How Does the Tube Drawing Reduction Percentage on 316 Stainless Steel Heater Sheaths Alter the Pitting Resistance Equivalent Number in Chloride Solutions

Influence of Mechanical Deformation on Passive Film Chemistry and Inclusion Distribution

The manufacturing history of 316 stainless steel sheathed electric heating tubes, particularly the percentage reduction due to cold drawing to which the tubing is subjected prior to the final annealing, has a measurable, but often ignored, effect on the intrinsic pitting resistance of the material in chloride containing media. The pitting resistance equivalent number (PREN = weight percent Cr + 3.3×Mo + 16×N) is computed from bulk chemistry and is generally a constant material attribute for a specific alloy grade . But cold drawing redistributes non-metallic inclusions, refines the grain structure and may change the effective concentration of chromium and molybdenum at the tube surface by strain-induced diffusion. High reduction percentages (>40-50%) can lead to the rupture of massive manganese sulphide inclusions, generating microcrevices that are favourable sites for pitting initiation, regardless of the bulk PREN. In this paper, the influence of the tube drawing % on effective pitting resistance of 316 sheaths in warm chloride solutions is quantified and a specification framework for the selection of drawing decrease based on corrosion severity is described.

The Metallurgical Relationship Between Cold Work and Pitting Susceptibility

The bulk chemical composition remains unchanged by the plastic deformation when 316 stainless steel tube is cold drawn to lower the outer diameter and wall thickness. It does, however, modify three microstructural characteristics that influence localised corrosion. The first effect of the deformation is the elongation of non metallic inclusions, mostly MnS and oxide clusters, in the drawing direction. The elongated inclusions have a larger continuous interface with the surrounding austenitic matrix and hence they increase the possibility of a pit formed at one inclusion to spread along the inclusion–matrix border instead of repassivating. Second, large MnS inclusions are fractured by heavy cold work (reductions > 40%) to produce microcracks which are not repaired by subsequent annealing unless the temperature is above 1050 °C. These microcracks act as crevices to hold chloride ions and to sustain locally aggressive chemistry, thereby beginning pitting at potentials much lower than the bulk pitting potential . Third, cold work raises the dislocation density three to four orders of magnitude from about 10{sup 8} cm{sup -2} in annealed material to 10{sup 11} to 10{sup 12} cm{sup -2} in 40% cold-drawn material. Dislocations are high energy areas where the passive film is thinner and more faulty and so more sensitive to chloride-induced breakdown. The net impact is that two tubes with equal PREN from bulk chemistry can have pitting temperature variances of 10-15 oC merely from differences in cold drawing reduction percentage applied prior to final annealing.

Annealing after drawing as a critical restorative step

The cold work induced loss of pitting resistance is not permanent. Full solution annealing at 1040-1100 °C and quick water quenching lead to recrystallisation of the grain structure, removal of dislocations and spheroidisation of elongated inclusions. annealing restores the pitting resistance of a highly cold-drawn tube to that of a lightly drawn tube of the same bulk chemistry . But the industrial practice of making heater sheaths differs greatly. Some manufacturers pull tubing to final dimensions, reducing the initial hollow billet by 60-70 %, then anneal before cutting to length and bending. Others draw several times with intermediate anneals leaving the tube in a half-hard or quarter-hard condition (10-25 % cold work) without a final full anneal, relying on the heat of future welding and bending to partially release tensions. For the latter technique, the residual cold work (normally 10-25 %) lowers the effective pitting resistance equivalent value by 2-4 points. In practice, the tube with the bulk PREN = 25 (e.g. 17%Cr, 2.2%Mo, 0.06%N) behaves as PREN = 22-23 in the cold-worked condition, which causes a decrease in the critical pitting temperature of 8-12 °C. At 80 °C and the same chloride concentration, pitting will occur on the cold-worked tube, whereas the fully annealed tube will be passive.

Quantification of the effect of drawing reduction on pitting temperature and induction time

Quantitative connections have been established from laboratory testing of 316 stainless steel tubing with controlled drawing reductions and subsequent pitting potential measurements in 1 M NaCl at 80 °C. For fully annealed material (0-5% cold work) the critical pitting potential is usually 300-350 mV vs. SCE and the critical pitting temperature (CPT) in 1 M NaCl is 55-65 °C. For materials with 15% cold work (quarter-hard) the CPT decreases to 48-55°C. For 25 % cold work (half-hard) it falls even lower to 40-48 °C. For 40% cold work (full-hard) CPT reduces to 30-38 °C. For service, a properly annealed 316 sheath will exhibit no pitting for 5-8 years in 2000 ppm chloride water at 90 °C. With the same bulk chemistry and 25 % residual cold work, pitting will commence in 12-24 months under the same fluid circumstances. The induction time to the commencement of pitting is exponential with cold work percentage. Each 5 % increase in cold work about halves the time to first pit in a given chloride-temperature environment. The table below shows these relationships for a typical 316 chemistry (PREN = 25) for a variety of chloride concentrations and operating temperatures.

