CC BY
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DOI: http://dx.doi.org/10.15688/)volsu10.2014.3.6
УДК 621.74 ББК 38.33
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STEEL SURFACE MODIFICATION
Igor Novak
Department of Welding and Foundry, Faculty of Materials Science and Technology in Trnava
upolnovi@savba.sk
Paulinska, 16, 91724 Trnava, Slovakia
Ivan Michalec
Polymer Institute of the Slovak Academy of Sciences upolnovi@savba.sk
Dubravska cesta, 9, 84541 Bratislava, Slovakia
Marian Valentin
P^' Department of Welding and Foundry, Faculty of Materials Science and Technology in Trnava
> upolnovi@savba.sk
■f Paulinska, 16, 91724 Trnava, Slovakia
^ Milan Maronek
§
Polymer Institute of the Slovak Academy of Sciences
eg upolnovi@savba.sk
Dubravska cesta, 9, 84541 Bratislava, Slovakia
Ladislav Soltes
Institute of Experimental Pharmacology of the Slovak Academy of Sciences
й upolnovi@savba.sk
§ Dùbravska cesta, 9, 84541 Bratislava, Slovakia
Jan Matyasovsky
VIPO
upolnovi@savba. sk
PUkinova, 1249/67, 95803 Partizanske, Slovakia
Peter Jurkovic
VIPO
> upolnovi@savba.sk
£ Pukinova, 1249/67, 95803 Partizanske, Slovakia
©
Abstract. Currently the reinforcement of steel surface treatment is widely used in various industries, especially in the automobile. The paper discusses the new economical methods of processing the steel surface, which can significantly improve the properties of the metal.
Key words: steel surface, chemical modification, mechanical properties of the material, surface energy, fracture morphology
1. Introduction
The treatment of steel surface is often used, especially in an automotive industry, that creates the motive power for research, design and production. New methods of surface treatment are also developed having major influence on improving the surface properties of steel sheets while keeping the price at reasonable level [6; 7; 11].
The nitrooxidation is one of the non-conventional surface treatment methods which combines the advantages of nitridation and oxidation processes. The improvement of the mechanical properties (Tensile Strength, Yield Strength) together with the corrosion resistance (up to level 10) can be achieved (5-8). The fatigue characteristics of the nitrooxidized material can be also raised (6).
Steel sheets with surface treatment are more often used, especially in an automotive industry which creates the motive power for research, design and production.
Previous outcomes [6; 9; 11; 12; 16] dealt with the welding of steel sheets treated by the process of nitrooxidation by various arc and beam welding methods. Due to high oxygen and nitrogen content in the surface layer, problems with high level of porosity had occurred in each method. The best results were achieved by the solid-state laser beam welding, by which the defect-free joints were
created. Due to high initial cost of the laser equipment, the further research was directed to the joining method that has not been tested. Therefore the adhesive bonding was chosen, because the joints are not thermally affected, they have uniform stress distribution and good corrosion resistance.
The goal of the paper is to review the adhesive bonding of steel sheets treated by nitrooxidation and to compare the acquired results to the non-treated steel.
2. Experimental
For the experiments, low carbon deep drawing steel DC 01 EN 10130/91 of 1 mm in thickness was used. The chemical composition of steel DC 01 is documented in Table 1.
2.1. Chemical modification
The base material was consequently treated by the process of nitrooxidation in fluidized bed. The nitridation fluid environment consisted of the Al2O3 with granularity of 120 mm. The fluid environment was wafted by the gaseous ammonia. After the process of nitridation, the oxidation process started immediately. The oxidation itself was performed in the vapours of distilled water. Processes parameters are referred in Table 2.
2.2. Adhesives
In the experiments, the four types of two-component epoxy adhesives made by Loctite Company (Hysol 9466, Hysol 9455, Hysol 9492
Table 1
Chemical composition of steel DC 01 EN 10130/91
EN designation C (%) Mn (%) P (%) S ( %) Si (%) Al (%)
DC 01 10130/91 0.10 0.45 0.03 0.03 0.01 -
Table 2
Process of nitrooxidation parameters
Parameters Nitridation Oxidation
Time (min) 45 5
Temperature (°C) 580 380
and Hysol 9497) were used. The properties of the adhesives are documented in Table 3.
