PSI - Issue 28

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

2153

6

11 H C , A/cm

a

b

H C , A/cm

I

II

III IV

I

II

III

IV

200

50

7,0

700

500



1

100 H EC

120

2

6,5

600

10

H r , A/cm 0

150

400

90 H EC

500

6,0

100 H r , A/cm

3

9

300

400

4

4

5,5

60

 , MPa

50

300  , MPa

8

200

-50

5,0

2

30

200

50

7

100

4,5

100

3

0

0

-100

0

4,0

0,0 0,2 0,4 0,6 0,8 1,0 0

6

0

0,0

0,2

0,4

0,6

0,8

1,0



 *

c

d

H C , A/cm , martensite cont.

I

II

III

IV

15

50

700

15

35

I

II

III IV

1000

1000

18 20 H C , А/cm 0 Н EC

3

600

30

800 H EC

1

800

10

10

500



25

Н r , А/cm

-50

600

600

400

20

2

3

5

H r , A/cm

4

martensite content

300  , MPa

400  , MPa

15

16

400

5

-100

4

2

200

10

0

200

200

14

100

5

-150

0

0

0

0

-5

0

0,0

0,2

0,4

0,6

0,8

1,0



0,0

0,2

0,4

0,6

0,8

1,0



Fig.3. Stress-relative strain diagrams (curves 1) of specimens from low-carbon (а), medium-carbon (b), austenitic stainless (c) and bainitic (d) steels, combined with deformation dependencies of self-magnetic field intensity H r (2), eddy current parameter H EC (3) and coercive force H C (4).

3.3. Assessment of damage parameters The estimation of damage evolution of the studied steels was performed during stops at different values of deformation. Microcrack patterns were obtained using an optical microscope (Fig.4). Since studied steels have different plasticity levels, it is important to compare the development of damage characteristics. To do this we plotted dependences of the damage parameter S * on the relative deformation ε* (Fig.5a). Analysis of the microcrack patterns showed that:  The process of the multiple damage development on the surface of the specimens (the appearance of slip bands and microcracks) begins with a relative deformation (ε*) not exceeding 0.38 for low-carbon steel specimens, 0.30 - from medium-carbon steel, 0.31 - from stainless steel and 0.38 bainitic steel specimens. The minimum length of microcracks was at least 0.5 μm.  In low- and medium-carbon steels specimens, as well as in bainitic steel specimen, slip bands and microcracks are oriented mainly at an angle close to 45 ° to the loading axis, while in stainless steel specimens - mainly along the loading axis (Fig.4).  The qualitative analysis of microcrack patterns for the steels under study showed that the least damage is observed in the bainitic steel specimens with relative deformations up to 0.8. With further deformation, damage in the bainitic steel specimen becomes substantially higher (Fig.5a). This indicates a higher rate of damage accumulation in this steel when the process of microcrack coalescence starts to prevail.  The stages of damage growth correspond to the fracture stages identified by analyzing the changes in acoustic emission parameters (Fig.2) and in magnetic characteristics (Fig.3).