Volume 04 - Heat Treating Part 3 pdf

Fig. 8 Effect of tempering time at six temperatures on room-temperature hardness of carbon-molybdenum steels with different carbon contents but with prior martensitic structures Fig. 9 Effect of carbon content and tempering temperature on room-temperature hardness of three molybdenum steels. Tempering time: 1 h at temperature Reference cited in this section 12. R.A. Grange and R.W. Baughman, Trans. ASM, Vol 48, 1956, p 165-167 Alloy Content The main purpose of adding alloying elements to steel is to increase the hardenability, that is, the capability of the steel to form martensite upon quenching from above its critical temperature. The general effect of alloying elements on tempering is a retardation of the rate of softening, especially at the higher tempering temperatures. Thus, to reach a given hardness in a given period of time, alloy steels require higher tempering temperatures than do carbon steels. Alloying elements can be characterized as carbide forming or non-carbide forming. Elements such as nickel, silicon, aluminum, and manganese, which have little or no tendency to occur in the carbide phase, remain essentially in solution in the ferrite and have only a minor effect on tempered hardness. Hardening due to the presence of these elements occurs mainly through solid-solution hardening of the ferrite or matrix grain size control. The carbide forming elements (chromium, molybdenum, tungsten, vanadium, tantalum, niobium, and titanium) retard the softening process by the formation of alloy carbides. The effect of the carbide-forming elements is minimal at low tempering temperatures where Fe 3 C forms; however, at higher temperatures, alloy carbides are formed, and hardness decreases slowly with tempering temperature. The increase in hardness due to the addition of alloying elements was plotted as a percent alloying element for various tempering temperatures from 205 to 705 °C (400 to 1300 °F). These graphs are shown in Fig. 10. Fig. 10 Effect of seven elements (chromium, manganese, molybdenum, nickel, phosphorus, silicon, and vanadium) on the ha rdness of martensite tempered in 55 °C (100 °F) increments ranging from 205 to 705 °C (400 to 1300 °F), each for a 1- h duration. Note that manganese, molybdenum, and phosphorus have no effect on hardness at 205 °C (400 °F). Source: Ref 13 Strong carbide-forming elements such as chromium, molybdenum, and vanadium are most effective in increasing hardness at higher temperatures above 205 °C (400 °F). Silicon was found to be most effective in increasing hardness at 315 °C (600 °F). The increase in hardness caused by phosphorus, nickel, and silicon can be attributed to solid-solution strengthening. Manganese is more effective in increasing hardness at higher tempering temperatures. The carbide-forming elements retard coalescence of cementite during tempering and form numerous small carbide particles. Under certain conditions, such as with highly alloyed steels, hardness may actually increase. This effect, mentioned previously, is known as secondary hardening. The effect of molybdenum content on the tempering behavior of a 0.35% C steel is shown in Fig. 11. As the alloy content increases, the magnitude of the secondary-hardening effect increases. Synergistic effects of various combinations of alloying elements can occur: Chromium tends to produce secondary hardening at a lower temperature than does molybdenum, and the combination of chromium and molybdenum produces a rather flat tempering curve, with the peak hardness occurring at a somewhat lower temperature than when only molybdenum is present. H11 steel is a widely used hot-working die steel that contains nominally 0.35% C, 5% Cr, 1.5% Mo, and 0.4% V. Figure 12 shows the room- temperature hardness of H11 as a function of tempering temperature. A very flat tempering curve results because of the specific combination of the three carbide-forming elements. Fig. 11 Influence of molybdenum content on the softening of quenched 0.35% C steels with increasing tempering temperature. Source: Ref 10 Fig. 12 Variation of room-temperat ure hardness with tempering temperature for H11 steel. All specimens air cooled from 1010 °C (1850 °F) and double tempered 2 h plus 2 h at temperature Tool Steels and Stainless Steels. Extensive data on the tempering of tool steels (including H11) and martensitic stainless steels are given in the articles "Introduction to Heat Treating of Tool Steels", "Heat Treating of Specific Classes of Tool Steels", "Heat Treating of Stainless Steels", and "Heat Treating of Superalloys" in this Volume. Additional information is available in the Section "Specialty Steels and Heat-Resistant Alloys" in Volume 1 of ASM Handbook, formerly 10th Edition Metals