Super Duplex Stainless Steel Welding for Offshore Applications

Material description

Stainless Steel (SS) contains significant amounts of chromium (Cr), nickel (Ni), and sometimes molybdenum (Mo) compared to carbon steel. Duplex Stainless Steel (DSS) consists of Fe-Cr-Ni-Mo, and N, with a typical chemical composition range provided in Table 1.

Table 1: Typical chemical composition of Duplex stainless steel and Super-duplex stainless steel

Common DSS grades are:

[A] UNS S32205 (Standard DSS): This is the most widely used duplex stainless steel, containing 22% Cr, 5-6% Ni, and 3% Mo. It offers a good balance of strength and corrosion resistance, with excellent resistance to pitting and crevice corrosion. It is used in chemical processing, oil and gas industries, desalination, and marine applications.The mechanical properties of DSS are shown in Table 1.

[B] UNS S32750, S32760 (Super Duplex Stainless Steel – SDSS): These grades contain 25% Cr, 7% Ni, and 4% Mo, offering higher mechanical strength (about twice as strong as austenitic stainless steels like 304 or 316) and superior corrosion resistance, especially in chloride-rich environments. These properties make SDSS suitable for more demanding applications. It has good toughness, although slightly less than austenitic stainless steels in cryogenic conditions. The Pitting Resistance Equivalent Number (PREN) of ~40–45 ensures resistance to stress corrosion cracking, pitting, and crevice corrosion.

It is typically used for:

Offshore oil and gas (subsea equipment, deep-sea pipelines)
Chemical processing
Desalination plants
Marine applications
Application where chloride stress corrosion or high strength is critical

Table 2 lists some standards that specify SDSS.

Table 2 lists some standards that specify Super-duplex stainless steel

In this article, we examine the properties, applicable standards, and challenges reported during qualification of welding procedures for SDSS S32760.

Welding Case study

Consider a welding project where SDSS grade UNS S32760 is selected for the fabrication of a 10 mm thick pressure vessel to be used in sour service. During welding procedure qualification, the client required the inclusion of ASTM A923 Test Method C (ferric chloride corrosion test) as part of the qualification process.

The purpose of this test is to assess the resistance to intermetallic phase formation by immersing the weld specimen in a ferric chloride solution. The acceptable weight loss is typically <10 mg/dm²/day (mdd). If the weight loss exceeds this, intermetallic phases are likely present, and the weld fails the test. It was mentioned in the report that ASTM A923-C specifies the corrosion requirement for base materials that is in the solution-annealed condition.

Fig. 1: Pressure Vessel Fabrication Using SDSS S32760

Note:

‘’Sour service” refers to environments where hydrogen sulphide (H₂S) is present, typically in oil and gas production. These conditions are highly corrosive and can cause severe hydrogen embrittlement, sulphide stress cracking (SSC), and stress corrosion cracking (SCC) in metals.

The solution-annealed condition implies that SDSS has been heat-treated to restore its desired microstructure and mechanical and corrosion-resistant properties after fabrication, welding, or cold working. This process usually involves heating the material to ~1050°C to 1120°C, holding it at this temperature depending on the section thickness, and then rapid quenching, typically in water, to restore the desired ferrite-austenite balance (around 50/50).

ASTM A923 Test Method A (Sodium Hydroxide Etch Test – Rapid screening method for intermetallic phases using visual inspection after etching) is used to confirm solution-annealed condition. Ferrite content check (target: ~40 – 60%) using instruments like a Ferritscope or metallography. Hardness testing can be implemented, and excessive hardness may suggest improper annealing or unwanted phase formation. Some relevant standards are shown in Table 3.

During qualification stages, the manufacturing company observed and reported the following:

ASTM A923 does not explicitly list SDSS S32760 as an applicable material.
Various welding parameters were tested, but meeting the corrosion requirement of ASTM A923-C (≤10 mdd at 40°C) proved challenging.
The supplied pipes, flanges, and end caps were in the solution-annealed condition and certified to meet the corrosion test requirement.
Welding consumable manufacturers could not guarantee the required corrosion resistance for S32760 weld metal.

In initial welding trials, pure Argon gas, an interpass temperature of 90°C, and a heat input of 1.55 kJ/mm were employed. A girth weld was made using manual TIG welding. About 10 trials were carried out and tested for mechanical, chemical (including PREN), and corrosion properties. The girth weld was not heat-treated, and ferrite content was tested according to ASTM E562, along with metallographic examination for any intermetallic precipitates.

Parameter Adjustments for Corrosion Requirement

To meet the corrosion requirement, the following parameters adjustments were reported:

Travel speed, TS was increased, particularly for the root and cold pass. This implies that welding heat input was reduced for root and cold pass.
The shielding and backing gas was changed from Argon to Ar+2% N₂.
The maximum interpass temperature was reduced from 90 °C to 50 °C.
The base material, consumable and fabrication tools were thoroughly cleaned.

The test analysis found that despite these adjustments, the corrosion requirement was still not met, and the results were inconsistent. It was difficult for the manually welded SDSS pipe to pass the ASTM A923-C requirement. The use of minimum TS of 65 mm/min resulted to inconsistent fusion to the parent material resulting in lack of fusion and undercut defects.

Comparison with S32750 Welds

The manufacturing company was surprised by the failure of S32760 grade to meet the ASTM A923-C requirement, judging from the point that S32750 Grade had been welded in the past and it met all chemical, mechanical, ferrite and corrosion requirements. The welding parameter for the S32750 welding were:

TS of 45 mm/min for root pass
Shielding & backing gas: Ar+2% N₂
Interpass temperature of 100° C maximum
Heat input for root pass in the range 0.6-1.2 kJ/mm and cold pass 0.5-0.9 kJ/mm.
Filler metal diameters: 1.6, 2.0 & 2.4 mm

Also, a purchased S32760 pipes, welded with mechanised process using a higher TS than the manual TIG process, and solution annealed met the ASTM A923-C corrosion test requirements.

