Laser alloying of 316L steel with boron using CaF 2

Transkrypt

1 Inżynieria Materiałowa 1 (209) (2016) 4 9 DOI / Copyright SIGMA-NOT MATERIALS ENGINEERING Laser alloying of 316L steel with boron using self-lubricating addition Daria Mikołajczak *, Adam Piasecki, Michał Kulka, Natalia Makuch Instytut 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 suitable 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 as a self-lubricating addition. Then the surface was remelted by laser beam. TRUMPF TLF 2600 Turbo CO 2 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 hardness 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. Key words: laser boriding, self-lubricating addition, microstructure, hardness, wear resistance. 1. INTRODUCTION AISI 316L austenitic stainless steel is well-known for its good corrosion 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 molybdenum 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 appreciable mechanical wear (adhesive or abrasive), this material should be characterized by suitable wear protection. A relatively low hardness (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 nitrogen, carbon or boron [2 11]. Glow discharge assisted low-temperature nitriding, carried out at 440 C for 6 h, resulted in the formation 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 thickness about 20 μm [3], and iron nitrides (Fe 4 N) were additionally visible in microstructure at a higher process temperature [3, 4]. The chromium nitrides Cr 2 N 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 lattice [6 9]. The chromium carbides, expanded austenite and martensite occurred after carburizing at higher temperature [6]. The layers were characterized by the thickness up to 50 μm. Austenitic steels could be also efficiently pack-boronized [10 12] in the range of temperature C ( K) without sacrificing corrosion resistance. Diffusion boronizing required a relatively 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 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 microstructure and properties of the metals and their alloys [15, 16]. Laser treatment allowed the superficial incorporation of hard particles into most metals and alloys. Therefore, laser alloying was successfully applied to improve the superficial hardness of various stainless steels by incorporating carbides [17, 18] or with using other alloying materials, i.e. NiCoCrB alloy [19]. Laser alloying with boron was also intensively developed for constructional steels [20], nodular 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 played an important role [28 31]. Therefore, in this study, lubricant was added to the alloying material. The paste, consisting of amorphous boron and 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 alloying with boron only [27]. 2. EXPERIMENTAL PROCEDURE AISI 316L austenitic steel was investigated. Its chemical composition 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. 4 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

2 The two-step laser-boriding process was carried out during laser alloying. The paste, consisting of amorphous boron and blended with a diluted polyvinyl alcohol solution, was used as an alloying material. Boron and were blended with a mass ratio of 10:1. The thickness of paste was equal to 200 μm. At first, the external 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 CO 2 laser of the nominal power 2.6 kw. Laser processing parameters were as follows: laser beam power P = 1.82 kw and scanning rate v l = 2.88 m/min. The diameter of the laser beam d was equal to 2 mm. TEM 01* multiple mode of the laser beam was applied. Hence, the averaging irradiance E of about 58 kw/cm 2 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 corresponded to the distance between the axes of the adjacent tracks and to the feed rate used. The obtained scanning rate v l (2.88 m/min) resulted from the rotational speed n (45.85 min 1 ) and feed rate v f (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: the ZWICK 3212 B apparatus. The tests were performed under the indentation load of 0.1 kgf (about 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/cm 3 and hardness of 1430 HV. The wear test was conducted under conditions 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 O = d f 100% (1) d 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 In order to reveal the microstructure, the etching solution, consisting of anhydrous glycerin, HCl and HNO 3, was used with a volume ratio of 2:3:1. Microhardness profile through the investigated layer was determined in the polished cross-section of specimen. The Vickers method was applied for microhardness measurements with using Table 1. Chemical composition of material used, wt % Tabela 1. Skład chemiczny stosowanego materiału, % mas. Material C Cr Ni Mo Mn Si Fe Fig. 2. Method of multiple tracks producing; d laser beam diameter (d = 2 mm), v f rate of feed, v l scanning rate, n rotational speed, v t tangential rate, f distance from track to track Rys. 2. Metoda wytwarzania ścieżek wielokrotnych; d średnica wiązki laserowej (d = 2 mm), v f posuw, v l prędkość skanowania wiązką, n prędkość obrotowa, f odległość między ścieżkami 316L balance Fig. 1. Two-step method of laser-boriding Rys. 1. Laserowe borowanie dwustopniową metodą przetapiania Fig. 3. Scheme of wear test; P = 49 N, rotational speed n = 250 min 1 Rys. 3. Schemat zużycia; P = 49 N, prędkość obrotowa n = 250 min 1 NR 1/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 5

