Nov_Dec_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 0 6 2 where quenchant concentration is critical to maintain via- ble heat-treating process. The CQI-9 specification for moni- toring quenchant concentration is once per day under nor- mal operating conditions. REFRACTOMETRY ON THE SHOP FLOOR There are several methods to determine quenchant concentration in aqueous quench media. The most accu- rate method to determine quenchant concentration is by kinematic viscosity. However, this method is usually con- ducted in a laboratory environment and not applicable to a shop floor manufacturing environment. Handheld refractometry developed around the diffu- sion of light caused by the concentration of sugar in water (Brix scale) has been the best option for measuring quen- chant concentration on the shop floor. Each quench has a refractometer adjustment based on its reflectivity relative to the Brix scale. The quenchant mentioned earlier, Aqua- Quench 245 has a scale factor of two. The actual concen- tration of quenchant would be two times the reading on the Brix scale. Light passing through a thin layer of quench media laid on a prism reflects light based on its purity to water (Fig. 5). The view on the scale between the unabated re- flected light and the diffusion from the quenchant can be clouded by extraneous contamination from pre-heat treatment processing fluids such as machining coolants and cleaning solutions if the parts are not washed prior to heat treatment. Dealing with unwanted contamination while taking a sample can be tricky. Most handheld refrac- tometers come with an eye dropper or pipet so that the surface of the quenchant can be penetrated and a sample taken from the solution of the actual quenchant. For ex- ample, upon sitting, any oil contamination will generally rise to the top of the quenchant solution due to the fact that the specific gravity of oil is lighter than that of water. There can also be some slight emulsification of oil in the quenchant solution. The principle of subsurface sample taking is sound, however, in practice sample preparation normally happens with a quick dip of the index finger and then catching the drip on the lens of the refractometer. Figure 6 shows a typical Brix reading from a handheld refractometer. The visual indication on the Brix percentage almost touches 10 while it shows darker blue up to 12. Each auditor may have their own standard for reading the scale, light or dark, touching or not, etc. It is an unavoidable characteristic of human intervention and one that has no place in precise concentration control. Compound human error by the correction factor of two times to convert to the actual quench concentration, and the standard error rate for the reading grows to almost 4% concentration. This fluctuation in con- centration has quite a drastic effect on cooling in the critical range of 800-1200 ° F as previously discussed. Fig. 6 — Typical Brix reading from a handheld refractometer. Fig. 7 — The principle of digital refractometry. Courtesy of Control Logic. Fig. 5 — Principle of refractometry. 8