Quenching steel

Quenching steel – mechanism and quenching media

Quenching steel plays a critical role in developing microstructure for high strength and hardness applications. Depending upon quenching media, the structure may contain martensite plus retained austenite or a mixture of pearlite, martensite and retained austenite. It’s been explained that microstructure can not be completely martensitic. Martensite due to the generation of stresses is always accompanied by retained austenite. For obtaining the maximum amount of martensite, practically, the best option is to use brine solution as salts formed on the surface of samples will further speed up the heat transfer process.

For details and experimentation, have a look around. But, if you want to understand how much austenitizing temperature is the important information of martensitic transformation than you should read, “Effect of Austenitizing temperature on Martensitic transformation”.  In the case of coarse carbon steel, there is, also, a chance for the formation of widmanstatten structure due to low heat transfer. Widmanstatten formation conditions and its application can be studied here….

Quenching steel and Quenching media

Cooing rate or heat transfer rate for carbon steel is of prime importance as it defines the required structure consequently essential properties. If the cooling rate is higher the critical cooling rate than quenching steel takes place. Quenching of carbon steel improves the hardness of materials due to the formation of martensite. 

Quenching steel can be carried out in either gas medium or liquid medium. In the case of gas quenching, heat transfer is slow because of gases are poor conductors of heat resultantly low cooling rate.

But, in liquid quenching, heat transfer is very fast. It can take place in three stages as follows: 

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  1. The vapor phase
  2. Nucleate boiling
  3. Convective stage

During initial contact of any liquid, surfaces get boiled up resulting in the formation of vapors. The vapors formed reduces heat transfer rate which causes, initially, slow cooling. When the temperature of liquid drops further, vapors that were formed earlier gets collapsed. This causes fresh liquid surfaces to be in contact with a heated body giving a high transfer rate. This stage of bubble bursting is called the liquid boiling stage. The stage where vapors were formed is called the Vapor blanket stage. Fastest heat transfer occurs during the fluid boiling stage.

Further decrease in temperature of the heated body gets us to the third stage of heat transfer which we call convective stage. During this stage, the heat transfer process occurs by laws of convection and conduction. The heated liquid which is in contact with the body gets moved to a new location and it gets replaced by new fresh liquid whose temperature is considerably lower. This transfer of liquid based on temperature variation happens by Law of convection.

Mostly liquid medium used is Brine solution and water. 

Brine quenching is faster than water quenching. Quenching rate is higher because of the removal of the vapor formation phase. Brine solution consists of salts that crystallize on the surface of the metal. Now, as soon as vapors are formed, they get destroyed due to salts present on the surface. This overall improves the quenching steel process. A higher quenching steel rate means a higher amount of martensite. For martensite, the austenitizing temperature is of prime importance. You can study about it in Martensitic Transformation in Steel.

There are two primary functions of Quenching Oils. One purpose of is to facilitate hardening process by controlling heat transfer. Second purpose is wetting of steel part as a whole during hardening treatment for avoiding formation of undesirable phases.

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The properties of quenching oil which are important for consideration are given below;

Commonly used vegetable oil used for cooking have attracted much attention as quenching media. The mechanical properties of material being hardened like tensile strength, hardness vary with oils used.

Oilve Oil lowers hardness. Same goes for plam kernel oil. These oils, on the other hand, improves toughness giving better impact energy.

Selection of these oils depends upon quenching temperature selected.

The low cooling rate can be used if we have low carbon in the steel. With high carbon, there is more martensitic formation meaning more stresses. High stresses may lead to quenching crack in samples. To avoid quench cracks, the cooling rate needs to be controlled according to requirements. Air quenching can be used in the case of high carbon steel but this may cause oxidation

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You can study about quenching Oils at AZOM.

Experimentation (quenching media)

For understanding the effect of cooling media, we took here samples of steel quenched in water, air and brine solution. The steel used in this case is AISI 1045. Steel is heated in a box furnace at 870-880oC for complete austenitization. This is essential for complete martensitic transformation. Samples were quenched in brine, water and air. Microstructural characterization and hardness of samples were carried out for understanding.

Microstructural Chracterization

Brine Quenching

Brine quenching steel microstructure
Optical Micrograph at 400X, AISI 1040 steel, dark region depicts martensite and light region depicts retained austenite.

Water Quenching

Water quenching steel microstructure
Optical Micrograph at 400X, AISI 1040 steel, dark region depicts martensite and light region depicts retained austenite.

Oil Quenching

Oil quenching steel microstructure
Optical Micrograph at 400X, AISI 1040 steel, dark region depicts martensite and light region depicts retained austenite.

Air Quenching

Air quenching steel microstructure
Optical Micrograph at 400X, AISI 1040 steel, dark region depicts pearlite and light region depicts ferrite.

Hardness (Quenching Media)

Hardness is calculated using the following formula and the results are tabulated as

Quenching media hardness effect

Comparison of hardness values of quenching steel in different quenching media are shown in table:

State of SampleLoad (P)Diameter of Indenter (D) (mm)Diameter of indent (d) (mm)BHN (kg/mm2)
Before Heat treatment3000103.5302
Brine Quenching3000103415
Water Quenching3000103.35330
Oil Quenching3000104.3197
Air Quenching3000104.9149

Conclusion (Quenching steel)

After seeing hardness results, we can have clear idea that brine quenched sample is hardest among all. High heat transfer rate due to absence of vapor blanket stage is main cause of high hardness of 415 BHN 10/300/10. In Brine solution, cooling rate was higher than critical cooling rate required for Ms, and this may cause martensitic formation.

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Water quenched sample has slightly low hardness as compared to that of brine solution. Since, vapor blanket stage appears in this case so cooling rate is lower than that of brine solution. In water quenching, retained austenite forms more than it forms in case of brine solution due to vapor blanket stage.

In oil quenching, the heat transfer rate is less than the heat transfer rate in brine and water due to enhanced viscosity of oil due to which Ms line went downward and the amount of martensite produced is lowest than the above two and some amount of lower bainite was formed due to continuous slow cooling. Hence the hardness was ranked third.

Now finally coming to air quenched sample, the cooling rate was too much low. So the transformation line was crossed leading to the formation of ferrite and pearlite. But percentage pro- eutectoid ferrite nucleated was less than that of equilibrium cooling. So it was the softest of all.