Quenching oil (heat treatment medium)

Quenching Oil is composed of base oil and additives, with base oil typically accounting for over 80% of the total composition. Therefore, the performance of quenching Oil depends on the base oil, the additives, and their compatibility.

One of the basic requirements of heat treatment is to ensure both hardening and minimal deformation, while large-scale industrial production primarily requires stability and reliability. However, quenching Oil is constantly in contact with high-temperature workpieces during use, and to reduce deformation, the quenching Oil’s operating temperature may be intentionally increased. Therefore, oxidation stability is a key performance indicator for quenching Oil. The difference in performance stability during use is one of the most important indicators for evaluating the quality of quenching Oil; only when the quenching Oil’s performance is stable can the stability of quenching quality be ensured.

A. Ordinary quenching Oil: used for quenching small-sized materials with good hardenability.

B. Rapid quenching oil: used for quenching large and medium-sized materials.

C. Ultra-rapid quenching oil: used for quenching large materials and materials with poor hardenability.

D. Bright quenching oil: Used for quenching small-to-medium cross-section bearing steel, tool steel, measuring and cutting tool steel, and instrument parts under a protective atmosphere.

E. Rapid bright quenching Oil: Used for quenching medium-sized materials and materials with poor hardenability under a controlled atmosphere.

F. Vacuum quenching oil: Used for quenching medium-sized materials under vacuum conditions.

The simplest way to evaluate whether the oil type and grade have been correctly selected is to heat a single workpiece using the process parameters to be used in production, quench it in the selected Oil, and verify that the required quenching hardness, hardening depth, core hardness, and deformation requirements are achieved. If the quenching of a single workpiece meets the heat treatment requirements, then the oil type and grade are correct. Two things remain: First, ensure uniform quenching of multiple workpieces simultaneously through equipment conditions and process methods; second, understand (or test) the stability of the Oil during use.

Key Technical Indicators for Quenching Oil Testing:

1. Cooling Characteristics

Controlling and testing cooling characteristics is crucial for determining whether the Oil meets the requirements of the heat treatment process. It also helps assess the Oil’s stability. The standard typically followed is ISO 9950 (domestic standard GB/T 30823).

Quenching cooling media are generally categorized into those undergoing phase changes during cooling, such as Water, salt solutions, alkaline solutions, Oil, emulsions, and polymer solutions, and those without phase changes, such as molten metal, molten salt, molten alkali, air, nitrogen, and solid particles. (Figure 2)

Figure a shows a phase change, while Figure b shows no phase change. Commonly used media undergo phase changes, which, during cooling, are sequentially classified as the vapor film stage (A), the boiling stage (B), and the convection stage (C).

Understanding these three stages, or rather tracing their origins, requires starting with the pool-boiling curve of a resistance wire continuously heated in Water, as shown in Figure 3.

The Image shows the temperature difference (superheat) between the heating element and the surrounding medium on the horizontal axis, and the heat flux density on the vertical axis. In stage AB, the superheat is relatively small, and the primary heat transfer method is convection, resulting in low heat transfer efficiency. After point B, as the superheat increases, boiling begins to occur on the workpiece surface. Bubbles, through nucleation, growth, and detachment from the heating element surface, cause strong disturbances to the surrounding medium, allowing the cold medium to directly and continuously contact the hot surface, rapidly accelerating heat transfer, reaching its maximum at point C. As superheat increases further, the medium continues to vaporize, leading to the vapor on the workpiece surface coalescing into a vapor film. This vapor film impedes direct contact between the hot surface and the cold medium, reducing heat transfer efficiency. At this stage, the vapor film is dynamic, localized, and unstable. With further increases in superheat and vaporization, the vapor film becomes more stable. Its area increases, while the heat transfer efficiency continues to decrease, eventually reaching point D. At this Leidenfrost point, a stable vapor film covers the entire surface of the hot workpiece, and the heat transfer efficiency reaches its minimum. Beyond point D, as superheat increases and radiative heat transfer increases, the heat transfer efficiency increases accordingly.

Interpretation of Oil Cooling Curve:

The workpiece quenching process is the opposite of the method described above. At the beginning of quenching and cooling, the temperature difference between the workpiece and the medium is significant, forming a stable vapor film in the ED segment. In the DC segment, due to the decrease in workpiece surface temperature and superheat, the vapor film becomes unstable and can only partially cover the surface, thus continuously increasing the heat transfer efficiency. Up to point C, the vapor film no longer exists, resulting in the highest heat transfer efficiency. The CB segment is the complete boiling stage; as bubbles form, grow, and leap off the workpiece surface, they displace the quenching medium. After the bubbles leap off the surface, the liquid flows back, and the cold quenching medium continuously contacts the workpiece surface, generating strong turbulence, thus resulting in high heat transfer efficiency and strong cooling capacity. Below point B, the superheat decreases further, and boiling cannot be maintained, entering the convection stage. Figure 4 shows the cooling curves during quenching cooling, measured with an IVF cooling rate instrument, indicating that quenching cooling has three stages: the vapor film stage (ED), the boiling stage (DCB), and the convective heat transfer stage (CA). The cooling rate is slow in the vapor film and convective heat transfer stages, and fast in the boiling stage. Points D and B are inflection points on the cooling curves. The temperature represented by point D is often called the upper characteristic temperature, while the temperature at point B is often called the lower characteristic temperature.

