Part surface cleanliness isbecoming a critical consideration for heattreaters as design requirements and serviceapplications become moredemanding. The automotive, aerospace, and consumer products industries are leading theway in requirements for thehighest quality in terms ofsurface cleanliness.

D. Scott MacKenzie, FASM*Mark FretzDon SchusterHoughton International Inc.Valley Forge, Pa.

*Member of ASM Internationaland member, ASM Heat Treating Society

he demand for heat treatedgoods to have the highestquality continues to increase;not only optimum mechanicaland dimensional properties, but

also meeting increasingly stringent vi-sual appearance specifications. Thus,parts cleaners are used in nearly all in-dustrial plants that perform metal-working operations, because almostall “soils” left of processed parts needto be removed to some extent. If aplant uses coolants, drawing com-pounds, rust preventatives, or heat-treating oils, then it will likely have astrong requirement for cleaning. Atyp-ical cleaning process schematic for heattreating is shown in Fig. 1.

Part 1 of this article (October 2008HTP) reviewed the types of soils, typ-ical cleaning problems, and types ofcleaners. This part addresses themethods of cleaning. Washer capabilityand capacity should be understood interms of the types of soils it is capableof removing.

Methods of CleaningWhile there are many different types

of equipment and numerous methodsavailable to remove soils, they can allbe classified as modifications of a fewbasic types including spray cleaning;power washers; soak tanks; andtumble, ultrasonic, and electrocleaning.

Spray cleaning (power wash). Powerwash spray cleaning is probably themost widely used method for soil re-moval in industry today. A powerwasher is an effective simple piece ofcleaning equipment. It contains a reser-voir for the cleaning solution andpump and piping to deliver the cleanerto the spray nozzles in the cleaningchamber. The cleaning solution im-pinges the soiled part causing a highdegree of agitation. It is important thatthe spray nozzles are placed in thecleaning chamber at the proper anglesand distances to obtain optimum im-pingement. The greatest advantage ofspray cleaning is the decrease in pro-cessing time that can be achieved.

Power washers may contain fromone chamber or stage up to five stages:

• One stage: Parts enter stage onewhere it is cleaned and exits washer;usually air blow off at exit point.

• Two stage: Parts enter Stage l

where they are cleaned. Stage 2 is nor-mally a water rinse. Avariation of thiswould be to use the cleaner in bothStages l and 2 where an in-house rustpreventive coating is desirable.

• Three stage: Parts are cleaned inStage l, rinsed in plain water in Stage2, and a rust-preventative is applied inStage 3.

• Five stage: This type of washer isfound in phosphating lines. Parts arecleaned, rinsed, and phosphated,rinsed, and sealed or coated in Stages 1through 5.

Cleaners used in power washers areformulated with low foaming ingredi-ents due to the high degree of agita-tion. An example of a spray washer isshown in Fig. 2.

Soak tanks. This type of cleaning isthe oldest, most economical methodused, and also has the advantage ofbeing able to accommodate parts ofunusual shape. Most soak tanks areequipped with a skim trough so thatfloating soil and debris can be removedby overflowing the solution by addingwater. Agitating the solution usingpumps, agitators, or air mixers im-proves soak-tank efficiency. Often, theagitation is augmented by a mecha-nism to raise and lower the work in thecleaner solution. Good oil skimmersmust be used in this type of washer, orthe parts washer will merely recoat theparts when they are extracted from thecleaner solution. An example of a soak

38 HEAT TREATING PROGRESS • JANUARY/FEBRUARY 2009

CLEANING FOR HEAT PART 2

T Machining

Austenitize andquench

Temper

Clean and rinse

Clean and rinse

Rust prevention

Fig. 1 — Typical cleaning process schematic for heattreating.

Clean.qxp 1/6/2009 11:01 AM Page 2

tank-type washer with vertical motionof the parts is shown in Fig. 3.

Tumble cleaning. Small, irregularshaped parts present a special problemin metal cleaning. They are not cleanedefficiently in immersion tanks becauseof “nesting” of the parts, which pre-vents contact with the cleaner. Theycannot be economically racked forspray cleaning. For these parts,cleaning in tumbling or rotating bar-rels is a logical answer. The barrel ispartially or completely filled with thealkaline cleaner, and the tumbling ac-tion, plus the rubbing of one part overanother, provides agitation. A typicaltumble cleaning washer is shown inFig. 4.

Ultrasonic cleaning. The value of ag-itation in cleaning cannot be over-stated, especially in the removal of soilscontaining finely divided solids. Amethod of high level agitation is avail-able through the use of high-frequencysound above the frequencies audibleto the human ear. Agitation is devel-oped by a mechanism of cavitation, orcollapse, of gas bubbles formed by thecompression and expansion of theliquid. Lesser agitation is also providedby movement of the solution under theinfluence of waves generated by thehigh-frequency vibrations. The cavita-

tion method produces bursts of veryhigh, instantaneous hydrostatic pres-sures, which literally explode adheringparticles from the solidsurface.

Major types of sound-generatingtransducers or vibrators are magnetostrictiveand piezoelectric devices. Commer-cially available equipment can be adjusted to desired frequency, and is designed to yield a rated maximum amount of power. The amount of power is significant;normally 3 W/in.2or greater is recom-mended. Removingthe bulk of the soilbefore ultrasonic cleaning is recom-mended for reduced cleaning time. Alarge untrasonic tankfor cleaning of aerospace componentsis shown in Fig. 5.

