MECH470: Transformation Induced Plasticity (TRIP) in Austenitic Steels

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MECH 470: Research Project

Topic: Transformation Induced Plasticity (TRIP)

TO: DR. KEITH PILKEY

FROM: ADAM BROOKS

APRIL 29, 2014



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Table of Contents

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The most important underlying factor behind stabilizing the austenite is the addition of

alloying elements to the steel. The simplest approach is through the addition of nickel

(ex. 20wt%), which however is expensive from an industry market perspective. [9]

Recent developments have been implemented towards 1.5wt% silicon and manganese.

The manganese directly stabilizes the austenite by lowering the martensitic start

temperature, which additionally lowers the starting temperature for the formation of

cementite preventing pearlite from forming during cooling. Silicon plays a key roll by

delaying the formation of carbides, during the bainite transformation (discussed section

3). The chemical content of carbon however is the most important in stabilizing the

austenite. Approximately 1wt% stabilizes the austenite the best, but from an industry

standard the steel is usually limited to 0.2wt% (hypo-eutectoid range, similar to dual

phase steels) for welding purposes. Additional elements such as Al, P, V, Nb, Cr, Mo can

also be added into TRIP steels. Fig. 1illustrates this:



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Strength of the steel formability is increased, thinner sheet can be used. Replacing conventional steel with advanced high-strength steel. Challenge is to design steel, which exhibits both high strength for weight

reduction and good formability for fitting complex structural shapes.

An example strain path of TRIP800 2mm thick steel (&UTS= 836 MPa &yl= 546 MPa)

is presented below [2] Fig. 7,with reported R2plastic anisotropy parameters using

standard Nakajima and Marciniak tests:

Fig. 6:This graph shows that the equiaxed microstructure has higher !!values than the lamellar

at small strains, as well as a higher maximum !! value. However, the lamellar microstructure

demonstrates higher hardening rates at tensile strains above 0.05.



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The TRIP 800 experiences relatively good drawability as R2~ 1, the TRIP 800

will be less prone to thinning/necking compared to conventional HSLA. The value of R2

does vary as a function of the angle between the tensile specimen and the processing

rolling direction due to planar anisotropy.

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The bainite hold is important in the stabilization of austenite at room temperature.

The bainite nucleates at the boundaries of the austenite. Forming on pre-existing

dislocations (from cold work), this produces fine lenticular sub units of super saturated

ferrite, referred to as bainite. The carbon content of the ferrite is higher than equilibrium,

and hence is partitioned to the surrounding austenite or forms carbides around the bainite

sheaves [9]. Its growth must be accommodated plastically by the surrounding austenite,

given that the specific volume of ferrite (BCC) is higher than that of austenite (FCC).

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Fig. 7:The forming limit curve (FLC) for TRIP 800, showing for two different strain path tests,Marciniak and Nakajima with reported R2anisotropy parameters



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So really the bainite rate controlling process can be thought of in two ways: i) the

thermodynamic driving force for nucleation decreases as the carbon content of the

remaining austenite increases. ii) The bainite must be plastically accommodated by the

austenite, which is increasingly strain-hardened by the transformation.

A good understanding comes from the work of Jacques et al. [6], where the length

of the bainite hold time must be carefully chosen as holds which are too short in duration

will not allow enough bainite to grow (hence not stabilizing the austenite), and the

austenite will transform into undesirable martensite upon quenching to room temperature.

On the contrary, if the bainite hold time is too long the high carbon austenite will

decompose into carbides, again reducing the volume percent of retained austenite. (Fig.

8) The silicon content of the TRIP steel is important because silicon is insoluble in

cementite and the silicon content will then delay the formation of carbides during the

bainite transformation, as there is a time delay for the silicon to diffuse away from the

bainite grain boundaries before cementite can form [6].



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MECH470: Transformation Induced Plasticity (TRIP) in Austenitic Steels

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