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Inspectioneering Journal

Elimination of Backing Gas in Austenitic and Duplex Stainless Steel Welds Using Semiautomatic Gas Tungsten-Arc Hot Wire Welding

By Charles Patrick, Director of Technical Services/Consultant at Republic Testing Laboratories, LLC, Juvenal Calvo, President at Republic Testing Laboratories, LLC, Scott Witkowski, Vice-President at Republic Testing Laboratories, LLC, and William F. Newell, Jr., PE, Co-Founder/Vice President – Engineering at Euroweld, Ltd. This article appears in the May/June 2022 issue of Inspectioneering Journal.
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Introduction

Open root welding of austenitic and duplex stainless steel with gas tungsten-arc welding (GTAW) plays a critical role in fabricating pressure components serving various industries. While several improvements in welding processes and equipment have been introduced, the basic welding practices have remained unchanged. For example, the only reliable method of welding austenitic and duplex stainless steel piping while preventing oxidation of the root bead was through either back purging with inert gas or using one of the following: flux agents, flux coated/flux-cored rods, and gas metal-arc welding using the waveform controlled short-circuiting transfer mode (GMAW-S) [1-4]. This article aims to focus on the elimination of backing gas for open root welding of austenitic duplex stainless steel welds using a flux-cored wire for the root pass.

To fully evaluate and provide validation of the resulting weldment, the weld test coupons were fabricated and subjected to a testing protocol consisting of radiographic examination, mechanical tests, ferrite tests, hardness tests, corrosion tests, and chemical tests, which verified that the weldments were not compromised by elimination of the backing gas.

Materials, Welding Equipment, and Monitoring

Materials Austenitic Stainless Steel

The base material for the austenitic stainless steel was SA-312 Type 304/304L, nominal pipe size (NPS) of 8-inch (203 mm), 0.500-inch (13 mm) wall thickness (schedule 80) pipe [5].

Table 1 lists the chemical composition per the Material Test Report (MTR).

Table 1. Chemical Composition of SA-312 Type 304/304L Pipe, Wt. %.
C Si Mn P
0.02 0.33 1.18 0.033
S Ni Cr N
0.011 8.07 18.26 0.065

Welding consumables were: SFA-5.22/SFA-5.22M (AWS A5.22/A5.22.M:2012), E308LT1/E308T1-4, 0.045 inch (1.14 mm) diameter flux-cored wire for the root pass SFA-5.9/SFA-5.9M (AWS A5.9/A5.9.M:2012), ER308/308L, 0.045 inch (1.14 mm) diameter bare wire for the balance of welding [6-9]. Chemical composition per Certified Material Test Report (CMTR) for each welding consumable is listed in Tables 2 and 3, respectively.

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