AJAX REPRINT No. 182

An Introduction to SALT BATH HEAT TREATING

by Q. D. Mehrkam V. P. Research 8 Development AJAX ELECTRIC COMPANY Philadelphia, Pa.

Reprinted from TOOLING & PRODUCTION Magazine, June & July, 1967 Figure 1 Rate of hearing curve shows the time required to heat a part of given dimensions.

Part I Salt bath heat treating

WHEN WORK is fully immersed ening) to the quench operation. into a salt bath furnace, no air Conduction heating. When the can contact it. Steel scaling, oxi- by Q. D. Mehrkam work is immersed, heat is trans- dation and decarburization can be V. P. Research & Development ferred by direct contact with a avoided. It is common to process AJAX ELECTRIC CO. heating medium of high capacity. low, medium, and high-carbon Philadelphia, Pa. This is 4 to 6 times faster than steels in the same medium without possible in radiation or convection surface carburization or decarburi- type methods. For example, the zation. This is particularly advan- time required to heat a part of tageous for a variety of batch-type given dimensions, at 1500°F,* the heat treating work. A thick salt rate of heating is approximately film adhering to the surface serves as protection which continues when the work is removed and *All temperatures in this discussion are transferred (in the case of hard- Fahrenheit. Immersing metals, ceramics, or plastics in a molten salt makes possible unique characteristics.

5 min/in of thickness or ¼ the time bath’s composition consists of one Design features or more salts, nitrates, chlorides, required in radiation type equip- Furnace size and shape can be carbonates, cyanides, caustics, or ment, see Fig. 1. almost anything desired. One de- additives in small amounts. The Automatic preheat. Upon immer- sign uses a metal pot for temper- additives can be sulphates, fluo- sion into molten salt, a frozen salt atures up to 1750°. With this rides, etc. cocoon forms around the part. arrangement, electrodes must Heat is transferred between Layer thickness and the time re- enter from the top and are usually work and molten salt by conduc- quired to melt it depend upon size located along the back wall free of tion. The salt lends itself to either and shape of the part, bath oper- o the work area, see Fig. 2. All ex- heating or cooling within a 300 ating temperature and salt melting o ternal walls are heavily insulated to 2400 range, as required. Com- point. As a result, the work is sub- and radiation losses from surfaces patible salt mixtures are selected jected to an automatic preheat are minimized by a suitable cover. with proper operating ranges and which prevents excessive heat The metal pot in this type furnace often are formulated to meet spe- shock and distortion damage of is used whenever salts contain cific temp-range requirements. cracking. Furthermore, heat is appreciable amounts of cyanides, transferred to the work over its carbonates, and caustic soda, or for entire surface uniformly. One side Operating principle low temperature applications using will not heat faster than the other. nitrates and nitrites. As the part gradually heats to the The electrode furnace generates A different type furnace is pre- salt’s melting point, the cocoon direct heat in salt by using its re- ferred for higher temperature o o melts leaving only the molten salt sistance to current passage. Salts, applications in the 1200 to 2400 while insulators in the solid state, contacting the work. It is impossi- range. The pot is of interlocking ble for the work to be higher or are excellent high-resistance con- ceramic tile. Electrodes are re- lower than the bath’s temperature ductors in the molten state. Poten- placed without costly downtime. tial is applied to the molten salt control. The tile is merely raised, the old by use of heavy electrode bars. electrodes removed, the new or Buoyancy. An important property These are connected to the sec- rebuilt electrodes installed, and the in molten salt is molten salt ondary of special multiple voltage tile replaced. weight; it may be 2 to 3 times that air cooled transformers. Electrodes A third type furnace, radically of water. Thus the piece’s im- are located in a recessed area of different from those above, is pri- mersed weight is appreciably de- the bath and cause salt circulation marily intended for cooling rather creased 25 to 35 per cent of the by electromagnetic forces when than heating. The isothermal original. This helps- hold work energized. A strong magnetic field quench furnace is for martemper- distortion to absolute minimum. is created between electrodes when ing, austempering and isothermal located close together. heat treating operations (of which Using Maxwell’s Law, this field Furnace development there are many hundreds). This