Avaliação da susceptibilidade à fragilização por hidrogênio in situ em microconstituintes por meio de nanoindentação instrumentada

Abstract

This work evaluated the hydrogen embrittlement susceptibility (HES) by means of electrochemical nanoindentation. Three steels (AISI 4130M, AISI 4137M and AISI 4140) and two Ni-alloys (UNS N07718 (Inconel 718®) and CMSX-4) were evaluated. Nanoindentation tests with different loading rates were performed under in situ hydrogen (H) charging. Hydrogen charging was carried out using cathodic polarization of samples immersed in a NaCl 3.5 % aqueous solution. For UNS N07718 tests were performed on grains with two different crystallographic orientations. In turn, for the AISI 4130M steel and CMSX-4 alloy, continuous multicycle (CMC) indentation type was carried out. Exclusively for the CMSX-4 alloy, incremental step loading indentations (N-ISL) were performed. It was possible to conclude that the HES evaluation by electrochemical nanoindentation is influenced by the proper selection of the loading rate. The N-ISL profile was efficient for HES evaluation since it favors the H accumulation under the indenter tip, enhancing its effect. Under the investigated experimental conditions all materials are susceptible to hydrogen embrittlement since the following H effects were observed: i) the H permeation increased the AISI 4137M and AISI 4140 steels nanohardness (HIT) in 9.8 % and 15.3 %, respectively, for a 5 mN/min loading rate. ii) The CMC indentation evidenced the H effect on the AISI 4130M steel HIT. In all loading cycles the HIT was reduced between 7 % and 10 % HIT. iii) For the UNS N07718, without the grain identification, the permeation of H under constant potential increased the HIT values in 16.2 % and 15.3 % for 15 mN/min and 480 mN/min loading rates, respectively. iv) Under galvanostatic H permeation and using a 5 mN/min loading rate the UNS N07718 presented a HIT reduction of 8.7 % and 3.6 % for grains with the lowest and the highest Schmid factor, respectively; v) using N-ISL indentations, it was observed that the H reduced the HIT values in 37 %.