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Home , Heat treating, Polishing (metalworking)

Clean.qxp 9/24/2008 8:06 PM Page 2

CLEANINGPART 1 FOR HEAT Part surface cleanliness is he demands of heat treated should run down the part in a contin- goods having the highest uous unbroken film, which indicates becoming a critical quality continue to increase. a water-break free part. A film that is consideration for heat These demands include not interrupted indicates some soil remains T only optimum mechanical and on the surface. treaters as design dimensional properties, but also re- Cleanliness should be determined quire meeting increasingly stringent by the customer. During a cleaner trial, requirements and service visual appearance specifications. Thus, it is good policy to allow the customer applications become more parts cleaners are used in nearly all in- to make the initial comments con- dustrial plants that perform metal- cerning cleanliness. A part that may demanding. The working operations, because almost appear to be dirty to an operator could automotive, aerospace, and all “soils” left of processed parts need be quite acceptable to the final cus- to be removed to some extent. If a tomer. consumer products plant uses coolants, com- industries are leading the pounds, rust preventatives, or heat- Cleaning Process Considerations treating oils, then it will likely have a To establish quality cleaning of parts, way in requirements for the strong requirement for cleaning. all process variables should be consid- highest quality in terms of Cleaning is the removal of dirt and ered, which includes previous soil from a surface. A cleaner is a for- processes and soils from previous surface cleanliness. mulated chemical system or product, processes. The subsequent process dispersed in water, which removes un- should also be considered, as residues wanted dirt, soil, grease, and other sim- and soils from the previous operation ilar matter from a surface. may not be tolerable. The washer ca- D. Scott MacKenzie, FASM*, Cleanliness is a relative term. The pability and capacity should be under- Mark Fretz, total process dictates the degree of stood in terms of the types of soils it is and Don Schuster cleanliness. For example, a part is capable of removing. The existing Houghton International Inc. being cleaned during various states of water quality should be taken into ac- Valley Forge, Pa. manufacture to remove chips or ma- count, and water treatment should be chinery fluids. The cleaner leaves be- considered if necessary (excessive hind a residual rust protective film, water ). The biostability of the and although there is residual film on cleaner should be reviewed to avoid the part, it is considered clean. In the the growth of bacteria and fungus and case of , blackening, or to avoid the use of biocides and fungi- enameling, the part must be chemically cides. Finally, the part orientation clean, often referred to by the term should be considered including exam- “water-break free.” After cleaning, if a ination of nesting of parts and the ori- part is rinsed in clear water, the water entation of the spray nozzles with re- lationship to the parts. Residues remaining after cleaning should also be considered; they should not cause smoking in operations, should not cause flash rusting, and should not result in discoloration of the parts. Figure 1 shows a typical cleaning process schematic for .

