Page 33 - SSAB Welding Handbook Edition 2

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Welding handbook 5.0 Properties of the heat-affected zone

The period of time during welding when the CGHAZ during welding even though carbide precipitations
reaches temperatures between 1500ºC -1100ºC is of small, hard particles can take place. The typical
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relatively short, which causes this zone to be narrow in microstructures at room temperature are tempered
relation to other parts of the HAZ. The microstructure martensite, bainite or a combination of the two.
in the HAZ at maximum temperatures is austenite. The toughness in this section can be somewhat low.
During the cooling process, when room temperature
is reached, the microstructure typically transforms
to one, or a combination, of martensite or bainite. 5.2 The width of the HAZ
Generally, with an enlargement of grains comes a
reduction in toughness in the parent metal as well as The width of each of the four regions in the HAZ
the HAZ. As a consequence, the coarse grain zone depends on the heat input and the preheat level for
normally has the lowest toughness in the HAZ. a specific welding procedure, which is illustrated
for a butt joint in fig. 5.3.

The fine grained zone
The fine grained zone is subjected to maximum
temperatures in the range of 1100°C and 900°C during High heat input
welding. This leads to small grain sizes in this section
after the welding performance, which contributes The coarse Low heat input
grained zone
to beneficial toughness values in the joint. The
microstructure is considered as austenite at the The fine
grained zone
peak temperature. At ambient temperature, the The partially
microstructure transforms to one, or a combination, transformed
zone
of martensite or bainite. Another name for this zone The annealed
is the normalized HAZ. zone

High heat input
The partially transformed zone
Low heat input
The welding thermal cycle leads to a peak temperature The coarse grained
between ~900ºC and 700ºC in this zone. During zone HAZ
the peak temperature, the microstructures consist Fig. 5.3: The extension of the HAZ at different heat inputs.
of austenite, tempered martensite and in some cases
bainite. After cooling to room temperature, this zone
typically includes one or more of the microstructures The hardness values in joints of Hardox
tempered martensite, martensite and bainite. The and Strenx
grain sizes remain relatively small during and after
the welding performance. Their sizes at ambient The hardness values in the HAZ for quenched/
temperatures are typically somewhat larger than in quenched and tempered steels have unique
the fine grained HAZ. characteristics.

In the Hardox and Strenx grades, the peak hardness
This part of the HAZ is also called the intercritical
HAZ. The toughness in this zone can be relatively in the HAZ is located immediately adjacent to the
low. weld metal. With further distance from the weld
metal, the hardness value decreases down to its
The annealed zone minimum level. The hardness values then gradually
increase in the HAZ until they reach the values of the
The annealed zone is the outer part of the unaffected parent metal.
HAZ where the temperature has peaked below
approximately 700°C. At these temperatures, there
is not a change in the microstructure or grain size

This handbook contains general suggestions and information without any expressed or implied warranty of any kind. SSAB hereby expressly disclaims
all liability of any kind, including any damages, in connection with the use of the information and for their suitability for individual applications. 33
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.