3 roughness of the sample, but it was not prepared before the wear test. Wear resistance was evaluated by relative mass loss of specimen and counter-specimen (Dm/m i ) according to the equation: Δm mi m = m m i where: Dm is mass loss, mg, m i is initial mass of specimen or counterspecimen,mg, m f is final mass of specimen or counterspecimen, mg. 3. RESULTS AND DISCUSSION The microstructure of laser-alloyed 316L steel with using boron and as alloying material was shown in Figure 4. The laser treatment was carried out at laser beam power of 1.82 kw. The continuous 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 invisible 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. i f (2) 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 performed along the axis of a laser track. According to the literature data [10 12], the diffusion boriding resulted in the presence of FeB and Fe 2 B phases in the surface of austenitic steel. Moreover, chromium and nickel borides could occur in the diffusion-borided layer. The laser alloying with boron caused the formation of a composite layer consisting of hard borides (iron, nickel and chromium borides) and soft austenite [27]. In the laser-fabricated Fe Ni Co Cr B austenitic alloy [19], the borocarbides also appeared. Hence, the similar phase composition was expected for the layers produced as a consequence of laser alloying with boron and. Only the difference should be the presence of 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 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 in alloying paste caused the decrease in microhardness of remelted zone, especially close to the surface, in comparison with the previous study [27]. Simultaneously, the depth of the alloyed layer (MZ), which was produced with using the lubricant, was greater. The lower melting point of, compared to boron, could be the reason for such a situation. Additionally, the slightly thinner paste coating was used in the presented 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 diminished hardness ( 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 ( HV) characteristic of the base material, i.e. austenitic 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 at laser beam power of 1.82 kw; 1 remelted zone (MZ), 2 substrate; LM Rys. 4. Mikrostruktura stali 316L laserowo stopowanej borem i 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 and with boron only [27] Rys. 5. Profile twardości stali 316L laserowo stopowanej borem i oraz wyłącznie borem [27] 6 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

4 Wear resistance was tested for 1 hour with a change in the counter-sample every half an hour. The results were presented in Figure 6 and in Table 2. The evaluation by relative mass loss of specimen and counter-specimen Δm/m i indicated the significant increase in wear resistance of the laser-alloyed layer with boron and in comparison with the laser-borided layer previously studied [27]. The specimen, laser-alloyed with using the lubricant, was characterized 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 which caused that tribofilm was still produced on the worn surface during the test. 4. CONCLUSIONS Laser alloying with boron using 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 heataffected zone. The laser alloying with boron and produced the composite layer consisting of hard borides and soft austenite as well as of 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 thickness. 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 remelted zone, alloyed with boron and. 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 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 tests Rys. 6. Wyniki prób odporności na zużycie Fig. 7. Microstructure of laser-alloyed layer with boron and (SEM) and EDS patterns of Fe, Cr and Ca from this surface Rys. 7. Mikrostruktura warstwy laserowo stopowanej borem i (SEM) oraz mapy rozmieszczenia Fe, Cr i Ca otrzymane metodą EDS Table 2. Relative mass loss of specimen and counter-specimen Tabela 2. Względny ubytek masy próbki i przeciwpróbki Relative mass loss Δm/m Material i Specimen Counter-specimen Laser-alloyed 316L steel with boron and Laser-alloyed 316L steel with boron only [27] was characterized by the hardness of about 650 HV. The diminished percentage of hard borides within the distance from the surface resulted in a decrease in hardness to 270 HV at the end of MZ. Next, the hardness gradually decreased up to about HV in the base material. The significant increase in wear resistance of laser-alloyed layer with boron and 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. NR 1/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 7

