ASTM 309S Forged Heat-resistant Steel Base

309S Forged Heat-resistant Steel Base

The 309S forged heat-resistant steel base is a key heavy-duty heat-resistant forging for grain-oriented silicon steel annular furnaces. It carries steel coils weighing several tons. It works for a long time under high temperature (900–1050°C), nitrogen-hydrogen protective atmosphere, heavy loads, and repeated heating and cooling cycles.

Many patents from 2025 to 2026 show that the material, forging density, and heat treatment structure of the base directly affect the temperature uniformity of the silicon steel annealing furnace. This part is the key tool for controlling "elephant foot" defects and local core loss problems in silicon steel.

Traditional cast bases or welded assembly bases often have creep deformation, thermal stress cracking, and oxidation peeling. These problems can ruin the whole furnace of silicon steel products.

Our company uses 309S (UNS S30908) open die forging. We strictly forbid any welding joints or welding repairs during the whole process. Our product meets the forced requirements of ASTM A473-23a for forgings. It is specially developed for the tough working conditions of grain-oriented silicon steel annular furnaces.

Material Grade and Chemical Composition

309S (UNS S30908) is a heat-resistant austenitic stainless steel with high chromium and nickel, and low carbon. The low carbon content helps reduce the risk of carbide precipitation during high-temperature service, which can cause sensitization. Compared with regular 309 (S30900, with C ≤ 0.20%), 309S is more suitable for parts that face thermal cycling or are used in annealing furnaces with protective atmospheres. It also helps avoid intergranular corrosion failure.

Forging process (ASTM A473 23a hot working requirements)

1. Forging Ratio
The main cross‑section forging ratio must be ≥ 5.0. This ensures that the coarse dendrite structure from the ingot is broken up, internal porosity and tiny voids are compressed, and the whole section becomes fully dense. Because austenitic stainless steel cannot be refined by heat treatment, grain size control depends only on the amount of forging deformation and the finish‑forging temperature.

2. Forging Temperature Range
Start forging at 980–1120 °C, and finish forging at or above 980 °C. Do not let the finish temperature drop too low, otherwise sigma phase (brittle particles) and micro‑cracks may form.

3. Internal Quality Control of the Forging
The forging must have no white spots, internal cracks, or shrinkage cavities. After forging, remove all surface scale by grinding. Local defects may be chipped out, but the chipping depth must not exceed 75% of the remaining machining allowance, so that all defects are completely removed during final machining.

4. Non‑Destructive Testing
Perform UT (ultrasonic testing) and PT (penetrant testing) according to the supplementary requirements of ASTM A788/A788M. These tests are used to find internal and surface micro‑defects.

Solution Heat Treatment（ASTM A473-23a）

1. Solution Treatment Temperature
Heat to 1040–1120 °C (≥1900 °F). This temperature must be high enough to fully dissolve chromium and nickel carbides into the austenite matrix.

2. Holding (Soaking) Time
Calculate the time based on the thickest section of the forging. Make sure the whole part is heated completely through.

3. Cooling Method
Cool quickly by water quenching or strong forced air. The cooling must be fast to pass through the sensitization temperature range of 425–815 °C quickly. This stops chromium carbides from forming at grain boundaries and gives a uniform single‑phase austenite structure.

4. Grain Size Check After Heat Treatment
After heat treatment, check the grain size. This is to prevent abnormal grain growth, which would weaken the creep properties of the material.

Mechanical Properties and High-Temperature Performance

Key High-Temperature Performance for Service (Actual Chassis Working Conditions: 900–1000°C)

1. Oxidation Resistance

• Can be used continuously in air up to 1000°C, and up to 1050°C in intermittent heat cycles.

• Forms a tight protective oxide layer on the surface that stops further oxidation and peeling.

• Prevents loose oxide flakes from falling off and contaminating the surface of silicon steel coils.

2. Creep Resistance

• Much better than 304 and 316 stainless steel.

• At 700°C, the stress to cause failure in 100,000 hours is about 25 MPa.

• This meets the needs of heavy steel coils under long-term high-temperature creep resistance, reducing the risk of permanent bending or sagging of the chassis.

3. Microstructure Stability

• Avoids the risk of forming harmful sigma phase during long-term use at medium temperatures.

• With the right solution heat treatment, it ensures a long service life inside the furnace.

Application

EPC Coil Chassis for High-Temperature Annular Furnace for High-End Grain-Oriented Silicon Steel

Work Condition Problems

The EPC chassis in the grain‑oriented silicon steel ring furnace carries very heavy steel coils. The furnace runs in cycles, with repeated heating and cooling. Inside the furnace, there is a high‑purity nitrogen‑hydrogen protective gas.
The main failure modes of the chassis are:

• Bending and sagging under high‑temperature creep,

• Cracking from thermal cycle fatigue,

• Oxide scale peeling off and dropping,

• Weld joints failing at high temperature.
  If the chassis fails, it causes uneven temperature at the bottom of the steel coil, which leads to batch quality defects like high iron loss and “elephant foot” shape problems on the silicon steel.

Why Our Product Fits Well

1. One‑piece forged, no welding – completely avoids the risk of weld joint failure at high temperature.

2. Made of 309S (UNS S30908), a low‑carbon heat‑resistant steel – gives good oxidation resistance, creep resistance, and resistance to damage from heat cycles. It works well for the 900–1050°C annealing process in the ring furnace.

3. Forging ratio ≥5 + standard solution heat treatment – makes the internal structure dense and uniform, so the mechanical properties are stable across the whole section.

4. Follows ASTM A473‑23a standard for stainless steel forgings – we can provide full material certificates, heat treatment reports, and non‑destructive test reports.

5. We can make rough parts or finished machined parts according to your drawings. We can also add extra tests like high‑temperature creep sampling and grain size inspection if your project requires.

Typical Industries We Serve

• High‑magnetic‑induction grain‑oriented silicon steel ring annealing furnaces (for transformer steel) – EPC coil support chassis.

• Other heavy‑duty heat‑resistant support parts for metallurgical ring furnaces and bell‑type furnaces.

• Custom heavy‑duty heat‑resistant forgings for petrochemical and heat‑treatment industries.

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