Types of Soils The soil is an important factor in de- termining the type of cleaner to be used, as well as conditions for cleaning. Soils can be broadly classified into six groups: • Pigmented drawing compounds. The presence of these pigments makes cleaning difficult, especially after the heat generated by a drawing operation *Member of ASM International and member, Fig. 1 — Typical cleaning process schematic for and the impacting of pigment into the ASM Heat Treating Society heat treating. metal. Mechanical cleaning methods 30 HEAT TREATING PROGRESS • OCTOBER 2008 Clean.qxp 9/24/2008 8:07 PM Page 3 TREATING such as ultrasonic, electrolytic, and spray cleaning aid in the removal of such compounds. • Un-pigmented drawing compounds, oils, and greases. Hot alkaline cleaning normally removes such soils. lf high viscosities are encountered, pre- cleaning in a solvent or solvent emul- sion cleaner will dissolve some soil and lower the viscosity, thus enabling the alkaline cleaner to perform more effi- ciently. • lubricants. Soluble oils and synthetic and emulsion cutting fluids are readily removable using syn- thetic and alkaline cleaners. The pres- Fig. 2 — Dried yellow solid material on a “cleaned” part after heat treating resulting from silicated ence of sulfur and chlorine compo- cleaner residue remaining on the part before heat treating. nents may make the soil more difficult to remove using alkaline cleaners, and finely divided particles can make soil solvents or emulsions are sometimes removal very difficult to remove. necessary. The presence of fatty acids • Corrosion or oxidation in the cutting fluids promotes better products in the soil. Proper cleaning through reaction with the al- neutralization of these soils is kaline cleaner. important so damage will • and buffing compounds. not occur to the part. Buffing compounds in general are dif- Cleaners work by both a ficult to remove because of the heat chemical action and a phys- generated during the buffing opera- ical action. The chemical ac- tion. Aging and over-buffing should tions are reaction, dissolving, be avoided. Aging causes polymeriza- emulsification, and wetting. Some tion, and over-buffing leaves excessive soils are removed by reacting amounts of compounds on the work- with the cleaner involved. For ex- Fig. 3 — Discoloration (chemical burn marks) on piece. Products containing high de- ample, alkaline cleaners will react with “cleaned” part after tempering resulting from grees of detergency and solvency are fatty soils to form soaps, and these residue remaining on the part from a highly normally recommended. soaps aid the cleaning process. Other contaminated cleaning solution. • Carbonaceous soils. Dry, finely di- soils are water soluble and will be re- vided particles are some of the moved by being dissolved in the In many cases, it is necessary to re- more difficult soils to remove. Direct cleaner. An emulsifying cleaner works move soils after one op- agitation, such as high pressure spray by dissolving oils and greases and eration and prior to another. An ex- impingement, sometimes removes a milky white emulsion. A ample would be if a part was such soils. An emulsion cleaner is often wetting agent, or surfactant, a key in- processed through a grinding process effective as is ultrasonic cleaning. gredient in a cleaner solution, functions and was scheduled to go into heat • Rust, scale, and paint. Removal of by lowering surface tension, thus al- treating. After grinding, the part would these soils is considered a specialty lowing cleaner solution to penetrate have grinding fluid and metal fines on area. High caustic alkaline cleaners between the soil and the surface. the surface, which would cause prob- may be used for removal of light rust, The physical actions are impingement lems in the subsequent heat treat paint, lacquer, etc. Heavy scale must and agitation. Impingement is involved process. Therefore, an in-process be removed using acids. Paint removal in spray cleaning processes. The part cleaner would be used. from aluminum and zinc metals must is hit by a cleaning solution flowing A finish cleaning operation is a be accomplished using a paint stripper. through a series of spray nozzles at process where the part goes through a Besides the type of soil considered rather high pressures. This scrubbing complete cycle such as painting, above, there are additional soil vari- action is critical to the removal of soil , enameling, , etc. ables that should be considered to de- from the surface. Agitation is found Cleaning is of the utmost importance termine the proper method of cleaning where soak cleaning is used. A soak for these operations. The part must including: cleaner depends solely on detergency have a water-break free surface prior • Physical form of the soil. This in- for removal of soil. Thus, agitation in to the application of a coating. cludes the viscosity of the and the form of air or steam being bubbled Rehabilitation cleaning is the the of any solids. through the solution is extremely cleaning of parts so they can be put • Fine particulate. The presence of helpful in soil removal. back in service. Removal of rust, paint, HEAT TREATING PROGRESS • OCTOBER 2008 31 Clean.qxp 9/24/2008 8:07 PM Page 4