will cause salt particles between About 30 years ago, the first im- electrodes to move downward in portant salt bath furnace installa- a whirlpool fashion. This electro- tion was made in this country. dynamic circulation assures abso- Since then they’ve gained wide lute temperature uniformity recognition and the number has through the bath and eliminates grown-not only in number but in danger of undesirable differentials. size and capacity. The 5000 in use today range in size to 43 ft L x 20 ft D x 8 ft W. In use, the unit con- tains 300,000 lbs of salt. Many operate with connected loads over Figure 2 Ajax Type H Furnace has a metal 2500 km. pot with electrodes entering from The furnace has a ceramic or the top. This type furnace is pre- metal container filled with molten ferred for cyanide hardening, car- salt. The work can be immersed burizing, descaling, desanding, and for either heating or cooling. The cleaning. is used for diffusing carbon and small amounts of nitrogen into steel surfaces. It will alter physi- cal properties of the surface by materially increasing strength and wear resistance. Liquid carburiz- ing is done in baths, operating at 1450o to 1750o. All carburizing steel grades, carbon and/or alloy, may be liquid carburized with case depths of 0.005" to 0.250". A uni- form carburization rate is con- trolled by maintaining sodium cyanide concentration-the source of carbon and nitrogen, see Fig. 3. Salt samples are removed from the bath daily, and are analyzed for cyanide content. Replenish- ment salt is then added to main- tain cyanide content at the re- quired operating level. Since cy- anide oxidizes in contact with air at these temperatures, the bath operates with a carbonaceous scum cover which serves two purposes: 1) It reduces radiation losses by 50 per cent or more-thus saving power; and, 2) it reduces cyanide oxidation. It’s interesting to note, a car- burizing bath doesn’t fluctuate chemically-radically or rapidly.’ It normally consists of a carbonate, chloride and cyanide mixture. With cyanide oxidation, carbonate is formed and this requires periodic dilution by adding fresh salt. Pro- prietary salt replenishments are available in varying concentra- tions of the above chemicals, to maintain proper chemical balance. Generally, it is only necessary to Figure 3 maintain the sodium cyanide oper- Carburizing curve shows time penetration needed to get desired care depth at various furnace temperatures. ating level. For example, the carbonate con- type furnace will always have a the work into the nitrate-nitrite, a centration in a deep case carburiz- metal port heated by either resist- separating chamber continuously ing bath should not exceed 30 to ance heaters or gas fired immer- clarifies the bath. Chloride salts 35 per cent with a sodium cyanide sion tubes. The external pot sur- settle to the chamber bottom and content of 8 to 12 per cent. Other- face is frequently finned and air are removed periodically. This wise, carburizing potential or the under pressure is circulated type furnace normally operates bath’s ability to do it is adversely o around the outside to extract heat. within 400 to 750 . affected. A higher cyanide con- Both heating and cooling cycles By eliminating pumps, separat- tent will also account for greater are automatically controlled to ing chamber and cooling chamber, breakdown to carbonate. If drag- maintain a constant and uniform the furnace can also be used for out losses on the work surface are o ±10 temperature. heating applications with in the low, it may be necessary to bail a o For quenching, salt must be range of 400 to 1100 . It can be portion of the bath each day to properly agitated. This is done heated by either resistance heaters maintain proper chemical balance with a propeller driven pump or gas-fired immersion tubes. by adding fresh salt. As we point which forces the molten nitrate- out above, chemical change is not nitrite salt through the quench rapid and considerable variation Applications header. Whenever high temper- can exist without affecting carbon ature chloride salts are carried by Liquid carburizing. This method potential. ure of its effect on the steel sur- Rectifiers incorporating silicon bath quench achieved a consist- face being treated. A more con- carbide, metallic silicon and silica ently higher uniform hardness lev- clusive examination is made by are commonly used effectively and el than obtainable by air cooling. processing steel test coupons, then will carry contamination to the Surface scaling is eliminated examining the surface hardness furnace bottom as sludge. This can since there is no contact with air using micro-hardness or superfi- be removed by desludging tools. while cooling through the oxidiz- cial