“Tempering” and “Bake Hardening”: The positive effect of zinc thermal diffusion on material properties - Post Image

Posted by Frank Schlipf | 9 OCT, 2018

“Tempering” and “Bake Hardening”: The positive effect of zinc thermal diffusion on material properties

Many steels require heat treatments in the form of tempering to achieve the desired material properties. These processes often operate in a similar temperature range as zinc thermal diffusion. So why not combine the excellent corrosion protection with the required heat treatment? We at Thermission call this the two-in-one effect. In this blog, metallurgist Dr. Vinzenz Bissig explains the metallurgical effects of the annealing process.

Typical thermal diffusion temperatures lie between 608°F (320°C) and 734°F (390°C). In this temperature range, no massive structural changes (e.g. grain growth) are expected. The temperature is high enough, nevertheless, to move, or rather diffuse, carbon and nitrogen atoms stored in the metal lattice over short distances, which partially heals crystal lattice defects. The material properties can be influenced, however, only if no state of equilibrium exists, in particular of the interstitial atoms.

The “Bake Hardening” Effect

Carbon and nitrogen atoms are preferably located close to displacements as interstitial atoms in the metal lattice. Through mechanical deformation (e.g. rolling) of a workpiece, displacements can be moved or even re-formed. The interstitial atoms, however, remain in place during this deformation. Once the workpiece is heated, the carbon and nitrogen mobilizes and moves back to the displacements. Interstitial atoms near displacements have the consequence that more force—that is to say, a higher load—is required to break the displacement. The result of bake hardening is therefore an increased and pronounced yield strength. The yield strength refers to the maximum deformation that a material can experience without being permanently deformed. For “bake hardening”, half an hour at approx. 392°F (200°C) is sufficient. It is often thus a desirable side effect when, for example, baking coatings.

Tempering as a sub-process of the compensation

When tempering, the material is first hardened and then tempered. For this purpose, the workpiece is briefly heated once or several times after hardening to adjust the mechanical properties. Put simply, when tempering, some errors in the metal lattice are eliminated. Forced-release carbon moves in the metal lattice to energetically better places. This is especially the case near displacements. Thus, tempering at lower temperatures below 752°F (400°C) always causes a simultaneous “bake hardening”, which has a positive effect on the properties of the material. Tempering at higher temperatures leads to an increase in the toughness of the base material. Since such temperatures are not achieved in zinc thermal diffusion, however, no increase in toughness is expected.

The Two-In-One Effect

The longer process time for zinc thermal diffusion compared to other coating processes is thus related to a potentially saved step in metal processing. Since the base material is fully and uniformly heated during the thermal diffusion process, a workpiece that has been through the zinc thermal diffusion process will benefit from a correction of the defects in the metal lattice, as explained above.

Want to know more about the process of zinc thermal diffusion? Download the process description here.

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Frank Schlipf

Written by Frank Schlipf

Chief Development Officer

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