Avaliação da susceptibilidade à fragilização por hidrogênio em aços de alta resistência e baixa liga usando ensaios de puncionamento esférico em meio assistido

Abstract

In this work it was evaluated the hydrogen embrittlement susceptibility (HES) of high strength low alloy steels (HSLA) used in the oil and gas offshore industry by means of small punch test (SPT). To evaluate the HES by using this technique, tests with in situ hydrogen charging were performed and its effects on the mechanical properties were investigated. For this purpose, three lines of work were followed: i) evaluation of SPT loading velocity effect on mechanical properties of 4130M, 4137M and 4140 steels through a single factor design on two levels (1.0 µm/s and 0.1 µm/s); ii) development of a methodology to evaluate the threshold force (P_th) by means of SPT, loaded in incremental steps (ISPT) using specimens with and without notch; iii) optimization of the yield strength calculation model in SPT. The results obtained showed that: i) an increase in the HES was observed when the test velocity was reduced from 1.0 µm/s to 0.1 µm/s, however, the test velocity factor caused statistically significant effects only on the tensile strength of 4137M steel, leading to a decrease of approximately 14.1 % in this property value; ii) the step time reduction from 2 h / 4 h to 20 min / 40 min did not affect the P_th values or the force x displacement curves. The P_(th_SPT) values obtained from the tests with notched specimens showed systematic errors lower than -3.0 %, when compared to those obtained with the conventional ISL tests, indicating excellent results accuracy. The methodology proposed to calculate P_th based on the material specimen stiffness proved to be robust, efficient and easy to implement, allowing the effects of yield and beginning of crack propagation to be separated during the ISPT; iii) the new proposed equation to determine the yield strength of steels with yield higher than 800 MPa using SPT exhibited errors lower than 5 % when compared to values obtained by conventional tensile test. When compared with the literature, the reduction in error values was of the order of up to 83%.