Fourth in a series » Among the many specifications applied to finished heat-treated goods, the cleanliness of those goods is increasingly important in appearance, performance, and customer satisfaction. In this series of articles, I’ll break down the why and how of cleaning for heat treatment.
In this article, I will discuss typical cleaning problems that may occur in the heat-treating shop and methods to control them.
Cleaning problems usually manifest themselves after the tempering operation. This is because of any residues left on the parts. Typical problems include rinse stages being contaminated, poor cleaning due to nesting of parts, alkaline residues from poorly rinsed parts, rust and caustic burns, and excessive staining from abused quench oils.
Rinse stage contaminated
After a recent shutdown, parts from a furnace were showing evidence of a dried yellow solid material on the “cleaned” part (Figure 1) after heat treatment. The parts are heat treated, quenched, and then cleaned and rinsed. The cleaner used was a silicate cleaner, and the machining is being done with a chlorinated oil that is made up on site from spent hydraulic fluid.
The dried yellow material was scraped out of the grooves and analyzed via KBr pellet-FTIR. It was identified as probable inorganic silicate and iron oxide. Since the parts were rinsed after the cleaning operation, it was thought that the rinse station had an excessive concentration of silicated cleaner from the cleaning cycle.
Poor cleaning due to nesting of parts
After the tempering operation, parts were found to be discolored (Figure 2). The discoloration matched the outlines of adjacent parts. The “cleaned” part was rinsed with ethyl ether, which was then evaporated. An oily residue remained. Fourier Transform Infrared Spectroscopy (FTIR) shows that it is an ester (possible vegetable oil) with an apparent high chlorine level.
The “cleaned” part shows that the rinse stage must be highly contaminated with cleaner, and with oil. This is drying on the parts and, at heat-treating temperatures, it will fuse and leave chemical burn marks as shown in the Figure 2. The solids from the quench oil were collected and were found to contain 7% iron, 2% nickel, 2% chromium, 1% sodium, 1% aluminum, 2% silicon, 3% sulfur, 10% calcium. This is similar to previously analyzed sludge samples, and confirms that parts are not clean, and are leaving swarf and “cooked” cleaner residue in the quenching oil. The oil portion was analyzed via FTIR and identified as a mixture of mineral oil and ester (estimated 10%). The ester could be due to hydraulic fluid contamination. FTIR analysis of quenching oil from the furnace shows a slightly elevated level of oxidation in the quench oil, but it was probably insignificant. This appears to be a cleaning-related issue, rather than a quenching oil problem.
Alkaline residue
A bolt manufacturer had a series of customer rejects for white residue in the threads. The white material (Figure 3) was analyzed via reflectance FTIR and identified as primarily carbonate residue. Investigation of the rinse tank found that high carbonate concentrations were present, which is drag-out from the cleaner operation. The customer was instructed to dump the rinse tank, and to check the alkaline levels in the rinse tank on a regular basis.
Caustic burns
A customer manufacturing bolts had a series of rejections from his customer for rusted and oxidized parts (Figure 4). A package containing various sizes was submitted to the Houghton Cleaning Laboratory for evaluation. Each bolt has an even black oily finish (from the rust preventative), but there are round orange areas visible. In addition, round areas that were bluish in color were observed.
The incoming bar stock is coated with calcium stearate (to aid in cold heading and threading). Parts are cold headed with chlorinated lube, washed with caustic cleaner, then go directly to heat treating (no rinse), quenched in a cold quench oil, washed in an alkaline cleaner, blow-off (no rinse), tempered at up to 1,100oF, and then dipped (hot) into an aqueous rust preventative which flashes off the water. The red spots are then observed.
After degreasing with ether in the laboratory to remove the rust preventative, the bolt was noticeably covered with round spots, and several red areas as described by the customer.
The spots are caused by a condition known as “caustic burn.” Essentially, caustic cleaner and/or chlorinated material and/or calcium compounds are burned into the steel during the heat treating process, leaving spotty alkaline areas on the surface of the part. The steel is quenched in an oil-based product, which does not remove the caustic material since it is burned onto the surface of the part from the high temperatures during austenitizing. The parts are then probably further impacted in the tempering furnace (although the damage is already present).
Recommendations to the customer were to thoroughly wash and rinse the part prior to heat-treatment to eliminate any caustic residues that would cause caustic burns on the part.
Inadequate rinsing
A customer was having rust issues on one of their induction hardeners, on one machine only (Figure 5). A polymer quenchant is used and delivered to the machines from a 14,000 gallon central system. All machines use the same quenchant. The rust is always on the inside of the part. Parts are formed with zinc stearate, machined with synthetic coolant, and cleaned with alkaline cleaner and rinse prior to induction hardening.
The inside of the part was rinsed with a small amount of distilled water and phenolphthalein. It turned a deep red color. The rusted area was rubbed onto a piece of ashless filter paper and placed into an EDXRF spectrometer and analyzed for elemental content. In addition to the iron oxide, large amounts of sodium, potassium, and sulfur were detected. The inside of the part is coated with alkaline cleaner, probably based on potassium hydroxide and sodium sulfonate (or similar chemistry). This caused subsequent rusting. It was recommended to investigate the rinse stage on the discrepant induction hardener, and to determine if either rinse nozzles were clogged, or the rinse had excessive contamination. More frequent dumping of the rinse station was also recommended.
Excessive staining
A customer was experiencing severe black streaks on parts coming out of several furnaces, immediately after quenching and prior to the quench washer (Figure 6). As a test, parts were hand cleaned in organic solvents and inorganic cleaners prior to heat treatment. The quenched parts still had the streaking on their surface, even after cleaning before heat treatment. The part (Figure 6) was collected immediately after quenching, and was hand cleaned with water and isopropanol (a total of 160 parts were cleaned in this manner). We were asked to analyze the black residue and determine corrective action.
The part was wiped thoroughly with a lab wipe and isopropanol, to remove any residual quenching oil. The part was then placed into an Energy Dispersive X-Ray Fluorescence Spectrometer (EDXRF) and the surface was analyzed for contaminants. Sulfur and calcium are present at significant levels. Following this, a piece of laboratory filter paper was dampened with isopropanol containing a small amount of acetic acid. This was vigorously rubbed on the black lacquered area and some of the black material was removed. The paper was analyzed in the same way (EDXRF). Calcium, sulfur, iron, manganese, and chromium are all prominent elements in the analysis.
The presence of calcium and sulfur in the stain (after removing residual quenching oil) shows that the material is baked-on quenching oil. What is also interesting is that the staining shows the progression of the vapor phase during quenching.
The mar-tempering quench oil was analyzed and found to be severely oxidized, with an excessively high Precipitation Number and Total Acid Number. Because of the extent of the oxidation, it was recommended to dump and recharge the system with fresh quench oil, and to institute proper maintenance and monitoring procedures.
Conclusions
In this column, I have illustrated some typical cleaning problems that may be experienced by the heat-treating shop. I highlighted some of the analysis methods used, and the recommended corrective action.
Should there be any questions regarding this article, or suggestions for any further topics, please contact the editor or writer.
