Specification
NSC 7
Technical Digest 2019
Eurocode requirements
A procedure to determine if improved through thickness properties are
required is given in Section 3 of BS EN 1993-1-10. Readers should note
that there is little enthusiasm in the UK for this procedure, and alternative
guidance is given in PD 6695-1-10. Despite the UK position, the guidance in
BS EN 1993-1-10 establishes important principles, reinforced by the PD. The
Eurocode notes that:
• The strain through the thickness of the material arises as welds to the
surface (see Figure 1) cool and shrink. If that shrinkage is restrained by other
stiff parts of the assembly, it is clear that the possibility of lamellar tearing
increases,
• Larger welds increase the possibility of tearing,
• Thoughtful weld detailing can reduce the risk, for example by avoiding
fusion faces which are parallel to the surface of the steel,
• The sulphur content in the steel is important – lower levels improve the
through thickness properties of the steel.
The procedure in BS EN 1993-1-10 is essentially a scoring system based on
a number of contributing factors. Criteria that increase the risk are awarded a
higher score, those that reduce the risk given a lower or negative score. The
required Z quality class (Table 1) must be greater than the summation of the
individual scores. Some examples illustrate the features of the system:
A fillet weld throat 5 mm scores zero, a throat of 14 mm scores 6. The table
includes fillet welds up to a 35 mm throat with a score of 15, but would be
unusual, one hopes!
Welds where the fusion faces are not parallel to the surface (Figure 3a) score
-25 (indicating that these are not a problem). Welds made to the surface of the
steel (Figure 3b) score 5, or 8, depending on the detail.
Thicker material, which provides more restraint, scores between 2 for 10 mm
material and 15 for 70 mm material.
Perhaps surprisingly, the degree of restraint offered by other portions of the
assembly is not so significant – a score of zero for low restraint to (a mere) 5 for
high restraint. The most significant contributions are therefore the weld size,
the thickness of the material and the joint type.
Guidance in PD 6695-1-10
The UK guidance is that through thickness testing is expensive, often
unnecessary, and should only be specified in ‘high-risk’ situations. High-risk
situations, illustrated in Figure 4, are identified as:
• Tee joints with butt welds where the thickness of the ‘incoming’ material
is greater than 35 mm, or if fillet welded the throat is greater than 35 mm
(again, a notable fillet weld!)
• Cruciform joints with butt welds where the thickness of the ‘incoming’
material is greater than 25 mm, or if fillet welded the throat is greater than
25 mm (still notable!)
In these high risk situations, the specification of quality class Z35 is
recommended. If Z35 material cannot be readily obtained, then the sulphur
content should be limited to 0.005%. This is significantly lower than the
maximum specified in BS EN 10025-2, which is typically 0.03%.
In addition, weld volume should be minimised by avoiding overspecification
– which is sensible advice in all situations. Both the designer
and steelwork contractor can contribute here: the designer by not specifying
conservative forces for the connection design and the steelwork contractor by
making a careful choice of joint preparation.
PD 6695-1-10 notes that steel with low sulphur levels is likely to have
improved through thickness properties (Z25 or even Z35) as a matter of
course. The sulphur levels which have such a significant influence on through
thickness properties may be verified by looking at the mill certificates. The PD
also lists a series of practical measures to reduce the risk of lamellar tearing.
These measures are primarily for the steelwork contractor and reflect the
contributions to the overall risk score noted above. Practice to reduce the risk
includes:
• Avoiding weld details where the fusion face is on the surface of the material.
• Managing the assembly of fabricated items to reduce restraint on
subsequent welds.
• Minimising shrinkage of the welds by process control.
• Ordering steel with lower maximum sulphur levels, or purchasing steel from
suppliers known to produce ‘cleaner’ steel.
Conclusions
In Western and other developed countries, steel is likely to be ‘clean’ (low
sulphur), the steelwork contractors undertaking complex welding of large
assemblies are likely to be highly experienced and the welding operations will
be managed by a Responsible Welding Coordinator (an essential individual
for the production of CE Marked steelwork). In these circumstances improved
through thickness properties need only be specified for the high risk situations
noted above.
Figure 3: Joint types
Figure 4: ‘High risk’
situations