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What is the Larson-Miller parameter?

 

The Larson-Miller parameter describes the equivalence of time at temperature for a steel under the thermally activated creep process of stress rupture. It permits the calculation of the equivalent times necessary for stress rupture to occur at different temperatures.

It has the general form:

faqbkl007e1.gif

L is the Larson-Miller parameter;
T is the temperature in degrees Rankine (°F + 460);
t is the time in hours for an isothermal condition.

The factor of 1000 is sometimes, but not always, used.

The factor of approximately 20 is applicable to C-Mn and low alloy steels; alternatively, a factor of 30 is sometimes applied in the case of higher alloy steels, e.g. 9%Cr steels.

It is also possible to include heating and cooling by adding an additional time, δt, to the existing time factor in the above expression:

faqbkl007e2.gif

K is the heating or cooling rate in Kelvin per hour.

The Larson-Miller parameter has been modified for use on creep-resistant alloys with a high degree of dispersion strengthening, e.g. some nickel alloys and steels [1] . Another modification of the Larson-Miller parameter - the Manson-Haferd parameter - has been proven to be more applicable to 9%Cr steels than the Larson-Miller parameter [2] :

faqbkl007e3.gif

P (σ) is the Mansen-Haferd parameter, and tr is the rupture duration (in hours).

The Larson-Miller parameter is similar to the Hollomon-Jaffe parameter, which deals with postweld heat treatment, but has different units.

References

  1. Adamovich  A, Zemzin VN, Rychkova LV, 'A double displacement parametric method of predicting the properties of welded joints in ageing', Paton Welding Journal, Vol.4, No.1, pp.9-15, 1992.
  2. Bell  K, 'An analysis of published creep rupture data for modified 9%Cr steel weldments', TWI Research Report 598/1997.

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