Zhou, S; Turnbull, A (2001) Environment assisted cracking of steam turbine disc and blade steels - description of test procedure and preliminary results. NPL Report. MATC(A)41

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Abstract

A facility has been established for investigating the resistance to environment assisted cracking of turbine steels. A distinctive feature is very tight control of water chemistry using a once-through water system for chloride-free environments and refreshing the reservoir weekly for the chloride-containing recirculated solutions.
Constant displacement tests have been initiated on a 3% NiCrMoV disc steel using smooth and pre-pitted specimens in three environments at 90 ºC: deaerated high purity water; aerated high purity water; aerated 1.5 ppm Cl - solution. In addition, the effect of oxygen and chloride content on the growth rate of long cracks has been studied using wedge opening loaded (WOL) specimens.
Electrode potential measurements indicate that chloride and oxygen content have a significant effect on the corrosion potential of the disc steel. In deaerated pure water, the potential was about –0.62 V (SCE) whilst in aerated pure water the potential was about +0.10 V (SCE); both steady values being attained in about 10-20 days. By contrast, about 250 days was required to attain the steady corrosion potential in aerated 1.5 ppm Cl - . The slow change in the corrosion potential in the more aggressive environment suggests caution in interpreting the results of short-term laboratory stress corrosion cracking tests.
To give insight as to the significance for service life of two-shifting operation, the impact of trapezoidal loading (the stress ratio was zero and the loading frequency was 4 cycles /day) on the growth rate of long cracks of disc and 12% Cr blade steels has been assessed using compact tension (CT) specimens in aerated 1.5 ppm Cl - solution at 90 ºC and in aerated 35 ppm Cl - solution at 120 ºC respectively. Tests are still ongoing, but preliminary results have shown that the long crack growth rate of the disc steel under transient loading is approximately four times higher than that under constant loading. A similar comparison is not possible at this stage for the blade steel but the growth rate under transient loading is about 1.87 x 10 -10 m/s.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	MATC(A)41
Subjects:	Advanced Materials Advanced Materials > Corrosion
Last Modified:	02 Feb 2018 13:17
URI:	http://eprintspublications.npl.co.uk/id/eprint/2514

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