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Investigation of weld joints of LPG pressure vessel made from A516 grade 70 steel

Quantitative Acoustic Emission Non-Destructive Inspection (QAE NDI) technology in combination with tension tests, optical and electron fractography, spectral and chemical analysis and micro-hardness tests were used to: Evaluate the real properties of ASTM A516 Grade 70 steel and establish their correspondence with the requirements for the steel of LPG tanks. Reveal and identify flaws, and assess the structural integrity of the equipment. Metallurgical examinations of specimens revealed that the structural integrity and chemical composition of the investigated steel generally is in accordance with the improved composition of A516 Grade 70 steel: Steel has a ferrite structure with numerous dispersed inclusions. Solid liquid transition zone consists of coarse needle-shaped grains. Inclusions mainly accumulated at the boundaries between the weldment and solid liquid transition zone as well as between the intercritical zone and base metal. It was established that utilizing continuous and burst AE in combination as an information tool makes it possible to reveal flaw initiation and growth, and to assess flaw danger level in steel in the original condition and in steel that has undergone stress corrosion cracking (SCC). In particular, it was found that: 1 There is linear interdependence between AE power and mechanical power for specimens from steel in original condition with fatigue cracks that have J-integral value below JIc ; and weld joint loaded to the limit of elasticity. 2 The presence of high stress concentrators, and/or interacting flaws, as well as intergranular cracking significantly increases the AE power released from specimens with fatigue cracks and welded specimens and changes the linear dependence between AE and mechanical power. 3 Methods used for segregating flaw-development related AE signals from background noise ensured that the accuracy of distinguishing between different stages of fracture development corresponded to J-integral values equal to 0-0.05 ; 0.05-0.075 ; and 0.075-0.3 of JIc would not be less than 92%. Intergranular cracking that has a J-integral value below 0.03JIc.

Quantitative Acoustic Emission Non-Destructive Inspection method; A516 GRADE 70 steel; flaw type and danger level; J-integral; continuous Acoustic Emission; stress corrosion cracking

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