Addressing H-Material Interaction in Fast Diffusion Materials-A Feasibility Study on a Complex Phase Steel

Abstract

Hydrogen embrittlement (HE) is one of the main limitations in the use of advanced high-strength steels in the automotive industry. To have a better understanding of the interaction between hydrogen (H) and a complex phase steel, an in-situ method with plasma charging was applied in order to provide continuous H supply during mechanical testing in order to avoid H outgassing. For such fast-H diffusion materials, only direct observation during in-situ charging allows for addressing H effects on materials. Different plasma charging conditions were analysed, yet there was not a pronounced effect on the mechanical properties. The H concentration was calculated while using a simple analytical model as well as a simulation approach, resulting in consistent low H values, below the critical concentration to produce embrittlement. However, the dimple size decreased in the presence of H and, with increasing charging time, the crack propagation rate increased. The rate dependence of flow properties of the material was also investigated, proving that the material has no strain rate sensitivity, which confirmed that the crack propagation rate increased due to H effects. Even though the H concentration was low in the experiments that are presented here, different technological alternatives can be implemented in order to increase the maximum solute concentration.

Keywords: advanced high-strength steels; hydrogen embrittlement; in-situ testing; plasma charging; scanning electron microscopy.

Figure 1

(a) Secondary electron image; and (b) inverse pole figure in the normal direction of the investigated CP1200 microstructure.

Figure 2

RD boundary conditions of the one-dimensional (1D) hydrogen permeation model. The time history of the sum of the incident ion flux and the recycling flux is applied as boundary condition at the H charging area in the three-dimensional (3D) model.

Figure 3

(a) Stress-strain curves of CP 1200 steel tested strain rates of 3 × 10⁻⁵ s⁻¹, 1.5 × 10⁻⁴ s⁻¹, and 3 × 10⁻⁴ s⁻¹; (b) ln flow stress-ln strain rate plot for the strains indicated in the box in (a), showing no significant strain rate sensitivity.

Figure 4

Fracture surfaces of complex phase (CP) steel tested with strain rates of (a) 3 × 10⁻⁵ s⁻¹; (b) 1.5 × 10⁻⁴ s⁻¹; (c) 3 × 10⁻⁴ s^−1.

Figure 5

(a) Load-indentation depth plots corresponding to jump tests with 0.05 s⁻¹, 0.005 s⁻¹, 0.05 s⁻¹, 0.001 s⁻¹, and 0.05 s⁻¹, with a change in the strain rate every 500 nm; (b) the exemplarily resulting hardness and Young’s modulus.

Figure 6

Load-elongation curves of three CP steel samples: uncharged, 3 h, and 6 h pre-charged.

Figure 7

Fracture surfaces of the (a) uncharged; (b) 3 h pre-charged; and, (c) 6 h pre-charged CP steel samples.

Figure 8

(a) Normalized crack length evolution; (b) crack growth rate of the uncharged, 3 h pre-charged and 6 h pre-charged CP steel samples.

Figure 9

Inverse Pole Figure maps in the normal direction of the (a) uncharged, (b) 3 h pre-charged, (c) 6 h pre-charged CP steel samples near the fracture and (d) misorientation map of 6 h pre-charged CP steel sample.

Figure 10

Load-elongation curves of CP 1200 steel tested under different plasma charging conditions. Power levels of 5 W, 6 W, 8 W, and 11 W were applied for running the plasma.

Figure 11

Fracture surfaces of the CP steel specimens tested with (a) 5 W; (b) 6 W; (c) 8 W; and, (d) 11 W. The red box in (c) indicates an area with shear fracture or specially oriented grains.

Figure 12

Load-elongation curves of an uncharged CP 1200 steel sample and one initially charged for 4 h and tested after discharging for additional 12 h in vacuum.

Figure 13

Fracture surfaces of the (a) uncharged and (b) charged and discharged CP steel specimens.

Figure 14

(a) H concentration distribution through the thickness of the CP steel sample, the arrows indicate the direction of H charging from the bottom of the samples; and, (b) H flux over time in an evaluation node on the top surface of the sample.