Nitrooxidation is a non-conventional surface treatment method that can provide significantly improved mechanical properties as well as corrosion resistance. However, the surface layer is a major problem during the welding process, and welding specialists face many problems regarding the weldability of steel sheets. This paper deals with the properties of a nitrooxidized surface layer, and evaluates ways of welding steel sheets treated by nitrooxidation using a Cold Metal Transfer (CMT) process. The limited heat input and the controlled metal transfer, which are considered as the main advantage of the CMT process, have a negative impact on weld joint quality. An excessive amount of porosity is observed,probably due to the high content of nitrogen and oxygen in the surface layer of the material and the fast cooling rate of the weld pool

Marônek, M.

Michalec, I.

Acta Polytechnica

English

I. Michalec

M. Marônek

Directory of Open Access Journals

Acta Polytechnica Vol. 52 No. 2/2012Influence of the Surface Layer when the CMT Process Is Used forWelding Steel Sheets Treated by NitrooxidationI. Michalec, M. MarônekAbstractNitrooxidation is a non-conventional surface treatment method that can provide significantly improved mechanical prop-erties as well as corrosion resistance. However, the surface layer is a major problem during the welding process, andwelding specialists face many problems regarding the weldability of steel sheets. This paper deals with the properties ofa nitrooxidized surface layer, and evaluates ways of welding steel sheets treated by nitrooxidation using a Cold MetalTransfer (CMT) process. The limited heat input and the controlled metal transfer, which are considered as the mainadvantage of the CMT process, have a negative impact on weld joint quality. An excessive amount of porosity is observed,probably due to the high content of nitrogen and oxygen in the surface layer of the material and the fast cooling rate ofthe weld pool.Keywords: steel sheet, nitrooxidation, Cold Metal Transfer, porosity.1 IntroductionSteel sheets with surface treatment are widely used,above all in the automotive industry. This researchwork is in response to a problem encountered in prac-tical applications of this material. An investigationis made of a new surface treatment method that hasa major effect on improving the surface properties ofsteel sheets, e.g. their corrosion resistance. Nitrooxi-dation in a fluidized bed is a non-conventional surfacetreatment method. This process consists of nitrida-tion with subsequent oxidation. Nitrooxidation leadsto a significant increase in corrosion resistance (up tolevel 10), and also an increase in mechanical prop-erties, e.g. yield strength and tensile strength (1, 2,3).The process of welding damages the surface layer,and it is necessary to find a way to minimize thisdamage. A pulse process is applied to limit the ther-mal input in arc welding, where there is periodic pul-sation of the welding voltage and the welding current.The CMT process, an innovative welding method,can also be used. CMT is a revolutionary short-circuit welding method, which works with digitally-controlled short-circuit metal transfer in the weldingarc. For the first time, this method integrates thewire motion into the welding process and into theoverall control of the process. In this way, the, ther-mal input can be limited. This is a major advantageof this welding method (1, 4, 5).AimsPrevious studies (3, 5, 6) dealt with ways of weldingsteel sheets treated by nitrooxidation. Several