2008 THERMAL PROCESS INFORMATION BOOK SELECTED TERMS 36 HEAT TREATMENT OF FERROUS METALS 37 HEAT TREATING PROCESSES 38 SURFACE ENGINEERING 39 FURNACE ATMOSPHERES 40 INDUCTION HEAT TREATING OF 42 COMBUSTION 44 45 CONTROLS/INSTRUMENTATION 46 FLOWMETERS 48 SELECTED HEAT TREATING TERMS atmosphere controlled furnaces. In many heat treating operations, drogen in a 90-10 blend, where the hydrogen serves as a re- the atmosphere must be controlled to prevent workpieces from ducing gas. oxidizing and/or decarburizing. Steel becomes more active as dissociated . Dissociated ammonia (N2 + H2) is produced the temperature increases, and severe oxidation of steel from anhydrous ammonia (NH3) by raising the temperature begins at about 425°C (795°F). Above 1200°C (2190°F), the ox- to 900–980°C (1650–1795°F) in a catalyst filled retort. The gas is idation rate increases exponentially. At high temperatures the then cooled for metering and transport. Dissociated ammonia at- carbon in steel also can react with the atmosphere to lower the mospheres are about 75% H2 and 25% N2,with less than 300 carbon content. ppm residual ammonia at a dew point below –60°C (–75°F). carbon potential. A measure of the ability of an environment con- The atmosphere provides a dry, carbon-free source of reducing taining carbon to alter or maintain, under prescribed condi- gas. Uses include bright copper and silver brazing, bright heat tions, the carbon level in steel. Control of carbon potential is treating of carbon and selected nickel and copper alloys, important in furnaces. Excessive carbon will per- and bright of electrical components. Dissociated am- meate the grain structure of the , causing embrittlement monia is also used as a carrier gas in certain processes. and eventual component failure. Preheat or burn-off muffles dry hydrogen atmospheres. Commercially available hydrogen is 98 require good atmosphere control to flush out these contami- to 99.9% pure. Cylinder hydrogen may contain trace amounts of nants. water vapor and oxygen. Dry hydrogen is used in furnaces for products of combustion. These result when fuel mixed with air is annealing stainless and low-carbon steels, electrical steels, and burned. If fuels such as methane and propane are burned in several nonferrous metals. optimized proportions with air, the by-products might be ideal endothermic atmospheres. Endothermic atmospheres are produced for certain heat treated products. If an excess of air exists (a lean by generators that use air and a hydrocarbon gas as fuel. The atmosphere), loose scale may form. When an excess of fuel is two gases are mixed, slightly compressed, and passed through used (a rich atmosphere), a tight, adherent oxide forms. Note a chamber filled with nickel catalyst. The chamber is heated ex- that water vapor is a by-product of combustion. ternally,thus, the term endothermic. Endothermic gas mixtures are used as carrier gases in carburizing and ap- PROCESSES plications (they offer a wide range of possible carbon poten- bright annealing. A process usually carried out in a controlled fur- tials). Other applications include bright of steel, nace atmosphere so that surface oxidation is reduced to a min- carbon restoration of steel forgings and bars, and sintering imum and the surface remains relatively bright. To limit oxi- powder that requires a reducing atmosphere. dation, the water vapor concentration must be limited. Bright exothermic atmospheres. Exothermic gas is produced by combus- annealing environments are typically purged with inert gases tion of a hydrocarbon fuel such as methane or propane to main- such as , argon, or dry air. Typically, the dew point tain a reaction temperature of 980°C (1795°F) for sufficient time temperature must be less than –50°C (–60°F). to reach equilibrium. Heat is obtained from the reaction, thus, carbonitriding. A process in which ammonia (NH3) added to a gas the term exothermic. The resultant gas is cooled and water vapor carburizing environment dissociates to produce hydrogen (H2) is removed either by a refrigerated or desiccant dryer. and nitrogen (N2). The addition of nitrogen has three impor- Exothermic atmospheres are used for clean and bright annealing tant effects: inhibits the diffusion of carbon, which favors pro- and clean hardening. Rich exothermic atmospheres are useful duction of a shallow case; enhances hardenability, which fa- for annealing, and for copper brazing of low-carbon steels, Cu- vors production of a hard, wear-resistant case that is easily Ni alloys, gold alloys, and some brasses. Applications for lean polished; and forms nitrides, which further enhance wear re- exothermic atmospheres include annealing of aluminum and sistance. copper and their alloys, bluing of steel parts, silver brazing of carburizing. A process in which ferrous metal is brought into con- nonferrous alloys, and nonflammable blanketing during var- tact with an environment of sufficient carbon potential to cause ious industrial processes. absorption of carbon at the surface, and by diffusion to create natural atmospheres (air). Air consists of about 78% nitrogen, 21% a carbon concentration gradient between the surface and the oxygen, 0.9% argon, and other trace gases. Air at room tem- interior of the metal. Carburizing is usually done at 850 to 950°C perature varies in moisture content from about 0.3 to 3%, nom- (1560 to 1740°F) in an atmosphere consisting of any of several inally. Although natural atmospheres are strongly oxidizing, carrier gases, principally nitrogen, carbon monoxide, and hy- they may be acceptable when workpieces are to be machined drogen, to which hydrocarbon gases (or vaporized hydrocarbon after heat treating. liquid) have been added. Methane or natural gas (CH4) is the steam atmospheres. Steam injection into furnaces is used for scale-free most commonly used source of carbon. For carburizing in the and stress relieving of ferrous metals in the temper- range of 0.8 to 1% C, the dew point temperature of the carrier ature range of 345 to 650°C (655 to 1200°F). The steam causes a gas is optimized at –7 to –1°C (19 to 30°F). Dew points below thin, hard, tenacious blue-black oxide to form on the surface of –12°C (10°F) may lead to accelerated sooting of generator cata- the part. Prior to processing in a steam atmosphere, parts must lyst. For low surface concentrations of carbon, the dew point be clean and oxide-free. To prevent condensation and rusting, may be adjusted to 0°C (30°F) or higher. furnace internal surfaces and the parts in the furnace must be at a temperature above 100°C (212°F). And air must be purged ATMOSPHERES AND GASES from the furnace to prevent formation of a brown coating in- argon. Provides an excellent inert atmosphere. It is used for gas- stead of the desired blue-black oxide. shielded arc welding and for heat treatment of special alloys. vacuum atmospheres. Heating metal parts at pressures below at- Generally, argon must be delivered at a dew point of less than mospheric is used for many semiconductor components, com- –60°C (–75°F) and an oxygen content of less than 20 ppm. posites, and metals. Vacuum heat treating prevents surface re- commercial nitrogen atmospheres. Nitrogen is used in many heat actions such as oxidation and , removes surface treating applications, sometimes replacing endothermic at- contaminants such as oxide films and lubricant residue, de- mospheres. Nitrogen serves as a pure, dry, inert gas that can gasses metals, removes dissolved contaminants from metals, provide efficient purging and blanketing. Typical specifications and joins metals by brazing or diffusion bonding. require the nitrogen to be delivered at dew point temperatures between –60 and –80°C (–75 and –110°F). Nitrogen is also used as a carrier gas for carbon control atmospheres in many com- mercial heat treating applications. Nitrogen is mixed with hy- 36 HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2007 HEAT TREATMENT OF FERROUS METALS INTRODUCTION Steel can be processed to produce a large variety of mi- which different phases are stable (producing beneficial phase crostructures and properties. The required results are transformations). The -carbon equilibrium phase dia- achieved by heating the material in temperature ranges gram is the foundation on which all steel heat treatment is where a phase or combination of phases is stable (producing based. The diagram defines the temperature-composition microstructural changes or distribution of stable phases) regions where the various phases in steel are stable, as well and / or heating or cooling between temperature ranges in as the equilibrium boundaries between phase fields.