and grease are examples of the soils probably insignificant. This appears to being removed. A typical application be a cleaning related issue, rather than is the cleaning of and diesel en- a oil problem. gines. Alkaline residue. A bolt manufacturer had a series of customer rejects for Typical Cleaning Problems white residue in the threads. The white Cleaning problems usually manifest material (Fig. 4) was analyzed via re- themselves after the tempering oper- flectance FTIR and identified as prima- ation. Typical problems are rinse stages rily carbonate residue. Investigation of being contaminated, poor cleaning due the rinse tank found that high car- to nesting of parts, alkaline residues bonate concentrations were present, from poorly rinsed parts, rust and which is drag-out from the cleaner op- caustic burns, and excessive staining eration. The customer was instructed from abused quench oils. to dump the rinse tank, and to check Rinse stage contaminated. After a re- the alkaline levels in the rinse tank on cent shutdown, parts from a furnace a regular basis. were showing evidence of a dried Caustic burns. A customer manufac- Fig. 4 — White residue (carbonate) on heat treated bolt threads resulting from high carbonate yellow solid material on the “cleaned” turing bolts had a series of rejections concentration in the rinse tank dragged out on the part Fig. 2. The parts are heat treated, from his customer for rusted and oxi- part prior to heat treating. quenched, and then cleaned and dized parts (Fig. 5). A package con- rinsed. The cleaner used was a silicated taining various sizes was submitted to cleaner, and the machining is being the Houghton Cleaning Laboratory for done using a chlorinated oil that is evaluation. Each bolt had an uniform, made up on site from spend hydraulic even black oily finish (from the rust fluid. The dried yellow material was preventative), but there were round scraped out of the grooves and ana- orange areas visible. In addition, lyzed via KBr pellet-FTIR. It was iden- round areas that were bluish in color tified as probable inorganic silicate and were observed. The incoming bar . Because the parts were stock is coated with calcium stearate rinsed after the cleaning operation, it (to aid in cold heading and was believed that the rinse station had threading). Parts are cold headed an excessive concentration of silicated using chlorinated lube, washed with cleaner from the cleaning cycle. caustic cleaner, then go directly to heat Poor cleaning due to nesting of parts. treating (no rinse), quenched in a cold After the tempering operation, parts quench oil, washed in an alkaline Fig. 5 — Caustic burn on heat treated part were found to be discolored (Fig. 3). cleaner, blow-off (no rinse), tempered resulting essentially from caustic cleaner and/ The discoloration matched the outlines at up to 1100°F, and then dipped (hot) or chlorinated material and/or calcium of adjacent parts. The “cleaned” part into an aqueous rust preventative, compounds burned into the during the was rinsed using ethyl ether, which which flashes off the water. The red heat treating process. was then evaporated. An oily residue spots are then observed. remained. Fourier transform infrared After degreasing using ether in the spectroscopy (FTIR) showed it is an laboratory to remove the rust preven- ester (possible vegetable oil) with an tative, the bolt was noticeably covered apparent high chlorine level. The with round spots, and several red areas cleaned part shows that the rinse stage as described by the customer. The must be highly contaminated with spots are caused by a condition known cleaner, and also with oil. This dries on as “caustic burn.” Essentially, caustic the part, and at heat-treating temper- cleaner and/or chlorinated material atures, fuses and leaves chemical burn and/or calcium compounds are marks as shown Fig. 3. The solids from burned into the steel during the heat the quench oil were collected and were treating process, leaving spotty alka- found to contain 7% Fe, 2% Ni, 2% Cr, line areas on the surface of the part. 1% Na, 1% Al, 2% Si, 3% S, 10% Ca. The steel is quenched in an oil base This is similar to previously analyzed product, which does not remove the Fig. 6 — Rust on inside of induction hardened part sludge samples, and confirms that caustic material because it is burned resulting from a alkaline cleaner residue parts are not clean, and are leaving onto the surface of the part from the prior to . swarf and “cooked” cleaner residue in high temperatures during austeni- the quenching oil. The oil portion was tizing. The parts probably are further analyzed via FTIR and identified as a impacted in the tempering furnace (al- mixture of mineral oil and ester (esti- though the damage is already present). mated 10%). The ester could be due to Recommendations to the customer hydraulic fluid contamination. FTIR were to thoroughly wash and rinse the analysis of quenching oil from the fur- part prior to heat treatment to elimi- nace shows a slightly elevated level of nate any caustic residues that would oxidation in the quench oil, but it was cause caustic burns on the part.

