Measurement of the gases volume diffused from solidified materials.

Ghenghea, Dan ; Nedelcu, Dumitru ; Dodun, Oana 等

1. INTRODUCTION

Mechanical characteristics of welded beads are imposed to be equal or higher as those of base materials, both the two elements and the heat affected zone, forming welded joints. At no matter what welded joint, quality acceptance requirements criteria do not permit existence of cracks and the pores are accepted only in specific volumetric dispersion. These two specific welding defects formation are generally assumed to be caused by diffusion of gases that are introduced, during welding process, in melted metals bath or to gases being in base materials from other manufacturing processes.

An objective of all welding technologies is to avoid the contact between welded joint and the atmosphere. The work presents the researches done to measure the volume of the gases diffused out side from welded joints using for this objective the eudiometer. Test pieces were made with two very used welding processes: shielded metal arc welding and gas tungsten arc welding in argon inert atmosphere.

2. LITERATURE CRITICAL REVIEW

In liquid state welding processes the melted metals bath has direct contact with dissociated gases in electric arc (Miclosi et al., 1982). Atomic hydrogen, oxygen, nitrogen and carbon oxide, diffuse or make chemical combinations and participate to solidification process. The theory of welding process assumes that atomic hydrogen has strong influence for welding joints quality and this is why many researchers investigate this topic. Presence of hydrogen embitterment like a common problem for API 5 L steel welded by electric arc was marked (Plascensia, G. et al. 2008) and ultrasonic measurements estimate its penetration rate. Cracks in welded zones produced by residual stress and [H.sup.+] diffusion from sulphured hydrogen molecule (Rogante, M. et al. 2006) were investigated. Susceptibility to hydrogen-induced cracking of weld beads on high-strength structural steel with weld beads subjected to hydrogenation under the conditions of cathodic polarization was studded (Malina, J. et al. 2005). The studied specimens fail in the heat-affected zone on the boundary between the main metal and the weld bead in the zone of application of the latter and, especially, near its end. This was explained by a higher hardness of the metal in this zone caused by the specific features of the temperature field formed in the process of welding. The gas tungsten arc welding (GTAW) process is used extensively for welding various grades of stainless steel. The addition of hydrogen to argon shielding gas improves weld pool wet ability, bead shape control, surface cleanliness, and heat input without causing hydrogen-induced cracking (Louthan Jr., M.R. Cannell, G.R., 2005) and it was observed that the use of hydrogen-containing shielding gases for the closure welding of austenitic stainless steel containers for nuclear materials packages does not lead to hydrogen-induced cracking (HIC) of the weld or weld heat-affected zone (HAZ).

Authors screening in technical reports did not find researches about volume of gases diffusion from welding joints and initiate a research about this subject taking account about utilisation of metallic joint in water environments. Results could be used in further researches to identify gases composition and each gas quantity.

3. EXPERIMENTAL PREPARATION

In the experimental part the shielded metal arc welding (SMAW, 111) and gas tungsten arc welding with argon protective inert atmosphere (GTAW, 136) were applied for welding bead deposition using same specific energy. The base materials for welding experiments were made in carbon steel, type OL 44.2, STAS 500/2-80, included in the first welding material group with a very good weld ability. Chemical composition and mechanical features for this type of material are presented in Table (1) .

Filler material in welded joint manufacturing was a wire type S10 1126-90 which chemical composition and mechanical features are presented in Table 2. In shielded metal arc welding process, outside of the wire, a shell of minerals and chemical products like titanium oxide are deposed. In experiments was used the type SUPERTIT with corresponding international codifications: STAS 1125/2 E 43.3.RR. 2.2, DIN 1913 E 43.32.RR.6, AWS A 5.1 E 6013 and ISO 2560 E 43.3. RR. 22 made at Fro Ductil Buzau, Romania. This shielded metal wire is used for welding of metallic structures, boilers, pipe vessels, railway cars and has Manganese in composition. It gives a stable electric arc with very good starts and restarts.

The melting process has fine drops, no sputtering and the slag is fluid, with good covering of the welded bed, easy to detach after the process of manufacturing. Real efficiency deposition rate is around 96%. Chemical composition and mechanical features of welding deposed material are presented in Table 3. Electrode can be used in all welding positions excepting down vertical with inverted polarity ([cc.sup.+], the plus of welding source at the shielded electrode) or in AC with a no charge minimum voltage of 50 V. SUPERTIT electrode is authorized to be used by following international authorities: Romanian Shipping Authority, Lloyd's Register of Shipping, Russian Shipping Register, American Bureau of Shipping, Veritias Bureau.

Argon utilized like protective atmosphere was in B class with purity 99.990% and other gasses like nitrogen, hydrogen under 0.0075%, oxygen under 0.0030%

The both welding processes were done with inverter's electrical source type TECHNOLOGY 200 made by TELWIN Company from Italy. It gives a CC current up to 150 A, for direct polarity ([cc.sup.-] minus source at electrode) or in inverted polarity ([cc.sup.+] plus source to the electrode).

Experimental pieces for research has following dimensions 100 X 100 X 12 mm, were polished to metallic surface to clean all oxides or superficial layers and using a 2.5 mm diameter wire electrode, welding bead were deposed with small deviation around 0. 5 of diameter. From its have been cutting test pieces with dimensions 50 x 5 x 12 mm notated: 1--test pieces from heat affected zone (zit 1 respectively 2), 2--test piece made with shielded metal arc welding (SMAW), 3--test piece made by gas tungsten arc welding process in argon inert protective atmosphere (GTAW). Polishing opposed welded bead edges the same weight for all test pieces was made.

