corrosion resistance of laser-borided inconel 600 alloy text.pdf · 2018-01-29 · inconel 600...

NR 3/2017 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 149

Corrosion resistance of laser-borided Inconel 600 alloyPiotr Dziarski*, Michał Kulka, Natalia Makuch, Daria Mikołajczak

Institute of Materials Science and Engineering, Poznan University of Technology, Poznan, Poland, *[email protected]

Inconel 600 alloy is used extensively for a variety of industrial applications involving high temperature and aggressive environments. However, under con-ditions of appreciable mechanical wear (adhesive or abrasive), this material has to be characterized by suitable wear protection. The diffusion boronizing efficiently improved the tribological properties of this alloy. Nevertheless, the long duration of this process was necessary in order to obtain the layers of the thickness up to about 100 µm. In this study, instead of the diffusion process, the laser alloying with boron was used for producing a boride layer on Inconel 600 alloy. During this process, the external cylindrical surface of base material was coated by paste, including amorphous boron, and remelted by a laser beam. In the remelted zone, the three areas were observed: compact borides zone consisting of nickel and chromium borides (close to the surface), zone of in-creased percentage of Ni–Cr–Fe matrix (appearing in the greater distance from the surface) and zone of dominant percentage of Ni–Cr–Fe matrix (at the end of the layer). The hardness was comparable to that-obtained in case of diffusion boriding. Simultaneously, the laser-borided layer was significantly thicker. In order to evaluate the corrosion behaviour, the immersion corrosion test in a boiling solution of H2O, H2SO4 and Fe2(SO4)3 was used. As a consequence of selective laser alloying, the difference in electrochemical potentials between the layer and base material caused the accelerated corrosion of the substrate in areas without laser-borided layer. The results showed that laser-borided Inconel 600 alloy could be characterized by the excellent corrosion resistance in such corrosive solution if the whole surface would be covered with laser-alloyed layer.

Key words: laser boriding, Inconel 600 alloy, microstructure, hardness, corrosion resistance.

Inżynieria Materiałowa 3 (217) (2017) 149÷156DOI 10.15199/28.2017.3.7© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTION

Nickel and its alloys are important materials in industries, which require excellent corrosion resistant and heat resistance. Most of nickel alloys are characterized by higher resistant to corrosion than the stainless steels, especially in solutions containing reducing ac-ids and in case of stress-corrosion cracking. The groups of nickel alloys resistant to corrosion can be categorized according to their major alloying elements: Ni–Cr, Ni–Cr–Mo, Ni–Cr–Fe, Ni–Cu and Ni–Mo [1].

Inconel series alloys (nickel–chromium–iron) are a standard engineering materials for applications which require resistance to heat and corrosion. These materials are characterized by excellent mechanical properties including combination of high strength and good workability. The high concentration of nickel results in resist-ance to corrosion by many organic and inorganic compounds and also to chloride-ion stress-corrosion cracking. The role of chromi-um in the Inconel series alloys is to facilitate the passive film forma-tion. Such a film provides protection in a wide range of oxidizing environments such as nitric (HNO3) and chromic (H2CrO4). A sec-ondary role of chromium is to provide some strengthening of the solid solution [1, 2]. However, an essential drawback of Ni-based alloys is their susceptibility to local types of corrosion (including intergranular corrosion). The intergranular corrosion (IGC) of nick-el alloys is caused by segregation of alloying elements at the grain boundaries (for example chromium carbides or nitrides). Upon ex-posure to a corrosive solution, the chemical and structural segrega-tion at grain boundaries leads to electrochemical heterogeneity and to dissolving metal surface and the development of IGC [3].

The secondary disadvantage of Ni-based alloys are their low re-sistance to abrasive or adhesive wear, which causes their limited application. The suitable surface treatment could increase the wear resistance of these materials. However, the production of hard sur-face layers should be accompanied by high corrosion resistance. Many methods of surface treatment of Ni-based alloys provided the protection against wear: nitriding [4÷6], boriding [7÷14] or laser al-loying [15÷18]. The low temperature gas ntriding of Nimonic series alloys in an atmosphere of NH3 caused the formation of thin layers

(maximal thickness up to 9.6 µm for process carried out at 400°C for 20 h) [4]. The obtained layers contained chromium nitrides CrN and were characterized by high hardness (970÷1230 HV0.005). The intensified plasma-assisted nitriding (IPAP) of Inconel 718 at 0.75 mA/cm2 and temperature below 500°C for 3 h caused the formation of surface layer consisting of a single nitride phase CrN (thickness about 4 μm) followed by a 25 μm thick nitrogen diffu-sion zone [6]. This layer was characterized by significantly im-proved hardness (up to 2350 HK) and wear resistance. The corro-sion behaviour of the nitrided layer was investigated in 0.1 M NaCl aqueous solution. The open circuit potential (OCP) of nitrided and untreated Inconel 718 alloy was measured as a function of time. The results showed that nitrided Inconel 718 alloy exhibited a signifi-cantly higher corrosion potential (+200 mV) compared to that of the untreated material (~0 mV). The increase in OCP can be attributed to passivation promoted by the presence of nitrogen [6].

