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A guideline document currently being drafted provides purchase instructions, with restrictions on chemistry (base and weld metal), hardness and heat treatment of material from the mill. It also contains manufacturing and processing specifications, including restrictions on heat treatment and welding practice not addressed in the codes (e.g., control of as-welded material prior to post-weld heat treatment), and identifying quality control requirements that must be satisfied to ensure material integrity (e.g., recommendations on thermocouple type and placement for temperature monitoring during heat treatment).
The use of this document will help utilities ensure that the components they receive meet the current material standards for CSEF steel.
A second focus of the EPRI project involves determining the best methods for evaluating Grade 91 material that has been installed in plants. This part of the project is identifying best practices with regard to:
- Inspection strategy, including which components to inspect
- Inspection methods, such as hardness testing and metallographic replication, and the appropriate surface preparation for each.
- Post-inspection activity, including sample removal, engineering analysis, weld repair and heat treatment.
One measure of a metal’s integrity is its hardness, which is the property of a metal that gives it the ability to resist being permanently deformed when a load is applied. The greater a metal’s hardness, the greater its resistance to deformation. Hardness tests determine the depth that a ball or cone will sink into a metal, under a given load, within a specific period of time. Hardness tests are often used for evaluation of materials because they provide a direct correlation to tensile strength and inferred creep strength. However, there might be great variability in hardness testing results.
To assist project members in evaluating their own inspection procedures, EPRI initiated a set of “round robin” hardness tests. For these tests, EPRI compiled samples of Grade 91 steel with different hardnesses and shipped the samples to the project members. The members were told to test the samples for hardness as they normally would, using their own procedures, equipment and surface preparations. The members reported their hardness readings to EPRI. Subsequently, EPRI conducted validation tests in its laboratory and compared the results. The findings indicated whether the utilities’ hardness readings were correct and where the utilities needed to improve their procedures.
Additionally, part of the project evaluated the effect of different variables on the accuracy of hardness tests. Evaluated under this part of the project were the affect of different testing equipment, surface preparation, operator experience level, inspection location access and material variability. The standard deviation and accuracy effect of each variable were fully quantified.
As a follow-on to the round-robin testing, the EPRI project team also prepared a guidelines document, entitled Field Hardness Testing Guidelines, which provide recommendations on training of personnel responsible for testing, selection of locations for testing, test point labeling, calibration, surface preparation and hardness measurements. The guidelines also include detailed procedures for the main testing methods, such as MIC 20/MIC 10, Telebrineller and Pin Brinell (see Figure 2).