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IWT Course Material

Module 2 Materials and Their

Behaviour During Welding

2.10 - Thermom echanic ally treated steels

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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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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 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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- 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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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.