Hydrogen Gas Effects on the Fatigue Crack Growth Behavior of Cr-Mo Steel CT Specimen in Extremely Low Rate Range

In order to investigate extremely slow fatigue crack growth characteristics of JIS SCM440 CT specimen in 9 MPa hydrogen gas environment, stress intensity factor range (ΔK) decreasing tests with in-situ observation were carried out. Fatigue crack growth rate (FCGR) in hydrogen gas did not show threshold behavior but FCGR in helium and air showed threshold behavior clearly. Fatigue crack in hydrogen gas showed sudden increased after a temporary stop in growth. The sudden increase in growth was induced by coalescing with a new micro-crack initiated in front of the main crack tip. The fractographic analysis showed the existence of intergranular facets. The intergranular facets were observed in all over the fracture surface. The amount of the intergranular facets in hydrogen gas decreased with decreasing the ΔK. However, facets were still observed in the extremely low rate region. On the other hand, there was no facet in fracture surface tested in helium and in air in the extremely low rate region. The formation of facets was supposed to be one of the causes of non-threshold behavior in hydrogen gas. Introduction It is well known that hydrogen degrades strength of material. In these days, a lot of researches have been carried out for developing the fuel cell vehicle. A lot of reports were made to the crack growth behavior in region of high ΔK and high FCGR in hydrogen gas environment. However, only few reports about the crack growth behavior in region of near ΔKth and low FCGR, which is very important, were made because of the difficulty with testing in the high pressure hydrogen gas environment. According to the report of Cr-Mo steel tested at 50 Hz in low pressure hydrogen gas, ΔKth in hydrogen went down and the ΔKth was smaller than that in air. The report concluded that the reason was the effect of oxide-inducted closure [1] [2]. However, it was reported that hydrogen affected the slip behavior [3] [4]. In addition, as the pressure rises, the amount of hydrogen in material increases. If the result of low pressure environment is applied for evaluation of high pressure environment, closer to real machine, it will have possibility to give danger decision. So, it is important to investigate extremely slow fatigue crack growth characteristics and the mechanism of near ΔKth in high pressure hydrogen environment. In this study, the effect of high pressure hydrogen environment to the fatigue crack growth characteristics of Cr-Mo steel, for storage cylinder of hydrogen station, is discussed with in-situ observation and SEM fractography. Experimental procedure Specimen. The metal used in this study was Cr-Mo steel JIS SCM440. Tables 1 and 2 show the chemical composition and mechanical properties. This steel was quenched at 1133 [K] for 2.5 hours and tempered at 733 [K] for 3 hours. Fig. 1 shows the geometry of CT specimen. Testing. In order to investigate the extremely slow fatigue crack growth characteristics, ΔK decreasing tests based on ASTM standard were carried out. Fig. 2 shows the load variation in the ΔK decreasing test. After the pre-crack was introduced, C constant (= 1/ΔK ×dΔK/da) satisfied the requirement of ΔK decreasing test (C > -0.08 [mm -1 ]). FCGR by in-situ observation was measured every 20 [μm] in the last 100 [μm] of load step at the observation surface. After the test, the crack length of the other side was measured and the ΔK was revised. In the region of large ΔK (> 8 [MPa √m]) FCGR was measured by the compliance method [5]. Conditions. The tests were conducted in 9 [MPa] high purity (99.999 %) hydrogen gas, 9 [MPa] high purity (99.999 %) helium gas and in air at room temperature. The reason why helium was selected is to exclude the effect of oxygen. Stress ratio (R = minimum load/maximum load) was 0.1. Cyclic loading frequency was 5 [Hz] (sine wave). In-situ observation. The environmental testing machine has a chamber with a viewing window on side. In-situ observation was realized through the window with the microscope camera. SEM observation. After the test, to investigate the vestige of hydrogen effect in fracture surface, SEM observations were carried out. To observe by SEM, the specimen with protection was cut carefully, soaked in liquid nitrogen and broken in the load direction. Table 1. Chemical composition [wt. %]. C Si Mn P S Cu Ni Cr Mo 0.36 0.26 0.8 0.014 0.025 0.24 0.07 1.13 0.15 Table 2. Mechanical properties. σY [MPa] σB [MPa] δ [%] φ [%] HV 118