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7/27/2019 2.10 Thermomechanically Treated Steelsl
1/9
The Mark of
Training
Excellence
IWT Course Material
Module 2 Materials and Their
Behaviour During Welding
2.10 - Thermom echanic ally treated steels
7/27/2019 2.10 Thermomechanically Treated Steelsl
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The Mark of
Training
Excellence
1
Common properties of all fine grained steels:
- Relativaly low C content 0.20%
- Limited no. of alloying elements.- Fine grain size
- Good hardenablility
Fine grained steels have been developed with yields 275-700 MPa with high
toughness, resistance to brittle fracture and good weldability but with significantly
less C than other normalised or Q & T fine grained steels.
Achieved by thermomechanical treatment. C < 0.20%.Fine grain, with precipitation
hardening by microalloying with Nb, Ti,V.
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The Mark of
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Full strength attained through reformation of grains and hardening, transformation,
and recrystallisation occur one after another and partly simultaneously- optimise this
process to achieve optimised material properties
Thermomechanical treatment sequence of deformation and heat treatment.
For TMCP steels, the mechanical properties are determined primarily by a
combination of mechanical working, recovery processes, recrystallisation, and grain
growth.
The mechanical properties introduced to the steel through this processing route are
virtually equivalent to those obtained by heat treating conventionally rolled or forged
steel. TMCP involves controlled hot working and microalloyed steel compositions.
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The Mark of
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The rough steel section is heated to a temperature regularly used for hot working
operations (about 1200C).
The initial hot working ("roughing") is carried out in a normal fashion, but the finalhot work reduction or "finishing pass" is carried out at a lower temperature than
would be used for older processes.
Plastic deformation at this lower temperature promotes fine grain sizes and retards
precipitation.
The final hot working may continue down to temperatures below the A3 critical
temperature (transformation from austenite to ferrite).
This requires heavy rolling equipment capable of deforming the steel at low hot
working temperatures.
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The optimum precipitate size and dispersion is obtained when the finish rolling
temperature is around 775C.
The cooling which follows brings the steel to the transformation temperature range,and the austenite to ferrite transformation results in fine ferrite grains and fine
dispersed precipitates.
For some TMCP steels, this last stage of cooling, during which transformation is
completed, is accelerated by water cooling, to give a finer grain size.
Accelerated cooling can sometimes result in bainite formation as well as, or instead
of ferrite formation.
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The Mark of
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The Mark of
Training
Excellence
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- Examples are the Domex steels.
- Relatively low loss of hardness max. 20 HV10 & loss of strength in HAZ.
- Choice of consumables based on matching properties of the base metal.
- TMCP steels (Thermo-Mechanically Controlled Processing).
For a given strength level, a TMCP steel will have a lower alloy content than a
normalised steel, and thus will be more readily weldable with regard to avoidance
of HAZ HIC, and achievement of maximum hardness limits.
Heat affected zone fracture toughness properties are generally better in TMCPsteels than normalised steels and adequate fracture toughness can often be
obtained up to higher heat inputs.
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There will be some degree of softening in the HAZs of TMCP steels afterwelding.
Reduction in joint strength, is unusual in those that have not had acceleratedcooling as part of the manufacturing process.
TMCP steels that have been manufactured using an accelerated cooling methodto achieve the desired properties are not generally resistant to high heat inputs, &a degradation of properties can occur on welding.
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The Mark of
Training
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Cooling rate in the weld region may be slower than that of production. In these
steels it is important that the cooling rate is high, so that the grain size of the
weld & HAZ can be maintained to give the desired properties.
This is achieved by the use of moderately low heat input levels, typically 2.5
kJ/mm for 15 mm plate in these steels.