Solutions to Insufficient Hardness During Quenching

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Solutions to Insufficient Hardness During Quenching

Quenching is a critical process in heat treatment that enhances the hardness and wear resistance of metal workpieces. However, a common defect encountered in production is insufficient hardness post-quenching, which typically manifests in two forms: overall low hardness of the workpiece or localized soft spots. To address this issue, it is essential to first identify the type of hardness deficiency using methods like hardness testing or metallographic analysis, then diagnose the root causes across raw materials, heating processes, cooling media, cooling methods, and tempering temperatures. Below is a detailed breakdown of the causes and corresponding solutions.

1. Issues with Raw Materials

Raw material-related problems are a primary contributor to insufficient quenching hardness, mainly involving improper material selection, misallocation, and uneven microstructures.

1.1 Improper Material Selection or Misallocation

Using low-carbon steel instead of medium/high-carbon steel, or ordinary high-carbon steel instead of alloy tool steel for parts that require specific hardness, directly leads to insufficient hardness.

Example 1: A gear designed to be made of 45 steel (target quenching hardness: ~60 HRC) was mistakenly manufactured with 25 steel, resulting in a final hardness of only ~380 HBS.
Example 2: A mold requiring 9Mn2V steel was incorrectly made with T8 steel. Due to the similar spark characteristics of 9Mn2V and T8 steel, the quenching process was executed according to 9Mn2V parameters (oil cooling), leading to a hardness of only ~50 HRC.

Both cases result in overall hardness deficiency, which can be verified via hardness testing or metallographic analysis (e.g., 25 steel forms low-carbon martensite after quenching, while 45 steel forms medium-carbon martensite).

Solutions:

Select materials that match the part’s hardness requirements during the design phase.
Strengthen material management: Conduct chemical analysis before storing materials, classify and label them to avoid misallocation.
Heat treatment operators should perform spark analysis before processing to roughly verify if the material matches the design specifications.
For workpieces with large cross-sections or significant thickness variations, switch to alloy steels with good hardenability (instead of tool steels with poor hardenability) to prevent low internal hardness in thick sections.

1.2 Uneven Microstructure of Raw Materials

Uneven microstructures—such as carbide segregation/agglomeration, ferrite clustering, graphite formation, or severe Widmanstätten structure—cause localized hardness deficiency or soft spots.

Solution:

Perform repeated forging or pre-heat treatment (e.g., normalizing or homogenizing annealing) before quenching to uniformize the microstructure.

2. Issues with Heating Processes

Heating process parameters (temperature, holding time, and surface protection) directly affect the formation of austenite (a key phase for martensite transformation post-quenching). Deviations in these parameters lead to incomplete phase transformation and insufficient hardness.

2.1 Low Quenching Temperature or Insufficient Holding Time

For hypoeutectoid steel: Heating between Ac₁ and Ac₃ (e.g., 25 steel heated below 860°C) prevents full dissolution of ferrite into austenite. After quenching, the structure becomes a mixture of ferrite and martensite, reducing hardness. Metallographic analysis reveals undissolved ferrite.
For high-carbon steel (especially high-alloy steel): Insufficient heating or holding time stops pearlite from transforming into austenite, failing to form martensite. Common causes include inaccurate temperature gauge readings (indicating higher temperatures than actual) or incorrect estimation of workpiece thickness (leading to short holding times).

Solutions:

Control heating rate to avoid uneven furnace temperature and premature timing of holding (which shortens effective holding time).
Regularly calibrate temperature-indicating instruments to ensure consistency between displayed and actual temperatures.
Strictly follow material handbooks to set heating rate and temperature, preventing underheating or overheating.
Accurately estimate workpiece thickness, especially for irregularly shaped parts.