Plasma arc welding of steel sheets treated by nitrooxidation.
Michalec, Ivan ; Maronek, Milan
Abstract: Nitrooxidation is a non-conventional thermochemical
treatment which significantly increases the mechanical properties as
well as the corrosion resistance of the steel sheat. The paper deals
with the welding possibility of this type of treated steel sheets by a
plasma arc welding. Elimination of the porosity, most common issue of
materials treated by nitrooxidation, was achieved. On the other hand,
undercuts were observed
Key words: nitrooxidation, plasma arc welding, undercuts
1. INTRODUCTION
The welding of surface treated steels is a challenge for all
welding specialists, due to the surface layer exposing to a strong
thermal affect by the welding process (Maronek et al., 2011).
Nitrooxidation is a non-conventional thermochemical treatment. It
consists of surface nitridation with subsequent oxidation. The base
material acquires maximum corrosion resistance (level 10) in comparison
to materials without nitrooxidation. Mechanical properties, such as
yield strength, microhardness and wear resistance are also improved as
well (Babul et al., 2008, Domankova et al., 2011).
From previous outcomes (Michalec et al. 2010), several arc and beam
welding methods were tested with unsatisfactory results. The most common
problems were the weld bead irregularity, spatter, surface layer damage
in the heat affected zone (HAZ) and excessive amount of porosity, which
was the main issue in each welding method. Only the solid-state laser
beam welding was considered as the most suitable method, due to superior
defects-free weld quality, 100 % repeatability and very narrow HAZ
(Barta, 2010).
In regard to high initial costs of the laser beam welding
equipment, there was a supposion, that the plasma arc welding could be
an appropriate substitution, mainly due to narrow HAZ, limited thermal
input and a good weld quality (Michalcakova, 2011).
2. MATERIALS AND METHODS
The material used for the experiments was thin steel sheet DC
01/DIN EN 10130-9 of 1 mm thickness. The chemical composition is
reffered in Table 1. The base material (BM) was put through the process
of nitrooxidation, which consisted of nitridation in environment of
[Al.sub.2][O.sub.3] grains of 120 [micro]m in diameter wafted by gaseous
ammonia. After the nitridation process, oxidation process started
subsequently. Oxidation itself was carried out in a vapours of distilled
water. The parameters of nitrooxidation are given in Table 2.
Experimenal actitivity was carried out at Faculty of Materials
Science and Technology in Tmava. Total amount of 33 specimens with
different welding parameters were welded on Fronius MagicWave 2200 with
PlasmaModule 10 device. Argon 4N6 was used as a shielding as well as
plasma gas.
3. RESULTS AND ACHIEVEMENTS
As the first step, visual inspection was done. There were
identified some defects, such as lack of fusion and undercuts. The best
specimens were chosen for further analysis. The macroscopic and
microscopic analysis as well as microhardness measurements were
performed.
The results of macroscopic analysis are presented in Fig. 1. The
weld had regular shape, it had no porosity and was spatter-free.
Nevertheless, undercuts were observed in whole weld length. The reasons
of creating the undercuts as well as their elimination is under further
investigation.
[FIGURE 1 OMITTED]
The microscopic analysis was carried out mainly in order to obtain
the information about microstructure of the weld metal (WM). In Fig. 2,
it is shown, that the microstructure was composed mainly of
coarse-grained acicular ferrite and polyedric ferrite as well.
[FIGURE 2 OMITTED]
Measurements according to Vickers HV0.1 with F = 981 mN of force
load and t = 10 s of loading time was carried out. The typical
mierohardness trend is given in Fig. 3. The highest microhardness values
were observed in the weld metal, due to coarse-grained microstructure,
which is in connection with the microscopic analysis. Towards to heat
affected zone and base material, the microhardness values had a
descending trend.
[FIGURE 3 OMITTED]
The mechanical properties were evaluated by a tensile test. The
main reason was to assess the joint quality regarding the presence of
the undercuts.
