Weldability of Steels
11.2 Hydrogen cracking
During fabrication by welding, cracks can occur in some types of steel, due to the presence of hydrogen. The technical name for this type of cracking is hydrogen induced cold cracking (HICC) but it is often referred to by other names that describe various characteristics of hydrogen cracks:
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.
Although most hydrogen cracks occur in the HAZ, there are circumstances when they may form in weld metal.
Figure 11.1 shows typical locations of HAZ hydrogen cracks.
Figure 11.2 shows hydrogen crack in the HAZ of a fillet weld.
WIS10-30816
Weldability of Steels 11-1 Copyright © TWI Ltd
11.2.1 Factors influencing susceptibility to hydrogen cracking
Hydrogen cracking in the HAZ of a steel occurs when 4 conditions exist at the same time:
Hydrogen level > 15ml/100g of weld metal deposited
Stress > 0.5 of the yield stress
Temperature < 3000C
Susceptible microstructure > 400HV hardness
These four conditions (four factors) are mutually interdependent so that the influence of one condition (its’ active level) depends on how active the others three factors are.
11.2.2 Cracking mechanism
Hydrogen (H) can enter the molten weld metal when hydrogen containing molecules are broken down into H atoms in the welding arc.
Because H atoms are very small they can move about (diffuse) in solid steel and while weld metal is hot they can diffuse to the weld surface and escape into the atmosphere.
However, at lower temperatures H cannot diffuse as quickly and if the weldment cools down quickly to ambient temperature H will become trapped - usually the HAZ.
If the HAZ has a susceptible microstructure – indicated by being relatively hard and brittle, there are also relatively high tensile stresses in the weldment then H cracking can occur.
The precise mechanism that causes cracks to form is complex but H is believed to cause embrittlement of regions of the HAZ so that high-localised stresses cause cracking rather than plastic straining.
11.2.3 Avoiding HAZ hydrogen cracking
Because the factors that cause cracking are interdependent, and each need to be at an active level at the same time, cracking can be avoided by ensuring that at least one of the four factors is not active during welding.
Methods that can be used to minimise the influence of each of the four factors are considered in the following sub-sections.
WIS10-30816
Weldability of Steels 11-2 Copyright © TWI Ltd
Hydrogen
The principal source of hydrogen is moisture (H2O) and the principal source of moisture is welding flux. Some fluxes contain cellulose and this can be a very active source of hydrogen.
Welding processes that do not require flux can be regarded as low hydrogen processes.
Other sources of hydrogen are moisture present in rust or scale, and oils and greases (hydrocarbons).
Reducing the influence of hydrogen is possible by:
Ensuring that fluxes (coated electrodes, flux-cored wires and SAW fluxes) are low in H when welding commences.
Low H electrodes must be either baked & then stored in a hot holding oven or supplied in vacuum-sealed packages.
Basic agglomerated SAW fluxes should be kept in a heated silo before issue to maintain their as-supplied, low moisture, condition.
Check the diffusible hydrogen content of the weld metal (sometimes it is specified on the test certificate).
Ensuring that a low H condition is maintained throughout welding by not allowing fluxes to pick-up moisture from the atmosphere.
Low hydrogen electrodes must be issued in small quantities and the exposure time limited; heated ‘quivers’ facilitate this control.
Flux-cored wire spools that are not seamless should be covered or returned to a suitable storage condition when not in use.
Basic agglomerated SAW fluxes should be returned to the heated silo when welding is not continuous.
Check the amount of moisture present in the shielding gas by checking the dew point (must be bellow -60°C).
Ensuring that the weld zone is dry and free from rust/scale and oil/grease.
Tensile stress
There are always tensile stresses acting on a weld because there are always residual stresses from welding.
The magnitude of the tensile stresses is mainly dependent on the thickness of the steel at the joint, heat input, joint type, and size and weight of the components being welded.
Tensile stresses in highly restrained joints may be as high as the yield strength of the steel and this is usually the case in large components with thick joints and it is not a factor that can easily be controlled.
The only practical ways of reducing the influence of residual stresses may be by:
Avoiding stress concentrations due to poor fit-up.
Avoiding poor weld profile (sharp weld toes).
Applying a stress-relief heat treatment after welding.
Increasing the travel speed as practicable in order to reduce the heat input.
Keeping weld metal volume to an as low level as possible.
