Tim Blok

Tim Blok

Tim Blok was trained as a metallurgist specializing in welding technology and has spent his entire working life in metal and welding technology. For many in the field, he is also known as a lecturer at the IWE and IWT courses.

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Welding imperfections

Cavities in welded joints

Cavities in welded joints have a detrimental effect on the integrity of a welded joint. The presence of large cavities may even cause the joint

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Pro’s and cons of preheating

What are the reasons to preheat, and what might be negative effects?  Master Welder Tim Blok shares his insights on the pros and cons of preheating.

Preheating; not as easy as it seems?

 If we want to keep it simple: you can say that preheating prevents cracking in the material and/or the weld. Since we are talking about various metals and various welding processes, in practice it is of course, much less simple. So what are the most important pros for preheating? 

Pros of preheating:  

  • Crack prevention  – Preheating reduces the cooling speed of the weld bead and its immediate surroundings. In particular, steels with a higher proportion of alloying elements (higher carbon equivalent) are sensitive to the formation of a brittle structure. In combination with tension stress and any hydrogen present, this can lead to cracking. The selected preheat temperature, during manufacture of the entire weld, must be maintained as a minimum. A good tool to determine the preheat temperature is the EN 1011 standard.

  • limiting shrinkage  – By preheating, we can reduce the shrinkage of the welded joint. With no or limited restraint, the shrinkage will cause deformation. In case of full restraint, this will result in tension stress. And, in combination with a brittle structure and any hydrogen present, this can lead to cracking. 

  • prevention of lack of penetration – In the case of metals with a high thermal conductivity, sometimes the arc cannot melt the base material, which results in bonding errors. Here, preheating can be a means of melting the base material in the immediate vicinity of the weld bead, so that a good connection can be established.

  • reducing of H2-content (HDM)  – It is claimed that preheating has a beneficial effect on the escape of any hydrogen present in the weld and the heat-affected zone. However, the effect is extremely limited and the preheat temperature must then be at least 150 – 200 °C.

Cons of preheating

It is of course important to also take into account the possible negative effects of pre-heating. In order to be able to assess this properly, knowledge of the materials to be welded is required. These are the five most common negative effects of preheating;

  • In the case of a ferritic structure in steel (especially in the case of corrosion-resistant steel), Undesired granular growth may occur, giving rise to embrittlement.
  • Austenitic stainless steel increases the chance of chromium carbide formation, which results in the risk of intergranular corrosion.
  • In the case of thermomechanically treated high yield strength steel (so-called TM steel), this treatment is partially undone, as a result of which a decrease of the yield strength can be observed (softening).
  • When using  duplex stainless steel, too much ferrite will be formed, which has a negative effect on the corrosion resistance.  
  • In the case of metals that have obtained their mechanical properties by means of precipitation hardening, the precipitates can grow, which leads to embrittlement.

When setting up a welding procedure, a well-considered choice must therefore be made between preheating and non-preheating. If a choice is made to do so, the next question is what will be the preheat temperature. The selected preheat temperature is the minimum temperature at which the welded joint to be produced must remain during the entire production of the weld.

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