Residual Cold Work after Drawing (%) Heat Treatment after Drawing Effective PREN (calculated) Critical Pitting Temperature in 2000 ppm Cl⁻ (°C) Time to Pit Initiation at 90 °C, 500 ppm Cl⁻ Recommended Service Environment 0-5 (completely annealed) Full solution anneal 1040 °C + quench 25 58 >8 years All chloride service up to 3000 ppm, 90 °C 5-10 Partial stress relief only 24-25 53-58 5-8 years Acceptable for moderate chlorides (<1500 ppm) 
10-15 No anneal; as drawn quarter-hard 23-24 48-533-5 years Chloride service (<800ppm)
15-20 No anneal, drawn half-hard 22-23 44-48 2-3 years Marginal, Low chlorides only (<400 ppm)
20-25 No anneal, hard-drawn21-22 40-44 12-24 months Not suitable for continuous service chlorine
25-35 No anneal; full-hard 19-21 34-40 6-12 months Unacceptable for any moist chloride environment
Any , post anneal Full solution anneal after all drawing Full PREN restored 55-65 (depends on bulk chemistry) >8 years (bulk PREN dependent) Preferred standard for all corrosive service
Practical Verification of the State of Cold Work and Adequacy of Annealing

There are three practical ways for buyers of 316 encased heaters for chloride use to ensure that the tube is fully annealed and has minimum residual cold work. The first is the measuring of magnetic permeability using a ferritescope. Fully annealed 316 austenitic stainless steel is nonmagnetic with relative permeability (μr) less than 1.02. Strain-induced martensite, which is ferromagnetic, is formed in cold wrought 316. A reading above 1.05 suggests at least 10-15% cold work, while 1.10 recommends 20-25% cold work. The second method is microhardness testing on a portion of the wall of the tube. Annealed 316 has a Vickers hardness of 150-180 HV. Quarter-hard material is 200-240 HV. Half-hard is 250-300 HV. Full-hard is 320 HV and beyond. The third test is an electrochemical potentiokinetic reactivation (EPR) test which measures the degree of sensitisation and cold work damage. EPR values less than 0.5 C/cm2 indicate thoroughly annealed material, while values greater than 2.0 C/cm2 suggest significant residual deformation. Specification of a certificate of full solution annealing (temperature, time, and quench method) along with a maximum magnetic permeability of 1.05 in the purchase specifications offers an enforceable quality assurance.

The relationship of cold work, welding and final annealing possibilities

A typical manufacturing constraint is the fact that a full solution annealing is not always possible after bending and welding. Long U-tube heaters, particularly those with attached flanges and thermowells, cannot be practically annealed, since the high temperature would distort the assembly, oxidise the internal MgO or harm non-metallic components. In those instances the most important factor for the final corrosion resistance is the cold work state of the raw tubing before shaping. If the tube is to be delivered in the fully annealed condition (0 to 5 percent cold work) prior to bending, then even with the addition of 10 to 15 percent cold work from the bending operation, the ultimate residual strain will be less than 20 percent. For heaters that have to be utilised in the as bent condition without final anneal, the choice of tubing with higher molybdenum content ( e.g. 316L with 2.5-2.7 % Mo instead of a minimum of 2.0 %) offers a compensating rise in bulk PREN. A tube with bulk PREN = 27 and 15% residual cold work has an effective PREN ≈ 25, the same as a fully annealed standard 316. This approach of using premium chemistry to compensate for manufacturing restrictions is typical for big, complicated heater assemblies where annealing after fabrication is not an option.

Conclusion – Defining Annealing Condition with Chemistry for Corrosive Service

The pitting resistance of 316 stainless steel sheathed heaters in chloride solutions is not a function of the bulk PREN as reported in the mill certificate only. The cold drawing reduction used during tube manufacture and whether the tube is given a full solution anneal after all forming operations can shift the effective pitting temperature by 10-20 °C and change the time to pit commencement by a factor of 5-10. For continuous service heaters in chlorides above 500 ppm or at temperatures above 80 °C, the purchaser should seek certification that the completed sheath (including bends and welds) has been completely solution annealed. When post-fabrication annealing is not practicable, using premium chemistry (316L with PREN ≥ 26) and requiring entering tube annealed condition (hardness < 180 HV, permeability ≤ 1.02) is a reasonable option. A paradigm developed in this paper correlates tube drawing reduction % and annealing history to observable pitting resistance. This enables engineers to choose 316 sheathed heaters that will give predictable corrosion performance, rather than depending on bulk chemistry alone.