Laser beam welding of nitrooxidation treated steel sheets.

Maronek, Milan ; Barta, Jozef ; Kolenic, Frantisek 等

1. INTRODUCTION

Lowering fuel consumption goes hand in hand with vehicle weight reduction. One of the possible ways to reach this goal is to use materials with low specific density and another one is to use materials with increased mechanical properties. Nitrooxidation treated low-carbon steel seem to be an effective alternative to common steel sheets due to its mechanical and chemical properties (Lazar, 2007).

Nitrooxidation treated low-carbon steel sheets have, in comparison to untreated material, many advantages. Improvement of the maximum strength of 41% can be observed and surface microhardness is increased by 653 %. Formability of these steel sheets is increased by 9% by Erichsen cupping test. Nitrooxidised steel showes also 10-times increased resistance to atmospheric corrosion compared to basic state steel. Nitrooxidation has a beneficial influence on formability as well as corrosion resistance (Kunikova, 2005), (Lazar, 2005).

In this article nitrooxidation treated steel sheets are used as a possible replacement for some ordinary sheets, used in industry where high corrosion resistance and surface hardness is required. On the other hand the positive corrosion resistance brought by surface oxidic layer causes problem during arc and resistance welding. The laser beam welding appears to be an alternative for welding this kind of material.

2. EXPERIMENTAL

Thin low-carbon steel sheets Cr 01 ISO 17/12N49-69 (DC 01 EN 10130-91) of 1 mm thickness were used for investigation. Table 1 shows the chemical composition of this material. This base material was treated by nitrooxidation in different parameters whereby the difference was in nitridation and oxidation temperature and time (table 2). Fluidised bed was composed of [Al.sub.2][O.sub.3] with grain size of 120 [micro]m. The waft of the fluidised layer during treatment was provided by gaseous ammonia, during oxidation using a vapour of distilled water, supplied to the furnace chamber (Maronek, 2005).

In contrast to zinc coated sheets ((Sejc, 2004), laser welding was chosen as a suitable joining method for this type of treated sheets . The welding parameters are shown in table 3.

3. RESULTS

[TABLE 4 OMITTED]

[TABLE 5 OMITTED]

[FIGURE 1 OMITTED]

Welding speed was a primary variable parameter in range from 30 to 60 mm/s. Majority of specimens has shown undesirable lack of penetration in 60 mm/s welding speed (Table 4). The specimens welded by welding speed of 30 mm/s had excess weld width as well as the heat affected zone. These welding speeds were evaluated as unsuitable.

The optimal shape of weld in cross-section was achieved at welding speeds 40 and 50 mm/s however in 50 mm/s welding speed, a very small lack of penetration was observed. It can be assumed that the optimal welding speed for this material is between 40 and 50 mm/s and it will be one of the subjects for further research. The appearance of weld surface and weld root for welding speed of 40 mm/s is shown in Table 5.

The nitrooxidation treatment produced a characteristic surface layer about 300 [micro]m in thickness. This was composed of two zones--about 70 [micro]m thick compound layer, with continuous thin [Fe.sub.3][O.sub.4] a [Fe.sub.2][O.sub.3] layer of approximately 3 to 5 [micro]m in thickness and [epsilon] phase ([Fe.sub.2-3]N), about 10 [micro]m in thickness. The welding process destroys this nitrooxidation layer and affects treated material as well as the corrosion resistance.

The microstructure of weld joint is shown on Fig. 1. where weld metal, heat affected zone and base metal can be seen. The microstructure analysis exposed the acicular ferrite and ferrite precipitated along boundaries of columnar crystals. Deep analysis proved that nitrides coming from nitrooxidation process were dissolved by laser beam welding process up to distance of 1 mm from the weld joint boundary. However, the structure showed no abnormalities in phase composition.

The microhardness measurements (Fig. 2.) showed maximum microhardness of weld metal 323 HV 0.1 in upper section and 355 HV 0.1 in root part of the weld. The base metal microhardness was approximately 150 HV 0.1.

Microhardness didn't show significant differences in top and root section of the weld joint. All in all the specimen of 60 mm/s welding speed had the highest values and differences of microhardness due to the highest cooling rate. On the other hand the specimen with lowest welding speed showed a well-balanced progress of microhardness due to the lowest cooling rate.

[FIGURE 2 OMITTED]

5. CONCLUSIONS

In contrast to arc and resistance welding (Sejc, 2004) laser beam welding appearesto be a suitable welding method for welding nitrooxidation treated low-carbon steel sheets.

The recommended welding parameters for C[O.sub.2] laser and 1 mm material thickness are 2000 W beam power, welding speed from 40 to 50 mm/s, beam defocus 0 mm and protective gas Ar 99,996% with 18 l/min flow rate.

Based on the mirostructure analysis and microhardness measurement which did not reveal any irregularities or abnormalities in weld metal and the heat effected zone it can be assumed that, the mechanical properties of the weld joints, currently subject to further research, will confirm our assumptions concerning weld quality.

6. ACKNOWLEDGEMENT

This paper was realised with the support of KEGA 3/4157/06 and APVV 0057-07 grants.

7. REFERENCES

Kunikova, T; Lazar, R; Domankova, M; Mokosova, E; Kamasova-Marekova, Z. (2005) Evaluation of corosion resistance of nitrided and nitrooxidised low-carbon steel. Proceedings of CO-MAT-TECH 2005 13th International Scientific Conference, pp. 671-678, ISBN 80-227-2286-3, Trnava Slovak Republic, October 2005, STU, Bratislava

Lazar, R.; Maronek, M.; Domankova, M. (2007) Low carbon steel sheets treated by nitrooxidation process. Available from: http://www.strojarstvo.sk/docwww/SK/285/285.pdf Accessed: 2008-06-17

Lazar, R; Domankova, M; Kotras, P (2005) Analysis of nitrooxidation layer of deep-drawing sheet metals with using TEM and X-Rays. Available from: http://www.mtf.stuba.sk/docs//internetovy_casopis/2005/mi morc/lazar.pdf Accessed: 2008-06-17

Maronek, M.; Lazar, R.; Domankova, M.; Kotras, P. (2005) Microstructure analysis of weld joints from nitrooxidation treated low carbon steel sheets. Zvarac, 2, 1, 24-28, ISSN 1336-5045

Sejc, P. (2004) Optimalization of selected MIG/MAG brazing parameters of zinc coated steel sheets. Zvaranie--Svafovdni 2004, 53, 3, 57-62, ISSN 0044-5525

Tab. 1. Chemical composition of examined steel

ISO code C Mn P S
 [max %] [max %] [max %] [max %]

DC 01 0,1 0,45 0,03 0,03

Tab. 2. Nitrooxidation parameters

Nitridation time 45 min
Nitridation temperature 540 [degrees]C
Oxidation time 5 min
Oxidation temperature 380 [degrees]C

Tab. 3. Laser welding parameters

Laser type Ferranti Photonics AF 8 C[O.sub.2]

Protective gas Ar 99,996% (18 l/min)
Welding speed [V.sub.s][mm/s] 30, 40, 50, 60
Laser power [W] 2000