ABSTRACT

Over the last five years, there have been numerous reports of failures of carbon steel A105 flanges, A234 Gr. WPB fittings and some A106 and A53 seamless pipe. The failures primarily occurred in newly constructed material and the nature of the failures was brittle fracture with very low Charpy V-notch values such as 4 Joules (3 ft-lbs) at temperatures where the notch toughness is expected to be higher than 54 Joules (40 ft-lbs). Typically these steels are exempt from impact testing for minimum design temperatures -29°C (-20°F) per ASME B31.3 for thicknesses up to 12.7 mm (0.5 in). It has been found that the cause of the unexpected low toughness was a combination of changes in the chemical composition of the steel due to changes in the steel making practices. These changes were primarily low manganese to carbon (Mn:C) ratios (even though within specification limits), addition of alloying elements like boron (B), heat treatment applied and grain size. This paper will identify and document how the different factors like low Mn:C ratio, addition of boron, heat treatment and resultant grain size affects the impact properties based on a review of currently available literature. A review of current industry best practices and recommendations to avoid brittle fracture due to metallurgical factors, without affecting other mechanical properties is presented.

INTRODUCTION

At the ASME B31.3 (1) Code Committee meetings held in Seattle April 2016, there was a discussion about recent in-service failures in ASTM (2) A105 flanges, A234 Gr. WPB fittings and A106 and A53 seamless pipe up to 12.7 mm (0.5 in) wall thickness during hydrostatic testing.1, 2 The nature of the failures was brittle fracture and the materials were of new construction. The failed material was found to be brittle with very low Charpy V-notch impact toughness values at temperatures which were well above the minimum design temperatures. This may be a concern since these materials are commonly exempted from impact testing per ASME B31.3 paragraph 323 for minimum design temperatures of 29°C (-20°F) or higher.3 Figure 323.2.2A is used for exemption from impact testing and these materials typically fall under Curve B (Figure 1).

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