Effect of the welding environment and storage time of electrodes on the diffusible hydrogen content in deposited metal

Jacek Tomków^a,*, Dariusz Fydrych^a, Grzegorz Rogalski^a, Jerzy Łabanowski^a

^aGdańsk University of Technology (GUT), Narutowicza 11/12, 80-233 Gdańsk, Poland

(*Corresponding author: jacek.tomkow@pg.edu.pl)

1. INTRODUCTIONTOP

Welding in underwater environment is most often applied as a method of repairs water constructions. The process is most often carried out with direct contact with water (wet welding) with the use of covered electrodes (Fydrych et al., 2015; Wang et al., 2018). Water environment intensifies some problems that have a negative impact on weldability of steel, such as a rapid cooling rate (Guo et al., 2017; Tomków et al., 2018a; Sajek, 2019) and the presence of residual stresses (Aloraier et al., 2004; Hu et al., 2017; Han et al., 2019; Wang et al., 2019a; Wang et al., 2019b). The environment also has an impact by increasing the diffusible hydrogen content (Fydrych and Łabanowski, 2015; Chen et al., 2018). The water is a source of potential hydrogen and diffusible hydrogen content in deposited metal depends on this amount. Hydrogen could be a source of corrosion (Świerczyńska et al., 2017a), and it is also a factor that is responsible for cracking (Yadav et al., 2017; Pascual-Guillamón et al., 2018), especially cold cracking, which is the biggest problem in underwater welding (Guo et al., 2015; Tomków et al., 2018b). Also, the hydrogen has a big influence on the mechanical properties of welded joints (Pandey et al., 2017a; Pandey et al., 2017b).

The diffusible hydrogen content in deposited metal depends on welding conditions such as the filler and the base material surface condition. It also depends on other parameters including the arc voltage, travel speed, welding current and polarity and stick out length (Fydrych and Łabanowski, 2015; Schaupp et al., 2017; Schaupp et al., 2018).

Hydrogen measurements are standardized (ISO 3690 (2012)). Literature recommended the use of the mercury method for hydrogen measurements (Padhy et al., 2015a; Padhy et al., 2015b). The most important disadvantage of the mercury method is the toxicity of mercury (López et al., 2014; López et al., 2015). The measurement error resulting from the solubility of hydrogen in glycerine is less important in the case of testing of high-hydrogen processes, for example: welding with rutile or cellulose electrodes, or welding in water environment (Świerczyńska et al., 2017b). As usage of the mercury is prohibited, one of the most prospective methods of low-temperature measurement of hydrogen content is glycerine method. Disadvantages of the results glycerin method in comparison with the mercury method are more difficulty in repeating the results and lower measurement accuracy. To compare results between these two methods, it is necessary to use of relatable data points (Fydrych and Łabanowski, 2015). However, the capability of recalculating the results is limited to 35 ml/100 g of diffusible hydrogen in deposited metal, so it cannot be used in underwater welding. Recent research showed that results up to 80 ml/100 g can be recalculated according to the following formula (Fydrych and Łabanowski, 2015):

HDme = 1.21 × HDgl + 2.60 (1)

Where HDme-hydrogen content in deposited metal determined with mercury method (ml/100 g) and HDgl-hydrogen content in deposited metal determined with glycerin method (ml/100 g).

The aim of the undertaken studies was to determine a relationship between the storage time of the electrodes and the amount of diffusible hydrogen content in deposited metal in different welding environments (air and water).

2. MATERIALS AND METHODSTOP

The diffusible hydrogen content was measured in the deposited metal by general purpose OMNIA (E42 0 RC11) rutile electrodes that are 4.0 mm in diameter. The welding polarity was assumed to be in accordance with the electrode manufacturer recommendations (DC-), at a test stand 0.15 m deep under the water with the use Aristo 4000i as a welding power source. These electrodes were chosen because they provide good plastic properties of the weld metal, which minimizes the possibility of cold cracking in a water environment. The test stand is presented in Fig. 1. The weld beads were made on 4×20×120 mm specimens from S235JR steel. The chemical composition of the materials are presented in Table 1 and Table 2.

Determination of the diffusible hydrogen content with the glycerin method was carried out in accordance with standard procedure BN-64/4130-01 (1971). The test stand is presented in Fig. 2.

