Application of NH4SCN Aqueous Solution to Hydrogen Embrittlement Resistance Evaluation of Ultra-high Strength Steels

Abstract

KEY WORDS: delayed fracture; immersion-type hydrogen charging method; ammonium thiocyanate. BackgroundIt is well known that the risk of hydrogen embrittlement fracture becomes significant when ultra-high strength steels (UHSS) with tensile strength over 980 MPa are used in atmospheric corrosion environments.1,2) Recently, application of UHSS sheets to automobiles has increased as a means of reducing auto body weight.3) Although the hydrogen embrittlement resistance of UHSS sheets has been evaluated by various methods, [4][5][6][7][8][9][10][11][12] it is difficult to predict the risk of hydrogen embrittlement fracture quantitatively. In order to expand the automotive applications of UHSS sheets, precise prediction of the risk of hydrogen embrittlement fracture is important.For evaluation of hydrogen embrittlement resistance, hydrogen must be charged to specimens. The immersiontype hydrogen charging method is more convenient than cathodic charging because a large number of specimens and automotive parts with various complex dimensions can be easily and uniformly charged with hydrogen at the same time simply by immersing the specimens in a solution. In contrast, the cathodic charging method requires various equipments, such as a power supply and electrodes, and the current density distribution in a specimen varies depending on the specimen shape. The latter point is important because the hydrogen charging condition in a specimen is considered to change depending on the local current density.On the other hand, the immersion test also has a demerit, in that specimens dissolve during immersion because strong acids with pH in the range of 1 to 3 are generally used. Dissolution of the specimen surface causes changes in surface stress and microstructure conditions during the test. For example, surface stress is reduced and microcracks disappear. Therefore, if the immersion test is applied to hydrogen embrittlement resistance evaluation, it is very important to inhibit dissolution during testing and to understand the effect of any dissolution on hydrogen embrittlement resistance.Recently, an ammonium thiocyanate (NH4SCN) solution with a concentration of several percent has been used in hydrogen charging tests.13) NH4SCN is well known as a catalyst in electrical hydrogen charging methods, and hydrogen embrittlement of bar steels is evaluated by the Fédération Internationale de la Précontrainte (FIP) test method, in which specimens are immersed in a 20% NH 4 SCN solution at 323 K.14) The pH of the 20% NH 4 SCN solution is reported around 4-5, and low specimen dissolution is predicted due to higher pH than conventional hydrogen charging solutions as HCl with pH 1. This suggests the possibility of using a NH 4 SCN solution in immersion tests for evaluation of hydrogen embrittlement in UHSS sheets with substantially reduced specimen dissolution. In this study, a low-dissolution hydrogen charging method was proposed. Experimental ProcedureAs test materials, 1180 MPa grade ferrite-martensite dualphase steel (1180DP) was used. T...

Hydrogen Embrittlement Resistance Evaluation of Ultra High Strength Steel Sheets for Automobiles

“…Our research group has been evaluating the hydrogen embrittlement resistance of UHSS sheets by various methods. [4][5][6][7][8][9][10][11] This paper introduces those results, and suggests the scope of future work in this field with the aim of promoting the application of UHSS sheets.…”

Method of Evaluating Delayed Fracture Susceptibility of Tempered Martensitic Steel Showing Quasi-Cleavage Fracture

“…The weight loss of steels in a 20 mass pct NH 4 SCN aqueous solution at a temperature of 323 K (50°C) was almost 0.001 g cm À2 after 20 hours, [20] which was smaller than that in an aqueous solution of HCl of 3.0 in pH at a temperature of 298 K (25°C) [7] by nearly one order of magnitude. In this study, the corrosion rate must be smaller than that study [7] since a 7 mass pct NH 4 SCN aqueous solution at a temperature of 303 K (30°C) was employed. The reduction of surface roughness probably led to increased fracture strength.…”

“…Though the corrosion rate was not measured in this study, the weight loss of steels in a 0.1 mass pct NH 4 SCN aqueous solution at a temperature of 298 K (25°C) [7] and in a 20 mass pct NH 4 SCN aqueous solution at a temperature of 323 K (50°C) [20] was reported. The weight loss of steels in a 20 mass pct NH 4 SCN aqueous solution at a temperature of 323 K (50°C) was almost 0.001 g cm À2 after 20 hours, [20] which was smaller than that in an aqueous solution of HCl of 3.0 in pH at a temperature of 298 K (25°C) [7] by nearly one order of magnitude. In this study, the corrosion rate must be smaller than that study [7] since a 7 mass pct NH 4 SCN aqueous solution at a temperature of 303 K (30°C) was employed.…”

“…This result indicates that the states of hydrogen present in the specimens were identical even though different hydrogen charging methods were employed, which is consistent with the previous papers. [7,8] The hydrogen contents charged by the immersion and cathodic electrolysis methods under the conditions used were measured to be 1.3 and 0.9 mass ppm, respectively. The scattering of the hydrogen concentration under the same charging condition was ±0.15 mass ppm.…”

“…Regardless of the type of hydrogen charging method used, the states of hydrogen present in steels are identical under the same charging temperature and hydrogen content. [7,8] However, immersion methods change the surface states of steels [8] and cause the blunting of notches [6] due to corrosion and dissolution, which affect fracture strength in delayed fracture tests. Hydrogen charging and stress application procedures differ among researchers.…”