Specific operational steps for re-quenching and standardized tempering:

To assure microstructure reconstruction and performance recovery, the particular operational methods for re-quenching and standardized tempering involve heating to 1020–1050°C, holding at that temperature, quenching, and then carrying out two tempering processes at 550–600°C, each holding for at least two hours.

 

1. Requenching

Procedures

Important Points

Goals

① Heating

Heat the workpiece gradually to 1020–1050°C. Segmented preheating (e.g., 600°C, 850°C)

advised to avoid cracking or distortion brought on by heat stress.

② Holding Period

determined using the workpiece's effective thickness

For every 25 mm of thickness, hold for at least 30 minutes. Make sure the core has been adequately austenitized.

③ Method of Cooling

Use high-pressure air cooling (vacuum furnace) or oil cooling (N32/N46 machine oil recommended).

achieve a fine, homogeneous martensitic structure.

Precautions: To prevent deformation, steer clear of uneven stirring and too high oil temperatures while cooling (<60°C is advised).

 

2. Conventional Tempering

Procedures

Important Points

Goals

① Initial Tempering

After quenching, hold at a temperature of 550–600°C (580°C is ideal) for at least two hours.

releases the majority of residual tension and starts the change of the microstructure.

② Reaching Room Temperature

After tempering, it must be air-cooled to room temperature (<50°C); fast cooling is not allowed.

stops fresh thermal stress from developing.

③ Second Tempering

Maintaining a constant temperature and duration, repeat the initial tempering procedure.

enhances microstructure stability and totally removes stress rebound.

Special Note: Hardness and toughness will not be stable if only one tempering stage is carried out.

 

3. Verification and Post-Remedial Testing

Hardness Testing: Test three to five locations on each piece using a portable hardness tester to make sure:

48–52 HRC on average;

Range ≤2 HRC for a single piece.

Metallographic sampling: at least 5% of samples are extracted from every batch to verify that the microstructure is composed of scattered carbides and homogenous tempered martensite;

Furnace temperature profile retention: For traceable management, temperature records are kept for the duration of the process.