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

992

4

is a linear correlation between the ferrite and  ´-martensite content reported by Talonen et al. (2004), in the present work the indicated magnetic fraction  is given in the volume percent of ferrite [FE-%]. For phase analysis and determination of residual stresses on the specimen surface and in the near surface regime, X ray diffraction measurements with Cu-K α -radiation operating at 40 kV and 40 mA at a scan speed of 0.0011 °/s and a spot size of 1.5 mm × 1.5 mm were performed. The penetration depth of Cu-K α -radiation in austenitic stainless steels is in the range of 4-10 µm. The quantitative analysis of the X-ray diffraction pattern was carried out by the Rietveld method Bish et al. (1988). To minimize the influence of texture, the diffraction profiles were measured at five different tilt angles in the range 0°    40°. Relative errors in the quantification of phases was about 1 – 3 % for each phase. The layer removal method for this kind of measurements was electrolytic polishing. The residual stress was determined by means of the sin 2  method from diffraction peak at the (220) austenite lattice plane. Additionally, measurements of the micro hardness were performed with a computer-controlled Fischerscope H100C system using a Vickers indenter with a force of 100 mN and a load time of 10 s for each indentation. The micro hardness HV0.01 of all surface morphologies was measured at the 40 µm distance below the specimen surface.

2.3. Chemical composition and microstructure

The investigated material was metastable austenitic stainless steel AISI 347 (X6CrNiNb1810, 1.4550) delivered as rolled bars with a diameter of 25 mm, stripped in solution annealed state from one single batch. To obtain a fully austenitic microstructure, an additional solution annealing heat treatment at 1050°C for 35 min and quenching in helium atmosphere in an industrial heat treatment furnace was performed. The chemical composition is given in Table 1 including the characteristic parameters of metastabilty, i.e. temperatures M s and M d30 calculated according to the empirical equations given by Eichelman and Hull (1953) and Angel (1954).

Table 1. Chemical composition of AISI 347 in weight-% and M s , M d30 temperatures

C

N 2

Cr

Ni

Nb

Si

Mn

Mo

Cu

P

S

Co 0.5

Fe

M S

M d30 46°C

0.024

0.019

17.29

9.25

0.41

0.63

1.55

0.19

0.21

0.023

0.008

bal.

-81°C

The fully austenitic microstructure after the additional solution annealing is shown in optical and scanning electron micrographs of a longitudinal section in Fig. 2. The rolling direction of the bars is oriented vertically. Figure 2a shows blue and brown band structure, grain boundaries and annealing twins as typical for austenitic steels. This optical micrograph was made after color etching using Bloch & Weld etching agent, revealing a band structure caused by slight inhomogeneities of the Cr and Ni content, which could not be removed during solution annealing. The blue band correlates with a lower Ni and higher Cr content, while the brown bands indicate higher Ni and lower Cr content Man et al. (2016). Note that this chemically induced band structure could not be observed in optical micrographs after etching with typical etching agents for stainless steels like V2A etchant (Fig. 2b), or in scanning electron micrographs using EBSD technique (Fig. 2c). The EBSD images show a homogeneous crystallographic microstructure with a grain size of 17 µm and very low defect density (Fig. 2c and Fig. 2d).

(a)

(b)

(c)

(d)

Fig. 2. Microstructure of AISI 347 steel in the initial state after additional solution annealing. (a) Optical micrograph after color etching with Bloech & Wedl, (b) optical micrograph after etching with V2A, (c) EBSD grain orientation map and (d) EBSD misorientation map.