PSI - Issue 16

Olena Berdnikova et al. / Procedia Structural Integrity 16 (2019) 89–96 Olena Berdnikova et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Purpose of research

The aim of the work is to determine the regularities of influence of the structural-phase composition of the metal of welded joints of low-alloyed high-strength steel on the physical and mechanical properties of these joints; revealing ways to optimize the welding process parameters (heating and cooling rates, heat input) for each method (arc, laser and hybrid laser-arc) fusion welding.

4. Approaches, equipment and materials

Studies were carried out on samples of welded joints of high-strength steel (0.14% C, Table 1) with an 8 mm thick bainite-ferrite structure produced by an arc (modes #1 – #4), laser (modes #5 – #7) and a hybrid laser-arc (modes #8 – #10) welding. In the case of arc and hybrid laser-arc welding, filler wire was used (Table 1). Nd: YAG-laser DY 044 was used as a source of laser radiation.

Table 1. The chemical composition of welding materials. Elements C Ni Mn Cr

V

Mo

Si

P

S

Fe

Steel, wt.% Wire, wt.%

0.14  0.1

2.07

0.98

1.19

0.08 0.15

0.22 0.22

0.33 0.24

0.018

0.005 0.007

Bal. Bal.

−

0.4

0.7

−

Studies of the microstructure of the welded joints metal were carried out using light, scanning (SEM) and transmission (TEM) electron microscopy (Markashova et al. (2019b)). Following areas of welded joints metallographic studies have been studied:  WM − weld metal or fusion zone (Guo et al. (2017));  I HAZ − heat affected zone or coarse grained HAZ (CGHAZ, Wang et al. (2019), Li et al. (2015));  BM − bas e metal. We compared the parameters of the welds: the weld width, the total width of the HAZ and the width of the overheating zone − I HAZ as the most important zone. It should b e noted that during the transition from arc welding to laser and hybrid welding, the widths of the weld bead and HAZ decrease by 2...3 times. The small dimensions of welds and HAZ should help reduce the level of local internal stresses in the metal of welded joints and increase their crack resistance. In the arc welding mode with heat input (energy per length unit) of 1519 J/mm (mode #1), a gradient coarse grained bainite-ferrite structure (mainly upper bainite, up to 65%) is formed in the metal of welded joints, Fig. 1a. During the transition from the weld to the HAZ (heat affected zones), the phase composition changes to bainite martensite one. With a decrease the heat input of arc welding to 546 J/mm (#4), the phase composit ion of the weld metal changes from bainite-ferrite to bainite-martensite (Fig. 1b and Fig. 1c). A more equiaxed grain structure is formed while the grain is crushed in 2...3 times. Microhardness increases by 30% (Table 2). At the same time, the volume fraction of upper bainite (B U ) decreases both in the weld (20%) and in the HAZ (45%). In the overheating area of HAZ for all welded joints, the phase composition of metal is bainite-martensite, Fig. 1d. However, the most significant (from the point of view of crack resistance) structural and phase changes (the greatest microhardness gradients 1.3 times, the coarse-grained structures of the B U ) are characteristic for mode with maximum heat input (1519 J/mm). In this case, in the weld metal along the grain boundaries (mainly along the B U boundaries), extended dislocation clusters up to ρ = (1...2)×10 11 cm – 2 are formed. This creates a high dislocation density gradient in such elements of the structure, Fig. 1a. This will probably lead to an uneven level of mechanical properties, as well as a significant reduction in the crack resistance of metal and brittle fracture. 5. Research results 5.1. Arc welding