de-fects were identified in most of the tested methods,e.g. spatter, porosity and weld bead irregularities.Only solid-state laser beam welding was consideredsatisfactory. Because of the high initial cost of laserequipment, our research focused on finding an ade-quate option in field of arc welding. The main aim ofthe research was to evaluate the possibility of weld-ing nitrooxidized material using the innovative CMTwelding method.2 Materials and methodsThin DC 01/DIN EN 10130-9 steel sheets 1 mm inthickness were used in the experiments. The chemicalcomposition of the steel is shown in Table 1. The ma-terial was subjected to the nitrooxidation process. Afluid environment consisting of Al2O3 grains 120 μmin diameter was used in the nitridation treatment.The oxidation process was carried out in vapoursof distilled water immediately after nitridation. Thenitrooxidation treatment parameters are presented inTable 2.Table 1: Chemical composition of DC 01 EN 10130/91 steelEN C Mn P S Si Aldesignation [%] [ %] [%] [%] [%] [ %]DC 01 max. 0.10 max. 0.45 max. 0.03 max. 0.03 0.01 –43Acta Polytechnica Vol. 52 No. 2/2012Table 2: The nitrooxidation parametersTemperature T Time t[◦C] [min]Nitridation 580 45Oxidation 350 5Table 3: The chemical composition of OK AristoRod12.50C Si Mn[%] [%] [%]0.10 0.90 1.50The experiments were performed at Fronius Ltd.,Trnava. The welding process configuration is shownin Figure 1. Robotised CMT welding was used. A to-tal of 26 specimens with dimensions of 25×10×1 mmwere produced. The welding parameters are pre-sented in Table 4. I joints, overlapped joints andflange joints were welded. OK AristoRod 12.50,1 mm in diameter, was used as the filler metal. COR-GON 18 (18% of CO2, 82% of Argon) with flow rate15 l · min−1 was applied as the shielding gas. Thechemical composition of the filler metal is shown inTable 3.Fig. 1: Welding process configurationTable 4: Welding parameters of the specimensSpecimen Welding Welding Wire Welding Shielding FillerNo. current voltage feed rate speed gas metal[A] [V] [m ·min−1] [mm · s−1]1 92 10.1 3.6 7 Corgon18 OK 12.502 92 10.1 3.6 7 Corgon18 OK 12.503 81 11.3 3.9 7 Corgon18 OK 12.504 90 11.4 3.6 20 Corgon18 OK 12.505 89 10.4 3.6 33 Corgon18 OK 12.506 90 10.1 3.7 10 Corgon18 OK 12.507 88 9.0 5.3 20 Corgon18 OK 12.508 88 9.0 5.3 20 Corgon18 OK 12.509 88 9.0 5.3 20 Corgon18 OK 12.5010 92 10.1 3.6 7 Corgon18 OK 12.5011 92 10.1 3.6 7 Corgon18 OK 12.5012 81 11.3 3.9 7 Corgon18 OK 12.5013 90 11.4 3.6 20 Corgon18 OK 12.5014 89 10.4 3.6 33 Corgon18 OK 12.5015 90 10.1 3.7 10 Corgon18 OK 12.5016 98 12.8 5.0 20 Corgon18 OK 12.5017 86 9.0 5.2 20 Corgon18 OK 12.5018 94 9.3 5.5 20 Corgon18 OK 12.5019 103 9.8 6.0 20 Corgon18 OK 12.5020 103 9.8 6.0 20 Corgon18 OK 12.5021 107 10.9 4.5 20 Corgon18 OK 12.5022 120 11.5 5.0 20 Corgon18 OK 12.5023 158 11.1 5.0 20 Corgon18 OK 12.5024 107 10.9 4.5 25 Corgon18 OK 12.5025 179 12.2 6.0 20 Corgon18 OK 12.5026 158 11.1 5.0 20 Corgon18 OK 12.5044Acta Polytechnica Vol. 52 No. 2/2012The evaluation of the material and of the speci-mens was performed at the Faculty of Materials Sci-ence and Technology in Trnava. Microscopic anal-ysis, spectroscopy and microhardness measurementswere performed as an analysis of the material proper-ties. Macroscopic analysis, microscopic analysis andmicrohardness measurements were performed for anevaluation of the joints.GD–OES — QDP (Glow Discharge — OpticalEmission Spectroscopy — Quantitative Depth Pro-filing) was performed with the following process pa-rameters: the current was 