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REPRODUCED - COURTESY TWI-UK

Defects - hydrogen cracks in steels - identification

Preaheating to avoid hydrogen cracking 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 after a short time after welding.

In this issue, the characteristic features and principal causes of hydrogen cracks are described.

Identification

Visual appearance

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 are essentially straight, 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.

Hydrogen cracks Fig. 1 Hydrogen cracks originating in the HAZ (note, 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.

Metallography

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 Crack along the coarse grain structure in the HAZ

Causes

There are three factors which combine to cause cracking:
  • hydrogen generated by the welding process
  • a hard brittle structure which is susceptible to cracking
  • residual tensile stresses acting on the welded joint

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 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.

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
  • parent material composition
  • parent material thickness
  • stresses acting on the weld
  • heat input

Weld metal hydrogen content

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. The amount of hydrogen generated is determined mainly 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 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.

Sources of hydrogen will include:

  • oil, grease and dirt
  • rust
  • 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.

equation

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, 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 will have a greater risk than a butt weld in the same material thickness.

Measurements for butt and fillet joints Fig.3 Combined thickness measurements for butt and fillet joints

Stresses acting 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.

Heat input

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:

equation

V = arc voltage (V)
A = welding current (A)
S = welding speed (mm/min)
k = thermal efficiency factor

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:

Submerged arc
(single wire)
1.0
MMA 0.8
MIG/MAG and flux cored wire 0.8
TIG and plasma 0.6

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 eg weave width /dwell.

Bill Lucas prepared this article with help from Gene Mathers and Dave Abson.

Copyright by TWI Ltd, 2000


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