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Inadequate rinsing. A customer was having rust issues on one of their in- duction hardeners, on one machine only (Fig.6). A quenchant is used, and delivered to the machines from a 14,000-gal central system. All machines use the same quenchant. The rust is always on the inside of the part. Parts are formed using zinc stearate, machined using synthetic coolant, and cleaned using alkaline cleaner and rinsed prior to . The inside of the part was rinsed using a small amount of distilled Fig. 7 — Baked-on quench oil due to contaminated and oxidized quenchant. water. Phenolphthalein was added to this, and it turned a deep red color. The mine if either rinse nozzles were rusted area was rubbed onto a piece of clogged, or the rinse had excessive con- Whatman ashless filter paper, which tamination. More frequent dumping was placed into the energy dispersive of the rinse station was also recom- X-ray fluorescence spectrometer mended. (EDXRF) and analyzed for elemental Excessive staining. A customer was content. In addition to the iron oxide, experiencing severe black streaks on large amounts of sodium, potassium, parts coming out of several furnaces, and sulfur were detected. The inside immediately after quenching, and of the part was coated with alkaline prior to the quench washer (Fig. 7). As cleaner, probably based on potassium a test, parts were hand cleaned in or- hydroxide and sodium sulfonate (or ganic solvents and inorganic cleaners similar chemistry). This caused subse- prior to heat treatment. The quenched quent rusting. It was recommended to parts still had the streaking on their investigate the rinse stage on the dis- surface, even after thorough cleaning crepant induction hardener, and deter- before heat treatment. The part shown

ASM Heat Treating Society furnace donation fires enthusiasm for high school student.

How can we thank you enough for being so generous to our Materials Science and Engineering class here at Westerville South High School? This is definitely Christmas in April. All year, we’ve wished that we could have a furnace just like this one and now you have made our dreams come true!” This is a story about an anonymous Founding Member of the ASM Heat Treating Society who saw an opportunity to reach out to young people with a message that “Materials are cool.” Or its corollary that “Heat treating is hot!” “We put your beautiful furnace to work immediately as our MSE class was working on their raku ceramics. All of the glazes we used contained various metal which reduced to their metals on the surface of the clay.” A small heat treating furnace had been obtained to support the efforts of the ASM Materials Education Society and our Materials Camp and Teachers Camp programs in central Ohio. It was meant to be portable so that it could be lent out to teachers at different schools. But the furnace was too large to be moved and shipped cost effectively. It would have to remain in one place. “With your furnace, we can also work with creating our own design with Nitinol, do a study of the solid changes for a Zn-Al , and make our own glass.” Rather than send the furnace back to its manufacturer, an HTS Founding Member had an idea. “I could write a check to cover the furnace and we would donate it to Westerville South High,” he thought. “Then we could buy another smaller unit that could be used by teachers at other schools.” A few phone calls were made, a check was sent and, almost immediately, the donated furnace was being used by eager students. “We are the first Materials Science and Engineering class in central Ohio, and I plan on doing everything I can to help this to spread. Our first year has been highly successful and as a result, we have at least four students who want to pursue MSE in college.” Through this initiative, an individual member of our Heat Treating Society – the proud heat treating roots of ASM – stepped up to join with the ASM Materials Education Foundation “to excite young people in materials, science, and engineering careers.” “Thank you for helping us to inspire students to pursue Materials Science and Engineering as a career. And thank you again for your generosity and for valuing the education of our young people so highly!” Very gratefully yours, Elizabeth L. Eddy Westerville South High School Teacher www.asmfoundation.org