2.3. Methods
The experiments were done at the Faculty of Materials Science and Technology, Department of Welding and Foundry in Trnava. The adhesive bonding was applied on the grinded as well as non-grinded surfaces of the material to determine the grinding effect on total adhesion of the material so as on ultimate shear strength of the joints. The grinded material was prepared by grinding with silicone carbide paper up to 240 grit.
Before the adhesive bonding, the bonding surfaces (both grinded as well as non-grinded) were decreased with aerosol cleaner. The overlap area was 30 mm. To ensure the maximum strength of the joints, the continuous layer of the adhesive was coated on the overlap area of both bonded materials. The thickness of the adhesive layer was 0.1 mm and it was measured by a calliper. The joints were cured under fixed stress for 48 hours at the room temperature. The dimensions of the joints are referred in Fig. 1.
The mechanical properties of the joints were examined by the static shear tests. As a device, the LaborTech LabTest SP1 was used.
The conditions of the tests were set in accordance with STN EN 10002-1. The static shear tests were repeated on three separate samples and an average value was calculated.
The fracture areas were observed in order to obtain the fracture character of the joints. The JEOL JSM-7600F scanning electron microscope was used as a measuring device.
The differential scanning calorimetry (DSC) was performed on Netzsch STA 409 C/CD equipment. As the shielding gas, Helium with purity of 99.999 % was used. The heating process starts at the room temperature and continued up to 400 °C with heating rate 10 °C/min. The DSC analysis on Hysol 9455 was done on Diamond DSC Perkin Elmer, capable of doing analyses from - 70°C.
3. Results and Discussion The material analysis represents the first step of evaluation. There are many factors having an influence on the joint quality. The properties of the nitrooxidized material depend on the treatment process parameters. For the adhesive bonding, the surface layer properties are important because of that, the high adhesion is needed to ensure the high strength of the joint.
Table 3
The characterisation of the adhesives
The characterisation Hysol 9466 Hysol 9455 Hysol 9492 Hysol 9497
Resin type Epoxy Epoxy Epoxy Epoxy
Hardener type Amin Methanethiol Modified Amin
Mixing ratio (Resin:Hardener) 2:1 1:1 2:1 2:1
Elongation (%) 3 80 0.8 2.9
Shore hardness 60 50 80 83
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Fig. 1. The dimension of the bonded joints ISSN 2305-7815. Вестн. Волгогр. гос. ун-та. Сер. 10, Иннов. деят. 2014. № 3 (12) 49
The overall view on the microstructure of the nitrooxidized material surface layer is referred in Fig. 2, a. On the top of the surface, the oxide layer (see Fig. 2, b) was created. This layer had a thickness of approx. 700 mm. Beneath the oxide layer, the continous layer of s-phase, consisting of nitrides Fe2-3N and with the thickness of 810 mm was observed.
The surface energy measurements were performed to obtain the properties of the material, which are important for adhesive bonding. For observing the grinding effect on the total surface energy, the measurements were done on the base as well as on the grinded material.
To determine the surface energy, the portable computer-based instrument SeeSystem was used. Four different liquids (distilled water, formamide, diiodomethan and ethylene glycol) were instilled on the material surface and contact angle was measured. The Owens-Wendt regression model was used for the surface energy calculation. The total
amount of six droplets in each liquid was analysed. The results (see Table 4) proved that the nitrooxidation treatment had a strong affect on material surface energy, where the decrease by 28 % in comparison to non-nitrooxidized material had occurred. The surface energies of grinded and non-grinded material without nitrooxidation were very similar, while the increase of surface energy of grinded nitrooxidized material by 35 % in comparison to non-grinded material was observed. In the case of barrier plasma modified steel is the surface energy higher than unmodified material and namely its polar component is significantly higher than polar component of surface energy for unmodified sample as well as for sample modified by nitrooxidation.
The mechanical properties of the material were obtained by the static tensile test. Total amount of three measurements were done and the average values are documented in Table 5.
a b
Fig. 2. The microstructure of the surface layer:
a - overall view; b - detail view on the oxide layer
Table 4
The surface energy of materials
Material type Total surface energy (mJ/m2) Dispersion component (mJ/m2) Acid-base component (mJ/m2)
DC 01 38.20 35.62 2.58
DC 01 grinded 38.80 33.66 5.14
Nitrooxidized 27.60 25.79 1.80
Nitrooxidized grinded 37.31 32.54 4.77
Barrier plasma treated 39.99 33.69 6.30
By the results, it can be stated, that after the process of nitrooxidation, the increase of Yield Strength by 55 % and Tensile Strength by 40 % was observed. The barrier plasma did not influence the mechanical properties after surface modification of steel, that remained the same as for unmodified sample.
