25 May

Cold cracking, consumables and high yield strength steels

Alain Laurent, business developer and welding metallurgist for Lincoln Electric’s Oerlikon brand of welding consumables, talks to African Fusion about the company’s consumable range for welding modern high yield strength steel (HYSS).

Products such as heavy mobile equipment; mobile, harbour and ship cranes and steel structures in the petrochemical, oil and gas and offshore industries are making increasing use of modern steels with high yield strength. “There is no real definition of a high yield strength steel, but it is generally accepted to refer to steels with a yield strength greater that 450 MPa and we are now looking at welding steels that yield at 690 MPa and higher,” says Laurent.

The crane and lifting industry, which needs the highest strengths possible while keeping the weight to a minimum, is one of the development drivers for these new steels. “Without compromising quality and safety, these steels enable the mass and volume used to be reduced, which significantly reduces the costs of the structures involved,” he tells African Fusion.

“When welding these steels, however, we need to take precautions in order to prevent hydrogen-induced cold cracking,” he says.

Preventing cold cracking

Three key factors combine to cause cold cracking: The first is the internal stress caused by restraint while welding, which is linked to the weld profile, most notably the thickness of the section. “As a consumable manufacturer, we can’t do very much to mitigate against this factor,” says Laurent.

“The second factor is the microstructure of the weld and heat-affected zone (HAZ), says Laurent, displaying a weld macrograph with a martensitic appearance in the HAZ. “When high cooling rates or quenchability prevail during and after welding, then brittle microstructures can form, which can easily become crack initiation sites,” he explains. “Here, the solution is to carefully control the heat input while welding, along with preheat and interpass temperatures and, after welding, the cooling rate,” he advises.

The third factor is hydrogen itself, which can be controlled via consumable design, by limiting the hydrogen level remaining in welding consumables after manufacture, preventing atmospheric hydrogen entering the flux through moisture pick-up, as well as mitigating against atmospheric conditions such as relative humidity and moisture on metal plates.

In almost all cases, low hydrogen consumables must be used when welding these steels in order to keep the levels of diffusible hydrogen in the weld metal and HAZ after welding below the crack initiating threshold.

Oerlikon manufactures an advance range of consumables for shielded metal arc welding (SMAW), submerged arc welding (SAW), flux-cored arc welding (FCAW) and solid wires for gas metal arc welding (GMAW) that have been carefully designed to help fabricators to overcome cold cracking problems. “In particular, we have recently released some advanced seamless flux-cored wires that are ideally suited to automatic welding in the offshore, petrochemical and heavy construction sectors,” he notes.

Focusing on the link between diffusible hydrogen (DH) in weld metal and the consumable, Laurent says that the hydrogen source can be induced by different parameters, but the consumable’s humidity content is the principle source.

He points to a graph showing how diffusible hydrogen in weld metal increases when consumables used are exposed to atmospheric humidity after removing the wrapping. In relative humidity (RH) of 80% at 27 °C, low-hydrogen consumables will contain over 11 g of moisture – the level associated with 5.0 ml/100 g of diffusible hydrogen in the weld metal – in less than 10 minutes. And within 90 minutes, 18 g of moisture, leading to between 7.0 and 9.0 g of diffusible of hydrogen will be absorbed.

Hence the need to be vigilant about the atmospheric conditions and the exposure times of consumables in those conditions after packages are opened, “to keep moisture pick-up as low as possible”.

Oerlikon solutions to help fabricators control moisture pick up include: Dry bag packaging for SAW fluxes; and vacuum packed medium and micro packaging for its SMAW electrodes, which have 24-hour permeability values of less than 0.005 g/m2. “In formulating sub-arc fluxes, electrode coatings and flux-cores, we strive to use raw materials with low hydroscopic levels, along with industrial baking procedures that expel moisture,” notes Laurent.

Lifting out the Oerlikon range of seamless flux-cored wires, he says that vacuum packaging is not necessary, since an impermeable metal sheath protects the flux. “Seamless cored technology is ideal for welding HYSS, because there is no folded seam to allow moisture to enter the flux,” he notes.

Oerlikon consumables for HYSS

Highlighting its purpose-designed range of consumables for welding high strength steels, Laurent begins with Oerlikon’s TENACITO 80CL double-coated stick electrode innovation, which has a current-conducting coating enclosed by an additional non-current-conducting ‘cover’.

“This is an excellent technology with many advantages. It is less sensitive to magnetic arc blow, more tolerant to poor joint preparation – narrow bevels, wide root gap, badly aligned plates – offers less porosity and undercut and better penetration,” he says…

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