The reliable operation of coke dry quenching (CDQ) systems is paramount for modern, environmentally conscious steel production. Central to this process is the coke quenching car, a mobile vessel responsible for transporting red-hot coke from the oven to the quenching chamber. Within this car, the integrity of the internal lining is the single most critical factor determining equipment longevity and operational safety. These liner plates operate in an exceptionally severe thermal regime, enduring direct exposure to coke at temperatures ranging from 900°C to 1100°C, followed by rapid cooling during the unloading cycle. This continuous thermal shock, or thermal fatigue, presents a formidable challenge to material selection and manufacturing quality. My extensive experience in foundry engineering has led to a focused investigation and subsequent optimization of both the metallurgy and the lost foam casting process for producing these essential components, resulting in a dramatic extension of service life.
The primary function of the liner plate is to shield the underlying carbon steel structure of the coke car from direct thermal assault. Failure is not merely a matter of wear; it is a critical safety and operational hazard. A plate that cracks or spalls can expose the structural frame to extreme heat, causing warping and failure. Worse, fragments of broken liner can fall onto conveyor belts below, causing catastrophic damage and protracted production stoppages. Therefore, the ideal material must possess a synergistic combination of properties: excellent high-temperature strength (creep resistance), outstanding resistance to thermal shock, good oxidation resistance at service temperatures, and sufficient toughness to resist crack initiation under mechanical and thermal stress.
Initial trials utilized high-chromium cast iron, specifically with a composition of 28-32% Cr and 2.0-2.2% C. While this material exhibited commendable high-temperature hardness and oxidation resistance, its performance was ultimately unsatisfactory. The high carbon content led to a continuous network of brittle carbides, severely compromising toughness. The fundamental issue was its poor thermal shock resistance, quantified by a low thermal shock parameter. Thermal stress ($\sigma_{th}$) induced during quenching can be approximated by:
$$
\sigma_{th} = \frac{E \alpha \Delta T}{(1 – \nu)}
$$
where $E$ is Young’s modulus, $\alpha$ is the coefficient of thermal expansion, $\Delta T$ is the temperature difference, and $\nu$ is Poisson’s ratio. High-chromium irons typically have a high modulus and a moderate CTE, but their low fracture toughness ($K_{IC}$) means the induced stress easily exceeds the material’s capacity, leading to crack propagation. Field results confirmed this, with plates failing catasthetically after only 2-3 months of service.
A shift was made to a heat-resistant steel with a preliminary composition focused on higher chromium and nickel. While an improvement, life remained limited to 4-5 months. Microstructural analysis revealed that the carbon content, though lower than in cast iron, was still too high (0.6-0.8%). This promoted the formation of excessive secondary carbides at grain boundaries during service, embrittling the structure. Furthermore, the levels of silicon and manganese were suboptimal for promoting the desired austenitic structure and toughness. This iterative learning process underscored that precise compositional control, not just broad categorization, was key.
The breakthrough came with the specification and meticulous control of the austenitic heat-resistant steel grade ZG30Cr24Ni7SiN. The target composition range was refined based on the thermodynamic and kinetic roles of each element, as summarized in the table below:
| Element | Target Range (wt.%) | Primary Metallurgical Function | Rationale for Range |
|---|---|---|---|
| C | 0.30 – 0.40 | Solid solution strengthener; carbide former. | Balances high-temperature strength with toughness. Lower carbon minimizes brittle Cr-rich carbide networks and improves thermal shock resistance by increasing thermal conductivity. |
| Si | 1.80 – 2.00 | Austenite stabilizer; improves oxidation resistance. | Promotes the formation of a stable, protective SiO2 layer beneath the primary Cr2O3 scale. Enhances fluidity in the lost foam casting process. Excessive Si (>2.2%) can promote embrittlement. |
| Mn | 1.30 – 1.50 | Austenite stabilizer; sulfide morphology control. | Partially substitutes for nickel in stabilizing austenite, reducing cost. Combines with sulfur to form globular MnS inclusions, preventing hot tearing along grain boundaries. |
| Cr | 23.0 – 26.0 | Oxidation and corrosion resistance; solid solution strengthener. | Critical for forming a dense, adherent Cr2O3 (chromia) scale at 1100°C. Levels >23% ensure excellent oxidation resistance. Also provides solid solution strengthening to the austenitic matrix. |
| Ni | 7.0 – 9.0 | Primary austenite stabilizer. | Guarantees a fully austenitic microstructure from room temperature to service temperature, eliminating detrimental phase transformations that generate stress during thermal cycling. |
| N | 0.15 – 0.25 (Typical) | Powerful austenite stabilizer and solid solution strengthener. | Dramatically increases yield strength at high temperatures without significantly impacting ductility. Acts synergistically with Cr and Mn. |
| P | ≤ 0.035 | Impurity. | Minimized to prevent severe segregation and grain boundary embrittlement. |
| S | ≤ 0.025 | Impurity. | Minimized to prevent hot shortness and reduce sulfide inclusions. |
The superiority of this composition is rooted in its stable, single-phase austenitic matrix. The absence of ferrite-to-austenite or martensitic transformations during thermal cycling is crucial. The thermal stress equation shows that stress is proportional to the modulus $E$ and the CTE $\alpha$. Austenitic steels, while having a higher CTE than ferritic steels, generally have a lower modulus at high temperatures, and more importantly, they avoid the volumetric strains associated with phase changes, which are a primary driver of thermal fatigue cracking.