DEFINITIONS OF IRON-CARBON EQUILIBRIUM PHASE DIAGRAM TRANSFORMATION The basis for understanding the heat treatment of steels is the iron-carbon TEMPERATURES IN (Fe-C) phase diagram. The Fe-C diagram is really two diagrams in one, IRON AND STEELS showing the equilibrium between cementite (iron carbide, or Fe C) and the Transformation temperature. The 3 temperature at which a change in several phases of iron, as well as the equilibrium between graphite and the phase occurs. The term is sometimes other phases. Steels are alloys of iron, carbon and other elements that con- used to denote the limiting tempera- tain less than 2% carbon (usually less than 1%), therefore the portion of the ture of a transformation range. The diagram below 2% C; that is, the iron-cementite (Fe-Fe3C) diagram, is more following symbols are used for iron pertinent to steel heat treatment. In cast , high carbon content (1.75-4.0% and steels. C) and high silicon content promote graphite formation. Therefore, technology is based more on the Fe-graphite diagram. Accm. In hypereutectoid steel, the tem- perature at which the solution of ce- mentite in austenite is completed during heating.

Ac1. The temperature at which austenite begins to form during heating, with the c being derived from the French chauffant.

Ac3. The temperature at which trans- formation of ferrite to austenite is com- pleted during heating.

Aecm, Ae1, Ae3. The temperatures of phase changes at equilibrium.

Arcm. In hypereutectoid steel, the tem- perature at which precipitation of ce- mentite starts during cooling, with the r being derived from the French re- froidissant.

Ar1. The temperature at which trans- formation of austenite to ferrite or to ferrite plus cementite is completed during cooling.

Ar3. The temperature at which austenite begins to transform to fer- rite during cooling.

Ar4. The temperature at which delta ferrite transforms to austenite during cooling.

Ms (or Ar’’). The temperature at which transformation of austenite to martensite starts during cooling.