hardness testers or microscop- While this rectification method is ing and scaling temperature range. ically examining a cross section. effective, it’s a disagreeable task. Since a better surface is obtained, Besides oxidants, the chloride salts In a large operation, it could in- less grinding stock is removed can be contaminated with other volve considerable time for the after hardening - thus reducing materials such as contaminated maintenance crew. finishing cost. work and fixtures in the bath. However, a “Chill Block Meth- An all-chloride hardening sys- This is particularly true if the od” is available, where a heavy tem is preferred for high-speed surfaces haven’t been properly steel section is quickly lowered to steel hardening installations. The cleaned. Naturally, any contam- the furnace bottom, allowed to preheating furnace (normally ination carried into the furnace remain a few seconds, then imme- 1450° to 1550°F) can be operated will cause costly maintenance for diately removed. Sludge attaches with either sodium and potassium removal. to the cocoon forming on the block chlorides or barium, sodium and To maintain acceptable bath surface and upon removal, the potassium chlorides. The high- neutrality, removing dragout loss sludge is quickly picked up and temperature furnace at 2000 to on the work surface and fresh salt removed. After cleaning, the block 2350° usually operates with 100 replenishments are usually suffi- is returned as many times neces- per cent barium chloride. The cient. However, where contamina- sary to remove sludge completely. pre-heating salt carry-over into tion is greater or dragout minimal, This method is particularly effec- the bath presents no problem be- a rectification practice may be tive in furnaces 15 to 20 ft deep cause lower melting salts will vol- used. Rectifier selections, either and equally successful for those atilize at higher operating tem- solid or gaseous, are available and only 2 ft deep but 10 to 20 ft long. perature. When transferring from each method is selected to meet high temperature furnace to salt individual requirements. quench, barium chloride is carried Methyl chloride gas rectification High speed tool steel hardening into the bath at about 1000 to o is used in chloride type salts oper- This requires the most exacting 1100 . Since the quench mixture ating at 1400° to 2000°. Its prin- control of all heat-treat applica- usually consists of barium, calci- cipal action is to chemically con- tions. A significant majority of um, sodium and potassium chlo- vert oxides to the original chloride quality tool manufacturers use salt rides, excess barium settles and and remove oxygen from the bath bath furnace treatment. Any type must be periodically removed. as carbon monoxide and carbon high speed steel can be hardened Metallurgical requirements are dioxide. Since by-products are with equal ease and safety merely easily met by quenching into this gaseous, there is no residual con- by a change in furnace tempera- salt since cooling rate exceeds that tamination and the bath can be ture setting. No special provision possible by air. restored to its original composi- is required to adjust bath chemis- On steel sections over 5 or 6”, o tion. However, metallic or non- try for the hardening of high- it is desirable to incorporate a 600 metallic contaminations are not re- chrome high-carbon, moly or tung- quench using an agitated nitrate- moved by this practice. It may be sten type steels. By providing close nitrite salt. This further insures o necessary to resort to use of solid control instruments using either cooling of the work from 1000 to o type rectifiers for more complete time proportioning or stepless con- 600 so that all sections, whether contamination removal. trol, temperatures can be con- light or heavy, will be brought to Commonly used solid-type rec- trolled within ±1o. a lower temperature from which tifiers are proprietary. Use of One customer uses the furnace the austenitic material can be cyanide as the organic Dicyanamid operating at 2200° for treating a safely air cooled to room temper- essentially neutralizes bath oxides Sendzimer Roll, see Fig. 4. The ature. In this case a more uniform by creating a temporary reducing furnace handles work pieces 12” steel transformation from surface action. But this is soon dissipated, dia x 108” long. Rolls to 3200 lbs to center is obtained, with mini- and additional rectifier must be each can be accommodated. Using mum thermal and transformation- added periodically. The main dis- the furnace for preheat, high-heat, al stresses. Also, surface oxidation advantage here is that cyanide quench and draw, the installation is avoided, and higher hardness is eventually decomposes into a car- enabled