Project Subjects
Stress Corrosion Cracking in Light Water Reactors
Background and Objective
Stress corrosion cracking (SCC) is one of the major degradation events to be considered in long-term
reactor operation. Countermeasures against SCC are residual stress improvement, water chemistry improvement, application of alternative material and so on. In addition, Fitness-for-Service code of the Japan Society ofMechanical Engineers has been applied on some plants. The code describes methods of inspection, evaluation
and repair/replacement. Operation with remaining ﬂaws is permitted if the detected ﬂaw is evaluated as nonhazardous during the evaluation period. The evaluation method will be more accurate if state of the art knowledge
on SCC is reﬂected on the code.
The object of our research is to obtain SCC related data in order to update the code or develop a countermeasure technique. Current programs focus on investigation of SCC propagation behavior around a weld joint
and evaluation of the relationships between microstructures of material and SCC.

Main results
1. SCC propagation behavior around a weld joint of low carbon stainless steel
SCC propagation behavior from base metal to weld metal was observed in detail in a previous study using
a weld joint of hot rolled low carbon stainless steel bar. It was observed that crack propagation from the matrix
to the weld metal hardly occurs. A new specimen obtained from an actual weld joint of a PLR piping is used forthe same experiment this time. Cracks in the new specimen propagate along a weld line accompanying branched
sub-cracks and hardly intrudes to a weld metal (Fig.1). This result is same as that of the weld joint of hot rolled
bar. Most of the cracks are arrested on a weld line. A few cracks propagate into the weld metal but the crack length
in weld metal is usually less than 100μm (Fig.2).
2. SCC initiation behavior of stainless steel
It is known that SCC initiation properties are inﬂuenced by surface hardness of material. The hardness
depends on a plastic strain that is changed with machined process. In order to establish a method to control hardness of a specimen surface, the effects of feed rate and shaft rotation rate of the milling machine on the surface
hardness were investigated (Fig.3). It is found that the feed rate changes the distribution of plastic strain on thespecimen surface. The number of cracks on the specimen in which high plastic strain regions are locally induced
is greater than that of the specimen in which high plastic strain are uniformly induced (Fig.4).
Other reports

8[Q09017]

Nuclear Technology

Base metal

EDM Notch

IGSCC
Fusion Line

Fatigue Crack

Weld metal

500 μm

Fig.1 SCC propagation behavior around weld joint
Load is applied on the direction of vertical in this ﬁgure, and SCC propagated from the fatigue crack tip to theleft side in this ﬁgure.

Fig.2

Crack length distribution in weld metal

Fig. 3 Relationships between settings of millingmachine and surface hardness of the specimen

High strain region

Milling conditions: Feed rate 50,000μm/min, Rotation rate: 800rpm
High strain region

High strain region

High strain region

High strain region

Milling conditions: Feed rate 300,000μm/min, Rotation rate: 800rpm

Fig.4 Distribution of plastic strain measured with electron beam diffraction and the portion of cracking
Dark parts on the image correspond to the regions of high plastic strain. Cracks tend to occur in the specimen locallystrained (lower image) rather than the specimen uniformly strained (upper image).

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