CEMP-E
TI 809-26
1 March 2000
2. WELDABILITY OF STRUCTURAL STEELS.
a. Chemical Composition. The chemical composition of the steel affects weldability and other
mechanical properties. Several elements are purposefully added in the production of structural steel, but
other undesirable elements may be present in the scrap materials used to make the steel. Carbon and
other elements that increase hardenability increase the risk of "cold" cracking, and therefore higher
preheat and interpass temperatures, better hydrogen control, and sometimes postheat are necessary to
avoid cold cracking.
(1) Carbon (C) is the most common element for increasing the strength of steel, but high levels of
carbon reduce weldability. Carbon increases the hardenability of the steel, increasing the formation of
undesirable martensite with rapid HAZ cooling. Higher preheats and higher heat input welding
procedures may be needed when welding a steel with relatively high carbon contents. Typical steel
specifications limit carbon below 0.27%, but some steel specifications have much lower limits.
(2) Manganese (Mn) is an alloying element that increases strength and hardenability, but to a
lesser extent than carbon. One of the principal benefits of manganese is that it combines with
undesirable sulphur to form manganese sulfide (MnS), reducing the detrimental effects of sulfur. With
high levels of sulfur, however, numerous large MnS inclusions may be present, flattened by the rolling
operation, increasing the risk of lamellar tearing when high through-thickness weld shrinkage strains are
created. Manganese limits are typically in the order of 1.40% or lower. A steel such as A36 does not
place limits on Mn content for shapes up to 634 kg/m (426 lb./ft.), or for plates and bars up to 20 mm (3/4
in.), inclusive.
(3) Phosphorous (P) is an alloying element that increases the strength and brittleness of steel.
Larger quantities of phosphorous reduce ductility and toughness. Phosphorous tends to segregate in
steel, therefore creating weaker areas. Phosphorous is typically limited to 0.04% to minimize the risk of
weld and HAZ cracking.
(4) Sulfur (S) reduces ductility, particularly in the transverse direction, thereby increasing the risk of
lamellar tearing, and also reduces toughness and weldability. Higher sulfur levels will form iron sulfide
(FeS) along the grain boundaries, increasing the risk of hot cracking. Manganese is used to form MnS to
reduce this tendency. A minimum Mn:S ratio of 5:1 to 10:1 is recommended. Typical steel specifications
limit sulfur to 0.05%.
(5) Silicon (Si) is a deoxidizer used to improve the soundness of the steel, and is commonly used
to "kill" steel. It increases both strength and hardness. Silicon of up to 0.40% is considered acceptable for
most steels.
(6) Copper (Cu) is added to improve the corrosion resistance of the steel, such as in weathering
steels. Most steels contain some copper, whether specified or not. When specified to achieve
atmospheric corrosion resistance, a minimum copper content of 0.20% is required. Generally, copper up
to 1.50% does not reduce weldability, but copper over 0.50% may affect mechanical properties in heat-
treated steels.
(7) Nickel (Ni) is an alloying element used to improve toughness and ductility, while still increasing
strength and hardenability. It has relatively little detrimental effect upon weldability. Where nickel is
reported as a part of steel composition, it is generally limited to a maximum value between 0.25% and
0.50%.
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