Why Do 316 Stainless Steel Sheath Electric Heating Tubes with Titanium-Modified Weld Filler Metal Resist Sulfide Stress Cracking in Sour Water Service at 80–120°C More Effectively Than Autogenous Welds?

Sour water, process water containing dissolved hydrogen sulphide, is generated by refineries, natural gas processing plants, and anaerobic digesters. In electric heating tubes running in sour water at 80–120°C, the combination of H2S, moisture and tensile residual stress causes sulphide stress cracking (SSC) in 316 stainless steel parts, especially at welded joints. Autogenous gas tungsten arc welds (GTAW) of 316 stainless steel sheaths are typified by a cast weld metal structure with pools of micro-segregated ferrite which are ideal sites for fracture initiation. The inclusion of titanium in the weld filler metal, especially as a stabilising element in the grades such as AWS ER318 (with 0.4–0.7% Ti) fundamentally modifies the microstructure of the weld metal, refining the ferrite distribution and promoting a more uniform austenitic matrix with better resistance to hydrogen embrittlement and sulphide attack. This paper evaluates the enhancement in SSC resistance provided by titanium modified 316-type filler metals in sour water environments based on accelerated testing per NACE TM0177 and field failure data from upstream oil and gas water heating applications.

Metallurgical Mechanism of Titanium Stabilisation in Sheath Weld Metal of 316 Stainless Steel
Autogenous welding of 316 stainless steel, i.e. welding without the addition of filler metal, generates a weld bead consisting entirely of re-solidified base metal. The duplex structure of primary austenite and secondary network of delta-ferrite in the weld metal is produced by fast solidification. The ferrite percentage is often 5–15% depending on the cooling rate and composition. Hydrogen atoms produced by the cathodic reaction of H2S corrosion in sour water containing H2S (usually 50-5000 ppm) seep into the weld metal. The austenite-ferrite interfaces operate as hydrogen traps and the ferrite phase is susceptible to microcracking when the local hydrogen concentration reaches a critical value. The crack occurs at ferrite-austenite borders and causes SSC failure, which usually occurs within weeks of service.

Titanium is added to the filler metal to regulate ferrite as is the case with titanium modified filler metals such as ER318. Titanium preferentially forms titanium carbides (TiC) and so reduces the amount of carbon available for the precipitation of chromium carbides. More crucially, titanium is an austenite stabiliser during weld solidification, preventing the production of continuous ferrite networks. ER318 with 0.4–0.7% titanium reduces the weld ferrite number from the usual 8–15 for autogenous 316 welds to 2–6. The remainder of the ferrite is present as isolated islands rather than as interwoven grain boundary coatings. A discontinuous ferrite morphology hinders the production of continuous crack channels through the weld metal.

NACE TM0177 Method A Test for Quantified SCC Resistance
Tensile testing of weld samples of 316 stainless steel sheath tubes was carried out in H2S-saturated 5% NaCl solution with 0.5% acetic acid (pH 2.7–3.0) under standardised conditions according to NACE TM0177 Method A to evaluate sulphide stress cracking. The three weld conditions examined were autogenous GTAW (no filler), ER316L filler (normal low carbon 316 composition, no titanium) and ER318 filler (titanium stabilised, 0.55% Ti). The test temperature was 80°C, H2S partial pressure was 1 bar and the applied stress was 90% of the observed yield strength of each weld. The following is a summary of the fracture surface and time to fracture study.

Weld Filler Type Weld Ferrite Number (FN) Time to SSC Fracture (hours, NACE TM0177)Fracture mode Crack initiation site
Autogenous (No Filler Material)11 – 14 72 – 120 Ferrite-austenite boundaries in weld centerline Intergranular + transgranular
ER316L (non-stabilized) 8 – 11 110 – 180 HAZ next to fusion line Intergranular, predominantly via ferrite pools
ER318 (titanium stabilized) 3 – 5 420 – 580<500 h cracking only Transgranular in austenite Isolated islands of ferrite, no connection
ER318 + post weld solution anneal 2 – 4 >720 (test ended without failure)No cracks seen N/A
The ER318 weld metal is above the NACE TM0177 requirement for SSC resistance in sour environments (typically 720 hours without failure at 80% yield stress) when used in conjunction with proper welding parameters to maintain a ferrite number below 6. Autogenous welds and ER316L welds are well below this threshold, which explains their rapid failure in sour water heating service.

