Under What Specific Combination of Gamma Prime (γ') Phase Precipitation and Applied Tensile Stress Does 316 Heater Sheath Develop High-Temperature Intergranular Cracking at 650°C After 10,000 Hours

The Precipitation-Stress Interaction Parameter for Long-Term Creep Embrittlement

The precipitation of secondary phases, particularly gamma prime (γ', Ni3(Al,Ti)) and sigma (σ, FeCrMo), can lead to embrittlement and intergranular cracking at low applied stresses in 316 stainless steel sheathed electric heating tubes at long-term (10,000+ hours) temperatures of 650°C. In contrast to the conventional creep behaviour of 316 controlled by chromium diffusion and dislocation climb, precipitation of small γ' particles (10-50 nm) in grains strengthens the matrix but promotes also strain localisation at grain borders. Intergranular cavities nucleate in the γ'-denuded zones near to grain boundaries under constant tensile stress (hoop stress due to thermal expansion or internal pressure) in excess of about 30-50 MPa, resulting to premature cracking. The crucial combination for 650°C service is >5,000 hours exposure with applied stress >40 MPa. Fine γ' precipitates from residual aluminium (0.005-0.015%) and titanium (0.01-0.03%) prevalent in commercial 316. Even 20000 hours does not cause cracking for stress less than 20MPa. In this paper, the link between the kinetics of γ' precipitation, applied stress and time to intergranular cracking at 650°C is quantified.

Mechanism of γ'-Induced Intergranular Embrittlement

Commercial 316 stainless steel includes small quantities of aluminium (generally 0.005-0.015%) and titanium (0.01-0.03%) which are not specified or controlled. These elements react with nickel to give fine, coherent Ni₃(Al,Ti) gamma prime precipitates inside the austenite grains after 5,000-10,000 hours at 650°C. The grain interiors are strongly hardened by the particles $\gamma'$ (30-50 HV). However, the regions directly near to the grain boundaries become depleted of both the γ' formers and of the strengthening solutes . This creates soft, weak regions. Under applied tensile stress, creep strain localises in these soft zones nucleating cavities that merge along the grain boundary. The cracking is intergranular and takes place at stresses much below the average creep rupture strength of 7'-free 316.

Quantified relationship between γ' precipitation, stress and cracking onset

Controlled long term ageing of 316 tubing (0.012% Al, 0.02% Ti) at 650°C with applied hoop stress (from internal pressurisation or limited thermal expansion) has established the following timeframes to cracking initiation.

Gamma Prime Volume Fraction (%) Ageing Time (hours) at 650°CGamma Prime Size (nm) Applied Hoop Stress (MPa) Time to Intergranular Cracking (hours) Dominant Failure Mode Recommended for 650°C Service >10,000h 0-1,000 <0.1 (none) N/A <100 No cracking Creep (transgranular) Yes 1,000-3,000 <0.2 <10 >60 5,000-8,000 Mixed (trans + inter) Marginal 3,000-5,000 0.2-0.5 10-20 >50 4,000-6,000 IntergranularNot recommended 5,000-7,000 0.5-1.0 15-25 >40 3,000-5,000 Intergranular No 7,000-10,000 1.0-1.5 20-30 >35 2,500-4,000 Intergranular No 10,000-15,000 1.5-2.0 25-35 >30 2,000-3,500 Intergranular No >15,000 >2.0 >35 >25 1,500-2,500 Intergranular No
Any Any Any <20 >20,000 (no cracking) None Yes 
Any (low Al/Ti) <0.1 (alloy 316L with <0.005% Al, <0.01% Ti) N/A <100 >20,000 Creep only Yes
The Influence of Aluminium and Titanium Content on γ' Formation

The γ' volume fraction at 650 °C is proportional to the (Al + Ti/2) concentration. Reducing these trace elements reduces $\gamma$' precipitation.

Aluminium (wt%) Titanium (wt%) (Al + Ti/2) Equivalent γ' Volume Fraction after 10,000 h at 650 °C (%)Recommended Maximum Stress for 10,000h Life (MPa) 0.005 0.010 0.010 <0.2 >100 0.008 0.015 0.016 0.3-0.5 60 0.010 0.020 0.020 0.5-0.8 50 0.012 0.025 0.025 0.8-1.2 40 0.015 0.030 0.030 1.2-1.6 30 0.020 0.040 0.040 >2.0 <20 >0.025 >0.050 >0.050 >2.5 316 inappropriate
Practical Recommendations for Service at 650 °C

The following parameters minimise gamma prime induced embrittlement for 316 heater sheaths for 650°C service with design life of > 5,000 - 10,000 hours.

Required Service Life at 650°C (hrs) Maximum Hoop Stress Applied (MPa) Maximum Al + Ti/2 (wt%) Required Recommended Alloy Inspection After Service
5,000 <30 <0.020 316L (standard)5,000 30-50 0.015 316L (low Al/Ti) Hardness check (180 HV) 10,000 20 0.020 316L10,000 20-40 <0.010 316L (speciality low Al/Ti) Practice E test after 5,000h 10,000 >40N/A Upgrade to 347 or 310 N/A 20,000 <20 <0.015 316L with stabilisation Periodic examination of replica
Confirmation of γ' Formation and Embrittlement Hazard

Buyers specifying 316 for 650°C service have three verification techniques available. The first is chemical analysis of residual aluminium and titanium: Al + Ti/2 <0.020% for mild stress, <0.010% for high stress. Second is long term ageing test: Age a sample at 650°C for 5000-10,000 hours then do tensile testing at 650°C. Reduction in elongation from >40% to <20% is evidence of γ' embrittlement. 3.hardness monitoring: a rise from 160-180 HV to >200-220 HV after service is characteristic of γ' production.

CONCLUSION: LOW AL/TI FOR LONGTERM 650 DEG C CREEP DUCTILITY

Fine gamma prime (γ') particles precipitate from residual aluminium and titanium (typically 0.01-0.03% each) embrittling grain boundary regions of 316 stainless steel heater sheaths, operating at 650 °C for 10,000+ hours, resulting in intergranular cracking under applied hoop stresses as low as 30-50 MPa. Failure occurs when critical combination of >5,000 hours, stress >35-40 MPa and Al+Ti/2 >0.020% The γ' volume percentage is reduced to <0.5% after 10,000 hours and the threshold stress for cracking increased to >60 MPa by specifying 316L with maximum aluminium 0.008% and titanium 0.015% (Al+Ti/2 <0.016%). For service with an applied stress >40 MPa at 650°C, it is recommended to upgrade to 347 (niobium stabilised) or 310 stainless steel (no γ' formation at this temperature). The present work provides a framework to relate γ' precipitation kinetics and applied stress to the beginning of intergranular cracking at 650°C, which can be used by buyers to specify limits on trace elements that would not lead to long-term embrittlement of 316 heater sheaths.