him to control distortion, usually obtained. n bonate, which is considered con- decarburization, maintain uniform tamination. As pointed out above, high-hardness and meet all rigid with methyl chloride rectification, QC specifications. By quenching the Dicyanamid doesn’t remove directly from the austenitizing solid contamination and if allowed temperature into a bath 1000°, to accumulate beyond a safe limit, then transferring to a second other means must be employed. quench about 600o, the double salt Steels carburize at faster rates 1400o to 1600° range, the salt is a tic so that it is file-hard but flexi- with higher temperatures - and mixture of cyanide, chlorides, and ble. It also permits, in some cases, this is another economy factor. carbonates. Cyanide hardening is severe part bending before case With increased temperature-car- usually applied to obtain a super- fracture. bon diffusion rate also increases. ficially high-quality, wear-resist- Obviously, greater production can ant surface for the more light-duty Neutral hardening be maintained from a given car- applications. burizing furnace size by raising Salt bath nitriding is carried out Hardening ferrous alloys in chlo- operating temperature; for exam- within the 950o to 1050o range ride salts without harmful surface ple: a case depth 0.070” at 1700o using a steel pot container for cy- attack (scaling, pitting, etc.), is requires 8 hrs. At 1800°, the same anide, chloride, carbonate and called “neutral-hardening.” Car- case depth can be obtained in less heated by over-the-top electrodes. bon, alloy, stainless, hot work and than 4 hrs. Thus, with an increase Thirty to 60 minute immersion in high-speed steels are commonly of 100o, capacity has doubled. the liquid nitriding bath is pre- neutral hardened in furnaces oper- Higher production rates are possi- ferred for some high-speed steel ating over a 1400o to 2350o range. ble, obviously; and it is necessary cutting tool applications w h e r e Since the salts provide the imme- to design the furnace with ade- maximum wear resistance is re- diate environment surrounding the quate power to accommodate the quired. Case depth is usually work, attention must be given to required lbs/hr. measured in 0.0001”-0.0005” terms. the bath’s chemical condition to Nitriding bath modifications maintain neutrality. Carburizing, cyaniding and nitrid- have been made to obtain special As the chloride salt is operating ing operations are applied to a effects; for example: by bubbling continuously with air contact, it is variety of work involving use of ammonia through the bath, a subject to contamination and ac- either sodium or potassium cyan- higher nitrogen potential is ob- counts for increase in salt alka- ides as the active agent. For ex- tained. This improves case depth linity. This is measured either in ample, one of our furnaces car- and nitrogen content in the steel. terms of pH or by wet chemical burizes outboard motor crank Adding sulphur compounds, a sul- analysis in terms of per cent sodi- shafts. A 0.045” case depth is ob- furizing treatment avoids seizing um oxide or barium oxide. It is tained at 1700o in 4½ hrs. The and galling. Still another process generally accepted when chloride work is then conveyed through the takes advantage of bubbling air in salt increases in alkalinity, it in- furnace with a push-pull mecha- the bath to purposely oxidize the creasingly tends toward decarbu- nism. Upon reaching the end, it is cyanide to a higher cyanate level. rization. However, alkalinity or removed and allowed to air cool This changes the case characteris- pH of the bath is not a direct meas- to a subcritical temperature. The work is then reheated in a neutral bath at 1500o. There is sufficient cyanide salt carry-over into the neutral chloride reheating bath to protect the surface. The work is then martempered in a nitrate- nitrite salt at 400° in an agitated isothermal quench operating with approximately 1 per cent water addition for increased quenching severity. The work is then allowed to air cool to room temperature and then is drawn at about 300o. After grinding, surface hard- nesses are held within 0.005” run- out. Cyaniding is similarly carried out in a salt bath furnace provided with a steel port and over-the-top electrodes. Operating 15 to 30 per cent with cyanide content, both carbon and nitrogen are added to the steel surface and case depth is usually limited to about 0.010”. Generally, operating within the

Figure 4 A Sendzimer Roll being removed from a salt bath furnace used for high speed tool hardening. PART II Salt bath heat treating

This final part of Mr. Mehrkam’s discussion will cover Annealing, Brazing, Aluminizing, isothermal Quenching, Martemper and Ausforming, and Cleaning.