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in Fig. 7 was collected immediately Alkaline Cleaners lated into an alkaline cleaner, inhibits after quenching, and was hand cleaned Alkaline cleaners consist of a blend attack of the cleaner on aluminum and using water and isopropanol (a total of alkaline salts (known as builders) zinc. Silicates reduce rusting or of 160 parts were cleaned in this and surfactants. They operate in a pH browning of steel during electro- manner). We were asked to analyze the range above 7. Each salt and surfactant cleaning. Alkaline cleaners com- black residue and determine corrective is added to provide specific cleaning pounded for general use almost always action. properties within economic and other contain silicates due to their versatility. The part was wiped thoroughly limitations. They are the most widely They provide a high degree of alkalinity using a lab wipe and isopropanol to used cleaners due to their ability to re- with little or no attack on the base metal. remove any residual quenching oil. move a wide range of soils at low A 1% solution has a pH of 12.3. The part was then placed into an costs. The builders used in alkaline Phosphates have a major function in EDXRF, and the surface was analyzed cleaner systems are hydroxides, sili- metal cleaners as water softeners. They for contaminants. Sulfur and calcium cates, phosphates, carbonates, borates, also impart alkalinity and peptize the are present at significant levels. Fol- sulfates, and organic chelants. soil, which is the breaking down of lowing this, a piece of laboratory filter Hydroxides provide the highest de- large particles to smaller ones. paper was dampened with iso- gree of alkalinity. They are relatively Carbonates serve as a low-cost source propanol containing a small amount inexpensive and small amounts will of alkalinity, as well as offer some de- of acetic acid. This was vigorously suffice to give the appropriate amount gree of water softening. They are poor rubbed on the black lacquered area cleaners, but supply a fair amount of al- and some of the black material was re- kalinity. However, the major function of moved. The paper was analyzed in the Cleaning is the removal carbonates in metal cleaners is to aid in same way (EDXRF). Calcium, sulfur, obtaining a free flowing, noncaking mix. iron, manganese, and chromium are of dirt and soil from Sodium carbonate, or soda ash, is by far all prominent elements in the analysis. a surface. the most widely used of the carbonates. The presence of calcium and sulfur Because of its low cost, it is used in some in the stain (after removing residual A cleaner is a formulated degree in almost all formulations. A 1% quenching oil) shows that the material chemical system or solution has a pH of 11.3. is baked-on quenching oil. What is also Borates are used for the development interesting is that the staining shows product, dispersed in of cleaners at lower pH ranges and also the progression of the vapor phase water, which removes for rust protection. A 1% solution of 10 during quenching. The mar-tempering ml sodium tetraborate (borax) is 9.2 quench oil was analyzed and found to unwanted dirt, soil, (pH?). be severely oxidized, having a exces- grease, and other Sulfates such as sodium sulfate is sively high Precipitation Number and used as filler. However, we choose to Total Acid Number. Because of the ex- similar matter use sodium carbonate for this purpose tent of the oxidation, it was recom- since it offers a greater alkalinity. mended to dump and recharge the from a surface. Organic chelants serve to sequester system with fresh quench oil, and to Cleanliness is a magnesium and calcium ions plus var- institute proper maintenance and mon- ious heavy metal ions. They are used itoring procedures. relative term. as replacements for complex phos- The total process phates in low to medium alkaline pH Types of Cleaners ranges. These products find applica- Many types of cleaners are available dictates the degree tions in areas of the country where and selection of appropriate one can of cleanliness. phosphates are not permitted. depend either on the type of soil or on As mentioned previously, builders the type of process washer used (spray, and surfactants comprise the compo- dunk, ultrasonic, etc.). The first classi- of alkalinity. They increase the electrical sition of alkaline cleaners. Builders fication of cleaners is based on chem- conductivity of a cleaner, which is im- alone will not clean satisfactorily. They istry. Alkaline cleaners are used for ag- portant for electrocleaning, and im- need to be combined with a surfactant gressive soils. Neutral cleaners are proves saponification. However, they to perform efficiently. A surfactant has used when a rust inhibitor is required are a poor rinser and don’t clean non- the property of concentrating at sur- and the soil is moderate. Quench oils saponifiable