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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,
*[email protected]
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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NR 1/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 5
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
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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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NR 1/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 7
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.
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8 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII
REFERENCES
[1] Glaeser W. A.: Materials for tribology. Tribology Series,
20, Elsevier (1992).
[2] Skołek-Stefaniszyn E., Kaminski J., Sobczak J., Wierzchoń
T.: Modifying the properties of AISI 316L steel by glow discharge
assisted low-tempera-ture nitriding and oxynitriding. Vacuum 85
(2010) 164÷169.
[3] Skołek-Stefaniszyn E., Burdynska S., Mroz W., Wierzchoń T.:
Structure and wear resistance of the composite layers produced by
glow discharge nitriding and PLD method on AISI 316L austenitic
stainless steel. Vacuum 83 (2009) 1442÷1447.
[4] Li Y., Wang Z., Wang L.: Surface properties of nitrided
layer on AISI 316L austenitic stainless steel produced by high
temperature plasma nitriding in short time. Applied Surface Science
298 (2014) 243÷250.
[5] Frączek T., Olejnik M., Jasiński J., Skuza Z.: Short-term
low-temperature glow discharge nitriding of 361L austenitic steel.
Metalurgija 50 (3) (2011) 151÷154.
[6] Sun Y., Li X., Bell T.: Structural characteristics of low
temperature plasma carburised austenitic stainless steel. Materials
Science and Technology 15 (1999) 1171÷1178.
[7] García Molleja J., Nosei L., Ferrón J., Bemporad E., Lesage
J., Chicot D., Feugeas J.: Characterization of expanded austenite
developed on AISI 316L stainless steel by plasma carburization.
Surface and Coatings Tech-nology 204 (2010) 3750÷3759.
[8] Ceschini L., Chiavari C., Lanzoni E., Martini C.:
Low-temperature car-burised AISI 316L austenitic stainless steel:
Wear and corrosion behav-iour. Materials and Design 38 (2012)
154÷160.
[9] Sun Y.: Tribocorrosion behaviour of low temperature plasma
carburized stainless steel. Surface and Coatings Technology 228
(2013) S342÷S348.
[10] Ozdemir O., Omar M. A., Usta M., Zeytin S., Bindal C.,
Ucisik A. H.: An investigation on boriding kinetics of AISI 316
stainless steel. Vacuum 83 (2009) 175÷179.
[11] Kayali Y., Büyüksagis A., Günes I., Yalçin Y.:
Investigation of corrosion behaviours at different solutions of
boronized AISI 316L stainless steel. Protection of Metals and
Physical Chemistry of Surfaces 49 (3) (2013) 348÷358.
[12] Kayali Y., Büyüksagis A., Yalçin Y.: Corrosion and wear
behaviours of boronized AISI 316L stainless steel. Metals and
Materials International 19 (5) (2013) 1053÷1061.
[13] Hsu C. H., Huang K. H., Lin M. R.: Annealing effect on
tribological prop-erty of arc-deposited TiN film on 316L austenitic
stainless steel. Surface and Coatings Technology 259 (2014)
167÷171.
[14] Zhang L., Yang H., Pang X., Gao K., Tran H. T., Volinsky A.
A.: TiN-coating effects on stainless steel tribological behaviour
under dry and lu-bricated conditions. Journal of Materials
Engineering and Performance 23 (4) (2014) 1263÷1269.
[15] Major B.: Chapter 7: Laser processing for surface
modification by remelt-ing and alloying of metallic systems. In
“Materials Surface Processing by Directed Energy Techniques” Edited
by Yves Paleau, Elsevier (2006).
[16] Goły M., Kusiński J.: Microstructure and properties of the
laser treated 30CrMnMo16-8 chromium steel. In: Problems of modern
techniques in aspect of engineering and education, eds.: Paweł
Kurtyka et al.. Monog-raphy, Institute of Technology, Pedagogical
University, Cracow (2006) 183÷188.
