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RESEARCH PAPERS

# Creep-Induced Microstructural Change in 304-Type Austenitic Stainless Steel

[+] Author and Article Information
Toshihiro Ohtani

Materials Lab.,  Ebara Research Co. Ltd., 4-2-1 Hon-Fujisawa, Fujisawa, Kanagawa 251-8502, Japanohtani.toshihiro@er.ebara.com

Hirotsugu Ogi, Masahiko Hirao

Graduate School,  Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka, 560-8531, Japan

J. Eng. Mater. Technol 128(2), 234-242 (Oct 24, 2005) (9 pages) doi:10.1115/1.2172629 History: Received July 14, 2005; Revised October 24, 2005

## Abstract

We studied microstructure changes of 304-type austenitic stainless steel subjected to a tensile stress at $973K$. We monitored the shear-wave attenuation and velocity using electromagnetic acoustic resonance (EMAR). The attenuation peaks at 40% to 50% and a minimum value at 70% of the creep life, being independent of the applied stress. A drastic change in dislocation mobility and arrangement interrupted this novel attenuation phenomenon, as supported by SEM and TEM observations. The relationship between attenuation change and microstructure evolution can be explained with the string’s model. EMAR demonstrates a potential for assessing damage advance and predicting the remaining creep life of metals.

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## Figures

Figure 1

Geometry and dimension of creep specimens in millimeters

Figure 2

Operation of the shear-wave EMAT. Lorentz force, F, excites the shear wave propagation in the Z direction

Figure 3

Frequency dependence of the shear-wave attenuation in the Interrupted test (973K, 100MPa). The polarization is parallel to the stress direction. Solid marks are the data of the reference sample and the dotted marks are for the creep sample. The rupture life is 826h.

Figure 4

Relationship between the creep strain and the attenuation coefficients for 120 and 100MPa at the 11th resonant mode in the Interrupted test. The rupture lives are 280h, 427h for 120MPa and 826h for 100MPa(973K).

Figure 5

Relationship between α, ΔV∕V0, creep strain, creep strain rate, and t∕tr, at the 11th resonant mode under 100MPa in the Interrupted test(973K)

Figure 6

Creep curves in the Continuous test (100MPa, 973K)

Figure 7

Relationship between the target creep strain and time under 120 and 100MPa in the Continuous test(973K)

Figure 8

Relationship between the attenuation coefficients at the 5th, 8th, and 11th resonant modes for 120 and 100MPa versus the creep strains in the Continuous test(973K)

Figure 9

Relationship between the estimated life fraction, t∕tr, and the attenuation coefficients of the 5th, 8th, and 11th under 120 and 100MPa and in the Continuous test(973K)

Figure 10

Attenuation and velocity evolution of the 11th resonant mode and creep strain under 100MPa in the Continuous test(973K)

Figure 11

Optical microstructure of the specimen before creep (t∕tr=0) and the creep specimen at t∕tr=0.38 (100MPa, 973K) in the Continuous test

Figure 12

Change of the average grain size during creep in the Continuous test (100MPa, 973K)

Figure 13

Scanning electron microstructure of the specimen before creep (t∕tr=0) and the creep specimen at t∕tr=0.38 (100MPa, 973K) in the Continuous test

Figure 14

Scanning electron micro microstructure of the creep specimen at t∕tr=0.38 in the Continuous test (100MPa, 973K)

Figure 15

Change of the number density and the average diameter of precipitates on grain boundaries during creep progress in the Continuous test (100MPa, 973K)

Figure 16

Change of the number density of voids during creep in the Continuous test (100MPa, 973K)

Figure 17

Transmission electron microstructure of creep specimens at t∕tr=0, 0.27, 0.48, 0.67, and 0.98 in the Continuous test (100MPa, 973K)

Figure 18

Change of the movable dislocation density and length during creep progresses in the Continuous test (100MPa, 973K)

Figure 19

A comparison between calculated and measured α’s in the 11th resonant mode in th Continuous test (100MPa, 973K)

Figure 20

Summary of the relationship between microstructure evolution (dislocations, precipitates, and voids) and change of ultrasonic properties as creep progressed

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