How to select the right grade of heat resistant steel castings for industrial furnaces?

Temperature Gradient: The Primary Selection Criterion

The actual temperature of components inside industrial furnaces is typically 50°C to 150°C higher than the workpiece temperature, and the heat source type (heavy oil, gas, or electric) directly affects temperature distribution uniformity. The performance degradation of heat-resistant steels is not linear but exhibits critical threshold points:

• 650°C to 900°C Danger Zone: This range is the sensitive temperature band for σ-phase (FeCr intermetallic compound) precipitation. For Fe-Cr-Ni series alloys (such as HH, HK), if compositional balance is improper, impact energy may decrease by more than 30% after long-term service at 750°C. Therefore, for components operating in this temperature range under cyclic loading (such as grate plates in clinker coolers), Fe-Ni-Cr series alloys with single-phase austenitic microstructure (such as HP, HT) should be prioritized, or nitrogen and rare earth elements should be added to inhibit σ-phase precipitation.
• 1000°C and Above Oxidation Resistance Threshold: Chromium content must be ≥20% to form a dense Cr₂O₃ protective film. According to GB/T 8492-2014 standard, ZG40Cr25Ni20 (commonly known as "2520") contains 23% to 27% Cr and can operate stably at 1150°C. Ordinary 304 stainless steel (18Cr-8Ni) is insufficient in chromium content and will experience oxidation spalling when used long-term above 800°C, and should never be substituted for dedicated heat-resistant cast steels.
• Quantitative Relationship Between Temperature and Oxidation Rate: For every 100°C increase in temperature, the oxidation rate may double. The annual oxidation weight gain of 310S stainless steel is approximately 1.2 mg/cm² at 1000°C, but this value may exceed 2.4 mg/cm² at 1100°C. This means that increasing the service temperature of HK40 from 1050°C to 1150°C may reduce its oxidation life by more than 50%.

Temperature Application Boundaries for Typical Grades

Furnace Atmosphere: The Overlooked Chemical Attack Factor

Industrial furnace atmospheres can be classified into six types: oxidizing, reducing, neutral, sulfur-containing, carburizing, and vacuum. The atmosphere type directly determines the failure mode of alloying elements:

Oxidizing and Sulfur-Containing Atmospheres

Chromium is the foundational element for oxidation resistance in all heat-resistant alloys. The Cr₂O₃ protective film it forms is crucial in oxidizing atmospheres. However, water vapor significantly accelerates oxidation of high-iron alloys, with relatively less impact on high-nickel alloys. In sulfur-containing atmospheres, sulfides penetrate the oxide film causing "sulfidation-oxidation" synergistic corrosion. In such cases, the HL series (29Cr-20Ni) with high chromium and low nickel should be selected, as its sulfidation resistance is superior to the HK series.

Carburizing Atmospheres and Metal Dusting

In carburizing atmospheres (such as methane or propane cracking environments), carbon atoms infiltrate the steel matrix forming brittle carbides. When carbon content exceeds 2%, most heat-resistant alloys completely lose ductility at room temperature. The HP series, due to its high nickel content (33% to 37%) which reduces the maximum carbon solubility, becomes the preferred choice for carburizing furnace components. For the more severe "metal dusting" — a catastrophic carbon corrosion occurring around 600°C — experience shows that high-nickel alloys such as RA333 and cast-grade Supertherm perform best, while RA330 and 801H perform significantly worse in this environment.

Vacuum and Reducing Atmospheres

In hydrogen or cracked ammonia atmospheres, decarburization embrittlement must be prevented. Grades with moderate carbon content (0.35% to 0.50%) and stable carbide-forming elements (such as Nb, W) should be selected. In modified HP-Nb grades, niobium forms NbC with carbon, preventing chromium depletion at grain boundaries and inhibiting hydrogen embrittlement.

Load Conditions: From Static Support to Dynamic Thermal Fatigue

The failure modes of heat-resistant steel castings in industrial furnaces depend not only on temperature and atmosphere, but are also closely related to load type:

Rupture Strength and Creep Resistance

For components under long-term static loading (such as furnace tubes and hangers), ISO 204:2018 standard requires: at 800°C and 100 MPa stress, creep rupture time must exceed 100,000 hours. HP40 (25Cr-35Ni) exhibits significantly higher rupture strength than HK40 at 900°C, because its higher nickel content stabilizes the austenitic matrix and promotes the dispersion of fine M₂₃C₆ carbides. If the operating temperature rises to 950°C with 50 MPa stress, nickel-based alloys such as Inconel 617 require rupture life ≥50,000 hours, at which point iron-based heat-resistant steels can hardly meet the requirements.

