4 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

Laser alloying of 316L steel with boron using CaF2 self-lubricating addition

Daria Mikołajczak*, Adam Piasecki, Michał Kulka, Natalia MakuchInstytut Inżynierii Materiałowej, Politechnika Poznańska, *daria.h.mikolajczak@doctorate.put.poznan.pl

Good resistance to corrosion and oxidation of austenitic 316L steel is well-known. Therefore, this material is often used wherever corrosive media or high temperature are to be expected. However, under conditions of appreciable mechanical wear (adhesive or abrasive), this steel have to characterize by suit-able wear protection. The diffusion boronizing can improve the tribological properties of 316L steel. However, the small thickness of diffusion layer causes the limited applications of such a treatment. In this study, instead of diffusion process, the laser boriding was used. The external cylindrical surface of base material was coated by paste including amorphous boron and CaF2 as a self-lubricating addition. Then the surface was remelted by laser beam. TRUMPF TLF 2600 Turbo CO2 laser was used for laser alloying. The microstructure of remelted zone consisted of hard ceramic phases (iron, chromium and nickel borides) located in soft austenite. The layer was uniform in respect of the thickness because of the high overlapping used during the laser treatment (86%). The obtained composite layer was significantly thicker than that-obtained in case of diffusion boriding. The remelted zone was characterized by higher hard-ness in comparison with the base material. The significant increase in wear resistance of laser-borided layer was observed in comparison with 316L austenitic steel which was laser-alloyed without CaF2.

Key words: laser boriding, self-lubricating addition, microstructure, hardness, wear resistance.

Inżynieria Materiałowa 1 (209) (2016) 4÷9DOI 10.15199/28.2016.1.1© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTION

AISI 316L austenitic stainless steel is well-known for its good cor-rosion resistance as well as good resistance to high temperature. It results from a single-phase austenitic microstructure as well as from an effective balance of carbon, chromium, nickel and molyb-denum content. Therefore, this steel is often used wherever a high temperature or aggressive corrosive media occur. However, this material is characterized by low hardness and poor wear resistance what causes the limited its applying. Under conditions of appreci-able mechanical wear (adhesive or abrasive), this material should be characterized by suitable wear protection. A relatively low hard-ness (200 HV) and an austenitic structure which cannot be hardened by the typical heat treatment causes, that there is no easy way to improve the wear resistance of this steel [1].

Therefore, many methods of surface treatment were developed to improve the tribological properties of this material. Some of them consisted in the diffusion alloying of the surface with nitro-gen, carbon or boron [2÷11]. Glow discharge assisted low-temper-ature nitriding, carried out at 440°C for 6 h, resulted in the forma-tion of a thin layer (4 μm) consisting of chromium nitrides (CrN) as well as of austenite supersaturated with nitrogen [2]. The layer produced at 550°C (823 K) for 6 h was characterized by the thick-ness about 20 μm [3], and iron nitrides (Fe4N) were additionally visible in microstructure at a higher process temperature [3, 4]. The chromium nitrides Cr2N were also identified in the nitrided layer [5]. Low temperature plasma carburizing at the temperature below 520°C (793 K) produced the layer consisting only of the austenite supersaturated with carbon, and characterized by an expanded lat-tice [6÷9]. The chromium carbides, expanded austenite and mar-tensite occurred after carburizing at higher temperature [6]. The layers were characterized by the thickness up to 50 μm. Austen-itic steels could be also efficiently pack-boronized [10÷12] in the range of temperature 800÷950°C (1073÷1223 K) without sacrific-ing corrosion resistance. Diffusion boronizing required a relative-ly high temperature and longer duration in comparison to typical boronized constructional and tool steels. Pack-boronizing of 316L steel at 950°C (1233 K) for 8 h resulted in producing the layer of

the thickness up to 90 μm [10]. Even for relatively thin boride layer (up to 25 μm), the corrosion resistance of the pack-borided 316L steel was acceptable [11, 12]. Titanium nitride (TiN) coatings were also applied in order to improve tribological properties of 316L steel [13, 14]. They were deposited by physical vapour deposition (PVD) which resulted in producing thin coatings: 1.4 μm [13] and 1.6÷2.4 μm [14], respectively.

