13 - q+t steels & welding q+t steels
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08/06/2004Istituto Italiano della saldaturaENTE MORALE
WELDABILITY OF LOW ALLOYED
QUENCHED AND TEMPERD
STEELS
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Introduction QUENCHED AND TEMPERED steels
are designed especially for welded
construction and fabrication.
They are furnished from steel producers
in the heat treated condition
Yield strengths range from 350 to 1000
MPa) depending on
their chemical composition
Thickness
type of heat treatment.
The heat treatment for most of these
steels consists of austenitizing,
quenching, and tempering.
A few are given a precipitation-
hardening (i.e., aging) treatment
following hot rolling or some other
thermal treatment.
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Metallurgy The carbon content of quenched and tempered steels generally does not
exceed 0.22% for good weldability.
The other alloying elements are carefully selected to provide the most
economically heat-treated steel with the desired properties and acceptable
weldability:
Microalloyng elements (Al, Ti, V, Ti, Zr, Nb)
Quenching elements (Cr, Mo, Cu, Ni)
Small quantities of Boron, 5 to10 ppm, in order to reduce grain size.
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Chemical composition EN 10137-2
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Tensile properties EN 10137-2
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Toughness EN 10137-2
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Chemical composition EN 10028-6
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Tensile properties EN 10028-6
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Toughness properties - EN 10028-6
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ASTM designations
f Q
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Commercial designations
T1 (grades T1, T1A, T1B e T1C)
US Steel patented, is classified
as
ASTM A514 structural
ASTM A517 pressure equipment
HY
US Navy specification
used for submarines
Good toughness properties at
low temperature
USS Skipjack, 1965
W ld bilit f Q T t l
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Chemical composition
W ld bilit f Q T t l
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Weldability
Cold cracking
The metallurgical structure is anon-equilibrium structure
Hydrogen must be as low as
possible
Preheat temperature must be
properly selected Hot cracking
Susceptivity is generally low
Some phenomena may occur
depending on Ni content
Lamellar tearing
The heat treatment generally
guarantees good properties
W ld bilit f Q+T t l
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Weldability
To obtain the same
properties of the materialafter welding:
Low heat input
Low preheat temperature
Low interpass temperature
W ld bilit f Q+T t l
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Weldability - PWHT
The mechanical properties of thewelded steel or the weld metal itselfmay be adversely affected by astress relief.
The alloying elements can causeadverse effects when the weldment
is postweld heat- treated: A PWHT in the temperature range of
510 to 650 C, with slow cooling speeds,may decrease the toughness of the weldmetal and the HAZ.
Reheat cracking can occur whenweldments of many high-strength steelsare heated above about 510 C.Intergranular cracking may take pIace inthe coarse-grained region of the weldHAZ.
31,83,3 VMoCrCS
The precipitation of carbides during stressrelaxation at elevated temperature alters thedelicate balance between resistance to grainboundary sliding and resistance to deformationwithin the coarse grains in the HAZ.
CS Is a Reheat Cracking parameter proposedby Nakamura Reference value is 0
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Weldability - PWHT
With any of the quenched and tempered steels, a postweld
stress-relief heat treatment should be used only afterassurance that worthwhile benefits can be achieved and
possible harmful effects can be tolerated. Stress relief is
desirable for some applications when:
Dimensional stability must be maintained during close tolerance
machining,
The weld is susceptible to stress corrosion after cold forming or
welding.
If postweld stress relief is required, the temperature must
not exceed that used for tempering the steel. A temperatureabout 30 to 60C lower than the tempering temperature is
desirable to avoid lowering the strength of the steel.
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Weldability Temper Beads
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Filler Metals- MMAW: EN 757
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Filler Metals- MMAW: EN 757
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Filler Metals GMAW: EN 12564
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Filler Metals - MMAW: EN 757
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