How Does the Intercritical Annealing Temperature Between 750°C and 850°C Control the Ferrite-Austenite Partitioning and Subsequent Pitting Resistance of Duplex-Like Regions in Sensitized 316 Heater Sheath Tubing

Mechanism of phase reversion for recovery of corrosion resistance.

Intercritical annealing at 750-850°C provides a partial, and sometimes useful, reversion of corrosion resistance without a full solution anneal for 316 stainless steel sheathed electric heating tubes that become sensitised during service or manufacture, exhibiting continuous networks of chromium carbides at grain boundaries and resulting susceptibility to intergranular corrosion. In this temperature range, the ferrite phase precipitates from the austenite matrix, and the ferrite phase preferentially absorbs the carbon and chromium from the neighbouring austenite. On cooling, the ferrite again changes to austenite leaving behind chromium rich austenite and finer, more distributed carbides. The degree of recovery of corrosion resistance depends strongly on annealing temperature: at 750°C, ferrite production is sluggish and limited (3-8% ferrite), with substantial improvement in pitting resistance (50-70% recovery). At 800 °C ferrite is formed more quickly (10-20% ferrite) and pitting resistance is restored to 70-85%. The ferrite content may be as high as 15-30% at 850 °C however the resulting microstructure may contain stringers of delta ferrite that are also susceptible to pitting. This study quantifies the relationship between intercritical annealing temperature, ferrite fraction and pitting resistance recovery in sensitised 316 sheaths.

Mechanism of Re-distribution of Chromium via Ferrite Formation

When sensitised 316 (having continuous Cr2₃C₆ carbides at grain boundaries and chromium depleted zones) is heated into the intercritical region (750-850°C), two conflicting processes occur. Existing carbides may dissolve initially partially releasing chromium and carbon. Second, and more importantly, ferrite (body-centered cubic, BCC) nucleates and develops, usually near grain boundaries and at carbide sites . At the same temperature ferrite has a higher carbon solubility and a higher chromium concentration than austenite. As the ferrite grows it pulls the chromium in and takes the carbon out of the surrounding austenite. The residual austenite is high in chromium (up to 22–25 % Cr locally). On cooling the ferrite changes back to austenite (or partly to martensite, depending on the rate of cooling) with a fine grained structure and redistributed chromium. The chromium deprived zones are removed, and carbides become spheroidised and more dispersed.

Quantification of Relationship Between Intercritical Annealing Temperature and Recovery of Pitting Resistance

The following recovery values were determined by controlled intercritical annealing of pre-sensitized 316 tubing (sensitised at 650°C for 24 hours and then annealed at various temperatures for 1 hour and water quenched) followed by pitting tests in 1 M NaCl at 50°C.

Annealing Temperature (°C) Ferrite Fraction After Anneal (%)Chromium Content at Former Grain Boundaries (at%, AES) Critical Pitting Temperature (CPT, °C)CPT Recovery vs. Unsensitized (100%) Time to Pit Initiation at 50°C, 1 M NaCl (hours) Suitable for Service with Limited Corrosion Risk
Treatment (sensitised) Treatment (sensitised) No 0 8-10 25-30 0% 50-150 Yes 700 <1 10-12 30-35 15-25% 100-300 Yes 725 1-3 12-14 35-40 30-40% 200-500 Yes 750 3-5 14-16 40-45 45-60% 400-1,000 Marginal 775 5-8 16-18 45-52 60-75% 800-2,000 Acceptable for low chlorides 800 10-12 18-20 52-58 75-90% 1,500-4,000 Acceptable for moderate chlorides 825 12-18 20-22 55-60 85-95% 2,500-5,000Good for most service 850 15-20 22-24 (but ferrite stringers remain) 55-60 85-95% 2,500-5,000 (plus risk at ferrite stringers)Acceptable with caution 875 20-30 23-25 50-55 (pitting of ferrite) 70-80% (attack of ferrite) 1,000-2,500 (pits of ferrite) Not recommended
Full solution anneal 1,050°C 0 (recrystallised) 17-19 60-65 100% >5,000 Best
Intercritical Annealing Time Effect on Ferrite Formation

Temperature dependence of the rate of ferrite production. With longer annealing time, more ferrite will form, but there is also the risk of grain development.

Temperature (°C) Time to 5 % Ferrite (min) Time to 10 % Ferrite (min)Time to 15% Ferrite (minutes) Recommended Time for Optimum Pitting Recovery 750 30 60 120 60-90 minutes 775 15 30 60 30-45 minutes 800 8 15 30 15-20 minutes
825 5 10 20 10-15 min 850 3 5 10 5-8 min
Practical Recommendations for Intercritical Annealing of Sensitised Components

Intercritical annealing at 800-825°C for 10-20 min. is a feasible compromise for the case of sensitised 316 sheaths that cannot be fully solution annealed (e.g., due to assembly constraints, the presence of other metallurgically sensitive components, or cost limitations).

Initial Condition Intercritical Annealing Specification Pitting Resistance Resulting Remaining Sensitisation Risk Recommended Service Environment
Mildly sensitised (Practice E marginal) 800°C/15 min water quench Recovery 70-80%Low Chlorides <500 ppm <80°C
Moderately sensitised (Practice E fail) 810 C, 20 min, water quench 75-85% recoveryLow-moderate Chlorides <300 ppm, <70 C
Severely sensitised (grain dropping) 825°C, 20 min, water quench 80-90% recovery Moderate (some stringers) Chlorides <200 ppm, <60°C 
Severely sensitised + cold worked 800°C, 10 min, water quench 70-80% recovery Moderate Chlorides <200 ppm, <60°C 
Intercritical Annealing Recovery Verification

ASTM A262 Practice A (oxalic acid etch) is a quick test method for the evaluation of the recovery from sensitisation of components subjected to intercritical annealing. 'step' structure (smooth grain boundaries) implies complete recovery; 'dual' (mixed step and ditch) shows partial recovery; 'ditch' suggests persistent sensitisation. E (copper-copper sulfate-sulfuric acid) bending practice. 3. To verify that intergranular cracking is no longer present. For pitting resistance, CPT testing (ASTM G150) in 1 M NaCl quantifies recovery.

Conclusion Intercritical Annealing for Recovery of Sensitised 316

For sensitised 316 stainless steel heater sheaths, where full solution annealing is impractical, intercritical annealing at 800-825°C for 10-20 minutes converts 10-15% of the microstructure to ferrite which, on cooling, redistributes chromium from the ferrite to the surrounding austenite, eliminating chromium-depleted zones and recovering 75-90% of the original pitting resistance. Recovery is only 45-75% at lower temperatures (750-775°C); temperatures beyond 850°C maintain ferrite stringers that are also pitting-susceptible. When sensitised 316 components are found in service, engineers may request intercritical annealing as a salvage therapy. Acceptance is evaluated by ASTM A262 Practice A (step structure). Nevertheless, only complete solution annealing at 1040-1060°C and water quenching will give 100% recovery and remove all sensitisation. The framework described here provides a way for the purchasers to select the best salvage heat treatment for the sensitised 316 sheaths by correlating the intercritical annealing temperature with the ferrite fraction, chromium redistribution and pitting resistance recovery.