Mitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (September 2021)
1

Development of Maintenance Guidelines for Dissimilar
Metal Welds of Low-Alloy Steel Boiler Tube

MASAKI HONDA*1

TAKASHI HONDA*2

TAISEI IKEMURA*2

MANABU KONDO*3

To determine the cause of the failure of the fusion line of boiler heat transfer tube dissimilar
metal welds (DMW), which is a common problem worldwide because the failure mechanism has
not been elucidated, Mitsubishi Heavy Industries Group (MHI Group) carried out research on theextraction of factors that influence failure and the proposal of a failure mechanism using
metallurgical investigation and creep analysis by FEM (Finite Element Method). In addition,
based on the proposed failure mechanism, we developed maintenance techniques against thefailure of the fusion line of heat transfer tube low alloy steel DMW(1).

|1. Introduction
Heat transfer tube of boilers, which are one of the main pieces of equipment of thermal
power generation facilities, are often exposed to severe environments under high temperatures andhigh pressures. This results in failures of the fusion line at DMW of heat transfer tube, which is thegeneration and propagation of cracks along the fusion line between the base metal and the weld
(Figure 1) (2). Since such failures found in the past in our group occurred only in DMW that had
undergone repair welding, we have taken measures such as not allowing repair welding whenmanufacturing joints. Since 2015, however, it has been confirmed that this failure occurs after
long-term use even in areas other than repair welds. In addition, according to a report (3) by EPRI
(Electric Power Research Institute), etc., similar failures have occurred not only in the boilers
manufactured by our group, but also in those manufactured by other companies, so this is acommon problem worldwide.

Figure 1

Example of failure on fusion line of dissimilar metal weld

*1Service Engineering Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*2

Strength Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*3

Nagasaki Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi Power, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (September 2021)
2In this way, the problem of the failure of the fusion line of boiler heat transfer tube DMW has
become apparent, but the failure conditions and failure mechanism have not been elucidated, so
there is an urgent need to establish maintenance techniques such as risk assessment and inspection.

|2. Failure morphology
Figure 2 shows the cross-sectional observation results of the failure of the fusion line that
occurred on a boiler heat transfer tube low alloy steel DMW. This failure is the generation andpropagation of a crack along the fusion line caused by voids (vacancies) generated in the fusionline on the low alloy steel side. Strictly speaking, voids and cracks often occur at a location on thelow alloy steel side, about several micrometers from the fusion line. Another characteristic of thisfailure is that a row of precipitates and a decarburized layer (Figure 3) are formed near the fusionline due to the atomic diffusion between the weld metal and the base metal.

Figure 2 Example of cross-sectional
observation of failure on fusion line
using scanning electron microscope

Figure 3 Observation example ofdecarburized layer formed near fusionline|3. Failure example
Figure 4 shows a typical example of the failure of a fusion line. Failures found in the past in
our group occurred only on the DMW of a superheater tube and no failures occurred on the DMWof a reheater tube. This failure was considered to be rare because there were few cases that led to aleak accident. However, recently a crack was detected by UT (Ultrasonic Test) as shown in Figure
4, so this failure should be subject to maintenance.

Figure 4 Examples of crack detected by UT inspection

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (September 2021)
3|4. Factors that affect failure
In order to investigate the mechanism of this failure, we first identified the factors that affectthis failure. Specifically, we compared and arranged the design conditions and operating conditionsof the failure material and non-failure material and conducted metallurgical investigations such as
cross-section survey and chemical analysis. During the metallurgical investigation, it was foundthat there is a difference in the structural composition of the HAZ (Heat Affected Zone) between
the superheater tube and the reheater tube (Figure 5 and Figure 6). As described above, since nofailure has occurred on the DMW of reheater tubes, it was hypothesized that this difference instructural composition is one of the factors that affect the failure.
In order to verify this hypothesis, we performed elastic － creep analysis by FEM under thesame conditions except for the structural composition. As a result of the FEM analysis, it wasclarified that the difference in structural composition causes a difference in the axial stress requiredfor the generation and propagation of cracks, so it was found that a difference in structuralcomposition can be one of the factors that affect failure (Figure 7). In addition, as a result of FEM
analysis of several models we created, it was clarified that the higher the base acting stress (axialstress due to internal pressure), the higher the axial stress acting on the fusion line, so it was foundthat the magnitude of this acting stress can also be one of the factors that affect failure (Figure 8).

