Gas turbines represent a pinnacle of modern engineering, serving as critical high-technology power sources in sectors ranging from electricity generation to marine propulsion. The performance and reliability of these machines are intrinsically linked to national energy security and industrial capability. Amidst the global push for higher efficiency and the strategic imperative of “dual-carbon” goals, the operational demands placed on gas turbines have intensified significantly. This evolution necessitates components capable of withstanding increasingly severe thermomechanical loads. The combustion chamber, a core component within the gas turbine, is particularly subjected to these extreme conditions. Consequently, the casting parts used in its construction must exhibit exceptional high-temperature creep resistance, superior oxidation resistance, and microstructural stability to ensure long-term operational integrity under cyclic thermal and mechanical stresses.
This research focuses on a large, geometrically complex combustion chamber casting part fabricated from the high-alloy martensitic heat-resistant steel, GX12CrMoVNbN9-1. This grade is selected for its balanced composition designed to deliver the requisite high-temperature strength and environmental resistance. However, the very properties that make this material suitable also present significant manufacturing challenges. The intricate design, substantial mass, and stringent quality requirements of such a casting part are compounded by the residual stresses introduced during precision machining and the inherent volumetric changes associated with the martensitic phase. A critical, yet underexplored, aspect for such finished components is surface protection and long-term stability enhancement through controlled oxidation. While pre-oxidation is a known concept for improving oxidation resistance, its systematic application to finished, precision-machined high-alloy martensitic steel casting parts remains inadequately documented. This gap presents a tangible risk to component longevity. This study, therefore, is dedicated to developing and elucidating a robust, controlled pre-oxidation methodology tailored for such demanding casting parts. The aim is to confer a uniform, adherent, and protective oxide scale on the finished surface, thereby mitigating storage corrosion, enhancing in-service oxidation resistance, and relieving detrimental machining stresses without compromising the casting part’s dimensional accuracy or underlying mechanical properties.
Material Characteristics and Technical Challenges
The foundation of this work lies in the specific material system and the associated challenges of the target component. The GX12CrMoVNbN9-1 steel is a meticulously engineered alloy where elements like Chromium (Cr), Molybdenum (Mo), Vanadium (V), Niobium (Nb), and Nitrogen (N) are balanced to achieve a stable martensitic microstructure with secondary precipitation strengthening. A typical specification for the chemical composition of this casting part is summarized in Table 1.
| Element | C | Cr | Mo | V | Nb | N | Si | Mn | Ni | Fe |
|---|---|---|---|---|---|---|---|---|---|---|
| Min | 0.10 | 8.50 | 0.80 | 0.18 | 0.05 | 0.03 | 0.10 | 0.30 | 0.30 | Bal. |
| Max | 0.15 | 9.50 | 1.20 | 0.25 | 0.10 | 0.07 | 0.50 | 0.80 | 0.80 | Bal. |
This chemistry yields the room-temperature mechanical properties essential for handling and assembly, as outlined in Table 2. However, the true test occurs in service under prolonged exposure to temperatures of 520–550°C.
| Property | Yield Strength (Rp0.2) | Tensile Strength (Rm) | Elongation (A) | Impact Energy (KV) |
|---|---|---|---|---|
| Requirement | > 620 MPa | 800 – 950 MPa | > 14 % | > 27 J (avg.) |
The combustion chamber casting part’s geometry is a primary source of complexity. It is typically manufactured as two symmetrical halves which are later assembled. This structure features significant variations in wall thickness, complex internal and external contours, and numerous precision-machined interfaces. The machining of such a massive, complex casting part inevitably induces non-uniform residual stresses. Furthermore, the martensitic transformation itself is accompanied by a volume expansion. The combined effect of machining stresses and the potential for transformation-induced stresses during thermal cycling in service can compromise dimensional stability and fatigue life. Post-machining, the clean steel surface of the casting part is highly susceptible to atmospheric corrosion (rusting) during storage and transport. Traditional protective methods like temporary coatings or oils are often undesirable as they can interfere with subsequent inspection processes (e.g., dye penetrant testing) or assembly operations, requiring thorough and often problematic removal.
Fundamentals of the Pre-Oxidation Methodology
The proposed pre-oxidation process is not merely a heat treatment; it is a holistic procedure integrating mechanical preparation, thermal management, and atmospheric control to achieve multiple synergistic benefits for the casting part. The core objectives are threefold: 1) To form a thin, dense, and adherent Cr-rich oxide layer that passivates the surface; 2) To act as a stress-relief anneal for machining-induced residual stresses; and 3) To accomplish these without inducing plastic deformation or degrading the base metal’s mechanical properties.