Handbook. Other Alloying Effects. In addition to ease of hardening and secondary hardening, alloying elements produce a number of other effects. The higher tempering temperatures used for alloy steels presumably permit greater relaxation of residual stresses and improve properties. Furthermore, the hardenability of alloy steels requires use of a less drastic quench so that quench cracking is minimized. However, higher hardenability steels are prone to quench cracking if the quenching rate is too severe. The higher hardenability of alloy steels may also permit the use of lower carbon content to achieve a given strength level but with improved ductility and toughness. Residual Elements. Residual elements, that is, elements not intentionally added to a steel, can cause embrittlement. The elements that are known to cause embrittlement are tin, phosphorus, antimony, and arsenic. A discussion of the specific effects of these elements can be found in the section "Temper Embrittlement" in this article. References cited in this section 10. E.C. Bain and H.W. Paxton, Alloying Elements in Steel, American Society for Metals, 1966, p 185, 197 13. R.A. Grange, C.R. Hribal, and L.F. Porter, Hardness of Tempered Martensite in Carbon and Low Alloy Steels, Metall. Trans. A, Vol 8A, 1977, p 1780-1781 Tempering Procedures Tempering can be accomplished by soaking entire parts in the furnace for enough time to bring the tempering mechanism to the desired point of completion or by selective heating of certain portions of the part to achieve toughness or plasticity in those areas. Bulk processing may be done in convection furnaces or in molten salt, hot oil, or molten metal baths. The selection of furnace type depends primarily on number and size of parts and on desired temperature. Table 2 gives temperature ranges, most likely reasons for use, and fundamental problems of these four types of equipment. Table 2 Temperature ranges and general conditions of use for four types of tempering equipment Temperature range Type or equipment °C °F Service conditions Convection furnace 50-750 120- 1380 For large volumes of nearly common parts; variable loads make control of temperature more difficult Salt bath 160- 750 320- 1380 Rapid, uniform heating; low to medium volume; should not be used for parts whose configurations make them hard to clean Oil bath ≤ 250 ≤ 480 Good if long exposure is desired; special ventilation and fire control are required Molten metal >390 >735 Very rapid heating; special fixturing is required (high density) Selective tempering techniques are used to soften specific areas of fully hardened parts or to temper areas that were selectively hardened previously. The purpose of this treatment is to improve the machinability, the toughness, or the resistance to quench cracking in the selected zone. Induction and flame tempering are the most commonly used selective techniques because of their controllable local heating capabilities. The immersion of selected areas in molten salt or molten metal can be accomplished, but with somewhat less control. Special processes are employed occasionally to achieve specific properties such as those derived from steam treating or the use of protective atmospheres. The tempering mechanism in certain steels is enhanced by cyclic heating and cooling. A particularly important procedure employs cycles between subzero temperatures and the tempering temperature to increase the transformation of retained austenite. The term used for this procedure, multiple tempering, is also applied to procedures that use intermediate thermal cycles to soften parts for straightening prior to the actual tempering operation designed to achieve the desired degree of toughness and plasticity. Equipment for Tempering Steel is usually tempered in either an air (convection) furnace or a salt bath (Fig. 13). Molten metal baths, oil baths, and flame or induction heating units are also used. Fig. 13 Types of furnaces used for tempering of steel Convection Furnaces. The most commonly employed tempering method utilizes the recirculating or forced-air convection furnace, and the equipment most commonly used in conjunction with convection furnaces includes continuous belt conveyor, roller rail, or dog beam pusher systems. Batch equipment such as box or pit types are also used. Forced recirculating air is the most common and efficient method of tempering because it lends itself to a wide selection of furnace designs to accommodate a variety of products and capacities. Moreover, the metallurgical results are very good in terms of price per unit weight of yield. Generally, convection furnaces are designed for tempering temperatures of 150 to 750 °C (300 to 1380 °F). For temperatures up to 550 °C (1020 °F), recirculated