Failure Modes and Strengthening Methods of Heat-Resistant Steel

I. Main Failure Modes of Heat-Resistant Steel

1. Oxidation and High-Temperature Corrosion Failure

• Mechanism:
  At high temperatures, metal atoms react with active elements such as oxygen to form oxide films (e.g., FeO, Fe₂O₃). If the oxide film is loose and easy to peel off, it cannot prevent the continuous invasion of the medium, leading to continuous consumption of the material and eventual failure due to wall thickness reduction or strength decrease.
  If there are elements like S and Cl in the environment (such as sulfur-containing flue gas and chloride media), high-temperature corrosion (such as sulfidation corrosion and chlorination corrosion) will occur. The generated sulfides (FeS) or chlorides (FeCl₃) have low melting points and are volatile, accelerating the corrosion process.
• Typical Case: When boiler superheater tubes serve in sulfur-containing flue gas, a loose mixed layer of FeS and FeO forms on the surface. After peeling, the tube wall thins rapidly and eventually bursts.

2. Creep Failure

• Mechanism:
  At high temperatures, the diffusion ability of atoms enhances, and dislocations inside the material move slowly, grain boundaries slide, or cavities grow, resulting in macroscopic deformation (such as elongation and bulging). When the deformation exceeds the critical value (usually 1%-5%), cracks will be initiated and propagate to fracture.
  The characteristic of creep failure is that the fracture surface shows intergranular fracture (grain boundaries are weak links, prone to cavity formation), and the deformation is irreversible.
• Typical Case: Steam turbine bolts serve under high temperature and pressure for a long time, leading to excessive elongation due to creep, which fails to ensure sealing and even breaks.

3. Thermal Fatigue Failure

• Mechanism:
  During temperature changes, the mismatch of thermal expansion coefficients between the internal material or adjacent components (such as shells and pipes) leads to periodic alternating thermal stress. When the stress exceeds the fatigue limit of the material, microcracks will be generated on the surface or at defects, and gradually expand to penetrating cracks.
  Thermal fatigue cracks are mostly reticular or radial and expand along or through grains (depending on material toughness).
• Typical Case: The exhaust manifold of an internal combustion engine generates a large number of thermal fatigue cracks on the surface due to repeated start-stop and severe temperature fluctuations, and eventually breaks.

4. Microstructural Degradation Failure

• Typical Cases:
  • Pearlitic heat-resistant steel (e.g., 12Cr1MoV): The cementite (Fe₃C) in pearlite spheroidizes, aggregates, and even transforms into graphite under long-term high temperatures, leading to a significant decrease in strength.
  • Austenitic heat-resistant steel: The strengthening phase (such as γ' phase Ni₃Al) coarsens or dissolves under long-term high temperatures, losing the strengthening effect and reducing creep resistance.

II. Strengthening Methods of Heat-Resistant Steel

1. Alloying Strengthening (Core Method)

• Improving Oxidation Resistance:
  Adding elements such as Cr (12%-30%), Al (2%-5%), and Si (1%-3%) to form a dense oxide film (such as Cr₂O₃, Al₂O₃, SiO₂) on the surface to block the invasion of media. For example, when the Cr content is ≥ 12%, a continuous Cr₂O₃ film can be formed on the steel surface, significantly improving oxidation resistance.
• Improving High-Temperature Strength (Creep Resistance):
  • Solid Solution Strengthening: Adding W and Mo (with large atomic radii, forming strong bonding with Fe) to improve the bonding force between matrix atoms and inhibit dislocation movement (for example, Mo can increase the creep activation energy of the iron matrix by more than 30%).
  • Precipitation Strengthening: Adding V, Nb, Ti, Ta, etc., to form high-temperature stable carbides/nitrides (such as VC, NbC) with C/N, pinning dislocations and grain boundaries, and hindering creep deformation (for example, V in 12Cr1MoV forms VC, significantly improving creep strength).
  • Phase Stabilization: Adding Ni (8%-20%) to form an austenitic matrix (more stable than ferrite with low diffusion coefficient), such as 310S austenitic steel (25%Cr-20%Ni) can serve for a long time above 1000℃.
• Improving Thermal Fatigue Performance:
  Adding Mn and Ni to reduce the thermal expansion coefficient (for example, Incoloy 800H has a 20% lower thermal expansion coefficient than ferritic steel due to containing 30% Ni), or adding Cu and Nb to improve material toughness (inhibiting crack propagation).

2. Heat Treatment Strengthening

• Solution Treatment + Aging Treatment:
  Suitable for austenitic heat-resistant steel (e.g., GH4169): Solution treatment (1000-1100℃) makes alloying elements (Nb, Ti) dissolve uniformly, and aging treatment (700-800℃) precipitates γ'' phase (Ni₃Nb) and γ' phase (Ni₃Al), significantly improving creep resistance through precipitation strengthening.
• Normalizing + Tempering:
  Suitable for pearlitic heat-resistant steel (e.g., 12Cr1MoV): Normalizing (950-1050℃) obtains fine pearlite structure, and tempering (750-800℃) eliminates stress and stabilizes carbides, preventing pearlite spheroidization under long-term high temperatures (spheroidization will lead to a strength decrease of more than 50%).
• Annealing Treatment:
  It is used to eliminate processing stress (such as the heat-affected zone after welding), refine grains, and avoid the initiation of thermal fatigue cracks.

3. Surface Strengthening

• Surface Alloying:
  Aluminizing and chromizing (such as aluminizing treatment): Forming an Al₂O₃ or Cr₂O₃ enriched layer (50-200μm thick) on the steel surface, which increases the oxidation resistance temperature by 200-300℃ compared with the matrix (for example, aluminized 20G steel can serve above 800℃).
• Coating Technology:
  Using high-temperature ceramic coatings (such as Al₂O₃, ZrO₂) or intermetallic compound coatings (such as NiAl), prepared by physical vapor deposition (PVD) or plasma spraying, which are both heat-insulating and corrosion-resistant (the thermal conductivity of the coating is only 1/10-1/20 of that of steel).
• Laser Surface Melting:
  Rapidly heating and cooling the surface with a laser to form a fine-grained or amorphous layer, improving surface hardness and wear resistance, and reducing oxide scale peeling.

4. Microstructure Control

• Grain Size Optimization:
  Coarse grains (with small grain boundary area) can reduce grain boundary sliding and improve creep strength (for example, turbine blades using coarse-grained austenitic steel have a 30% increase in creep life); fine grains can improve toughness and thermal fatigue performance (grain boundaries hinder crack propagation, for example, heat exchanger tubes using fine-grained ferritic steel have a 50% extension in thermal fatigue life).
• Duplex Structure Design:
  Such as ferrite-austenite duplex steel (e.g., 2205), ferrite provides good oxidation resistance, austenite provides high strength, and it has both corrosion resistance and creep resistance (the applicable temperature range is 100-150℃ wider than that of single-phase steel).

Summary