The report concluded that:

SDSS grade S32760 is not recommended for fabrication where welding procedures must meet the ASTM A923-C corrosion test requirements, whereas S32750 generally meets these requirements.
S32760 can achieve corrosion requirements under less stringent standards like ISO 17781, which accepts a corrosion rate of 4.0 g/m² at 35°C, much easier than the ASTM A923-C.
That the specified corrosion criteria can be achieved if S32760 is welded within the following parameters limits:
1. Travel speed for root pass: 65 mm/min minimum
2. Travel speed for cold pass: 100–125 mm/min
3. Shielding and backing gas: Ar+2% N₂
4. Interpass temperature: Limit to 50°C for low thickness materials
5. Oxygen content during back purging: Limit to 500 ppm
6. Heat input: 0.6–0.9 kJ/mm for root pass, 0.52–0.86 kJ/mm for cold pass
7. Filler metal diameter: 2.0 mm for root pass, 2.4 mm for cold, fill, and cap passes
8. Craftsmanship: Competent craftsmen should be employed

Commentary

In this report, the author noted that tests and analyses of the SDSS weld showed the corrosion requirement of <10 mg/dm²/day at a test temperature of 40 °C was not met using manual TIG welding, and no post-weld heat treatment was applied to the sample. However, the author suggested that this criterion could be achieved under certain conditions, which were outlined in the conclusion, although the justification for these recommendations was not clearly explained.

To weld SDSS successfully, it is critical to understand the behaviour of the phases in its microstructure before selecting welding parameters. Both DSS and SDSS exhibit a balanced microstructure of approximately 50% austenite and 50% ferrite. This balance combines the strength of ferritic steels with the corrosion resistance and toughness of austenitic steels. The mechanical properties of these grades are summarized in Table 1.

Welding SDSS presents several challenges due to its unique metallurgical and mechanical characteristics. Welding can disturb the phase balance. If the ferrite content is too low (e.g., <25%), it can lead to reduced strength and increased susceptibility to pitting or stress corrosion cracking in chloride or seawater environments. Conversely, if the ferrite content is too high (e.g., >65%), there is a risk of embrittlement (or brittle weld metal) , poor low-temperature toughness, and reduced corrosion resistance.

The heat-affected zone (HAZ) is particularly prone to the formation of unwanted intermetallic phases (e.g., sigma or chi phases), especially when exposed to critical temperature ranges (600 – 950 °C) for extended periods or under excessive heat input. These phases are brittle and significantly degrade corrosion resistance and toughness. Additional risks include hot cracking during solidification and hydrogen-induced cold cracking.

Therefore, precise control of welding parameters and energy input is essential. Excessive heat input promotes intermetallic formation, while too little may result in high ferrite content and poor fusion. Post-weld evaluation must include testing for phase balance, mechanical properties, and corrosion resistance. An acceptable ferrite-austenite balance is typically within ±10% of the desired 50/50 ratio. Sometime the acceptable value is quoted in the range (35% – 65%) which is less stringent.

Welding conditions that would help maintain the phase balance within acceptable range are:

Avoiding slow cooling, which promotes sigma phase formation
Avoiding preheating, which slows cooling and increases risk of intermetallic formation
Keeping interpass temperatures strictly below 100 °C
Ensuring rapid but controlled cooling rates
Maintaining consistent energy input, say, between 0.5 -1.5 kJ/mm
Using TIG for the root pass and MIG/MAG for fill passes, though only TIG can be used for the whole process
Applying short arc lengths and stringer beads (no weaving) to avoid overheating
Maintaining consistent torch angle and travel speed
Using matching or over-alloyed filler metals
Avoiding post-weld heat treatment (PWHT) unless necessary

One critical factor not addressed in the welding case study is the composition of the filler wire. A carefully selected, often over-alloyed filler is essential to promote proper austenite formation in the weld metal. The PREN of the filler should be equal to or greater than that of the base metal – ideally around 47.

Using an inappropriate filler can cause phase imbalance and corrosion failures, which may explain why the weld in the case study failed to meet ASTM A923 Method C requirements.

Additionally, cooling must be rapid but controlled. This may also explain why the mechanized weld, with its more consistent cooling rate, met the corrosion criteria. The welding technique itself is also an important factor – and involved if backhand, vertical or forehand is employed. Therefore, to consistently achieve a weld that meets corrosion testing requirements, all welding parameters must be carefully and comprehensively optimized.

Contributor: VCI

Table 3: Important standards relevant to Duplex stainless steel

Table 3: Important standards relevant to this article

Welding case study culled from:

Raju, A. (2025) ‘Challenges in qualifying welding procedures for super-duplex steel S32760’, Welding and Joining Matters, Issue 11, TWI, pp. 42–43.

Question:
What are the main challenges in welding Super Duplex Stainless Steel (SDSS) for offshore applications?

Welding Super Duplex Stainless Steel (SDSS), especially UNS S32760, presents challenges such as maintaining the correct ferrite-austenite phase balance, avoiding intermetallic phase formation, and meeting stringent corrosion test requirements like ASTM A923 Method C. Precise control of heat input, interpass temperature, shielding gas, and travel speed is essential. Manual TIG welding without post-weld heat treatment often fails corrosion testing, unlike mechanized processes.