In order to increase the case depth of the surface layer and, as a consequence, to extend the range of operating conditions (i.e. range of load), the alternative methods of surface treatment were also developed for austenitic steels. Laser processing was being used for a wide range of applications in order to modify the micro-structure and properties of the metals and their alloys [15, 16]. La-ser treatment allowed the superficial incorporation of hard particles into most metals and alloys. Therefore, laser alloying was success-fully applied to improve the superficial hardness of various stainless steels by incorporating carbides [17, 18] or with using other alloy-ing materials, i.e. NiCoCrB alloy [19]. Laser alloying with boron was also intensively developed for constructional steels [20], nodu-lar cast iron [21], titanium and its alloys [22÷24], Ni-based alloys [25, 26] as well as for austenitic steel [27].

Recently, the self-lubricating coatings were often applied in order to improve the tribological properties of various materials. These coatings contained the lubricants among which CaF2 played an important role [28÷31]. Therefore, in this study, CaF2 lubricant was added to the alloying material. The paste, consisting of amor-phous boron and CaF2 was used in order to improve tribological properties of laser-alloyed 316L steel. The microstructure and some mechanical properties were compared to the effects of laser alloy-ing 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) were used for the study.

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The two-step laser-boriding process was carried out during la-ser alloying. The paste, consisting of amorphous boron and CaF2 blended with a diluted polyvinyl alcohol solution, was used as an alloying material. Boron and CaF2 were blended with a mass ratio of 10:1. The thickness of paste was equal to 200 μm. At first, the ex-ternal cylindrical surface of the specimen was coated by this paste. Then, this surface was remelted by the laser beam (Fig. 1). Laser treatment was carried out with using the TRUMPF TLF 2600 Turbo CO2 laser of the nominal power 2.6 kW. Laser processing param-eters were as follows: laser beam power P = 1.82 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. Hence, the averaging irradiance E of about 58 kW/cm2 was calculated. The focusing mirror was characterized by curvature 250 mm, diameter 48 mm and focal length 125 mm. The laser tracks were arranged as multiple tracks (Fig. 2), with the distance f = 0.28 mm. It corre-sponded to the distance between the axes of the adjacent tracks and to the feed rate used. 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). Laser processing was conducted in argon shielding at a pressure of 0.2 Pa.

The relatively high overlapping of the laser tracks (86%) was applied during laser treatment. This value was calculated using the equation as follows:

O d f

d= − ⋅100%

(1)

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

The microstructure of polished and etched cross-section of the specimen was observed by an light microscope (LM) and scanning electron microscope (SEM) Tescan Vega 5135. In order to reveal the microstructure, the etching solution, consisting of anhydrous glycerin, HCl and HNO3, was used with a volume ratio of 2:3:1.

Microhardness profile through the investigated layer was de-termined in the polished cross-section of specimen. The Vickers method was applied for microhardness measurements with using

the ZWICK 3212 B apparatus. The tests were performed under the indentation load of 0.1 kgf (about 0.981 N).

Wear resistance test was applied to evaluate the tribological properties of the produced layer. The frictional pair consisted of a cylindrical specimen with laser-alloyed layer and of a plate made of sintered carbide S20S as counterspecimen. 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. Such material obtained a mass density of 10.7 g/cm3 and hardness of 1430 HV. The wear test was conducted under condi-tions of dry friction (unlubricated sliding contact) with using the load P = 49 N and the specimen speed of 0.26 m/s, resulting from the rotational speed n = 250 min–1 and the external diameter of the specimen (20 mm). The laser treatment caused a change in surface

Table 1. Chemical composition of material used, wt %Tabela 1. Skład chemiczny stosowanego materiału, % mas.

Material C Cr Ni Mo Mn Si Fe

316L 0.023 17.45 12.92 2.88 0.56 0.45 balance

Fig. 1. Two-step method of laser-boridingRys. 1. Laserowe borowanie 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 wiązki laserowej (d = 2 mm), vf – posuw, vl – prędkość skanowania wiązką, n – prędkość obrotowa, f – odległość między ścieżkami

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

6 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

roughness of the sample, but it was not prepared before the wear test. Wear resistance was evaluated by relative mass loss of speci-men and counter-specimen (Dm/mi) according to the equation:

Δmm

m mmi

i f

i=

−

(2)

where: Dm is mass loss, mg, mi is initial mass of specimen or counterspecimen,mg, mf is final mass of specimen or counterspeci-men, mg.