directly from the bath. The sur- tically for solution annealing in face is scale-free and exhibits only chloride salt. Parts are immersed at 1825o for 30 min, and rods are by Q. D. Mehrkam a slight oxidation responding to V. P. Research & Development mild pickling to prepare for draw- then quenched in water. As AJAX ELECTRIC CO. ing lubricants. The process is done treated, the surface has only a Philadelphia, Pa. at 1250 to 1300o and cycle time is slight oxidation and is completely within 20 to 30 min for 200 to scale-free. 300-lb coils. Nitrate salts provide an excel- Salt baths have also been suc- lent heating medium for solution cessfully used to anneal stainless heat treatment of aluminum al- SALT BATH FURNACES are used and nickel-chrome alloy products. loys. High-heat reflectivity prop- in a wide variety of annealing Both 300 and 400 series stainless erties are no problem since salt and solution heat treating appli- fittings have been annealed within heating is by conductivity as salts cations. We have an installation 1550 to 2100o. Total cycle time contact the surface. Salt heating where low carbon steel wire (C- falls within the 5 to 30 min range. is several times faster than air 1008), in 36” dia x 30” coils, is Work is water-quenched directly furnaces. After a few minutes im- annealed in a nitrate type salt at after removal from the furnace. mersion time, the work is water 1000o, see Fig. 1. It is done in a The surface is slightly oxidized, quenched leaving the surface me- steel pot with over-the-top elec- but it is quickly restored by a tallically clean, while nitrate salt trodes. The furnace is 42” L x short acid dip. oxidation, by air, causes increase 48” W with a 48” salt depth. The Maintaining bath chemical bal- alkalinity in the bath and may connected load is 160 kw. which is ance presents no problem. While cause slight etching. In most ap- capacity rated at 1000 lbs/hr. AS alkaline earth and other metallic plications, this condition is avoided the wire surface enters the salt, it oxides may build up, they are not by replenishing dragout losses has an organic lubricant which harmful to low carbon stainless with fresh salt. Otherwise, ‘the oxidizes and burns away. The re- steels. However, the baths can be salt can be rectified by adding so- sulting surface exhibits a bluish- rectified in the same manner pre- dium or potassium dichromate. black oxide readily removed by viously discussed. mild pickling. When using methyl chloride or Steels of 0.35 carbon or less are cyanide rectification methods, suf- Brazing given a “process anneal” in a car- ficient time should elapse after- Salt bath brazing is a limited but bonate-chloride or tenary salt con- ward to prevent possible carbon attractive method for a number of sisting of barium, sodium and po- pick-up. jobs. All types of brazing materi- tassium chlorides. This treatment Copper and copper base alloys als can be used: silver solder, is applied as a relief to recover are successfully annealed in car- aluminum, brass and copper. Both original ductibility as wire is bonate-chloride salts (1100- ferrous and non-ferrous base ma- o drawn to smaller diameter. Coils 1500 ). Copper-chrome alloy rods, terials can be joined or combina- are either air cooled or quenched 12 ft long, can be suspended ver- tions thereof which influence joint Figure 1 Low carbon steel wire is annealed in this steel pot furnace with over- the-top electrodes. The furnace uses nitrate salt at 1000F.

