Page 182 - CW E-Magazine (1-4-2025)

Special Report

from the surface to the interior of steel
can lead to the following types of
failures (Fig.2):
Hydrogen Blistering (HB): Blister
formation in low-strength steels ini-
tiated in pores/cracks and interface
of non-metallic inclusions, such as
sulphides, and the matrix;
Hydrogen Induced Cracking (HIC),
also known as Step Wise Cracking Fig. 4: Step wise microcracks over a time period grow, join, and become large in size leading to
(SWC); bigger blisters.
Sulphide Stress Cracking (SSC) grow, join, and become large in size, by SSC. SOHIC is characterized by
occurring in steels of high strength/ leading to bigger blisters (Fig. 4). interlinking microscopic cracks oriented
hardness; and both in the direction perpendicular to
Stress-Oriented Hydrogen-Induced Hydrogen embrittlement (HE)/(SSc) that of the stress and in the plane
Cracking (SOHIC). The former term is more generic, defined by non-metallic inclusions.
while the latter refers to HE in sulphide
HB and SWC/HIC are similar in containing environments. Sulphides Process variables affecting hydrogen-
the sense that the hydrogen atoms form promote hydrogen uptake of steels. In related cracking phenomena
molecular hydrogen at pores, cracks, and these cases, the hydrogen remains in Presence of water is essential to all
inclusion/matrix interface and develop the lattice in atomic form and promotes the above four categories of hydrogen
internal pressure. When the pressure brittleness through several mecha- damage, since hydrogen is produced
reaches a critical value, micro-cracks nisms. Notably, higher the strength from water through cathodic part of the
develop and grow. As these defects are and hardness of steel/alloy, higher corrosion reaction.
generally planar in nature, these cracks will be its susceptibility to cracking.
look like steps, leading to the term Steel welds are more prone to HE/SSC Generally, wet hydrogen attack
SWC(Fig.3).These cracks over time as they exhibit residual tensile stresses does not occur in neutral environments.
as well as high hardness. The environment needs to be acidic (pH
below 4) if only H S is present and even
2
SoHIc alkaline environments (pH above 8)
In simple terms, SOHIC has mixed can be effective if dissolved cyanide is
characters of HB and HE. So applied also present.H S levels above 50 ppm
2
stresses as well as stresses developed are normally required for the attack.
by molecular hydrogen are responsible
for cracking. Notably, SOHIC tends to Most wet hydrogen-related attack
occur in the base metal region adjacent occurs at operational temperatures bet-
Fig. 3: defects/cracks generally planar in to hard weldments, mostly in plate ween ambient and about 150°C. SSC is
nature looking like steps (SWc Microcracks). steels, where cracking might initiate quite common around 80°C.
table 1: controlling low temperature hydrogen-induced cracking
HE/SSc SWc/HIc/HB
Lower the tensile stresses. Reduce the proportion of non-metallic inclusions
by lowering the sulphur (S) content.
Providing right tempering heat treatment of steel to eliminate detrimental Modify the morphology of S segregation by
effect of martensite. For aluminium alloys, provide an over-aging adding calcium (Ca) which spheroidize non-
treatment. metallic inclusions.
Prefer presence of Cu (>0.2%) as alloying element.
Prefer plate mill products over hot strip mill products for steel.
Avoid hydrogen entry into steel during surface treatment and fabrication.
If not possible, bake the components to remove hydrogen.

182 Chemical Weekly April 1, 2025

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