10CRMO9-10 AND INCOLOY 825
COMPENSATORS FOR NUCLEAR PLANT

Four Tied Compensators made from 10CrMo9-10 and Incoloy 825, with special protective covers, have been installed in the steam extraction system from the turbines in a nuclear power plant in Northern Europe.

 

CLIENT ISSUE
The Compensators were needed to replace some old Expansion Joints that had reached the end of their life expectancy. The client forwarded the drawings of the original Expansion Joints, as they needed a 1:1 replacement of the design. The nuclear power plant was very satisfied with the solution and its operation and therefore wanted the 1:1 replacement of the original Expansion Joints. However, it was crucial for the nuclear power plant that the loads were as low as possible, especially as one end of the Expansion Joints would be used to carry loads from the piping system.  Therefore, the client was looking for a manufacturer that was experienced with the nuclear sector and that was able to design and manufacture a 1:1 solution which was CE-marked. In this way the nuclear power plant avoided renewed design appraisal.

 

BELMAN SOLUTION – 10CrMo9-10 and Incoloy 825 Compensators
To meet the demands, Belman expertly designed a 1:1 solution and forwarded it to the client for approval. The design comprises Tied Expansion Joints with double Bellows to absorb larger lateral movements. Due to the high flow velocity, a smooth interior was needed; therefore an inner sleeve design was used. This also served the purpose of protecting the bellows against the high flow velocity. The Bellows are made from Incoloy 825 while the inner sleeves and the piping ends are made from 10CrMo9-10, as the Expansion Joint is welded onto the mating pipe at one end, which is also made of 10CrMo9-10, whereas the other mating pipe is made from P265GH. The media is steam; however, the client requested that the Expansion Joints be made in the materials Incoloy 825 and 10CrMo9-10. The material is the same for both ends to avoid adding a third material to the construction and thus new welding procedures. 10CrMo9-10 is a material with a high yield strength at elevated temperatures that requires special handling during welding with pre-heating and post weld heat treatment (PWHT).

The client requested special covers as they needed them to be permanent covers during operation and thus not only for protection during transport. Being relatively thin, it can be a good idea to protect the Bellows on the outside from damage during plant construction, operation, maintenance etc. Naturally, the covers on these Expansion Joints are designed to permit free movement of the Expansion Joint.

 

Why post weld heat treatment (PWHT)?
Post weld heat treatment (PWHT) is done to reduce the hardness in the welded sections of the 10CrMo9-10. Post weld heat treatment (PWHT) is essential for steel grades like 10CrMo9-10 (1.7380), a chromium-molybdenum alloy steel commonly used in high-temperature and high-pressure applications such as boilers, heat exchangers and pressure vessels. Here’s why PWHT is necessary for 10CrMo9-10:

1. Reduction of residual stresses: 10CrMo9-10 is often used in high-stress environments, and welding creates residual stresses in the heat-affected zones (HAZ) due to rapid heating and cooling. PWHT relieves these stresses, helping prevent warping, cracking and potential failures, especially in thick-walled applications.
2. Improvement in toughness and ductility: Chromium-molybdenum steels, such as 10CrMo9-10, tend to become brittle in the HAZ after welding due to martensitic transformation, a hard microstructure formed from fast cooling. PWHT refines the microstructure, restoring ductility and toughness in the welded areas and making the material less prone to brittle fracture.
3. Reduction of hydrogen embrittlement and cracking: Hydrogen absorbed during welding can lead to hydrogen-induced cracking (also called cold cracking) in alloy steels. PWHT allows the hydrogen to diffuse out of the welded areas, reducing susceptibility to delayed cracking. This is especially important in high-pressure applications where failure could be catastrophic.
4. Stress relaxation at elevated temperatures: In high-temperature applications, 10CrMo9-10 must maintain structural integrity under sustained loading. PWHT ensures that the material’s creep resistance—the ability to withstand prolonged stress at elevated temperatures—is optimised, which is critical for long-term service life in high-temperature environments.
5. Enhanced corrosion resistance: Although chromium-molybdenum steels have good oxidation resistance, PWHT improves corrosion resistance by stabilising the material’s microstructure. This is particularly valuable in environments where the steel is exposed to corrosive agents, as PWHT can reduce the risk of localised corrosion in welded areas.

To sum up, performing PWHT on 10CrMo9-10 (1.7380) is crucial to ensure the weld joint’s reliability and longevity. It enhances ductility, reduces residual stresses, mitigates the risk of hydrogen-induced cracking, and increases resistance to high-temperature creep and corrosion.

 

High demands for quality assurance and extended warranty
Due to the nature of the application, quality assurance was very important. Prior to production it was a client request to have the following for review and approval: drawings, part lists, a model of the Expansion Joint including cover as step or sat files to ensure there was sufficient room and free movement on the Expansion Joint, the stiffness values of the Expansion Joints, etc. After a stamp of approval had been given for the drawing and related documents, a kick-off meeting was held and Belman started the production. The client requested to review all WPSs and that the Expansion Joints were CE marked. Furthermore, the Expansion Joints were pressure tested, penetrant tested, magnet tested, radiographic tested, ultrasonic tested, leak tested and visual tested. All production, test, quality assurance and documentation was done according to 2014/68/EU and national standards and the Expansion Joints were CE marked.

As documentation, detailed drawings and calculations were delivered to ease future ordering of more Expansion Joints for this position in the application at the nuclear power plant.

 

Belman – unparalleled expertise in Expansion Joints for nuclear power plants
Belman brings unparalleled expertise in designing and manufacturing custom Expansion Joints, with over 900,000 Compensators produced for various applications worldwide. Over decades, Belman has supported nuclear power plant projects worldwide, building a robust track record and extensive industry knowledge of the nuclear sector’s needs and requirements. Among many interesting projects, it is worth mentioning Expansion Joints with a manhole for internal inspection of the pipeline, rush order delivery for a nuclear power plant in urgent need and some DN 3000 specially designed Expansion Joints with a built-in length of 8 metres for a nuclear power plant. This covers a large variety of designs and requests, but this is everyday work for Belman, making us the Expansion Joints manufacturer with unparalleled expertise.