Article
Evolution of Precipitated Phases during Creep of
G115/Sanicro25 Dissimilar Steel Welded Joints
Maohong Yang
1
, Zheng Zhang
1,
* and Linping Li
2
Citation: Yang, M.; Zhang, Z.; Li, L.
Evolution of Precipitated Phases
during Creep of G115/Sanicro25
Dissimilar Steel Welded Joints.
Materials 2021, 14, 5018. https://
doi.org/10.3390/ma14175018
Academic Editor: Arkadiusz Gola
Received: 2 August 2021
Accepted: 25 August 2021
Published: 2 September 2021
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1
School of Material Science and Engineering, Beihang University, 37 Xueyuan Road, Beijing 100191, China;
y970094835@163.com
2
Shenhua Guohua (Beijing) Electric Power Research Institute Co., Ltd., No.75 Jian Guo Road,
Chaoyang District, Beijing 100025, China; linping.li@chnenergy.com.cn
* Correspondence: zhangzh@buaa.edu.cn
Abstract:
This paper studies the evolution of the microstructure and microhardness in the G115
side of the G115/Sanicro25 dissimilar steel welded joint during the creep process. The joints were
subjected to creep tests at 675
◦
C, 140 MPa, 120 MPa and 100 MPa. A scanning electron microscope
equipped with an electron backscattering diffraction camera was used to observe the microstructure
of the cross-section. The fracture position of the joint and the relationship between the cavity and the
second phase were analyzed. The microstructure morphology of the fracture, the base metal and
the thread end was compared and the composition and size of the Laves phase were statistically
analyzed. The results show that the fracture locations are all located in the fine-grain heat-affected
zone (FGHAZ) zone, and the microstructure near the fracture is tempered martensite. There are
two kinds of cavity in the fracture section. Small cavities sprout adjacent to the Laves phase; while
large cavities occupy the entire prior austenite grain, there are more precipitated phases around the
cavities. The Laves phase nucleates at the boundary of the M
23
C
6
carbide and gradually grows up by
merging the M
23
C
6
carbide. Creep accelerates the coarsening rate of the Laves phase; aging increases
the content of W element in the Laves phase.
Keywords: dissimilar steel welded joint; creep; laves phase; cavity
1. Introduction
High-parameter ultra-supercritical thermal power plants are currently the first choice
for clean and efficient power generation. Studies have shown that the thermal efficiency
of the unit can be increased by about 0.25% for every 10
◦
C increase in the main steam
temperature [
1
]. However, the improvement of steam parameters, such as temperature
and pressure, is largely restricted by the performance of high-temperature heat-resistant
materials. As a candidate material for the 650
◦
C parameter unit, G115 steel has made
certain improvements to the composition of MARBN steel on the basis of Abe et al. [
2
] and
combined with the existing research results of traditional 9%–12% Cr steel. It has excellent
microstructure stability performance, high temperature creep performance and steam
oxidation resistance in the range 630–650
◦
C. Its creep rupture strength at 650
◦
C is 1.5 times
that of P92 steel, and its high temperature steam oxidation resistance and weldability are
equivalent to P92 steel [
3
]. At present, the creep and oxidation resistance properties of G115
have been studied. However, there are few reports on its join to austenitic steel and creep
properties. This severely restricts the development of high-parameter ultra-supercritical
units at 630–650
◦
C. Therefore, there is an urgent need for a more reasonable and mature
welding technology and corresponding high-temperature performance corresponding to
the new heat-resistant steel.
In a few cases, the Laves phase precipitated after heat treatment [
4
]. In most cases, the
Laves phase precipitates during high temperature aging and creep [
5
,
6
]. The size of the
observable Laves phase is about 200 nm [
6
]. This phenomenon has been verified in the
Materials 2021, 14, 5018. https://doi.org/10.3390/ma14175018 https://www.mdpi.com/journal/materials