The glow discharge assisted nitriding of Inconel 625 alloy [5], carried out at temperature of 560°C in a gaseous mixture of nitro-gen and hydrogen (N2:H2 = 4:1) under a pressure of 3 mbar for 6 h, resulted in the formation of about 5 µm thick nitride layer (CrN-phase). The microhardness measured on the outer surface of the produced layer increased from up to 1480 HV0.05. The corrosion resistance tests were performed in a 0.5 M NaCl solution using the potentiodynamic method. The results show that the chromium ni-tride layer caused slightly increase in corrosion resistance of In-conel 625 alloy. The corrosion potential increased from –232 mV for untreated Inconel 625 alloy to –136 mV for nitride alloy, at cur-rent densities of 13.7 mA/cm2 and 197 mA/cm2, respectively. The presence of chromium nitrides altered the character of corrosion: from pitting character to uniform corrosion, for untreated Inconel 625 and chromium nitride layer, respectively [5].

Boriding was also a suitable treatment of nickel and its alloys to increase their hardness and wear resistance. Many boriding meth-ods were used for producing the borided layer on nickel and its al-loys: powder-pack [7÷11], paste [12] or gas boronizing [13, 14]. In case of powder-pack method, the use of special agents without SiC was required. The commercial powders, containing SiC, resulted in the formation of silicides which were characterized by high porosity

150 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

and diminished hardness of the diffusion layer. Boriding of Nickel 201 in Ekabor-II powder at 900°C for 4 h resulted in the formation of a thick (220 µm) multiphase layer (NiB, Ni2B, Ni3B and Ni4B3) with maximal hardness of 1778 HV [10]. However, Ekabor II pow-der contained SiC and, hence, the presence of nickel silicides was very probable. Pack boriding of Inconel 625 and Inconel 718 with-out SiC in powder at a temperature in the range of 850÷950°C for 4 h caused the formation of thick layers, 70÷76 µm and 66÷72 µm, respectively [11]. Producing the multhiphase layers (containing nickel borides Ni2B, Ni3B, and Ni4B3) increased the hardness up to 2400 HK. Boriding of pure nickel and Nimonic 90 alloy using the paste boriding method [12] also improved hardness and wear resistance of these materials. The maximal thickness of the borided layer produced on Nimonic 90 alloy was obtained after boriding at 1050°C for 5 h (75 µm), and following phases were detected: Ni2B and CrB. Corrosion resistance tests were carried out in an ar-tificial seawater solution for 1 month (31 days). Boriding of pure nickel improved corrosion resistance, probably, due to the forma-tion of a single phase layer (Ni2B) on the surface. Simultaneously, the borided Nimonic 90 was characterized by diminished corrosion resistance in comparison with untreated alloy. The presence of two co-existing phases (CrB and (Ni, Co)2B), which were characterized by different galvanic potentials, was the reason for the diminished corrosion resistance, because many galvanic microcells were cre-ated in a seawater solution [12]. Gas boriding was a very interesting process because of the possibility of controlling the composition of boriding medium [13, 14]. In case of Inconel 600 alloy, the gas-borided layer was 1.5 times thicker compared to the layers produced on Inconel series alloys using pack-boronizing method. The boride layer, formed on Inconel 600 alloy, was composed of the mixture of hard ceramic phases (nickel and chromium borides) and was char-acterized by a high hardness (up to 2180 HV) and tenfold increase in wear resistance of the examined alloy [13].

Nickel-based alloys were also laser-alloyed in order to improve their hardness and wear resistance [15÷18]. The laser alloying of Nimonic 80A alloy with Si + Al and pure Al powder caused obtain-ing the multiphase layers with hardness up to 500 HV [15, 16]. The presence of SiO2 and Al2O3 oxides was the reason for improved oxidation resistance of Nimonic 80A alloy [15]. Laser alloying of Inconel 600 alloy with a paste containing amorphous boron [17] re-sulted in the formation of multiphase boride layer (nickel, chromium or iron borides). The produced layers were thick (up to 467 µm) and were characterized by high hardness and wear resistance. Seawater corrosion behaviour of laser surface modified Inconel 625 alloy was presented in paper [18]. Laser melting with simultaneous injection of TiC or WC particles was carried out in order to improve wear resistance of Inconel 625 alloy. After laser surface modification the microstructure consisted of non-melted particles (WC or TiC) sur-rounded by the remelted Inconel alloy matrix. Seawater exposure tests for 6 months were performed in order to determine the corro-sion resistance of the produced layers. Both types of carbides were the reason for the depletion of alloying elements from the metal matrix and the formation of galvanic microcells between it and the adjacent metal. In case of laser alloying with WC, corrosion oc-curred within the particulate itself. Whereas, in the case of injected TiC particles, corrosion also occurred at the eutectic carbide areas. Unfortunately, to minimize seawater corrosion of laser-modified Inconel 625, the produced layers required homogenization of mi-crostructure [18].

Most of the corrosion studies of surface layers produced on Ni-based alloys were concentrated on reporting the results of the cor-rosion resistance test. However, the important aspect of corrosion behavior studies is an analysis of the changes in the microstructure of treated material. Therefore, this study discussed the various kinds of corrosive attack observed in the microstructure of laser borided and untreated Inconel 600 alloy.

2. EXPERIMENTAL PROCEDURE

2.1. MaterialAs-received Inconel 600 alloy without heat treatment was the se-lected experimental substrate material (nominal composition of Cr 15.72 wt %, Fe 8.63 wt %, Cu 0.04 wt %, Al 0.06 wt %, Si 0.18 wt %, Mn 0.16 wt %, C 0.078 wt %, S < 0.001 wt %, Ni bal-ance). The ring-shaped specimens were prepared with following di-mensions: external diameter ca. 16.5 mm, internal diameter 9.5 mm and height 12 mm.