Figure 2 Typical parts that are aluminized for better corrosion resistance. design and brazing alloy location, the same for salt bath brazing and other methods. Here brazing alloy can be applied in the form of wire, shim, washers, sheets, etc. And, in joint areas which do not permit preplacement of preformed mate- rial, paste form brazing alloy can be applied. Salts are generally a chloride mixture. but also can be modified by adding reactive ingredients. then it is selectively immersed in brazing. The reason brass brazing While salt immersion excludes air a chloride salt at 1600o so that the is not more widely used with other contact and oxidation, it is neces- carbide is exposed above the bath. heating methods is because of sary to provide a fluxing environ- The alloy begins melting in melting instability. Zinc is read- ment to obtain good flow and about 6 to 8 minutes, and the bit’s ily volatilized, when melted, when penetration of the brazing alloy shank is thoroughly austenitized in contact with the atmosphere. throughout the joint area. while the carbide slots are ade- When lost from the base alloy, the For silver solder, brass, and cop- quately flooded with molten silver physical properties of the remain- per brazing, it is not uncommon to solder. The bit is then lifted ver- ing alloy are completely altered. apply a fluoride-borax type flux tically and transferred to a selec- Dezinctification is avoided and to the joint area before immersion. tive isothermal quench where the brass alloy fluidity is easily main- Since these fluxes are usually shank rapidly cools with the car- tained in a neutral chloride or car- water mixtures, it is essential that bide exposed. The alloy is added burizing type salt used to impart a slight carburizing case to the part’s surface. In an application using SAE- 1010 for bicycle forks, for instance, flux is applied by dipping or spraying. Alloy rings of 60-40 brass are located in contact with the joints for brazing. This in- stallation indicates what can be done in a fully mechanized opera- tion using a revolving unit with six stations through which parts pass. They are: 1) Load and un- load, 2) dry, 3) furnace braze, 4) air cool, 5) hot water wash and 6) hot water rinse. The mechanism is operated by a hydraulic cylinder which raises and lowers the central shaft and rotates one position every 3 min. The operator loads 14 assembled forks on each fixture at the first Figure 3 station, with a 30 second interval Photomicrograph shows cross section of a 5/8” diameter between part transfer. In this, the o bolt. Note the uniformity of the aluminum coating. furnace is maintained at 1700 temperature. the flux assembly be thoroughly continuously to the carbide area The forks can also be copper dry before immersion to prevent until the silver solder is complete- brazed. Here the assembly is splattering. Applying flux to the ly solidified. This replaces shrink- fluxed, dried and partially im- joint locally creates a temporary age cavities that tend to develop. mersed in a neutral chloride salt fluxing atmosphere, necessary for For brazing ferrous assemblies, at 2050° for 2 min. Assemblies steel surface etching and oxide the trend has been toward the use are then air cooled to about 1500° contaminant removal, just prior to of brass alloys, such as 60-copper for 15 to 20 seconds, then water wetting and flow of the brazing 40-zinc. Compared to silver sol- quenched followed by a hot water alloy. The flux is normally dis- der, brass is only a fraction of the wash. The operation is semi-auto- placed by the alloy and dissipated cost. Also, brass brazing is done matic, by a push-button causing into the molten chloride salt. Since in the 1675 to 1750o range. When the fixture to be raised and low- these fluxes are flushed away from replacing copper, it eliminates ered for a specified time cycle. the part surface, there is no clean- grain coarseness and requires only An interesting modification ing problem. half the power. Brass alloys ac- combines carburizing and brass Carbides are successfully silver- commodate a greater variation in brazing in one step. A carburizing solder brazed to 9” dia medium joint tolerance without strength bath provides an excellent self- alloy steel drill bits. The bit is loss; clearances to 0.010” have fluxing heating medium, simulta- held vertical with the carbide been brass brazed successfully, al- neously giving a superficial carbu- end up. And the carbide is gen- though this is not recommended. rized surface for wear resistance. erously coated with a brazing flux. Brass brazing with a 60-40 type A good example is a pair of vise- The silver solder alloy and carbide will yield average shear stress val- grip pliers, where all component is then inserted. The bit is pre- ues within 42,000 to 46,000 psi. parts are SAE-1010. The small heated to approximately 1000o to This compares favorably with re- jaw insert is carburized at 1675o, drive off moisture from the flux; sults usually