Minimizing Distortion: Mechanical Integrity Management
For a large, thin-walled casting part, distortion during any heat treatment is a paramount concern. To maximize structural rigidity during the thermal cycle, the two halves of the combustion chamber are bolted together (“married” or “dummy assembled”) prior to furnace loading. This assembly mimics the final installed condition, significantly increasing the moment of inertia and resistance to sagging or warping. The support system inside the furnace is equally critical. The casting part is placed on an array of uniformly high, rigid, and level support blocks (pads) distributed to bear weight at structurally robust locations, avoiding thin sections or unsupported spans. The final alignment is achieved using precision levels and adjustable jacks to ensure the assembly is perfectly horizontal before locking it in place. This meticulous setup minimizes thermal gradients and uneven stress development within the casting part during heating and cooling.
Oxide Layer Formation: Thermodynamic and Kinetic Principles
The formation of a protective oxide scale relies on the selective oxidation of alloying elements, primarily Chromium. The growth of a continuous Cr2O3 layer follows parabolic kinetics, where the scale thickness \(x\) is related to time \(t\) by:
$$x^2 = k_p \cdot t$$
where \(k_p\) is the parabolic rate constant, strongly dependent on temperature via an Arrhenius relationship:
$$k_p = A \cdot \exp\left(\frac{-Q}{RT}\right)$$
Here, \(A\) is a pre-exponential factor, \(Q\) is the activation energy for oxidation, \(R\) is the universal gas constant, and \(T\) is the absolute temperature. The selected pre-oxidation temperature of approximately 600°C is a strategic compromise. It is sufficiently high to ensure adequate diffusion rates of Cr cations to the surface to form a continuous layer, as the thermodynamics favor Cr2O3 formation at this temperature. Crucially, this temperature is kept below the original tempering temperature of the martensitic casting part (typically >650°C). This ensures that the carefully engineered microstructure and precipitation state responsible for the casting part’s strength are not altered (over-tempered), preserving its mechanical properties as summarized in Table 2.
Atmosphere control in the natural gas-fired furnace is aimed at maintaining a weakly oxidizing environment. The burner flames are adjusted to a slightly fuel-lean condition, producing an excess of oxygen. The flame length and direction are controlled to prevent direct impingement on the casting part, ensuring even, radiant/convective heating and uniform gas access to all surfaces. This promotes the formation of a uniform oxide scale.
Stress Relief Mechanism
The pre-oxidation cycle simultaneously serves as a sub-critical stress relief anneal. The residual stress \(\sigma\) induced by machining can be relaxed through thermally activated creep processes. The rate of stress relaxation can be conceptually related to temperature and time. Holding the massive casting part at 600°C allows for dislocation glide and climb, enabling the redistribution and relaxation of locked-in elastic strains without recrystallization. The extent of relaxation contributes to the long-term dimensional stability of the casting part under thermal cycling.
Detailed Process Flow and Operational Parameters
The successful implementation of the pre-oxidation method for the high-value casting part requires a strict, sequential protocol. The key steps and their scientific/engineering rationale are expanded below.
| Process Step | Key Actions & Objectives | Critical Parameters & Controls |
|---|---|---|
| 1. Dimensional Inspection | Establish pre-process baseline. Identify measurement datums on the casting part. | Use CMM or laser tracking. Document all critical interface and mating dimensions. |
| 2. Casting Part Preparation | Remove all contaminants to ensure uniform oxide nucleation and growth. | Alkaline or solvent cleaning → Thermal degreasing (>300°C) → Thorough drying. |
| 3. Assembly & Fixturing | Bolt two halves together to enhance rigidity. Prepare furnace support system. | Use torque-controlled tightening sequence. Pads must be level, rigid, and thermally stable. |
| 4. Furnace Loading & Leveling | Place casting part on supports without inducing local stress. Ensure perfect horizontality. | Use 4-point lifting. Employ precision levels and jacks for fine adjustment. Shim gaps. |
| 5. Thermocouple Attachment | Monitor actual casting part temperature, not just furnace air temperature. | Attach T/Cs to thick, thin, upper, and lower sections of the casting part. |
| 6. Pre-Oxidation Thermal Cycle | Execute controlled heating, hold, and cooling to form oxide and relieve stress. | See detailed parameters in Table 4. |
| 7. Unloading & Post-Process Inspection | Cool completely, handle carefully, and verify results. | Check for uniform dark gray oxide. Re-measure dimensions against baseline datums. |
The heart of the process is the thermal cycle, whose parameters are derived from the principles discussed earlier and are detailed in Table 4.
