1. CRACK REPAIR

Before starting a crack repair it should be known what kind of material one is dealing with to be able to choose the right welding procedure; and here enters the term: weldability. The weldability of a steel depends on the chemical composition and the thickness.

The background for this is that a steel with increasing carbon-content and other elements is disposed to harden, which means that the heat affected zone HAZ can become hard and brittle if the heat escapes too fast, and that can cause cracks.

A common way to express the weldability of a steel is the so-called "carbon-equivalent", which is calculated as follows:

The annexed file, 'preheating temperature for welding' will calculate the required temperature, based on the carbon equivalent of the metal at hand.

If the result of this (carbon equivalent) is higher than 0.41 it is necessary to preheat. It must be pointed out that even if the result is lower than this, it does not harm to preheat, especially with heavy sections and if the weather is cold.

As long as we are dealing with kiln- and mill-plate, coolers with supports, the mentioned carbon-equivalent will always be lesser than 0.41, as those parts are manufactured of mild steel plate or weldable boiler plate.

When a crack is detected, the first part of the program will be:

1. Finding the length of the crack

2. Finding the depth of the crack

This is done through methods such as Ultrasonics, Magnetic Particles (MPI) or Dye-penetrant. If the crack is throughgoing, e.g. in a kiln shell it is normally necessary to work from both inside and outside.

A normal procedure will then be:

1. Gouging from one side to about 2/3 of the thickness

2. The gouging must start a little away from the crack ends in sound material, gouging towards the crack

3. Careful cleaning by grinding

4. Possibly preheating

5. Welding with a normal basic electrode as AWS E 7016 or E 7018

Usage of Inconel electrodes is also possible Castolin 2222 or OK 92.26 (recommended for cast steel)

6. The electrodes have to be dry (dried for at least 2 hours at 200 °C in a furnace)

7. Each run should be overpeened to reduce stresses, and carefully cleaned for slags

8. Grinding down the surface of the weld

9. In case of throughgoing cracks, the root weld has to be removed (from the inside, around 1/3 of thickness) and re-welded (through either manual arc welding or an automated procedure). Grind surface of weld.

10. Control with Ultrasonics, MPI and/or Dye-penetrant

All this seems to be generalisations, but new crack formations can be caused by non-compliance with these simple rules. It is not to be too difficult to repair that kind of cracks if the mentioned rules are followed.