15 mA at voltage 1000 V.The analysis was performed on a nitrooxidized mate-rial and also on a material without surface treatment.Vickers microhardness measurements were per-formed using Buehler IndentaMet 1100 Series equip-ment. The load force applied to the specimen was981 mN, and the loading time was 10 s. The mea-surements were carried out on nitrooxidized and non-nitrooxidized steel sheets 1 mm in thickness. Themeasurements were made transversely to the materialthickness (from top to bottom), and were repeatedon three separate specimens (nitrooxidized and with-out nitrooxidation) in order to obtain average values.The distance between the indents was 80 μm, andthe depth of the first indent beneath the surface was60 μm.3 ResultsMaterial analysisThe microstructure of the DC 01 steel after the ni-trooxidation process is shown in Figure 2. Beneaththe very thin oxide layer (up to 680 nm), an ε-phase7–10 μm in thickness was observed. It was composedof Fe2-3N. A compound layer was created under theε-phase. It consisted of ferritic matrix with needle-shaped γ′–Fe4N nitrides. The compound layer wasapproximately 200 μm in thickness. These phaseswere identified by scanning electron microscopy, us-ing a JEOL 7600-F scanning electron microscope.Fig. 2: Microstructure of the DC 01 steel after nitrooxi-dationThe results of the GD-OES analysis (Figure 3)showed that, after the nitrooxidation process, the ni-trogen content had increased up to five times in thematerial surface. This increase was mainly at the ex-pense of the iron content. The oxygen content at adepth of 1 μm was more than 50% greater. Thiscorresponds with the microscopic analysis, wherean oxide surface layer 680 nm in thickness was ob-served.Fig. 3: GD-OES analysis of the material surface a) material without nitrooxidation b) material after nitrooxidation45Acta Polytechnica Vol. 52 No. 2/2012Fig. 4: Microhardness trend comparison of nitrooxidizedmaterial (black) and material without surface treatment(red)The results of the microhardness measurementsare shown in Figure 4. The graph compares the mi-crohardness of the nitrooxidized material with themicrohardness of the material without surface treat-ment. It can be observed that, after the nitrooxi-dation process, the material is affected to a depthof 400 μm. The highest microhardness values wereobserved on both surfaces of the steel sheet (dis-tances 0 and 1 000 μm in the graph). They were47% higher than the values for the material withoutsurface treatment.Due to limitations of the testing device, the mi-crohardness values in the area of the ε-phase, wheremuch higher values were expected (4), could not beobserved.Analysis of the joints:Inappropriate joints were rejected in a visual inspec-tion. Several defects, e.g. weld bead irregularity andweld bead reinforcement were detected. Only speci-mens with the minimum amount of defects were anal-ysed.The macroscopic analysis results are presented inFigure 5. The picture shows an excessive amount ofporosity, distributed over almost the full length ofthe joint. Weld bead irregularity together with lackof fusion was also detected. The probable reason forthe high porosity lies in the surface layer, which isrich in nitrogen and oxygen. However, the fast cool-ing rate may also have caused increased porosity, dueto insufficient time for the escape of gases originat-ing in the nitrooxidized surface layer and dissolvedin WM during its melting process. These are untilnow just hypotheses, and will be a topic for furtherresearch.The microscopic analysis