PSI - Issue 33

M.R. Tyutin et al. / Procedia Structural Integrity 33 (2021) 765–772 M.R. Tyutin , L.R. Botvina, A.V. Ioffe / Structural Integrity Procedia 00 (2019) 000–000

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2.5. Effect of operation on the kinetics of changes in the acoustic and magnetic characteristics of 12Cr18Ni10Ti steel in tension Fig. 4 shows the acoustic emission diagrams characterizing the changes in the acoustic emission and magnetic characteristics at different stages of tensile deformation of 12Cr18Ni10Ti steel specimens in the initial state and after the operation. The fracture stages were highlighted similarly to ones of the 15Cr2MnMoV steel specimens (Fig. 2). a b

I

II

III

IV

I

II

III

IV

600

15

8000

15

2000

600

1,6

σ

6000 ΣN AE

10 Ṅ , s -1

3

1500 ΣN AE

10 Ṅ , s -1

σ

500

Σ N AE

Σ N AE

500

1,4

400

2 b AE

400

b AE

Ṅ

1,2 b AE

300

4000

1000

300

b AE

200 σ , MPa

200 σ , MPa

5

5

1,0

1

Ṅ

2000

500

100

100

0,8

0

0

0

0

0

0

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

ε*

ε*

c

d

I

II

III

IV

I

II

III

IV

35

1000

40

1000 H EC

600

600

H EC

σ

30

500

800

800

500

30

25

H C

400

400

600

600

20

H EC

20

300

200 H C , A/cm, % martensite 400

300

H EC

H C , A/cm 400

15

200 σ , MPa

200 σ , MPa

H C

10

10

200

100

%martensite

100

5

0

0

0

0

0,0

0,2

0,4

0,6

0,8

1,0

0

0

ε∗

0,0

0,2

0,4

0,6

0,8

1,0

ε*

Fig. 4. Relative strain dependences of stress σ , accumulated number of AE signals Σ N AE , AE activity Ṅ AE , b AE -value (a, b), coercive force H C , eddy-current parameter H EC and martensite content (c, d) for 12Cr18Ni10Ti steel specimens in the initial state (a, c) and (b, d) after operation. As can be seen in Fig. 4a maximum AE activity corresponds to the strain hardening stage. At tension of the specimen after operation, a uniform acoustic emission activity is observed until the ultimate strength is reached. AE activity peak at the yield point is not observed at tension of specimen in initial state (Fig. 4a) unlike the specimen after operation (Fig. 4b). Fig. 4c, d shows the results of measuring the magnetic characteristics during tension. It should be noted that in the initial state, stainless steel specimens are non-magnetic, therefore magnetic characteristics practically do not change at stages I and II. When loading specimen, deformation-induced martensitic transformation takes place, and the magnetic properties appear, as can be seen in the Fig. 4c. After reaching strain ε*=0.5, there is a sharp increase in the coercive force and martensite content. It should be noted that the growth of the eddy current parameter begins earlier – at ε*= 0.3. This can be explained by the fact that the eddy current method, in contrast to the coercive force method, controls structural change in the surface layer of the specimen, whose damage degree is higher than in the volume of specimen. During tension of the specimen after operation, the kinetics of changes in the b AE -value and magnetic characteristics are similar. However, it can be observed that the beginning of the growth of these parameters is shifted to an earlier stage (Fig. 4d).