Figure 5 Difference in the structural composition between
superheater tube and reheater tube

Figure 6 Typical example of superheater tube structuralcomposition

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (September 2021)
4

Figure 7

FEM analysis result image

Figure 8Comparison of FEM analysis results using multiple models|5. Failure mechanism
Based on the aforementioned factors that affect failure, we investigated the failure
mechanism of the fusion line of boiler heat transfer tube low alloy steel DMW. The failure
mechanism estimated at this time is shown below.
(1) The structural composition of a heat transfer tube DMW differs between superheater tubes
and the reheater tubes (Figure 5 and Figure 6). In the case of superheater tubes, the fined
grain area of HAZ is located from the inner surface side of the tube near the fusion line tothe center of the tube thickness. In a low alloy steel dissimilar joint, since the creep
deformation resistance of the fine grained area of HAZ is higher than the creep
deformation resistance of the coarse grained area of HAZ*, stress redistribution due to creep
during operation occurs and high axial stress is generated on the inner surface side of thetube or near the center of the tube thickness (Figure 7). This axial stress includes not only
the axial stress due to the internal pressure, but also the tensile bending stress caused by thedifference in thermal expansion between the stainless steel, weld metal and low alloy steel.
(2) In the case of superheater tubes, generally the difference between the nominal wall
thickness and tsr (minimum required thickness) is smaller than that for reheater tubes andthe axial stress caused by the internal pressure, which is one of the factors that affect failure
as mentioned above, is relatively high in many cases.
(3) In the case of superheater tubes, in addition to the high stress on the fusion line on the innersurface side and near the center of the tube thickness, the presence of a decarburized layer

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (September 2021)
5with less creep deformation resistance than the surrounding structure makes it easier toaccumulate creep strain. This decarburized layer is formed by atomic diffusion between theweld metal and the parent phase due to heat input during welding and is observed in bothsuperheater tubes and reheater tubes.
(4) Furthermore, due to the difference in Cr concentration between the parent phase and theweld metal, precipitates such as M23C6 and M6C are formed in the decarburized layer
mainly in a row shape (Figure 9).
(5) Due to the accumulation of creep strain during operation, atomic vacancies are likely toaccumulate at the fusion line between the precipitate in the decarburized layer and theparent phase and creep voids are preferentially generated (Figure 9).
(6) As the formation of voids progresses, they are connected to each other, leading to thegeneration and propagation of microscopic cracks and eventual fractures .
Note that even with the same tube specifications and working conditions, the degree of thisfailure may differ and the reason and mechanism have not been clarified at this time, so
further research is needed.
* As a result of a creep characteristic survey using minute test pieces collected from both the coarse grained area of HAZ and
the fine grained area of HAZ of a 1Cr steel DMW for confirmation of the creep deformation resistance of the structure of a1Cr steel DMW, it was confirmed that the creep deformation resistance was lower in the coarse grained area of HAZ.

Figure 9 Schematic view of failure|6. Maintenance techniques
Based on the factors that affect failure and the failure mechanism mentioned above, we
examined maintenance techniques against the failure of the fusion line of boiler heat transfer tube
low alloy steel DMW and formulated effective maintenance guidelines to prevent failure.
Figure 10 shows the outline of the maintenance guidelines.
The statistical risk assessment method, which has been developed this time, included in thesemaintenance guidelines is described below. This assessment method builds a learning modelthrough machine learning of the factors that affect failure and some of the working conditions
described in Chapter 4, as well as actual failure cases. By analyzing data including tube
specifications and working conditions with this learning model, the risk of failure occurrence foreach tube specification and each working condition can be assessed in a relative manner. By
incorporating the factors that affect failure into the variables of the learning data, this method
enables highly accurate risk assessment and can prioritize the targets to be inspected from a huge
number of DMW. Furthermore, by regularly learning additional inspection results, the accuracy of
the risk assessment can be improved (Figure 11).

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (September 2021)
6Since this failure often occurs from the inside of the wall thickness or the inner surface sideof the tube, UT inspection was adopted. UT inspection uses our proprietary thin UT probe and theunique flaw detection sensitivity and detection standard value are set based on a simulation andactual equipment inspection results, which results in the minimum detectable failure size of 1 mm.
In this way, by utilizing maintenance techniques that combine statistical risk assessment, UT
inspection, etc., the prevention of failures on boiler heat transfer tube low alloy steel DMW andreduced maintenance costs can be expected.

Figure 10 Conceptual diagram of maintenance flow
against failure on weld interface of low alloysteel dissimilar welds

Figure 11

Conceptual diagram of risk assessment with statistical analysis|7. Conclusion
Failure of the fusion line of boiler heat transfer tube DMW is a common problem worldwide
because the failure mechanism has not been elucidated. In order to solve this problem, MHI Groupextracted some factors that affect the failure by metallurgical investigation and creep analysis
through FEM of the failure material and non-failure material and proposed a probable failure
mechanism. We also established maintenance techniques based on the failure mechanism.
We will continue research on this failure—which is still partially unclear—and contribute tothe prevention of failures and the improvement of the reliability of boilers by using the establishedmaintenance techniques.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (September 2021)
7References
(1)
(2)
(3)

Masaki Honda et al., Weld Interface Failure of Dissimilar Metal Weld of Low-alloy Steel Boiler Tube andMaintenance Guideline for the Failure, The thermal and nuclear power Vol.71 No.1(2020), p.29～33
EPRI Boiler and Heat Recovery Steam Generator Tube Failures: Theory and Practice, Volume3:
Steam-Touched Tubes, Fig.47-14
EPRI Workshop Dissimilar Welds in High Temperature Applications，ASME PVP(2017) conference,https://archive.asme.org/events/pvp2017/program/epri-workshop