Why Welds Crack

A weld failure is never good.  Whether  a weld cracks on a truck’s axle or on the post holding your house’s fence, there are always consequences.  Hopefully the consequences are limited to time and money.  Unfortunately, weld failures can cause injury.   Welds can fail due to design flaws, but the majority of weld failures occur due to reasons we as welders can prevent.

Probably the single, most important tool for preventing cracks is the Welding Procedure Specification (WPS).    The use of WPS is not simply to comply with an internal or customer requirement, it is to insure quality.  With this being said, we should not follow a WPS blindly, if there is something we believe is not right we must bring it up to our supervisor’s attention.

Lets look at the most common types of cracks, their possible causes, and how to prevent them.

1. Heat Affected Zone Crack

The heat affect zone (HAZ) of a weld is not limited to the weld itself but to the immediate area of parent material surrounding the weld. A HAZ crack can originate at the toes of the weld or a few millimeters from the weld altogether.  The most common causes for this type of crack are: excess hydrogen, high residual stress levels on the weld, and high carbon content on the base material.

Crack in the Heat Affected Zone

Crack in the Heat Affected Zone

Etched cross-section of weld with HAZ crack

Etched cross-section of weld with HAZ crack

 

 

 

 

 

 

 

To minimize the susceptibility or prevent HAZ cracks consider:

  • using low hydrogen electrodes
  • pre-heating the base material
  • slow cooling the base material after welding

2. Centerline Crack

Also know as longitudinal crack, a centerline crack will typically extend the length of the weld.  This type of crack is most often caused by an improper width-to-depth ratio, low melting point of tramp elements in the base material, and a concave weld surface.  Although a single one of these can be the root of the problem, most often it is a combination of these that cause a weld to crack.

 

Centerline (Longitudinal) crack

Centerline (Longitudinal) crack

To reduce the chance of a centerline crack consider:

  • aiming for a width-to depth ration of 1:1 to 1.3:1
  • run lower current to decrease excess penetration
  • decrease welding voltage or slow travel speed to achieve a flat to convex weld surface

3. Crater cracks

Most common in aluminum welds, the crater crack is due to lack of fill at the the end of the weld.  Carbon steel and stainless steel welds are not as susceptible to crater cracks, but care must still be taken when welding these materials.

 

Crater Crack

Crater cracks at the end of aluminum welds

 

To eliminate crater cracks insure proper crater fill by:

  • backstepping at the end of the weld (reach the end and then weld back over the bead for 1/2 to 1 inch
  • use welding equipment crater fill option (if available)

4. Transverse cracks

Transverse cracks are not very common outside of harfacing welds.  However, they due occur  and are caused by: high residual stress, excessive weld strength, and excess hydrogen.

Transverse crack 1

Transverse and longitudinal cracks

To minimize susceptibility of transverse cracks cosider:

  • pre-heating the base material
  • using lower strength consumables
  • slow cooling after welding

Have you had issues with weld cracking?  What actions have you taken to prevent cracks in the future?

 

Please note: I reserve the right to delete comments that are offensive or off-topic.

Leave a Reply

Your email address will not be published. Required fields are marked *

2 thoughts on “Why Welds Crack

  1. Your first line of your article mentions a truck axle. We have been seeing a lot of axle failures and the axle company in central MO says they use a robot weld and it is not there fault. All failures have been the weld. All of the re-welded welds hold but they clam no responsibility and they note that they know what they doing. By the looks of all failures they are clueless about welding. There are cracks on the top and the root of the weld is incomplete. The fusion to the axle is little to none. Again they state that it meets there expectations. What do we do?

    • A manufacturer that claims no responsibility for weld failures without proper investigation certainly sounds a bit fishy. Assuming that the axles are being used per the manufacturers specifications (not exceeding limits or being misused), they should not fail. We can learn a lot simply by looking at the failed part. If it is evident that there is lack of penetration at the root then it is NOT a good weld. The strength of the weld is determined by the effective throat, which extends form the face of the weld all the way to the point of deepest penetration. If a weld size is called out it assume there will be root penetration. In this case the strength of the weld is reduced drastically.Putting the axle under “normal” working conditions could certainly create a weld failure in this case.

      Welding with a robot does not guarantee quality. Torches crash, operators touch up points, wire may not be coming out straight, etc. All these things can cause bad wire placement and prevent good root penetration.

      The first step to get some resolution (if your supplier is not helping) would be to contact the manufacturer of the equipment and/or welding wire being used. Depending on the company they can sometimes provide a lot of assistance in failure analysis at little or no cost.