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Factors Affecting Impact Toughness in Carbon Steels

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Over the last five years there have been numerous reports of failures of carbon steel A105 flanges A234 fittings and some A106 seamless pipe. The failure primarily occurred in newly constructed material and the nature of the failures was Brittle Fracture with very low Charpy V-notch values (3 ft-lbs) at room temperatures when the expected notch toughness was higher than 40 ft-lbs. Typically these steels are exempted from impact testing for upto temperatures -20°F (-29°C) per ASME B31.3. It has been found that the cause of low toughness was a gradual change in the chemical composition of the steel during the steel making process primarily low Mn to C ratios (even though within specification limits) addition of alloying elements like boron vanadium kind of heat treatment applied grain size etc.This paper will identify and document how the different factors like low Mn high C addition of boron and resultant grain size affects the impact toughness of steels based on a review of currently available literature. A comparison with impact properties of other product forms of carbon steel like plates etc. will be presented. A review of the current industry best practices and recommendations to avoid brittle fracture due to metallurgical factors without affecting other mechanical properties during hydrostatic testing cold startups and other low temperature in service conditions will be documented and presented.

Product Number: 51319-13361-SG
Author: Sangeetha Cathapuram
Publication Date: 2019
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2,25 Cr 1 Mo steel is required to comply with very stringent technical requirements when such a steel is applied in the Refinery Industry.Most of the attention is focused on the impact toughness performance of the weld metal which provides information on the structural stability of the pressure vessel taking into account the defect’s growth resistance in the welded joint.The assessment of welded joint toughness by performing CVN tests as a measure of this resistance remains the simplest and most widespread. Bainitic microstructure with very low ferrite content and small grain size of the primary austenite are effective in improving the CVN impact toughness at low temperature after min and max PWHT. The aforementioned microstructure is promoted by lowering the temperature of the Bs in addition to the heat input. Carbide precipitation at grain boundaries doesn’t significantly affect the impact properties of the weld metal after the industrial min and max PWHTs. The micro slag inclusions are investigated. Their chemical composition, volume and size is limited in the weld metal and they do not affect the CVN weld metal property. Min PWHT and max PWHT transform the Martensite/Austenite microstructure developed in the weld metal in as welded condition to Bainitic microstructure and the size and distribution of the carbide precipitated into the matrix. The PWHTs do not modify significantly the grain size of the microstructure which is mainly managed by chemistry and heat input. The type of current DC+ and AC significantly affect the chemistry of the weld metal providing different content of C, Mn, Si, Oxygen. Their contributions enhance the CVN impact properties especially when AC is used
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