The tensile test of the adhesives was carried out on the specimens, which were created by curing of the adhesives in special designed polyethylene forms for 48 hours. The results are shown in Table 6. In three of the adhesives (Hysol 9466, Hysol 9492 and Hysol 9497), very similar values were observed while in case of Hysol 9455, only tensile strength of 1 MPa was observed.
The differential scanning calorimetry was performed due to obtain the glass transition temperature as well as the melting points of the adhesives. The results are in Table 7. To measure the glass transition temperature of Hysol 9455, the measurements had to be started from the cryogenic temperatures. The results of such a low glass transition temperature explained the low tensile strength of the Hysol 9455, where at the room temperature, the mechanical behaviour changed from rigid to rubbery state.
In order to obtain mechanical properties of the joints, the static shear tests were carried out. Results (Table 8) showed that the highest shear strength was observed in grinded nitrooxidized
The mechanical properties of the base materials
Material type Yield Strength (MPa) Tensile Strength (MPa)
DC 01 200 270
Nitrooxidized 310 380
Barrier plasma treated 200 270
The mechanical properties of the adhesives
Adhesive type Hysol 9466 Hysol 9455 Hysol 9492 Hysol 9497
Tensile Strength (MPa) 60 1 58 65
The glass transition temperatures of adhesives
Adhesive type Hysol 9466 Hysol 9455 Hysol 9492 Hysol 9497
Glass transition temperature (°C) 52.6 5.0 61.2 62.4
Melting point (°C) 315.7 337.0 351.8 327.3
The results of shear test of the joints
Material Shear strength (MPa)
Hysol 9466 Hysol 9455 Hysol 9492 Hysol 9497
DC 01 9.0 2.8 7.1 6.0
DC 01 grinded 8.9 2.9 7.2 6.1
Nitrooxidised 12.9 3.1 7.8 5.4
Nitrooxidised grinded 12.9 5.9 12.7 7.0
Barrier plasma treated 13.8 6.1 14.2 7.8
Table 5
Table 6
Table 7
Table 8
joints. The Hysol 9466 provided joints with the highest shear strength
The mechanical properties of the joints made on non-nitrooxidized material did not depend on the surface grinding. The mechanical properties of the joints made on nitrooxidized material, in comparison to DC 01, were higher by 43 % in case of Hysol 9466, 11 % in case of Hysol 9455 and 10 % in case of Hysol 9492. In the case of adhesive Hysol 9497, the decrease of the shear strength had occurred. The joints produced on grinded nitrooxidized material, in comparison to DC 01, had a higher shear strength by 43 % in case of Hysol 9466, 110 % in case of Hysol 9455, 79 % in case of Hysol 9492 and 15 % in case of Hysol 9497. The shear strength of adhesive joint was for barrier plasma modified steel for all kinds of adhesive Hysol higher compared unmodified and nitrooxidised steel.
Results of fracture morphology of the joints made of non-nitrooxidized material are shown in Fig. 3, a. Only the adhesive type of fracture morphology (see Fig. 3, b) was observed in every type of the adhesive. Cohesive and combined fracture type were not observed. It can be stated that the adhesion forces were not strong enough so that the joints were fractured between the material and adhesive.
The results of the fractographic analysis of the non-grinded nitrooxidized steel are documented in Fig. 4. No oxide layer peeling was observed. The cleavage fracture pattern was observed only as well as adhesive fracture morphology. The closeup view on the cleavage fracture is shown in Fig. 4, b.
Differential Scanning Calorimetry revealed that three of the adhesives had a very similar glass transition temperature, so the meshing of the adhesives will start in the same way.
a b
Fig. 3. The fractographic analysis of the fractured adhesive joint of non-nitrooxidized material:
a - overall view; b - close-up view
a b
Fig. 4. The fractographic analysis of the fractured adhesive joint of nitrooxidized material:
a - overall view; b - close-up view
The evaluation results of mechanical properties of the joints proved that the material after the nitrooxidation process had a better adhesion to the epoxy adhesives than plain material DC01. Due to this fact the higher shear strength was achieved. It can be explained by the surface oxide layer porosity, which helped the adhesive to leak in.
