X-ray Diffraction Applied to Hydropower Plant Components

A repair may return an impeller to service, yet leave behind invisible changes that can accelerate new failures. In hydroelectric turbines, welding, grinding, and machining alter residual stresses and crystallographic conditions that conventional NDT methods do not always account for. X-ray diffraction provides quantitative evidence to verify repairs before authorizing a component’s return to service, strengthen component integrity, and guide hydroelectric maintenance decisions.

What does X-ray diffraction actually measure?

In practice, X-ray diffraction measures how much the material has been stretched or compressed in a given area. The equipment analyzes the reflected signal and provides a stress value, in megapascals. This allows us to check the condition of the surface after welding, machining, or grinding.

Bragg’s law explains the measurement as follows: when the distance between atoms changes, the angle of the signal also changes. The software compares readings taken at different orientations and calculates the residual stresses on the surface, indicating whether they are tensile or compressive.

XRD answers a different question than other tests. Magnetic particles reveal surface cracks, and ultrasonic testing looks for internal discontinuities; XRD reveals a condition that may contribute to damage. Taken together, some methods locate defects, while others help assess their cause or verify a repair.

Why residual stresses matter

Residual stresses remain in a component even after the external load that caused them has been removed. They can be generated during casting, forging, welding, machining, grinding, sandblasting, heat treatment, or repair welding. Their distribution depends on the material, geometry, thermal history, and manufacturing sequence.

In hydroelectric turbines, numerous components are subjected to varying loads over millions of cycles. Residual tensile stress can superimpose itself on the in-service stress and promote cracking. Compressive stresses can delay some crack initiation mechanisms, but their effect must be interpreted within the metallurgical, geometric, and operational context.

A single MPa value is not enough. It must be interpreted in light of the material, orientation, location, surface condition, manufacturing, previous repairs, operational loads, and damage mechanism. This context transforms the XRD data into useful information for maintenance.

Where to apply XRD in a hydroelectric power plant

X-ray diffraction should be used primarily on critical metal surfaces that have been welded, machined, ground, or subjected to wear. These include turbine impellers and blades, shafts, areas repaired by welding, and surfaces where cracks appear.

It is not used to indiscriminately inspect all equipment. Measurement points should be selected based on the material, failure history, geometry, accessibility, and the maintenance decision to be made.

Turbine impellers and blades

Francis and Pelton impellers convert hydraulic energy into mechanical motion. The Francis impeller is a reaction type and operates submerged with medium heads and high flow rates. The Pelton impeller is an impulse type and receives high-velocity jets onto blades for high heads. Cavitation, erosion, grinding, and repair by welding can alter the surface, thermal, and mechanical properties of both designs.

The technical literature documents the use of X-ray diffraction on welds in 13Cr-4Ni martensitic stainless steel used in hydroelectric turbines. This method allows for a comparison of areas before and after a repair and enables the evaluation of the weld bead and the heat-affected zone. The results should be interpreted in conjunction with hardness, microstructure, and other non-destructive testing (NDT) methods.

Areas repaired by welding

Weld repair introduces localized thermal cycles. Non-uniform expansion and contraction generate a complex field of residual stresses around the deposited metal and the heat-affected zone. A repair may pass visual inspection and surface testing but may still require additional information about its stress state when the criticality of the component warrants it.

Post-repair inspection may combine visual inspection, penetrant testing, magnetic particle testing, hardness testing, ultrasonic testing, and X-ray diffraction, depending on the material, geometry, and damage mechanism. In hydroelectric maintenance, this combination makes it possible to verify not only the absence of indications but also aspects of the metallurgical and mechanical condition of the repaired area.

Machined shafts and surfaces

Turning, grinding, and finishing processes can also alter the surface condition. Excessively aggressive machining can leave undesirable residual stresses without producing any visible indication. On shafts, bearings, ground surfaces, and other critical areas, XRD can be incorporated when warranted based on failure analysis, operational history, or quality control requirements.

Case study: An impeller repaired by welding

Suppose that during a scheduled shutdown, cavitation damage and surface cracks are detected in an impeller. The hydroelectric maintenance team removes the affected material, verifies that the defects have been removed, and performs a weld repair in accordance with an approved procedure.

For a critical component, the inspection may include an X-ray diffraction map of the weld bead, the heat-affected zone, and the base material, as well as a reference region. The profile makes it possible to determine how residual stresses vary and to identify concentrations that warrant further analysis.

Practical procedure

Identify the material and its metallurgical condition.
Review the history of damage, manufacturing, and repairs.
Define critical areas and measurement directions through engineering analysis.
Prepare the surface without unnecessarily altering its stress state.
Verify the alignment and configuration of the XRD instrument.
Select appropriate crystallographic planes and parameters.
Perform measurements along the established orientations.
Calculate residual stresses and document the uncertainty.
Compare the repair, the heat-affected zone, and the reference material.
Integrate results with other NDT methods and with component integrity assessments.

