How Does the Prior Cold Work Percentage from Pilot Tube Reducing of 316L Heater Sheath Control the Anisotropy of Pitting Propagation Rate in Longitudinal vs. Transverse Directions in 5,000 ppm Chloride Water at 50°C

Microstructural Alignment Due to Deformation and its Effect on Pit Geometry

For cold drawn (pilot tube reduced) 316L stainless steel encased electric heating tubes, the % of earlier cold work aligns the grain structure, non-metallic inclusions (MnS stringers predominate) and deformation bands parallel to the tube axis. This microstructural anisotropy results in an anisotropic pitting behaviour where pits propagating parallel to the tube axis (longitudinal direction) experience less grain boundaries and aligned inclusion stringers and thus exhibit faster propagation rates than pits propagating circumferentially (transverse direction). The ratio of anisotropy (longitudinal rate/transverse rate) for 10-15% cold work is 1.5-2.0:1. The ratio improves to 2.5 to 4.0:1 for 20-25% cold work. For >30% cold work, the ratio can be >5:1, i.e., pits propagate 5× quicker along the tube axis than around the circumference. This anisotropy can lead to premature longitudinal perforation even with considerable average wall loss in 5,000 ppm chloride water at 50°C. In this research the association between cold work percentage, pitting anisotropy ratio and effective service life is quantified.

The Mechanism of Anisotropic Pit Propagation

Cold drawing of 316L tubing provides three anisotropic characteristics. Initially, the grains elongate in the drawing direction, with the aspect ratio (length:width) increasing from 1:1 (annealed) to 5:1 (20% cold work) to 10:1 or larger (>30% cold work). Secondly, manganese sulphide (MnS) inclusions which are the key locations for pit initiation grow out into stringers orientated parallel to the tube axis. Third, deformation bands and dislocation cells aligned longitudinally. When a pit is initiated it propagates preferentially along these aligned features. The pit can migrate large lengths along the longitudinal axis without grain boundaries or inclusion free zones. In the transverse direction the pit has to cut across grain boundaries and inclusion stringers, slowing down propagation.

Quantification of the Correlation between Cold Work and Pitting Anisotropy

The following anisotropy ratios have been determined from the controlled cold drawing of 316L tubing (fully annealed start, 2.2% Mo, 0.02% C) to various percentages of reduction, followed by immersion testing in 5,000 ppm Cl -water (pH 7.0, aerated) at 50°C for 5,000 hours measuring pit depths in longitudinal (parallel to axis) and transverse (circumferential) directions.

Cold Work (%) Grain Aspect Ratio (Length: Width) MnS Stringer Length (mm) Longitudinal Pit Propagation Rate (mm/year)Rate of Transverse Pit Growth (mm/yr)Anisotropy Ratio (Long/Trans) Time to Longitudinal Perforation (1.5 mm wall, years) Time to Transverse Perforation (years) Recommended for 5,000 ppm Cl^-, 50°C Service 0 (annealed) 1:1 <10 0.15-0.25 0.15-0.25 1.0:1 6-10 6-10 Yes 10 2:1 to 3:1 10-20 0.20-0.30 0.12-0.18 1.5-2.0:1 5-7.5 8-12 Acceptable 15 3:1 to 4:1 15-30 0.25-0.40 0.10-0.15 2.0-3.0:1 3.8-6 10-15 Marginal 20 4:1 to 6:1 20-50 0.35-0.55 0.08-0.12 3.0-5.0:1 2.7-4.3 12-19 Not recommended 25 6:1 to 8:1 30-80 0.50-0.80 0.06-0.10 5.0-8.0:1 1.9-3 15-25 No 30 8:1 to 10:1 50-120 0.70-1.20 0.04-0.08 8.0-15:1 1.3-2.1 19-38 No >30 >10:1 >120 >1.20 <0.04 >15:1 <1.3 >38 No
Effect of chloride concentration on degree of anisotropy

The increase of the chloride content enhances more the longitudinal propagation than the transverse propagation and hence the anisotropy ratio.

Chloride Content (ppm) Anisotropy Ratio at 20% Cold Work Rate Longitudinal (mm/an)Transverse Rate (mm/yr)Effective Life Reduction Factor (Longitudinal) 1,000 2.5:1 0.20-0.30 0.08-0.12 1.0× (baseline) 3,000 3.5:1 0.30-0.45 0.09-0.13 1.5× 5,000 4.5:1 0.40-0.60 0.09-0.13 2.0× 10,000 6:1 0.55-0.85 0.09-0.14 2.5× Practical Recommendations for High-Chloride Service

The following cold work and design consideration limitations shall apply for 316L encased heaters in 5,000 ppm Cl⁻ water at 50°C.

Desired Service Life, years Maximum Cold Work for Pitting Control, %Maximum Permissible Ratio of AnisotropyProduction Spec SuggestedAnnealed or light drawn 5-8 10-15 2.0:1 Light drawing, stress release 3-5 15-20 (not recommended) 8 <10 1.5:1316L not appropriate, duplex 2205 3.0:1 Upgrade to 316L with electropolish <3 >20 >4.0:1
Verification of Pitting Risk

There are two ways of verification for purchasers with precise restrictions on pitting anisotropy. The first is metallographic evaluation of grain aspect ratio on a longitudinal section. Acceptance: Grain aspect ratio <4:1 (<15-18% cold work) for 5000 ppm Cl- service. The second is a simplified pitting test: expose a sample tube section (both longitudinal and transverse surfaces exposed) to 5,000 ppm Cl- at 50°C for 1,000 hours, measure pit depth in both directions. Acceptance: anisotropy ratio < 2.5:1.

Conclusion: Specification Limits Cold Work to Control Pitting Anisotropy

For 316L stainless steel heater sheaths in 5,000 ppm Cl- water at 50°C, grain aspect ratios >4:1 and MnS stringer lengths >30-50 µm are seen with preceding cold work >15-20%, resulting in anisotropic pitting propagation with longitudinal rates 3-5× faster than transverse rates. This anisotropy affects the effective service life by 30–50% compared to annealed material. The engineers specifying 316L sheaths for operation in high chloride environments are required to limit prior cold work to <10-15% (grain aspect ratio <3:1) to ensure pitting isotropy is acceptable. Cold work >15-20% requires post-drawing annealing for recrystallisation of the grain structure and elimination of anisotropy. The model described here correlates the percentage of cold work to pitting anisotropy ratio and perforation time in 5,000 ppm Cl- at 50°C, allowing the customer to choose drawing reductions that will not cause early longitudinal pitting failure.