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Welding handbook 21.0 Microstructures in the heat affected zone (HAZ) and the unaffected parent metal

21.0 Microstructures in the heat-affected
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zone (HAZ) and the unaffected parent metal

The microstructures in the joint and the unaffected the atoms of iron (Fe) are oriented according to
parent metal are viewed under high magnification. fig. 21.1. This atomic structure is called face
The kind of microstructure that is present in the centered cubic (fcc) and the black circles illustrate
joint affects its mechanical properties. This chapter the atomic center for each Fe atom.
will examine the microstructures that can be present
in the HAZ of the Hardox and Strenx grades.
Furthermore, it will discuss the microstructures that
are present in the unaffected parent metal of these
steel grades in order to point out differences in the
properties between the HAZ and the unaffected
parent metal.

The characteristics of the microstructure in the
HAZ are related to the chemistry of the steel,
the outcome of the thermal cycle and the number
of passes in the joint. The microstructure in the Fig. 21.1: Austenite.
unaffected steel, on the other hand, depends on
the chemistry of the steel and how it is produced.
Studies of microstructures are performed during Austenite in mainly found in the Strenx and
metallographic evaluations using microscopes. Hardox grades in:
4 The plate during the quenching process of
During and after the welding performance, the production when the plate is heated to
microstructures for both the HAZ and the unaffected temperatures of around 900°C.
parent metal of the Hardox and Strenx grades 4 The part of the HAZ – and at a time – where
include primarily austenite, martensite, tempered temperatures of around 700°C-1500°C are
martensite and bainite. Each of these structures has reached during welding.
specific properties that are discussed in this chapter.
However, there is seldom a need for the user of the In addition, there may be small portions of
Hardox and Strenx grades to map out the austenite still left in both the HAZ and the
microstructures in a welded structure.
unaffected parent metal when they reach room
temperature. If so, the microstructure is called
retained austenite. However, this kind of
21.1 Austenite microstructure is hard to detect with the
common metallographic methods.
Austenite is primarily present at elevated
temperatures, which occurs when the Hardox
and Strenx grades are heated in the solid state to
temperatures of about 700°C-1500°C. An attribute
of this microstructure is its toughness. In austenite,

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