Mf. The temperature at which marten- site formation finishes during cooling. Note: All of these changes, except the formation of martensite, occur at lower temperatures during cooling than during heating and depend on the rate of change of temperature. Expanded iron-carbon phase diagram showing both the eutectoid and eutectic regions. Source: ASM Handbook, Vol. 4, Heat Treating, ASM Source: Practical Heat Treating, Second Edition, ASM International, Materials Park, Ohio, 2006. International, Materials Park, Ohio, 1991, pp. 3-4. HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2007 37 HEAT TREATING PROCESSES NORMALIZING SURFACE HARDENING TEMPERING Normalizing is a homogenizing or These treatments impart a hard, In tempering, a previously hard- grain refining treatment, with the aim wear resistant surface to parts, while ened or normalized steel is usually being uniformity in composition maintaining softer, tough interior. heated to a temperature below the throughout a part. It consists of an Hardness is obtained through lower critical temperature and cooled austenitizing heating cycle followed quenching rapidly from above a steel’s at a suitable rate, primarily to increase by cooling in still or slightly agitated transformation temperature, and duc- ductility and toughness, but also to in- air. Typically, work is heated to a tem- tility is obtained via tempering. The crease grain size of the matrix. Steels perature of approximately 55°C hardened surface of the part is referred are tempered by reheating after hard- (100°F) above the upper critical line of to as the case, and its softer interior is ening to obtain specific values of me- the iron-carbide phase diagram, and known as the core. chanical properties and to relieve the heating portion of the process Gas carburizing is one of the most quenching stresses and ensure di- must produce a homogeneous widely used surface hardening mensional stability. Tempering usu- austenitic phase. The actual tempera- processes. Carbon is added to the sur- ally follows quenching from above the ture used depends upon the compo- face of low-carbon steels at tempera- upper critical temperature. Most steels sition of the steel; but the usual tem- tures ranging from 850-950°C (1560- are heated to a temperature of 205- perature is around 870°C (1600°F). 1740°F). In quenching, austenite is 595°C (400-1105°F) and held at tem- transformed to martensite. Other perature for an hour or more. Higher ANNEALING methods of case hardening low- temperatures increase toughness and Annealing is a generic term de- carbon steels include cyaniding, fer- resistance to shock, but at the expense noting a treatment consisting of ritic nitrocarburizing, and carboni- of lower hardness and strength. heating to and holding at a suitable triding. describes an inter- temperature, followed by cooling at a rupted quench from the austenitizing suitable rate; used primarily to soften QUENCHING temperature to delay cooling just metals and to simultaneously produce Steel parts are rapidly cooled from above martensitic transformation for desired changes in other properties or the austenitizing or solution treating a period of time to equalize the tem- in microstructures. Reasons for an- temperature. Stainless and high-alloy perature throughout the piece, which nealing include improvement of steels may be quenched to minimize minimizes distortion, cracking, and machinability, facilitation of cold the presence of grain boundary car- residual stress. work, improvement in mechanical or bides or to improve the ferrite distri- isothermally trans- electrical properties, and to increase bution, but most steels, including forms a steel at a temperature below dimensional stability. In ferrous alloys, carbon, low-alloy, and tool steels, are that for pearlite formation and above annealing usually is done above the quenched to produce controlled that of martensite formation. Steel is upper critical temperature. amounts of martensite in the mi- heated to a temperature within the In full annealing, steel is heated 90- crostructure. The ability of a quen- austenitizing range; quenched in a 180°C (160-325°F) above the A3 for chant to harden steel depends upon bath maintained at a constant tem- hypoeutectoid steels and above the the cooling characteristics of the perature, usually in the range of 260- A1 for hypereutectoid steels, and slow quenching medium. Quenching ef- 400°C (500-750°F); allowed to trans- cooled. In full annealing, the rate of fectiveness is dependent on steel com- form isothermally to bainite in the cooling must be very slow, to allow position, type of quenchant, or quen- bath; then cooled to room tempera- the formation of coarse pearlite. In chant use conditions, as well as the ture. Benefits of the process are in- process annealing, slow cooling is design and maintenance of a creased ductility, toughness, and not essential because any cooling quenching system. strength at a given hardness. rate from temperatures below A1 re- sults in the same microstructure and QUENCHING MEDIA COLD AND CRYOGENIC hardness. Selection of a qunchant depends on TREATMENT OF STEEL the hardenability of the steel, section Cold treatment can be used to en- STRESS RELIEVING thickness and shape involved, and the hance the transformation of austenite In the stress relief process, steel is cooling rates needed to achieve the de- to martensite in case hardening and heated to around 595°C (1105°F), en- sired microstructure. Typically, quen- to improve the stress relief of castings suring that the entire part is heated chants are liquids (water, oil that could and machined parts. Practice identi- uniformly, then cooled slowly back to contain a variety of additives, aqueous fies -84°C (-120°F) as the optimum cold room temperature. Care must be polymer solutions, and water that treatment temperature. By compar- taken to ensure uniform cooling, es- could contain salt or caustic additives), ison, at a tempera- pecially when a part has varying sec- and gases (inert gases including he- ture of around -190°C (-310°F) im- tion sizes. If the cooling rate is not con- lium, argon, and nitrogen). Other proves certain properties beyond the stant and uniform, new residual quenchants include fogs and fluidized capability of cold treatment. stresses, equal to or greater than ex- beds. isting originally, can be the result.

38 HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2007 SURFACE ENGINEERING CHARACTERISTICS OF DIFFUSION TREATMENTS Process Typical Case temperature, case depth, hardness, Typical Process Type of case °C (°F) m (mils) HRC base metals Process characteristics Carburizing Pack Diffused carbon 815-1090 125-1500 50-63(a) Low-carbon steels, Low equipment costs, difficult to (1500-2000) (5-60) low-carbon alloy control case depth accurately steel Gas Diffused carbon 815-980 75-1500 50-63(a) Low-carbon steels, Good control of case depth, (1500-1800 (3-60) low-carbon alloy suitable for continuous steels operation, good gas controls required, can be dangerous Liquid Diffused carbon 815-980 50-1500 50-65(a) Low-carbon steels, Faster than pack and gas and possibly (1500-1800) (2-60) low-carbon alloy processes, can pose salt disposal nitrogen steels problem, salt baths require frequent maintenance Vacuum Diffused carbon 815-1090 75-1500 50-63(a) Low-carbon steels, Excellent process control, bright (1500-2000) (3-60) low-carbon alloy parts, faster than gas carbur- steels izing, high equipment costs Nitriding Gas Diffused nitrogen, 480-590 125-750 50-70 Alloy steels, Hardest cases from nitriding nitrogen (900-1100) (5-30) nitriding steels, steels, quenching not required, compounds stainless steels low distortion, process is slow, is usually a batch process Salt Diffused nitrogen, 510-565 2.5-750 50-70 Most ferrous Usually used for thin hard cases nitrogen (950-1050) (0.1-30) metals including <25 mm (1 mil), no white layer, compounds cast irons most are proprietary processes. Ion Diffused nitrogen, 340-565 75-750 50-70 Alloy steels, Faster than gas nitriding, no nitrogen (650-1050) (3-30) nitriding, white layer, high equipment compounds stainless steels costs, close case control. Carbonitriding Gas Diffused carbon 760-870 75-0.75 50-65(a) Low-carbon steels, Lower temperature than carbur- and nitrogen (1400-1600) (3-30) low-carbon alloy izing (less distortion), slightly steels, stainless steel harder case than carburizing gas control critical. Liquid Diffused carbon 760-870 2.5-125 50-65(a) Low-carbon steels Good for thin cases on noncrit- (cyaniding) and nitrogen (1400-1600) (0.1-5) cal parts, batch process, salt disposal problems. Ferritic Diffused carbon 565-675 2.5-25 40-60(a) Low-carbon steels Low-distortion process for thin nitro- and nitrogen (1050-1250) (0.1-1) case on low-carbon steel, most carburizing processes are proprietary Other Aluminizing Diffused 870-980 25-1000 <20 Low-carbon steels Diffused coating used for oxida- (pack) aluminum (1600-1800) (1-40) tion resistance at elevated temperatures. Siliconizing by Diffused silicon 925-1040 25-1000 30-50 Low-carbon steels For corrosion and wear resis- chemical vapor (1700-1900) (1-40) tance, atmosphere control is deposition critical. Chromizing by Diffused 980-1090 25-50 <30 Low-carbon steel, Chromized low-carbon steels chemical vapor chromium (1800-2000) (1-2) high-carbon steel yield a low-cost stainless steel, deposition 50-60 High- and low- high-carbon steels develop a carbon steels hard corrosion-resistant case. Titanium Diffused carbon900-1010 2.5-12.5 >70(a) Alloy steels, Produces a thin carbide (TiC) case carbide and titanium, (1650-1850) (0.1-0.5) tool steels for resistance to wear, high temp- TiC compound erature may cause distortion. Diffused boron, 400-1150 12.5-50 40->70 Alloy steels, Produces a hard compound boron, compound (750-2100) (0.5-2) tool steels, cobalt layer, mostly applied over hard- and nickel alloys ened tool steels. High process temperature can cause distortion. (a) Requires quench from austenitizing temperature.

HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2007 39 FURNACE ATMOSPHERES Atmospheres serve a variety of func- form oxides. It also reacts with carbon supply of carbon dioxide is exhausted tions; acting as carriers for elements used dissolved in steel, lowering surface and the steel surface is free of carbon. in some heat treating processes, clean- carbon content. ing surfaces of parts being treated in Nitrogen in its molecular state is pas- CLASSIFICATION OF PREPARED other processes, and providing a pro- sive to ferrite and can be used as an at- ATMOSPHERES tective environment to guard against mosphere in annealing low-carbon The American Gas Association is the source of the adverse effects of air when parts are ex- steels; as a protective atmosphere in following classifications: posed to elevated temperatures. Prin- heat treating high-carbon steels, ni- • Class 100, exothermic base: formed by the combustion cipal gases and vapors are air, oxygen, trogen must be completely dry— small of a gas/air mixture; water vapor in the gas can be removed to get the required dew point nitrogen, carbon dioxide and carbon amounts of water vapor in nitrogen • Class 200, prepared nitrogen base: carbon dioxide and monoxide, hydrogen, hydrocarbons (i.e., cause decarburization. Molecular ni- water vapor have been removed methane, propane, and butane), and trogen is reactive with many stainless • Class 300, endothermic base: formed by the reaction inert gases, such as argon and helium. steels and can’t be used to heat treat of a fuel gas/air mixture in a heated, catalyst filler Air provides atmospheres in fur- them. Atomic nitrogen, which is cre- chamber naces in which protective atmospheres ated at normal heat treating tempera- • Class 400, charcoal base: air is passed through a bed are not used. Air is also the major con- tures, is not a protective gas—it com- of incandescent charcoal stituent in many prepared atmos- bines with iron, forming finely divided • Class 500, exothermic-endothermic base: formed by pheres. The composition of air is ap- nitrides that reduce surface hardness. the combustion of a mixture of fuel gas and air; water proximately 79% nitrogen and 21% Carbon dioxide and carbon monoxide vapor is removed and carbon dioxide is reformed to carbon oxygen, with trace elements of carbon are used in steel processing atmos- monoxide by reaction with fuel gas in a heated catalyst dioxide. As an atmosphere, air behaves pheres. At austenitizing temperatures, filled chamber like oxygen, the most reactive con- carbon dioxide reacts with surface • Class 600, ammonia base: can consist of raw ammonia, stituent in air. carbon to produce carbon monoxide, dissociated ammonia, or combusted dissociated ammonia with a regulated dew point Oxygen reacts with most metals to a reaction that continues until the CLASSIFICATION AND APPLICATION OF PRINCIPAL FURNACE ATMOSPHERES Nominal composition, vol%

Class Description Common applications N2 CO CO2 H2 CH4 101 Lean exotherimc Oxide coating of steel 86.8 1.5 10.5 1.2 — 102 Rich exothermic Bright annealing; copper brazing; 71.5 10.5 5.0 12.5 0.5 sintering 201 Lean prepared nitrogen Neutral heating 97.1 1.7 — 1.2 — 202 Rich prepared nitrogen Annealing; brazing stainless steel 75.3 11.0 — 13.2 0.5 301 Lean endothermic Clean hardening 45.1 19.6 0.4 34.6 0.3 302 Rich endothermic Gas carburizing 39.8 20.7 — 38.7 0.8 402 Charcoal Carburizing 64.1 34.7 — 1.2 — 501 Lean exothermic- Clean hardening 63.0 17.0 — 20.0 — endothermic 502 Rich exothermic- Gas carburizing 60.0 19.0 — 21.0 — endothermic 601 Dissociated ammonia Brazing; sintering 25.0 — — 75.0 — 621 Lean combusted Neutral heating 99.-0 — — 1.0 — ammonia 622 Rich combusted Sintering stainless powders 80.0 — — 20.0 — ammonia Source: Heat Treater’s Guide: Practices and Procedures for Irons and Steels, ASM International, 1995

ATMOSPHERE CLASSIFICATION Protective Reactive Neutral Inert Application Case hardening Annealing, Hardening, Tempering, Vacuum operations Decarb-Annealing Brazing, Sintering HIP

Components Active: CO, CO2, H2O, CxHy, H2, Active: CO, CO2, H2O, CxHy, H2, Ar/He, N2/He, N2/Ar/He NH3, etc. Base: N2, Ar NH3, etc. Base: N2 Ar, H2 Surface changes Composition, time, temperature Composition, time, temperature NO Other surface Not usual if controlled Not usual Possible structural changes Furnace type Batch, Vacuum, Continuous Vacuum, HIP Chamber, Autoclave Batch, Vacuum, Continuous Source: The Theory and Economics of Atmosphere Selection, Meri Lazar and Rob Edwards, Heat Treating Progress, Jan. / Feb. 2005 40 HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2007 FURNACE ATMOSPHERES Hydrogen reduces iron oxide to iron. carbon monoxide and hydrogen. It is burizing tendency to furnace atmos- Under certain conditions, hydrogen can reactive with steel surfaces at very low pheres. decarburize steel, an effect that depends temperatures and partial pressures. It Inert gases are especially useful as on furnace temperature, moisture con- is also the principal cause of bluing protective atmospheres in the ther- tent (of gas and furnace), time at tem- during cooling cycles. mal processing of metals and alloys perature, and carbon content of the steel. Carbon hydrocarbons are methane that can’t tolerate the usual con- Water vapor is oxidizing to iron and (CH4), ethane (C2H6), propane (C3H8), stituents in protective reactive metals combines with carbon in steel to form and butane (C4H10). They impart a car- and their alloys. Process Atmosphere Suitability Gas

