Hydrogen-induced cracking in welds

I.Introduction

In this article, we will address the phenomenon caused by stress from diffused hydrogen in the weld pool, which is known by various names, including:

• Hydrogen-Induced Cracking (HIC) (AWS D1.1)
• Delayed Cracking
• Underbead/Toe Cracking
• Hydrogen Embrittlement. (ASME BPVC II, NONMANDATORY APPENDIX A, A-702.1.1 Hydrogen Embrittlement.)

First, we will describe the phenomenon of Weld-Induced Hydrogen Cracking (HIC).

2.? the phenomenon of Weld-Induced Hydrogen Cracking (HIC).

• When hydrogen atoms or ions enter the welding pool, they diffuse easily into the heated and molten metal due to the high temperatures. As the metal begins to cool, these hydrogen atoms or ions start to migrate out of the metal. However, the rapid cooling rate prevents all of them from escaping. The trapped hydrogen atoms or ions accumulate in interstitial spaces and other cavities, eventually forming diatomic hydrogen gas (H?). This gas generates pressure as it accumulates, increasing the residual stresses within the metal. When these stresses exceed the material’s yield strength, they can lead to the formation of cracks.
• The amount of hydrogen absorbed depends on the thickness of the metal in the welding pool, the concentration of hydrogen atoms present, and the time available for hydrogen to diffuse out of the metal (above 390°F).
• The stress generated by diatomic hydrogen pressure alone does not cause damage. Instead, it combines with residual stresses and applied stresses/restraints. When the combined stress exceeds the material's yield strength, it can cause cracking, especially in relatively low fracture toughness materials.

3. The susceptibility of weld hydrogen-induced cracking (HIC)

The susceptibility of weld hydrogen-induced cracking (HIC) is influenced by several factors:

• Hydrogen Absorption: The amount of hydrogen atoms absorbed in the base or welded metal is governed by the metal thickness, cooling rate, and the amount of hydrogen in the welding pool.
• Microstructure of Base Metal Heat affected Zone: This is determined by the hardenability and the microstructure of the heat-affected zone (HAZ) and the cooling rate.
• ·Stresses (Residual and Applied): The susceptibility is affected by the level of restraint, applied stress, and residual stress present in the metal

4. Mitigate hydrogen-induced cracking (HIC)

To mitigate hydrogen-induced cracking (HIC), we should take the following measures:

a)????? Reduce the Amount of Absorbed Hydrogen:

• Use basic low-hydrogen electrodes for SMAW or inherently low-hydrogen welding processes like GTAW, GMAW, and SAW.
• Eliminate sources of moisture by properly baking electrodes and preheating.
• Ensure the weld joint is clean.
• Adjust the cooling rate to allow hydrogen to diffuse out of the metal through preheating and/or bakeout treatments and PWHT .

b)????? Microstructure of the Base Metal Heat-Affected Zone:

During welding, the temperature gradient through the metal thickness results in the formation of various microstructures at different temperatures. The specific microstructure depends on factors such as the cooling rate and the Carbon Equivalent (CE). If martensite forms in the heat-affected zone (HAZ) due to rapid cooling and high CE, it can become very brittle. This brittleness makes the martensite prone to cracking under hydrogen-induced pressure, especially if it is not tempered adequately just after the welding finished.

To mitigate this risk:

• Maintain Adequate Preheat/Interpass Temperature: Ensure high preheat or interpass temperatures to facilitate the migration of hydrogen out of the HAZ and to temper the formed martensite.
• Manage Hardness: Although the hardenability of the material cannot be adjusted during the welding process, proper preheating and post-weld treatments can help manage the effects of martensite formation.

C) Stresses (Residual and Applied):

• Proper Weld Joint Design: Ensure weld joints are symmetrically designed to manage induced stress, allowing for compensation of thermal expansion and contraction effects.
• Stress Relieving Heat Treatment: Components subjected to forming operations that exceed a specific stress threshold should undergo stress-relieving heat treatment to reduce residual stresses.