Welding of high yield strength structural steels

Practical guidelines

The following provides a short summary of important points that should be taken into consideration when welding high strength steels.

•฀ Always investigate if the specification or

component design will permit use of an

under-matching weld metal. Many structures

can be fabricated so that the welded joints are

positioned where stress levels are relatively

low, permitting undermatching welds.

The term “high strength” refers to steels with a

minimum yield strength of around 460 MPa

This group of structural steels generally obtains its

strength level by rapid cooling and is delivered in the

quenched and tempered condition, according to EN

10025-6. This manufacturing process enables high

strength to be achieved with a relatively low alloy

content and thereby low Carbon Equivalent (CE),

which in turn gives good weldability. This thermal

history also produces a microstructure that gives a

combination of high strength and good toughness.

•฀ Only use low hydrogen welding consumables.

•฀ Ensure that safe routines are in place for storage and handling of welding consumables in

order to minimize risk of exposure to moisture.

Coated electrodes packed in a cardboard box

with shrink plastic wrapping must always be

re-dried before use.

•฀ Tack welds and root passes require special

attention, and the needs for preheating should

be carefully observed.

In order to produce weld metal with matching mechanical properties, however, a significantly higher

alloying level than that of the steel is required,

since control of microstructure via the quenching

and tempering operation is not available to the weld

deposit. This results in the weld also having a higher

CE, with consequential increased risk for hydrogen

induced cracking.

As a result, the weld metal is often the controlling

factor when calculating the CE, together with the

critical minimum cooling time t8-5. The cooling

time is determined by the preheat, heat input, joint

design and plate thickness.

•฀ Quenched and tempered steels normally have

an increased CE with increased plate thickness

and welding parameters may have to be adjusted accordingly if different joint thicknesses

arise in the same structure.

•฀ Base welding parameter calculations on the

actual chemical composition of plate and do

not rely on nominal values.

•฀ Cooling rates can differ significantly depending on whether two-dimensional or threedimensional cooling is involved, which in

turn is determined by joint design and plate

thickness. For this reason calculate welding

parameter settings according to EN 1011-2.

•฀ Plate manufacturers often offer software to

facilitate the above calculations.

254