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Up Topic Welding Industry / Technical Discussions / Toe crack
- - By indranil_68 Date 05-22-2004 13:19
highly restrained joint(cross joint) of carbon steel is giving me too much problem.The material is AWS GROUP II material with low sulphur content & impact tested at -40 degree.After welding with E 7016, we found some crack at weld toe in numer of T joint & cross and that too after quite long time.We discovered this after one month.May be it is a delayed crack bot we failed to understand why only T & cross joint.
After removing the crack by grinding followed by MPI,we rewelded it with all precautions e.g baking of electrode , preheat.Yet surprisingly we found small hair line crack on weld bead .Though not deep but we failed to understand why it happened.Material was OK, Electrode KOBE make was also good.

Can any body give me some idea?
Parent - By GRoberts (***) Date 05-24-2004 02:44
Sounds like you need to preheat due to the joint restraint (and thickness?) It is also possible that your heat input or bead size is too small with respect to the size of the members.
Parent - By Niekie3 (***) Date 05-24-2004 13:46

Your first crack certainly sounds like hydrogen cracking. (Delayed cracking.) There are a number of factors that give rise to H cracking. They are:

1) Susceptible microstructure.
2) Presence of Hydrogen.
3) Presence of a stress.

In cross and T joints, you typically have high constraint, which typically leads to high residual stresses. This would explain why you get the cracking in these joints in preference to the other joints.

The second incidence of cracking does not sound typical of H cracking, but it may be. Did you bake the electrodes at a high enough temperature for a long enough time? Was there any other contamination present? (e.g. from the MPI testing?) If it was not H cracking, it may be due to contamination of the weld with something else, that may lead to hot cracking (unusual in C/Steel) or your width to depth ratio was too small or too large.

Hope this helps.

Niekie Jooste
Fabristruct Solutions
Parent - - By swnorris (****) Date 05-24-2004 16:46
There is an old theory which suggests that hydrogen produces a pressure, which when combined with shrinkage stresses and any hardening effect from the chemistry of the steel, causes underbead and toe cracking. Slower cooling by welding slower or by higher preheat temperatures allows more hydrogen to escape and helps to control the problem. As you probably know, D1.1 says that preheat shall be sufficient to prevent cracking, and that the temperatures in Table 3.2 are minimum temperatures. A higher temperature may be required to allow a more rapid hydrogen diffusion which in turn reduces the tendency for cold cracking.
For highly restrained thicker joints where metal to metal contact exists, the material cannot move, and so the joint is restrained. As the weld cools and contracts, all the shrinkage stresses are taken up by the weld. This restraint may cause the weld to crack, especially on the first pass on the second side of the material. Joint restraint can be minimized by providing a 1/32" to 1/16" gap between the members, which allows movement as the weld cools. Although the gap is minimal, a slight movement of members during welding will reduce welding stresses. Soft wire spacers can be used in the gap between the members. The wire flattens out as the weld shrinks.
You may want to make sure that your welds have a convex finish rather than concave. Years ago, a concave weld was preferred by designers because of the smoother stress flow it offers to resist a load on the joint. Experience has since shown that single pass concave fillet welds have more tendency to crack during cooling than convex welds, because when a concave bead cools and shrinks, the outer surface is in tension and may crack. A convex bead has considerably reduced shrinkage stresses in the surface area, and the possibility of cracking during cooling is slight.
Also, with the SMAW process, it is assumed that in hot humid weather the arc atmosphere will contain more hydrogen as water vapor than in cool, dry weather. Any tendency to minimize the importance of preheat, of keeping the joint hot, or possibly of postheat in summer months, could be at the root of cracking problems on highly restrained joints. This can also be magnified if the weld metal or the base metal is hardenable because of alloy or carbon content. Low heat input with interruptions in the welding cycle tends to aggravate the cracking problem, but the welding position and its influence on bead size, number of layers, restraints such as those present in cross and T joints that produce stresses on the weld, hydrogen pick up, contaminants on the electrode or material, etc. also has a direct influence on cracking tendency.

Here's a link to Chapter 6 of the Army's training circular, which has some good information about cracking and remedies:

Scroll down to Section VI, Welding Problems and Solutions. Scroll down to 6-29, Stresses and Cracking.

Parent - By rodofgod (**) Date 05-30-2004 22:23

Hi All!

Interesting thread!

I'd agree with previous post's, that the first cracks were Hydrogen induced!

The second batch of cracks suggest more of a procedural problem, either with the 'repair' welding or (more likely) the heat treatment applied!

All of swnorris comments are valid, We regularly use the 'compressed' wire spacer when welding high restraint 'T' joints.

Don't forget, when you grind out the defects, you are ,effectively, reducing the cross-sectional area of the joint. Also, as has been mentioned pre/post heat might be altered because of this!

Another point worth mentioning is the 'heat' generated by the 'grinding' process??

This can be high enough to introduce stresses into the weld metal!


Up Topic Welding Industry / Technical Discussions / Toe crack

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