Citation: Shin, J.-H.; Song, J.-Y.; Kim,
S.-D.; Park, S.-J.; Ma, Y.-W.; Lee, J.-W.
Microstructure, Tensile, and Fatigue
Properties of Large-Scale Austenitic
Lightweight Steel. Materials 2022, 15,
8909. https://doi.org/10.3390/
ma15248909
Academic Editors:
Alberto Campagnolo and
Alberto Sapora
Received: 6 November 2022
Accepted: 2 December 2022
Published: 13 December 2022
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Article
Microstructure, Tensile, and Fatigue Properties of Large-Scale
Austenitic Lightweight Steel
Jong-Ho Shin
1
, Jeon-Young Song
1
, Sung-Dae Kim
2
, Seong-Jun Park
3
, Young-Wha Ma
1
and
Jong-Wook Lee
1,
*
1
Corporate Research and Development Institute, Doosan Enerbility Co., Ltd., 22 Doosanvolvo-Ro,
Changwon 51711, Republic of Korea
2
Department of Materials Science & Engineering, Pukyong National University, Busan 48513, Republic of Korea
3
Department of Steels, Advanced Metals Division, Korea Institute of Materials Science, 797 Changwondae-Ro,
Changwon 51508, Republic of Korea
* Correspondence: jwook.lee@doosan.com
Abstract:
High-Mn lightweight steel, Fe-0.9C-29Mn-8Al, was manufactured using steelmaking, ingot-
making, forging, and rolling processes. After the final rolling process, a typical austenite single phase
was observed on all sides of the thick plate. The microstructural changes after annealing and aging
heat-treatments were observed, using optical and transmission electron microscopy. The annealed
coupon exhibited a typical austenite single phase, including annealing twins in several grains; the
average grain size was 153
µ
m. After aging heat treatment,
κ
-carbide was observed within the grains
and on the grain boundaries. Additionally, the effect of aging heat treatment on the mechanical
properties was analyzed, using a tensile test. The fine
κ
-carbide that precipitated within the grains
in the aged coupon improved the 0.2% offset yield and the tensile stresses, as compared to the
as-annealed coupon. To estimate the applicability of high-Mn lightweight steel for low-pressure (LP)
steam turbine blades, a low-cycle fatigue (LCF) test was carried out at room temperature. At a total
strain amplitude of 0.5 to 1.2%, the LCF life of high-Mn lightweight steel was approximately three
times that of 12% Cr steel, which is used in commercial LP steam turbine blades. The LCF behavior
of high-Mn lightweight steel followed the Coffin–Manson equation. The LCF life enhancement in the
high-Mn lightweight steel results from the planar dislocation gliding behavior.
Keywords: lightweight steel; κ-carbide; low cycle fatigue (LCF); steam turbine; blade
1. Introduction
Lightweight steel has high strength and toughness, and its density is approximately
10% lower than that of conventional steels. Thus, it is being developed as a candidate
material to improve energy efficiency and reduce CO
2
emissions in automobiles [
1
–
3
].
Lightweight steel is classified into Fe-Al and Fe-Mn-Al-C-based steels. This study investi-
gated the effect of aging heat treatment on the microstructural evolution and mechanical
properties of Fe-Mn-Al-C-based steels, including high-Mn and C concentrations that were
added to stabilize the austenite structure at room temperature [
4
,
5
]. Fe-Mn-Al-C-based
steels are precipitation-hardened by
κ
-carbide ((Fe,Mn)
3
AlC) with a perovskite E21 struc-
ture within the grains during aging heat treatment. However, when the steel is aged at high
temperatures for an extended period, coarse
κ
-carbide precipitates on the grain boundaries,
reducing the toughness of the steel [
5
]. For steels containing high concentrations of Mn
and Al,
β
-Mn is also precipitated on the grain boundaries during aging heat treatment at a
temperature of 550
◦
C or higher, thereby reducing the toughness of the alloy [6].
As compared to the conventional 12% Cr steel used for steam turbine blades, the
investigated steel has a lower material cost and lower density, which can reduce centrifugal
force. A lower centrifugal force on the blades can allow for longer blades, improving
the efficiency of the power plant. Thus, it can be considered for use as a blade material
Materials 2022, 15, 8909. https://doi.org/10.3390/ma15248909 https://www.mdpi.com/journal/materials
