PSI - Issue 62

Edoardo Proverbio et al. / Procedia Structural Integrity 62 (2024) 285–298 Author name / Structural Integrity Procedia 00 (2019) 000–000

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the presence of localized corrosion attack. Since the beginning of 1974 a new prestressing steel quality 1080/1320 melted with high silicon, manganese and chromium content were produced with a very good combination of strength and ductility, but with extremely unsatisfactory resistance to hydrogen cracking and therefore forbidden (Manuel Elices et al., 2003). The addition of vanadium and a microstructure refinement allowed to rise strength values with improved resistance to hydrogen embrittlement. Both the wire (up to a diameter of 7 mm) and the strand obtained from cold drawing processes are produced from patented wire rods (obtained by hot rolling). After drawing the wires are additionally heat treated while under stress to improve their performance characteristics and reduce relaxation. Cold drawing significantly reduces the sensitivity of steel to hydrogen as plastic deformation introduces a large number of dislocations and therefore sites towards which hydrogen tends to migrate and distribute itself and making it more difficult to achieve the critical levels required for embrittlement. Cold drawing also produces a highly anisotropic microstructure, with pearlitic lamellae preferentially aligned parallel to the drawing axis. The interlamellar spacing within pearlite decreases with the degree of cold working (Toribio & Ovejero, 1998), as does the inhomogeneity of the microstructure. The cold drawing process is also responsible for inducing large residual stresses in the steel which, in the longitudinal direction, are compressive near the axis but tensile near the surface (Atienza et al., 2005). A correlation has been observed between longitudinal residual stresses and failure times in hydrogen embrittlement tests (Toribio & Ovejero, 2005). The fracture behavior of eutectoid cold-drawn steel typically depends on the drawing intensity (Toribio et al., 2013). Increasing strength goes with an increased tendency towards hydrogen-induced stress corrosion in the presence of corrosion-promoting influencers (water, carbonated concrete, chloride). However, it must be observed that increasing the strength of cold-deformed steel from 1700 to 2000 MPa leads to a drop in the service life by a factor of 100. Indeed, based on these results, FIP bulletin (FIP, 1996) suggests a maximum strength for different steel types:  1400 MPa·for hot-rolled  1700 MPa for quenched and tempered  1950 MPa·for cold-drawn As a result of this, the maximum strength of prestressing steels is limited in Germany, and since about 1980 high strength steel rods St 1080/1320 (St 110/135) have been taken out of the prestressing steel market. Due to the same reasoning German authorities raised objections to the new European standard for prestressing steels prEN 10138 which allows nominal tensile strength up to 2160 MPa for strand (in particular when considered that actual strength values are even up to 10% higher than these nominal strength grades). To assess the susceptibility of these steels to hydrogen embrittlement, a simple test—based on a solution of ammonium thiocyanate (NH 4 SCN)—was proposed in 1981 by the FIP (FIP, 1981). In 1982, the Deutsches Institut für Bautechnik (Berlin, Germany) proposed another hydrogen embrittlement test, the DIBt, being less aggressive and more realistic (following the results from chemical analyses of water samples taken from prestressing ducts) (Grimme, 1983). Since then, some authors claimed that the DIBt test is superior to the FIP test, about its suitability to differentiate prestressing steels according to their sensitivity to hydrogen embrittlement (Mietz & Isecke, 2002). Anyway, both tests are now standardized, with each enjoying advantages and drawbacks, and included in the standard EN ISO15630-3 (CEN, 2002). In any case it should be taken into accoun that their effect on failure mode and crack propagation are remarkable different (Fig. 5). More recently some authors based on experimental results (M Elices et al., 2008) concluded that either FIP (solution A in ISO 15630-3) and DIBt (solution B in ISO 15630-3) tests can discriminate between the hydrogen embrittlement susceptibility of steels with the same strength and different post-drawing thermo-mechanical treatments, between steels with the same manufacture (by cold-drawing) and different tensile strength and between steels with different microstructure (quenched and tempered or cold-drawn). Notwithstanding, it was evidenced that to better differentiate the effects of the tensile surface residual stresses reduced load condition (0.70 R m or 0.65 R m ,