Civil Engineering Reference
In-Depth Information
establish notch ductility or material toughness based on energy absorbed at different
test temperatures. CVN testing is done at a rapid load rate, so adjustments are made
to the specified test temperature to account for the greater ductility associated with
the slower strain rate application of railway traffic. For design purposes, temperature
service zones are established with a specified minimum energy absorption at a spec-
ified test temperature for various steel types and grades. CVN requirements are often
specified independently for FCM and non-FCM.
Tables 2.1
and
2.2
show the specified
CVN test requirements for steel railway bridges recommended by AREMA (2008).
2.2.4 W
ELDABILITY
If the carbon content of steel is
<
0.30%, it is generally weldable. Higher-strength
steels, where increased strength is attained through increased carbon and manganese
content, will become hard and difficult to weld. The addition of other alloy elements
to increase strength (Cr, Mo, and V) and weathering resistance (Ni and Cu) will also
reduce the weldability of steel.
The weldability of steel is estimated from a carbon equivalency equation, given as
Mn
+
Si
Ni
+
Cu
Cr
+
Mo
+
V
CE
=
C
+
+
+
,
(2.6)
6
15
5
where C, Mn, Si, Ni, Cu, Cr, Mo, andV are the percentage of elemental carbon, man-
ganese, silicon, nickel, copper, chromium, molybdenum, and vanadium in the steel,
respectively. Carbon equivalence (CE) of about 0.5% or greater generally indicates
that special weld treatments may be required.
Weld cracking generally results from resistance to weld shrinkage upon cooling.
Thickerelementsaremoredifficulttoweld.Preheatandinterpasstemperaturecontrol,
in conjunction with the use of low hydrogen electrodes, will prevent welding-induced
hardening and cracking.
Modern structural steels have been developed with excellent weldability.
∗
The
increase in weldability enables limited preheat requirements and postweld treat-
ments (translating into fabrication savings), and may eliminate hydrogen-induced
weld cracking.
2.2.5 W
EATHER
R
ESISTANCE
Atmospheric corrosion-resistant (weathering) steel chemistry (chromium, copper,
nickel, and molybdenum alloys) is such that a thin iron oxide film forms upon initial
wetting cycles and prevents the further ingress of moisture. This type of corrosion
protection works well where there are alternate wetting and drying cycles. It may not
be appropriate in locations where deicing chemicals and salts are prevalent, in marine
environments, or where there is a high level of sulfur content in the atmosphere.
Weldability is slightly compromised because CE is raised through the addition of
alloy elements for weathering resistance. However, these steels have about 4 times
∗
For example, HPS for bridges such as ASTM A709 HPS 50W, 70W, and 100W.