However, achieving this optimal microstructure is only half the battle. The manufacturing process must not introduce defects that act as stress concentrators and crack initiation sites. This is where the lost foam casting process presents both unique advantages and specific challenges. The ability to create complex geometries with internal re-entrant angles (like the backing ribs on a liner plate) without cores is a major benefit. Yet, the process demands extreme attention to detail to prevent distortion, burn-on, and porosity.
The following procedural optimizations within the lost foam casting process were implemented and are critical to success:
1. Pattern Design and Assembly: The EPS pattern is machined with strict adherence to fillet radii. All intersections between ribs and the plate body, as well as corners within internal cavities, are designed with a minimum radius of R=10 mm. This is not merely a suggestion; it is a requirement to prevent sharp thermal concentrations during solidification and service. The stress concentration factor ($K_t$) for a sharp corner is theoretically infinite, while a fillet reduces it significantly:
$$
K_t \approx 1 + 2\sqrt{\frac{t}{r}}
$$
where $t$ is a characteristic length (like plate thickness) and $r$ is the fillet radius. A larger $r$ directly reduces $K_t$, making the part more resistant to crack initiation from casting micro-shrinkage or inclusions located in these high-stress zones.
Patterns are assembled in vertical pairs with interconnecting gates and runners. Crucially, distortion control braces (or “pull bars”) are integrated between the patterns. These are not simple wax sticks but are designed as part of the gating system with a specific cross-section (e.g., 50 mm x 10 mm). If too thick, they create a hot spot and shrinkage porosity; if too thin, they freeze prematurely and fail their purpose. Their dimensions are calculated to ensure they solidify after the main casting sections, allowing them to restrain distortion during the critical solidification contraction period. The entire cluster is then externally reinforced with bamboo or wooden stakes to maintain dimensional stability during the demanding coating, drying, and sand-filling stages.
2. Coating and Dewaxing Dynamics: The refractory coating formulation is tailored for high-temperature steel. Its permeability is critical. During the pour, the polystyrene decomposes into gaseous products that must escape through the coating. The pressure in the mold ($P_{mold}$) must be managed:
$$
P_{mold} = P_{atm} + \rho g h – \Delta P_{coating} – \Delta P_{sand}
$$
where $\Delta P_{coating}$ is the pressure drop across the coating. An impermeable coating leads to high back-pressure, causing poor filling or “blows.” An overly permeable coating can allow sand penetration. We optimized for a coating that provides a strong, gas-permeable barrier, applied to a consistent thickness of 0.8-1.2 mm.
3. Process Parameter Optimization: Rigorous control of three key parameters is non-negotiable:
- Pouring Temperature ($T_{pour}$): Maintained at 1580°C – 1600°C. Lower temperatures risk mistruns and cold shuts, especially in the thin ribs. Higher temperatures increase the gas volume from foam degradation, escalate mold erosion, and promote coarse grain structure, reducing toughness.
- Vacuum Pressure ($P_{vac}$): Controlled at 0.03 MPa – 0.04 MPa (0.3 – 0.4 bar) during pour and for a sustained period after. This negative pressure enhances metal velocity, ensures precise replication of the pattern geometry, and, most importantly, swiftly removes pyrolysis gases from the mold cavity, minimizing carbon pickup and porosity in the casting.
- Solidification and Cooling Time: The casting cluster is held under vacuum for a minimum of 6 minutes post-pour to ensure complete filling and initial solidification under controlled conditions. The total time in the sand mold before shakeout is mandated to be no less than 24 hours. Premature extraction subjects the casting to rapid, non-uniform cooling, inducing residual stresses that can cause immediate distortion or provide a driving force for later failure in service.
4. Geometrical Tolerances for Service: Design for manufacture extends to design for assembly and thermal expansion. Bolt holes are positioned no closer than 50 mm from any plate edge to prevent breakout cracks from stress concentration. Furthermore, the as-cast dimensions of the plate’s length and width are intentionally held at the lower limit of the drawing tolerance. This provides a deliberate assembly gap between adjacent plates when installed on the coke car frame. This gap is essential to accommodate the thermal expansion of the plates during heating, preventing them from buckling or imposing excessive compressive stresses on each other and on the mounting structure.
The culmination of these metallurgical and process engineering efforts is a quantifiable performance leap. Liner plates produced under the old schemes failed in 2-5 months. Plates produced via the optimized lost foam casting process with the tightly controlled ZG30Cr24Ni7SiN composition consistently achieve a service life of 7-8 months in the same demanding application. This represents a 60-100% improvement. The economic impact is substantial, significantly reducing spare part consumption, maintenance downtime, and risk of secondary equipment damage.
In conclusion, the successful production of coke quenching car liner plates is a paradigm of integrated materials and process engineering. It demonstrates that the lost foam casting process is not merely an alternative to conventional molding but a superior route for complex, high-integrity heat-resistant components, provided it is mastered with scientific rigor. The synergy between a rationally designed austenitic steel chemistry—which prioritizes thermal fatigue resistance through phase stability and controlled strengthening—and a meticulously controlled casting process—which eliminates stress raisers and ensures sound, dimensionally stable castings—is the foundation for this success. This holistic approach, focusing on the interplay of composition, solidification mechanics, and in-service thermo-mechanical demands, provides a replicable framework for advancing the reliability of critical components in extreme industrial environments.