PSI - Issue 23

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

550

4

2 A S    ,

E

(1)

r

c

whereas ß [m -1 ] is a constant which depends on the geometry of used Cube Corner indenter and A c [m 2 ] is the contact area of indentation. For all performed indentation standard trapezoid load curve with the maximum test load F = 2000 µN and t = 2 s holding period is used. These parameters have been proven in previous research focused on high strength 33NiCrMoV15 steel (Barényi et al., 2019).

3. Results & Discussion

3.1. Evolution of microstructure in relation to the cooling rate

Before nanoindentation tests, all treated samples were observed by standard metallographic analysis. There are shown all microstructures corresponding to all used heat treatment regimes in Fig. 3. The microstructures for all cooling rates from 100 to 0.5 °C·s -1 (Fig. 2 a÷ e) are full martensitic (M) with retained austenite (A). The difference is in volume fraction between M and A only as dilatometric analysis for these regimes showed. The microstructure for 0.1 °C·s -1 (Fig. 2f) is martensitic and bainitic (B). In contrast to previous microstructures, martensite is present here in a form of soft needles where the rest dominant phase is bainite. The microstructure for 0.05 °C·s -1 (Fig. 2g) is almost fully bainitic with small amount of martensite and ferrite (F) what was also confirmed by dilatometric analysis. Last microstructure (Fig. 2h) for 0.01 °C·s -1 cooling rate is very close to equilibrium state. There are visible polygonal ferrite grains (light areas) in pearlite matrix. The surface hardness of samples treated by each of the used thermal regimes was measured. The values of the hardness HV5 versus cooling rate (Table 3) also supports the conclusions from the metallographic analysis.

Fig. 2 Evolution of microstructure in relation to the cooling rate ( °C·s -1 ): a) 100, b) 10, c) 5, d) 1, e) 0.5, f) 0.1, g) 0.05, h) 0.01

Table. 3 Course of hardness HV5 in relation to the cooling rate

Cooling rate [ °C·s -1 ]

100 850

10

5

1

0.5

0.1

0.05 582

0.01 423

Hardness [HV5]

815

810

800

790

730