Blog: Building the Technical Case for New Strain Limits

The fabrication of pressure vessel heads is a critical step in the manufacturing of boilers, pressure vessels, and nuclear components. These components must maintain structural integrity throughout their service life, often under demanding temperature and pressure conditions.  

For decades, industry standards have relied on forming strain calculations to determine when post-forming heat treatment is required after cold forming operations. However, evidence accumulated through both experimental studies and finite element analysis has increasingly shown that the existing ASME rules for one-piece cold-formed heads may significantly underestimate the actual strain introduced during fabrication. This concern prompted the launch of a collaborative research initiative, the Strain Criteria for Heads project, aimed at establishing a sound technical basis for revised strain acceptance criteria.  
 

Behind the work 

A long-standing technical issue within the ASME Boiler and Pressure Vessel Code prompted this effort. Industry discussions dating back more than 20 years, including several open code items, have acknowledged that the strain calculation method used in ASME Section VIII, Division 1 for one-piece heads underestimates actual forming strains. While engineers generally agree that the Bouhelier equation provided in Section VIII, Division 2 is more appropriate, they have not reached a consensus on the applicable strain acceptance limits.  

Forming strain directly influences material properties and the need for post-forming heat treatment. If the actual strain exceeds acceptable limits, residual stresses and reduced fracture resistance may increase the risk of in-service failures. Current rules may therefore permit conditions that are not fully supported by experimental data. At the same time, overly conservative requirements can increase manufacturing costs and lead to unnecessary heat treatment operations. Establishing technically justified strain criteria is essential for balancing safety, reliability, and economic efficiency. 

This project’s overall objective is therefore to generate the experimental evidence, material property data, and engineering analyses required to establish strain acceptance criteria that accurately reflect the behavior of cold-formed carbon steel and low-alloy steel heads.  
 

Involved parties

Several organizations with complementary expertise in pressure vessel technology, manufacturing, testing, and code development came together to find a solution. 

The prime contractor is MPR Associates, Inc., a world-leading technical consulting firm providing diversified engineering, project and risk management, and other strategic and innovative solutions. MPR is responsible for overall technical project management, execution of the engineering analyses, development of recommendations, and preparation of the final technical reports. The company serves as the technical lead and is accountable for all project deliverables.  

Supporting MPR is Doosan Enerbility Co., Ltd., a globally recognized manufacturer with extensive experience in heavy industrial and nuclear components. Doosan’s role focuses on fabrication and testing activities. The company will manufacture six formed heads, conduct comprehensive testing, and prepare the measurement and test report. Its expertise in forming, materials testing, and fracture mechanics provides the experimental foundation for the project.  

The project has just begun and is being sponsored and administered by ASME Standards Technology, LLC (ST-LLC), which oversees the technical research needed to support future code improvements. ST-LLC will coordinate reviews of project deliverables and obtain feedback from ASME subject matter experts and a Peer Review Group (PRG). This review structure helps ensure that the findings are technically sound and suitable for eventual consideration by ASME code committees.  

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The collaboration represents a strong combination of research, manufacturing, and standards development expertise. MPR contributes advanced engineering analysis capabilities, Doosan contributes real-world fabrication and testing resources, and ST-LLC provides the framework for translating research findings into potential code improvements.  
 

A systematic approach 

The project is structured in two distinct phases designed to move systematically from data collection to code-relevant recommendations. 
 

Phase I: Fabrication and Testing 

The first phase will focus on generating experimental data through the fabrication and testing of six cold-formed heads. Doosan will manufacture heads using two commonly used pressure vessel materials: SA-516 Grade 70 carbon steel and SA-387 Grade 11 low-alloy steel. Three representative head geometries will be produced for each material, creating a broad range of forming strain conditions. 

Measurements will be performed to quantify the actual plastic deformation occurring during forming. Head thicknesses and strain distributions will be evaluated using established measurement techniques, including grid-based methods and advanced scanning technologies.  

The fabricated heads will then undergo an extensive testing program. Mechanical testing includes tensile testing, Charpy impact testing, and fracture toughness testing conducted at both room temperature and low-temperature conditions. The resulting data will provide insight into the relationship between cold-forming strain and the mechanical properties that govern structural reliability.  

The Phase I effort culminates in a detailed test report documenting fabrication methods, testing procedures, measured results, and raw data. This report forms the technical basis for the analytical activities that follow.  
 

Phase II: Analysis and Development of Criteria 

Following testing completion, MPR will lead a comprehensive engineering evaluation of the results. The first task is to establish correlations among head geometry, measured forming strain, and material toughness properties. Understanding these relationships will allow engineers to identify the strain levels at which material performance begins to degrade significantly.  

The project team will then perform fracture mechanics analyses using the experimentally measured fracture toughness data. These evaluations are intended to determine the effect of forming strain on structural integrity and to identify strain limits that will prevent in-service failures.  

For the final analytical step, the research team will develop recommended allowable forming strain criteria. These recommendations will include guidance regarding when post-forming heat treatment should be required and will specifically consider implementation using the ASME Section VIII, Division 2 (Bouhelier) equation.  

More importantly, the project’s findings are expected to support potential revisions to multiple ASME Code sections, including provisions in Sections I, III, VIII Divisions 1 and 2, and XII. By establishing a rigorous technical basis for strain acceptance limits, the project has the potential to improve consistency across the Code while maintaining a high level of public safety. 
 

Guiding change 

The Strain Criteria for Heads project represents an important investment in the continued improvement of ASME pressure technology standards. By addressing a recognized weakness in current forming strain calculations, the project seeks to replace historical assumptions with experimentally validated engineering criteria. 

Through the collaborative efforts of MPR Associates, Doosan Enerbility, ASME Standards Technology, LLC, and ASME technical reviewers, the project will provide the data and analysis needed to establish defensible forming strain limits and heat treatment requirements.  

Ultimately, the outcome will support safer, more consistent, and more technically justified code rules for the fabrication of cold-formed pressure vessel heads.  

Dan Andrei is senior project manager, research at ASME. Michael McKenna is project manager, research at ASME.

If you would like to learn more about this project or get involved, please contact Dan Andrei at andreid@asme.org.