How Does the Volume Fraction of Primary Ferrite in 316L Heater Sheath Tubing After High-Temperature (1,150°C) Solution Annealing Control the Solidification Cracking Susceptibility During End Cap Welding

The Ferrite Window for Weldability of Austenitic Stainless Steels

The volume fraction of retained primary delta ferrite after solution annealing at 1,150°C (higher than the 1,040-1,080°C typically specified) directly governs the susceptibility to solidification cracking during autogenous gas tungsten arc welding (GTAW) of 316L stainless steel sheathed electric heating tubes requiring end cap closure welding. Ferrite content in 316L is approximately 3-8% (depending on chemistry) at common solution annealing temperatures (1,040-1,080°C). However, solution annealing at 1150°C (often employed to entirely dissolve carbides or to coarsen grains for creep resistance) moves the phase balance towards austenite at the higher temperature, giving a ferrite volume fraction of 0-3% on cooling. It was shown that during welding, weld metal with ferrite concentration less than 2-3% solidifies in a fully austenitic mode, which is especially sensitive to hot cracking due to segregation of low-melting-point impurities (P, S, Si) to grain boundaries. The crucial ferrite threshold for crack-free welding is 3-8% ferrite in the weld metal (4-10% ferrite in the base metal for autogenous welds). In this paper the link between solution annealing temperature, ferrite volume fraction and weld hot cracking susceptibility is quantified.

Mechanism of Ferrite Controlled Solidification Cracking

316L The first phase to develop from the liquid on solidification of 316L weld metal will be delta ferrite if the composition lies within the ferrite-austenite range of the Schaeffler diagram. The solubility of impurities (P, S, Si) in the ferrite phase is higher than that in the austenite. During solidification these impurities partition to the ferrite and inhibit the production of low-melting-point liquid films at the austenite grain boundaries. The amount of ferrite remaining at room temperature (usually 3-8%) indicates that solidification took place in the ferrite-austenite mode (FA mode), which is crack-resistant. If the ferrite concentration is less than 2-3%, solidification proceeds in the austenite mode (A mode). In this mode, austenite dendrites are formed straight from the liquid and impurities are concentrated in the last-to-solidify interdendritic regions forming liquid films which shatter under the stress of thermal contraction.

Quantified Relationship Between Solution Annealing Temperature, Ferrite and Weld Cracking

The following ferrite content and cracking incidence is revealed by controlled solution annealing of 316L tubing (0.02%C, 11.5% Ni, 2.2% Mo, balance typical) at many temperatures followed by autogenous GTAW (no filler, heat input 0.5 kJ/mm) and transverse bend testing.

Solution Annealing Temperature (°C) Base Metal Ferrite Volume Fraction (%) (ferritescope) Weld Metal Ferrite Volume Fraction (%) (autogenous) Solidification Mode Hot Cracking Incidence (% visual at 10x) Welded End Cap Service 1,400 <0.1 <0.1 A (austenitic) >80% No 1,150 0-1 0-1 A (austenitic) 50-80% No 1,120 1-2 0.5-1.5 A (austenitic) 30-60% No 1,100 2-3 1-2 AF (austenite-ferrite) 15-30% Marginal 1,080 3-5 2-4 FA (ferrite-austenite) 5-15% Acceptable 1,060 4-6 3-5 FA 2-8% Yes 1,040 5-8 4-6 FA 1-5% Yes (best) 1,020 6-10 5-8 FA <2%Yes (but sigma risk) 980 8-12 7-10 FA <2% Yes (but carbide risk)
The Role of Niobium and other Austenite Stabilisers

The inclusion of austenite-stabilizing elements (Ni, N, C, Cu) displaces the ferrite content downward at a given annealing temperature.

Alloy Ni (wt%) N (wt%) Ferrite @ 1,040 °C (%) Ferrite @ 1,100 °C (%)Recommended Max Annealing Temp for >3% Weld Ferrite 316L (low Ni) 10.0-10.5 0.04-0.06 6-10 3-5 1,100°C 316L (standard) 10.5-11.0 0.06-0.08 5-8 2-4 1,080°C 316L (high Ni) 11.0-11.5 0.08-0.10 4-6 1-3 1,060°C 316L (very high Ni) 11.5-12.0 0.10-0.12 3-5 0-2 1,040°C 347 (Nb-stabilized) 9.0-10.0 N/A 5-8 2-5 1,080°C Practical Recommendations for Welded Heater Manufacturing

316L heater sheaths needing end cap welding shall comply with the following solution annealing and ferrite requirements.

Desired Weld Integrity Level Minimum Weld Metal Ferrite for Crack-Free Welding (%) Solution Annealing Temperature Range (°C) Base Metal Ferrite Target (%) Verification Procedure
Low (dry service, not critical)2 1,060-1,080 3-5 Ferritescope <5 %
Standard (water immersion) 3 1,040-1,060 4-6 Ferritescope 4-8%
High (pressurised, corrosive) 4 1,020-1,040 5-8 Ferritescope >5% + WPS qualification
Critical (nuclear, high pressure) 5 1,000-1,020 6-10 Full weld procedure qualification
Ferrite and Weldability Verification

Buyers that need weldable 316L sheath tubing can verify this in two ways. The first is ferritescope measurement on the base metal in the as-received condition. GTAW standard: >4% ferrite in base metal (weld metal ferrite >3%). The second is a weldability test: produce an autogenous GTAW weld on a sample tube, cross-section, polish, etch and analyse for micro-cracks. Acceptance requirement No fractures at 200× magnification

Conclusion: Specification of Solution Annealing Temperature for Weldable 316L

For 316L stainless steel heater sheaths that require crack-free end cap welds, the solution annealing temperature should be managed to maintain a base metal ferrite volume fraction of 4-8%, producing weld metal ferrite of 3-6% and ensuring FA (ferrite-austenite) solidification mode. This range of ferrite is produced by solution annealing at the usual 1,040-1,080°C. Annealing at 1,100°C and above results in <2-3% ferrite, resulting in full austenitic solidification. Hot cracking rates are 30-60%. The fabrication of the welded heater is carried out with 316L sheaths having annealed tubing and an allowable solution annealing temperature of 1080°C with a minimum base metal ferrite of 4% (by ferritescope). For alloys with higher nickel contents (Ni > 11.5%), a lower annealing temperature (≤1,040°C) may be necessary to retain sufficient ferrite. The framework given here allows the purchasers to set the tube annealing parameters to assure reliable end cap welding by correlating solution annealing temperature with ferrite volume fraction and weld hot cracking susceptibility .