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Quenching Oils

Quenching oil

Quenching oil and heat treatment fluids are designed for rapid or controlled cooling of steel or other metals as part of a hardening, tempering or other heat-treating process. Quenching oil serves two primary functions. ... Oil has a major advantage over water due to its higher boiling range.Quenching oil and heat treatment fluids are designed for rapid or controlled cooling of steel or other metals as part of a hardening, tempering or other heat-treating process. Quenching oil serves two primary functions. ... Oil has a major advantage over water due to its higher boiling range.



The Quenching Process

When heat treatment fluids are used to quench metals, cooling occurs in three distinct stages: film boiling, nucleate boiling and convective heat transfer.

Film Boiling

Film boiling, also known as the "vapor blanket" stage, occurs upon initial immersion. Contact between the hot metal surface and quenchant creates a layer of vapor (known as the Leidenfrost phenomenon) due to the supply of heat being greater than that which is carried off. The stability of the vapor layer, and thus the ability of the oil to harden steel, is dependent on the metal's surface irregularities, oxides present, surface-wetting additives (which accelerate wetting and destabilize the layer), and the quench oil's molecular composition (including the presence of more volatile oil degradation by-products). Cooling in this stage is a function of conduction through the vapor envelope and is relatively slow since the vapor blanket acts as an insulator.

Nucleate Boiling

As the part cools, the vapor blanket collapses at points and nucleate boiling (violent boiling of the quenchant) results. Heat transfer is fastest during this stage, with heat transfer coefficients sometimes over two orders of magnitude higher than during film boiling, largely due to the heat of vaporization. The boiling point of the quenchant determines the conclusion of this stage. The points at which this transition occurs and the rate of heat transfer in this region depend on the oil's overall molecular composition.

Oil Selection

When selecting quenching oils, industrial buyers will need to consider the chemistry, properties, and features of the fluid that are needed for the application.

GENERAL TECHNICAL ADVANTAGES OF THERMAL OIL FOR ITS APPLICATION IN INDUSTRIAL SECTORS

  • Straight oils are non-emulsifiable products used in machining operations in an undiluted form. They are composed of base mineral or petroleum oils, and often contain polar lubricants like fats, vegetable oils, and esters, as well as extreme pressure additives such as chlorine, sulfur, and phosphorus. Straight oils provide the best lubrication and the poorest cooling characteristics among quenching fluids. They are also generally the most economical.
  • Water soluble and emulsion fluids are highly diluted oils, also known as high-water content fluids (HWCF). Soluble oil fluids form an emulsion when mixed with water. The concentrate consists of a base mineral oil and emulsifiers to help produce a stable emulsion. These fluids are used in a diluted form with concentrations ranging from 3% to 10%, and provide good lubrication and heat transfer performance. They are used widely in industry and are the least expensive among all quenching fluids. Water-soluble fluids are used as water-oil emulsions or oil-water emulsions. Water-in-oil emulsions have a continuous phase of oil, and superior lubricating and friction reduction qualities (i.e. metal forming and drawing). Oil-water emulsions consist of droplets of oil in a continuous water phase and have better cooling characteristics (i.e. metal cutting fluids and grinding coolants).
  • Micro-dispersion oils contain a dispersion of solid lubricant particles such as PTFE (Teflon®), graphite, and molybdenum disulfide or boron nitride in a mineral, petroleum, or synthetic oil base. Teflon® is a registered trademark of DuPont.
  • Synthetic or semi-synthetic fluids or greases are based on synthetic compounds like silicone, polyglycol, esters, diesters, chlorofluorocarbons (CFCs),and mixtures of synthetic fluids and water. Synthetic fluids tend to have the highest fire resistance and cost.They contain no petroleum or mineral oil base, but are instead formulated from organic and inorganic alkaline compounds with additives for corrosion inhibition. Synthetic fluids are generally used in a diluted form with concentrations ranging from 3% to 10%. They often provide the best cooling performance among all heat treatment fluids. Some synthetics, such as phosphate esters, react or dissolve paint, pipe thread compounds, and electrical insulation. Semi-synthetic fluids are essentially a combination of synthetic and soluble petroleum or mineral oil fluids. The characteristics, cost, and heat transfer performance of semi-synthetic fluids fall between those of synthetic and soluble oil fluids.

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