2.2. Laser boriding

The laser-alloying process was arranged as a two-step process (Fig. 1). First, the external cylindrical surface of the specimen was coated with a paste containing amorphous boron and the diluted polyvinyl alcohol used as a binder. The average thickness of the preplaced paste was about 230 µm. During the second step, the la-ser beam caused the simultaneous remelting of the preplaced paste and the thin surface layer of the substrate material. The continuous wave CO2 laser (TRUMPF TLF 2600 Turbo) of the nominal power 2.6 kW was applied. The parameters of laser boriding were as fol-lows: laser beam power (P) 1.95 kW, scanning rate (vl) 2.88 m/min, laser beam diameter (d) 2 mm. The details of the experimental pro-cedure for laser alloying were described in the previous study [17].

Fig. 1. Two-step method of laser-alloying with amorphous boron; 1 – focusing optic, 2 – laser beam, 3 – preplaced paste with alloying ma-terial, 4 – molten pool, 5 – substrate material, 6 – heat affected zone, 7 – remelted zone Rys. 1. Dwustopniowa metoda laserowego stopowania amorficznym bo-rem; 1 – optyka skupiająca, 2 – wiązka laserowa, 3 – pasta z materiałem stopującym, 4 – ciekłe jeziorko, 5 – materiał podłoża, 6 – strefa wpływu ciepła, 7 – strefa przetopiona


2.3. Microstructure analysisThe microstructure observations (LM) were carried out for three types of specimens: laser-borided specimen (directly after the pro-cess), laser-borided specimen after the corrosion immersion test and untreated Inconel 600 alloy after the corrosion immersion test. The metallographic specimens were performed. The samples were ground using the abrasive paper of the different granularity, and, finally, they were polished with applying Al2O3. The etching solu-tion consisting of CuSO4, HCl and H2O was used in order to reveal the microstructure. In case of laser-borided Inconel 600 alloy, first, the specimen was cut perpendicular to the produced layer. After the corrosion resistance test, the specimens weren’t cut, but were only ground and polished. The microstructure analysis was carried out at two different depths.

2.3. Microhardness profiles

The microhardness was measured vs the distance from the surface us-ing the metallographic specimens which were etched in Marble’s rea-gent. To determine the microhardness profiles across the laser-borided layer (also after the immersion corrosion test), the apparatus ZWICK 3212 B equipped with Vickers indenter was used. The load of 100 G (0.981 N) and a loading time of 15 s were used during measurements.

2.4. Immersion corrosion resistance tests

In this study, the corrosion rate of laser-borided layer produced on Inconel 600 alloy was compared to the results obtained for untreat-ed Inconel 600 alloy. According to the ASTM G 28 standard, a boil-ing solution of H2O, H2SO4 and Fe2(SO4)3 was used for the tests [19]. Before the test, the specimens were cleaned and weighed. The total exposed surface area of both specimens was equal to about 12.65 cm2 and the test duration was equal to 8 hours. The typical apparatus for a ferric sulfate–sulfuric acid test was applied (Fig. 2). During immersion corrosion resistance tests (Streicher tests), the investigated specimens were totally immersed in boiling corrosive solution (boiling temperature of 145°C). The corrosion rate of the tested specimen (CR) was calculated using millimeters per year (mm/year) according to the following equation:

CR W

A T D= ⋅

⋅ ⋅87600

(1)

where: T is the time of exposure, h, A is the area of the tested sam-ple, cm2, W is the mass loss, g, D is the density of investigated material, g/cm3.

After the test, the specimens were cleaned in order to remove all the corrosion products. Then, the metallographic specimens were prepared and observed using light microscope (LM). The microstructure analy-sis of partially laser-borided sample required the special metallograph-ical preparation. The scheme of this specimen was presented in Figure 3. Laser boriding was arranged as a selective process, in which only the external surface (marked as surface 3 in Figure 3) was remelted. The rest of surfaces weren’t borided. LM observations were performed at two different depths which were obtained by successive grinding and polishing. First observation was carried out at the depth marked as D1 (0.376 mm from the surface 2). After second grinding the micro-structure observation was performed at the depth D2 (3.62 mm from the surface 2). The microhardness measurements were performed at the depth D2. The microstructure of untreated Inconel 600 alloy was also analysed, but only one grinding and polishing was applied.

3. RESULTS AND DISCUSSION

3.1. Microstructure

Microstructure of the laser-borided layer, produced on Inconel 600 alloy at laser beam power of 1.95 kW, was shown in Figure 4. The

characteristic shape of multiple laser tracks was obtained. Howev-er, the high overlapping of the adjacent tracks (86%) was the rea-son for the relatively uniform thickness of the remelted zone (Fig. 4b). An average thickness of the laser-borided layer was equal to 467 µm. The relatively high layer thickness was possible to obtain because of the high laser beam power during laser irradiation. The laser-alloyed layer was produced by simultaneous remelting and mixing of the paste with amorphous boron and substrate material.

