False Concentration Issue of THI®F-502 Water-based Quenching Fluid

(1) Why Do Workpieces Still Crack When the THI® F-502 Quenching Fluid Concentration Is Within the Normal Range?

For newly prepared quenching fluid, a refractometer is used to test its concentration. However, after a period of use, the quenching fluid will be contaminated with oil, rust, oxide scale, and other impurities, which have a high refractive index. If the concentration is still calculated by multiplying the refractometer reading by the refractive index coefficient of 2.5 under such circumstances, the resulting concentration value will be higher than the actual one. This leads to misunderstandings about the real concentration among operators and may eventually cause workpiece cracking.

Take a user case as an example: the recommended concentration range of the quenching fluid is 6-8%, and the new PAG fluid was initially used at a concentration of 6%. Half a year later, in winter, the operator obtained a refractometer reading of 2.4, and calculated a concentration of 6% by multiplying it by the coefficient of 2.5. In reality, the quenching fluid had been contaminated, and its actual concentration was only about 4%. Coupled with the low ambient temperature in winter, which accelerates the cooling rate of the fluid, the workpieces either developed cracks or showed a tendency to crack. This situation must be guarded against.

(2) How to Determine the True Concentration of THI® F-502  Quenching Fluid After a Period of Use?

For quenching fluid that has been used for a long time and is heavily contaminated, a cooling characteristic tester should be used to measure its cooling rate first. Then, the measured cooling rate is converted into the true concentration value, which is further used to calibrate the daily refractive index coefficient.

Specifically, the cooling curve of the existing working fluid is tested first. Based on the measured cooling rate at 300℃ and the maximum cooling rate, the current concentration of the quenching fluid is deduced by referring to the corresponding concentration-cooling rate standard table. After that, the new refractive index coefficient of the product is obtained by dividing the deduced true concentration by the current refractometer reading. This new refractive index coefficient can be used as a reference for self-testing the concentration in a short period.

(3) Auxiliary Self-Testing Method for Concentration in Emergency Situations

In emergency scenarios where the true concentration of the quenching fluid needs to be determined immediately, the following method can be used for a rough self-test of the PAG quenching fluid concentration. This method may have errors and is for reference only.

First, calibrate the refractometer to zero with tap water.

Measure the total refractometer reading (N_total) of the quenching fluid.

Take 250ml of the quenching fluid with a 300ml beaker, heat the fluid to above 100℃, immediately remove it from the electric furnace, and let it stand for 20 minutes until all PAG settles at the bottom of the beaker and the upper layer becomes a clear liquid.

Use a pipette to slowly draw half a tube of the clear liquid from 1cm below the liquid surface, and measure its refractometer reading (N_contamination).

Calculate the actual effective concentration (C_true, %) of the quenching fluid using the formula:

Tags:False Concentration, Water-Based Quenching Fluid, PAG Quenchant, Heat Treatment Problem, Quenchant Maintenance, THIF-502, Honorhonest, Refractometer, Quenching Quality