3. RESULTS AND DISCUSSION

The microstructure of laser-alloyed 316L steel with using boron and CaF2 as alloying material was shown in Figure 4. The laser treat-ment was carried out at laser beam power of 1.82 kW. The continu-ous laser-borided layer was obtained at the surface. Two zones were visible in the microstructure: MZ – laser remelted zone (1) and the substrate (2). It was previously characteristic of the laser-alloyed layer with boron only [27] that heat-affected zone (HAZ) was invis-ible below MZ. The similar effect of laser processing was observed in this study. HAZ didn’t appear below the MZ even at the lowest magnification (Fig. 4a). The microcracks as well as gas pores were not detected in the laser-alloyed layer.

The compact and uniform remelted zone was observed in respect of the thickness. It resulted from the relatively high overlapping of the laser tracks (86%) and was observed during other processes of laser alloying [24÷27]. The depth of the laser-alloyed layer (MZ) varied between 320 and 520 μm, obtaining the averaging value of 460 μm. The thinner layer was measured at the contact of multiple tracks while the thicker layer occurred for the measurements per-formed along the axis of a laser track.

According to the literature data [10÷12], the diffusion boriding resulted in the presence of FeB and Fe2B phases in the surface of austenitic steel. Moreover, chromium and nickel borides could oc-cur in the diffusion-borided layer. The laser alloying with boron caused the formation of a composite layer consisting of hard bo-rides (iron, nickel and chromium borides) and soft austenite [27]. In the laser-fabricated Fe–Ni–Co–Cr–B austenitic alloy [19], the boro-carbides also appeared. Hence, the similar phase composition was expected for the layers produced as a consequence of laser alloying with boron and CaF2. Only the difference should be the presence of CaF2 lubricant. In the future, X-ray diffraction method should be used in order to identify the phases in the investigated layer.

The hardness profiles of laser-borided layers, produced with and without using CaF2 self-lubricating addition were shown in Figure 5. The measurements were performed perpendicular to the laser-alloyed surface. The occurrence of composite microstructure, reinforced by hard ceramic phases (borides, boro-carbides), was the reason for hardness increase close to the surface and in the entire remelted zone. The presence of CaF2 in alloying paste caused the decrease in microhardness of remelted zone, especially close to the surface, in comparison with the previous study [27]. Simultane-ously, the depth of the alloyed layer (MZ), which was produced with using the lubricant, was greater. The lower melting point of CaF2, compared to boron, could be the reason for such a situation. Additionally, the slightly thinner paste coating was used in the pre-sented study in comparison with the layer alloyed with boron only [27]. These conditions caused the increase in dilution ratio what resulted in the higher depth of remelted zone as well as in its dimin-ished hardness (600÷700 HV). The hardness of about 650 HV was obtained close to the surface (Fig. 5). Then, the hardness gradually decreased to about 270 HV at the end of MZ. It resulted from the diminishing percentage of hard borides within the distance from the surface. Below the laser remelted zone, the hardness obtained the values (170÷205 HV) characteristic of the base material, i.e. auste-nitic 316L steel. The measurements of hardness did not indicate that heat-affected zone appeared below MZ.

Fig. 4. Microstructure of laser-alloyed 316L steel with boron and CaF2 at laser beam power of 1.82 kW; 1 – remelted zone (MZ), 2 – substra-te; LMRys. 4. Mikrostruktura stali 316L laserowo stopowanej borem i CaF2 przy mocy wiązki laserowej 1,82 kW; 1 – strefa przetopiona, 2 – podłoże; mikroskop świetlny

Fig. 5. Hardness profiles of laser-alloyed 316L steel with boron and CaF2 and with boron only [27] Rys. 5. Profile twardości stali 316L laserowo stopowanej borem i CaF2 oraz wyłącznie borem [27]

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Wear resistance was tested for 1 hour with a change in the coun-ter-sample every half an hour. The results were presented in Fig-ure 6 and in Table 2. The evaluation by relative mass loss of speci-men and counter-specimen Δm/mi indicated the significant increase in wear resistance of the laser-alloyed layer with boron and CaF2 in comparison with the laser-borided layer previously studied [27]. The specimen, laser-alloyed with using the lubricant, was character-ized by a few times lower value of relative mass loss. SE image of the investigated layer as well as EDS patterns of iron, chromium and calcium (Fig. 7) confirmed the presence of CaF2 which caused that tribofilm was still produced on the worn surface during the test.

4. CONCLUSIONS

Laser alloying with boron using CaF2 self-lubricating addition was applied in order to improve tribological properties of austenitic 316L steel. Two zones characterized the microstructure obtained: laser remelted zone (MZ) and the substrate without visible heat-affected zone. The laser alloying with boron and CaF2 produced the composite layer consisting of hard borides and soft austenite as well as of CaF2 particles what should be confirmed by phase analysis in the future.

