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.

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.

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.

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 at a temperature ≥~600°C