These measures are particularly important when welding some low alloy steels that have particularly sensitivity to hydrogen cracking.
WIS10-30816
Weldability of Steels 11-3 Copyright © TWI Ltd
Susceptible HAZ microstructure
A susceptible HAZ microstructure is one that contains a relatively high proportion of hard brittle phases of steel - particularly martensite.
The HAZ hardness is a good indicator of susceptibility and when it exceeds a certain value a particular steel is considered to be susceptible. For C and C-Mn steels this hardness value is ~ 350HV and susceptibility to H cracking increases as hardness increases above this value.
The maximum hardness of an HAZ is influenced by:
Chemical composition of the steel.
Cooling rate of the HAZ after each weld run is made.
For C and C-Mn steels a formula has been developed to assess how the chemical composition will influence the tendency for significant HAZ hardening - the carbon equivalent value (CEV) formula.
The CEV formula most widely used (and adopted by IIW) is:
CEViiw = % C + %Mn + %Cr + %Mo + %V + %Ni + %Cu
6 5 15
The CEV of a steel is calculated by inserting the material test certificate values shown for chemical composition into the formula. The higher the CEV of a steel the greater its susceptibility to HAZ hardening and therefore the greater the susceptibility to H cracking.
The element with most influence on HAZ hardness is carbon. The faster the rate of HAZ cooling after each weld run, the greater the tendency for hardening.
Cooling rate tends to increase as:
Heat input decreases (lower energy input).
Joint thickness increases (bigger heat sink).
Avoiding a susceptible HAZ microstructure (for C and C-Mn steels) requires:
Procuring steel with a CEV that is at the low-end of the range for the steel grade(limited scope of effectiveness).
Using moderate welding heat input so that the weld does not cool quickly (and give HAZ hardening).
Applying pre-heat so that the HAZ cools more slowly (and does not show significant HAZ hardening); in multi-run welds, maintain a specific interpass temperature.
For low alloy steels, with additions of elements such as Cr, Mo and V, the CEV formula is not applicable and so must not be used to judge the susceptibility to hardening. The HAZ of these steels will always tend to be relatively hard regardless of heat input and pre-heat and so this is a ‘factor’ that cannot be effectively controlled to reduce the risk of H cracking. This is the reason why some of the low alloy steels have greater tendency to show hydrogen cracking than in weldable C and C-Mn steels, which enable HAZ hardness to be controlled.
WIS10-30816
Weldability of Steels 11-4 Copyright © TWI Ltd
Weldment at low temperature
Weldment temperature has a major influence on susceptibility to cracking mainly by influencing the rate at which H can move (diffuse) through the weld and HAZ. While a weld is relatively warm (>~300°C) H will diffuse quite rapidly and escape into the atmosphere rather than be trapped and cause embrittlement.
Reducing the influence of low weldment temperature (and the risk of trapping H in the weldment) can be effected by:
Applying a suitable pre-heat temperature (typically 50 to ~250°C).
Preventing the weld from cooling down quickly after each pass by maintaining the preheat and the specific interpass temperature during welding.
Maintaining the pre-heat temperature (or raising it to ~250°C) when welding has finished and holding the joint at this temperature for a number of hours (minimum 2) to facilitate the escape of H (called post-heat *).
*Post-heat must not be confused with PWHT which is performed at a temperature ≥~600°C.
11.2.4 Hydrogen cracking in weld metal
Hydrogen cracks can form in steel weld metal under certain circumstances. The mechanism of cracking, and identification of all the influencing factors, is less clearly understood than for HAZ cracking but it can occur when welding conditions cause H to become trapped in weld metal rather than in HAZ.
However it is recognised that welds in higher strength materials, thicker sections and using large beads are the most common areas where problems arise.
Hydrogen cracks in weld metal usually lie at 45° to the direction of principal tensile stress in the weld metal and this is usually the longitudinal axis of the weld (Figure 11.3). In some cases the cracks are of a V formation, hence an alternative name chevron cracking.
There are not any well-defined rules for avoiding weld metal hydrogen cracks apart from:
Ensure a low hydrogen welding process is used.
Apply preheat and maintain a specific interpass temperature.
BS EN 1011-2 entitled Welding – Recommendations for welding of metallic materials – Part 2: Arc welding of ferritic steels gives in Annex C practical guidelines about how to avoid H cracking. Practical controls are based principally on the application of pre-heat and control of potential H associated with the welding process.