How Does the Post-Weld Heat Treatment Cooling Rate from 400°C to 200°C Affect the Residual Stress Relaxation and Pitting Resistance of 316 Stainless Steel Heater End Caps

The Critical Temperature Window for Stress Relief sans Sensitisation

The end cap welding technique creates residual tensile strains in 316 stainless steel wrapped electric heating tubes which may cause stress corrosion cracking (SCC) and impair pitting resistance in chloride conditions. These residual stresses are often eliminated by post-weld heat treatment (PWHT) at 400-450°C for 1-2 h. However, the cooling rate from the PWHT temperature down to 200 °C has a considerable but generally neglected effect on both the degree of stress relaxation and the corrosion resistance of the material. Standard 316 will precipitate chromium-rich carbides at the grain boundaries (sensitisation) if cooled slowly across the range 300-450 °C. Rapid cooling (air or water quench) retains the stress alleviated structure without sensitisation. The link between PWHT cooling rate, residual stress reduction, and pitting resistance is quantified in this article, offering a specification framework for optimised post-weld treatment.

The Counteracting Effects of Stress Relief and Sensitisation

Two thermally activated reactions occur simultaneously during PWHT at 400-450 °C in 316 stainless steel. The first one is stress relief. Residual stresses due to welding (usually 100–300 MPa) are relaxed via dislocation motion and recovery. The process is logarithmic and 70-80% of the residual stress is released after 1 hour at 450 °C. The second phase is carbide precipitation: above temperatures of about 450 °C, chromium carbides can nucleate, although precipitation is sluggish. As the material cools from the PWHT temperature it again goes through the sensitisation range (450-850 °C). However, because the PWHT temperature (400-450 °C) is below the sensitisation range, no additional carbides precipitate during the hold. The risk occurs when the furnace is slow cooled and the material temperature increases above 450 °C during cooling? It does not – PWHT is carried out at 400-450 °C therefore the cooling will start from below the sensitisation range. The real risk is for PWHT at 500-600 °C (used sometimes for more thorough stress release) when gradual cooling through 450-850 °C produces sensitisation. During the 400-450 °C PWHT, the key concern is not sensitisation, but the re-introduction of residual stresses due to differential cooling. Any non-uniform cooling (one side of the weld cools quicker than the other) might generate new thermal stresses of 50-100 MPa which largely negate the advantage of the stress relieving hold.

Quantification of Influence of Cooling Rate on Pitting Resistance and Residual Stress

Final residual stress was assessed by X-ray diffraction and pitting resistance was evaluated by critical pitting temperature (CPT) testing in 1 M NaCl, over a range of cooling rates from a 450 °C, 1-hour PWHT of welded 316 end caps (1.5 mm wall thickness) in controlled tests. The results reveal that the cooling pace has a minor effect on sensitisation (all the specimens remained unsensitized as the maximum temperature never crossed the sensitisation range) but a strong effect on the final residual stress due to thermal gradient effects.

PWHT Cooling Method from 450°C Estimated Cooling Rate (450°C to 200°C) Final Residual Stress at Weld (MPa) Stress Relief Efficiency (% of as-welded stress)Critical Pitting Temperature (°C) (1 M NaCl) Recommended for chloride services
Cool furnace (power off)5-10 °C/hour 60-80 60-70 % 52-56 No (not effective)
Still air cold (horizontal) 100-200 °C/hour 40-60 70-80 % 54-58 Suitable for moderate chlorides
Forced air fan 300-500 °C/hour 30-45 80-85 % 55-59 Yes
Water quench (immersion) 1000-2000 °C/hour 20-30 85-90% 56-60Yes , best high chlorides
No PWHT (as-welded) N/A 150-250 0 % 50-54 Not for chloride service
Special Case of PWHT at 500-600 °C

In some situations where more comprehensive stress relief is required (e.g. heaters for high pressure duty or severe thermal cycling), PWHT at 500-600 °C for 1-2 hours is occasionally specified. Stress reduction efficiency at these temperatures is 90-95 % however the material is passed through the sensitisation range during heating and cooling. Severe sensitisation is seen in standard 316 (max 0.08 % C) with CPT decreasing to 35-40 °C and slow cooling from 550 °C, inappropriate for any chloride service. For these PWHT temperatures, the cooling rate is crucial. Water quenching from 550 °C (cooling rate >1000 °C/hour to 850-450 °C) suppresses sensitisation in thin-walled tubing, keeping the CPT around 55-60 °C. Thermal shock from water quenching a welded part, however, can induce distortion or breaking. The preferred solution for any heater requiring PWHT above 480 °C is to utilise 316L (maximum 0.03 % C). 316L is substantially less susceptible to sensitisation and has barely minor carbide precipitation even when cooled slowly from 550 °C.

Practical Recommendations for PWHT Cooling

For most of 316 sheathed heaters in aqueous service, the suggested PWHT is 450 °C for 1 hour followed by forced air cooling (cooling rate of 300-500 °C/hour). This treatment lowers the residual stresses by 80-85 % and does not produce sensitisation, because the maximum temperature reached is below the sensitisation range. Water quenching offers a somewhat greater stress reduction but is more likely to distort and is rarely essential. The table below serves as a decision guide for the selection of PWHT temperature, cooling rate and material grade depending on service corrosivity.

Service Environment Chloride Level (ppm) Maximum Temperature (°C) Recommended PWHT Cooling Rate Material Grade
Dry (non-chloride)Any Any None (as-welded) N/A 316L
Low chloride <200 <80 450°C, 1 hour Forced air 316 
Moderate chloride 200-500 <80 450°C, 1 hour Forced air or water quench Preferred 316L
High chloride 500-1500 <70 450°C, 1 hour Water quench 316L necessary
Very high chloride >1500 <60 500-550oC, 1 hour Water quench 316L (or upgrade to duplex)
Effectiveness of PWHT on Field Verification

For in-service heaters, where the PWHT history is unknown, two methods may be used to determine if residual stresses have been sufficiently released. The first is the ASTM G38 U-bend test, in which a tiny sample from the straight part of the heater is bent 180° and placed in boiling magnesium chloride. If cracking develops before 100 hours, the residual stresses surpass 100 MPa. The second way is to quantify the residual stress at the weld by X-ray diffraction but this requires specific equipment. This is the realistic solution for most customers . Put PWHT specifications on the purchase order and seek verification that the procedure ( time , temperature , cooling method ) was followed .

Conclusion: Specification of Cooling Rate as Part of PWHT for Chloride Service

For 316 stainless steel heater end caps, post-weld heat treatment at 450 °C for 1 hour followed by forced air cooling gives 80-85 % stress alleviation without sensitisation, greatly enhancing resistance to SCC and pitting in chloride environments. It is not recommended to slow cool (in a furnace) since this would diminish the efficiency of stress alleviation. Water quenching provides maximal stress reduction, but also increases complexity and risk. 316L should be specified and water quenching is required for PWHT at higher temperatures (500-600 °C) to prevent sensitisation. For chloride service, engineers specifying PWHT must also define the hold temperature and the method of cooling. The methodology given here allows customers to pick PWHT parameters that maximise corrosion resistance while preventing the unwanted sensitisation that might occur with inappropriate cooling, by relating cooling rate to residual stress and pitting resistance.