Microstructures of creep-fatigued 9–12% Cr ferritic heat-resisting steels

doi:10.1016/j.ijfatigue.2005.04.013

International Journal of Fatigue

Volume 28, Issue 3, March 2006, Pages 300-308

https://doi.org/10.1016/j.ijfatigue.2005.04.013 Get rights and content

Abstract

FE-SEM and TEM observations revealed a multiscale structure consisting of prior austenite grains, packets, blocks and subgrains for 9–12 mass% Cr ferritic heat-resisting steels. During a creep-fatigue test the microstructure showed significant changes in subgrains within the block and the particle size and the precipitation density on the prior austenite grain boundaries. The block size showed little changes. The creep-fatigue life depends on the occupancy of the precipitates on the prior austenite grain boundaries as well as the local recovery processes near prior γ grain boundaries.

Introduction

Nine to 12 mass% chromium containing ferritic steels have a high creep rupture strength and much smaller thermal expansion coefficients than austenitic stainless steels [1], [2], [3]. Mod.9Cr–1Mo steel, which is strengthened with vanadium- and niobium-carbides and molybdenum, is already in use as a main steam pipe in ultra super critical power plants operated at 873 K. Tungsten-strengthened 12Cr–2W steel has also recently been developed and standardized as a material for next advanced power plant operated at 898 K [1], [2], [3].

In previous studies [4], [5], [6], the authors evaluated the creep and creep-fatigue properties for 11 kinds of high chromium (9–12 mass%) ferritic heat-resisting steels. Among them, 9Cr–3W–0.0139B steel was superior in creep rupture strength and creep-fatigue life [4], [5], [6].

The ferritic steels have a tempered martensitic structure. As for the tempered martensitic structures of carbon steels, and as schematically shown in Fig. 1 [7], [8], [9], [10], the microstructure consists of prior γ grains, packets, blocks, and laths. One of the authors defined it multiscale structure [7], [8], [9]. By using the same electropolished method the multiscale structure was investigated for the ferrite heat-resisting steels.

In addition, the authors investigated a change in the microstructure during the creep-fatigue tests. Focusing on a morphology of precipitates at prior γ grain boundaries, occupancy of the precipitates on the grain boundaries was proposed as a new definition of creep damage in creep–fatigue interaction in this study.

Equipments for microscopic observation are a field emission-type scanning electron microscope (FE-SEM) with high-resolution and a transmission electron microscope (TEM).

Section snippets

Material information

Samples were a 12Cr–2W pipe steel, a 12Cr–2W plate steel, and a boron-added 9Cr–3W–0.0139B plate steel [4], [5], [6]. Table 1 shows the chemical compositions, and Table 2 shows the product form, condition of heat-treatment such as normalizing and tempering temperatures and the duration times, and average maximum and minimum diameter of prior γ grain. Fig. 2 shows the optical microstructures of the three materials.

The data on tensile tests, creep rupture tests, and creep-fatigue tests were

Microstructure observation

Fig. 3, Fig. 4, Fig. 5 show FE-SEM photographs, and Fig. 6, Fig. 7, Fig. 8 are TEM photographs for the pipe, plate, and boron-added steels, respectively, before and after the creep-fatigue test. In the figures, (a) designates before the test, and (b) designates after the test for each specimen.

Schematic illustration for Fig. 3(b) as a typical example is shown in Fig. 9. The multiscale structure of the ferritic steel consisted of blocks, packets and prior γ grain boundaries as shown in Fig. 1.

Quantification of precipitates on prior γ grain boundaries

The relationship between precipitates on prior γ grain boundaries and temper brittleness has been much discussed [11], [12], [13] for tempered martensitic carbon steels. The precipitates on prior γ grain boundaries accelerate intergranular fracture, and lead to temper brittleness. Under creep–fatigue interaction tests the precipitates on prior γ grain boundaries are supposed to play an important roll for intergranular creep-fatigue fracture. The grain boundary precipitates of high-chromium

Conclusions

The microstructures of three kinds of ferritic heat-resisting steels before and after creep-fatigue tests were investigated. The results obtained in this study are as follows.

1.
FE-SEM and TEM observations revealed a multiscale structure consisting of prior γ grains, packets, blocks and subgrains.
2.
After the creep-fatigue tests, the microstructure showed significant change in the subgrains within the blocks and particle size and the precipitation density on the prior austenite grain boundaries. The

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Microstructures of creep-fatigued 9–12% Cr ferritic heat-resisting steels

Abstract

Introduction

Section snippets

Material information

Microstructure observation

Quantification of precipitates on prior γ grain boundaries

Conclusions

ASME code approval for NF616 and HCM12A

R&D of advanced ferritic steels for 650 °C USC boilers

Creep-fatigue properties for base metals and welded joints of ferritic heat-resisting materials

Tetsu-to-Hagane

Strain range partitioning analysis for creep-fatigue lives of heat-resisting materials

J High Pressure Inst Jpn

Creep and fatigue properties of newly developed ferritic heat-resisting steels for ultra super critical (USC) power plants

Mater Sci, Res Int

Microstructural analyses of modified-ausformed medium-carbon steel with high resistance to hydrogen embrittlement by atomic force microscopy

J Jpn Inst Met

Microstructural observation of tempered martensite in medium-carbon low-alloy steel by atomic force microscopy

J Jpn Inst Met