Material Selection for the Bottom Support of a Pit Furnace

 

The bottom support of a pit furnace is primarily made of high-chromium-nickel austenitic heat-resistant cast steel. The core requirements are high-temperature creep resistance, oxidation resistance, thermal shock resistance, and dimensional stability to support heavy-load workpieces and withstand long-term high temperatures and corrosive atmospheres within the furnace.

 

I. Commonly Used Materials and Selection

 

1. ZG35Cr24Ni7SiN(Re)

 

Applicable operating conditions: Most commonly used, long-term operating temperature 950~1050℃, short-term up to 1100℃. Suitable for general pit-type annealing, tempering, and carburizing furnaces.

 

Features: Contains rare earth (Re) elements, good resistance to thermal fatigue, carburizing, and thermal shock, suitable for operating conditions with frequent hot and cold cycles.

 

2. ZG40Cr25Ni20Si2 (2520 / 310S casting grade)

 

Applicable operating conditions: High temperature and heavy load, long-term stability 1050~1150℃. Used in high-temperature solution treatment, carburizing furnaces, and other demanding applications.

 

• Features: High creep resistance, oxidation resistance, stable structure, not easily deformed or collapsed at high temperatures.

 

3. 310S (0Cr25Ni20) Stainless Steel

 

• Applicable working conditions: Light-duty, non-heavy-duty supports.

 

• Features: Welded sheet metal, flexible forming, smooth surface, but high-temperature strength is lower than cast steel.

 

4. Ultra-high temperature materials (e.g., ZG40Cr28Ni48W5Si2)

 

• Applicable working conditions: 1200℃ ultra-high temperature, extremely heavy-duty areas.

 

• Features: Contains high alloying elements such as tungsten (W), extremely high high-temperature strength, expensive.

 

II. Core Performance Requirements

 

1. High-Temperature Mechanical Properties

 

• Creep Resistance: Core indicator. No significant plastic deformation or sagging under long-term load at 1000℃.

 

• High-Temperature Strength: Sufficient high-temperature tensile/yield strength to support the workpiece without fracture.

 

2. High Temperature Resistance and Oxidation Resistance

 

 Operating Temperature: Matches furnace temperature, maintaining long-term stability at the design temperature (typically 1050~1100℃).

 

 Oxidation Resistance: Forms a dense oxide film (e.g., Cr₂O₃) at high temperatures, preventing oxidation, peeling, and powdering.

 

3. Thermal Shock Resistance and Microstructure Stability

 

 Thermal Fatigue Resistance: Does not crack or chip when the furnace body experiences frequent temperature increases and decreases.

 

 Metallographic Stability: Austenitic matrix, no phase transformation at high temperatures, ensuring dimensional accuracy.

 

4. Corrosion Resistance

 

 Resistant to carburizing, nitriding, sulfur/chlorine atmospheres, and dust corrosion within the furnace.

 

5. Structure and Process

 

 Casting/Welding Quality: Dense microstructure, free of shrinkage cavities and cracks, high rigidity, and impact resistance.

 

 Thermal Expansion Compatibility: Coefficient of thermal expansion matches the furnace body, with pre-reserved expansion joints.