Mechanistic interpretation on acidic stress-corrosion cracking of NiCrMoV steam turbine steel

This work evaluates the effect of film-forming amines (FFA) on the acidic stress-corrosion cracking (SCC) resistance of NiCrMoV turbine steel. Contact angle measurements show an increased hydrophobicity of the surface when coating the steel with oleyl propylene diamine (OLDA). According to potentiodynamic measurements and post-mortem scanning electron microscopy (SEM) analysis, anodic dissolution and hydrogen embrittlement still occur when the steel is FFA coated. In situ constant extension rate testing (CERT) in acidic aqueous environment at elevated temperature of FFA-coated steel shows a ductility gain compared to non-coated steel, explained by a decrease in both corrosion rate and hydrogen uptake.

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Study on Stress Corrosion Cracking Sensitivity of CrNiMoV Steam Turbine Rotor Steels

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.795.102 ◽

2019 ◽

Vol 795 ◽

pp. 102-108 ◽

Cited By ~ 1

Author(s):

Shu Xian Lin ◽

Yu Hui Huang ◽

Fu Zhen Xuan ◽

Shan Tung Tu

Keyword(s):

Stress Corrosion ◽

Steam Turbine ◽

Stress Corrosion Cracking ◽

Turbine Rotor ◽

Optical Microscope ◽

Corrosion Cracking ◽

Slow Strain Rate Test ◽

Operation Condition ◽

Steam Turbine Rotor ◽

Rate Test

The stress corrosion sensitivities of 25Cr2Ni2MoV, 26NiCrMoV10-10 and 30Cr2Ni4MoV low-pressure rotor steels in simulated nuclear steam turbine operation condition were investigated by slow strain rate test (SSRT), and the stress corrosion cracking (SCC) mechanisms were studied by optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Results revealed that the SCC sensitivity of 25Cr2Ni2MoV steel was highest in 3.5wt.%NaCl solution at 180°C, while the SCC sensitivity of 26NiCrMoV10-10 steel and 30Cr2Ni4MoV steel are similar. The SCC sensitivity of CrNiMoV steam turbine rotor steels could be decreased by the increase of Ni element and the decline of mechanical intensity. Cracks initiate from metal surface and then propagate to the inner metal, which showed a form of transgranular cracking.

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Stress Corrosion Cracking in Steam Turbine: Two Case Studies

Volume 8: Microturbines, Turbochargers and Small Turbomachines; Steam Turbines ◽

10.1115/gt2017-63665 ◽

2017 ◽

Cited By ~ 1

Author(s):

Vamadevan Gowreesan ◽

Kirill Grebinnyk

Keyword(s):

