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Metal Fabrication: A Look At Cold Cracking In Steels And Prevention

15 April 2015 by Ramona Lawrence

When welding metals during the fabrication process, it is important to pay particular attention to any hydrogen dissolving in the weld metal at the heat affected zones (HAZ). At temperatures below 600 degrees, steels are particularly vulnerable to cold cracking, also known as hydrogen-induced cracking. Cold cracking is a serious welding defect that needs to be addressed immediately, as it significantly compromises and alters the structural integrity of the steels.

In-Depth Review of Cold Cracking

Thicker steels tend to be more susceptible to cold cracking because there is a higher restraint that can serve as a heat sink. This leads to rapid cooling rates that are difficult for the crystalline structures within the metals to tolerate. 

Rapid cooling results in the formation of martensite, which is a crystalline microstructure that is very brittle and lacks ductility. Hydrogen accumulation at the martensite builds pressure in the HAZ, which eventually causes the steels to crack. 

Techniques That Prevent Cold Cracking

Once cold cracks form, the structural integrity of the steels have been compromised permanently, and the steels should be scrapped and recycled, as they no longer have the tensile strength required. To prevent cold cracking, implement the following techniques:

• Pre-heat the base materials that are used before welding them, as this will reduce the speed of cooling and prevent the formation of martensite. When pre-heating the base, a pearlitic structure forms instead of martensite. In comparison to martensite, the pearlitic structure has better mechanical properties and is more ductile. 
• Rely on welding techniques that use only a minimal amount and concentration of hydrogen to prevent hydrogen from diffusing into the steels. In short, make sure that you select an appropriate welding process. 
• Back step the steels. To back step, you will have to weld in one direction for short lengths. Weld over the same area several times, so that the heat from subsequent welds can relieve stresses in the steels. 
• Keep the welded steels at a heated temperature after the job is done. This helps slow down the rate of cooling, which will allow any hydrogen that has diffused into the metal to diffuse out easily.

Conclusion

Cold cracking in steels is a serious issue that signifies that there are unaddressed problems in the welding procedures. In addition to maintaining temporal conditions during the welding process, maintaining constant storage conditions and using proper filler metals can help prevent cold cracking as well. For more information, contact Moorhead Machinery & Boiler Co. or a similar company.

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