Investigation of warm working process of an austenitic creep resisting steel (ST17/13W)

Cattin, Daniel (2020). Investigation of warm working process of an austenitic creep resisting steel (ST17/13W). University of Birmingham. M.Sc.

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Abstract

St17/13W is an austenitic creep resisting steel used by Alstom/GE and produced by Acciaierie Valbruna.
This type of steel responds to warm forging when hardening increase occurs, which does improve subsequent creep life.
The mechanism of this hardening process has not been identified and so an optimisation of process parameters in forming components in this class of alloys is not possible at this stage.
The MSc is proposed to address these aspects.
From the industrial point of view, the main difficulty is to ensure the mechanical characteristics requested for the entire range of flat bars ordered by Alstom: starting from 56 x 40 (the smallest size) to 120 x 60 mm (the biggest), understanding what happens metallurgically speaking during this process; this will guarantee what requested by Alstom.
In order to have a complete knowledge about all processes involved, up to the finished product, each step was covered by a metallurgical analysis, starting from the ingot down to the flat bars used by Alstom for blade production using a mixture of optical, scanning electron and transmission electron microscopy and X-ray diffraction. Characterisation of the industrial process was backed up by Gleeble simulations and related to room temperature and elevated temperature testing. Comparisons were also made with AISI 316.
The principal findings from this study are:

(i) The grain size in forging is sensitive to local strain and temperature at the surface of the billet.
(ii) In the as-forged flat bar state, room temperature mechanical properties are dominated by work hardening and grain size hardening so that both AISI 316 and St17/13W have similar structures and properties. W and Ti do not appear to have fully precipitated in St17/13W but the contribution from solute solution strengthening and precipitation hardening is not significant.
(iii) The as-forged strength is then very dependent on surface temperature for the final reduction and hence to product size.
(iv) During elevated temperature testing, precipitation of M23C6 occurs along grain boundaries in both materials; in St17/13W precipitation of (Ti,W)C also occurs. These impart better hot tensile and stress rupture properties to St17/13W compared with AISI 316.
(v) Short-term creep properties are superior for St17/13W compared with AISI 316, but scatter occurs in the creep life values for St17/13W, which has been related to refined grain size resulting in formation of a continuous grain boundary carbide network and so easier cavitation with more rapid creep rupture.
(vi) St17/13W can provide good creep performance but the process parameters and resulting microstructure require careful control. The precise relationships between these still need to be established.

Type of Work:

Thesis (Masters by Research > M.Sc.)

Award Type:

Masters by Research > M.Sc.

Supervisor(s):