At first the specimen was weighed with an accuracy of 0.01 g before welding. The specimen was then placed in a copper fixture and the weld bead was deposited with a short arc. When the welding has been finished, the slag has been removed and the specimen was quenched in water at 20 °C. After 30 seconds, the specimen was cleaned, dried and placed in an apparatus. The time between finishing welding the sample and beginning the hydrogen content measurements did not exceed 2 min. The pressure, temperature and during extraction the temperature were measured. After being removed from the measurement vessel the specimen was cleaned, dried and weighed to determine the mass of deposited metal. Adjusting the results to normal operating conditions was completed according to the formula from BN-64/4130-01 (1971) standard. The glycerin solution was changed every two weeks as recommended (Fydrych and Łabanowski, 2015).

3. RESULTSTOP

The tests were carried out on 10 specimens welded in the air (series 1 and 2) and 10 specimens made in the water environment (series 3 and 4). The first part of the specimens was welded immediately after opening the package of the electrodes (series 1 and 3). Then the electrodes were stored in an open packages in laboratory conditions (temperature in range 18-21 °C, and humidity 50-60%), and they had direct contact with the air. After 3 years, the second part of the samples (series 2 and 4) was made to verify the effect of the storage time of electrodes on the diffusible hydrogen content in deposited metal in different welding environments. Additionally, the visual testing of electrode covering surfaces showed no changes in their appearance. The results of the determination of the diffusible hydrogen content are presented in Table 3. The exemplary specimens are presented in Fig. 3.

The results showed in Table 3 were subjected to statistical analysis. The first aim of the analysis was to verify is there was any statistically important difference in the diffusible hydrogen content for the specimens made in two environments. The second aim was to verify the effect of the storage time of electrodes on the diffusible hydrogen content in deposited metal in different welding environments.

4. DISCUSSIONTOP

The statistical analyses were performed in a Statistica software package using the ANOVA module (one-way analysis of variance) with an assumed significance level α = 0.05. In the first stage, a comparison of average results was carried out as a variable that groups a series of tests (type of environment and time of welding). Random variables in the considered populations (groups) are independent and measurable. It is necessary to verify the normal distribution of variables in each population and the homogeneity of variance in all populations. The assumption of normality was verified by the Shapiro-Wilk test. Based on the results shown in Table 4, it was assumed that the assumption is met (p > α = 0.05).

The homogeneity of variance was verified by the Levene test. Because the p value is bigger than the assumed level of significance α = 0.05, the assumption is met. The Levene test results are presented in Table 5.

The results of the variance analysis for all of the series are summarized in Table 6. Equality of means hypothesis test should be rejected because the value of p < α = 0.05. It means that there is a statistically significant difference in the hydrogenation levels of the analyzed specimens.

In the next step, post-hoc analyzes were made. The aim was to determine which averages are significantly different. Scheffe, Tukey, NIR Fisher, Newman-Keuls and Duncan tests were used. Results obtained in different environments were assigned to two separate groups. It was shown that welding environment changes cause statistically significant differences in the level of the hydrogenation of the deposited metal. The post-hoc results are presented in Table 7.

Possible changes in the diffusible hydrogen content in deposited metal resulting from storage time of electrodes (3 years) were verified by Student’s t-test. Table 8 presents the results of the analyzes for results obtained in the air environment and Table 9 presents results obtained in the water environment. In both environments, the values of p > α = 0.05, so it was assumed that there are no significant differences in the content of hydrogen in a series of samples made before and after the three year period.

The graphical results of the analysis are presented in Fig. 4. Vertical error bars representing 0.95 confidence intervals overlap for series 1 and 2 (air environment) and 3 and 4 (water environment). There is a significant difference between results in different welding environments.

The literature stated, that the storage time of welding consumables could provide to increasing of the diffusible hydrogen content in deposited metal (Kiefer, 1996; Harwig et al., 1999). The results of presented in this paper experiment proved the statement that storage time of the electrodes has resulted in the hydrogenation of electrodes covering, which was affected by moisture in the air. The expected result was an increase in the diffusible hydrogen content in deposited metal in the specimens made under water, which is due to the increase in the potential hydrogen amount in the welding environment (Tomków et al., 2018c). The increase in the content of potential hydrogen caused by this phenomenon has no impact on the statistically significant increase in the diffusible hydrogen content in deposited metal. This was observed for welding experiments done in the air and in the water. The key achievement of presented results is statement that in the case of rutile electrodes, the storage time can be at least three years without changes in their appearance and harmful effect on hydrogenation of deposited metal. The key achievement of presented results is statement that in the case of rutile electrodes, the storage time can be at least three years without changes in their appearance and harmful effect on hydrogenation of deposited metal.

5. CONCLUSIONSTOP

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