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Experimental phase balance assessment for duplex stainless steel welded pipe

This paper discusses the assessment of weld metal ferrite content in duplex stainless steel pipe welded at a 45˚angle

S Chidambaram
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Article Summary
The mechanical and corrosion properties of the weld joint depend on the proportion of ferrite and austenite phases in the microstructure. The ferrite number is measured using a Feritscope on the pipe weld joint for determining the ferrite phase and it is measured between 40-47 FN. The welding consumable ER-2209/E-2209 is used for a welding pipe with a known chemical composition and it is mapped in a WRC diagram to determine the theoretical ferrite number. The importance of the ferrite phase for a balanced ferrite and austenite dual phase microstructure largely depends on the ferrite number. In experimental measurements, a slightly higher ferrite number is obtained as compared to a theoretical value. The probable causes of variations in values and the importance of the phase balance test on welder qualification are analysed. 

Wrought duplex stainless steel (DSS) consists of both ferrite and austenite phases in equal volumes. There are numerous advantages in DSS over austenitic stainless steel, such as high yield strength, pitting resistance, stress corrosion cracking resistance and suitability of material in chloride environments. When comparing DSS with ferritic stainless grades, DSS shows superior weldability. However, its weldability is inferior to austenitic stainless steel. Therefore, DSS alloys were developed as an intermediate between austenitic and ferritic grade stainless steels.1,3 Reactor effluent air fin coolers and the critical pipes of static equipment are made of DSS for superior corrosion and pitting resistance. The performance of that equipment directly depends on the quality of welding.2, 5

The welder performance qualification test is conducted for each welder, to evaluate their ability to produce a sound weld deposit of duplex grade.1 It is mandatory to conduct a welder qualification test as per the design codes and standards for any material. In this grade, the ferrite phases formed during the initial solidification will resist solidification cracking during welding.5 Those ferrite phases ensure resistance from chloride stress corrosion cracking and pitting corrosion. However, a greater ferrite phase proportion leads to hydrogen attack on the DSS.3 Therefore, the ferrite phase in duplex grade for multiple positions is important.   

Experimental test
The test coupon is made up of ASTM SA 790 with UNS S31803 equivalent to 2209 DSS grade, with dimensions of ϕ355.6 x t 12.70 mm2 and 14 mm length, prepared for a single V groove joint. The chemical composition of the test coupon is shown in Table 1.3 It was recommended to test the welder for the 6G position so that they may qualify for both pipe and plate welding in field joints during construction and turnaround.1 The ASME Section IX design code for the standard 6G position before the start of welding test coupons is shown in Figure 1, and the appropriate groove joint position for the welder qualification test is shown in Figure 2. Note in Figure 2 that the leveller indicates horizontal and vertical positions, and the axis of the pipe is oriented at 45˚ to both the horizontal and vertical axis. The pipe orientation was ensured before the start of welding until its completion, as shown in Figure 3. The axis of the pipe was in a similar position after the first pass welding.1 The fillet weld of the pipe to the L section beam fixture ensures a 45˚ pipe angle. The root pass of the welding was produced by ER2209 filler wire using the GTAW process, and the remaining hot passes were produced by an E-2209 welding rod using the SMAW process. The chemical composition of the welding consumables is shown in Table 2.3 Preheating and post-weld heat treatment of the weld joints is not recommended by the design code. The ferrite number (FN) of the weld joint was measured by the Feritscope FMP30 from Fischer, Germany. This was calibrated using stainless steel blocks of a known ferrite number and no error was obtained (see  Figure 4).  

Results and discussions
The quality of the weld joint is assessed by radiography, and the quality of the microstructure is assessed by Feritscope readings. The welded joint was subjected to radiography for quality assessment, and it was observed that linear indications such as slag, incomplete penetration and fusion were absent.4 A few rounded indications, almost like porosity, were observed on the hot pass of the weld run in a radiography film. However, these are accepted as per the QW 191 ASME Section IX standards4 and were present due to the SMAW welding process.

Completion of the DSS alloy 2209 test coupon is shown in Figure 5, and the weld joint is inspected visually after completion of the root and final passes separately. No significant indications were observed. The Feritscope measurements were taken on the final pass of the weld joint, as shown in Table 3, and results were obtained in ferrite number. The ferrite number is an indication of the delta ferrite content in a filler wire and welding rod, and its value mainly depends on the chemical composition of the welding consumables. Therefore, matching or slightly over-alloyed with nickel consumables are essentially required for the welding procedure qualification. An over-alloyed consumable is more advantageous than matching counterparts.5 Matching consumables require mandatory post-weld heat treatment, while nickel-rich over-alloy consumables do not require post-weld heat treatment, and should be used in as-welded condition.2,3

An unwanted sigma (σ) phase might have formed from the previous delta ferrite phase during the post-weld heat treatment (PWHT) when welded with a matching consumable.5 Welding with a nickel-enriched consumable avoids PWHT so the σ phase is avoided. However, it produces slightly lower weld metal strength than matching consumables due to the slightly higher austenite and lower ferrite phase proportions present in the welded microstructure.2,3 It is noted that welding filler wire contains 2-3.5% higher nickel content than the base metal, as shown in Tables 1 and 2. ASME Section IX specifies additional mandatory tests for weldments such as phase balance, pitting corrosion and microstructural tests apart from transverse root and face bend tests for welder qualifications.1

One such phase balance assessment was conducted and the results are shown in Table 3. The measurement of the ferrite number in a dual phase weldment microstructure measures the ferrite proportion in the weld metal.5 Measuring the ferrite number on the weld metal is easier and more accurate than in the heat-affected zone (HAZ) where it varies locally due to various weld thermal cycles. The weld metal ferrite number was predicted from a constitutional diagram, and the duplex welding consumable (ER-2209 and E-2209) solidification mode was mapped in a WRC diagram, as shown in Figure 6. It clearly reveals that the ferrite number of the weld metal is 40 FN and that the weld metal solidifies in a ferrite austenite FA mode for the 6G weld position, highlighted by the dotted red line.

It is very well understood that solidification pattern and ferrite number measurements are independent of welding positions. The ferrite number measured by the Feritscope was found to be 40-47 FN, as shown in Table 3. A few increases in the weld metal ferrite content were observed. Comparing it to the WRC diagram reveals that some error may have been encountered while measuring the ferrite number in the weld metal due to the presence of fine ferrite phases.5 These phases are difficult to measure by Feritscope it was reported for field joints, thick sections and large industrial static equipment. The high ferrite number obtained from experiments reveals that welding with such consumables may result in a slightly higher ferrite phase in the ferrite austenite microstructure and it also prevents weld solidification cracking.1,2,5    

The ferrite number measurement in weld metal is considered an important test for qualifying a welder for the aforementioned material grades. The ferrite number measurement matches the theoretical ferrite number and is shown in a WRC diagram. However, a slight difference may be due to unresolved ferrite phases present in the microstructure. All the mechanical tests of the test coupon were passed and fulfilled the code criteria in addition to the above phase assessment test. Hence, it is recommended that the welder is qualified for multiple positions of duplex stainless steel grade.  
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