PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 98–103 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

99

2

1. Introduction High-strength pearlitic steels are used as (i) constituent materials of rails (pearlitic rail steels) after hot rolling, as described by Masoumi et al. (2019) and Ferreira et al. (2022); (ii) constituent materials of pre-stressed concrete structures, bridge cables and wire ropes in wire form in civil engineering (cold drawn pearlitic steel wires) after cold drawing, as reported by Toribio (1992, 2006) and Borchers and Kirchheim (2016); (iii) reinforcement materials in vehicle tires in the form of tiny wires after heavy cold drawing (cold drawn pearlitic steel wires), as explained by Yan et al. (2019) and Mihaliková et al. (2017). In these materials, hydrogen embrittlement (HE) is a deleterious phenomenon affecting the structural integrity. Such a phenomenon has received many names in the scientific literature such as hydrogen degradation (HD), hydrogen assisted fracture (HAF) or hydrogen assisted cracking (HAC) when a crack is present in the material. The manifestations of HE/HD/HAF/HAC consist of lack of ductility or decrease of fracture toughness, or more complicated phenomena involving locally ductile effects (localized plasticity or hydrogen-dislocation interactions). This paper analyzes the hydrogen-assisted micro-physical degradation in pearlitic microstructures, i.e., the hydrogen assisted microdamage (HAMD) in the form of tearing topography surface (TTS), a term coined by Thompson and Chesnutt (1979) and Costa and Thompson (1982) but just described as a new, non-conventional, fractographic mode but, in principle, not associated with a fracture phenomenon of specific nature (either brittle or ductile) or linked to a particular material (although all provided examples deal with metals and alloys). At the macro -, micro - and nano -levels, the TTS fractographic mode has been associated in the past with HE/HD processes and HAMD in pearlitic steel (Toribio et al., 1991, 1992; Toribio and Vasseur, 1997; Toribio, 1997, 2012). This paper analyzes further HE/HD/HAF/HAC and HAMD of hot rolled pearlitic steel in the presence of notches ( hydrogen-assisted notch-induced fracture and notch tensile strength ), with emphasis on local triaxiality effects in the vicinity of the notch on the microscopic appearance and progression of the TTS region from the initiation (sub critical) to the fracture (critical) point, to elucidate specific features of the evolution of this hydrogen-affected region. 2. Experimental programme A high strength pearlitic steel was used, whose chemical composition and mechanical properties are respectively given in Tables 1 and 2. It presents a coarse pearlitic microstructure, with a pearlite interlamellar spacing of 0.3  m, an average size of the cleavage facet of 75  m, and an average pearlite colony size of about 15  m.

Table 1. Chemical composition (wt %) of the steel. C Mn Si P S

Cr

Ni

Mo

0.85

0.60

0.26

0.010

0.030

0.02

0.02

0.001

Table 2. Mechanical properties of the steel. Young's Modulus E (GPa) Yield Strength (MPa) UTS (MPa)

Ramberg-Osgood parameters  =  e +  p =  / E +(  / P ) n P (MPa) n

Elong. at UTS (%)

199

600

1151

6.1

2100

4.9

Four notched geometries were used of different depth and radii, as sketched in Fig. 1, so as to achieve very different triaxiality (constraint) levels in the vicinity of the notch tip. The samples were subjected to slow strain rate testing with displacement rates between 10 – 10 and 2.10 – 6 m/s. The test environment was an aqueous solution of 1 g/l calcium hydroxide plus 0.1 g/l sodium chloride. The pH value was 12.5 and tests were performed at a constant electrochemical potential of – 1200 mV SCE (saturated calomel electrode) to achieve HE environmental conditions.