Hydrogen-induced cracking (also referred to as cold cracking
), is caused by the diffusion of hydrogen in the crystal lattice of HAZ near the fusion boundary
. Atomic hydrogen is dispersed throughout the weld material, making it weak in mechanical properties. The entrapped hydrogen makes cavities in metal that results in cracking due to weak ductility and tensile strength. When metal is subjected to tensile stress with high level of hydrogen content, hydrogen induced cracking occurs. Hydrogen cracking is also named as toe cracking or under-bead cracking due to its location of occurrence. It primarily occurs in High strength Low alloy steels. During welding, hydrogen is induced due to the moisture and organic compounds by the usage of cold cellulosic electrodes
. Proper cleaning of metal surface and pre-heating are the preventive steps to avoid alarming hydrogen induced cracking. Below listed are the ways to reduce Hydrogen cracking in welded items.
Pre-heating is one of the best step to achieve good solidify microstructure as it slows down the cooling rate to facilitate solidification. In this way controlled cooling provides the desired microstructure that is not prone to hydrogen cracking. In worst the rapid cooling may result in sensitive microstructure to HIC.
2. Interpass Temperature
Welder associates must define the peculiar interpass temperature. Once this temperature is specified, the base metal must be pre-heated to that temperature before depositing the other weld pass. In this way cooling rate is controlled and the mechanical properties of both weld metal and HAZ are secured, avoiding hydrogen cracking.
After welding is finished, apply post heat to eliminate any remaining hydrogen. On post weld heating, the metal gives off the hydrogen that lessens the risk of hydrogen induced cracking. Moreover, post weld heat treatment (PWHT)
is also applied for the removal of harmful residual stresses, in order to achieve the desired microstructure as per requirement of service.
4. Suitable arc process
5. Suitable filler metal
As mentioned above cellulosic stick electrodes are the mean of hydrogen embrittlement. Moisture and the organic compounds are the source of hydrogen. Such electrodes provide hydrogen gas shielding. For this, basic electrodes (minerals like calcium and magnesium carbonates) are available that give low hydrogen shielding, reducing the risk of cracking in low alloy steels.
6. Multipass technique
Multipass method renders tempering effect, modulating the microstructure in order to get rid of hydrogen content. Every next pass tempers the previous deposit pass and protects it from cracking. On the other hand hot pass is also applied for same purpose. We know hot pass is used to clean the root pass by the removal of slag content. Moreover it is also applied to render heat treatment effect so that root may not be subjected to hydrogen cracking.
7. Temper bead technique
Temper bead technique is introduced to provide the heat treatment effect in repair welds where post weld heat treatment is difficult. In this technique overlapping of beads, assists the self-heat treatment that refines the course grain zones, due to which desired microstructure with improved properties in both weld metal and HAZ, is obtained. Furthermore, it also minimizes the residual tensile stresses. All these features encourage the reduction of hydrogen induced cracking.
8. Shielding gases
As it is stated above cellulosic electrodes offer hydrogen gas shielding. For this moist free (low hydrogen) shielding gas must be used in case of metals that are prone to hydrogen cracking. Use of baked electrodes, is the recommended practice in order to lower the hydrogen content.
9. Rust and scale removal
Before welding, cleaning of joint with removal of rust and scale, is always endorsed. Since the presence of moisture or grease on parent metal, is the major source of hydrogen. So making the joint rust free and proper cleaning are the paramount activities before the commencement of weld.
Perfect fusion is achieved by the merging the weld metal at weld toes, to provide reinforcement. As stress concentration is one of the cause for hydrogen cracking/toe cracking. Reinforcement at weld toes reduces the stress concentration and thus avoiding the toe cracking.
In brief residual stresses, high hydrogen content and sensitive micro structure are the primary causes for hydrogen cracking. Control of these elements gives a cracking free weld. In order to achieve quality welds, following practices must be exercised;
- Proper cleaning of joint
- Pre-heating of joint
- Use of nickel and austenitic filler materials
- Use multi-pass method in case of those weldable materials that are sensitive to HIC.