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.
High Strength Steels
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.
Hydrogen Entrapment
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.