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To determine whether the residual stress in the carbon steel of the heat exchanger has been reduced to the required level, a comprehensive assessment is necessary by combining quantitative testing with practical engineering verification. Absolute “zero stress” is neither necessary nor possible; the key is whether the residual stress level meets the design and service requirements. I. Core testing methods and standard references: X-ray diffraction (the preferred non-destructive method) is suitable for detecting residual stresses on the surface or near the surface; it offers high precision and is non-destructive, making it the \"gold standard\" for acceptance testing. Measurement depth: from a few micrometers to several dozen micrometers, suitable for detecting weld areas. Acceptance criterion: The maximum residual tensile stress after treatment ≤ 30%~40% of the base material’s yield strength, or it must meet the limits specified in the drawings. If the goal is to introduce compressive stress (such as to increase fatigue life), it is necessary to ensure that the compressive stress level reaches the desired intensity and depth. The blind hole method (semi-destructive, high precision) involves attaching strain gauges to critical areas and drilling small holes (about 2 mm in diameter), after which residual stresses are calculated based on the changes in strain.