PSI - Issue 23

Igor Barényi et al. / Procedia Structural Integrity 23 (2019) 547–552 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

551

5

3.2. Nanoindetation study of selected structure components

Two characteristic types of the treated samples were chosen for quasistatic nanoindentation. First one was heat treated by rapid cooling rate 5 °C·s -1 with martensitic structure as the final result. The second one corresponds to very slow cooling (0.01 °C· s -1 ) with ferritic - pearlitic microstructure as a result. The sample surfaces were first scanned by SPM (Scanning probe microcopy). To form images, indenting tip oscillates with specific frequency over the surface while scanning the selected area without causing any physical changes.

Fig. 3 In Situ SPM scans of measured surfaces with indentation locations: a), b) cooling rate 5 °C·s -1 ; c), d) cooling rate 0.01 °C·s -1

Table 4 Local mechanical properties of microstructure constituents for cooling rate 5 °C·s -1

Location

b0

b1

b2

b3

b4

b5

b6

b7

H [GPa]

52

61

43

41

38

20

44

42

E r [Gpa]

496 546 484 576 352 275

512 586

phase (assumed)

P1

P2

M1 M1 P3

matrix M2 M2

Table 5 Local mechanical properties of microstructure constituents for cooling rate 0.01 °C·s -1

Location H [GPa] E r [GPa]

c0 13

c1 14

c2 14

c3 17

c4 14

d0 20

d1 19

d2 14

d3 22

d4 15

d5 13

274

361

386

306

372

302

263

237 320

232 336

phase (assumed)

FG1 FG1 FG1 FG2 FG2 PC1 PC2 PF1 PC3 PF2 matrix

The SPM scans acquired for these two samples are depicted in Fig. 3. Scan area 20  20 µm large consisting of the fully martensitic structure is in Fig 3a. The structure is too fine to reliably identify any phase and perform sufficiently precise indentation, therefore more detailed scan (3  3 µm) was realized with some clearly visible structure constituents (Fig. 3b). The nanohardness and reduced Young modulus of two martensitic needles (M1 M2) and some structure particles (P1÷P3) were measured while the results are shown in Table 4. Based on measured local mechanical properties, we assume that these are carbides of alloying elements. Figure 3c shown 20  20 µm scan of the structure after slow cooling with ferritic grains and matrix between them consisting of pearlite. Figure 3d shown 5  5 µm scan of the area between ferritic grains in more detail. Particles with lower nanohardness were estimated as pearlitic ferrite (PF1, PF2), where PC1÷PC3 particles with higher nanohardness correspond to the pearlitic cementite. Local mechanical properties of all these components are in Table 5.