Page 20 - SSAB Welding Handbook Edition 2

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4.0 Hydrogen cracks Welding handbook

Hydrogen crack Unaffected The purpose of these studies is to attain optimized
Weld metal metal
recommendations with regard to the individual
©SSAB
characteristics of each Hardox and Strenx grade.

As a compliment to the recommendations from SSAB,
other general models can be used for the assessment
of hydrogen cracks in different types of high strength
Initiation point of Hydrogen crack steels. The established models describe the resistance
hydrogen crack to hydrogen cracks for a certain plate according to its
Fig. 4.2: A surface breaking hydrogen crack seen transverse to, and carbon equivalent, which is calculated from the
above the joint. The extension of these cracks can be relatively long. chemical content of the plate. A lower carbon
A length of up to several dm is not unusual, and surface breaking
cracks can often be identified visually. equivalent value corresponds to a higher resistance
to hydrogen cracks.

In general, cracks located in the weld metal are Several models for the carbon equivalent exist and each
related to welding with high strength consumables. formula is derived from studies based on specific
If cracks of this kind occur, the crack propagation is steels. The most common international carbon
typically restricted to the weld metal. Their equivalents are according to the CET and CEV models.
orientation is most commonly perpendicular to the SSAB prefers the CET formula for the Hardox and
direction of the joint. The cracks are usually small Strenx grades because this carbon equivalent is
and narrow and they can either be embedded or especially designed to suit high strength steels. The
surface breaking. Their presence can be hard to find CEV formula is also valid for high strength steels;
with non-destructive methods due to their small size. however, its carbon equivalent focuses on unalloyed
and low-alloyed steels with lower strengths than the
Hardox and Strenx grades. Formulas for the
4.2 The influence of alloy elements calculation of the CET and CEV value are defined
in the steel below. A synonymous name for CEV is CE. When
a carbon equivalent value is calculated, the alloy
The Hardox and Strenx grades are high strength content stated on the inspection certificate of the plate
steels with relatively low levels of alloys. However, the is to be used. All alloy elements are stated by their
amounts of alloy elements that are added to a specific weight percentage in the formulas presented below.
steel are crucial to achieving the required mechanical
properties in the parent metal. The alloy content, in CET=C+ (Mn+Mo)/10+ (Cr+Cu)20+ Ni/40 [%] Formula 4.1
general terms, rises with larger plate thicknesses for
each steel grade. This is because higher levels of alloy CEV= C+ (Cr+Mo+V)/5 +Mn/6+ (Ni+Cu)/15 [%] Formula 4.2
elements are, in general terms, required in order to
achieve the desired mechanical properties. Calculating a carbon equivalent is a mean for
determining if – and to what level – preheating is
Certain alloys can promote the formation of hydrogen needed in order to avoid hydrogen cracks. The
cracks. As these elements increase in content level, so methods for determining the preheat temperature
does the sensitivity of the steel, making more welding in this context are the CET method and the CEV
restrictions necessary in order to minimize the risk of method which refers to the respective carbon
hydrogen cracks. equivalents. These two methods are described in
the European Standard EN1011-2.
The recommendations for the prevention of hydrogen
cracks in the Hardox and Strenx grades are based on SSAB has special recommendations for preventing
careful evaluations performed by SSAB. hydrogen cracks because the other methods
typically result in measures that are too conservative
for the Hardox and Strenx grades. It is very likely
20 This handbook contains general suggestions and information without any expressed or implied warranty of any kind. SSAB hereby expressly disclaims
all liability of any kind, including any damages, in connection with the use of the information and for their suitability for individual applications.
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.