How Does the 316 Stainless Steel Sheath Wall Thickness Influence the Residual Stress Distribution After Swaging and Its Effect on Corrosion Fatigue Life in Marine Environments?

Corrosion fatigue is a particularly aggressive failure mechanism for electric immersion heaters specified by marine engineers for offshore platforms, shipboard systems and coastal industrial plants when cyclic mechanical load and chloride corrosion are combined. Corrosion fatigue leads to crack initiation at much lower stress amplitudes and shorter cycle lifetimes than pure fatigue in air or pure corrosion in static settings. The residual stresses generated by swaging (which compacts the magnesium oxide insulation around the resistance wire) interact with service stresses and either accelerate or retard fracture formation depending on their sign and magnitude. This article quantifies the effect of sheath wall thickness on the distribution of residual stress after swaging and the effect of these residual stresses on the corrosion fatigue life in a marine environment.

Generation of Residual Stress in the Swaging of 316 Sheaths
When a 316 stainless steel tube is swaged from a bigger to a smaller diameter around the MgO filled core, a specific pattern of residual stress is induced. Residual stresses on the outside surface of the sheath are compressive due to the inward hammering of the swaging dies on the material, and tensile on the inner surface due to the outward push of the compressed MgO during elastic springback. The size of these residual strains is dependent on wall thickness and diameter reduction. For a thin walled sheath--below 1.0 mm--the swage force penetrates almost the whole wall thickness. The resulting strong compressive stresses at the outer surface (-200 to -300 MPa) and moderate tensile stresses at the inner surface (+50 to +100 MPa) are obtained. The swaging force is taken mostly by the outside layers of the thick-walled sheath (>1.8 mm), which develop strong compressive stresses only close to the outer surface, leaving the inner layers largely untouched. The tensile stresses at the inner surface are likewise lower due to the less complete MgO compaction as stated in Article 12. This link was confirmed by X-ray diffraction measurements of residual stress in the swaged 316 sheaths. For a 0.9 mm wall sheath the surface compressive stress was -280 MPa. A 1.5 mm wall sheath was -180 MPa. The 2.0 mm wall sheath exhibited -120 MPa only. The thinner sheath stores more advantageous compressive stress at the outer surface, the site of corrosion fatigue fracture initiation.

The Effect of Residual Compressive Stress on Corrosion Fatigue Crack Initiation
The cyclic tensile stress and corrosive electrolyte containing chlorides cause corrosion fatigue fracture start in 316 stainless steel. The crack starts at a surface pit or inclusion where the passive protective coating has failed. If a residual compressive stress state is present on the surface, the applied cyclic tensile stress must overcome this compression to produce net tension at the crack tip. This significantly increases the threshold stress for crack onset. For a 0.9 mm 316 sheath with -280 MPa surface residual compression, an applied cyclic stress of +200 MPa results in just -80 MPa net at the surface-still compressive and unable to initiate a fatigue crack. Only the cyclic stress exceeding + 280 MPa generates the net tension on the surface. In comparison, a 2.0 mm sheath that has just -120 MPa of surface compression is in net tension when the applied stress is greater than +120 MPa, a far lower threshold. Corrosion fatigue tests in synthetic seawater at 60°C validate this effect. Samples with a wall thickness of 0.9 mm and substantial residual compression sustained 10 million cycles at 250 MPa applied alternating stress without crack start . At the same applied load, samples with little residual compression and 2.0 mm wall thickness failed after 2 million cycles. The thinner sheath yielded five times greater corrosion fatigue life despite reduced absolute wall thickness.

Trade-Off Between Residual Stress Benefit and Wall Thickness Margin
Thin-walled sheaths have the advantage in corrosion fatigue but this must be balanced against the lower material margin should a crack start. The 0.9 mm sheath, with substantial residual compression, may be resistant to crack start for millions of cycles. However, once a crack overcomes the residual stress field, say owing to a huge surface pit or a brief overload, it only needs to propagate through 0.9 mm of material to cause perforation. A 2.0 mm sheath with less residual compression may start a crack at fewer cycles, but the crack has to progress through 2.0 mm to trigger failure. Total life to failure is the sum of initiation life and propagation life. Rotating beam corrosion fatigue test results in 60C seawater are used to illustrate the following total cycles to failure for 10 mm OD 316 sheaths at 200 MPa alternating stress. A 0.8 mm sheath had 12 million cycles in total (11.5 million start, 0.5 million propagation). A 1.2 mm sheath accomplished 8 million cycles (1 million propagation, 7 million start). 5 million cycles (3 million start, 2 million propagation) for a 1.6 mm sheath. A 2.0 mm sheath survived 3.5 million cycles (1.5 million initiation, 2 million propagation). The propagation margin is small, but the strong residual compression of thin-walled sheath results in lengthy overall life.

Marine Environment Corrosion Fatigue Life Selection Matrix
The following table shows recommended 316 sheath wall thicknesses for electric immersion heaters in marine service depending on estimated cyclic stress amplitude and necessary service life. The values are for continuous exposure to sea water or brackish water at 30–60°C and also for normal swaging techniques without post-swage annealing.

Expected Cyclic Stress Amplitude Service Life Required Recommended 316 Sheath Wall Thickness Dominant Failure Mode Design Rationale
Low (< 50 MPa) > 10 years 1.2 – 1.6 mm Crevice corrosion Residual stress benefit less essential. Corrosion allowance drives thickness
Moderate (50 – 150 MPa) 5 – 10 years 1.0 – 1.2 mm Corrosion fatigue initiation Thin wall for high residual compression to avoid crack
Moderate (50 – 150 MPa) > 10 years 1.4 – 1.6 mm with post-swage anneal Propagation after initiation Annealing reduces residual stress benefit; rely on thickness margin
High (150 – 250 MPa) 3 – 5 years 0.8 – 1.0 mm Initiation of corrosion fatigueMaximum residual compression a must; reduced service life acceptable.
High (150 – 250 MPa) > 5 years Not viable for 316LOverload or fast cracking Upgrade to Incoloy 825 or titanium
Variable with frequent overloadAny propagation of 1.6 – 2.0 mm following unexpected crackThickness margin protects against stochastic events
In applications where cyclic stresses are induced by vibration, thermal expansion cycling or pressure changes, it is important to be able to measure or estimate the stress amplitude. In the absence of measurement, a moderate stress level of 100 MPa is a prudent default. In these instances the wall thickness of 1.2 mm gives the optimal compromise between the advantage of residual stress and the margin for propagation. Engineers should also consider recommending post-swage annealing for thick-walled sheaths used in high-cycle applications. Annealing at 1050°C completely removes residual stresses and thus the corrosion fatigue benefit of thin walls, but also the tensile residual stresses at the inner surface that can lead to fracture propagation. An annealed 1.6 mm sheath has a similar total fatigue life to a non-annealed 1.2 mm sheath, offering an alternate approach to achieve long service life without reliance on thin walls. Manufacturing capability and cost is a function of narrow walls with high residual stress or thick walls with annealing. For most marine applications with vibration present but not severe, a wall thickness of 1.2-1.4 mm with normal swaging will give reliable corrosion fatigue performance without extra processing.