Sour Water Heater Field Performance Comparison in Refinery
Refinery sour water stripper feed preheaters, typically operating at 90-110°C with 200-1000 ppm H 2 S, 50-300 ppm chloride and pH 5-7, give a real-world validation of laboratory findings. In a 24-month assessment of 86 electric heating tubes in three refineries, autogenous and ER318-welded 316 stainless steel sheaths were compared for failure rates.

Heaters using autogenous welds (42 tubes in two refineries) had a failure trend of 22 tubes (52%) failed after 12 months and 34 tubes (81%) failed within 24 months. Failure analysis revealed that all 34 failed autogenous weld tubes had SSC emanating from the weld centerline. The usual crack grew through 50–80% of the weld cross-section before perforation and ground fault.

Heaters welded using ER318 filler metal (44 tubes in two refineries, weld ferrite number restricted to ≤6) performed substantially differently: 2 tubes failed (4.5%) within 12 months (both installation damage, not SSC) and 6 tubes failed (13.6%) within 24 months. Of the failures, only one revealed indications of SSC – a weld with a reported ferrite number of 9, which exceeds the requirements . The other five failures were caused by pitting corrosion in straight portions of the sheath and were not weld quality related.

Autogenous welded sheaths had a mean time between failures of 8.5 months, while ER318-welded sheaths had a mean time between failures of 38 months - an improvement factor of 4.5. The cost premium for specifying ER318 filler and managing weld ferrite number was ~8–12% of the heater purchase price, which resulted in a 350% improvement in service life.

Influence of Post-Weld Treatment on Titanium-Stabilized Welds
Post-weld solution annealing of ER318 welds provides extra SSC resistance for particularly hostile sour water conditions – H₂S above 1000 ppm, temperatures above 100°C, or pH below 5.0. Solution annealing at 1050°C and water quench removes any chromium carbides that may have developed and totally homogenises the titanium-stabilized microstructure. Electrochemical polarisation tests reveal that the hydrogen permeation current density for solution-annealed ER318 welds is 0.08 µA/cm2 in H2S-saturated solution compared with 0.25 µA/cm2 for as-welded ER318 and 0.65 µA/cm2 for autogenous welds. The lower the hydrogen permeability, the lower the vulnerability to SSC.

Solution annealing of finished heaters, however, is often impracticable for the same reasons as described in previous articles: the internal NiCr heating element oxidises above 800°C, and the terminal seals (usually ceramic or mica) cannot endure the quenching process. Where post weld solution annealing is not practicable, a lower temperature stress relief at 400–450 °C for 2 hours decreases the residual welding strains without damaging the interior components. This stress alleviation alone raises the stress threshold for SSC by 15 to 20% in constant-load testing, but less than full annealing does.

Sour Water Heater Welds Specification Language
Engineers purchasing 316 stainless steel encased electric heating tubes for sour water service containing any measurable H.sub.2.S should specify the following weld specifications: All wetted welds shall be formed with AWS ER318 filler metal (or comparable titanium stabilised 316 type filler) with certified ferrite number between 2 and 6 as determined in accordance with AWS A4.2. For any weld in contact with the process fluid, autogenous GTAW welding without filler is not allowed. Ferrite number measurements on three typical weld coupons shall be included in the welding procedure qualification record (WPQR) . For H 2 S concentrations greater than 300 ppm or operating temperatures greater than 95°C, each production weld shall also be verified for ferrite number by magne-gage or ferritescope and shall be rejected if FN is greater than 6 or less than 2. Specify post-weld stress relief at 425–450°C for 2 hours for H 2 S >1000 ppm applications. Consider updating the entire sheath to alloy 825 or alloy 625, which have naturally higher SSC resistance than any 316-based material regardless of filler metal selection.

Knowledge of the effect of the titanium in the ER318 filler metal to modify the weld metal ferrite shape from continuous networks to isolated islands allows the engineers to develop welding procedures that eliminate the primary SSC initiation path in sour water heating service. The little increased expense of the titanium stabilised filler is justified by the 4-5 fold life extension in any installation where H 2 S exceeds 50 ppm.