Brief description

The Open Hearth Process, also known as the Siemens-Martin process, is an industrial method for producing high-quality steel from molten pig iron and steel scrap. Unlike the rapid Bessemer process, it operates slowly in a shallow, open furnace bed (hearth) heated by gas and air preheated through regenerative brick chambers. This slower pace allows for precise control over the chemical composition of the steel, making it the dominant steelmaking method for most of the 20th century.

Use / Function

High-Quality Steel Production: Delivers exceptional consistency and strength, ideal for railways, bridges, ships, and building skeletons.
Scrap Metal Recycling: Uniquely capable of melting and refining large quantities of solid scrap steel, reducing the need for raw ore extraction.
Carbon and Alloy Control: Allows the steel composition to be tested and adjusted mid-melt to produce specific steel grades.
Scale: Heavy industrial scale, producing batches from 50 to hundreds of tons per melt.

Operating principle

The fundamental innovation of the Open Hearth process is the regenerative preheating system, which achieves temperatures exceeding 1600°C by recycling waste heat:

Regenerative checkerwork: Below the furnace lie chambers filled with a grid of firebricks (checkers).
Reversing Combustion Gas Flow: Hot waste gases exiting the hearth pass through one set of checker chambers, heating the bricks to extreme temperatures. Simultaneously, fresh air and fuel gas are drawn in through a second, already hot set of checker chambers, absorbing their heat before entering the hearth.
The Cycle: Every 20 to 30 minutes, valves reverse the direction of the gas flow. The cold intake air/gas now flows through the newly heated chambers, while the exhaust gas begins reheating the cooled chambers. This continuous cycle minimizes heat waste and maximizes furnace temperature.
Decarburization: In the shallow pool of the hearth, the carbon and other impurities in the Pig Iron are slowly oxidized by adding iron ore or exposing the melt to the hot furnace gases.

How to implement

1. The Siemens Furnace and Regenerators

Construct a large, shallow hearth lined with refractory silica brick (acid process) or magnesite/dolomite brick (basic process).
Beneath the furnace level, build four regenerative chambers (two for air, two for fuel gas) packed with firebrick checkerwork.
Install a system of reversing valves to control and redirect the intake and exhaust flows.

2. Preheating and Fuel supply

Burn gasified coal or liquid fuel. Run the preheating cycle until the checkerwork chambers are glowing red-hot.
Reverse the valves periodically to stabilize the hearth at refining temperatures (1600°C–1700°C).

3. Charging the Furnace

Introduce solid steel scrap, solid Pig Iron, and Limestone (for the basic process, to act as a flux).
Once the solid charge is partially melted, pour in molten pig iron from a Blast Furnace.

4. Refining and Testing

Keep the bath boiling for several hours. The limestone reacts with phosphorus and silica to form a slag layer on top of the liquid steel.
Take small spoon samples of the molten metal periodically, quench them, and fracture them to inspect the grain structure or perform chemical testing.
Add iron ore to speed up carbon oxidation, or add ferromanganese/spiegeleisen to adjust final carbon and alloy levels before tapping.

Materials needed

Raw Feedstocks: Molten Pig Iron, steel scrap, and iron ore.
Refractory Linings: Silica bricks, dolomite, or magnesite bricks to withstand extreme heat.
Fluxes: Limestone or lime to bind impurities into slag.
Fuel: Coal gas, natural gas, or oil.

Variants and improvements

Acid Open Hearth: Lined with silica bricks, useful only for raw materials low in phosphorus and sulfur.
Basic Open Hearth: Lined with magnesite or dolomite bricks. It allows limestone to be used to remove phosphorus and sulfur, making it highly versatile for low-grade ores.
Tilting Open Hearth (Talbot Process): A furnace that can tilt to pour off slag and steel continuously, improving throughput.

Limits and risks

Time Intensity: A single batch takes 8 to 15 hours, making it far slower than the Bessemer process (which takes 15 minutes) or modern basic oxygen steelmaking.
Energy Requirements: Requires constant external heat supplied by burning fuel throughout the long refining cycle.
Structural Integrity: The roof of the furnace is subject to severe chemical attack and thermal shock; a roof collapse can destroy the furnace and endanger workers.
Explosion Hazards: Accidental introduction of wet scrap steel or water can cause instantaneous, cataclysmic steam explosions.