Technical
22 NSC
February 19
Properties of quality
class steel
David Brown of the SCI discusses the specification of steel with improved through
thickness properties. It should be noted that steel with through thickness properties
(so-called “Z grade”) is only needed in high risk situations.
Steel with improved through thickness properties is often referred
to as “Z grade”, although the formal description is ‘Quality class’.
The “Z” is simply because the dimensions in-plane are “x” and “y”
and out-of-plane, through the thickness of the material, is the
“z” direction. The word “improved” is important, as steels to the
EN 10025 Standards will generally have resistance to stress in
the z direction. The common arrangement used to demonstrate
the potential need for improved through thickness properties is
shown in Figure 1 – tensile stress is applied through the ‘incoming’
plates, leading to possible lamellar tearing in the ‘through’ plate.
Lamellar tearing is when the steel in the ‘through’ plate separates
internally.
Internal tearing may be occur due to areas of inclusions or
impurity which can be detected by ultrasonic testing, or when
through thickness loading causes tearing to propagate between
micro imperfections. Micro imperfections cannot readily be
detected by ultrasonic testing, but would be revealed by through
thickness testing to EN 10164.
Material specification
Steel may be examined for the two types of imperfections
mentioned above by specifying certain options at the time of
order. Within EN 10025, which covers the steel sections and plate
normally used in construction, options 6 and 7 apply to plate and
sections with parallel flanges respectively, and require the steel to
be examined for internal defects by ultrasonic testing. If through
thickness properties are required, this must be selected by
specifying option 4, which is testing in accordance with EN 10164.
For rolled sections to EN 10025-2, clause 7.3.3 indicates that
option 4 can only be specified for sub-grades J2 and K2.
If through thickness testing to EN 10164 is specified, this
automatically includes ultrasonic testing to EN 10160 (for plate)
or EN 10306 (for sections) as applicable, so there is no need to
separately specify option 6 or 7.
Through thickness testing
Through thickness testing to EN 10164 requires samples cut
from the plate (or section) to be subject to a tensile force in the
z direction until the sample fractures. The test is examining the
capacity of the steel to ‘neck’ before fracture, which is a measure
of material ductility in the z-axis. The samples are machined to
have a circular cross section, typically of 6 mm or 10 mm diameter,
with a “headed” portion of the form shown in Figure 2, so that it
can be gripped in a testing machine. EN 10164 specifies where the
samples are to be taken – typically at 1/3 of the web depth and
1/3 of the flange outstand (measured from the tip).
The obvious question relates to the testing of thin material –
how can this be prepared in such a way to be gripped in a testing
machine? For thin material, extension pieces are welded to the
sample. Because welding will change the material properties
locally, the original sample must be at least 15 mm thick. To
minimise the effect of the welding, EN 10164 suggests that
extension pieces be friction welded to ensure the heat affected
zone is minimised. Fracture in the weld or heat affected zone
invalidates the results. Extension pieces are mandatory for
samples up to 20 mm thick, optional for samples between 20 and
80 mm thick, and cannot be used for samples thicker than 80 mm.
Three samples are tested and in each case the reduction
Figure 1 – Cruciform joint Figure 2 – Testing sample profile
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/Material_selection_and_product_specification#Through_thickness_properties
/Steel_construction_products#Standard_open_sections
/Construction
/Material_selection_and_product_specification#Fracture_toughness
/Steel_construction_products#Flat_products_-_plates
/Welding