PSI - Issue 5

Marek Smaga et al. / Procedia Structural Integrity 5 (2017) 989–996 Marek Smaga et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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3. Results and discussion

3.1. Monotonic properties at ambient and elevated temperature T = 300°

At ambient temperature, an ultimate tensile strength of 621 MPa and an elongation after specimen fracture of 51 % were measured (Table 2). The  ´-martensite formation was measured (only at AT) in situ during monotonic loading by a Feritscope TM magnetic sensor (Fig. 3). After tensile testing at 300 °C no  ´-martensite was detected due to higher austenite stability at elevated temperature. The achieved tensile strength is 428 MPa and the elongation after specimen fracture was 37 %.

Fig. 3. Stress-strain response and  ´-martensite formation in AISI 347 steel during tensile tests at AT and T = 300 °C.

Table 2. Mechanical properties and  ´-martensite fraction  at specimen failure

Temperature in °C

R p0.2 in MPa

UTS in MPa

A in %

 in FE-%

AT 300

220 155

621 428

51 37

33 0.0

3.2. Surface and near surface morphology

Figure 4a shows an example of X-ray diffractograms obtained at the surface and at defined distances from the surface up to 285 µm after electrochemical removing of thin material layers from specimens of the MSL t, f=0.15 type. Three phases were detected in this type of surface morphology, namely f.c.c.  -austenite, b.c.c.  ´-martensite and h.c.p.  -martensite.

Fig. 4. (a) X-ray diffractograms at different distances from the surface of specimen with MSL t, f=0.15 surface morphology. Distribution of  ´-martensite (b),  -martensite (c) residual stresses (d) and micro hardness (e) as a function of distance from the surface for three surface morphologies ASL t, f=0.15 , MSL t, f=0.15 and MSL t, f=0.35 . Figure 4b and 4c show the phase distribution of ASL t, f=0.15 , MSL t, f=0.15 and MSL t, f=0.35 specimens. Fully austenitic microstructure was confirmed by X-ray measurements for ASL t, f=0.15 surface morphology where no martensite fraction was detected both at the surface and in the near surface layer. A comparison of both MSL t specimens showed that the feed significantly influenced the distribution of  ´-martensite (see Fig. 4b). A continuous decrease of the α´ -martensite fraction from the maximum value of 18 vol.-% at the surface to 0 vol.-% at 150 µm was found for the small feed of 0.15 mm/rev. At higher feed an i ncrease of α´ -martensite fraction from 4 vol.-% at the surface to 32 vol.-% at 37 µm