During welding, cracks may occur in some type of steels due to the presence of Hydrogen. The technical name for this type of cracking is...
- Cold cracking - cracks occur when the weld has cooled down.
- HAZ cracking - cracks tend to occur mainly in the HAZ.
- Delayed cracking - cracks may occur some time after welding has finished (possibly up to ~48h).
- Underbead cracking - cracks occur in the HAZ beneath a weld bead.
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.
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 maybe 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
(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 oxidized, 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 that 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.
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| Fig. 2 Crack along with the coarse grain structure in the HAZ |
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.
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
- 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.
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
- heat input
Weld metal hydrogen content
The principal source of hydrogen is the 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. 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.
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
The 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.
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| 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, 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 an arc 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 arc efficiency must be taken into consideration. The arc efficiency factors are given in EN 1011-1: 2001 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, e.g. weave width /dwell.
Prevention and best practice
Techniques and practical guidance to avoid hydrogen cracks:
- Preheating
- interpass and post-heating
There are three factors which combine to cause cracking in arc welding:
- hydrogen generated by the welding process
- a hard brittle structure which is susceptible to cracking
- tensile stresses acting on the welded joint
Cracking generally occurs when the temperature has reached a normal ambient. In practice, for a given situation (material composition, material thickness, joint type, electrode composition, and heat input), the risk of hydrogen cracking is reduced by heating the joint.
Preheat
Preheat, which slows the cooling rate, allows some hydrogen to diffuse away and prevents a hard, crack-sensitive structure being formed. The recommended levels of preheating for carbon and carbon-manganese steel are detailed in EN 1011-2: 2001 (which incorporates the nomograms from BS 5135). The preheat level may be as high as 200°C for example, when welding thick section steels with a high carbon equivalent (CE) value.Interface and post-heating
As cracking rarely occurs at temperatures above ambient, maintaining the temperature of the weldment during fabrication is equally important. For susceptible steels, it is usually appropriate to maintain the preheat temperature for a given period, typically between 2 to 3 hours, to enable the hydrogen to diffuse away from the weld area. In crack sensitive situations such as welding higher CE steels or under high restraint conditions, the temperature and heating period should be increased, typically 250-300°C for three to four hours.
Post-weld heat treatment (PWHT) may be used immediately on completion of welding, i.e. without allowing the preheat temperature to fall. However, in practice, as an inspection can only be carried out at an ambient temperature, there is the risk that 'rejectable,' defects will only be found after PWHT. Also, for highly hardenable steels, a second heat treatment may be required to temper the hard microstructure present after the first PWHT.
Under certain conditions, more stringent procedures are needed to avoid cracking than those derived from the nomograms for estimating preheat in Fig. C2 of EN 1011-2. Section C.2.9 of this standard mentions the following conditions:
- high restraint, including welds in section thicknesses above approximately 50mm, and root runs in double bevel joints
- thick sections (approximately 50mm)
- low carbon equivalent steels (CMn steels with C 0.1% and CE approximately 0.42)
- 'clean' or low sulphur steels (S approximately 0.008%), as low sulphur and low oxygen content, will increase the hardenability of steel.
- alloyed weld metal where preheat levels to avoid HAZ cracking may be insufficient to protect the weld metal. Low hydrogen processes and consumables should be used. Schemes for predicting the preheat requirements to avoid weld metal cracking generally require the weld metal diffusible hydrogen level and the weld metal tensile strength as input.
Use of austenitic and nickel alloy weld metal to prevent cracking
In situations where preheating is impractical, or does not prevent cracking, it will be necessary to use an austenitic consumable. Austenitic stainless steel and nickel electrodes will produce a weld metal which at ambient temperature, has a higher solubility for hydrogen than ferritic steel. Thus, any hydrogen formed during welding becomes locked in the weld metal, with very little diffusing to the HAZ on cooling to ambient.
A commonly used austenitic MMA electrode is 23Cr:12Ni, e.g. from EN 1600: 1987. However, as nickel alloys have a lower coefficient of thermal expansion than stainless steel, nickel austenitic electrodes are preferred when welding highly restrained joints, to reduce the shrinkage strain. Figure 1 is a general guide on the levels of preheating when using austenitic electrodes. When welding steels with up to 0.2%C, a preheat would not normally be required. However, above 0.4%C a minimum temperature of 150°C will be needed to prevent HAZ cracking. The influence of hydrogen level and the degree of restraint are also illustrated in the figure.
