What is the quantitative impact of the heat treatment process deviation of UNS N06625 control pipeline?

1. Insufficient annealing temperature (<1050℃): niobium solid solution defects and intergranular corrosion sensitivity
Annealing temperature is the core parameter of N06625 solid solution treatment, which directly determines the degree of solid solution of niobium (Nb) element and the uniformity of distribution of carbides (NbC). When the annealing temperature is lower than 1050℃, the diffusion kinetic energy of niobium atoms is insufficient, resulting in the aggregation of undissolved NbC particles at the grain boundaries (Figure 1a). This non-uniform distribution forms a local micro-galvanic effect, which induces preferential destruction of the passivation film in a Cl⁻-containing medium.

Quantitative impact analysis:
Intergranular corrosion rate: Electrochemical potentiodynamic polarization test shows that the intergranular corrosion sensitivity index of the alloy annealed at 1050℃ in 3.5% NaCl solution is 0.82, while that of the alloy annealed at 1020℃ rises to 1.21 (sensitivity threshold 1.0), and the corrosion rate increases by 35%.
Niobium element distribution: Atomic probe tomography (APT) shows that the concentration of niobium at the grain boundary stabilizes at 3.8±0.2 wt% after annealing at 1050℃, while the fluctuation range of the annealing state at 1020℃ is 2.1-4.9 wt%, and the local niobium-poor area becomes a corrosion breakthrough.
Engineering verification: Due to the low annealing temperature (1030℃) of a condenser pipeline on an offshore platform, the intergranular corrosion depth reached 0.32mm after 18 months of operation, far exceeding the designed corrosion margin (0.15mm).
Solution:
Medium-frequency induction heating combined with infrared temperature measurement system is used to ensure that the core temperature of the tube reaches 1080-1120℃, and the insulation time is calculated as 1.5 minutes per millimeter of wall thickness to achieve full solid solution of niobium elements.

2. Too slow cooling rate (air cooling): δ phase precipitation and mechanical property degradation
Cooling rate control is a key follow-up link in solid solution treatment. When slow cooling methods such as air cooling are used, the alloy stays in the 700-900℃ range for a longer time, triggering the precipitation of Ni₃Nb (δ phase) (Figure 1b). The coherence relationship between the orthorhombic structural phase and the matrix is ​​destroyed, resulting in a decrease in the resistance to dislocation movement.

Quantitative impact analysis:
Hardness and toughness: The hardness of the air-cooled alloy decreases by 18HB (320HV→302HV) compared with the water-quenched state, and the Charpy impact energy decreases by 37% (145J→91J), and the corresponding fracture mode changes from ductile fracture to quasi-cleavage fracture.
Stress corrosion cracking (SCC) risk: The critical stress intensity factor (K_ISCC) of the slow-cooled sample in the boiling MgCl₂ solution is 28.3MPa√m, which is 31% lower than that of the water-quenched state (41.2MPa√m).
Engineering case: Due to the air cooling process, the heat transfer tube of a nuclear power steam generator was found to have intergranular SCC cracks after 3 years of operation, with a depth of 1/3 of the wall thickness.
Solution:
Implement graded water quenching process: After the tube billet is taken out of the furnace at 1080℃, it is immediately immersed in 25℃ circulating water to ensure that the cooling rate is ≥120℃/s, while avoiding quenching cracks.

3. Overheating treatment (＞1150℃): grain coarsening and creep strength attenuation
When the annealing temperature exceeds 1150℃, the grain boundary migration rate is significantly enhanced, resulting in the abnormal growth of the original fine grains (ASTM 8-9 grade) to ASTM 6-7 grade (Figure 1c). This kind of microstructure coarsening reduces the grain boundary strengthening effect and accelerates creep damage under high temperature and long-term load.

Quantitative impact analysis:
Creep performance: The steady-state creep rate of the 1150℃ annealed alloy under 650℃/100MPa conditions is 3.2×10⁻⁸ s⁻¹, which is 2 times higher than that of the 1120℃ annealed state (1.1×10⁻⁸ s⁻¹).
Grain boundary strengthening effect: Electron backscatter diffraction (EBSD) analysis shows that the proportion of high-angle grain boundaries after overheating treatment drops from 68% to 52%, and the contribution of grain boundary strengthening is reduced by about 40MPa.
Engineering Lessons: Due to overheating (1180℃), the maximum creep deformation of a high-temperature reactor coil after 5 years of operation reached 1.8%, far exceeding the design limit (0.5%).
Solution:
A vacuum heat treatment furnace combined with temperature field simulation is used to ensure that the axial temperature difference of the tube billet is less than ±15℃, and the traditional long-term low-temperature process is replaced by a short-term high temperature (1120℃/15min) during the insulation stage.

4. Systematic solution for precise process control
In order to eliminate the impact of process deviation on the performance of N06625 control pipeline, a closed-loop system of "process design-process monitoring-organization verification" needs to be constructed:
Process window optimization: The solid solution temperature-time parameter envelope (Figure 2) is determined by thermodynamic calculation (Thermo-Calc) to ensure that the solid solubility of niobium element is greater than 98%.
Online monitoring technology: Infrared thermal imager is used to monitor the surface temperature field of tube billet in real time, and the core temperature gradient is predicted by combining finite element model.
Quantitative evaluation of organization: Image analysis software is used to count grain size, carbide size and distribution, and establish a database of correlation between microstructure and corrosion rate.