Heat Treatment of Forgings

 

Heat Treatment of Forgings

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Issuing time:2025-07-06 15:43Author:Timothy Forgings

Heat Treatment of Forgings
Almost all cold-, warm-, and hot-stamped parts must undergo heat treatment to become qualified components. The quality of the heat treatment process plays a decisive role in ensuring and improving part performance, precision, stability, and service life. Raw materials, heating, and heat treatment processes are critical to ensuring part quality. The forging process itself also impacts the heat treatment of forgings.

The heat treatment of forgings is tailored based on the type of forging and the raw materials used. Key parameters include heating temperature, holding time, and cooling rate. By coordinating heating processes (equipment, heating speed, and atmosphere) and cooling processes (equipment and medium), forgings achieve the required metallographic structure and mechanical properties while avoiding defects (e.g., deformation, cracks, or abnormal structures). Thus, heat treatment is an essential step in forging production.


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Types of Heat Treatment for Steel Forgings
Steel forgings undergo various heat treatments. For small-to-medium forgings, treatments are categorized into two groups based on purpose:

1. Preparatory Heat Treatment

o Prepares forgings for subsequent processes like machining or final heat treatment.

o For carburized steel or bearing steel forgings, this step optimizes machinability, reduces hardness, refines grains, and improves microstructure. Common methods include normalizing, isothermal normalizing, and annealing.

Example:

o Automotive gear forgings (e.g., 20CrMnTiH) are normalized after forging to reduce hardness, homogenize structure, and minimize deformation during carburizing.

o Isothermal normalizing is often preferred over standard normalizing to ensure uniform microstructure and hardness. This involves reheating forgings to 900–950°C, cooling rapidly to 600–650°C, holding for pearlite transformation, and air-cooling.

2. Final Heat Treatment

o Directly imparts the required mechanical properties and microstructure for end use.

o Medium-carbon and alloy steel forgings (e.g., engine crankshafts, connecting rods) typically undergo quenching and high-temperature tempering to balance strength, hardness, ductility, and toughness.

Example:

o For high-precision forgings, annealing or normalizing is applied before tempering to refine grains and improve machinability. A forging company might use isothermal normalizing with residual heat to achieve a Grade 3 microstructure in engine connecting rods.

Common Heat Treatment Processes for Steel Forgings

1. Normalizing

o Heating to 30–50°C above austenitizing temperature (Ac3/Acm), holding, then air-cooling.

o Refines microstructure, eliminates Widmanstätten structures or carbide networks, reduces stress, and enhances machinability.

2. Isothermal Normalizing

o Rapid cooling (e.g., 30–42°C/min) to 600–650°C after austenitizing, followed by air-cooling.

o Ensures uniform pearlite-ferrite structures, minimizes carburizing distortion (used for gear steels like 20CrMoH, 20CrNiMoH).

3. Annealing

o Slow furnace cooling after heating to refine grains, relieve stress, or adjust hardness.

4. Quenching

o Rapid cooling from austenitizing temperatures (30–50°C above Ac3/Accm) to form martensite.

o Enhances strength and hardness (for hypoeutectoid/hypereutectoid steels) or corrosion resistance (for austenitic stainless steels).

5. High-Temperature Tempering

o Heating to 500–700°C (below Ac1) to stabilize quenched structures, reduce brittleness, and relieve residual stress.


http://www.forging-shafts.com/Heat-Treatment-of-Forgings.html

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