Thermal Fatigue and Thermal Shock

For components experiencing frequent start-up/shut-down cycles or temperature fluctuations (such as heat treatment trays and radiant tubes), thermal fatigue is the primary failure mode. Through 1,000 thermal cycles between 20°C and 800°C, crack growth rates can be evaluated. HH Type 1, due to its partial ferrite content, exhibits better ductility under such conditions than the fully austenitic Type 2; while the HT series (15Cr-35Ni), due to its high nickel content, has the best thermal shock resistance and can operate up to 1150°C in oxidizing conditions and 1100°C in reducing conditions.

Wear and Mechanical Impact

In environments with material erosion such as cement rotary kilns and pellet shaft furnaces, wear resistance must be enhanced on the basis of heat resistance. For ZG40Cr25Ni20, carbon content can be increased to 0.40% to 0.50%, or trace molybdenum (0.5% to 1.0%) can be added to form hard carbides. After replacing ordinary carbon steel with ZG40Cr25Ni20 in a cement kiln lining, service life extended from 6 months to 3 years, fully demonstrating the exponential improvement that proper material selection brings to service life.

Standard Systems and Engineering Practice in Composition Optimization

There are systematic differences in the compositional specifications for heat-resistant cast steels among major global standard systems. Understanding these differences helps in precise material selection:

Chinese Standards (GB/T 8492) and International Benchmarking

ZG40Cr25Ni20 specified in GB/T 8492-2014 corresponds to HK40 in ASTM A297, but with a slightly lower minimum nickel content (18% to 21% vs 19% to 22%). Chinese standards tend to compensate for performance losses from reduced nickel content by adding nitrogen (N, 0.15% to 0.25%) and rare earth (RE) elements, thereby controlling costs. For example, ZG35Cr24Ni7SiN, through nitrogen solid solution strengthening, achieves high-temperature strength close to HK40 at 1050°C, but with material cost reduced by approximately 15% to 20%.

ASTM A297 HP Series Modifications

Traditional HP grades (Cr 24% to 28%, Ni 33% to 37%) have evolved into several modified branches:

1. HP-Nb: Addition of 0.8% to 1.2% niobium forms Nb(C,N) precipitates, improving rupture strength at 1100°C by 20% to 30% while enhancing weldability.
2. HP-Mo: Addition of 1.0% to 1.5% molybdenum enhances solid solution strengthening effects, suitable for conditions with mild sulfidation corrosion.
3. HP-W-Nb: Combined addition of tungsten (0.5% to 1.0%) and niobium, used for ethylene cracking furnace radiant tubes, with synergistic optimization of carburization resistance and creep resistance.

Composition Testing and Quality Control

Composition deviations in heat-resistant steel castings significantly affect performance. For example, silicon content exceeding 3%, while enhancing oxidation resistance, severely reduces room temperature toughness; carbon content exceeding 0.50% accelerates high-temperature embrittlement. Engineering practice recommends using Optical Emission Spectrometry (OES) or Inductively Coupled Plasma (ICP) for composition testing, with error control within ±0.01%. For critical components, 500-hour oxidation testing (GB/T 13303-2020) is also required, calculating average oxidation rate V = (g₂ - g₁) / (S · t), in units of g/m²·h.

Economic Trade-offs: Life Cycle Cost Rather Than Initial Purchase Price

The final material selection decision must transcend unit material price and calculate the full Life Cycle Cost (LCC). Taking petrochemical ethylene cracking furnace radiant tubes as an example:

• Selecting HK40 offers lower initial material costs, but requires replacement every 2 to 3 years due to creep deformation or carburization embrittlement, resulting in massive shutdown maintenance losses.
• Selecting modified HP-Nb increases initial costs by approximately 25% to 30%, but service life can reach 5 to 7 years. Moreover, due to reduced wall thinning rates, fuel savings from improved thermal efficiency can reach twice the material cost difference.

In the ultra-high temperature range of 1095°C to 1205°C, even though iron-nickel-based alloys such as HL, HU, and HX have higher initial costs, their reduced downtime frequency and maintenance labor often recover the material cost difference within 18 months. Therefore, the essence of heat-resistant steel selection for industrial furnaces is finding the optimal balance among five dimensions: temperature, atmosphere, load, service life, and cost, rather than simply pursuing the extreme of any single indicator.