Fig. 2. Apparatus for the immersion corrosion resistance tests accord-ing to ASTM G 28 standardRys. 2. Aparatura do immersyjnych prób odporności korozyjnej zgodnie z normą ASTM G 28

Fig. 3. Scheme of laser-borided specimen with marked grinding depths; surface 1 – untreated internal surface, surface 2 – untreated butting face, surface 3 – laser-borided external surface, surface 4 – un-treated butting face, D1, D2 – the grinding depths from the surface 2 equal to 0.376 mm, 3.62 mm, respectivelyRys. 3. Schemat laserowo borowanej próbki z zaznaczonymi głęboko-ściami szlifowania; powierzchnia 1 – nieobrobiona powierzchnia we-wnętrzna, 2 – nieobrobiona powierzchnia czołowa, 3 – laserowo boro-wana powierzchnia zewnętrzna, 4 – nieobrobiona powierzchnia czołowa, D1, D2 – głębokości szlifowania od powierzchni 2 równe odpowiednio: 0,376 mm i 3,62 mm


The melting point of the alloying material (2075°C) was higher than the melting point of the substrate (1354÷1413°C). This dif-ference in melting points of used materials and the rotary motion of the treated sample was the reason for well mixing the boron coating and the substrate. It was visible in Figure 4a, that micro-structure in remelted zone had dendritic character. Moreover, the observation of the top surface and cross-sections of the whole sample indicated that laser-borided layer was crack-free. The gas

pores were also not detected. The detailed microstructure analysis of laser-borided layer produced on Inconel 600 alloy was carried out using SEM and was presented in previous study [17]. The re-sults indicated, that this layer was characterized by a changeable microstructure. The following areas were observed in the remelted zone: compact zone with only nickel and chromium borides (close to the surface), zone of increased percentage of Ni–Cr–Fe matrix, and zone of dominant Ni–Cr–Fe matrix percentage. The presence of nickel borides (Ni3B and Ni2B) and chromium borides (CrB and Cr2B) was detected by XRD [17]. Because of the high laser beam power used (1.95 kW), the Ni–Cr–Fe matrix also occurred in alloyed zone. At the bottom of remelted zone (Fig. 4c), the microstructure was characterized by a diminished percentage of chromium and nickel borides.

3.2. Microhardness profiles

The microhardness profiles of laser-borided layer, produced on Inconel 600 alloy, were presented in Figure 5. The measurements were performed on the etched sample perpendicular to the laser al-loyed surface. Two types of profiles were performed: along the axis of a selected track and along the contact of the adjacent tracks. It was clearly visible that the thickness of the produced layer in the axis of track was higher compared to the thickness, observed at the contact of adjacent tracks. This situation was caused by the charac-teristic irradiance profile of a laser beam.

The differences in the microstructure of remelted zone were the reason for fluctuations in microhardness. The presence of nickel and chromium borides resulted in hardness increase. The hardness ranged from 1119 to 1663 HV0.1 in this zone. However, in comparison with the borided layer produced on Inconel 600 alloy using gas-boriding method [13, 14], the obtained values were slightly lower. The presence of the Ni–Cr–Fe matrix was the reason for such a situation. Although only nickel and chromium borides were observed close to the surface, the increase in the distance from the surface resulted in an increased percentage of Ni–Cr–Fe matrix. It caused a gradual decrease in hardness. The fine-grained microstructure was characteristic of the remelted zone. During hardness tests at load 100 G, the indenter penetrated the areas in which both the borides and the matrix appeared. As a consequence, the increase in the distance from the surface was accompanied by the diminished hardness to about 1119 HV0.1 at the end of remelted zone. The average hardness of substrate mate-rial (Inconel 600 alloy) was equal to about 210 HV0.1.

Fig. 4. Microstructure of laser-borided Inconel 600 alloy at laser beam power of 1.95 kW: a) the dendritic structure of remelted zone, b) the laser-borided layer with visible multiple laser tracks, c) the micro-structure at bottom of remelted zone; LMRys. 4. Mikrostruktura stopu Inconel 600 borowanego laserowo z mocą 1,95 kW: a) struktura dendrytyczna strefy przetopionej, b) warstwa bo-rowana laserowo z widocznymi wielokrotnymi ścieżkami laserowymi, c) mikrostruktura na dnie strefy przetopionej; mikroskop świetlny

Fig. 5. Microhardness profiles of laser-borided Inconel 600 alloy at laser beam power of 1.95 kW; measurements along the axis of a track and along the contact of the adjacent tracksRys. 5. Profile mikrotwardości stopu Inconel 600 laserowo borowanego z mocą wiązki laserowej 1,95 kW; pomiary wzdłuż osi ścieżki i wzdłuż styku sąsiednich ścieżek

a)

b)

c)


3.3. Immersion corrosion resistance testsThe results of immersion corrosion resistance tests were calculated in millimeters per year. In case of untreated Inconel 600 alloy, the ex-pected value of corrosion rate was measured (CR = 3.49 mm/year) because of the susceptibility of this material to intergranular attack. It was confirmed by the observation of the microstructure after cor-rosion resistance test of untreated Inconel 600 alloy. In Figure 6, the relatively strong intergranular corrosion of this specimen was visible. In order to reveal the mechanism of corrosion, the speci-men was non-etched. The depth of intergranular attack obtained even 160 μm. Microstructure revealed the grain boundary corro-sion, caused by corrosive solution used. The Inconel 600 alloy was characterized by susceptibility to intergranular corrosion because of the segregation of carbon at the grain boundaries and the forma-tion of carbides rich in chromium, e.g. Cr7C3 and Cr23C6 [20]. As a consequence, some grains were extracted from material (Fig. 6). Probably, the use of a solution heat treatment could improve the re-sistance of Inconel 600 alloy to intergranular attack. The influence of such a treatment should be checked in the future.

