Issue 21

R. Valentini et alii, Frattura ed Integrità Strutturale, 21 (2012) 30-36; DOI: 10.3221/IGF-ESIS.21.04 36 during the heat treatment phase at high temperature, create discontinuity in the metallic matrix. Due to this, matrix decohesion is eased around the precipitates and larger dimples occur. In particular, in Fig. 7 (b) is shown a precipitate fractured in three parts. As far as diffusivity of hydrogen in HSS is concerned, all the materials involved in this study were previously characterized using a Devanathan–Stachurski[11] permeation apparatus. The highest value is related to AISI (is 1·10 -7 cm 2 /s) whereas diffusion in both grades of Maraging 250 is an order of magnitude lower (8.20·10 -9 cm 2 /s for peak- aged at 480°C and 9.58·10 -9 cm 2 /s for peak-aged at 535°C) [12]. Known the diffusion coefficient of hydrogen in the steel, it is possible to calculate the distance at which hydrogen concentration is about 50% ( X ) inside the metallic matrix, during the test duration, by the following formula [13] Dt X 2  (1) where D is the diffusion coefficient in cm 2 /s and t is the time in seconds. Referring to SSRT curves presented in Figures 4 and 5, it is reasonable to take 500 s as average duration of hydrogen diffusion through the steel. By calculation, it is obtained a length of penetration of 140 µm and about 40 µm for AISI 4340 and Maraging 250 respectively. Initiation area for fractures shown in Figure 6 is congruent with the order of magnitude of hydrogen penetration through AISI 4340. Even the result for Maraging is consistent with its experimental evidence, because such a low value of penetration through the steel could not enhance its embrittlement. In conclusion, SSRT duration results insufficient to start HRE for Maraging 250. In order to obviate this problem, other tests may be conducted with a strain rate of an order of magnitude lower or by constant load tensile test of standard duration (200 h maximum) [14]. C ONCLUSIONS  AISI 4340 exhibited a relevant susceptibility to HRE whereas both grades of Maraging 250 appeared immune to the phenomenon in the present experimental conditions.  Fracture surface analysis by SEM is consistent to hydrogen depth of penetration model both for AISI 4340 and Maraging 250.  SSRT duration is too short to enhance brittle cracking of Maraging 250, further test must be conducted to deepen the knowledge of influence of HRE for this class of steel.  Maraging 250 peak-aged at 535°C showed a higher percent elongation than the grade peak-aged at 480°C. R EFERENCES [1] G. F. Lovicu, C. Colombo, M. De Sanctis, R. Valentini, Met. Trans. A, 42 (2011) 3577. [2] G. Chalaftris, M.J. Robinson, Corr. Eng. Sci. Tech., 40 (2005) 28. [3] D.J. Figueroa Gordon, Ph.D. Dissertation, Cranfield University, Cranfield, UK, (2005). [4] D. Figueroa, M.J. Robinson, Corr. Sci., 50 (2008) 1066. [5] E. E. Nelson, T. S. Humphries, J. R. Lowery, NASA, MSFC Memorandum EH24 (1981). [6] R. P. Gangloff, Hydrogen Assisted Cracking Of High Strength Alloys, Elsevier Science, (2003) 31. [7] W. J. Pollock, Prevention and Control ASTM STP 962, L. Raymond Ed., ASTM, Philadelphia, (1988) 372. [8] R. M. Nageswara, Corr. Sci., 51 (2009) 1645. [9] J. C. Scully, NATO Scientific Affairs Division, (1971). [10] P. Novak, R. Yuan, P. Somerday, P. Sofronis, R. O. Ritchie, Journal Mech. Phys. Solids, 58 (2010) 206. [11] M.A.V. Devanathan, Z. Stachurski, Proc. R. Soc. London, Ser. A , 270 (1962) 90. [12] R. Lontano, Bachelor Dissertation, University of Pisa (2011) [13] J. Crank, The Mathematics of Diffusion, Oxford University Press (1975) [14] ASTM F 519 Standard test method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments.