Role of a complex carbonitride of niobium and vanadium in creep strength of 9% Cr ferritic steels

The martensitic 10CrMoVNb 91 (P91) steel has been developed for applications under high ultra supercritical pressure for steam power plants. The creep rupture strength of welded joints of this steel is limited by the fine grained region of its heat affected zone (HAZ). The microstructural changes occurring in this region during long term creep tests at 600uC and 70 MPa were examined using optical microscopy and scanning and transmission electron microscopy for comparison with the microstructure of this region after post-weld heat treatment. The factors which result in the reduction of creep rupture strength of the fine grained region of HAZ are discussed. It was found that the most effective factors reducing the creep rupture strength of the fine grained region of the HAZ of a welded joint in P91 steel, in comparison with the coarse grained region of the HAZ and the base metal of the welded joint, are the finer prior austenite grain size of the fine grained region, which accelerates the rate of growth of martensite lath subgrains and enhances creep cavitation, and also the lower peak temperature during welding, which results in a softer martensite matrix.Keywords: P91 weld joint, Long term creep at 600uC, Creep rupture strength, Microstructural instability, Fine grained region of HAZ, Prior austenite grain size Creep testCreep tests were performed on the base metal, the welded joint, and specimens simulating the fine grained HAZ region. Tests were carried out at 600uC, in air and at various constant applied loads by using single and

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“…9a Table 2. These phases were also observed in tempered martensites of 9%Cr steels 7,15 and are known as VN and (V,Nb)(CN).…”

Section: Microstructural Changes During Creepmentioning

confidence: 91%

Microstructural instability of a welded joint in P91 steel during creep at 600°C

El-Azim¹,

Nasreldin²,

Zies³

et al. 2005

Materials Science and Technology

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“…Vanadium nitrides are good in this context. It has been shown that plate-like V-nitrides are effective obstacles against a dislocation glide, and the length of the major axis of plate-like V-nitrides is considered to be one of the controlling factors of a creep resistant [10]. Meanwhile, previous studies have also shown that in 9% chromium steel the volume fraction of the VN precipitates remains constant during a creep [11], and the VN precipitates are highly resistant to a coarsening at both 600 and 650°C, implying that this phase contributes equally to a creep strength at these two temperatures [12].…”

Section: Introductionmentioning

confidence: 99%

Vanadium nitride precipitate phase in a 9% chromium steel for nuclear power plant applications

Shen

Kim

Han

et al. 2008

Journal of Nuclear Materials

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“…For example, the continuous cyclic softening observed during fatigue loading [ 2,4,5] can be related primarily to the formation of the equiaxed subgrain structure. The presence of carbides and carbonitrides [ 6,7] and especially their interparticle spacing affect the creep behaviour; the minimum creep strain rate was found to increase with the cube of the interparticle spacing [ 61. That is why the long-term stability of carbides and their resistance to coarsening are required.…”

Section: Introductionmentioning

confidence: 99%

INTERACTION OF HIGH CYCLE FATIGUE AND CREEP IN 9%Cr‐1%Mo STEEL AT ELEVATED TEMPERATURE

Vašina

Lukáš

Kunz

et al. 1995

Fatigue Fract Eng Mat Struct

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High-cycle-fatigue/creep experiments were performed on a 9%Cr-1 %Mo temperered martensite ferritic steel at 873 K in air. The stress ratio R = u,,,,,,/umX ranged from -1 ("pure" fatigue) to 1 ("pure" creep). The maximum stress u , , was kept constant at 240 MPa. The lifetime depends on the stress ratio R in a non-monotonic way.In the stress ratio interval 0.6 < R < 1.0 both the creep strain rate and the lifetime are controlled by mean stress a , , , of the stress cycle. In the stress ratio interval -1 < R < 0.2 the lifetime is controlled by the stress amplitude na. The fatigue/creep interaction occurs in between these intervals. The fatigue/creep loading induces transformation of the tempered martensite ferritic structure into an equiaxed subgrain structure. The resulting subgrain size depends strongly on the stress ratio.

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Role of a complex carbonitride of niobium and vanadium in creep strength of 9% Cr ferritic steels

Cited by 14 publications

References 4 publications

Microstructural instability of a welded joint in P91 steel during creep at 600°C

Microstructural instability of a welded joint in P91 steel during creep at 600°C

Vanadium nitride precipitate phase in a 9% chromium steel for nuclear power plant applications

INTERACTION OF HIGH CYCLE FATIGUE AND CREEP IN 9%Cr‐1%Mo STEEL AT ELEVATED TEMPERATURE

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