Electrocleaning. Most electrocleaning

HEAT TREATING PROGRESS • JANUARY/FEBRUARY 2009 39

TREATING

Fig. 2 — Example of spray washer with parts conveyor and fume hood. Courtesy of Guyson Corp. Fig. 4 — A tumble-type washer for small parts. Courtesy of Ransohoff Corp.

Fig. 3 — Example of soak tank type washer with vertical motion of parts for agitation. Courtesy of Ransohoff Corp.

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follows other cleaning steps prior toplating. The part to be cleaned is madean electrode in a solution (the cleaner).Upon passage of direct current, wateris electrolyzed into hydrogen gas at thecathode and oxygen gas at the anode.The generation of gas in large volumesprovides a high level of agitation. Amajor reason to use electrocleaning isto remove smut, which, if not removedwill impair the quality of electro-plating. To blast off smut, it is desirableto have gas generation as great as pos-sible. Therefore, cleaners of high con-ductivity are used at high concentra-tion (10-16 oz/gal) and at elevatedtemperatures 180 to 200°F. These pa-rameters keep the internal resistanceof the bath at a minimum and permit

development of maximum currentdensity.

Factors Affecting Industrial CleaningWith few exceptions, there are cer-

tain principles that apply to all typesof cleaning including:

• Increased temperature usually im-proves cleaning.

• Agitation to move the soil (ratherthan the cleaner) improves cleaning.Agitation to move the cleaner is im-portant when the layer of cleaner nextto the surface has become heavily con-taminated with soil or has cooled off.A minimum concentration of cleaneris needed for cleaning; above this level,cleaning improves with increased con-centration, but each increment of

cleaner has a lesser effect than the pre-vious. A point of diminished returnsis eventually encountered.

• Adequate time must be providedfor detergency, or reaction of cleanerwith soil. Otherwise agitation or me-chanical removal effects become moreimportant.

• Rinsing away of soil and cleaneris necessary and must take into con-sideration factors such as the amountthat can be left behind without harm,how much cleaner residue can be toler-ated. Pressure rinse water or an agi-tated rinse is far more effective than astill rinse.

• Soil must be kept from rede-positing on the work. This might takethe form of cleaner components to sus-pend the soil, or design of the vesselholding the cleaner to provide roomfor soil to settle to the bottom (awayfrom the cleaning area), or an overflowfor oil to float away from the surfaceof the rinse.

Other ConsiderationsConcentration. The concentration nec-

essary for good cleaning is geared tothe method of agitation. Thus, disre-garding any foam problems, it is pos-sible to get similar cleaning from thesame alkaline cleaner at 8 to 12 oz/galby soaking, 0.5 to 1 oz/gal by spraying,0.1 to 0.5 oz/gal by steam cleaning,and 2 to 4 oz/gal in a high-pressurewater spray at room temperature.

Time required for cleaning. This is de-termined by the conditions of temper-ature, soil, concentration, agitation, etc.Note that it does take some time forthe detergency process to occur. In astatic bath, cleaning may take 5 to 15min; in a high-pressure spray of qualitydetergent, it may take a few seconds.It is expensive in equipment, labor, andchemicals to reduce cleaning time, andthere are situations in which the ex-pense is not justified.

The time available for cleaning isvery closely related to the economicsof the cleaning operation not only froma direct production viewpoint, but alsoon the basis of time lost in recleaning,rejects, or customer complaints wheninadequate cleaning results from an unsuccessful balance of cleaningfactors.

Temperature. The effect of tempera-ture depends on the type of soil andcleaner. A first consideration is thatenough temperature to melt a soil thatcan be melted (e.g., a fat, grease, orwax) makes a tremendous differencein the rate of cleaning. For most oils,

40 HEAT TREATING PROGRESS • JANUARY/FEBRUARY 2009

Control Procedures for WashersIt is important that the washer system be properly monitored to achieve

the desired cleaning. A disciplined approach to washer and cleaner mainte-nance is critical if quality cleaning is to be achieved. This can include daily,by shift, and weekly. This is in addition to the routine preventative mainte-nance done on the quenching system and quench oil.

Daily or shift control measures (depending on use):• Concentration determination by titatration.• Verification of system pressures and temperatures.• Verify cycle time.• Visual verification that the oil skimmer or overflow is working.• Check filtration equipment (screens or filters).• Rinse stations require the same tests. If an air blow off is used, proper

operation should be verified to ensure that all nozzles are operating correctly.

Weekly control measures:• Inspect exhaust equipment for proper functioning.• Visually inspect nozzle arrangement for misalignment or improper

functioning (clogged nozzles, etc.).• Interior spray guards for proper location and function.

Fig. 5 — Ultrasonic

cleaner tankfor cleaning

of aerospacecomponents.Courtesy of

RansohoffCorp.

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Should there be any questions on this paper or anything else in the HOUGHTON LINE, please contact your local Technical Representative

Mark Fretz, CMFS Metal Cleaning Marketing Manager

Houghton International, Inc. mfretz@houghtonintl.com

610-666-4193