Test pieces have been introduced in the eudiometer for 72 de hours, reading at 1, 10, 24 and 72 hours the volume of diffused gasses. At the end of analyzed period of time once again the test pieces have been weighted but no significant difference have been observed.

4. RESULTS AND DISCUTION

Results given by the experimental activity made in Theory of Welding Processes laboratory at Faculty of Machine Manufacturing, "Gh. Asachi" Technical University of Iasi, are presented in table 4.

Welded joints diffused gasses could arrive from atmospheric humidity, filler materials asked by fabrication or from impurities existing on welding edges.

High temperatures in welding zone create very good conditions for dissociation process of diatomic gasses ([H.sub.2], [O.sub.2], [N.sub.2]) respecting following reactions:

2 H [left and right arrow] [H.sub.2] + 432.7 kJ/mol (1)

2 O [left and right arrow] [O.sub.2] + 494.6 kJ/mol (2)

2 N [left and right arrow] [N.sub.2] + 712.3 kJ/mol (3)

In atomic state these gasses has a strong chemical activity and react with melted metal drops from electric arc column and with metallic melted bath formed by base and filler materials. In steels nitrogen combined with alloying elements conduct to nitrides that do not remain in the gas state; oxygen gives oxides with all alloying elements but with carbon gives the monoxide that could be diffused from welding joints; hydrogen do not have chemical combinations with Fe, Ni, Cr, Mn, Si, from metallic materials used in welding joints but could have combinations with sulphur or carbon resulting gasses witch will diffuse from welded joints. These arguments offer the possibility to appreciate that diffused gasses from studied welded joints could be carbon monoxide CO, methane C[H.sub.4], sulphurated hydrogen [H.sub.2]S and [H.sub.2].

Experimental data analyse from table 4 shows that shielded metal arc welding process produced a volume of diffused gasses with a value near the industrial electrode producers feature related to diffused hydrogen. For the gas tungsten arc welding in protective inert argon atmosphere, the volume of diffused gasses is smaller and this could be caused by low quantities of hydrogen from inert gasses compared with the hydrogen from shielded electrode used in surrounding atmosphere. The values for diffused gasses from test pieces originate from heat--affected zone, presented are closed to those manufactured with shielded metal arc welding. This shows the presence of gasses in base material and could be one explanation for the cold cracking process in heat-affected zone.

5. REFERENCES

Louthan Jr., M.R., Cannell G.R, (2005), Impact of [H.sub.2] in shielding gas for welding austenitic stainless steels, Welding Journal, v 84, n 4, p 38-40, ISSN: 0043-2296.

Malina, J., Samardzic, I., Gliha, V., (2005) Materials Science, v 41, n 2, ISSN: 1068-820X.

Miclosi, V. et al. (1982), Fundamentals of welding processes, E.D.P, Bucharest, Romania.

Plascensia, G. et al (2008), Estimation rate of hydrogen penetration in a weld API 5L steel pipe, Defect and Diffusion Forum, 273-276, pp. 500-505.

Rogante, M. Batistella, P. Cesari, F. (2006). International Journal of Hydrogen Energy, v 31, n 5, pp. 597-601, ISSN 0360-3199.

Tab. 1. Chemical composition and mechanical features of OL
44.2, carbon steel.

Chemical element and Concentration and
 mechanical feature feature values

 C [%] Maxim 0.20
 Mn [%] 1.05 - 1.55
 Si [%] Maxim 0.04
 P [%] Maxim 0.04
 Al [%] Minim 0.025
 [R.sub.p]0,2 [MPa] 255
 [R.sub.m] [MPa] 420 - 500
 A5 [%] Minim 22
 KV [J] 28

Tab. 2. Chemical composition and mechanical features of filler
welding wire S 10.

Chemical element and Concentration and
 mechanical feature feature values

 C [%] Maxim 0.10
 Mn [%] 0.4-0.6
 Si [%] Maxim 0.3
 Cr [%] Maxim 0.2
 Ni [%] Maxim 0.3
 P [%] Maxim 0.03
 S [%] Maxim 0.03
 [R.sub.m] [MPa] 1180 [phi] 2-2.5)or980 [phi] 3.15-5)

Table 3. Chemical composition and mechanical features of
welded deposed material.

Chemical element or Concentration or values
mechanical feature

 C [%] 0.06-0.1
 Mn [%] 0.4-1.7
 Si [%] 0.2-0.6
 S [%] Maxim 0.04
 P [%] Maxim 0.04
[R.sub.p]0,2 [MPa] 430-490
 [R.sub.m] [MPa] 490-550
 A5 [%] Minim 24

Tab. 4. Experimental results of diffused gases from welded
bead and heat affected zones.

 Test 1 HAZ, 2 SMAW 3 GTAW
 ml/100grames ml/100g ml/100g

Welding 111 136 111 136
 1 hour 0.2 0.1 0.1 0.1
10 hours 2.1 1.9 1.5 0.6
24 hours 4.0 4.1 2.7 2.1
72 hours 5.5 5.3 5.5 3.6