| Phase | Temperature Target (T) | Rate / Duration | Furnace Atmosphere & Key Objective |
|---|---|---|---|
| Loading | T_furnace ≤ 100°C | N/A | Prevent thermal shock to the casting part. |
| Heating | 100°C → ~600°C | Controlled Rate ≤ 80°C/h | Minimize thermal gradients in the massive casting part. Oxidizing (excess O2). |
| Hold (Soak) | 595°C – 605°C | 3 – 5 hours | Enable Cr diffusion and oxide growth (kinetics). Stress relaxation. |
| Cooling | ~600°C → ≤ 50°C | Controlled Rate ≤ 60°C/h (in furnace) | Prevent quench-like stresses in the casting part. Allow scale to set. |
| Unloading | T_casting part < 50°C | N/A | Final handling of the treated casting part. |
The controlled heating and cooling rates are vital for the integrity of the casting part. A simplified view of the heat transfer consideration involves preventing excessive temperature differences (\(\Delta T\)) within the thick and thin sections of the casting part, which could cause yield and distortion. The rate is often governed by the limiting thermal diffusivity \(\alpha\) of the steel:
$$\alpha = \frac{k}{\rho c_p}$$
where \(k\) is thermal conductivity, \(\rho\) is density, and \(c_p\) is specific heat capacity. Slower rates allow heat to equilibrate, reducing \(\Delta T\) and associated thermal stresses.
Results, Analysis, and Performance Validation
The efficacy of the developed methodology is evaluated based on qualitative and quantitative outcomes for the treated casting part.
Visual and Metrological Inspection: A successfully processed casting part exhibits a uniform, matte dark gray surface appearance. The oxide layer is thin (typically 1-3 microns) and adherent, passing simple tape adhesion tests. Crucially, post-treatment dimensional metrology using the same datums as the pre-treatment inspection shows minimal deviation. The total indicated distortion, measured as movement at key flange faces or interface points, is consistently maintained within tight engineering tolerances (e.g., < 0.5 mm over several meters), confirming the effectiveness of the rigidity and leveling strategy.
Oxide Layer Characterization: While advanced surface analysis (like SEM/EDS or GDOES) provides definitive proof, the process success is inferred from the uniformity of color and the performance outcome. The formed scale is predominantly a chromium-rich oxide (Cr2O3), possibly with minor contributions from iron and other alloying elements. The pre-oxidation layer acts as a diffusion barrier, significantly slowing further oxidation during subsequent storage or the initial service hours of the casting part.
Stress Relief Confirmation: Although direct residual stress measurement (e.g., by X-ray diffraction) on such a large casting part is complex, indirect evidence is strong. The absence of distortion and the maintenance of dimensional stability during subsequent handling and simulated thermal cycles indicate a significant reduction in the unstable, locked-in machining stresses. This enhances the geometric stability of the casting part under operational thermal transients.
| Aspect of Casting Part | Challenge Before Treatment | Benefit After Treatment | Mechanism |
|---|---|---|---|
| Surface State | Active, prone to rust during storage. | Passivated with protective, adherent Cr2O3 scale. | Selective high-temperature oxidation. |
| Dimensional Stability | High machining residual stresses. | Stresses reduced; distortion minimized. | Sub-critical thermal exposure enabling stress relaxation. |
| In-Service Oxidation | Initial oxidation in service would be less protective. | Head-start with a dense, continuous oxide layer. | Pre-formed diffusion barrier. |
| Mechanical Properties | Risk of over-tempering if temperature too high. | Base material strength and toughness preserved. | Hold temperature below original tempering point. |
Conclusion and Broader Implications
This study has comprehensively detailed a controlled pre-oxidation methodology specifically designed for large, precision-machined high-alloy martensitic heat-resistant steel casting parts, as exemplified by the GX12CrMoVNbN9-1 combustion chamber component. The process is a synthesis of meticulous mechanical preparation—including assembly and precise leveling—and a carefully designed thermal cycle in an oxidizing atmosphere. The selection of a ~600°C hold temperature is pivotal, enabling the growth of a chromia-rich protective scale and providing stress relief without altering the casting part’s core microstructure. The results demonstrate that this integrated approach successfully transforms the vulnerable, freshly machined surface of a complex casting part into a passivated, stable state. It effectively solves the dual problems of storage corrosion and internal stress instability, thereby enhancing the component’s readiness for service and contributing to its long-term performance and reliability in demanding high-temperature applications.
The significance of this work extends beyond the specific casting part studied. It provides a validated technical framework and a set of engineering principles that can be adapted and applied to other large, complex casting parts made from similar high-performance alloys used in the energy, aerospace, and heavy industrial sectors. By ensuring the surface integrity and dimensional stability of such critical components, this pre-oxidation method contributes directly to the advancement of manufacturing quality and the extension of service life, supporting the development of more efficient and reliable high-temperature machinery.