The considerable difference in corrosion rate between laser-borided and untreated Inconel 600 alloy was observed. Laser borid-ing caused a significant increase in corrosion rate (CR) in com-parison with untreated alloy (up to 521.56 mm/year). In case of laser-borided specimen after immersion corrosion resistance test, the corrosion effect was easily visible in macrostructure (Fig. 7). At the edge between butting face and external surface (areas marked as A and C in Figure 7), the strong corrosion effects in the substrate material were observed. In the area marked as A, the substrate mate-rial (Inconel 600 alloy) was destroyed from the butting face to about 1 mm depth as a result of the contact with boiling solution. In the area marked as C, the corrosion of Inconel 600 alloy was so strong that laser-borided layer lost the contact with substrate material. As a consequence, spalling of the layer was observed. If only the ex-ternal surface was analyzed (area B in Figure 7), it was visible that the corrosive solution didn’t have the destructive influence on the laser-borided layer. On the top surface of layer even the multiple la-ser tracks were clearly visible. These observations indicated that the relatively high corrosion rate of the laser-borided specimen resulted from an accelerated corrosion of the base material (Inconel 600 al-loy). However, the corrosion behaviour of this specimen required an additional analysis.

Such a difference between the corrosion behaviour of the laser-borided and untreated Inconel 600 alloy resulted from the surface condition. The laser boriding was the process of selective treatment, which caused the formation of a thick surface layer only on the

external surface of specimen. Other surfaces were composed of only the substrate material (Inconel 600 alloy). In case of untreated specimen, the whole surface was identically prepared. In order to explain the corrosion behaviour of laser-borided specimen, the mi-crostructure observations were performed after the immersion cor-rosion test.

Microstructure analysis of laser-borided Inconel 600 alloy after the immersion corrosion test was carried out at two differ-ent distances from butting face, according to a scheme presented in Figure 3. First, the effects of corrosion were observed at depth D1 (0.376 mm from the surface 2). The observation of the external specimen surface (Fig. 8) indicated that laser-borided layer (marked as A) was completely separated from the substrate material (marked as B). However, the intergranular attacks were not detected, where-as the strong intergranular corrosion occurred for the untreated In-conel 600 alloy (Fig. 6). The microstructure of the internal surface (marked as 1 in Figure 3) of the laser-borided specimen was shown in Figure 9. The uniform corrosion symptoms, without localized corrosion attack, were observed in this area. The uniform corrosion consisted in uniform thinning of a base material.

Based on these results, it could seem that laser boriding provided an inadequate corrosion protection of Inconel 600 alloy. However, this problem required a more detailed analysis. The selective laser boriding produced the surface layer on the external surface of sam-ple only. As a consequence, at butting faces two different materials (laser-borided layer and base material) were in contact with a boil-ing corrosive solution. The mixture of borides and Inconel 600 alloy differed in electrochemical potentials. This difference was the rea-son for creation of local microcells. The more noble potential was characteristic of the mixture of borides. Therefore, the anodic disso-lution of substrate material occurred. The intensive digestion of the substrate from the butting faces caused the loss of bonding between the layer and base material. During the preparation of metallograph-ic specimen, the mounting pressure was the reason for chipping the layer which lost the contact with the substrate. Therefore, the butt-ing face 2 of the tested specimen was ground to the greater depth (3.62 mm) in order to prove that the laser-borided layer protected the base material from corrosion.

The microstructure after second grinding (to a depth D2 = 3.62 mm from the surface 2) was shown in Figure 10. At this depth, the borided external surface of the sample was in a relatively great distance from the base material which was being in a con-tact with the corrosive solution. Therefore, the laser-borided layer (marked as A in Figure 10) was completely free of the corrosion symptoms. The compact zone of nickel and chromium borides, ap-

Fig. 6. Microstructure of Inconel 600 alloy after the immersion corro-sion resistance test; LMRys. 6. Mikrostruktura stopu Inconel 600 po badaniu odporności na ko-rozję metodą zanurzeniową; mikroskop świetlny

Fig. 7. Macrostructure of laser-borided Inconel 600 alloy after the im-mersion corrosion resistance testRys. 7. Makrostruktura borowanego laserowo stopu Inconel 600 po ba-daniu odporności na korozję metodą zanurzeniową


pearing close to the surface, was a reason for such a situation. The difference in electrochemical potentials of various types of borides had to be relatively small, not causing the visible effects of corro-sion on the surface. Simultaneously, the substrate material (marked as B in Figure 10) was also free from corrosion defects. It confirmed that laser-boriding could provide a suitable corrosion protection for Inconel 600 alloy. However, the whole surface of this alloy should be subjected to laser alloying with boron.

The microhardness measurements were carried out after second grinding at depth D2 (3.62 mm from the surface 2). The hardness profile across the laser-borided layer along the axis of track was presented in Figure 11. The remelted zone was characterized by high microhardness (910÷1744 HV0.1). The obtained values were comparable to those-measured before the immersion corrosion re-sistance test. These results indicated, that laser-borided layer was resistant to the corrosion in boiling solution of H2O, H2SO4 and Fe2(SO4)3. Simultaneously, below the remelted zone the hardness measured in base material was equal to about 200 HV0.1.

The macro- and microstructure observations after the corrosion resistance test as well as the measured microhardness profile ena-bled the conclusion that the laser alloying with boron could provide the excellent corrosion protection of Inconel 600 alloy if the whole surface of base material would be covered with laser-borided layer.

