Analysis of the Degradation in the Creep Strength of High-Cr Martensitic Steels

To investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel, the creep curves of Grades T91, T92, and P92 were analyzed along with the experimental steels of 9Cr-1W and 9Cr-4W by applying an exponential law to the temperature, stress, and time parameters. The activation energy (Q ), activation volume (V ), and Larson-Miller constant (C ) were obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening occurs due to dislocations that are swept out of the sub-grains, which is followed by strengthening due to the rearrangement of M23C6 and the precipitation of the Laves phase. After Q  reaches a peak, heterogeneous recovery and subsequent heterogeneous deformation begin at an early stage of transient creep in the vicinity of several of the weakest boundaries due to coarsening of the precipitates. This activity triggers an unexpected degradation in strength due to the accelerated formation of the Z-phase. Stabilization of M23C6 and the Laves phase is important for mitigating the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for the Cr-Mo systems because Fe2Mo is easily coarsened at ~600 °C as compared to Fe2W. Lowering the hardness and Si content also prevents excess hardening due to the Laves phase, which also mitigates the degradation. The online monitoring of creep curves and the QVC  analysis render it possible to detect signs of long-term degradation under targeted conditions within a relatively short period.

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Root Cause of Degradation in the Creep Strength of Martensitic Steel

Journal of Materials Science Research ◽

10.5539/jmsr.v11n1p1 ◽

2021 ◽

Vol 11 (1) ◽

pp. 1

Author(s):

Manabu TAMURA

Keyword(s):

Martensitic Steel ◽

Rupture Strength ◽

Early Stage ◽

High Strength ◽

Transient Creep ◽

Laves Phase ◽

Grade 92 Steel ◽

Grade 91 ◽

Heterogeneous Deformation

Creep curves of Grade 91 and 92 steels were analyzed by applying an exponential law to the temperature, stress, and time parameters to investigate the formation process of the Z-phase, which lowers the long-term rupture strength of high-Cr martensitic steel. The activation energy (Q ), activation volume (V ), and Larson–Miller constant (C ) were obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening occurs because of dislocations that are swept out of the sub-grains, and this is followed by strengthening owing to the rearrangement of M23C6 and the precipitation of the Laves phase. Heterogeneous recovery and subsequent heterogeneous deformation start at an early stage of transient creep near several of the weakest boundaries because of the coarsening of the precipitates; this results in the simultaneous decreases in Q , V , and C  even in transient creep. Further, this activity triggers an unexpected degradation in strength because of the accelerated formation of the Z-phase even in transient creep. The stabilization of M23C6 and the Laves phase is important to mitigate the degradation of the long-term rupture strength of high-strength martensitic steel. The stabilization of the Laves phase is especially important for Cr-Mo systems because Fe2Mo is easily coarsened at approximately 600 °C compared to Fe2W in Grade 92 steel.

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Analysis on Degradation in Creep Strength of 9Cr-W Martensitic Steel

Journal of Materials Science Research ◽

10.5539/jmsr.v10n1p1 ◽

2021 ◽

Vol 10 (1) ◽

pp. 1

Author(s):

Manabu TAMURA

Keyword(s):

Activation Volume ◽

Martensitic Steel ◽

Rupture Strength ◽

Early Stage ◽

Laves Phase ◽

Creep Mechanism ◽

Steel Grades ◽

Time Parameters ◽

Heterogeneous Deformation

In order to clarify the creep mechanism of high Cr martensitic steel, creep curves of 9Cr-1W and 9Cr-4W steels were analyzed applying an exponential law to the temperature, stress, and time parameters. The activation energy, Q, the activation volume, V, and the Larson-Miller constant, C, are obtained as functions of creep strain. At the beginning of creep, sub-grain boundary strengthening by swept dislocations out of sub-grains occurs followed by strengthening due to the rearrangement of M23C6 and the precipitation of Laves phase. After Q reaches a peak, heterogeneous recovery and subsequent heterogeneous deformation begin at an early stage of transient creep in the vicinity of some weakest boundaries due to coarsening of the precipitates, which triggers the unexpected degradation in strength due to the accelerating coarsening of precipitates. Stabilizing not only M23C6 but also Laves phase is important to mitigate the degradation of rupture strength of martensitic steel. The above creep mechanism for martensitic steel can be applicable to the explanation for the degradation in long term rupture strength of high Cr martensitic steel, Grades 91 and 92.

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Long Term Creep Rupture Strength of 9Cr1MoNbV Narrow Gap Welded Joints Improved by Normalizing and Tempering After Welding

Volume 9: Eighth International Conference on Creep and Fatigue at Elevated Temperatures ◽

10.1115/creep2007-26570 ◽

2007 ◽

Cited By ~ 1

Author(s):

Takashi Sato ◽

Kohji Tamura ◽

Koichi Mitsuhata ◽

Ryuichi Ikura

Keyword(s):

Welded Joints ◽

Power Plants ◽

Rupture Strength ◽

Thermal Power ◽

High Strength ◽

Creep Rupture ◽

Narrow Gap ◽

Parent Metal ◽

Welding Procedure

With the increase of steam parameters of coal-fired thermal power plants, high strength 9%Cr steel containing niobium and vanadium became major material in high temperature boiler components. As the microstructure of these steels is tempered martensite, it is known that the softening occurs in HAZ of the weldment. In the creep rupture test of these welded joints the rupture strength is lower than that of the parent metal, and sometimes this reduction of strength is caused by Type IV cracking. To develop an effective method to improve the rupture strength of welded joints, a normalizing-tempering heat treatment after weld was proposed. 9Cr1MoNbV plates with a thickness of 40–50 mm were welded by 10 mm width automatic narrow gap MAG welding procedure using specially modified welding material. After normalizing at 1050C and tempering at 780C, material properties of the welded joints were examined. Microstructure of the HAZ was improved as before weld, and rupture strength of the welded joints was equal to that of the parent metal. The long term rupture strength of the welded joints as confirmed in the test exceeded 30,000 hours. This welding procedure has been applied to the seam weld of boiler hot reheat piping in USC plants successfully.

