Study of the hydrogen diffusion out of press hardened boron steel

Abstract:

This master thesis was performed at the R&D department at Gestamp HardTech AB
in Luleå. Gestamp HardTech is a global leader in the development and manufacturing of safety components for the automotive industry and the production is based on the press hardening technique.

Hydrogen is known to cause degradation of steel and this may be seen as
either a decrease in ductility or as spontaneous fracture for internally or
externally stressed components. Studies of the hydrogen diffusion out of
steels are of great importance for the steel industry as this provides more
knowledge about the hydrogen degradation phenomenon and better possibilities
of preventing problems caused by hydrogen.

The purpose of this thesis was to study how the diffusible hydrogen content
in uncoated, Al-Si pre-coated and zinc electroplated boron steel changes with
time after press hardening. The influence of post heat treatments on the
diffusible hydrogen content was also examined.

In order to study the hydrogen diffusion out of boron steel, hydrogen was
introduced into sample pieces. The introduction methods were electrochemical
charging, charging in a laboratory furnace by varying the dew point
temperature and zinc electroplating. The samples were then stored at room
temperature and heat treated to accelerate the hydrogen diffusion out of the
samples.

Hydrogen charging in a furnace by varying the dew point temperature was
established to be the most suitable charging method. Linear correlations
between the diffusible hydrogen content in the samples and the dew point
temperatures in the furnace were found.

The hydrogen diffusion behavior of press hardened boron steel proved to be
different for uncoated, Al-Si pre-coated and zinc electroplated boron steel.
Post heat treatments were shown to be effective in accelerating the hydrogen
diffusion out of Al-Si pre-coated and zinc electroplated boron steel.