obtained by copper for 30 minutes, in a water soluble carburizing salt before assembly. loy flow and penetration failures After an air-cool to room tem- are often indications of water con- perature and a water wash, the tent. The bath is easily rectified jaw and a hairpin clip are pressed by immersing an aluminum sheet, into position. The round bushing where oxygen is picked-up on the and brass alloy ring are inserted surface and the hydrogen is driven at the other end, and the parts are from the bath surface burning on rack loaded. They are then im- contact with air. The sheets are mersed in the bath for 20 minutes reimmersed, after cleaning, as at 1675o, then quenched in oil, needed for bath dehydration. washed and drawn at 500o. The Aluminum dip brazing furnaces resulting hardness is Rc 48-51. can be built to meet most dimen- The brazed joints pass an accept- sional requirements. Furnaces 20 ance test of 40,000 psi-minimum. ft deep and 15 ft long have been The handle portion of the assem- built successfully. They are al- bly has a slight case approximate- most always heated by immersed ly 0.004”-0.005” which is abrasion electrodes and have ceramic tile resistant and serves well as a final pots. finish. When brazing aluminum, it is Aluminizing necessary to penetrate the thin Coating the surface of ferrous ma- oxide coating to obtain a sound terial with aluminum gives better bond. This requires an active flux corrosion resistance. This finish during the entire brazing cycle to resists many types of atmospheres insure good flow and alloy pene- from room temperature to 1500°, tration. The aluminum brazing see Fig. 2. alloy is particularly vulnerable to To prepare for the process, parts oxides or oxygen in the bath; and (such as pole line hardware and it is, therefore, essential to pre- industrial fasteners, see photomi- clean the assembly thoroughly crograph Fig. 3) are descaled in a with various alkaline and acid be- descaling furnace salt bath, pick- fore brazing. The function of the led, preheated in a flux-type bath is to melt the alloy in an en- chloride-fluoride salt at 1300° and vironment completely oxidation remain there about 10 to 15 min- free and provide fluxing for sur- utes. During this time the baskets face coat removal of the parent containing parts are progressively metal. moved from right to left, while the The aluminum dip brazing salt bath reacts with the oxides on the is essentially a chloride mixture part surfaces so that aluminum with small percentages of either wetting-out is precluded. The bas- aluminum or sodium fluoride. The kets are then quickly transferred fluoride acts as reducing agent and to. a pot of molten aluminum at reacts with any oxides present. 1290° and held there for 1 to 3 The bath operating at 1080 minutes. During this time parts to 1120o from which the volatile are agitated so all surfaces are and corrosive fumes are removed covered completely with a coating by an exhaust system. Two basic of from 0.002 to 0.025” thickness procedures maintain a proper depending upon dipping time and bath: since it operates relatively temperature. close to freezing, it is necessary to The bath performs two func- make periodic additions of lithium tions: it preheats material to prop- chloride for fluidity; and the self- er temperature and chemically re- fluxing characteristic is continu- acts to residual oxides that would ously depleted so it is necessary to interfere with the aluminum wet- make periodic fluoride additions. ting the surface and diffusing into In a bath with sufficient drag- the base material. Bath mainte- Figure 4 Martempering the 9-ft. wing flap out, adding fresh salt is usually nance is similar to that for alumi- track in the Boeing 722 jetliner. sufficient to maintain proper num brazing and dragout loss re- chemical balance; but, due to the plenishment is sufficient for chem- hydroscopic nature of the alumi- ical balance. num brazing salt, moisture enters Isothermal quenching the bath and becomes a contamina- tion which must be removed. Al- In steel hardening, distortion and cracking are often associated with distortion is minimized and crack- room temperature. Dimensions are water and oil quenching. How- ing eliminated. held accurately, and the part is ever, distortion is considerably re- Martempering is generally for fully heat treated ready to draw duced and cracking avoided by any steel that responds to oil without distortion. This heat-treat quenching in a molten nitrate-ni- quench hardening. By martem- and forming method has been suc- trite salt within the 400 to 750o pering within the 400 to 750o cessful for alloy and stainless steel range. Martempering and austem- range, lower thermal and trans- sheet, plate and rod. The process pering processes are also successful formation stresses are involved so also improves physical properties for parts ranging in size from less distortion is encountered. As over conventional treatment. typewriter bars to automotive in