PSI - Issue 23

Aleš Materna et al. / Procedia Structural Integrity 23 (2019) 425–430 Author n me / Structural Int grity Procedia 00 (2019) 000 – 000

429 5

100 150 200 250 300

Hardness HV5

Thickness reduction:

0 % 10 % 20 %

0 50

0

1

2

3

4

5

6

Annealing time @700 ° C [h]

Fig. 4. Effect of increasing annealing time on hardness of undeformed and cold-deformed 08Ch18N10T steel

Similar results found Singh (1985) for AISI 321 type steel. In his study, cold-rolling with thickness reduction 20 % increased hardness by 40 % which was followed by 10 % decrease of hardness after 1 h annealing at 700 °C.

3.3. Effect of cold-forming process on hardness homogeneity

As can be seen in Table 3, measured hardness of cold-deformed 08Ch18N10T prior the annealing exhibits higher scatter. It was explained ex-post by the 3D FEM analysis of used cold-working proccess. Due to the friction between the pressure plates and sample surfaces, shape of the specimen is no longer rectangular (see Fig. 5) and plastic strains are unevenly distributed as is shown in Fig. 6. From the simplified 2D analysis (chapter 2.7) was estimated that indentation hardness HIT is linearly proportional to the value of equivalent plastic strain , ( ) according to equation [MPa] = 7243.4 , ( ) + 1528.3. (1) Resulting through-width hardening in Fig. 7 was estimated by the combination of both 3D and 2D FEM analyses and compared with the measured one. Athough the measured values of hardness are higher compared to numerically estimated, both trends are the same. Hardness on majority of measured upper sample surface lies within the 10 % interval. Exceptions are sample boundary and sample middle, where hardness significantly increases and decreases, respectively. Increase of material hardness near the center of rectangular block after the unlubricated upsetting was numerically and experimentally observed also by Pillinger at all (1985). Therefore, to obtain representative values of hardness, 2 mm thin region in the middle of 12 mm wide sample should be avoided during the hardness measurement.

Fig. 5 Shape of the specimen after 20% reduction of its thickness . 5 rows of “dots” in the specimen’s center are indents from measurement of hardness profile along the specimen ’s width