results are shown in Fi-gure 6. The Weld Metal (WM — see Figure 6a)was mainly composed of acicular ferrite. Globularferrite was observed along the column grain bound-aries. Widmanstätten ferrite was also observed spo-radically. Figure 6b shows the interface betweenthe High Temperature Heat Affected Zone (HTHAZ)and WM. HTHAZ consisted of upper bainite, inwhich the Fe3C needles were secreted, together withcoarse-grained acicular ferrite and proeutectoid fer-rite. Coarsening of the primary austenitic grains wasalso identified. The Heat Affected Zone (HAZ — seeFigure 6c) was composed of a fine-grained structureconsisting of globular ferrite.Fig. 5: Macrostructure of the jointsFig. 6: Microstructure of the joints a) WM area b) WM — HTHAZ interface c) HAZ — BM interface46Acta Polytechnica Vol. 52 No. 2/2012Fig. 7: Microhardness values trendThe results of the microhardness measurements(Figure 7) proved that the highest microhardness val-ues were detected in WM. A continuous drop in mi-crohardness was identified towards HAZ. In the areaof BM, the microhardness stabilized, because therewas no thermal affection within this area. The re-sults presented here correspond with the microscopicanalysis, where acicular ferrite was observed in theWM area.4 ConclusionThe results show that for steel sheets treated by ni-trooxidation there was a radical increase in micro-hardness values, up to 47%, in comparison with thevalues for the same material without surface treat-ment. Microscopic analysis identified individual sur-face phases of the material treated by nitrooxidation.On the basis of the research results presented here, itcan be stated that the given parameters of the CMTprocess were not suitable for welding steel sheetstreated by nitrooxidation, due to the high level ofporosity. Future research will focus on changing theshielding gas and the filler metal, and on optimizingthe parameters.AcknowledgementThis paper was prepared with support from theSlovak Research and Development Agency, grantNo. 0057-07 and from the Scientific Grant Agency,grant No. 1/0203/11.References[1] Michalec, I.: CMT Technology Exploitation forWelding of Steel Sheets Treated by Nitrooxidation.Master’s thesis, 2010.[2] Michalec, I. et al.: Metallurgical joining of steelsheets treated by nitrooxidation by a hybridCMT – laser process. In Metal 2011 – 20th An-niversary International Conference on Metallurgyand Materials, Brno : Tanger, 2011.[3] Marônek, M. et al.: Welding of steel sheetstreated by nitrooxidation, In JOM-16: 16th In-ternational Conference on the Joining of Materi-als & 7th International Conference on Educationin Welding ICEW-7, Tisvildeleje, 2011.[4] Dománková, M. et al: Influence of nitridation andnitrooxidation processes on microstructure andcorrosion properties of low carbon deep-drawingsteels. In Materials Science and Technology [on-line]. Vol. 11, No. 1, 2011, p. 40–51.[5] Bárta, J. et al.: Joining of thin steel sheets treatedby nitrooxidation. In 15th seminar of ESAB: Pro-ceedings of lectures of the 15th ESAB + MTF-STU seminar in the scope of seminars aboutwelding and weldability. Trnava : AlumniPress,2011, p. 57–67.[6] Jančár, J. et al.: Laser beam utilisation in weld-ing of steel sheets treated by nitrooxidation. InVyužit́ı laseru v průmyslu. Plzeň : 2011, p. 25–35.Ivan MichalecE-mail: ivan.michalec@stuba.skMateriálovotechnologická fakulta STUKatedra zváraniaJ. Bottu 25, 917 24 TrnavaMilan MarônekE-mail: milan.maronek@stuba.skMateriálovotechnologická fakulta STUKatedra zváraniaJ. Bottu 25, 917 24 Trnava47

