Page 133 - SSAB Welding Handbook Edition 2

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Welding handbook 23.0 The influence of the alloys and contaminants on the Weldox and Hardox grades

Issues regarding grain refinement 23.7 Silicon (Si)
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During welding performances, the parts of the HAZ Silicon has a similar function to aluminum in the
that are closest to the weld metal are subjected to respect that it removes oxygen during steel
peak temperatures above ~1100°C. At these production. Silicon reacts with oxygen and forms
temperatures, considerable grain growth takes place slag. The slag floats to the surface of the liquid steel
and the grains remains enlarged throughout the where it is removed.
welding process.

A second effect of silicon is that it adds yield
Precipitations of Nb, Ti, V and Al can reduce grain strength, tensile strength and hardness to the parent
growth in the HAZ where the precipitations metal through substitutional solution hardening of
including Nb and Ti have the largest effect. Each the steel.
kind of precipitate exists up to a certain temperature.
If the temperature is increased, the precipitates
dissolve in the microstructure. Grain growth is 23.8 Sulfur (S) and phosphorus (P)
counteracted as long as the precipitations are
present. Sulfur and phosphorus are contaminants in the steel.
Sulfur can react with other elements and form
Examples of the maximum temperature for sulfides in the parent metal, which can reduce the
different precipitates are: toughness of the steel. The addition of manganese in
4 Vanadium nitrides are present at temperatures the Hardox and Strenx grades counteracts the
up to ~950-1050°C negative effects of sulfur.

4 Niobium carbides are present at temperatures
up to ~1100-1250°C The level of phosphorus is to be kept as low as
4 Titanium nitrides are present at temperatures possible. High levels of this element would impair
up to ~1400°C the toughness in the grain boundaries of the steel,
which would lead to an overall reduced toughness
of the parent metal.
Precipitates form again during the cooling phase
of welding. Once they are re-established, they can The Hardox and Strenx grades have a very low
continue to reduce the growth of the grains.
content of both sulfide and phosphorous. As a
result, these steel grades have a substantially
The ability for these precipitates to obstruct grain strong resistance to the negative effects from
growth is also used to minimize the final grain size these contaminants.
in the steel during the production of the Hardox
and Strenx grades.

Supplementary aspect of aluminum 23.9 Nitrogen (N)

Another purpose for adding aluminum is that it Nitrogen acts in different ways in the parent metal.
reduces the oxygen content of the melt during the An increase in yield strength, tensile strength and
metallurgical part of the steel production. Oxygen hardness of the parent metal can be attained since
must be diminished in the melt in order to produce it contributes to precipitation hardening, interstitial
the Hardox and Strenx grades. Aluminum reacts solution hardening and grain refinement. However,
with oxygen and forms slag which floats to the nitrogen leads to a significant reduction in toughness
surface of the liquefied steel where it is removed. in the parent metal when present at levels that are
too high. Consequently, the nitrogen levels are kept
low in the Hardox and Strenx grades.

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