ I METODYKA BADAŃ

Do badań zastosowano stal austenityczną 316L o składzie chemicz-nym przedstawionym w tabeli 1. Skład chemiczny proszku Stelli-te-6 zamieszczono w tabeli 2. Próbki do badań miały kształt pier-ścienia o średnicy zewnętrznej 20 mm, wewnętrznej 12 mm i wyso-kości 12 mm. Stopowanie laserowe przeprowadzono metodą dwu-stopniową (rys. 1). Pierwszy etap polegał na pokryciu zewnętrznej powierzchni cylindrycznej próbek materiałem stopującym, który składał się z amorficznego boru i proszku Stellite-6 (w proporcji mas. 1:1), wymieszanych z organicznym spoiwem w postaci alko-holu poliwinylowego. Grubość nałożonej pasty wynosiła 200 μm.

Drugi etap obróbki polegał na przetopieniu laserowym tak przy-gotowanej powierzchni za pomocą lasera technologicznego CO2 TRUMPF TLF 2600 Turbo. Parametry obróbki laserowej były na-stępujące: moc wiązki laserowej P = 1,56 kW, prędkość skanowa-nia wiązką vl = 2,88 m/min, średnica wiązki d = 2 mm. Prędkość skanowania wiązką była wypadkową ruchu obrotowego próbki (45,85 obr./min) oraz posuwu głowicy laserowej (0,28 mm/obr.), co pokazano na rysunku 2. Stosowano dość duży stopień zachodzenia ścieżek laserowych (86%).

Po obróbce laserowej wykonano zgłady metalograficzne w kierun-ku prostopadłym do wytworzonych ścieżek laserowych. W celu ujaw-nienia mikrostruktury próbki trawiono odczynnikiem składającym się z bezwodnej gliceryny, HCl i HNO3 w proporcji obj. 2:3:1. Skład fazowy strefy przetopionej badano za pomocą dyfraktometru rentge-nowskiego PANalytical EMPYREAN. Profile zawartości głównych pierwiastków występujących w strefie przetopionej (żelaza, chromu, niklu, kobaltu i boru) badano mikroanalizatorem rentgenowskim PGT Avalon metodą EDS. Profil twardości w funkcji odległości od po-wierzchni wyznaczono sposobem Vickersa przy obciążeniu 0,1 kG (0,98 N). Do badań odporności na zużycie przez tarcie wytworzonej

warstwy zastosowano przeciwpróbkę z węglika spiekanego S20S (rys. 3). Ocenę tej odporności przeprowadzono wyznaczając wględny ubytek masy próbki i przeciwpróbki po próbie trwającej 1,5 godziny ze zmianą położenia przeciwpróbki co pół godziny.

3. WYNIKI I ICH DYSKUSJA

Mikrostrukturę laserowo stopowanej stali 316L z zastosowaniem materiału stopującego w postaci boru i Stellite-6 pokazano na ry-sunku 4. Na powierzchni otrzymano ciągłą warstwę powierzchniową pozbawioną mikropęknięć i pęcherzy gazowych. Stwierdzono wy-stępowanie dwóch stref w materiale: strefy przetopionej (1) i podłoża (2). Strefy wpływu ciepła nie zaobserwowano, co było spowodowa-ne brakiem możliwości oddziaływania na mikrostrukturę stali za po-mocą typowej obróbki cieplnej, niezależnie od szybkości chłodzenia. Strefa przetopiona była zwarta i miała dość jednorodną grubość dzię-ki stosowaniu dużego stopnia zachodzenia ścieżek (86%). W stre-fie przetopionej otrzymano strukturę kompozytową, składającą się z twardych borków żelaza, chromu i niklu w miękkiej osnowie auste-nitycznej, co potwierdziły badania metodą dyfrakcji rentgenowskiej (rys. 5) i obrazy SE (rys. 6). Nie stwierdzono występowania borków kobaltu, a jedynie jego zwiększoną zawartość w austenicie (rys. 7).

Profil twardości w wytworzonej warstwie porównano z otrzyma-nym wcześniej profilem dla stali 316L stopowanej wyłącznie borem (rys. 8). Stwierdzono zmniejszenie twardości (do 462÷771 HV) w strefie przetopionej, przy jej porównywalnej grubości, pomimo różnic w parametrach obróbki laserowej skutkującej różnym stop-niem rozcieńczenia (tab. 3).

Odporność na żużycie przez tarcie również porównano z odporno-ścią warstwy stopowanej laserowo wyłącznie borem. Wyniki zesta-wiono w tabeli 4 i pokazano na rysunku 9. Okazało się, że pomimo mniejszej twardości warstwy jej odporność na zużycie przez tarcie była większa w porównaniu z próbką stopowaną wyłącznie borem. Względ-ne ubytki masy próbki stopowanej laserowo borem z dodatkiem prosz-ku Stellite-6 i współpracującej z nią przeciwpróbki były mniejsze od analogicznych wartości otrzymanych dla stopowania wyłącznie borem.

4.PODSUMOWANIE

Laserowe stopowanie stali austenitycznej 316L borem i proszkiem Stellite-6 zastosowano w celu zwiększenia jej twardości i odpor-ności na zużycie przez tarcie. Głównym założeniem pracy było wprowadzenie do strefy przetopionej kobaltu (głównego składnika proszku Stellite-6), co powinno pozytywnie oddziaływać na od-porność korozyjną. W wyniku przeprowadzonej obróbki wytwo-rzono w strefie przetopionej strukturę kompozytową składającą się z twardych borków żelaza, chromu i niklu oraz miękkiej osnowy austenitycznej wzbogaconej kobaltem. Sprawdzono jednocześnie, jaki wpływ ma ten dodatek na właściwości użytkowe powstałej warstwy. Pomimo pewnego zmniejszenia twardości warstwy w po-równaniu z materiałem stopowanym wyłącznie borem, odporność na zużycie przez tarcie zwiększyła się.