Influence of the Surface Layer when the CMT Process Is Used for Welding Steel Sheets Treated by Nitrooxidation

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

Acta Polytechnica Vol. 52 No. 2/2012Influence of the Surface Layer when the CMT Process Is Used forWelding Steel Sheets Treated by NitrooxidationI. Michalec, M. MaroˆnekAbstractNitrooxidation is a non-conventional surface treatment method that can provide significantly improved mechanical prop-erties as well as corrosion resistance. However, the surface layer is a major problem during the welding process, andwelding specialists face many problems regarding the weldability of steel sheets. This paper deals with the properties ofa nitrooxidized surface layer, and evaluates ways of welding steel sheets treated by nitrooxidation using a Cold MetalTransfer (CMT) process. The limited heat input and the controlled metal transfer, which are considered as the mainadvantage of the CMT process, have a negative impact on weld joint quality. An excessive amount of porosity is observed,probably due to the high content of nitrogen and oxygen in the surface layer of the material and the fast cooling rate ofthe weld pool.Keywords: steel sheet, nitrooxidation, Cold Metal Transfer, porosity.1 IntroductionSteel sheets with surface treatment are widely used,above all in the automotive industry. This researchwork is in response to a problem encountered in prac-tical applications of this material. An investigationis made of a new surface treatment method that hasa major effect on improving the surface properties ofsteel sheets, e.g. their corrosion resistance. Nitrooxi-dation in a fluidized bed is a non-conventional surfacetreatment method. This process consists of nitrida-tion with subsequent oxidation. Nitrooxidation leadsto a significant increase in corrosion resistance (up tolevel 10), and also an increase in mechanical prop-erties, e.g. yield strength and tensile strength (1, 2,3).The process of welding damages the surface layer,and it is necessary to find a way to minimize thisdamage. A pulse process is applied to limit the ther-mal input in arc welding, where there is periodic pul-sation of the welding voltage and the welding current.The CMT process, an innovative welding method,can also be used. CMT is a revolutionary short-circuit welding method, which works with digitally-controlled short-circuit metal transfer in the weldingarc. For the first time, this method integrates thewire motion into the welding process and into theoverall control of the process. In this way, the, ther-mal input can be limited. This is a major advantageof this welding method (1, 4, 5).AimsPrevious studies (3, 5, 6) dealt with ways of weldingsteel sheets treated by nitrooxidation. Several de-fects were identified in most of the tested methods,e.g. spatter, porosity and weld bead irregularities.Only solid-state laser beam welding was consideredsatisfactory. Because of the high initial cost of laserequipment, our research focused on finding an ade-quate option in field of arc welding. The main aim ofthe research was to evaluate the possibility of weld-ing nitrooxidized material using the innovative CMTwelding method.2 Materials and methodsThin DC 01/DIN EN 10130-9 steel sheets 1 mm inthickness were used in the experiments. The chemicalcomposition of the steel is shown in Table 1. The ma-terial was subjected to the nitrooxidation process. Afluid environment consisting of Al2O3 grains 120 μmin diameter was used in the nitridation treatment.The oxidation process was carried out in vapoursof distilled water immediately after nitridation. Thenitrooxidation treatment parameters are presented inTable 2.Table 1: Chemical composition of DC 01 EN 10130/91 steelEN C Mn P S Si Aldesignation [%] [ %] [%] [%] [%] [ %]DC 01 max. 0.10 max. 0.45 max. 0.03 max. 0.03 0.01 –43Acta Polytechnica Vol. 52 No. 2/2012Table 2: The nitrooxidation parametersTemperature T Time t[◦C] [min]Nitridation 580 45Oxidation 350 5Table 3: The chemical composition of OK AristoRod12.50C Si Mn[%] [%] [%]0.10 0.90 1.50The experiments were performed at Fronius Ltd.,Trnava. The welding process configuration is shownin Figure 1. Robotised CMT welding was used. A to-tal of 26 specimens with dimensions of 25×10×1 mmwere produced. The welding parameters are pre-sented in Table 4. I joints, overlapped joints andflange joints were welded. OK AristoRod 12.50,1 mm in diameter, was used as the filler metal. COR-GON 18 (18% of CO2, 82% of Argon) with flow rate15 l · min−1 was applied as the