Banded structure of ferrite and perarlite was varied by soaking in ASTM A516 Grade 70 steel plates which are popular steel plates for perssure vessels of oil refinery. The effect of banded structure on the susceptibility to both HIC (hydrogen induced craking) and SOHIC (stress oriented hydrogen induced cracking) has been examined. The SOHIC tests were also conducted with specimens containing HIC preformed by preceding immersion in NACE solution.
Fading-out of banded structure by soaking makes the steel plates less susceptible to HIC though, the susceptibility to SOHIC is hardly affected. Even if the plates had blisters, which are produced by immersion in NACE solution and are parallel to the plate surface, the plates were not suffered from premature failure in SOHIC tests The fractured surface in SOHIC test contains many disclike hydrogen embrittled fracture surfaces perpendicular to both the applied stress and the rolled surface of the plate. These hydrogen embrittled fracture surfaces are linked by ductile fracture.
By hardness measurement, the applied stresses in the present SOHIC tests are estimated to be almost comparable to the yield strength of ferrite phase. The neiboring pearlite phase would restrain ferrite phase from yielding; the stress in ferrite could exceed its yield strength. This highly stressed state in ferrite phase would be the reason for the hydrogen embrittled fracture surfaces which lead the failure in SOHIC tests. Strengthenig of ferrite phase and softening of pearlite phase could be suggested for the improvement of SOHIC resistance.

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In applying HT780 steel to welded constructions, preheating over 100°C has been needed in order to prevent cold cracking. This leads a strong desire to decrease required preheating temperature of HT780 steel. So this paper presents a newly-developed HT780 steel with superior weldability. In order to improve weldability lowering P_CM value should be taken into account because of their good relationship. In this newly-developed HT780 steel, lowering P_CM value has been achieved by decreasing carbon contents and has been compensated by utilizing niobium. Because of its excellent shop workability this steel is expected to pioneer a great demand for HT780 steel.

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The Sub-commitee on Mechanical Properties of the JPVRC-Elevated Temperature Design Committee (J-ETD) surveyed several material data bases on typical high temperature resistant steels to contribute to future establishment of the material strength codes or standards, which will be utilized under the simplified procedures for design and structural serviceability assessment for non-nuclear pressure vessels. Several accessible data bases, issued by public institutes, research councils and committees etc., are already available or will be available in the near future. But those data bases are not necessarily sufficient to establish the material strength codes or standards by themselves. Consolidation of several data bases and further acquisition of insufficient data, especially on creep deformation and creep-fatigue properties into the data bases, are required to prepare for establishment of the material strength codes and standards. Data bases on weldment properties will be also needed.

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Referring to literature on crack analysis, flaw interaction criteria of existing codes i. e. WES 2805 mainly, are critically investigated. Those arguments are based on stress intensity factors in various situation of cracks. It is shown that PD 6493 and ASME code XI are more conservative than WES 2805 in many cases.
To verify existing codes, fracture tests are carried out by means of PMMA plate with two surface or through-thickness cracks. Comparing fracture stress in the case of two cracks with that of combined crack according to the codes, it is concluded that moderate sefety margin is set for most cases but excessive in some cases. Particularly WES rules for both-sided surface crack seem to be too conservative.

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