Process N2 N2/NH3 N2/CxHy N2/CH3OH N2/H2 H2 Ar Ar/H2 He Endo(a) Exo(a) Bright anneal • • • • NonFe anneal • • • • • • • • Decarb anneal • • • Alloy steel anneal • • • • • • Stainless steel anneal • • • • • Neutral hard • • • • • • Carburize • • Nitride • • Temper • • • Braze •• • • • Sinter • • • • • • • Galvanize • HIP •• Vacuum • • • • Cryo treat • (a)atmosphere generator gas (all others industrial gas). Source: The theory and Economics of Atmosphere Selection; Meri Lazar and Rob Edwards, HTP, Jan./Feb. 2005.

Reasons for Atmosphere Monitoring Atmosphere components Processes Common concerns/reasons for measurement

O2 Ferrous Processes: Carburizing, carbonitriding, Control of oxidation, decarburization, and carburization neutral hardening, normalizing, subcritical Optimizing atmosphere generator operation annealing, stress relieving Optimizing synthetic, industrial gas-based atmospheres Nonferrous processes: Annealing, solution Calculating dew point and carbon potential annealing, stress relieving, vacuum annealing, Reducing discoloration vacuum solution annealing, vacuum brazing Improve brazing properties

Dew point or H2O Ferrous Processes: Carburizing, carbonitriding, Control of oxidation, decarburization, and carburization neutral hardening, normalizing, subcritical Optimizing atmosphere generator operation annealing, stress relieving, vacuum quenching, Optimizing synthetic, industrial gas-based atmospheres sinter hardening Calculating carbon potential indirectly Nonferrous processes: Annealing, solution Reducing discoloration annealing, stress relieving, vacuum annealing, Improve brazing properties vacuum solution annealing, vacuum brazing Verify incoming inert-gas quality and integrity of gas distribution system

CO, CO2, H2, CH4 Ferrous Processes: Carburizing, carbonitriding, Control of oxidation, decarburization, and carburization neutral hardening, normalizing, subcritical Optimizing atmosphere generator operation annealing, stress relieving, vacuum quenching, Optimizing synthetic, industrial gas-based atmospheres sinter hardening Calculating carbon potential indirectly Nonferrous processes: Annealing, solution Indication of lubricant burn-off (in sintering and annealing, stress relieving, vacuum annealing, aluminum annealing) vacuum solution annealing, vacuum brazing Detect leaks in radiant tubes Source: Atmosphere Monitoring and Control; Robert Oesterreich, Shahab Kazi, Richard Speaker, and John Buonassisi, HTP, April/May 2002.

HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2007 41 INDUCTION HEAT TREATING OF STEEL Principles of pattern, is connected to the power workstation, an inductor (heating) coil, induction heating supply so a magnetic field is gener- controls, and workpiece handling The basic components of an induc- ated from the current flow. The mag- units. When a steel or cast iron is being tion heating system are an induction nitude of the field depends on the hardened, the system may also in- coil, an alternating-current (ac) power strength of the current and the clude a quenching system, depending supply, and the workpiece itself. The number of turns in the coil. on the hardenability of the particular coil, which may take different shapes An induction heating system typi- grade. It is common practice to include depending on the required heating cally consists of a power supply, a other auxiliary equipment such as line

Frequency Selection for of Steel Frequency, kHz Case depth,mm (in.) Diameter,mm (in.) 1 3 10 50 450 0.38-1.27 (0.015-0.050) 6.35-25.4 (0.25-1) ————Good 1.29-2.54 (0.051-0.100) 11.11-15.88 (0.4375-0.625) — — Fair Good Good 15.88-25.4 — — Good Good Good 25.4-50.8 — Fair Good Good Good >50.8 Fair Good Good Good Poor 2.56-5.08 (0.101-0.200) 19.05-50.8 — Fair Good Good Good 50.8-101.6 Fair — Good Good Poor >101.6 Good Good Fair Good Poor 5.08-10.0 (0.200-0.400) >8 Good ———— Through hardening 1.59-6.35 (0.0625-1.0) ————Good 6.35-12.7 (0.250-0.5) — — Fair Fair Good 2.7-25.4 (0.5-1.0) — Fair Good Good Fair 25.4-50.8 (1.0-2.0) Fair Good Fair Poor — 50.8-76.2 (2.0-3.0) Good Good Poor — — 76.2-152.4 (3.0-6.0) Good Poor Poor — — >152.4 (>6.0) Poor Poor Poor — — Good indicates most efficient frequency. Fair indicates the frequency is less efficient. Poor indicates not a good frequency for this depth. The coil power density must be kept within the recommended ranges. Source: Richard E. Haimbaugh, Practical Induction Heat Treating, ASM International, 2001.

