PSI - Issue 37

Mihaela Iordachescu et al. / Procedia Structural Integrity 37 (2022) 203–208 Iordachescu M. et al / Structural Integrity Procedia 00 (2019) 000 – 000

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occurred well before being detected). 2. Microstructure, surface state and fracture mechanisms of the bolts

The hot-dip galvanized bolts of the dismantled joint were of metric series M20, PC8.8 class, with a threaded shank length of 90 mm. The nominal yield and tensile strength of the steel were respectively 640 and 800 MPa, with the chemical composition being that given in Table 1. In order to determine the microstructure of the bolt steel, as well as that of the galvanized surface layer, metallographic samples were machine cut from the failed bolt, in both longitudinal and transverse directions. These were mounted in epoxy-resin molds, grinded and polished in several successive steps. Then, after cleaning with distilled water, alcohol and drying, the samples were chemical-etched with 2.5% Nital.

Table 1. Chemical composition of the bolt steel, in weight percent. C Mn Si P S Cr

Ni

Cu 0.2

Mo

Fe

0.48

0.25

0.2

0.0025

0.025

0.8

0.1

0.04

Bal.

Fig. 2. Bolt steel microstructure: (a) in transverse direction; (b) in longitudinal direction

According to the SEM images of Fig. 2, the microstructure of the bolt steel is a martensitic matrix composed of laths and plates that contains disperse spheroidal carbide particles (Fig. 2.b) formed during the quenching treatment. This microstructure is slightly anisotropic because the orientation and size of the martensite crystals differ between the bolt transverse and longitudinal directions. This is likely a consequence of the differences in the thermal gradients and cooling speeds developed during the tempering treatment that follows the quenching process.

Fig. 3. (a) Tread profile and Zn-layer of the failed bolt; (b) SEM image of the bolt damage in the nut-gripping zone closer to the fracture surface; (c) higher magnification detail showing hydrogen-assisted cracking initiation of the bolt steel.

Fig. 3a shows the thread profile of the failed bolt far from the nut gripping area and from the fracture surface. The measured values of the pitch length, thread height and angle do not deviate from the nominal ones. The average thickness of the Zn-layer is 60 µm. The Zn coating and the subjacent bolt steel are damaged in the nut-gripping zone close to the threaded bolt rupture. The damage of the Zn coating consists of generalized transverse cracking and local detachment or disappearance (Fig. 3b), whereas an incipient hydrogen-assisted cracking process is developing in the steel, with the damage concentration zones of the Zn-layer acting as initiators (Fig. 3c). The environmentally-induced cracking and dissolution of the Zn layer propitiate the access of the aggressive media to the steel with the subsequent local diffusible H up-take, embrittlement and cracking of the steel crest (Fig. 3c). These singular cracking sites suggest that the damage process