MS Calculator Android App

A researcher at GIFT has produced an metallurgical application for android phones for calculation of MS, ε-MS, Ae3, Ae1, and conversion of weight percent to atomic percent.

MS Calculator Android App, screen shot 1

MS Calculator Android App

MS Calculator App webpage

MS Calculator Android App, screen shot 1

MS Calculator Android App

Tempering of Martensite

Stage 1
upto – 250 C – precipitation of eta-carbide, partial loss of tetragonality of martensite.
Steels above 0.25 wt% carbon precipitate hexagonal close-packed eta-carbide within the supersaturated martensite until 0.25 wt% carbon level is reached, martensite preserves some tetragonality. The orientation relationship between the laths or rodlets and the cube planes of the matrix was first described by Jack [1].

Stage 2
200-300 C – decomposition of retained austenite.

Stage 3
200-350 C – replacement of eta-iron carbide by cemeneite,; martensite loses tetragonality

Stage 4
350 C – cementite coarsens and speroidizes, recrystallisation of ferrite.

[1] Steels 2nd Ed, Honeycombe and Bhadeshia, Edward Arnold, 1981, p172.

Making Podcasts

More podcasts this weekend…

Crystallographic texture and intervening transformations

Stress of Strain affected martensitic transformation

Quantitative Metallography of Deformed grains

Steels for Fusion

New Podcasts

I made three new podcasts with Prof. Harry Bhadeshia on his latest papers on transformation texture, the new delta-Trip steels and on prediction of Hot Strength of ferritic steels.

The work on transformation texture is from Saurabh Kundu’s thesis were Patel and Cohen’s model has been shown to correctly predict the orientation relationship between ferrite and austenite after martensitic transformation. It’s shown that variants are selected by free energy differences that can be calculated depending on the orientation.

The delta-Trip steels were developed as a result of the prediction of neural networks, were after the neural network was made computer optimisation was used to try and maximise the mechanical properties. This work was done with Saurabh Chatterjee in collaboration with Murugananth Marimuthu. Both Saurabh Kundu and Saurabh Chatterjee completed their PhD’s at Cambridge while visiting from Tata Steel, Murugananth Marimuthu is a previous member of the phase transformations group, and has now also joined Tata Steel’s research and development section.

The work on Hot Strength of ferritic steels is the part of Radu Dmitriu’s topic of research. A neural network model of the hot-strength of ferritic steels. It was observed from the neural network that the strength is expected to suddenly start to decrease at 800 Kelvin, which can has been explained to be due to changes in the mobility of dislocations.

Addendum

According to wordpress documentation these links should get added to the rss feed of this bainite blog as enclosures.

Podcast: Hot Strength

Podcast: Delta Ferrite

Podcast: Transformation Texture

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Subscibe to RSS Feed for posts I remember to add to Podcasts category

Martensite kinetics

Martensite transformation usually occurs on cooling, with the volume fraction of martensite depending upon the temperature as shown in the first two examples below. Martensite transformation is often thought only to depend upon the temperature, but has been shown to also depend upon the time. It should therefore be represented by it’s own C-curve on a time-temperature-transformation graph. The strong temperature dependence may be caused by the large driving force necessary before transformation can proceed.

The Koistinen-Marburger equation

This equation due to Koistinen[1] and Marburger can be used to estimate the fraction of martensite as a function of temperature.

Steels were quenched to various temperatures and the austenite volume fraction determined by X-ray diffraction. The results plotted logarithmically against Ms-Tq, where Ms is the martensite start temperature and Tq is the quench temperature.

V_gamma = exp [-b(Ms-Tq)] where b is 1.1×10^-2

Magee[2] later showed that an equation of this form can be justified based on martensite nucleation theory.

Rearranging the formula is simple to give the volume fraction of austenite transforming to martensite on cooling to room temperature as a function of the MS temperature of the steel.

Khan and Bhadeshia [3] have produced a modified equation which also considered autocatalytic nucleation of martensite to allow better matching to dilatometry results. This changed the goodness of the fit by regression from 0.9 to 0.94, when analysing the data of volume fraction against temperature obtained from their own experiments. This work used one alloy (300M) but changed the starting conditions by partial transformation to bainite.

[1] D. P. Koistinen and R. E. Marburger, Acta Metallurgica, 7, p59-60, 1959
[2] C. L. Magee, The nucleation of martensite, In “Phase transformations”, p115-156, Ed: H. I. Aaronson and V. F. Zackay, ASM, Metals Park, Ohio, 1970.
[3] S. A. Khan and H. K. D. H. Bhadeshia “Kinetics of Martensitic Transformation in Partially Bainitic 300M Steel” Materials Science and Engineering A, Vol. A129, 1990, pp. 257-272.