soils very well. A one per- faces because it contains two separate are generally considered to be mod- cent solution has a pH of 1.3. Both portions: one soluble in oil and the erate soils, unless the oil has deterio- sodium and potassium hydroxide are other in water. They have the ability to rated significantly. The use of emulsion used in alkaline cleaners where ferrous penetrate the surface, prevent re-dep- cleaners is usually dictated by the sub- metals are being processed. osition of soil, and to lower surface ten- sequent operation. In this case, an oily Silicates make up the largest portion sion of a solution. Types of surfactants film consisting of a rust preventative of most heavy-duty alkaline cleaners. found in alkaline cleaners are non- is left on the part. Solvent cleaners are Sodium orthosilicate and sodium ionics, anionics, and cationics. used when it is necessary to achieve metasilicate are excellent emulsifiers, Nonionic are the most widely used exceptionally clean parts. Unfortu- are good buffers at pH above 9, hold type due to their versatility. By varying nately, the use of solvent cleaning is soil in suspension, and provide active the nonionic surfactants, different fea- declining because of environmental alkalinity. Sodium metasilicate is the sil- tures can be derived, such as greater considerations, while the demands for icate most commonly used in metal detergency, greater oil , and exceptionally clean parts is increasing. cleaners. This material, when formu- less foam.

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Anionic: The negatively charged par- method for removal of general shop The emulsifiers and detergents in the ticles of the molecule are largest and soils. General cleaning is accomplished system fasten on to the oil and grease most important. These types are par- on the alkaline side. However, acid particles and pull them into solution ticularly effective in removing fatty cleaning is very effective for the re- where they dissolve into the solvents oils. Usually a combination of nonionic moval of a specific soil; that is, oxide. and oils present in the cleaner. An and anionic would be found in a for- This process is generally known as emulsion cleaner is a straight oil mula. pickling. Acid is used for removal of product in its concentrated form and Cationic: The positively charged por- mill scale; scale developed during when diluted in water, forms a milky tion is largest and most important. ; scale developed during heat white solution. Cationics are poor cleaners and are treating; superficial oxide that inter- generally used as germicides and feres with plating, painting, etc.; rust Detergent Cleaners fabric softeners. and corrosion products; and hard Detergent cleaners contain solvent, water deposits. surfactants and water as their major Synthetic Cleaners Acids generally used are sulfuric, components. They differ from emul- Synthetic cleaners are alkaline in pH, hydrochloric, phosphoric, nitric, and sion cleaners in that they are straight but differ from standard alkaline organics such as citric and acetic acid. solutions rather than emulsions. Their cleaners in their chemical composition. The first three are predominant with major uses are as maintenance Whereas standard alkaline cleaners are the others used in certain applications. cleaners for cleaning floors, ma- inorganic in nature, the synthetics are Acid cleaners consist of the base acid, chinery, walls, etc. HTP organic consisting of amine bases. an inhibitor to prevent attack on the These type products are designed to metal surface, and a surfactant to add eliminate alkaline residues encoun- detergency to the package. For more information: Dr. D. Scott MacKenzie, FASM, is Technical Specialist tered in single-stage cleaning opera- - Heat Treating Products, Houghton Inter- tions, and to impart good in-plant rust Solvent Emulsion Cleaners national Inc., Madison and Van Buren protection. They are designed for A solvent emulsion cleaner usually Aves. Valley Forge, PA 19482; tel: 610-666- medium duty cleaning, such as consists of a soil, solvent, emulsifiers, 4007; fax: 610-666-5689; e:mail: smackenzie quench oils, soluble and synthetic detergents, rust inhibitors, and a small @houghtonintl.com; site: www. coolants, etc. amount of water to dissolve some of houghtonintl.com. the emulsifiers. The cleaner’s purpose Acid Cleaners is to dissolve soils and leave a tempo- Part 2 of this article will appear in the an- Acid cleaning is a rather ineffective rary rust preventive film on the work. uary 2009 issue of Heat Treating Progress.

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