Recrystallisation and grain size development during forging in power generation steels

Kalinowski, Piotr (2017). Recrystallisation and grain size development during forging in power generation steels. University of Birmingham. Ph.D.

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Abstract

Martensitic 9-12%Cr-based creep-resistant steels are currently employed in a number of power generation applications as a cheaper substitute for the more expensive Ni-based superalloys. A tempered martensite microstructure with a uniform, fine prior austenite grain size is desired to give the required strength and creep-resistant properties at service entry.
Refinement of austenite grain size can be achieved via recrystallisation. During hot opendie forging of high-volume rotor discs utilised in power plants non-uniformities of parameters determining austenite grain size, such as temperature, initial grain size, strain and strain rate across the component occur. The main focus of the research was to gain a comprehensive understanding of the influence of the aforementioned parameters on the development of grain size and kinetics of recrystallisation. Experimental efforts were channelled towards the investigation of static recrystallisation as the process governing the grain size development between the forging sequences.
Experimental material for this study was provided by the industrial partner, FOMAS, in a form of a 5-tonne ESR ingot of FB2 steel, stabilised at 760 °C for 24 hours, heat treated at 1150 °C for 10 hours and tempered. Cylindrical axisymmetric test specimens were deformed on the Gleeble 3500TM Thermomechanical Simulator in uniaxial compression mode in the range of temperatures 900-1200 °C, strains 0.15-0.45, strain rates 0.1-5 s-1 and two different initial grain sizes, utilising stress relaxation and double hit methods.
Stress relaxation softening behaviour over the range of experimental conditions has been analysed with the final recrystallised fraction being confirmed by optical microscopy. Aspect ratio development, along with the grain size, have been employed as a recrystallisation progress criterion. Considerable scatter and unusual shapes have been shown by the softening curves during the stress relaxation. Deviations in the amount of recrystallisation, when compared with predictions based on the Dutta-Sellars equations have been identified and discussed, based on a modification of the Avrami exponent used. Grain size measurements allowed the necessary parameters for the modelling of recrystallised grain size to be obtained.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):