Power Density Required for Surface Hardening Input W/mm2 kW/in.2(e) Depth of hardening(a), Frequency, kHz mm (in.) Low (b) Optimum (c) High (d) Low (b) Optimum (c) High (d) 500 0.38-1.14 (0.015-0.045) 10.9 15.5 18.6 7 10 12 1.14-2.29 (0.045-0.090) 4.7 7.8 12.4 3 5 8 10 1.52-2.29 (0.060-0.090) 12.4 15.5 24.8 8 10 16 2.29-3.05 (0.090-0.120) 7.8 15.5 23.3 5 10 15 3.05-4.06 (0.120-0.160) 7.8 15.5 21.7 5 10 14 3 2.29-3.05 (0.090-0.120) 15.5 23.3 26.35 10 15 17 3.05-4.06 (0.120-0.160) 7.8 21.7 24.8 5 14 16 4.06-5.08 (0.160-0.200) 7.8 15.5 21.7 5 10 14 1 5.08-7.11 (0.200-0.280) 7.8 15.5 18.6 5 10 12 7.11-9.14 (0.280-0.360) 7.8 15.5 18.6 5 10 12 Note: This table is based on use of proper frequency and over-all operating efficiency of equipment. Values may be used for static and progressive methods of heating. However, for some applications, higher inputs can be used when hardening progressively. (a) For greater depth of hardening, o lower kW input is used. (b) Low kW input can be used when generator capacity is limited. The kW values can be used to calculate ;argest part hardened (single-shot method) with a given generator. (c) For best metallurgical results. (d) for higher production when generator capacity is available. 9e) kW is read as maximum during heat cycle. Source: Richard E. Haimbaugh, Practical Induction Heat Treating, ASM International, 2001.

42 HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2007 INDUCTION HEAT TREATING OF STEEL starters, electrical disconnects, and sign. Coil design is built on equipment cooling systems as part of a large store of empirical the induction heating package. data whose development The basic architecture of an induc- has sprung from theoret- tion heating system includes a work- ical analyses of several station (or heat station), which con- rather simple inductor tains load matching components such geometries. Consequently, as output transformers and capacitors, coil design generally is plus high-frequency contactors, pro- based on experience. tective devices, cooling water mani- Frequency is the first pa- folds, and quench valves. The prin- rameter considered for in- cipal function of the workstation is to duction heating. Primary provide proper electrical impedance considerations in the selec- match between the output of the tion of frequency are depth power supply and the inductor (in- of heating, efficiency, type duction heating coil) for optimum of heat treatment (such as power transfer into the heated load. surface hardening versus The coil is normally mounted on the subcritical annealing), and Interrelationship among heating time, surface power den- front of and close to the workstation. the size and geometry of sity, and hardened depth for various induction generator fre- Coil design is influenced by many the part. Lower frequencies quencies. factors including the dimensions and are more suitable as the size of the part profiles can be produced by varying configuration of the workpiece, the and the case depth increase. However, the power density and heating time. number of parts to be heated, the many variations are possible because Selection of these two heating para- temperature required, the pattern of power density and heating time also meters depends on the inherent heat heat desired, and whether the work- have an important influence on the losses of the workpiece (from either piece is to be heated at one time or depth to which the part is heated. radiation or convection losses) and the progressively. Applied frequency and Once the frequency has been se- desired heat conduction patterns of a level of power also enter into coil de- lected, a wide range of temperature particular application.

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HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2007 43 COMBUSTION Flue gas Excess losses or make up air losses

Industrial heating and heat treating Wall furnaces are heated by gaseous or (conduction liquid fuels, or by electric heating losses) elements. Exhaust Opening losses (radiation) Natural gas is the principal gaseous losses fuel used in the United States. It has a gross heating value of about 37 MJ/m3 Gross input (1000 Btu/ft3). For combustion, natural (purchased Net energy) output 3 3 Available gas requires about 0.28 m (10 ft ) of heat (heat to load) air per cubic foot of gas. Other fuels of this type include liquefied petroleum gases such as Heat storage propane and butane. Conveyor loss Six steps to good furnace fuel efficiency Furnace heat balance represented as a Sankey diagram. Keeping heat treat furnace energy ef- ficiency and productivity at peak levels requires a comprehensive approach to the factors affecting fuel consumption. 100 This approach can be summarized in Fuel: Birmingham natural gas 90 (1002 Btu/ft3, 0.6 sp. gr.) six steps: with 10% excess air 1. Monitor burner air-gas ratios. Use 80 no more excess air than necessary and 70 no excess fuel. 2. Keep insulation and refractories in 60 good repair. Explore replacing old 50 dense refractories with lower density 400°F Air 70°F Air

insulation. % of input 40 3. Don’t overload the furnace. 1000°F Air 600°F Air 800°F Air 4. Pay attention to scheduling, mini- Exhaust gas heat loss, 30 mize furnace idle time, and run fur- 20 naces as close to 100% of capacity as possible. 10 5. Consider converting ambient air combustion systems to preheated air. 1000 1200 1400 1600 1800 2000 2200 6. Investigate the possibility of re- Furnace exhaust temperature, °F ducing the weight of baskets, trays, and fixtures. Exhaust heat losses vs. exhaust gas temperatures, % excess air, and combustion air temperature.

1200 100 300% Conditions: 90 600% 200% 80°F Ambient temperature, 100% 50% 25% 400% still air Sides 80 1200% 300% 10% 150% 900 1000% 70 800%

of surface 60 2 Top 600 50 0% Excess air

Bottom % of input 40

Exhaust gas heat loss, 30 200 20 Heat loss, Btu/hr·ft Fuel: Birmingham natural gas 10 (1002 Btu/ft3, 0.6 sp. gr.) 0 100 200 300 400 1000 2000 3000 Outer skin temperature, °F Exhaust gas temperature, °F Chart for estimating furnace wall losses. Exhaust heat losses vs. exhaust gas temperatures and % excess air. 44 HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2007 QUENCHING TYPICAL COOLING RATE CURVES FOR SELECTED LIQUID MEDIA Selection of the most appropriate quenchant depends on the hardenability of the steel, the section thickness and shape of the part being quenched, and the cooling rates needed to produce the required microstructure.

