Page 127 - SSAB Welding Handbook Edition 2

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Welding handbook 22.0 Hardening mechanisms in the heat-affected zone and the unaffected parent metal

22.0 Hardening mechanisms in the

heat-affected zone and the unaffected

parent metal

The hardening mechanisms, which occur in both the The dimensions of the grains in the unaffected
HAZ and the unaffected parent metal, have a parent metal vary to some extent depending on the
significant role in the outcome of the mechanical steel grade and plate thickness. As an example, the
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properties of the Hardox and Strenx grades. The unaffected parent metal of Strenx 1100 with a plate
essential mechanisms addressed are grain refinement, thickness of 10 mm has a grain size of approximately
solid solution hardening, and precipitation hardening. 10 μm -15 μm.
The occurrence of these mechanisms depends on the
character of the grains in steel structures as well as the The thermal heat treatment from welding affects
presence of dislocations. Therefore, an initial the sizes of the grains in the HAZ. The grain sizes
description of grains and dislocations is first throughout the HAZ vary; the largest difference is
presented. adjacent to the weld metal, see fig. 22.2. This zone is
called the coarse grained zone (CGHAZ). Even though
In the following context, the term parent metal the grains in the CGHAZ are small, they are typically
includes both the HAZ and the unaffected parent up to several times larger than those in the unaffected
metal. parent metal. The sizes of the grains throughout the
complete HAZ depend on the characteristics of the
steel and the welding performance. More information
22.1 The grains regarding characteristics of the different parts of the
HAZ is found in chapter 5.
The structure of steel consists of small grains, including
both the welded joint and the unaffected parent metal,
which can be observed under magnification with a
microscope.

A characteristic of an individual grain is that all the
iron atoms are oriented in the same direction, which The coarsed grained zone
is exemplified in fig. 22.1. The area between the Other parts of the HAZ
grains is called the grain boundary. Unaffected parrent metal
Fig. 22.2: A butt joint in the Strenx/Hardox grades.

In all types of steels and joints, each grain has line
defects in its atomic structure. These defects are
called dislocations.

Fig. 22.1: The atomic structure, where the orientation of the iron
atoms are marked with dots. The gray area illustrates the grain
boundaries.
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