Page 62 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Materials and Their Weldability

the material properties and the specific mechanical properties
required. Tempering can continue up to the lower critical
temperature of 723 8C, at which point most of the extra
hardness produced by thermal hardening will have been
removed, but the fine grain structure produced by the
hardening process will remain. Quenched and tempered
(QT) steels are normally tempered from between 550 and
650 8C giving them good toughness and strength
Quenching and tempering of ‘precipitation hardening’
alloys
Precipitation hardening metal alloys have their alloying
elements trapped in solution during quenching, resulting in a
soft material. Ageing a ‘solutionised’ metal will allow the
alloying elements to diffuse through the microstructure and
form intermetallic particles, which fall out of solution and
increase the strength of the alloy. Alloys may age naturally at
room temperature, or artificially at elevated temperatures.
Some naturally ageing alloys can be prevented from age
hardening until needed by storing at subzero temperatures.
Precipitation hardening alloys include 2000, 6000 and 7000
series aluminium alloys, some superalloys and some stainless
steels. An age hardening alloy can be tempered after
quenching by heating at temperatures below the solutionising
temperature. During tempering, the alloying elements will
diffuse through the alloy and react to form intermetallic
compounds. These precipitate out and form small particles
that strengthen the metal by impeding the movement of
dislocations through the crystal structure of the alloy.
The mechanical properties of an alloy can be determined
by careful control of the tempering time and temperature,
affecting the size and amount of precipitates. Artificially aged
alloys are tempered at elevated temperature, while naturally
ageing alloys may be tempered at room temperature. Some
superalloys may be subjected to several tempering operations
where a different precipitate is formed during each operation.
This results in a large number of different precipitates that

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Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200904QG Welding chap4.3d Page 47 of 48