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Tribological Characteristics of Nickel-Aluminium Bronze CuAl10Ni5Fe4 against 30CrMnSiA Steel after the Prior Corrosion Treatment

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.201-202.73 ◽

2012 ◽

Vol 201-202 ◽

pp. 73-77 ◽

Cited By ~ 1

Author(s):

Zhi Hai Tan ◽

Qiang Guo ◽

Wen Kai Zhai ◽

Zheng Ping Zhao

Keyword(s):

Wear Resistance ◽

Friction Coefficient ◽

Wear Rate ◽

Early Stage ◽

Tribological Characteristics ◽

Sliding Bearing ◽

Short Period ◽

Aluminium Bronze ◽

Resistance Performance

The tribological characteristics of nickel-aluminium bronze CuAl10Ni5Fe4 against 30CrMnSiA steel after the prior corrosion treatment was studied at the M2000A model wear tester. The wear rate of corroded specimen was less than that of uncorroded specimen at the early stage of prior corrosion treatment, while the wear resistances deteriorated and the average friction coefficient slightly declined with the extension of prior corrosion period. The tests of CuAl10Ni5Fe4 sliding bearing at PLS-100 swing tribometer were used to validate the results. The wear resistances of CuAl10Ni5Fe4 sliding bearing increased slightly after a short period of prior corrosion, and the average friction coefficient decreased slightly after a long term of prior corrosion. A suitable period of prior corrosion treatment can enhance the wear resistance performance of CuAl10Ni5Fe4 bearing. The tribological characteristics of CuAl10Ni5Fe4 sliding bearing tested by PLS-100 rack swing tribometer are consistent with the conclusions of the tests at M2000A model wear tester.

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Laves-phase in the China Low Activation Martensitic steel after long-term creep exposure

Materials & Design (1980-2015) ◽

10.1016/j.matdes.2014.06.028 ◽

2014 ◽

Vol 63 ◽

pp. 333-335 ◽

Cited By ~ 16

Author(s):

Lixin Huang ◽

Xue Hu ◽

Wei Yan ◽

Wei Sha ◽

Furen Xiao ◽

...

Keyword(s):

Martensitic Steel ◽

Laves Phase ◽

Term Creep ◽

Creep Exposure

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Heuristic Design of Advanced Martensitic Steels That Are Highly Resistant to Hydrogen Embrittlement by ε-Carbide

Metals ◽

10.3390/met11020370 ◽

2021 ◽

Vol 11 (2) ◽

pp. 370

Author(s):

Michio Shimotomai

Keyword(s):

Hydrogen Embrittlement ◽

Hydrogen Absorption ◽

Martensitic Steel ◽

High Strength ◽

Literature Survey ◽

Martensitic Steels ◽

Tempered Martensite ◽

Grain Structures ◽

Controlled Nucleation ◽

Experimental Survey

Many advanced steels are based on tempered martensitic microstructures. Their mechanical strength is characterized by fine sub-grain structures with a high density of free dislocations and metallic carbides and/or nitrides. However, the strength for practical use has been limited mostly to below 1400 MPa, owing to delayed fractures that are caused by hydrogen. A literature survey suggests that ε-carbide in the tempered martensite is effective for strengthening. A preliminary experimental survey of the hydrogen absorption and hydrogen embrittlement of a tempered martensitic steel with ε-carbide precipitates suggested that the proper use of carbides in steels can promote a high resistance to hydrogen embrittlement. Based on the surveys, martensitic steels that are highly resistant to hydrogen embrittlement and that have high strength and toughness are proposed. The heuristic design of the steels includes alloying elements necessary to stabilize the ε-carbide and procedures to introduce inoculants for the controlled nucleation of ε-carbide.

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Change of Mechanical Properties of FM Steels after Thermal Aging

Solid State Phenomena ◽

10.4028/www.scientific.net/ssp.124-126.1413 ◽

2007 ◽

Vol 124-126 ◽

pp. 1413-1416

Author(s):

Sung Ho Kim ◽

Chang Hee Han ◽

Woo Seog Ryu

Keyword(s):

Mechanical Properties ◽

Thermal Aging ◽

Nuclear Power ◽

Rupture Strength ◽

Early Stage ◽

Martensitic Steels ◽

Creep Rupture Strength ◽

Absorbed Energy ◽

Service Behavior ◽

The Impact

The effects of thermal aging on microstructural evolution and mechanical properties are important in the understanding of the in-service behavior of ferritic/martensitic steels in advanced nuclear power system. Ferritic/martensitic steels have been aged at 600oC for times up to 20,000 hrs. The change of mechanical properties has been examined for these aged materials. The strength and hardness was hardly changed after the thermal aging at 600oC for 20,000 hrs in all specimens. The impact absorbed energy decreased with the aging time. But the decrease of the impact absorbed energy was larger at the early stage of aging in tungsten added steels. This is attributed to the formation of Laves phase. Nitrogen which is known to increase the creep rupture strength had no effect on the degradation of the microstructure and mechanical properties during thermal aging.

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