The Investigation On The Fatigue Crack Growth On Welded Pressure Vessel Steel

Mohd Khairul Adli , Mohd Zin (2009) The Investigation On The Fatigue Crack Growth On Welded Pressure Vessel Steel. Project Report. UTeM, Melaka, Malaysia. (Submitted)

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Abstract

This project (Projek Sarjana Muda) studied the fatigue crack growth (FCG) of commonly industry used welded pressure vessel steel. In the oil and gas industries, the welded pressure vessel steel was affected by the welding heat output that causes many changes to the mechanical and metallurgical properties on near as-welded region (HAZ) effecting fatigue crack growth (FCG) rate on its interface. This leads to many accidents involving failure of pressure vessels. The objective of this project is to investigate and study about the fatigue crack growth (FCG) on the base metal (BM) region, and heat affected zone (HAZ) region of the welded pressure vessel steel. The type of tested steel is ASTM 516 Grade 70 of thickness 12mm. The steel was welded using submerged arc welding, and undergone post weld heat treatment. Then, it is fabricated to compact tension C(T) specimen according to ASTM E647-03. Two different notch location of compact tension specimens were fabricated relatively according to the heat affected region, and base metal region. The microstructure analysis was done and resulted ferrite-pearlite in BM region, and ferrite, austenite and cementite in the HAZ, while the WM has ferrite, widmanstatten ferrite and bainite. The hardness test was also done using Rockwell Hardness testing and found HAZ region had the highest value than BM region. These two specimens were fatigue cyclic loaded according ASTM E 647-03 and found that the HAZ region had the highest FCG rate compared to the BM FCG rate because of the brittle structures of the HAZ regions. Finally, the ruptured surface of the specimens was done fractograph analysis under scanning electron microscope (SEM) and found that modes of fracture formations involved in both HAZ and BM regions are the same and slightly differences that are transgranularmixed and equiaxed dimples, quasi and mixed modes fracture, and fine cleavage and transition from large to small dimples of fracture on pre-crack, FCG and rupture regions respectively.

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