austempering, water addition Another contributing advantage bumpers. results in improved hardness. of hot forming is the salt’s lubri- Austempering is usually applied A 9-ft-long wing flap track for cating effect. Whether the form- to carbon steels 3/8“ or less, alloys the Boeing 722 jet is a god ex- ing machinery operates beneath to 2”, where hardness falls be- ample of martempering, Fig. 4. the quench bath surface or parts tween Rc 35 and 54, and where The parts are a modified SAE-4330 are removed from the bath and toughness or bendability without and 4340 steel held to ±0.0l0” ma- formed, the salt film adhering to breaking is required. chining tolerances. Both austen- the surface provides lubrication. itizing and isothermal quenching In drawing cups from discs, fairly In one installation for austem- high reductions have been made pering rotary mower blades of will handle pieces up to 2000 lbs. possible due to the lubrication ef- SAE-1070, they are rack-loaded, The austenitizing furnace operates at 1600o with a connected load of fect of salt at 600o. austenized in a neutral chloride o 140 kw, and measures 5 ft dia x type salt at 1550 , and transferred Cleaning 16 ft deep. The quench furnace to the bath where they are o is rated also at 140 kw, having Maintaining and operating extrud- quenched at 640 for 27 minutes. working dimensions of 5 x 7 x 15 ing and forming equipment is a The final structure is bainitic and ft deep. problem that has plagued the plas- provides a toughness superior to When parts are removed from a tics industry. Dies, plates, screens, oil quench and temper. Salt nitrate-nitrite salt, a thin salt film etc. become contaminated with quenching provides a means by adheres to the surface. The mix- plastic materials that resist re- which heat can be extracted quick- ture is one of the most soluble moval by alkali and acid solvents. ly to prevent the steel from trans- used in heat treating and will re- Hand cleaning was necessary and forming into undesirable high- spond to a hot water wash and costly. Using salt bath cleaning temperature material. Austenite rinse, leaving a bluish-black oxide furnaces in the industry, they now can transfer to the desirable bain- finish. Contrasting with oil re- find cleaning time reduced from itic product by the isothermal moval by alkaline cleaners, water hours to minutes. quench temperature. washing for salt removal permits Polymers have been successful- Quenching severity is enhanced savings in equipment and mainte- ly removed from steel mold sur- by adding water-particularly suc- nance. This is in addition to the faces by immersing them in an cessful for borderline harden- metallurgical advantages. oxidizing type salt. All polymer ability applications. Water is materials under go decomposition added to the nitrate-nitrite by Martemper and ausforming by complete removal unless spraying it on the surface of the Meeting space-age demands has brought into contact with molten salt then mixing mechanically in- required new design and fabricat- salt. to the bath-water being about ing methods, even though many Using a caustic-nitrate type salt half the weight of salt explaining steels have common fabricating at 650 to 950°, polymers will de- why the two must be mechanically problems. Many materials are dif- compose and burn-off as combus- mixed. ficult to form to the desired shape tion products in minutes. Total Agitation is essential for ex- without excessive part distortion time cycle is usually less than 30 tracting heat, because it is neces- during heat treatment. Instead of minutes for complete organic con- sary to move the quenching medi- forming structural members, then tamination removal, leaving the um past the surface of the work heat treating them, it is possible part with a bluish-black oxide being quenched. Salt, completely by using isothermal quenching coat. dehydrated, does not have a vapor equipment to form and harden In some cases, a residue results phase within the normal operat- simultaneously. The steel is aus- from material breakdown. Inert ing temperature range; however tenitized at normal temperature, substances not reacting with the by adding water, moisture is driv- then isothermically quenched in a bath will fall to the bottom as en off-which provides faster cool- nitrate-nitrite salt within the sludge which must be removed ing. The addition of water also steel’s bainitic range. periodically. There is little chem- improves quenching severity so While the steel is still austenitic, ical deterioration in the bath; so that heavier steel sections can be forming machinery (part of the adding salt from time to time, to quenched satisfactorily. Because isothermal equipment) forms the replace that lost in dragout, is thermal and transformation part. After this operation, parts adequate to maintain cleaning stresses are held to a minimum, are removed and air cooled to efficiency. n AT AJAX, WE’VE ANSWERED A LOT OF

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