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6 Inżynieria Materiałowa 1 (209) (2016) 4 9 DOI / Copyright SIGMA-NOT MATERIALS ENGINEERING Laserowe stopowanie stali 316L borem z dodatkiem samosmarującym Daria Mikołajczak *, Adam Piasecki, Michał Kulka, Natalia Makuch Instytut Inżynierii Materiałowej, Politechnika Poznańska, * daria.h.mikolajczak@doctorate.put.poznan.pl Słowa kluczowe: borowanie laserowe, dodatek samosmarujący, mikrostruktura, twardość, odporność na zużycie. 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 (ściernego 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 mieszaniny amorficznego boru i dodatku samosmarującego. Bor amorficzny miał prowadzić do wytworzenia w strefie przetopionej twardych borków żelaza, chromu i niklu podstawowych pierwiastków występujących w stali 316L. Spodziewano się znacznego zwiększenia twardości oraz odporności na zużycie przez tarcie wytworzonej warstwy powierzchniowej w porównaniu ze stalą 316L nie poddaną żadnej obróbce. Zastosowanie dodatku samosmarującego w postaci fluorku wapnia miało prowadzić do jeszcze większej odporności na zużycie dzięki wytworzeniu na żużywającej się powierzchni tribofilmu. 2. MATERIAŁ I METODYKA BADAŃ Do badań zastosowano stal austenityczną 316L o składzie chemicznym 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 zewnętrznej powierzchni cylindrycznej próbek materiałem stopującym, który 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 przetapiano laserowo. Stosowano laser technologiczny CO 2 TRUMPF TLF 2600 Turbo. Parametry obróbki laserowej były następujące: moc wiązki laserowej P = 1,82 kw, prędkość skanowania wiązką v l = 2,88 m/min, średnica wiązki d = 2 mm. Uśrednione natężenie promieniowania E wynosiło 58 kw/cm 2. 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 stosunkowo duży stopień zachodzenia ścieżek laserowych (86%). Po obróbce laserowej wykonano zgłady metalograficzne w kierunku prostopadłym do wytworzonych ścieżek laserowych. W celu ujawnienia mikrostruktury próbki trawiono odczynnikiem składającym się z bezwodnej gliceryny, HCl i HNO 3 w proporcji objętości 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 pokazano na rysunku 4. Na powierzchni otrzymano ciągłą warstwę powierzchniową pozbawioną mikropęknięć i pęcherzy gazowych. Stwierdzono występowanie dwóch stref w materiale: strefy przetopionej (1) i podłoża (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 zachodzenia 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 otrzymanym wcześniej profilem dla stali 316L stopowanej wyłącznie borem (rys. 5). Stwierdzono nieznaczne zmniejszenie twardości (do 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 odpornością warstwy stopowanej laserowo wyłącznie borem. Wyniki zestawiono w tabeli 2 i pokazano na rysunku 6. Okazało się, że względny ubytek masy próbki stopowanej laserowo borem i dodatkiem samosmarującym ( ) jest kilkakrotnie mniejszy od otrzymanego dla próbki stopowanej wyłącznie borem. Przyczyny tego należy szukać w wytworzeniu na zużytej powierzchni tribofilmu zawierającego fluorek wapnia. Występowanie cząstek w strefie przetopionej potwierdziły obserwacje prowadzone z zastosowaniem skaningowego mikroskopu elektronowego (rys. 7). Mapy rozmieszcenia 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 wapnia można wprowadzać do stali austenitycznej metodą stopowania laserowego równocześnie z borem. Powodowało to wytworzenie w strefie przetopionej struktury składającej się z twardych borków żelaza, chromu i niklu oraz miękkiej osnowy austenitycznej z cząstkami. Pomimo pewnego zmniejszenia twardości warstwy w porównaniu z materiałem stopowanym wyłącznie borem, odporność na zużycie przez tarcie zwiększyła się kilkakrotnie. NR 1/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 9