Hydrogen cracking also known as cold cracking or delayed cracking. The main feature of this type of crack is that it occurs in ferritic weldable steels, and generally occurs immediately on welding or after a short time after welding, but usually within 48hrs.
Visual appearance of Hydrogen Cracking
Hydrogen cracks can be usually have the following characteristics:
On breaking open the weld, the surface of the cracks will normally not be oxidised, even if they are surface breaking, indicating they were formed when the weld was at or near ambient temperature. A slight blue tinge may be seen from the effects of preheating or welding heat.
Cracks, which originate in the HAZ, are usually associated with the coarse grain region, (Fig 1). The cracks can be intergranular, transgranular or a mixture. Intergranular cracks are more likely to occur in the harder HAZ structures formed in low alloy and high carbon steels. Transgranular cracking is more often found in C-Mn steel structures.
There are three factors, which can cause hydrogen cracking:
Cracking is caused by the diffusion of hydrogen to the highly stressed, hardened part of the weldment.
In C-Mn steels, because there is a greater risk of forming a brittle microstructure in the HAZ, most of the hydrogen cracks are likely to be found in the parent metal. Using the correct choice of electrodes, the weld metal will have a lower carbon content than the parent metal and, hence, a lower carbon equivalent (CE). However, transverse weld metal cracks can occur especially when welding thick sections.
In low alloy steels, as the weld metal structure is more susceptible than the HAZ, cracking may be found in the weld bead.
The effects of specific factors on the risk of cracking are::
Weld metal hydrogen content
One of the principal source of hydrogen is the moisture contained in the flux ie the coating of MMA electrodes, the flux in cored wires and the flux used in submerged arc welding. Mainly the electrode type determines the amount of hydrogen generated. Basic electrodes normally generate less hydrogen than rutile and cellulosic electrodes.
It is important to note that there can be other significant sources of hydrogen eg moisture from the atmosphere or from the material where processing or service history has left the steel with a significant level of hydrogen. Hydrogen may also be derived from the surface of the material or the consumable, or from oil and paint etc,.
Sources of hydrogen include:
Parent metal composition
This has a major influence on hardenability and, with high cooling rates, the risk of forming a hard brittle structure in the HAZ. The hardenability of a material is usually expressed in terms of its carbon content or, when other elements are taken into account, its carbon equivalent (CE) value.
The higher the CE value, the greater the risk of hydrogen cracking. Generally, steels with a CE value of <0.4 are not susceptible to HAZ hydrogen cracking as long as low hydrogen welding consumables or processes are used.
Material thickness will influence the cooling rate and therefore the hardness level, microstructure produced in the HAZ and the level of hydrogen retained in the weld.
The 'combined thickness' of the joint, i.e. the sum of the thicknesses of material meeting at the joint line, will determine, together with the joint geometry, the cooling rate of the HAZ and its hardness. Consequently, as shown in Fig. 3, a fillet weld will have a greater risk than a butt weld in the same material thickness.
Stresses which act on the weld
The stresses generated across the welded joint as it contracts will be greatly influenced by external restraint, material thickness, joint geometry and fit-up. Areas of stress concentration are more likely to initiate a crack at the toe and root of the weld.
Poor fit-up in fillet welds markedly increases the risk of cracking. The degree of restraint acting on a joint will generally increase as welding progresses due to the increase in stiffness of the fabrication.
The heat input to the material from the welding process, together with the material thickness and preheat temperature, will determine the thermal cycle and the resulting microstructure and hardness of both the HAZ and weld metal.
A high heat input will reduce the hardness level.
Heat input per unit length is calculated by multiplying the arc energy by an arc efficiency factor according to the following formula:
V = arc voltage (V)
In calculating heat input, the arc efficiency must be taken into consideration. The arc efficiency factors given in BS EN 1011-1: 1998 for the principal arc welding processes, are:
In MMA or stick welding, heat input is normally controlled by means of the run-out length from each electrode which is proportional to the heat input. As the run-out length is the length of weld deposited from one electrode, it will depend upon the welding technique eg weave width /dwell.
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