Water quenching Polymer quenching Water is the quenchant of choice where a severe quench does In a number of applications, aqueous polymers provide a not result in excessive distortion and cracking. Use generally quench rate (quench severity) that falls between those of water is restricted to quenching simple, symmetrical parts made of and oil).Cooling rate can be tailored to requirements by changing shallow-hardening grades of steel. Other applications: austenitic the concentration of polymer, the solution temperature, and the stainless steels and other metals that have been solution treated at degree of agitation of the bath. high temperatures. Polymer quenchants are nontoxic, which makes them safer to work with and easier to dispose of than traditional quenchants. They also are nonflammable, improving working conditions be- cause there is no chance of fire, smoke, and fume during quenching. In practice, polymer quenchants are more cost effec- tive to use. They cost less initially, have reduced drag-out, top off mainly with water to compensate for evaporation losses, and have low viscosity. Polymer quenchants also have a high spe- cific heat, leading to reduced temperature rise during quenching, higher production rates, and lower cooler-rating requirements. Also, no cleaning is required before tempering, eliminating al- kali or solvent degreasing. Polymer quenchants offer technical advantages over mineral oils. Quenching speed is flexible, which enables heat treaters to select a cooling rate that matches their specific requirements. It also allows leaner alloy steels to be used and results in better physical properties on some steels. Polymer quenchants improve tolerance to water contamination, as quenching speed is not influenced significantly compared with quenching oils. Organic polymers used as a basis for water-based quenching fluids include: polyvinyl alcohol (PVA), polyalkylene glycol (PAG), acrylate (ACR), polyvinyl pyrrolidone (PVP), and poly- ethyl oxazoline (PEO). Polyvinyl-alcohol-type quenchants have Oil quenching been replaced by polyalkylene glycols due to their greater flexi- Normal-speed oil is used where the hardenability of a steel is bility and ease of control and maintenance. PAGs are widely used high enough to provide specified mechanical properties with slow in a variety of applications, including immersion quenching of cooling. Typical applications are highly alloyed steels and tool steel, induction hardening and spray quenching, and solution steels. Medium-speed oils are typically used to quench medium- treatment of aluminum alloys. ACR, PVP, and PEO provide more to high-hardenability steels. High-speed oils are selected for low- oil-like quenching characteristics than PAGs, and are generally hardenability alloys, carburized and carbonitrided parts, and large- used for higher hardenability steel applications. cross-section, medium-hardenability steel parts that require very high rates of cooling to produce maximum mechanical properties.

Cooling characteristics of water, PAG, ACR, and oil

Note: All agitated at 1,000 rpm and tested at 40°C (100°F) using Wolfson probe

HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2007 45 CONTROLS/INSTRUMENTATION Standard thermocouples Base metal elements: Main advantages are economy, good reliability, and reasonable accuracy. Used primarily for low to moderately high temperatures (–200 to 1250oC). More than 90% of all thermocouples used are in this group. Types J and K prevail. ANSI Type and Recommended common name1 temperature range2 Applications and conditions Type E –200 to 900oC Can be applied in atmospheres ranging from vacuum to mildly oxidizing. Chromel/constantan Excellent choice for cryogenic applications. Has the highest electromotive force (emf) per degree of all the common elements. Positive thermoelement or leg: Chromel. Type J 0 to 760oC The standard selection for its recommended temperature range. Good reliability Iron/constantan at lower temperatures. The positive leg (iron) will oxidize rapidly above 500oC. Very economical. Used extensively in the plastics industry but applicable to most processes within its operating range. Type K –200 to 1250oC The industry standard for use up to the recommended maximum temperature. Chromel/Alumel While stable in oxidizing atmospheres, it is prone to corrosion in reducing environments. Protection tubes are always recommended. Positive leg: Chromel. Type N –200 to 1250oC Similar to Type K, but more resistant to oxidation and less subject to the large Nicrosil/Nisil drop in emf found in the positive leg (Chromel) of Type K thermocouples operating at approximately 500oC. Positive leg: Nicrosil. Type T –200 to 350oC Widely used in the food processing industry. More stable than Types E or J for Copper/constantan low-temperature applications. Has been used at a temperature as low as –269oC (boiling helium). Positive leg: copper. Ni-0.8Co/Ni-18Mo –200 to 1300oC Designed for operation in vacuum or hydrogen atmosphere, environments that will degrade most other elements. Although its emf output is unique at higher temperatures, it is nearly identical to Type K below 120oC. This permits the use of Type K extension wire. Has not been assigned an ANSI Type code. Positive leg: Ni-0.8Co.

Noble metal elements: Offer improved accuracy and stability over base metal elements. Most are manufactured from combinations of platinum and rhodium. Commonly used in high-temperature applications up to 1700oC. Also applied as reference standards when testing base metal elements. Highest cost of all thermocouples. ANSI Type and Recommended common name1 temperature range2 Applications and conditions Type R 0 to 1450oC Industry standard noble metal thermocouple for high-temperature applications. Pt/Pt-13Rh Platinum is prone to contamination if in contact with other metals. A ceramic protection tube must be used. Very stable in an oxidizing atmosphere but will degrade rapidly in vacuum or a reducing atmosphere. Positive leg: Pt-13Rh. Type S 0 to 1450oC Applications and conditions similar to Type R. Type S was traditionally Pt/Pt-10Rh considered the “laboratory thermocouple,” while Type R was considered the “industrial thermocouple.” Type S is now used extensively as an industrial sensor. Positive leg: Pt-10Rh. Type B 870 to 1700oC Applications and conditions similar to Types R and S, but more stable than either Pt-6Rh/Pt-30R at high temperatures. Very low output and high nonlinearity at low temperatures. Generally not considered usable below 250oC. Positive leg: Pt-30Rh.

Refractory metal elements: Combinations of tungsten and rhenium. Very brittle and prone to breakage. Used for very high-tempera- ture applications up to 2315oC. Must be used in vacuum or a totally inert atmosphere. ANSI Type and Recommended common name1 temperature range2 Applications and conditions Type C 0 to 2315oC These elements must be used in vacuum, hydrogen, or inert atmosphere. W-5Re/W-26Re Tungsten has no oxidation resistance. Sometimes supplied with open-end protection tubes for use with vacuum; otherwise made as a sealed assembly purged with argon. Element must not be in contact with metal. Brittle and prone to breakage. Generally considered a limited-life product. Positive leg: W-5Re. 1 ANSI is the American National Standards Institute. Chromel (~Ni-10Cr, UNS N06010) and Alumel (~Ni-2Mn-2Al, UNS N02016) are trade names of Hoskins Mfg. Co., Hamburg, Mich. Nicrosil (84 Ni, 14.2 Cr, 1.4 Si) and Nisil (95 Ni, 4.4 Si, 0.15 Mg) are trade names of Driver-Harris Co., Harrison, N. J. Similar alloys having different trade names are available from other manufacturers. Composition of constantan is ~46Ni-54Cu. 2. The recommended temperature range is that for which limits of error have been established.