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Fig.1 Guide to preheat temperature when using austenitic MMA electrodes at 1-2kJ/mm a) low restraint (e.g. material thickness <30mm) b) high restraint (e.g. material thickness >30mm) |
Reduction in weld metal hydrogen
The most effective means of avoiding hydrogen cracking is to reduce the amount of hydrogen generated by the consumable, ie by using a low hydrogen process or low hydrogen electrodes.
Welding processes can be classified as high, medium, low, very low, and ultra-low, depending on the amount of weld metal hydrogen produced. The weld metal diffusible hydrogen levels (ml/100g of deposited metal), and the hydrogen scale designations of EN 1011-2: 2001 are as follows:
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| Fig.2 General relationships between potential hydrogen and weld metal hydrogen levels for arc welding processes |
High
|
>15
|
Scale A
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Medium
|
10
|
Scale B
|
Low
|
5
|
Scale C
|
Very low
|
3 - 5
|
Scale D
|
Ultra-low
|
≤3
|
Scale E
|
Figure 2 illustrates the relative amounts of weld metal hydrogen produced by the major welding processes. MMA, in particular, has the potential to generate a wide range of hydrogen levels. Thus, to achieve the lower values, it is essential that basic electrodes are used and they are baked in accordance with the manufacturer's recommendations. For the MIG process, cleaner wires will be required to achieve very low hydrogen levels.
General guidelines
The following general guidelines are recommended for the various types of steel but requirements for specific steels should be checked according to EN 1011-2: 2001
Mild steel (CE <0.4)
- readily weldable, preheat generally not required if low hydrogen processes or electrodes are used
- preheat may be required when welding thick section material, high restraint and with higher levels of hydrogen being generated
C-Mn, medium carbon, low alloy steels (CE 0.4 to 0.5)
- thin sections can be welded without preheat, but thicker sections will require low preheat levels, and low hydrogen processes or electrodes should be used
Higher carbon and alloyed steels (CE >0.5)
- preheat, low hydrogen processes or electrodes, post weld heating and slow cooling required.
More detailed guidance on the avoidance of hydrogen cracking is described in EN 1011-2: 2000.
Practical Techniques
The following practical techniques are recommended to avoid hydrogen cracking:
- clean the joint faces and remove contaminants such as paint, cutting oils, grease
- use a low hydrogen process, if possible
- dry the electrodes (MMA) or the flux (submerged arc) in accordance with the manufacturer's recommendations
- reduce stresses on the weld by avoiding large root gaps and high restraint
- if preheating is specified in the welding procedure, it should also be applied when tacking or using temporary attachments
- preheat the joint to a distance of at least 75mm from the joint line, ensuring uniform heating through the thickness of the material
- measure the preheat temperature on the face opposite that being heated. Where this is impractical, allow time for the equalisation of temperature after removing the preheating before the temperature is measured
- adhere to the heat input requirements
- maintain heat for approximately two to four hours after welding, depending on crack sensitivity
- In situations where adequate preheating is impracticable, or cracking cannot be avoided, austenitic electrodes may be used
Acceptance standards
As hydrogen cracks are linear imperfections which have sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of EN ISO 5817.
Detection and remedial action
As hydrogen cracks are often very fine and may be sub-surface, they can be difficult to detect. Surface-breaking hydrogen cracks can be readily detected using visual examination, liquid penetrant, or magnetic particle testing techniques. Internal cracks require ultrasonic or radiographic examination techniques. Ultrasonic examination is preferred as radiography is restricted to detecting relatively wide cracks parallel to the beam.
Most codes will specify that all cracks should be removed. A cracked component should be repaired by removing the cracks with a safety margin of approximately 5mm beyond the visible ends of the crack. The excavation is then re-welded.
To make sure that cracking does not re-occur, welding should be carried out with the correct procedure, ie preheat and an adequate heat input level for the material type and thickness. However, as the level of restraint will be greater and the interpass time shorter when welding within an excavation compared to welding the original joint, it is recommended that a higher level of preheat is used (typically by 50°C).
References
BS 5135:1984 Arc Welding of Carbon and Carbon Manganese Steels (now superseded by EN 1011: 2001)
EN 1011-1: 2001 Welding - Recommendations for Welding of Metallic Materials
Part 1- General Guidance for Arc Welding
Part 2- Arc Welding of Ferritic Steels
EN ISO 13916: 1997 Welding - Guidance on the Measurement of Preheating Temperature, Interpass Temperature, and Preheat Maintenance Temperature
EN ISO 5817: 2003 Welding - Fusion-welded joints in steel, nickel, titanium, and their alloys (Beam welding excluded). Quality levels for imperfections
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