[17] Kim T. H., Kim B. C.: Chromium carbide laser-beam
surface-alloying treatment on stainless steel. Journal of Materials
Science 27 (1992) 2967÷2973.
[18] Tassin C., Laroudie F., Pons M., Lelait L.: Improvement of
the wear resis-tance of 316L stainless steel by laser surface
alloying. Surface and Coat-ings Technology 80 (1996) 207÷210.
[19] Kwok C. T., Cheng F. T., Man H. C.: Laser-fabricated
Fe–Ni–Co–Cr–B austenitic alloy on steels. Part I. Microstructures
and cavitation erosion behaviour. Surface and Coatings Technology
145 (2001) 194÷205.
[20] Kulka M., Makuch N., Pertek A.: Microstructure and
properties of laser-borided 41Cr4 steel. Optics & Laser
Technology 45 (2013) 308÷318.
[21] Paczkowska M., Ratuszek W., Waligora W.: Microstructure of
laser bo-ronized nodular iron. Surf. Coat. Technol. 205 (2010)
2542÷2545.
[22] Filip R., Sieniawski J., Pleszakov E.: Formation of surface
layers on Ti-6Al-4V titanium alloy by laser alloying. Surf. Eng. 22
(1) (2006) 53÷57.
[23] Guo C., Zhou J., Zhao J., Guo B., Yu Y., Zhou H., Chen J.:
Microstructure and friction and wear behaviour of laser boronizing
composite coatings on titanium substrate. Appl. Surf. Sci. 257
(2011) 4398÷4405.
[24] Kulka M., Makuch N., Dziarski P., Piasecki A., Miklaszewski
A.: Micro-structure and properties of laser-borided composite
layers formed on com-mercially pure titanium. Optics and Laser
Technology 56 (2014) 409÷424.
[25] Kulka M., Dziarski P., Makuch N., Piasecki A., Miklaszewski
A.: Micro-structure and properties of laser-borided Inconel
600-alloy. Applied Sur-face Science 284 (2013) 757÷771.
[26] Kulka M., Makuch N., Dziarski P., Piasecki A.: A study of
nanoindentation for mechanical characterization of chromium and
nickel borides’ mixtures formed by laser boriding. Ceram. Int. 40
(4) (2014) 6083÷6094.
[27] Kulka M., Mikołajczak D., Makuch N., Dziarski P.: Laser
alloying of 316L steel with boron. Inżynieria Materiałowa 6 (2014)
512÷515.
[28] Xu C. H., Wu G. Y., Xiao G. C., Fang B.: Al2O3/(W,
Ti)C/CaF2 multi-component graded self-lubricating ceramic cutting
tool material. Int. J. Refract. Met. H. 45 (2014) 125÷129.
[29] Xiang Z.-F., Liu X.-B., Ren J., Luo J., Shi S.-H., Chen Y.,
Shi G.-L., Wu S.-H.: Investigation of laser cladding high
temperature anti-wear compos-ite coatings on Ti6Al4V alloy with the
addition of self-lubricant CaF2. Appl. Surf. Sci. 313 (2014)
243÷250.
[30] Hua Y., Jie Z., Peilei Z., Zhishui Y., Chonggui L., Peiquan
X., Yunlong L.: Laser cladding of Co-based alloy/TiC/CaF2
self-lubricating composite coatings on copper for continuous
casting mold. Surf. Coat. Technol. 232 (2013) 362÷369.
[31] Lingqian K., Shengyu Z., Qinling B., Zhuhui Q., Jun Y.,
Weimin L.: Fric-tion and wear behaviour of self-lubricating
ZrO2(Y2O3)–CaF2–Mo–graph-ite composite from 20°C to 1000°C. Ceram.
Int. 40 (2014) 10787÷10792.
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NR 1/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 9
Laserowe stopowanie stali 316L borem z dodatkiem samosmarującym
CaF2
Daria Mikołajczak*, Adam Piasecki, Michał Kulka, Natalia
MakuchInstytut Inżynierii Materiałowej, Politechnika Poznańska,
*[email protected]
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.