hot air is supplied to the product from a chamber separate from the work-holding area to avoid uneven heating by radiation. For temperatures of 550 to 750 °C (1020 to 1380 °F), either forced convection or radiant heating is used, depending on the metallurgical requirements of the product. To obtain closer control of metallurgical properties, recirculated forced hot air is employed; but for greater efficiency, radiant heating is used because the transfer of radiant heat is greater as the temperature approaches 750 °C (1380 °F). The most important phase of convection furnace design is determining the proper amount of forced air. The objective of the blower is to furnish enough hot air to the complete work area so that it is efficiently used to heat the product in the shortest time thermophysically allowed. The type of product and the material being processed dictate the required forced- air supply, which is measured at the operating temperature. Consultation with fan manufacturers can help achieve maximum efficiency of heat transfer. Heat for the furnace can be supplied by electricity, gas, or oil. In most furnace designs, a dual heat source capability can be built in, such as gas and electricity. This allows for more than one choice of utility when there is a shortage or a cost advantage of one over the other. Temperature control is accomplished by positioning a thermocouple at the hot-air side of the recirculating system close to the product. When this technique is used, there is minimal danger of overheating, and loads of various sizes can be handled. This method also allows the duration of processing (holding time) to be varied by moving the thermocouple location, but only within the limits of the furnace size (and/or conveyor speed, for continuous-type furnaces). Temperatures generally are held within ±5 °C (±9 °F). If modern temperature controllers are used, baffle plates are positioned properly and furnace curtains are installed. The efficient use of a continuous furnace cannot be attained when production quantities are small or when parts vary in size, shape, and mechanical requirements. A batch furnace is better suited for work of this type. When a continuous furnace is used for such applications, production time is lost when the furnace temperature is raised or lowered. Sometimes, when the process variables must be changed, a dummy load must be placed in the furnace to accelerate a desired reduction of temperature, or production must be stopped until the temperature is stabilized. Salt bath furnaces may be used for tempering at 160 °C (320 °F) and above. Good heat transfer and natural convection in the bath promote the uniformity of workpiece temperature. All moisture must be removed from parts before they are immersed in the molten salt because hot salt reacts violently with moisture. If dirty or oily parts are immersed in the bath, the salt will become contaminated and will require more frequent rectification. Rectification with chemical or gaseous compounds controls the soluble oxides within proper limits. A carbon rod rectification procedure is used to remove the insoluble metallics. All parts tempered in salt must be cleaned soon after being removed from the bath because the salt that clings to them is hygroscopic and may cause severe corrosion. Parts with small or blind holes from which salt is difficult to clean should not be tempered in salt. Additional information is available in the article "Salt Bath Equipment" in this Volume. Salt bath compositions and operating temperature ranges presented in Table 3 pertain to baths in common use for tempering and are classified according to Military Specification MIL-S-10699A (Ordnance). Table 3 Compositions and operating temperatures for salt baths used in tempering Composition of bath, % Operating temperature Fuming temperature Class NaNO 2 NaNO 3 KNO 3 Na 2 CO 3 NaCl KCl BaCl 2 CaCl 2 °C °F °C °F 1 37-50 0-10 50-60 . . . . . . . . . . . . . . . 165-595 325-1100 635 1175 2 . . . 45-57 45-57 . . . . . . . . . . . . . . . 290-595 550-1100 650 1200 3 . . . . . . . . . 45-55 . . . 45-55 . . . . . . 620-925 1150-1700 935 1720 4 . . . . . . . . . . . . 15-25 20-32 50-60 . . . 595-900 1100-1650 940 1725 4A . . . . . . . . . . . . 