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Gigacycle fatigue behaviors of two SNCM439 steels with different tensile strengthes were experimentally studied by rotating bending tests, to investigate the effects of the tensile strength obtained by different heat treatment processes on very high cycle fatigue failure mechanisms. The material with higher tensile strength of 1 710 MPa exhibited typical gigacycle fatigue failure characteristics, whereas one with lower tensile strength of 1 010 MPa showed only traditional fatigue limit during the tests and no gigacycle failure could be found even when the specimen ran up to more than 10 8 cycles. Metallographic and fractographic analysis were carried out by an optical microscope (OM) and scanning electron microscope (SEM). It showed two different crack initiation mechanisms that for the specimen with lower tensile strength the crack prefers surface initiation and for that with higher strength the crack initiates from subsurface inclusions revealed by a fish-eye like microstructure.

SummaryThe application of Laser welds close to the notch of toughness specimen was made to create a highconstrained condition near the notch tip. Laser welds enabled a very thin specimen (thickness, B = 2.5 mm) to promote brittle fracture at a lower CTOD level than the full-thickness (B = 20 mm) homogeneous specimen. This is due to the plastic constraint induced by the strength mis-matching.Controlling factors for the plastic constraint were addressed by 3 D FE-analysis.It was pointed out that the location of Laser welds (distance from Laser welds to the notch) and the shape of Laser welds give a marked effect on the plastic constraint around the notch. By contrast, the influence of the width of Laser welds was marginal. By using strength mis-matching effect, the thickness requirement for KICmeasurement could be reduced to a large extent. ExperimentsThree-point bend specimens were extracted from a long-term used 20 mm thick pressure vessel of a chemical plant. Material of the pressure vessel was a C-1/2 Mo steel of ASTM A 204 A class. The chemical composition and mechanical properties of the material extracted from the long-term used vessel are shown in Table 1. The yield strength and tensile strength met the value specified in the standard, while the impact energy showed a decrease by about 15 J from the standard value. This loss in the impact energy resulted from the embrittlement by long-term usage.The size of the test specimen extracted from the pressure vessel in service should be as small as possible to keep the structural integrity of the pressure vessel. In this study 2.5 and 5mm thick specimens were extracted. As a reference specimen, the full-thickness (B = 20 mm) specimen was also used. A couple of Laser welds were laid for thin specimens (B = 2.5, 5 mm) as shown in Fig. 1

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