Using Resistance Heating to Create Full-Scale API RP 2Z CTOD Samples
Authors: Gallagher M.L. , Gould J.E. , Babu S.S. 
Published: December 2011

API RP 2Z testing is used by the oil and gas industry to qualify steel plate for service in offshore environments. Part of this qualification process involves assuring "that the steel to be supplied is inherently suitable for welding" (API 1992). Suitability for welding is demonstrated in part through CTOD testing of the HAZs of plates welded over a wide range of heat-inputs. This CTOD testing determines the fracture toughness of the HAZs, which becomes increasingly important as a design criterion as the lowest anticipated service temperature decreases (i.e. arctic applications). Unfortunately, the time and cost associated with welding CTOD samples can become burdensome as the qualified plate thickness increases. Additionally, the current RP2Z welding procedure may not lead to uniform microstructures in the HAZ of new generation steels. Therefore, using resistance heating to directly/rapidly fabricate thick-section HAZ CTOD samples should be very attractive to those in the Oil and Gas Industry who must qualify steel plate for offshore service. EWI is developing a large-scale resistance heating machine to fabricate full-scale CTOD heat-affected-zone samples.

Introduction

The API RP 2Z specification is the recommended practice for preproduction qualification of steel plates for offshore structures. This recommended practice essentially prequalifies "specific steelmaking and processing procedures for the manufacture of steel of a specified chemical composition range by a specific steel producer" to be used for fabrication of offshore structures. One of the major objectives of this recommended practice is "to assure that the steel to be supplied is inherently suitable for welding."

Section 3 of API RP 2Z (Qualification for HAZ Toughness) details the welding that must be performed as part of this prequalification. " Each prequalification shall consist of preparation of three butt welds using test plates with either a K-bevel or single-bevel joint preparation (i.e. straight wall weld-joint for subsequent CTOD testing). Test plates shall be of the maximum thickness intended for inclusion within the scope of the prequalification. Prequalification test welds shall be made with the mechanized SAW process. The three butt welds shall be welded with pre-heat, heat input, and interpass temperatures that span the prequalification range. The heat input range for prequalification shall encompass at least 1.5 – 4.5 kJ/mm (38 – 114 kJ/in). One weld shall be made at the lowest heat input to be prequalified with preheat/interpass temperature at 100°C or lower at the start of each weld pass. Another weld shall be made at the highest heat input to be prequalified with preheat/interpass temperature at 250°C or higher at the start of each weld pass. The third weld shall be made with heat input at 3 kJ/mm (76 kJ/in); the preheat temperature shall be 100°C, and the interpass temperature shall be allowed to build naturally, but not to exceed 250°C.?? These three butt welds are subsequently used to fabricate CTOD specimens, which test the HAZ toughness of the steels seeking the RP 2Z prequalification.