Page 124 - SSAB Welding Handbook Edition 2

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

21.2 Martensite and tempered temperatures in the range of 700ºC-1500ºC. First,
martensite the austenite is formed and, if the cooling rate is
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rapid enough, the martensite forms at the same
Martensite is established in the Strenx and Hardox locations.
grades during and after the quenching process of the
plates during production. Austenite must first form in Tempered martensite forms in the part of the
order for martensite to form. The austenite develops HAZ that has reached peak temperatures of
when the hot rolled plate is heated in the furnace to approximately 150ºC-700ºC during welding. In
temperatures of approximately 900°C -1100°C. addition, martensite that is formed in the HAZ can
Martensite is then formed during the quenching transform in to tempered martensite due to a heat
process when the plate is rapidly cooled to ambient treatment from a subsequent weld pass if there is
temperature. sufficient heating. The exact requirements for
tempered martensite to appear depend on the
Carbon is an alloy element that is necessary for the carbon content of the steel.
formation of martensite and its atomic structure is
displayed in fig. 21.2. At ambient temperatures, both martensite and
tempered martensite can attain a combination of
very high strength and beneficial toughness in both
the unaffected parent metal and the HAZ.

21.3 Bainite
Bainite is a microstructure of high strength and
hardness. The toughness can be more so or less so
advantageous. The HAZ and parts of the unaffected
steel in the Hardox and Strenx grades can include
Fig. 21.2: Martensite. The black atoms illustrate the positions of
the iron atoms. The white circles are possible locations for carbon bainite at ambient temperature.
atoms. The position of each iron and carbon atom represent its
atomic center.
The formation of bainite is similar to martensite.
Initial heating of the parent metal to a temperature in
the range of 700°C-1500°C is necessary in order for
Some of the Hardox and Strenx grades are austenite to form partly or completely. This happens
tempered after the quenching process. Updated during the production process of the Hardox and
information for a specific steel grade is found in Strenx grades as well as during welding when parts
its datasheet. The holding temperatures during of the HAZ reach the peak temperatures. A relatively
tempering can be in the order of 200°C-600°C. high cooling rate is then required for the steel
This type of heat treatment is primarily used as a although it is not as rapid as when martensite is
measure to achieve the desired mechanical properties formed.
in the steel. When tempering is performed in the
mid and upper region of this temperature interval, Bainite consists of ferrite and carbides. The
the microstructure becomes tempered martensite, austenitic structure dissolves carbon to a greater
which means that carbon in the martensite reacts extent than the ferrite. During the cooling process,
with other chemical elements in the steel and there is enough time for carbides to form. Bainite
forms carbides.
can be further sub-divided according to its
appearance.
If martensite forms in the HAZ during welding, it
occurs in the sections that have been heated to

124 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.
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.