2008 THERMAL PROCESS INFORMATION BOOK SELECTED HEAT TREATING TERMS 36 HEAT TREATMENT OF FERROUS METALS 37 HEAT TREATING PROCESSES 38 SURFACE ENGINEERING 39 FURNACE ATMOSPHERES 40 INDUCTION HEAT TREATING OF STEEL 42 COMBUSTION 44 QUENCHING 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 ammonia. Dissociated ammonia (N2 + H2) is produced the temperature increases, and severe oxidation of carbon 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 steels and selected nickel and copper alloys, important in carburizing furnaces. Excessive carbon will per- and bright annealing of electrical components. Dissociated am- meate the grain structure of the alloy, causing embrittlement monia is also used as a carrier gas in certain nitriding 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 carbonitriding 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 hardening 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 nitrogen, 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 tempering 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 decarburization, 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 iron-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 irons, high carbon content (1.75-4.0% and steels. C) and high silicon content promote graphite formation. Therefore, cast iron 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.