PSI - Issue 28

M.R. Tyutin et al. / Procedia Structural Integrity 28 (2020) 2148–2156 TyutinM.R./ Structural Integrity Procedia 00 (2020) 000–000

2150

3

2. Materials and methods The study was carried out under tension conditions on flat specimens (Fig.1). Four steel grades widely used in industry were selected as materials for the study – low - (0.2%C) and medium-carbon (0.45% C) ferrite-perlite steels, austenitic corrosion-resistant Cr-Ni-Ti and bainitic (15Cr2MnMoV) steel. Table 1 presents the chemical composition and standard mechanical properties of the investigated steels.

Table 1. Chemical composition (%, mass.) and standard mechanical properties of studied steels.

Steel

C

Si

Mn

Ni

Cu

Cr

Mo

Ti

El., %

σ YS , MPa

σ U , MPa

Low-carbon

0,169

0,212

0,357

0,0354

0,0693

0,0468

- -

- -

37 25 71 15

283 363 234 890

435 642 593

Medium-carbon

0,43 0,07 0,16

0,25

0,55

0,05 9,52 0,30

0,11

0,11

Stainless

0,278

0,909

0,358 0,075

18

0,246

0,35

Bainitic (15Cr2MnMoV)

0,28

1,12

2,12

0,19

-

1040

a

b

Fig.1. The geometry of the specimens: a) low-, medium-carbon steel, and austenitic stainless steel; b) bainitic steel.

The acoustic emission (AE) parameters and the self-magnetic field intensity were evaluated in the course of tensile deformation. The magnetic field intensity by the eddy current method and the coercive force was measured at different stages during stops in loading. The percentage of the martensitic phase in stainless steel was evaluated by eddy current technique. Damage assessment of the studied steels was performed using an optical microscope during stops at different stages of deformation. The following characteristics were evaluated:  intensity of acoustic emission signals Ṅ AE ;  slope coefficient of the amplitude distributions of the AE signals ( b AE -value);  intensity of the resulting self-magnetic field ( H r ) estimated by the method of magnetic memory of metals;  magnetic field intensity ( H EC ) estimated by the eddy current method;  coercive force ( H C );  length ( l c ), total number (Σ N c ) of microcracks, and relative area of the damaged surface ( S *). According to the measurement results, dependencies of the estimated characteristics on the relative deformation ε* were plotted. The relative deformation ε* was defined as the ratio of the current strain to the strain at specimen failure. 3. Results and discussions 3.1. Assessment of acoustic emission parameters Figures 1a, c, e shows the stress-strain diagrams of studied steels, the dependencies of the cumulated number of AE events (Σ N AE ), acoustic emission activity ( Ṅ AE ) and the b AE -value on the relative strain (ε*). The achievement of yield stress and ultimate strength with an increase in the load cause corresponding changes in the acoustic parameters. As can be seen from Fig.2 the dependencies of the AE activity of the steels under study are different both in the shape and in the values of the measured parameter. When testing the low-carbon steel specimens (Fig.2a), the AE activity maximum occurs at the beginning of the plastic flow, which is connected with the dislocation movement.