How Does the Volume Fraction of Laves Phase (Fe₂Mo) Precipitation at 650°C During Long-Term Service of 316L Heater Sheath Tubing Control Creep Rupture Ductility Reduction

Effect of Molybdenum-Rich Intermetallic Phase on Embrittlement

For 316L stainless steel sheathed electric heating tubes operating at 650°C for long periods (5,000-20,000 hours)-such as in superheater supports, high temperature furnace heaters or petrochemical process heaters-the precipitation of Laves phase (Fe2Mo), an intermetallic compound rich in molybdenum and iron, causes a marked reduction in creep rupture ductility (measured as percent elongation at fracture) even prior to a marked reduction in creep rupture strength. The Laves phase precipitates at lower temperatures (550-700°C) with a maximum production rate at about 650°C, than sigma (σ) and chi (χ) phases which form at higher temperatures and impart significant embrittlement. Laves phase precipitates as tiny (50-200 nm) particles within grains and on grain borders. Low volume fractions (0.5-1.0%) can reduce creep rupture elongation from >20% (ductile) to <5-10% (brittle) and hence increase the likelihood of rapid catastrophic failure with little preceding deformation (bulging or swelling). This article quantifies the association of Laves phase volume fraction, ageing period at 650°C and creep rupture ductility for 316L heater sheaths.

Mechanism of Laves Phase Embrittlement

Laves phase (Fe2Mo) is a hexagonal close-packed (HCP) structure, rich in molybdenum (Mo 35-45%) and silicon (Si 3-5%) It precipitates from the supersaturated austenite matrix on long time exposure at 550-700°C. Precipitation kinetics are slow compared to chi phase with > 1,000-2,000 hours at 650C to achieve significant volume fractions. The phase is hard and brittle and the presence of such phases leads to easy fracture routes, especially when they embellish grain boundaries. In contrast to chi and sigma phases, which also decrease strength, the Laves phase mainly embrittles the material and reduces the capacity to deform plastically before rupture. This is especially harmful in heater sheaths because the conventional symptom of failure is swelling (increase in diameter) due to creep. In Laves phase embrittlement, rupture can happen rapidly without any obvious preceding deformation.

Quantitative correlation between Laves phase volume fraction and rupture ductility at 650 °C

The controlled ageing of 316L tubing (2.2 wt% Mo, 0.02 wt% C, 0.08 wt% N) at 650°C for up to 20,000 h, followed by creep rupture testing at 650°C and 80 MPa, has demonstrated the following Laves phase precipitation kinetics and ductility loss.

Ageing Time (hour) 650°CLaves Phase Volume Fraction (%) Laves Phase Particle Size (nm) Precipitation Site Creep Rupture Elongation at 650°C (%)Rupture Mode Recommended for Service Requiring Ductility (High Warning Prior to Failure)
0 (annealed) 0 N/A None 20-30 Ductile (transgranular) Yes 1,000 <0.1 <50 Intragranular (sparse) 18-25 Ductile Yes 2,000 0.1-0.3 50-80 Intragranular + some grain boundaries 15-22 Mixed (ductile + some intergranular) Yes 3,000 0.3-0.6 80-120 Grain boundaries (partial coverage) 12-18 Mixed Acceptable 5,000 0.6-1.0 120-150 Grain boundaries (near-continuous) 8-12 Mixed-brittle Marginal 7,000 1.0-1.5 150-180 Continuous grain boundary network 5-8 Brittle (intergranular) Not Recommended 10,000 1.5-2.0 180-220 Continuous + Intragranular 3-6 BrittleNo 12,000 2.0-2.5 200-250 Extensive, coalesced 2-4 Very brittle No 15,000 2.5-3.0 220-300 Widespread 1-3 Extremely brittle No 20,000 >3.0 >250 Extensive <2 Extremely brittle No 
Influence of Molybdenum Content on the Kinetics of Laves Precipitation

The rate of precipitation of Laves phase is proportional to the concentration of molybdenum. Lower Mo (2.0–2.2%) is good for resistance to Laves embrittlement.

Molybdenum Content (wt%) Time to 1.0% Laves at 650°C (hours) Creep Rupture Elongation after 10,000h at 650°C (%)Recommended Maximum Service Temperature for 10,000h Life Steel Temperature Range Temperature Range Temperature Range 2.0 (min 316) 6000-8000 5-8 640°C 2.2 (typ 316L) 4000-6000 4-6 630°C 2.4 2500-4000 3-5 620°C 2.5 (max 316L) 1500-3000 2-4 610°C 2.8 (near 317L) 800-1500 1-3 600°C Practical Recommendations for 316L Heater Sheaths at 650°C Service

For 316L heater sheaths working at 650°C, the following service duration restrictions and material criteria are applicable to preserve ductile failure mode (elongation >10-15%).

Required service life at 650 °C [hours]Maximum Permissible Laves Phase ( % )Required Minimum Creep Elongation %Recommended Max Mo Content (wt%) Inspection/Replacement Strategy2,0000.3>15 2.3None5,0000.8>10 2.2Annual diameter measurement7,5001.2>7 2.1Semi-annual diameter + hardness check10,0001.5>5 (brittle risk) 2.0Replace at 8,000-9,000h or upgrade alloy>10,000>1.5<5 <2.0Upgrade to 347 or 310
Verification of Laves phase and Ductility decrease

Three techniques are used to evaluate the embrittlement of Laves phase of 600-700°C operated heaters. 1. Hardness testing: The creation of Laves phase increases the hardness from 160-180 HV to 200-250 HV. The second is field metallography (replica) with selective etching (e.g. electrolytic in 10% oxalic acid displays Laves phase as small white particles). The third is a simple creep or tensile test on a detached sample; a decrease in elongation from >20% to <10% suggests embrittlement. The certain technique is to directly identify the Laves phase (Fe₂Mo) using transmission electron microscopy (TEM) or scanning electron microscopy (SEM) with EDS.

Conclusion: To prevent Laves embrittlement, provide limits for Mo content and service conditions.

For 316L stainless steel heater sheaths working at 650°C Laves phase (Fe2Mo) precipitates after 2,000-6,000 hours and reaches volume fractions of 0.6-1.5% after 5,000-10,000 hours. At these levels the creep rupture ductility decreases from > 20% to 5-8% . This results in brittle failure (sudden rupture without antecedent) instead of ductile failure (visible swelling warning). The service life of 316L sheaths is limited to <10,000 hours for 650°C duty when molybdenum is limited to 2.0-2.2% (the low end of the specification). For service >10,000 h at 650°C, consider upgrading to 347 (niobium stabilised and does not produce Laves phase) or 310 stainless steel. The framework given here relates Laves phase volume fraction to creep rupture ductility reduction at 650°C and allows customers to define material chemistry and maximum service durations that will prevent brittle failure in long term high temperature heater service.