For years, HCs with zero ODP and low GWP have been used as alternative B.A. (blowing agents) in the appliance industries. The improvement of insulation performance and lowering costs are significant factors in the development of R-PU (rigid polyurethane) foams especially blown with HCs. The development of R-PU foam with a lower density for cost effectiveness is progressing at present. Positive experiences as to the long-term dimensional stability of pentane blown R-PU foam accelerated this movement. The most important factor in the development of R-PU foam with a lower density is long-term dimensional stability. In this report, we propose a novel long-term dimensional stability test and its appropriateness is discussed. The influence of formulation and the boiling point (B.P.) of B.A. on long-term dimensional stability have also been studied. The use of a blowing agent mixture of cyclopentane and iso- or n-pentane with a lower B.P. allowed the achievement of density savings. The understanding of the relation between the formulation and long-term dimensional stability is an important issue for the development of R-PU foam.
A large 3.5 inch thick compact tension specimen (CT) was hydrogen pre-charged in an extra high capacity autoclave in order to introduce hydrogen into steel, then hydrogen cracking tests were conducted for an extended period of time in ambient air. The anticipated out gassing from the specimen was significantly less in the 3.5T-CT specimen than in the conventional 1.0T-CT specimen. The residual hydrogen after 2 weeks of exposure to ambient air was as much as 80% (≥2 ppm) of the original hydrogen introduced. The threshold stress intensity factor for the onset of cracking (= KIH) for the high toughness, recently manufactured 2.25Cr-1Mo steel was severely degraded to KIH = 42 MPa√m (46 ksi√in) under ultra slow strain rate (dK/dt = 0.005 MPa√m/sec. (0.006 ksi√in/sec)) and subcritical cracking continued over 100 hours. The crack growth rate was kept almost constant regardless of slow change of increase or decrease in K. On the other hand, the temper embrittled 60’s 2.25Cr-1Mo steel showed brittle, unstable fracture at very low stress intensity factor (KIC-H = 33 MPa√m(36 ksi√in)) with no subcritical crack occurring before the fracture event. The fracture point KIC-H turned out to be as low as 10% of the fracture toughness KIC. Finally, comparisons were made between 2.25Cr-1Mo and 2.25Cr-1Mo-1/4V steels by tests of small specimens, since the 2.25Cr-1Mo-1/4V steel substantially retains hydrogen for its lowest diffnsion coefficient. The degree of hydrogen embrittlement is higher at room temperature for both steels.
In order to simulate temper and hydrogen embrittlement in 2.25Cr-1Mo pressure vessel steel in the laboratory, test specimen exceeding 100 mm (4 in.) in thickness containing repair welds made with the shielded metal arc welding (SMAW) process were exposed to hydrogen environment in an autoclave. By investigating the dilution, hardness and microstructure characteristics of the specimen repair welds, it was possible to determine a standard repair technique including minimum stainless steel overlay thickness. From the test results, it was concluded that a minimum of 3.0 mm (1/8 in.) residual overlay thickness was recommended as part of the repair technique by SMAW. In addition, tensile tests of the hydrogen exposed specimens confirmed the serviceability of recent and old generation 2.25Cr-1Mo pressure vessel steel repair welds. In particular, the effect of temper and hydrogen embrittlement on serviceability was examined by detailed observation of microstructure and fracture surface of the tensile specimens.
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