How to integrate XRD into the maintenance plan

The application must answer a specific question: Did the repair result in an unfavorable stress state? Did a treatment reduce internal stresses? Are there differences between procedures? Does the area where cracks appear exhibit a particular pattern? Thus, XRD is linked to real-world maintenance decisions.

The report must identify components, location, material, surface, direction, method, radiation, crystallographic plane, elastic constants, value, sign, and uncertainty. It must also state any limitations and relate the results to the engineering decision; it should not merely present a figure.

XRD also identifies crystalline phases

X-ray diffraction is not limited to measuring stresses. It also allows for the identification of crystalline phases and the study of metallurgical transformations in materials used in hydroelectric turbines, coatings, and weld deposits. In martensitic stainless steels, the thermal cycles involved in a repair can alter the microstructure; therefore, XRD can be combined with metallography, hardness testing, microscopy, and chemical analysis.

This integration is useful in failure investigations where a visible discontinuity alone does not explain the damage mechanism. Crystallographic information helps to link the thermal history and surface condition to the component’s behavior.

Standards for measuring stresses using XRD

EN 15305:2008 establishes a method for determining residual or applied stresses near the surface of polycrystalline materials using XRD and serves as a reference for structuring general procedures.

ASTM E915-21 addresses the alignment of XRD instruments. ASTM E1426-14(2024) covers the X-ray elastic constants used to convert crystallographic strains into stress.

ASTM E2860-20 describes measurements using XRD. Its scope is specifically intended for quasi-isotropic bearing steels. The standard should be selected based on the material, purpose, equipment, and geometry.

Advantages and limitations in the field

Conventional XRD allows for non-destructive measurements of near-surface stresses, before-and-after comparisons following an intervention, and evaluations in different directions. Portable equipment is available for certain field applications, which facilitates its use during shutdowns and quality control checks.

The main limitation is the depth analyzed: in metals, conventional XRD reflects conditions near the surface. Geometry, access, roughness, texture, and grain size affect the results. Subsurface profiles may require controlled electrolytic removal; for greater depth, methods such as incremental drilling, cutting, or neutron diffraction are evaluated.

XRD as part of an integrity strategy

The key is not to perform more tests, but to select those that can influence a decision. In maintenance management, XRD can be incorporated into a risk-based strategy for areas with a history of cracking, repetitive interventions, or critical machining.

If an impeller has been repaired several times, repeating the penetrant testing alone confirms whether there are any surface indications at that time. Assessing hardness, microstructure, and stress state can help explain why the damage recurs. The recommended sequence is to detect, characterize, evaluate, intervene, verify, and monitor.

Conclusions

XRD enhances the integrity of components by quantifying internal stresses and characterizing crystallographic conditions. It is particularly useful after welding, machining, and surface treatments, or in cases of recurring failures. When integrated with other NDT methods, metallurgical analyses, and engineering, it enables the verification of corrective actions and improves hydroelectric maintenance. Its surface-level scope and limitations related to geometry, texture, and grain size must be taken into account when interpreting the results.

Frequently Asked Questions (FAQs)

Is X-ray diffraction a non-destructive method?

Yes. For conventional measurements of internal stresses, XRD can be applied without destroying the component. However, certain depth profiles may require controlled removal of material, so the procedure must be defined in advance.

Which hydroelectric components can be evaluated?

This technique can be used on metallic materials with a crystalline structure, such as impellers, blades, welded areas, shafts, and other metallic surfaces, provided that the geometry, material, and accessibility are suitable. Its application must be justified based on the damage mechanism and criticality.

Can XRD detect a crack?

It is not its primary function. Penetrant testing, magnetic particles, ultrasonics, or other NDT methods are used to detect discontinuities. The XRD technique provides complementary information on crystalline structure and internal stresses.

Can a welded repair be verified using XRD?

Yes. A welded repair is of interest because the thermal cycle can cause metallurgical changes and internal stresses. The measurement can compare the weld bead, the heat-affected zone, and the base material.

Does XRD measure stresses throughout the entire thickness?

Not with conventional XRD. In metals, the measurement primarily corresponds to a region near the surface. To characterize stresses at greater depths, complementary methods must be evaluated.

Which standard can be used as a reference?

EN 15305 is a general standard for stress analysis using XRD. ASTM E915 addresses instrument alignment, and ASTM E1426 covers the elastic constants required for the calculations. Applicability must be verified for each material and component.

References

ASTM International. (2021). ASTM E915-21: Standard practice for verifying the alignment of X-ray diffraction instruments for residual stress measurement.
ASTM International. (2024). ASTM E1426-14(2024): Standard test method for determining the X-ray elastic constants for use in the measurement of residual stress using X-ray diffraction techniques.
ASTM International. (2020). ASTM E2860-20: Standard test method for residual stress measurement by X-ray diffraction for bearing steels.
European Committee for Standardization. (2008). EN 15305:2008: Non-destructive testing—Test method for residual stress analysis by X-ray diffraction. CEN.