Environmental Engineering Reference
In-Depth Information
tensile strength is 1000 MPa, which can be lower in acidic environments.
Wet hydrogen sulfide environments are considered to be among the most
aggressive in promoting hydrogen entry. Common metals and alloys are
graded according to strength level and/or heat treatment in terms of their
resistance to hydrogen-induced cracking. The steels are generally restricted
to a maximum hardness of 22 HRC (35 HRC for other alloys). In aerospace
applications, steel parts are restricted in strength to below 1380 MPa for
tensile loading applications.
2.
Heat treatment
. For the same stress level, the susceptibility to hydrogen em-
brittlement of steels is dependent on their microstructure. Untempered mar-
tensite is the most susceptible phase. Quenched and tempered microstructures
are more resistant than normalized and tempered ones. Accordingly, the heat
treatment procedure may be selected.
The removal of hydrogen in steels can be carried out by heat treatment
at low temperatures (up to 200
C), a process known as baking. In the absence
of inversible damages inside the material, a baking treatment can restore the
mechanical properties almost to its normal level (Fig. 8.6).
Steel parts having tensile strengths greater than 965 MPa that are processed
in potentially embrittling fluids must receive in-process and final baking for
23 h at 190
°
C in order to reduce hydrogen content. Longer bathing times
may be required for parts that have large cross-sections, bright cadmium
treatments, or very high strength. Hydrogen can be removed from titanium,
zirconium, and their alloys by annealing in vacuum.
°
3.
Alloying additions
. Alloying steels with strong hydride-forming elements
such as titanium, molybdenum, and vanadium reduces susceptibility. How-
ever, the concentration of the alloying element is an important factor. For
example, molybdenum up to about 0.75% reduces susceptibility of AISI
4130 steel to sulfide stress cracking, Beyond this concentration, a tempering
treatment at and above 500
C leads to the precipitation of Mo
2
C and the
resistance to sulfide stress cracking decreases.
°
4.
Proper plating conditions and coatings
. Hydrogen pickup during plating can
be controlled by the proper choice of plating parameters, i.e., the composition
of the bath and the plating current. Cadmium plating and hot-dip galvanizing
should be avoided in the case of very-high-strength steels. Hydriding of tita-
nium can be minimized by anodizing or thermal oxidizing treatments to in-
crease the thickness of the protective oxide film.
5.
Use of inhibitors
. The source of hydrogen pickup by steels is often the pick-
ling operation. Addition of inhibitors to reduce the corrosion of the base
metal can largely decrease hydrogen pickup.
6.
Proper welding practice
. Solutions to the hydrogen embrittlement problems
associated with welding include proper cleaning and degreasing procedures
for prepared weld joints, use of dry electrode, and the maintenance of dry