The high overlapping of multiple laser tracks (86%) caused the formation of the uniform laser-alloyed layer in respect of the thick-ness. The microcracks as well as gas pores were not detected in the laser-alloyed layer.

The occurrence of various types of borides (iron, chromium or nickel borides) was the reason for an increase in hardness of the re-melted zone, alloyed with boron and CaF2. However, the presence of lubricant in alloying paste caused the decrease in microhardness of remelted zone. It was caused by the lower melting point of CaF2 in comparison with boron what resulted in the increased dilution ratio as well as in higher depth of the MZ. Hence, the percentage of hard ceramic phases diminished compared to the 316L steel after laser alloying with boron only. Close to the surface, the produced layer

Fig. 6. Results of wear resistance testsRys. 6. Wyniki prób odporności na zużycie

Table 2. Relative mass loss of specimen and counter-specimenTabela 2. Względny ubytek masy próbki i przeciwpróbki

MaterialRelative mass loss Δm/mi

Specimen Counter-specimen

Laser-alloyed 316L steel with boron and CaF2

0.000616681 0.0000515426

Laser-alloyed 316L steel with boron only [27] 0.003375454 0.000203919

Fig. 7. Microstructure of laser-alloyed layer with boron and CaF2 (SEM) and EDS patterns of Fe, Cr and Ca from this surfaceRys. 7. Mikrostruktura warstwy laserowo stopowanej borem i CaF2 (SEM) oraz mapy rozmieszczenia Fe, Cr i Ca otrzymane metodą EDS

was characterized by the hardness of about 650 HV. The diminished percentage of hard borides within the distance from the surface re-sulted in a decrease in hardness to 270 HV at the end of MZ. Next, the hardness gradually decreased up to about 170÷205 HV in the base material.

The significant increase in wear resistance of laser-alloyed layer with boron and CaF2 was observed in comparison with the layer alloyed with boron only. The wear test indicated that relative mass loss was more than four times lower than that-obtained for the layer produced without lubricant.

8 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

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Laserowe stopowanie stali 316L borem z dodatkiem samosmarującym CaF2

Daria Mikołajczak*, Adam Piasecki, Michał Kulka, Natalia MakuchInstytut Inżynierii Materiałowej, Politechnika Poznańska, *daria.h.mikolajczak@doctorate.put.poznan.pl

Inżynieria Materiałowa 1 (209) (2016) 4÷9DOI 10.15199/28.2016.1.1© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. CEL PRACY

Stal austenityczna 316L jest znana z dużej odporności na korozję i utlenianie. Dlatego materiał ten jest stosowany często tam, gdzie jest spodziewane agresywne środowisko lub wysoka temperatura. Jednakże w warunkach znacznego zużycia mechanicznego (ścier-nego czy adhezyjnego) materiał ten powinien charakteryzować się odpowiednią odpornością na zużycie.

Celem pracy było przeprowadzenie stopowania laserowego stali 316L z zastosowaniem materiału stopującego w postaci mie-szaniny amorficznego boru i dodatku samosmarującego CaF2. Bor amorficzny miał prowadzić do wytworzenia w strefie przetopionej twardych borków żelaza, chromu i niklu — podstawowych pier-wiastków występujących w stali 316L. Spodziewano się znacznego zwiększenia twardości oraz odporności na zużycie przez tarcie wy-tworzonej warstwy powierzchniowej w porównaniu ze stalą 316L nie poddaną żadnej obróbce. Zastosowanie dodatku samosmarują-cego w postaci fluorku wapnia miało prowadzić do jeszcze więk-szej odporności na zużycie dzięki wytworzeniu na żużywającej się powierzchni tribofilmu.

2. MATERIAŁ I METODYKA BADAŃ

Do badań zastosowano stal austenityczną 316L o składzie chemicz-nym przedstawionym w tabeli 1. Próbki do badań miały kształt pierścienia o średnicy zewnętrznej 20 mm, wewnętrznej 12 mm i wysokości 12 mm. Stopowanie laserowe przeprowadzono metodą dwustopniową (rys. 1). Pierwszy etap polegał na pokryciu zewnętrz-nej powierzchni cylindrycznej próbek materiałem stopującym, któ-ry składał się z amorficznego boru i fluorku wapnia (w proporcji masowej 10:1) wymieszanych z organicznym spoiwem w postaci alkoholu poliwinylowego. Grubość pasty wynosiła 200 μm.

