Influence of Specific Features of Twin Arc Welding on Properties of Weld Joints

The present article covers the influence of standard and narrow gap twin arc welding on properties of weld joints from high-strength steels. While analyzing microsections we established that distribution of micro structure and phase terms, as well the distribution of micro-hardness, were more homogeneous under narrow gap twin arc welding.


Introduction
Regarding the manufacturing process of special-purpose machinery the most expensive and timeconsuming things to produce are thick-walled housings where welding takes over 50% of the total amount of work [1][2]. Why this process is so time-consuming might be explained by thickness of weld materials and the usage of joints with bevel preparation. Use of high tensile medium alloy steels with derated weldability for manufacturing of housings of special-purpose machinery requires the development of special welding technique that will allow us to get solid weld seams. It is important to note, that the straight welds and fillet or I-shaped welds are pretty extended [3]. All that predetermines worthwhileness of usage of automatic welding. Increase of productivity and decrease of defect formation, improvement of welding process might be achieved by gas-shielded multi arc welding [4][5][6][7][8].
Mechanical, physical-chemical and performance characteristics of weld seam metal and heataffected zone (HAZ) are determined by whole range of factors, such as structural phase composition of all layers of weld seam because of that a comprehensive research of their formation under different welding techniques appears to be a relevant applied research task.

Experimental method
To determine special aspects of the influence of multi arc welding on characteristics of weld joints we carried out a series of experiments. The welding was done as gas-shielded single / multi arc welding (Ar+CO2) with welding wire such as: ОК 12.51 -for the first arc, ER307Ti for the second arc. An angle of V-shaped preparation (groove angle) of welded sheets with thickness 20 mm was 60° (area of preparation 231 mm 2 ) or 12° (area of preparation 122 mm 2 ). As the material for samples we used high-tensile-strength steel 30CrMnSiN, which is commonly used for manufacture of housings for special-purpose machinery.
After welding of samples, the templates were cut out for micro structural analysis ( Figure 1). The analysis was carried out using raster electron microscopy of high resolution Quanta-200 (produced by  Figure 1. Scheme of templates cutting out from the sample weld seam By spectrometer, produced by "EDAX", recorded characteristic X-ray excited chemical elements and measurement of their weight (and atomic) ratio in the test material, the elements are detected in a range from Borium to Uranium; energy resolution on line KαMn not less than 160 electrone-volts.
The welding mode is given in Table 1. Regulation of thermocycle under twin arc welding was carried out automatically due to variations of the distance between arcs from 50 to 200 mm. During welding the heating temperature of samples was recorded with 6 thermocouple-sensing elements, 3 of them were set in the middle of the sample in heat-affected zone (HAZ) within different distances from weld seam axis (4, 6, 8 mm) and at the end of the seam within equal distances from weld seam axis, the same as in point 1 (4 mm) in template positions 4, 5, 6. Example of thermal cycles of heat supply and cooling after the second pass in analyzing points, the allocation of thermocouple-sensing elements under twin welding of sample 07 (distance from 100 mm within arcs) are given on Figure 2. Indicative macrosections of weld seams are presented on Figure 3.

Results and Discussion
To analyze the influence of twin arc welding on the properties of weld seams we measured microhardness at the cross-section. Indicative allocation of micro-hardness is presented on Figure 4. HAZ in all examined cases is more distinctive within the width surface of a seam in comparison with HAZ, that is spread at a seam room narrower.
A higher value has micro-hardness of the upper part of HAZ than of the lower part of HAZ joins the root of a weld seam.
In a way micro-hardness scans are similar under single and twin arc welding. Less shown as for micro-hardness angle 12°.
Besides measurement of micro microstructure of seams, heat affected zones and main metal parts of cross sections is presented on Based on the analysis of microstructure we can come to the following conclusions: 1) In close to the weld seams areas, HAZ in all cases in prior austenite grain we observed colonies of laminar morphology particles, typical for initial steel structure of a plate and determined by intense overheating of steel in a close distance to the weld seam reverse change of perlite to austenite and its bainitic hardening.

2)
In microstructure of HAZ located between bainitic and far from the weld seam we can see domination of thin plate perlite with a certain amount of perlitic areas with globalized finely dispersed cementite occurred as a result of dissolution decay to thin plate perlite.

3)
As for the depth of HAZ where the root weld is locatedperlite changes: the thickness of bainetic paltes increases as well as the thickness of plates and equiaxed particles of cementite in феррита (on photos wide black layers fundamental state of steel perlite.
Typical microstructure of a sample in HAZ under twin arc welding Based on the analysis of microstructure we can come to the following conclusions: In close to the weld seams areas, HAZ in all cases in prior austenite grain we observed colonies of laminar morphology, but within borders there were no globular crystal typical for initial steel structure of a plate. Such structure might be assigned to bainitic phase and determined by intense overheating of steel in a close distance to the weld seam enite and its bainitic hardening. In microstructure of HAZ located between bainitic and main globular perlitic (matrix) far from the weld seam we can see domination of thin plate perlite with a certain amount of perlitic y dispersed cementite. This microstructure set alongside with bainite occurred as a result of dissolution of the initial under heating process and then under cooling in the As for the depth of HAZ (from the surface of a cross-section -the first line to the bottom the third line) thin structure of bainite and neighboring intermediate the thickness of bainetic paltes increases as well as the thickness of plates and es of cementite in perlite colonies. On the borders of grains we can see areas of on photos wide black layers). In total such morphology is typical more for main fundamental state of steel perlite.
Typical microstructure of a sample in HAZ under twin arc welding (distance Based on the analysis of microstructure we can come to the following conclusions: In close to the weld seams areas, HAZ in all cases in prior austenite grain we observed but within borders there were no globular crystalline α-phase Such structure might be assigned to bainitic phase and determined by intense overheating of steel in a close distance to the weld seam, that assured a main globular perlitic (matrix) that is far from the weld seam we can see domination of thin plate perlite with a certain amount of perlitic This microstructure set alongside with bainite of the initial under heating process and then under cooling in the the first line to the bottom thin structure of bainite and neighboring intermediate the thickness of bainetic paltes increases as well as the thickness of plates and perlite colonies. On the borders of grains we can see areas of α-In total such morphology is typical more for main

4)
Under twin arc welding with bevel angle of 60°, microstructure of bainite and intermediate perlite in HAZ is more organized.

5)
In case of single / twin arc welding with bevel angle of 12° in HAZ we can also see beinite layer and only then finely dispersed perlite. With that a perlite area hardly differs from neighboring matrix of steel out of HAZ, but bainite morphology in forms of packs was more structured and contained particles of finely dispersed cementite, in general it represents less distortion and lower initial stresses of microstructure. This correlates with data obtained from measurements of microhardness. 6) In general HAZ with narrow gap stood out by larger microstructure and phase homogeneity, that makes her closer to a main metal of steel plate. This results from the measurements of microhardness.

Conclusion
As can be seen from the above, according to the results of carried examinations we can come to a conclusion that usage of narrow gaps and twin arc welding influence in a positive way microstructure and characteristics of weld seams of high-tensile-strength steel.