shielding gas. Thechemical composition of the filler metal is shown inTable 3.Fig. 1: Welding process configurationTable 4: Welding parameters of the specimensSpecimen Welding Welding Wire Welding Shielding FillerNo. current voltage feed rate speed gas metal[A] [V] [m ·min−1] [mm · s−1]1 92 10.1 3.6 7 Corgon18 OK 12.502 92 10.1 3.6 7 Corgon18 OK 12.503 81 11.3 3.9 7 Corgon18 OK 12.504 90 11.4 3.6 20 Corgon18 OK 12.505 89 10.4 3.6 33 Corgon18 OK 12.506 90 10.1 3.7 10 Corgon18 OK 12.507 88 9.0 5.3 20 Corgon18 OK 12.508 88 9.0 5.3 20 Corgon18 OK 12.509 88 9.0 5.3 20 Corgon18 OK 12.5010 92 10.1 3.6 7 Corgon18 OK 12.5011 92 10.1 3.6 7 Corgon18 OK 12.5012 81 11.3 3.9 7 Corgon18 OK 12.5013 90 11.4 3.6 20 Corgon18 OK 12.5014 89 10.4 3.6 33 Corgon18 OK 12.5015 90 10.1 3.7 10 Corgon18 OK 12.5016 98 12.8 5.0 20 Corgon18 OK 12.5017 86 9.0 5.2 20 Corgon18 OK 12.5018 94 9.3 5.5 20 Corgon18 OK 12.5019 103 9.8 6.0 20 Corgon18 OK 12.5020 103 9.8 6.0 20 Corgon18 OK 12.5021 107 10.9 4.5 20 Corgon18 OK 12.5022 120 11.5 5.0 20 Corgon18 OK 12.5023 158 11.1 5.0 20 Corgon18 OK 12.5024 107 10.9 4.5 25 Corgon18 OK 12.5025 179 12.2 6.0 20 Corgon18 OK 12.5026 158 11.1 5.0 20 Corgon18 OK 12.5044Acta Polytechnica Vol. 52 No. 2/2012The evaluation of the material and of the speci-mens was performed at the Faculty of Materials Sci-ence and Technology in Trnava. Microscopic anal-ysis, spectroscopy and microhardness measurementswere performed as an analysis of the material proper-ties. Macroscopic analysis, microscopic analysis andmicrohardness measurements were performed for anevaluation of the joints.GD–OES — QDP (Glow Discharge — OpticalEmission Spectroscopy — Quantitative Depth Pro-filing) was performed with the following process pa-rameters: the current was 15 mA at voltage 1000 V.The analysis was performed on a nitrooxidized mate-rial and also on a material without surface treatment.Vickers microhardness measurements were per-formed using Buehler IndentaMet 1100 Series equip-ment. The load force applied to the specimen was981 mN, and the loading time was 10 s. The mea-surements were carried out on nitrooxidized and non-nitrooxidized steel sheets 1 mm in thickness. Themeasurements were made transversely to the materialthickness (from top to bottom), and were repeatedon three separate specimens (nitrooxidized and with-out nitrooxidation) in order to obtain average values.The distance between the indents was 80 μm, andthe depth of the first indent beneath the surface was60 μm.3 ResultsMaterial analysisThe microstructure of the DC 01 steel after the ni-trooxidation process is shown in Figure 2. Beneaththe very thin oxide layer (up to 680 nm), an ε-phase7–10 μm in thickness was observed. It was composedof Fe2-3N. A compound layer was created under theε-phase. It consisted of ferritic matrix with needle-shaped γ′–Fe4N nitrides. The compound layer wasapproximately 200 μm in thickness. These phaseswere identified by scanning electron microscopy, us-ing a JEOL 7600-F scanning electron microscope.Fig. 2: Microstructure of the DC 01 steel after nitrooxi-dationThe results of the GD-OES analysis (Figure 3)showed that, after the nitrooxidation process, the ni-trogen content had increased up to five times in thematerial surface. This increase was mainly at the ex-pense of the iron content. The oxygen content at adepth of 1 μm was more than 50% greater. Thiscorresponds with the microscopic analysis, wherean oxide surface layer 680 nm in thickness was ob-served.Fig. 3: GD-OES analysis of the material surface a) material without nitrooxidation b) material after nitrooxidation45Acta Polytechnica Vol. 52 No. 2/2012Fig. 4: Microhardness trend comparison of nitrooxidizedmaterial (black) and material without surface treatment(red)The results of the microhardness measurementsare shown in Figure 4. The graph compares the mi-crohardness of the nitrooxidized material with themicrohardness of the material without surface treat-ment. It can be observed that, after the nitrooxi-dation process, the material is affected to a depthof 400 μm. The highest microhardness values wereobserved on both surfaces of the steel sheet (dis-tances 0 and 1 000 μm in the graph). They were47% higher than the values for the material withoutsurface treatment.Due to limitations of the testing device, the mi-crohardness values in the area of the ε-phase, wheremuch higher values were expected (4), could not beobserved.Analysis of the joints:Inappropriate joints were rejected in a visual inspec-tion. Several defects, e.g. weld bead irregularity