Electron Microscope ◽

Fracture Surface ◽

Scanning Electron Microscope ◽

Stress Corrosion ◽

Steam Turbine ◽

Stress Corrosion Cracking ◽

Corrosion Cracking ◽

Steam Turbines ◽

Root Section ◽

Scanning Electron

Stress corrosion cracking in steam turbines had been an old problem though some modern steam turbines have almost eliminated this problem by several methods. The methods include design modification to reduce the stress levels below the threshold stress level for stress corrosion cracking, inducing compressive stress by different means and using pure steam [1, 2]. Some of the older steam turbine discs are prone to stress corrosion cracking. Two cases where such machines experienced stress corrosion cracking in their discs are discussed here. The row 6 disc of an integral steam turbine rotor developed cracks in the root sections. Some of the cracks were mechanically opened for the evaluation. Evaluation of the fracture surfaces with a scanning electron microscope showed evidence of intergranular mode of cracking. Optical microscopy of a cracked root confirmed intergranular mode of cracking. In addition, it showed branching of cracks. Based on these findings, it was concluded that stress corrosion cracking was the reason for the cracks. In addition, finite element analysis was used to calculate the stress distribution in the blade root of the disc. The location of the maximum equivalent stress coincided perfectly with that of the actual crack location in the disc root section. Unfortunately, redesign of the root geometry to minimize the local stress concentration is very difficult due to the size limitation of the blade roots. Small amount of chlorine was identified on the fracture surface and the chlorine could have come from the steam used. The customer was advised to analyze their steam quality and to improve the quality of the steam if needed. The cracked portion was removed from the disc and weld-build up to machine new root sections with the same type of roots. Root section of the row 6 disc of another steam turbine developed failure. This disc had radial entry type blades. Portion of the disc root and some blades were liberated from the disc due to the cracking. The fracture surface had heavy oxide layer on it. Evaluation of the fracture surface with a scanning electron microscope revealed intergranular mode of failure. Energy dispersive spectroscopy analysis of the fracture surface found oxides on the fracture surface. Optical microscopy showed secondary cracking and branched cracking. All these evidences confirmed that the failure occurred due to stress corrosion cracking. In addition, it was suspected that forging was not heat treated properly due to measured lower toughness and different microstructure. The lower toughness was believed to be a result of improper heat treatment rather than that of embrittlement. Methods to mitigate the risk of stress corrosion cracking were proposed.

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Stress Corrosion Cracking and Corrosion Fatigue Analysis of Nuclear Steam Turbine Rotor

Volume 1B: Marine; Microturbines, Turbochargers and Small Turbomachines; Steam Turbines ◽

10.1115/gt2014-25583 ◽

2014 ◽

Cited By ~ 1

Author(s):

Gang Chen ◽

Puning Jiang ◽

Xingzhu Ye ◽

Junhui Zhang ◽

Yifeng Hu ◽

...

Keyword(s):

Stress Corrosion ◽

Steam Turbine ◽

Corrosion Fatigue ◽

Stress Corrosion Cracking ◽

Nuclear Power ◽

Power Plants ◽

Fatigue Cracking ◽

Corrosion Cracking ◽

Major Influence ◽

Steam Turbines

Although stress corrosion cracking (SCC) and corrosion fatigue cracking can occur in many locations of nuclear steam turbines, most of them initiate at low pressure disc rim, rotor groove and keyway of the shrunk-on disc. For nuclear steam turbine components, long life endurance and high availability are very important factors in the operation. Usually nuclear power plants operating more than sixty years are susceptible to this failure mechanism. If SCC or corrosion fatigue happens, especially in rotor groove or keyway, it has a major influence on nuclear steam turbine life. In this paper, established methods for the SCC and corrosion fatigue-controlled life prediction of steam turbine components were applied to evaluating a new shrunk-on disc that had suffered local keyway surface damage during manufacture and loss of residual compressive stress.

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Prediction of Failures of Low-Pressure Steam Turbine Disks

Journal of Pressure Vessel Technology ◽

10.1115/1.2842321 ◽

1997 ◽

Vol 119 (4) ◽

pp. 393-400 ◽

Cited By ~ 18

Author(s):

C. Liu ◽

D. D. Macdonald

Keyword(s):

Stress Corrosion ◽

Steam Turbine ◽

Stress Corrosion Cracking ◽

Wilson Line ◽

Corrosion Damage ◽

Low Pressure ◽

Corrosion Cracking ◽

Localized Corrosion ◽

Pressure Steam ◽

Turbine Disks

Localized corrosion phenomena, including pitting corrosion, stress corrosion cracking, and corrosion fatigue, are the principal causes of corrosion-induced damage in electric power-generating facilities and typically result in more than 50 percent of the unscheduled outages. In this paper, we describe a deterministic method for predicting localized corrosion damage in low-pressure steam turbine disks downstream of the Wilson line, where a condensed, thin electrolyte layer exists on the steel disk surfaces. Our calculations show that the initiation and propagation of stress corrosion cracking (SCC) is not very sensitive to the oxygen content of the steam, but is sensitive to the conductivity of the condensed liquid layer and the stresses (residual and operational) that the disk experiences in service.

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