On the other hand, increase of mechanical properties of j oints prepared from grinded nitrooxidized material can be explained by removing the surface oxide layer and thus resulting into rapid increase of surface energy.
The only adhesive type of fracture was observed and the fractographic analysis showed, that only cleavage type of fractures has been created. It can be stated, that the surface energy of the materials was not appropriate for the cohesive fracture pattern.
4. Conclusion
Joining of steel sheets treated by the process of nitrooxidation represents an interesting technical as well as technological problem. The fusion of welding methods with high energy concentration, e.g. laser beam welding are one of the possible options, however even with high effort of minimising the surface layer deterioration, it's not possible to completely avoid it.
Adhesive bonding of nitrooxidized steels presents thus the second alternative, when the surface layer is not damaged and the adhesive joint keeps its properties after it has been cured. Adhesive bonding of metallic substrates, often requires removing the surface oxide layer from the areas to be bonded. In case of materials treated by nitrooxidation, this is possible, however the damage of the formed surface layer will occur. The goal of this paper was to review the effect of surface layer, created by the process of nitrooxidation and/or by barrier discharge plasma treatment, on final mechanical properties of the joints, evaluation of fracture morphology and results comparison for both, treated and untreated material.
The research havs revealed, that the presence of nitrooxidation surface layer caused decrease of free surface energy by 28 %. The surface energy and namely its polar component were higher for barrier plasma modified steel than for unmodified and nitrooxidized steel. On the other hand, this surface layer brings on the joints
shear strength increase by 10-43 % in dependence of the adhesive used. In case of Hysol 9497, the decrease of the joint shear strength by 10 % was observed. In the case of barrier plasma modified steel were the strength of adhesive joint higher compared unmodified and nitrooxidized material. We can presume that the increase of the shear strength was mainly due to porous structure of the surface layer, which enabled the adhesive to leak in.
The adhesive type of fracture morphology was observed during fractographic analysis. Regarding the characteristics of used adhesives, the cleavage fracture morphology of the joints occurred.
The authors concluded on the base of received results, that the epoxy adhesive bonding represents the suitable alternative of creating the high quality joints of steel sheets treated by nitrooxidation as well as treated by barrier discharge plasma.
NOTES
1 This paper was prepared with the support of Slovak Research and Development Agency, grant No. 0057-07 and Scientific Grant Agency VEGA, grant No. 1/0203/11 and 2/0199/14.
This publication was prepared as an output of the project 2013-14547/39694:1-11 "Research and Development of Hi-Tech Integrated Technological and Machinery Systems for Tyre Production - PROTYRE" co-funded by the Ministry of Education, Science, Research and Sport of the Slovak Republic pursuant to Stimuli for Research and Development Act No. 185/ 2009 Coll.
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МОДИФИКАЦИЯ СТАЛЬНОЙ ПОВЕРХНОСТИ Игорь Новак
Кафедра сварочного и литейного производства,
Факультет материаловедения и технологии материалов в Трнаве
upolnovi@savba.sk
Paulinska, 16, 91724 г. Трнава, Словакия
Иван Михалек
Институт полимеров Словацкой академии наук upolnovi@savba.sk
Dúbravská cesta, 9, 84541 г. Братислава, Словакия
Мариан Валентин
Кафедра сварочного и литейного производства,
Факультет материаловедения и технологии материалов в Трнаве
upolnovi@savba.sk
Paulinska, 16, 91724 г. Трнава, Словакия
Милан Маронек
Институт полимеров Словацкой академии наук upolnovi@savba.sk
Dúbravská cesta, 9, 84541 г. Братислава, Словакия
Ладислав Солтес
Институт экспериментальной фармакологии Словацкой академии наук upolnovi@savba.sk
Dúbravská cesta, 9, 84541 г. Братислава, Словакия
Ян Матысовски
VPO
upolnovi@savba.sk
PUkinova, 1249/67, 95803 r. Партизанске, Словакия
Питер Юркович
VIPO
upolnovi@savba.sk
PUkinova, 1249/67, 95803 r. Партизанске, Словакия
Аннотация. В настоящее время упрочняющая обработка поверхности стали широко используется в разных отраслях промышленности, особенно в автомобильной. В работе рассматриваются новые экономичные методы обработки стальной поверхности, которые позволяют значительно повысить свойства металла.
Ключевые слова: стальная поверхность, химическая модификация, механические свойства материала, поверхностная энергия, морфология разрушения.