The probable work of corrosion microcell, occurring in laser-borided specimen, was shown in Figure 12. Before the immersion corrosion resistance test, the laser-borided layer was tight and con-tinuous, covering only the external surface of the sample (Fig. 12a). After immersion of sample in boiling solution, the corrosion of sub-strate started (Fig. 12b). The detailed analysis of this phenomena was schematically presented in Figure 12c. Chromium and nickel borides co-existed in the laser-borided layer. These borides are marked as MexBy phases, where Me is metal (Cr or Ni). In the se-lectively laser-borided sample, the microcells were created between the boride layer and the substrate material (Fig. 12c). The borides were characterized by the more noble electrochemical potential. Therefore, they performed a role of the cathode. In corrosive solu-

Fig. 8. Microstructure of laser-borided Inconel 600 alloy at grinding depth D1 (0.376 mm from surface 2) after the immersion corrosion re-sistance test; LM Rys. 8. Mikrostruktura laserowo borowanego stopu Inconel 600 szlifo-wanego na głębokość D1 (0,376 mm od powierzchni 2) po badaniu od-porności na korozję metodą zanurzeniową; mikroskop świetlny

Fig. 9. Microstructure of internal surface of laser-borided Inconel 600 alloy at grinding depth D1 (0.376 mm from surface 2) after the immer-sion corrosion resistance test; LMRys. 9. Mikrostruktura wewnętrznej powierzchni laserowo borowane-go stopu Inconel 600 szlifowanego na głębokość D1 (0,376 mm od po-wierzchni 2) po badaniu odporności na korozję metodą zanurzeniową; mikroskop świetlny

Fig. 10. Microstructure of laser-borided Inconel 600 alloy at grinding depth D2 (3.62 mm from surface 2) after the immersion corrosion re-sistance test; LM Rys. 10. Mikrostruktura laserowo borowanego stopu Inconel 600 po szli-fowaniu na głębokość D2 (3,62 mm od powierzchni 2) po badaniu odpor-ności na korozję metodą zanurzeniową; mikroskop świetlny

Fig. 11. The microhardness profile of laser-borided Inconel 600 alloy at 1.95 kW after the immersion corrosion resistance testRys. 11. Profil mikrotwardości stopu Inconel 600 laserowo borowanego z mocą 1,95 kW po zanurzeniowym teście odporności korozyjnej


Fig. 12. Scheme of work of corrosion microcells for laser-borided In-conel 600 alloy: a) the specimen before corrosion test, b) and c) the specimen during corrosion test; 1 – laser-borided layer, 2 – substrate material, 3 – corrosion symptoms of substrate materialRys. 12. Schemat pracy korozyjnych mikroogniw dla laserowo borowa-nego stopu Inconel 600: a) próbka przed próbą korozyjną, b) i c) próbka w trakcie próby korozyjnej; 1 – warstwa borowana laserowo, 2 – materiał podłoża, 3 – symptomy korozji materiału podłoża

tion, a metal (Cr or Ni) of the substrate, giving valence electrons off, passes into solution in the form of ions (oxidation). Electrons in metal migrate to the cathodic area (chromium or nickel boride MexBy). Then, they combined with a depolarizer (D), i.e. ion having the ability to electron addition (reduction). As a consequence, the strong dissolution of substrate material (anode) was observed.

4. SUMMARY AND CONCLUSIONS

Laser boriding was used in order to produce the hard and wear resist-ant surface layer on Inconel 600 alloy. Such a layer was character-ized by the significantly higher thickness compared to the diffusion borided layers. After laser alloying of Inconel 600 with boron, the surface layer was created by the remelted zone. Its microstructure consisted of three zones: tight and compact zones with only nickel and chromium borides, the mixture of borides and Ni–Cr–Fe matrix with a predominant percentage of borides, and the zone in which Ni–Cr–Fe matrix dominated. The differences in the microstructure of remelted zone were the reason for a fluctuation in microhardness from 910 to 1744 HV0.1. The increased percentage of Ni–Cr–Fe matrix resulted in a hardness decrease towards the substrate.

The corrosion behavior of laser-borided Inconel 600 alloy was studied based on the immersion corrosion test. The results were compared to the corrosion resistance of the untreated alloy. The cor-rosion rate of Inconel 600 alloy obtained a value of 3.49 mm/year which was in agreement with expectation. The surface of untreated alloy was characterized by the typical effects of intergranular attack. Microstructure observation revealed the strong grain boundary cor-rosion, caused by corrosive solution used. Laser boriding caused a significant increase in corrosion rate (up to 521.56 mm/year) in comparison with untreated alloy. It could suggest that laser-borided layer didn’t protect the base material from corrosion. However, the laser alloying with boron was used in this study as a selective pro-cess in which only the selected surface was subjected to laser treat-ment. As a consequence, the two different materials (laser-borided layer and base material) were in contact with a boiling corrosive solution. In the selectively laser-borided sample, the microcells were created between the boride layer and the substrate material. The borides performed a role of the cathode because of the more noble electrochemical potential. Therefore, the strong dissolution

of substrate material (anode) was observed. The laser-borided ex-ternal surface of the sample was completely free of the corrosion defects. The compact zone with only nickel and chromium borides, appearing close to the surface, was a reason for such a situation. Hence, it was obvious that the laser alloying with boron could pro-vide the excellent protection of Inconel 600 alloy against corrosion if the whole surface of base material would be covered with laser-borided layer.

ACKNOWLEDGEMENTS

This work has been financially supported by Ministry of Sci-ence and Higher Education in Poland as a part of the 02/24/DSMK/4517 project.

REFERENCES

[1] Cramer S. D., Covino B. S.: ASM Handbook: Volume 13B: Corrosion: Materials. ASTM International, Materials Park, Ohio (2005).

[2] Craig B. D., Anderson D. B.: Handbook of corrosion data. ASM Interna-tional, Ohio (1995).

[3] Kasparova O. V.: Intergranular corrosion of nickel alloys (Review). Pro-tection of Metals 36 (6) (2000) 524÷532.

[4] Eliasen K. M., Christiansen T. L., Somers M. A. J.: Low temperature gas-eous nitriding of Ni based superalloys. Surface Engineering 26 (4) (2010) 248÷255.