W drugim etapie tak przygotowaną powierzchnię przetapia-no laserowo. Stosowano laser technologiczny CO2 TRUMPF TLF 2600 Turbo. Parametry obróbki laserowej były następujące: moc wiązki laserowej P = 1,82 kW, prędkość skanowania wiązką vl = 2,88 m/min, średnica wiązki d = 2 mm. Uśrednione natężenie promieniowania E wynosiło 58 kW/cm2. Prędkość skanowania wiązką była wypadkową ruchu obrotowego próbki (45,85 obr./min) oraz posuwu głowicy laserowej (0,28 mm/obr.), co pokazano na ry-sunku 2. Stosowano stosunkowo duży stopień zachodzenia ścieżek laserowych (86%).

Po obróbce laserowej wykonano zgłady metalograficzne w kie-runku prostopadłym do wytworzonych ścieżek laserowych. W celu ujawnienia mikrostruktury próbki trawiono odczynnikiem składa-jącym się z bezwodnej gliceryny, HCl i HNO3 w proporcji obję-tości 2:3:1. Profil twardości w funkcji odległości od powierzchni wyznaczono sposobem Vickersa pod obciążeniem 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 względny

ubytek masy próbki i przeciwpróbki po teście godzinnym 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 CaF2 pokazano na rysun-ku 4. Na powierzchni otrzymano ciągłą warstwę powierzchniową pozbawioną mikropęknięć i pęcherzy gazowych. Stwierdzono wy-stępowanie dwóch stref w materiale: strefy przetopionej (1) i pod-łoża (2). Już wcześniejsze badania wykazały brak strefy wpływu ciepła pod strefą przetopioną, która była zwarta i dość jednorodna pod względem grubości dzięki stosowaniu dużego stopnia zacho-dzenia ścieżek (86%). W strefie przetopionej otrzymano strukturę składającą się z twardych borków żelaza, chromu i niklu w miękkiej osnowie austenitycznej wzbogaconej fluorkiem wapnia.

Profil twardości w wytworzonej warstwie porównano z otrzy-manym wcześniej profilem dla stali 316L stopowanej wyłącznie borem (rys. 5). Stwierdzono nieznaczne zmniejszenie twardości (do 600÷700 HV) oraz zwiększenie grubości warstwy przetopionej, na co wpływ miało występowanie w mikrostrukturze fluorku wapnia o niższej temperaturze topnienia w porównaniu z borem i nieco mniejsza grubość pasty z materiałem stopującym. To skutkowało nieco mniejszym udziałem twardych borków w mikrostrukturze.

Odporność na zużycie przez tarcie również porównano z od-pornością warstwy stopowanej laserowo wyłącznie borem. Wyni-ki zestawiono w tabeli 2 i pokazano na rysunku 6. Okazało się, że względny ubytek masy próbki stopowanej laserowo borem i dodat-kiem samosmarującym (CaF2) jest kilkakrotnie mniejszy od otrzy-manego dla próbki stopowanej wyłącznie borem. Przyczyny tego należy szukać w wytworzeniu na zużytej powierzchni tribofilmu za-wierającego fluorek wapnia. Występowanie cząstek CaF2 w strefie przetopionej potwierdziły obserwacje prowadzone z zastosowaniem skaningowego mikroskopu elektronowego (rys. 7). Mapy rozmiesz-cenia pierwiastków otrzymane metodą EDS wykazały zmniejszone stężenie żelaza i chromu w miejscach występowania cząstek fluorku wapnia wprowadzonych metodą stopowania laserowego.

4.PODSUMOWANIE

Laserowe stopowanie stali austenitycznej 316L borem i dodatkiem samosmarującym prowadzono w celu zwiększenia jej twardości i odporności na zużycie. Jednym z ważniejszych osiągnięć pracy jest wykazanie, że dodatek samosmarujący w postaci fluorku wap-nia można wprowadzać do stali austenitycznej metodą stopowania laserowego równocześnie z borem. Powodowało to wytworzenie w strefie przetopionej struktury składającej się z twardych bor-ków żelaza, chromu i niklu oraz miękkiej osnowy austenitycznej z cząstkami CaF2. Pomimo pewnego zmniejszenia twardości war-stwy w porównaniu z materiałem stopowanym wyłącznie borem, odporność na zużycie przez tarcie zwiększyła się kilkakrotnie.

Słowa kluczowe: borowanie laserowe, dodatek samosmarujący, mikrostruktura, twardość, odporność na zużycie.