As discussed in previous article, activation of four factors simultaneously influences hydrogen cracking in HAZ. Methods to minimize influence of each factors is described here:
The main source of hydrogen is moisture (H2O) and principle source of moisture is welding flux. Some fluxes contain cellulose and this can be a very active source of hydrogen.
Reducing the influence of hydrogen is possible by:
- Ensuring that fluxes are low in H when welding commences.
- Low hydrogen electrodes must be either baked and then stored in a hot holding oven or supplied in a vacuum sealed packs
- Basic agglomerated SAW fluxes should be kept in a heated silo before issued for welding
- Checking the amount of moisture present in the shielding gas by checking the dew point(must be below -60oC)
- Ensuring the weld zone in 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 tensile stresses is mainly dependent on the thickness of the steel at joint, heat input, joint type and the size and weight of the components being welded.
Practical ways of reducing the influences of residual stresses may be:
- Avoiding stress concentrations due to poor fit up.
- Avoiding poor weld profile
- Increasing the travel speed to reduce heat input
- Keeping weld metal volume as low as possible
- Susceptible HAZ microstructure
A Susceptible HAZ microstructure is one that contains a relatively high proportion of hard brittle phases of steel, particularly martensite.
Avoiding a susceptible microstructure requires:
- Using moderate welding heat input so that the weld does not cool quickly
- Apply pre-heat so that the HAZ cools more slowly; in multi run welds, maintain a specific interpass temperature
- Weldment at low temperature
Weldment temperature has a major influence on susceptibility to cracking mainly by influencing the rate at which H can move through weld and HAZ.
Reducing the influence of low weldment temperature can be effected by:
- Applying a suitable preheat temperature(typically 50 deg. C to 250 deg. C)
- Maintaining preheat temperature and interpass temperature
- Hold preheat temperature after completion of welding for atleast 2 hours so that H gets time to escape from solid metal
- Post-heat(De hydrogenation treatment)
Reference: TWI literature
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