Preheating to avoid hydrogen cracking
Hydrogen cracking may also be called cold cracking or delayed cracking. The principal distinguishing feature of this type of crack is that it occurs in ferritic steels, most often immediately on welding or a short time after welding.
In this issue, the characteristic features and principal causes of hydrogen cracks are described.
Hydrogen cracks can be usually be distinguished due to the following characteristics:
- In C-Mn steels, the crack will normally originate in the heat affected zone (HAZ), but may extend into the weld metal (Fig 1).
- Cracks can also occur in the weld bead, normally transverse to the welding direction at an angle of 45° to the weld surface. They follow a jagged path, but may be non-branching.
- In low alloy steels, the cracks can be transverse to the weld, perpendicular to the weld surface, but are non-branching, and essentially planar.
Fig. 1 Hydrogen cracks originating in the HAZ and weld metal. (Note that the type of cracks shown would not be expected to form in the same weldment.)
On breaking open the weld (prior to any heat treatment), 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 2). 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.
In fillet welds, cracks in the HAZ are usually associated with the weld root and parallel to the weld. In butt welds, the HAZ cracks are normally oriented parallel to the weld bead.
Fig. 2 Crack along the coarse grain structure in the HAZ
There are three factors which combine to cause cracking:
- hydrogen generated by the welding process
- a hard brittle structure which is susceptible to cracking
- tensile stresses acting on the welded joint
Cracking usually occurs at temperatures at or near normal ambient. It 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 to be found in the parent metal. With 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 section components; the risk of cracking is increased if the weld metal carbon content exceeds that of the parent steel.
In low alloy steels, as the weld metal structure is more susceptible than the HAZ, cracking may be found in the weld bead.
The main factors which influence the risk of cracking are:
- weld metal hydrogen
- parent material composition
- parent material thickness
- stresses acting on the weld during welding or imposed (shortly) after welding
- heat input
Weld metal hydrogen content
The principal source of hydrogen is moisture contained in the flux, i.e. the coating of MMA electrodes, the flux in cored wires and the flux used in submerged arc welding. The amount of hydrogen generated is influenced by the electrode type. 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, e.g. from the material, where processing or service history has left the steel with a significant level of hydrogen or moisture from the atmosphere. Hydrogen may also be derived from the surface of the material or the consumable.
Sources of hydrogen will include:
- oil, grease and dirt
- paint and coatings
- cleaning fluids
Parent metal composition
This will have 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.
Parent material thickness
Material thickness will influence the cooling rate and therefore the hardness level, the microstructure produced in the HAZ and the level of hydrogen retained in the weld.
The 'combined thickness' of the joint, ie 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 is likely to have a greater risk than a butt weld in the same material thickness.
Fig.3 Combined thickness measurements for butt and fillet joints
Stresses acting on the weld
Cracks are more likely to initiate at regions of stress concentration, particularly at the toe and root of 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. Poor fit-up (excessive root gap) 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 the weld metal.
Increasing the heat input will reduce the hardness level, and therefore reduce the risk of HAZ cracking. However, as the diffusion distance for the escape of hydrogen from a weld bead increases with increasing heat input, the risk of weld metal cracking is increased.
Heat input per unit length is calculated by multiplying the arc energy by a thermal efficiency factor, according to the following formula:
V = arc voltage (V)
A = welding current (A)
S = welding speed (mm/min)
k = thermal efficiency factor
In calculating heat input, the thermal efficiency must be taken into consideration. The thermal efficiency factors given in EN 1011-1: 2009 for the principal arc welding processes, are:
|MIG/MAG and flux cored wire
|TIG and plasma
In MMA 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, e.g. weave width /dwell.
Bill Lucas prepared this article with help from Gene Mathers and David Abson.
This Job Knowledge article was originally published in Connect, January/February 2000. It has been updated so the web page no longer reflects exactly the printed version.