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Broad Base. Best Solutions. www.sglcarbon.com FLOWMETERS Flowmeters typically measure ei- rate is calculated from the pressure applications because of their very ac- ther volumetric or mass flow. Volu- loss across a pipe restriction. Pressure curate and repeatable measurements. metric flow measurement looks at the drop through these devices is well un- Variable-area and thermal-mass flow of a given volume of the medium derstood, and a wide variety of con- flowmeters are most often used in heat over time (e.g., cubic feet per hour). figurations areavailable. Variations of treating and processing applications. This technology uses primarily me- differential pressure flow measure- For most heat treating applications, chanical flow rate indication, with ment include the use of pitot tubes. important flowmeter selection criteria electronic output normally available • Mechanical: Flow is measured ei- are reliability, accuracy, ruggedness, as an option. Mass flow measurement ther by passing isolated, known vol- ease of calibration, and ease of main- looks at the flow of a given mass over umes of a fluid (gas or liquid) through tenance. time (e.g., pounds per hour). a series of gears or chambers (positive- General types of flowmeters are displacement type), or via a spinning variable area, differential pressure, turbine or rotor. Measurements using mechnical, electronic, and thermal a positive-displacement flowmeter are Variable-area flowmeters: mass. obtained by counting the number of a) glass / plastic tapered tube rotameter; • Variable area: Fluid flow rate is passed isolated volumes. (b) metal tapered-tube rotameter; measured as the flowing medium • Electronic: Magnetic, vortex, and (c) slotted metal cylinder; passes through a tapered tube. The ultrasonic devices have either no (d) vane type; position of a float, piston, or vane moving parts or vibrating elements (e) piston meter with spring-loaded orifice placed in the flow path changes as and are relatively nonintrusive. piston over a tapered plug; and higher flows open a larger area to pass • Thermal mass: These flowmeters (f) tapered tube with spring. the fluid, providing a direct visual are essentially immune to changes in Source: A Flowmeter Primer, indication of flow rate. gas temperature and pressure. They Vytas Braziunas and Daniel Herring, • Differential pressure: Fluid flow are used in critical flow measurement Mar. / April Heat Treating Progress, 2004]

COMMONLY USED FLOW MEASUREMENT INSTRUMENTS

Disassembly Sensitivity Robust without to dirty spare Industrial flowmeter type Style Manufacturer* unpiping fluids parts Variable-area, including Metal tube Waukee Engineering Co. Inc. Yes Moderate Delicate rotameters Metal cylinder tube Meter Equipment Mfg. Inc. Yes Low Moderate Glass or plastic tube Fisher-Porter, Brooks No Sensitive Moderate Instrument, King Instrument Co., Dwyer Instruments Inc., Key Instruments Vane type Universal Flow Monitors Inc., No Moderate Moderate Erdco Engineering Corp., Orange Research Inc. Moving orifice Hedland, Div. Racine No Moderate Robust Federated Inc.

Piston (with spring) Insite, by Universal Flow No Moderate Delicate Monitors Inc. Differential pressure/Orifice Orifice Lambda Square Inc., No Moderate Robust Flowell Corp.

Venturi Flowell Corp., Fox Valve No Moderate Robust Development Corp.

Turbine/Impeller Rotary impeller Roots (BNC Industrial Co. Ltd., No Sensitive Moderate TokicoTechno Ltd., and others)

Turbine Hoffer Flow Controls Inc., No Sensitive Delicate Sponsler Inc., Great Plains Industries Inc. Thermal mass Thermal mass Sierra Instruments Inc., MKS No Sensitive Delicate Instruments, Brooks Instrument * Instruments also may be supplied by companies other than those listed. Source: A Flowmeter Primer, Vytas Braziunas and Daniel Herring, Mar. / April Heat Treating Progress, 2004] 48 HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2007 Flow P1 Vane P2 Indicator P2 Flow Flow Flow DTube g P2 DTube DFloat (d) DFloat Moving orifice g Metering cone P1 P2 Fspring g Slot P1 Flow Flow P1 Flow Flow (e)

g P2 (a) Open slot length Flow P1 P2 P1 F spring Flow Flow Indicator Flow

(b) (c) (f)

Typical Mechanical Mechanical Electronic full-scale flow flow reading/ flow accuracy, % Typical Pressure Special installation requirements reading scale type reading of reading turndown drop Vertical mounting Yes Easy/linear Available 3.5 3:1 Low Vertical mounting Yes Easy/linear Available 1–2 25:1 Low Vertical mounting Yes Easy/linear No 1–2 10:1 Low

No special requirements Yes Easy/linear No 2–5 5:1 High/average

Straight pipe upstream and Additional Complex/ Available 2–3 3:1–10:1 Low/average downstream required instrumentation square required root No special requirements Yes Easy/linear No 1–5 5:1 Low/average

Straight pipe upstream and Additional Hard/ Available 0.5–2 3:1–10:1 High downstream required instrumentation square required root Straight pipe upstream and Additional Hard/ Available 0.5–2 3:1–10:1 Average downstream required instrumentation square required root No special requirements Additional Moderate/ Available 0.5–2 10:1–20:1 Average instrumentation linear, total required flow counter Straight pipe upstream and Additional Moderate/ Yes 0.5–3 10:1–20:1 Average downstream required instrumentation linear required Straight pipe upstream and No Not applicable Yes 1–2 10:1–100:1 Average/high downstream required

HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2007 49