[5] Borowski T., Brojanowska A., Kost M., Garbacz H., Wierzchoń T.: Modi-fying the properties of the Inconel 625 nickel alloy by glow discharge assisted nitriding. Vacuum 83 (2009) 1489÷1493.

[6] Singh V., Meletis E. I.: Synthesis, characterization and properties of inten-sified plasma-assisted nitrided superalloy Inconel 718. Surface & Coatings Technology 201 (2006) 1093÷1101.

[7] Ozbek I., Akbulut H., Zeytin S., Bindal C., Ucisik A. H.: The characteriza-tion of borided 99.5% purity nickel. Surface and Coatings Technology 126 (2000) 166÷170.

[8] Lou D. C., Akselsen O. M., Solberg J. K., Onsoien M. I., Berget J., Dahl N.: Silicon-boronising of Nimonic 90 superalloy. Surface & Coatings Technology 200 (2006) 3582÷3589.

[9] Mu D., Shen B. I., Yang C., Zhao X.: Microstructure analysis of boronized pure nickel using boronizing powders with SiC as diluents. Vacuum 83 (2009) 1481÷1484.

[10] Gunes I., Kayali Y.: Investigation of mechanical properties of borided Nickel 201 alloy. Materials and Design 53 (2014) 577÷580.

[11] Petrova R. S., Suwattananont N., Samardzic V.: The effect of boronizing on metallic alloys for automotive applications. Journal of Materials Engi-neering and Performance 17 (2008) 340÷345.

[12] Lou D. C., Solberg J. K., Akselsen O. M., Dahl N.: Microstructure and property investigation of paste boronized pure nickel and Nimonic 90 su-peralloy. Materials Chemistry and Physics 115 (2009) 239÷244.

[13] Makuch N., Kulka M., Dziarski P.: Gas boriding of Inconel 600 alloy. Inżynieria Materiałowa 6 (2013) 745÷748.

[14] Makuch N., Kulka M., Piasecki A.: The effects of chemical composition of Nimonic 80A alloy on the microstructure and properties of gas-borided layer. Surface & Coatings Technology 276 (2015) 440÷455.

[15] Majumdar J. D., Manna I.: Laser-surface alloying of nimonic 80 with sili-con and aluminum and its oxidation behavior. Metallurgical and Materials Transactions A 43A (2012) 3786÷3796.

[16] Rodriguez G. P., Garcia I., Damborenea J.: Effects of laser surface modi-fication of nimonic with aluminum on oxidation behavior. Oxidation of Metals 58 (2002) 235÷248.

[17] 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.

[18] Cooper K. P., Slebodnick P., Thomas E. D.: Seawater corrosion behavior of laser surface modified Inconel 625 alloy. Materials Science and Engi-neering A206 (1996) 138÷149.

[19] ASTM G28 Norm, Standard test methods of detecting susceptibility to intergranular corrosion in wrought, nickel-rich, chromium-bearing alloys. ASTM International (2003).

[20] Aspden R. G., Economy G., Pement F. W., Wilson I. L.: Relationship between magnetic properties, sensitization, and corrosion of lncoloy Alloy 800 and Inconel alloy 600. Metallurgical Transactions 3 (1972) 2691÷2697.


Odporność na korozję borowanego laserowo stopu Inconel 600

Piotr Dziarski*, Michał Kulka, Natalia Makuch, Daria MikołajczakInstytut Inżynierii Materiałowej. Politechnika Poznańska, *[email protected]

Inżynieria Materiałowa 3 (217) (2017) 149÷156DOI 10.15199/28.2017.3.7© Copyright SIGMA-NOT MATERIALS ENGINEERING

Słowa kluczowe: borowanie laserowe, Inconel 600, mikrostruktura, twardość, odporność na korozję.

1. CEL PRACYCelem pracy było wytworzenie warstwy borków na stopie niklu Inconel 600 z zastosowaniem laserowego stopowania borem oraz ocena odporności korozyjnej stopu z wytworzoną warstwą. War-stwa ta powinna charakteryzować się zwiększoną odpornością na zużycie mechaniczne (adhezyjne lub ścierne) oraz wykazywać znaczną odporność na działanie agresywnego środowiska. Boro-wanie dyfuzyjne skutecznie poprawia właściwości tribologiczne tego stopu, jednak do uzyskania warstwy o grubości ok. 100 µm dotychczas był potrzebny długi czas procesu. Zamiast tradycyjnego procesu dyfuzyjnego w pracy do wytworzenia warstwy borków na stopie Inconel 600 zastosowano nowoczesną, bardziej ekologiczną metodę laserowego stopowania borem.

2. MATERIAŁ I METODYKA BADAŃ

Do badań zastosowano stop niklu Inconel 600, którego główne do-datki stopowe to Cr (15,72%) i Fe (8,63%). Próbki miały kształt pier-ścienia, a borowaniu laserowemu poddano tylko zewnętrzną cylin-dryczną powierzchnię. Proces borowania laserowego zrealizowano metodą dwustopniową (rys. 1). Najpierw próbki pokryto pastą zawie-rającą bor amorficzny o średniej grubości powłoki 230 µm. Następ-nie przetopiono wiązką laserową. Zastosowano laser CO2 TRUMPF TLF 2600 Turbo. Parametry borowania laserowego były następują-ce: moc wiązki laserowej (P) 1,95 kW, prędkość skanowania wiązką (vl) 2,88 m/min, średnica wiązki lasera (d) 2 mm. Twardość wytwo-rzonych warstw zmierzono sposobem Vickersa, stosując obciążenie 100 G. W celu oceny odporności korozyjnej stosowano próbę zanu-rzeniową we wrzącym roztworze H2O, H2SO4 i Fe2(SO4)3 zgodnie z normą ASTM G 28. Próbę prowadzono przez 8 godzin, stosując aparaturę pokazaną na rysunku 2, a wyniki obliczono w mm/rok.

