Delayed Cracking also known as Hydrogen Induced Cracking or Cold cracking, occurs when the following three factors are present at the same time; high strength steel, high stress level (residual or mechanical/structural loading) and a source of hydrogen. Moisture in welding filler material or environment and joint contamination all are the typical source of hydrogen in a weld metal. Elimination of one or more of the factors can prevent the delayed cracking to occur. So all you need to do is to control these factors to make crack free welds.
Steels with high strength level may be susceptible to hydrogen induced cold cracking. Example of this includes; Precipitation Hardened Steels, high strength nickel base alloys, 400 Series Stainless Steels. Welding of above mentioned materials may prone to hydrogen embrittlement in their micro-structure consequently results in cold cracks.
High Stress Level
A weld joint with high stress level is more likely to prone to delayed cracking. It may include residual stresses from cold working or shop fabrication and mechanical or structural loading (for example: wind loadings). These stresses can be reduced by using streamline joint designs and elimination of any sharp edges, corners or notches. Sharp joints and notches are stress raisers where stress concentration may cause catastrophic failure.
As states above hydrogen comes from high moisture content in atmosphere, welding consumables and joint. Thorough cleaning of weld joint at least 25mm on either side, inside and outside if joining a pipe removes grease, oil, coatings, water etc. Preheat the base metal at the minimum of 200 F degrees and maintain the preheat temperature throughout the whole process. It must be ensured that coated consumables are dry stored and baked in ovens prior to use. The Use of low hydrogen rods (H8 or less) helps to prevent cracking. If using SMAW process, the weldment after completion of welding must maintained to about 500 F (260 C) degrees for about two hours without allowing it to cool down to specified minimum pre-heat temperature.
Prevention of Delayed (Hydrogen Induced) Cracking
- Joint must be thoroughly cleaned, dried and free from any kind of rust, scale, mill, oil, grease, coating etc. About one inch from either sides of joint must be cleaned using specific type of cleaning process.
- Make sure to complete at the minimum of 1/3 length of weld deposit without any interruption to avoid any thermal stresses to develop due to high thermal gradient. Before restart, make sure the metal temperature should not below the minimum pre-heat temperature.
- Rods must be oven baked according to manufacturer’s specific product details, procedure and recommendation or sealed pack prior to use. Opened rods must be discarded after 7-8 hour of exposure. Use diffusible-hydrogen designator H8 or less filler metals.
- Use of low strength materials with good weldable composition and properties, is a best way to avoid cracking.
If you found a welder using AWS 5.1 EXX10 rods for root pass disallow him. Because it is a general practice in several fabrication shop to use EXX10 rods for roots pass and EXX18 for subsequent hot, fill and cap passes. For this I must say that it’s not a good practice as EXX10 rods are not low hydrogen and not suitable for welds subjected to cold cracking. Try to use rods with low hydrogen content and with essential pre-heat and post-heat requirements.