The tensile test was carried out on four specimens with a
dimensions of 100 x 20 x 1 mm. The results are presented in Fig. 4.
Picture shown, that the fracture area of all specimens was observed out
with the joint area and therefore the results could be marked as a
suitable, not regarding the presence of the undercuts. The average force
to failure was F = 6.3 kN, which represented the material tensile
strength of 315 MPa.
[FIGURE 4 OMITTED]
Because of the DC 01 steel suitilization for deep-drawing
applications, Erichsen cupping test was carried out. The testing
procedure was performed in accordance to STN EN 1001-5. The experimental
specimens have a dimensions of 250 x 50 x 1 mm. Total amount of three
specimens were made. The results are presented in Fig. 5. The fracture
character, in comparison to the joint orientation, was evaluated as the
main criceria in the choice of suitability of the joints. As it is shown
in Fig. 5, all tested specimens had a transverse type of the fracture
both on the face and the root side of the joint. Therefore the joints
were marked as a suitable regarding the fracture character.
Regarding the depth of the indent, there was an increase by 11% in
comparison to the joints welded by a solid-state laser beam welding
(Barta, 2010).
[FIGURE 5 OMITTED]
4. CONCLUSION
Based on the previous outcomes, the laser beam welding was marked
as the most suitable welding method for welding of steel sheets treated
by nitrooxidation. However, the high initial cost of laser beam
equipment accelerated the effort to find the other suitable welding
method. The plasma arc welding, which had not been previously tested,
was considered as an appropriate option.
Results showed that the elimination of porosity, which was the main
issue in welding of steel sheets treated by nitrooxidation, was
achieved. Mechanical properties of the joints were satisfactory as well.
Nevertheless, the presence of undercuts in weld joints requires further
investigation and subsequent elimination.
Therefore, the influence of a torch-to-work distance and
torch-to-work angle together with change of the plasma as well as the
shielding gas will be evaluated in the research to come.
5. ACKNOWLEDGEMENTS
This paper was prepared within the support of Slovak Research and
Development Agency, grant No. 0057-07 and Scientific Grant Agency, grant
No. 1/0203/11.
6. REFERENCES
Babul, T.; Obuchowicz, Z. & Grzelecki, W. (2008).
Nitrooxidation of tools manufactured from high-speed steel, 2nd
International conference on heat treatment and surface engineering of
tools and dies, May 25-28th, Bled, Slovenia, pp. 89-90
Barta, J. (2010). Welding of special treated thin steel sheets:
Dissertation thesis, Trnava, 2010
Domankova, M.; Kebiskova, J.; Lazar, R. & Kusy, M. (2011).
Influence of nitridation and nitrooxidation processes on microstructure
and corrosion properties of low carbon deep-drawing steels, Materials
Science and Technology [online], ISSN 1335-9053, Vol. 11, No. 1, pp.
40-51
Maronek, M.; Barta, J.; Bartova, K. & Drimal, D. (2011).
Welding of steel sheets treated by nitrooxidation, JOM-16 : 16-th
International Conference On the Joining of Materials & 7-th
International Conference on Education in Welding ICEW-7, May 10-13th,
Tisvildeleje, Denmark, ISBN 87-89582-19-5
Michalec, I.; Jancar, J. & Maronek, M. (2010). Metallurgical
joining of steel sheets treated by nitrooxidation by a hybrid CMT--laser
process, 20th Anniversary International Conference on Metallurgy and
Materials, May 18-20th, Brno, Czech Republic, ISBN 978-80-87294-22-2
Michalcakova, A. (2011). Plasma arc welding of steel sheets treated
by nitrooxidation, Diploma thesis, Trnava, 2011
Tab. 1. Chemical composition of DC 01/DIN EN 10130-9 steel
EN C Mn P S Si
Marking [%] [%] [%] [%] [%
DC 01 0.12 0.60 0.045 0.0451 0.1
Tab. 2. Parameters of nitrooxidation
Temperature [[degrees]C] Time [min.]
Nitridation 540 45
Oxidation 350 5
