As welding residual stress is a primary factor in serious failure or fracture of plants, many testing methods such as X-ray diffraction, hole-drilling, and saw-cutting are used to evaluate this welding residual stress. Instrumented Indentation Testing (IIT) is an attractive testing method for evaluating welding residual stress because it is nondestructive and can be used in-field. The material properties can be obtained by Instrumented Indentation Testing in field using attachments like magnetic device or chain device. In general, the principle of evaluation of residual stress with IIT is based on comparing the load-depth curves obtained from stress-free state and stressed state which are same material. However, as the mechanical properties of welds are altered during welding process, it is difficult to obtain the load-depth curve for stress-free state of welds. In this research, in order to estimate the load-depth curve for the stress-free state, we consider the relation between k value of Kick’s law which is related to the slope of the load-depth curve and the hardness value which is independent of residual stress state. The relation between maximum load and maximum indentation depth which is k value of Kick’s law is similar to the concept of hardness and as a result, the ratio of k value between two materials is almost equal to the ratio of their hardness values. To validate this research, we compare the load-depth curve of stress-free material with the load-depth curve which is estimated from hardness ratio.

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