WELDABILITY OF STRUCTURAL STEELS.
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 heattreated 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%.
(8) Vanadium (V) is an alloying element used for increasing strength and hardenability. Weldability
may be reduced by vanadium. When vanadium is reported as a part of steel composition, vanadium is
generally limited to a maximum value between 0.06% and 0.15%.
(9) Molybdenum (Mo) is an alloying element which greatly increases hardenability and helps
maintain strength and minimize creep at higher temperature. When molybdenum is reported as a part of steel composition, it is generally limited to a maximum value between 0.07% and 0.10%.
(10) So-called “tramp” elements such as tin (Sn), lead (Pb), and zinc (Zn), may be present in steel
from the scrap material melted for steel-making. They have a low melting point, and may adversely
affect weldability and cause “hot” cracking. Other low-melting point elements that create a risk of hot
cracking include sulfur, phosphorous, and copper. When welding with high levels of these elements, it may be necessary to use low heat input welding procedures to minimize dilution effects.
b. Carbon Equivalency. The weldability of a steel can be estimated from its composition, using a
calculation system termed the carbon equivalent (CE). The most significant element affecting weldability is carbon. The effects of other elements can be estimated by equating them to an additional amount of carbon. The total alloy content has the same effect on weldability as an equivalent amount of carbon.
There are numerous carbon equivalent equations available and in use.
(1) The following equation is used in AWS D1.1 Annex XI.
CE = C + Mn/6 + Cr/5 + Mo/5 +V/5 + Ni/15 + Cu/15 + Si/6
Where C = carbon content (%)
Mn = manganese content (%)
Cr = chromium content (%)
Mo = molybdenum content (%)
V = vanadium content (%)
Ni = nickel content (%)
Cu = copper content (%)
Si = silicon content (%)
A carbon equivalent of less than 0.48 generally assures good weldability.
(2) Another common carbon equivalent equation is:
CE = C + Mn/6 + Cr/10 + Ni/20 + Cu/40 - V/10 - Mo/50.
If the CE from this equation is below 0.40, the material is considered readily weldable, and AWS D1.1 Table 3.2 guidance for the given steel strength should be adequate. For values between 0.40 and 0.55, the use of preheat and low-hydrogen electrodes is generally necessary, regardless of thickness. Carbon equivalent values above 0.55 indicate a high risk that cracks may develop unless special precautions are implemented.
(3) The Dearden and O’Neill equation, applicable for steels with C greater than 0.12%, is similar:
CE = C + Cr/5 + Mo/5 + V/5 + Mn/6 + Ni/15 + Cu/15
A CE of 0.35% or lower is considered a steel with good weldability
(4) For steels with C between 0.07% and 0.22%, the Ito and Bessyo equation may be used.
The Ito-Bessyo equation is also termed the composition-characterizing parameter, Pcm.
CE = C + 5B + V/10 + Mo/15 + Mn/20 + Cu/20 + Cr /20 + Si/30 + Ni/60
Where B = boron content (%)
A CE of 0.35% or lower is considered a steel with good weldability.
(5) The Yurioka equation may also used to calculate CE for steel with C between 0.02% and
0.26%, as follows:
CE = C + A(C) * {5B + Si/24 + Mn/6 + Cu/15 + Ni/20 + Cr/5 + Mo/5 + Nb/5 + V/5}
Where Nb = niobium content (%)
A(C) = 0.75 + 0.25 * tanh [ 20 (C- 0.12) ]
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