Quenching of steel refers to a process in which heated steel is cooled at an appropriate rate, generally for the purpose of hardening the material.

Steel containing a suitable amount of carbon is heated to the austenite phase and then rapidly cooled to form a martensitic structure. The oil used for this rapid cooling is referred to as quenching oil, and oils used for other heat treatment processes such as tempering or annealing are collectively referred to as heat treatment oils.

1. Cooling Mechanism

As standardized in KS M 2170, most heat treatment oils are mineral oil-based. In general, “heat treatment oil” primarily refers to quenching oil.

The goal of quenching is to harden the steel without causing cracks or distortion. Cracks or deformation during quenching can occur due to the steel’s hardenability, shape, dimensions, residual stresses from prior processes, and the cooling capacity of the quenchant. The hardening of steel is determined by its hardenability, size, and the cooling performance of the quenchant.

Therefore, a key to successful quenching is to cool rapidly at first and slowly later. Water quenches quickly but has a higher risk of cracking or warping, whereas mineral oil quenches more slowly but results in less distortion.

The cooling performance of a liquid is generally lower when its vapor pressure, boiling point, surface tension, and viscosity are high, and greater when its latent heat of vaporization, specific heat, and thermal conductivity are high. Within the same type of mineral oil, heavier oil fractions have higher molecular weight and boiling point, lower vapor pressure, and higher flash point and viscosity. Consequently, both the characteristic temperature and onset temperature of convection stage also increase.

During rapid cooling using mineral oil-based heat treatment oils, the cooling process generally occurs in three stages:

1. Vapor Blanket Stage – Characterized by low cooling rate
2. Boiling Stage – Characterized by rapid cooling
3. Convection Stage

The characteristic temperature is defined as the temperature at which the vapor blanket collapses and nucleate boiling begins. The convection onset temperature is the point where cooling begins solely by convection. These two temperatures are crucial in governing the cooling behavior of heat treatment oils.

The higher the characteristic temperature and the lower the convection onset temperature, the greater the cooling performance. However, representing cooling performance solely by cooling curves is problematic, so the Grossman H-value is generally used to quantitatively express the cooling capacity.

2. Temperature Effects

The cooling performance of a quenchant changes with temperature. Water, for example, experiences a sharp drop in cooling efficiency as its temperature increases due to the easier formation and prolonged duration of the vapor blanket stage.

In the case of quenching oil, as the oil temperature increases, viscosity decreases, leading to an increase in characteristic temperature. Cooling performance reaches its peak at around 80°C, but decreases if the oil temperature rises further.

Vegetable oils such as rapeseed oil, soybean oil, peanut oil, and whale oil generally have higher characteristic temperatures and relatively good cooling performance. However, due to their high cost and rapid degradation, mineral oils are predominantly used today.

As the oil temperature rises, the H-value also increases and reaches its maximum between 60–80°C. Compared to water, the influence of liquid temperature on quenching oil is less significant, and 60–100°C is considered the optimal range for practical use.

For marquenching (hot bath quenching), where oil is used at around 200°C, the cooling performance is intentionally reduced. In this case, light oil with low flammability risk is recommended. Among oils with similar flash points, naphthenic oils provide better cooling performance than paraffinic oils. However, in general, mineral oils have inferior cooling capacity compared to vegetable oils. To improve performance, methods such as reducing surface tension or inhibiting vaporization are under investigation.

While enhancing cooling performance is relatively simple, it often leads to shorter oil lifespan. Therefore, it’s generally better to use long-life mineral oils and supplement any cooling deficiencies with agitation. However, this can lead to uneven quenching and distortion due to different cooling rates on various parts of the component.

3. Effect of Agitation

Agitation during quenching significantly influences cooling performance.

It is standard practice in heat treatment facilities to equip quenching oil tanks with agitation devices. These devices, or manual shaking of the parts in the oil, help achieve appropriate cooling rates for the parts.

Agitation disrupts the formation of the vapor blanket, shortening that stage and enhancing the boiling and convection phases, leading to improved overall cooling.

When agitation begins in a static bath, cooling efficiency rises sharply at first but then levels off or even decreases with excessive flow rates. On cooling curves, strong agitation can make the three cooling stages less distinct.

4. Non-Aqueous Heat Treatment Oils

Paraffinic base oils generally have higher characteristic temperatures than naphthenic base oils. Adding bright stock improves cooling performance. Mixing low-viscosity and high-viscosity mineral oils results in intermediate values for both characteristic and convection onset temperatures.

Cooling enhancers include polyisobutylene, methacrylate polymers, ashless dispersants, terpene polymer resins, petroleum sulfonates, oxidized asphalt, and coal tar. Among these, polyisobutylene, petroleum heavy fractions, and sulfonates are commonly used. These are all high molecular weight substances that are difficult to evaporate, contributing to an increase in characteristic temperature.

However, depending on the additive used, negative effects on stability or brightness may occur, meaning selection cannot be based on cooling enhancement alone. Hence, there are few additives suitable for practical use.

5. Types of Heat Treatment Oils in KS Standards

Type	Use Description
Type 1, No. 1	Quenching oil for materials easy to harden
Type 1, No. 2	Quenching oil for materials difficult to harden
Type 2, No. 1	For marquenching at around 120°C
Type 2, No. 2	For marquenching at around 160°C
Type 3, No. 1	For tempering at oil temperature around 150°C
Type 3, No. 2	For tempering at oil temperature around 200°C

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