andweld bead reinforcement were detected. Only speci-mens with the minimum amount of defects were anal-ysed.The macroscopic analysis results are presented inFigure 5. The picture shows an excessive amount ofporosity, distributed over almost the full length ofthe joint. Weld bead irregularity together with lackof fusion was also detected. The probable reason forthe high porosity lies in the surface layer, which isrich in nitrogen and oxygen. However, the fast cool-ing rate may also have caused increased porosity, dueto insufficient time for the escape of gases originat-ing in the nitrooxidized surface layer and dissolvedin WM during its melting process. These are untilnow just hypotheses, and will be a topic for furtherresearch.The microscopic analysis results are shown in Fi-gure 6. The Weld Metal (WM — see Figure 6a)was mainly composed of acicular ferrite. Globularferrite was observed along the column grain bound-aries. Widmansta¨tten ferrite was also observed spo-radically. Figure 6b shows the interface betweenthe High Temperature Heat Affected Zone (HTHAZ)and WM. HTHAZ consisted of upper bainite, inwhich the Fe3C needles were secreted, together withcoarse-grained acicular ferrite and proeutectoid fer-rite. Coarsening of the primary austenitic grains wasalso identified. The Heat Affected Zone (HAZ — seeFigure 6c) was composed of a fine-grained structureconsisting of globular ferrite.Fig. 5: Macrostructure of the jointsFig. 6: Microstructure of the joints a) WM area b) WM — HTHAZ interface c) HAZ — BM interface46Acta Polytechnica Vol. 52 No. 2/2012Fig. 7: Microhardness values trendThe results of the microhardness measurements(Figure 7) proved that the highest microhardness val-ues were detected in WM. A continuous drop in mi-crohardness was identified towards HAZ. In the areaof BM, the microhardness stabilized, because therewas no thermal affection within this area. The re-sults presented here correspond with the microscopicanalysis, where acicular ferrite was observed in theWM area.4 ConclusionThe results show that for steel sheets treated by ni-trooxidation there was a radical increase in micro-hardness values, up to 47%, in comparison with thevalues for the same material without surface treat-ment. Microscopic analysis identified individual sur-face phases of the material treated by nitrooxidation.On the basis of the research results presented here, itcan be stated that the given parameters of the CMTprocess were not suitable for welding steel sheetstreated by nitrooxidation, due to the high level ofporosity. Future research will focus on changing theshielding gas and the filler metal, and on optimizingthe parameters.AcknowledgementThis paper was prepared with support from theSlovak Research and Development Agency, grantNo. 0057-07 and from the Scientific Grant Agency,grant No. 1/0203/11.References[1] Michalec, I.: CMT Technology Exploitation forWelding of Steel Sheets Treated by Nitrooxidation.Master’s thesis, 2010.[2] Michalec, I. et al.: Metallurgical joining of steelsheets treated by nitrooxidation by a hybridCMT – laser process. In Metal 2011 – 20th An-niversary International Conference on Metallurgyand Materials, Brno : Tanger, 2011.[3] Maroˆnek, M. et al.: Welding of steel sheetstreated by nitrooxidation, In JOM-16: 16th In-ternational Conference on the Joining of Materi-als & 7th International Conference on Educationin Welding ICEW-7, Tisvildeleje, 2011.[4] Doma´nkova´, M. et al: Influence of nitridation andnitrooxidation processes on microstructure andcorrosion properties of low carbon deep-drawingsteels. In Materials Science and Technology [on-line]. Vol. 11, No. 1, 2011, p. 40–51.[5] Ba´rta, J. et al.: Joining of thin steel sheets treatedby nitrooxidation. In 15th seminar of ESAB: Pro-ceedings of lectures of the 15th ESAB + MTF-STU seminar in the scope of seminars aboutwelding and weldability. Trnava : AlumniPress,2011, p. 57–67.[6] Janča´r, J. et al.: Laser beam utilisation in weld-ing of steel sheets treated by nitrooxidation. InVyuzˇit´ı laseru v průmyslu. Plzeň : 2011, p. 25–35.Ivan MichalecE-mail: ivan.michalec@stuba.skMateriálovotechnologická fakulta STUKatedra zváraniaJ. Bottu 25, 917 24 TrnavaMilan MaroˆnekE-mail: milan.maronek@stuba.skMateriálovotechnologická fakulta STUKatedra zváraniaJ. Bottu 25, 917 24 Trnava47

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Influence of the Surface Layer when the CMT Process Is Used for Welding Steel Sheets Treated by Nitrooxidation

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