3. WYNIKI I ICH DYSKUSJA

Borowanie laserowe spowodowało wytworzenie warstwy o dużej grubości, średnio 467 µm (rys. 4). Duży stopień nakładania sąsied-nich ścieżek laserowych spowodował, że warstwa charakteryzowa-ła się dużą jednorodnością pod względem grubości. W warstwie przetopionej zaobserwowano trzy strefy różniące się mikrostruk-turą: przy powierzchni strefę zwartych borków niklu (Ni3B i Ni2B) i borków chromu (CrB i Cr2B), poniżej strefę o zwiększonym udziale osnowy Ni–Cr–Fe oraz strefę o dominującym udziale tej osnowy (pod koniec strefy przetopionej). Z powodu różnic w mi-krostrukturze twardość zmierzona w strefie przetopionej wahała się w zakresie 1119÷1663 HV0,1. Zwiększający się udział osnowy Ni–Cr–Fe był powodem obniżonej twardości.

Próbom korozyjnym poddano stop Inconel 600 borowany lasero-wo i nie poddany obróbce. Jak się spodziewano, stop bez wytworzo-nej warstwy wykazał dużą wrażliwość na korozję międzykrystaliczną. Symptomem tego zjawiska były widoczne na zdjęciach z mikroskopu świetlnego (rys. 6) silnie wytrawione granice ziaren. Niektóre z ziaren

zostały nawet odseparowane i wykruszone z próbki. Szybkość korozji próbki nieborowanej wynosiła 3,49 mm/rok. Powodem wrażliwości stopu Inconel 600 na korozyjny atak międzykrystaliczny jest wydzie-lanie na granicy ziaren węglików bogatych w chrom Cr7C3 i Cr23C6. Sytuację mogłoby poprawić zastosowanie przesycania. Odporność korozyjna borowanego laserowo stopu Inconel 600 była niewielka (521,56 mm/rok). Jednakże problem odporności korozyjnej tej próbki był bardziej złożony i wynikał ze specyfiki procesu obróbki lasero-wej. Na rysunku 7 przedstawiono zdjęcie próbki borowanej laserowo po próbie korozyjnej. Widoczne są silne skutki korozji podłoża od strony powierzchni czołowych, natomiast sama warstwa borowana nie uległa tak silnej destrukcji. Jednakże można zauważyć również, że w wyniku ubytku podłoża warstwa straciła z nim kontakt, w efek-cie czego uległa wykruszeniu. Analizę zachowania korozyjnego tej próbki przeprowadzono, badając mikrostrukturę na dwóch głęboko-ściach szlifowania zgodnie ze schematem na rysunku 3. Pierwsze szlifowanie na głębokość D1 (0,376 mm od powierzchni czołowej) ujawniło silną korozję podłoża (rys. 8). Warstwa natomiast wolna jest od skutków korozji, niemniej jednak brak kontaktu z podłożem i ciśnienie stosowane podczas inkludowania spowodowało pękanie. Stwierdzono konieczność ponownego szlifowania na głębokość D2 (3,62 mm), spodziewając się braku korozji podłoża. Mikrostruk-tura po drugim szlifowaniu (rys. 10) potwierdziła, że w przypad-ku braku kontaktu ośrodka korozyjnego z granicą między warstwą a podłożem materiał podłoża nie ulega korozji. Zmierzono również mikrotwardość i stwierdzono, że na głębokości D2 ani warstwa nie uległa korozji (zmierzona twardość po próbie korozyjnej wynosiła 910÷1744 HV0,1). Na podstawie otrzymanych wyników powstał model zachowania korozyjnego próbki borowanej laserowo (rys. 12). Powodem silnej korozji podłoża w próbce borowanej laserowo były różnice w potencjałach elektrochemicznych warstwy i stopu Inconel 600. Bardziej szlachetnym materiałem były borki MexBy (Me oznacza metal Cr lub Ni) znajdujące się w warstwie, dlatego w utworzonym mikroogniwie przejęły one rolę katody. Anodę stanowiło podłoże ze stopu Inconel 600, w zwiazku z tym ono ulegało silnej korozji.

4. PODSUMOWANIE

Borowanie laserowe powoduje wytworzenie grubej warstwy o twardości do 1663 HV. W wyniku przeprowadzonych badań ko-rozyjnych stwierdzono, że stop Inconel 600 bez obróbki wykazuje silną tendencję do korozji międzykrystalicznej. Próbka borowana laserowo charakteryzowała się większą szybkością korozji z powo-du przyspieszonej korozji podłoża. Podczas kontaktu z wrzącym roztworem elektrolitu między warstwą borowaną a podłożem two-rzyły się mikroogniwa, w których stop Inconel 600 stanowił ano-dę. Stwierdzono, że borowanie laserowe może skutecznie ochronić stop Inconel 600 przed korozją, ale tylko w przypadku wytworzenia warstwy na całej powierzchni elementu. Selektywne laserowe sto-powanie borem nie stanowi żadnej ochrony przed korozją. Wręcz przeciwnie, przyspiesza ją.

corrosion resistance of laser-borided inconel 600 alloy text.pdf · 2018-01-29 · inconel 600...

Documents