What is Hydrogen Cracking or Delayed Cracking or Cold Cracking?

What is Hydrogen cracking or cold cracking?

Hydrogen Induced Cracking (HIC) which is commonly known as Hydrogen cracking or Delayed Cracking or Cold cracking occurs in carbon or low-alloy steels (If I say precisely it is Ferritic Steels) when atomic hydrogen diffuses into it and forms molecular hydrogen during Welding of Carbon steel and low alloy steels. . It is caused by the diffusion of hydrogen to the higher stresses, hardened part of the weldment. They occur generally in Heat Affected Zone (HAZ) in C-Mn steels but can extend to the weld metal as there is a greater risk of forming a brittle microstructure in the HAZ, most of the cold cracks are to be found in the parent metal. Fewer changes in the weld metal as welding consumables contain lower carbon content.

It is known that a certain amount of hydrogen is usually present in the weld pool. This comes from the breakdown of moisture that is generally present in fluxes (electrode coating, submerged flux, filling in flux-cored wires) and that may also be present in shielding gases. Occasionally, high humidity on certain days can also increase the amount of hydrogen that might be introduced into the weld pool.

Why we say it Cold cracking or Delayed Cracking

Cold cracking because cracks form only when the weld has cooled down to below about 100°C; and
Delayed hydrogen cracking because cracks can form several hours or days after weld

Hydrogen Cracking Phenomena

The hydrogen dissociates (separate) in the arc and transform in atomic or ionized state into the molten metal.

H2O = H2 + O,  H2 = 2H,  H = H+ + e

The amount of hydrogen absorbed in weld metal depends on the hydrogen partial pressure and the temperature. Therefore, hydrogen solubility in the weld metal is 35ml H2/100g weld metal at 1800°C. With decreasing temperature, the highest portion is again diffused.  For iron, equilibrium solubility depends apart from temperature also on the lattice structure, on the type of elementary cell (cbc, cfc).

Image Courtesy: GSI SLV Duisburg

Figures above shows the maximum hydrogen solubility that can present in the weld metal even in case of fast cooling. But in fast cooling, it can be force released molecular hydrogen embedded in the lattice cavities but also in spaces. Especially the element is concentrated in the range of dislocations and grain boundaries.

Because of its very low atomic radios (25 pm), (140 pm for iron) the hydrogen element is already be able to diffuse noticeable at room temperature. Now in microstructure and lattice areas with increased energy, the atomic hydrogen recombines into the gas molecules. Due to recombination as well as large number of hydrogen molecules in such microstructure areas, the hydrogen gas pressure rises strongly locally, thus the microstructure bonds can break apart locally resulting pores or cracks.

This diffusion process, including recombination or dissociation mechanisms, can extend over periods lasting from minutes to several weeks. 

Factors Affecting the Formation of Hydrogen Cracks

The tendency to form hydrogen cracks depends on the following factors:
1. Amount of hydrogen present in the weld pool: The greater the amount of hydrogen present in the weld pool, the greater the chance of forming hydrogen cracks. The amount of hydrogen introduced into the pool depends on such factors as the welding process used; the design of the welding consumable and its storage conditions; and the presence of moisture, oil, grease, etc. on the workpiece to be welded. Generally, the GTAW and GMAW introduce the smallest amount of hydrogen into the weld pool and these are called low-hydrogen processes. The amount of hydrogen introduced in SMAW and FCAW processes will depend on the electrode designation, electrode manufacturer, and the conditions under which the electrodes have been stored. For example, E7018 electrodes can be provided in vacuum-sealed packaging and these electrodes should introduce a very low amount of hydrogen into the weld pool. However, if after opening the package and before use, the electrodes are allowed to stay in the open in a high humidity environment for a period of a few hours or days, then the amount of hydrogen getting into the weld pool will be higher and this will increase the chances of hydrogen cracking.

2. Locked-in stresses present: The higher the magnitude of the locked-in stresses, the easier it is for the hydrogen cracks to form. Residual stresses (weld zone shrinkage against the colder steel) are always present in welds. In addition, stresses may be present due to high restraint (the workpiece is too rigid to move).
Also, if notches are present, then stresses are magnified at these locations and hydrogen cracks can form more easily. Some such locations include the root pass in a groove weld, one-sided weld on the backing bar, or unspliced backing bar for a longitudinal weld. 
3. Steel hardness/microstructure: Generally speaking, the harder the heat-affected zone, the greater its tendency to form hydrogen cracks. A harder microstructure means that it has a smaller proportion of ferrite and more martensite-like phases. Depending on the hydrogen content and stress, the hardness above which hydrogen cracking may occur varies from 300 to 400 HV. Whether a hard heat-affected zone forms on welding or not depends on the steel’s hardening capacity, which in turn depends on the composition of the steel (amount of C, Mn, Cr, Ni, Mo, etc.) and the rate at which the weld cools. As mentioned in the previous section, the rate at which the weld cools depends on the arc energy, steel thickness, and preheat.

How to Avoid Hydrogen Cracking

Once a steel has been selected and purchased for welding, the options available to counter the

possibility of hydrogen cracking include:

  • Minimize weld joint restraint.
  • Avoid notches in the area of the weld.
  • Use a low hydrogen process.
  • Use low hydrogen consumables and ensure their proper storage.
  • Use high arc energy to reduce the cooling rate (but this may reduce other properties such as strength and toughness).
  • Use preheat (and post-heat); its main function is to slow down the cooling rate below 100°C and give more time for hydrogen to diffuse out.

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