2. Trace Moisture in Quenching Oil

According to ASTM D1533 and ASTM D6304, Water should be prohibited in quenching Oil. Moisture is the most significant hazard in oil quenching; it not only accelerates oil aging and affects cooling rate and quenching quality, but also poses a potential fire threat due to foaming and sudden boiling. The Karl Fischer method is generally used to detect trace moisture.

Water ingress into quenching Oil:

1. First, it will cause uneven hardness in the quenched parts.

2. It will cause cracking in the quenched parts.

3. It will change the cooling performance of the quenching medium;

4. It will affect the processability of the entire machining process. Solutions depend on the amount of water ingress. Heating methods can be used (for small amounts, depending on the boiling point), oil-water separation technology (for larger amounts), or vacuum dehydration. Alternatively, the entire tank of Oil can be drained, replaced with new Oil, and the old Oil returned to the manufacturer for processing. The manufacturer can easily resolve these issues.

3. Kinematic Viscosity of Oil

Follows ISO 3104/ASTM D445 standards.

Viscosity measurement and changes reflect the degree of aging of the quenching Oil. Under normal circumstances, oil aging is mainly characterized by oxidation and polymerization, reflected in an increase in viscosity. However, in a furnace under a protective atmosphere, oil aging may be principally characterized by decomposition, reflected in a decrease in viscosity.

4. Flash Point of Oil

Follows ASTM D92 standards. As the quenching process proceeds, the amount of light fractions in the quenching oil increases, thus lowering the flash point. A decrease in flash point has adverse effects on safety, the surface quality of the workpiece after quenching, and quenching deformation.

5. Acid-Base Value (also called Neutralization Value) of Oil

Follows ASTM D664 standards. Quenching oils may exhibit acidity or alkalinity, depending on their source and additives. Titration with an acid or alkali can be performed. Generally, the neutralization value increases as oil oxidation and aging progress, but it should be considered alongside other indicators to assess the degree of oil aging.

6. Saponification Value

For mineral oils without fatty acid additives, the saponification value increases as unsaturated hydrocarbon components oxidize. However, some oils contain fatty acid additives, so even new oils may have a saponification value of 2-3 mg KOH/g.

7. Ash Content

Complete mineral oils produce no ash after combustion; therefore, the ash content of mineral oils indicates the level of contaminants. However, it should be noted that additives in mineral oils can also form ash, so it is essential to verify whether any ash-forming additives are present before testing.

8. Carbon Residue

In the past, carbon residue was used to measure the tendency of quenching oils to form precipitates. However, because oils now contain additives, their meaning may be completely different. For example, adding a high-molecular-weight additive to Oil may increase ash content and carbon residue, but reduce precipitate formation.

9. Oil Sediments

Following standard ASTM D91, this measures the amount of compounds in the Oil that form sludge, and is an indicator of quenching Oil aging.

10. Sludge

Sludge is a product of oil oxidation and polymerization, and is an essential indicator of oil aging. Sludge adversely affects the surface quality of quenched workpieces, impacts cooling rate, and can clog systems, making it the most significant hazard besides moisture.

High-quality new quenching Oil will not produce sludge, but some lower-quality new quenching oils tend to do so. For a high-quality quenching surface finish, its content should be controlled below 0.2%.

11. Specific Gravity

The specific gravity of new Oil indicates the type of base oil used. For Oil in use, it has little practical significance.

Safety Issues in Quenching Oil Use

Flash point is an essential indicator for oil use, transportation, and production. The operating temperature of the Oil should be at least 60°C below its flash point. When quenching oil ages or becomes contaminated with other light components, its flash point may decrease, reducing safety performance. Regularly checking the flash point of quenching Oil is a good practice.

During continuous production, the temperature rise of the Oil should be monitored. The Oil should be continuously stirred during use to prevent localized overheating.

A drop in the quenching oil level or slow immersion of workpieces in the Oil poses a significant fire hazard. During quenching, workpieces should be quickly immersed in the Oil and completely covered (at least 50mm).

The presence of Water in the quenching Oil also threatens safety. Water in the Oil may originate from cooling Water, pre-rinsing Water, or other mixed Water. Extra caution is needed when using oils in furnaces.

Fire-fighting equipment, such as foam extinguishers, should be stored near the quenching oil tank. Foam extinguishers should be used upside down. Oil that has been extinguished with a fire extinguisher may contain significant amounts of Water and impurities; it should be tested and treated before reuse.