10-15 25-30 40-45 15-20 550-760 1025-1400 790 1450 Class 1 and class 2 salts are reasonably stable and seldom require rectification. If chlorides are added by carryover from a higher-temperature bath, they will cause an increase in the viscosity of the bath. Chlorides can be removed by filtering through fine screens or by cooling and settling out the insoluble chlorides as a sludge. Occasionally, carbonates become excessive. These can be removed by reaction with dilute nitric acid-base rectifiers. Upper temperature limits must not be exceeded or salt will become highly oxidizing, even toward alloy steels. Class 3 salts seldom require rectification. However, their high melting points (about 560 °C, or 1040 °F) severely restrict their useful temperature range. Also, they are decarburizing to steels at temperatures above about 705 °C (1300 °F). Class 4 salts, which are all-chloride neutral salts, are quite stable. They seldom require rectification but are restricted to temperatures above 595 °C (1100 °F). Class 4A salts are close relatives of class 4 salts but contain calcium chloride, which lowers the minimum working temperature to 550 °C (1025 °F). The upper limit for these salts is more restricted than that for class 4 salts. Commercial Availability. All of the salts for these baths are commercially available. The reader is referred to the military specification cited above for the chemical and other control procedures applicable to the various bath composition. For additional information, see the articles"Martempering of Steel", "Austempering of Steel" , "Introduction to Heat Treating of Tool Steels", and "Heat Treating of Specific Classes of Tool Steels" in this Volume. Caution: The introduction of cyanide salts or other reducing agents into nitrite tempering baths will cause violent explosions. Oil bath equipment for tempering may be similar in design to that used for salt baths, or a steel tank over hot plate burners will serve satisfactorily. Submerged electric heating elements may also be employed. Stirring is essential for temperature uniformity and satisfactory oil life. Simple, oven-type temperature controls may be employed but localized overheating must be avoided to prevent fire and the rapid decomposition of the oil. A standard thermometer of the proper range may be used to check the temperature of the oil. Low-temperature tempering in a hot oil bath is a simple and inexpensive method that is especially useful for holding work at temperature for long periods of time. The practical temperature limit is about 120 °C (250 °F) without special ventilation or fire protection equipment and about 250 °C (480 °F) with such precautions, which may include extremely efficient ventilators or inert-gas blanketing systems. When a tempering temperature above 205 °C (400 °F) is required, a salt bath is usually preferable to an oil bath. Oils for tempering must resist oxidation and have a flash point well above the operating temperature. The most commonly used oils are high-flash-point paraffinic oils with antioxidant additives. Oils used for martempering (see the article "Martempering of Steel" in this Volume) are also satisfactory for tempering. Molten metal baths for tempering have largely been replaced by salt baths. When employed, commercially pure lead, which melts at about 327 °C (620 °F), has proved to be the most generally suitable of all metals and alloys. For special applications, however, lead-base alloys having lower melting points are used. Lead oxidizes readily. Although lead itself will not adhere to clean steel, the adherence of lead oxide to steel surfaces is a problem, especially at higher temperatures. Within the range of temperatures usually employed, a film of molten salt will protect the surface of the lead bath, and the work will be easily cleaned. Above 480 °C (900 °F), granulated carbonaceous material, such as charcoal, may be used as a protective cover. Because of its high thermal conductivity relative to the gaseous atmosphere, lead is useful for rapid local heating and selective tempering. A typical application is the tempering of a ball joint. The part is carburized and quenched to a minimum case hardness of 59 HRC and a core hardness of 30 to 40 HRC. The thread and taper are then tempered in lead to produce a maximum case hardness of 40 HRC. Because of the high specific gravity of lead, parts tempered in molten lead will float unless held down by fixtures. All parts and fixtures must be dry when immersed in the bath to prevent the formation of steam in, and resultant violent expulsion of, the molten lead. Precautions also must be taken to protect personnel from industrial lead poisoning; hoods and ventilating equipment are required. Temperature Control. For either gas or electric heat, properly adjusted potentiometers of the on-off type will control the tempering temperature within ±6 °C (±10 °F) at the thermocouple location. With proportioning controls, these instruments can maintain temperatures within ±1 °C (±2 °F) at the thermocouple location. Selection of Tempering Equipment The selection of equipment for tempering is based principally on the temperature requirements and the quantity and similarity of the work to be treated. Temperature requirements are dictated by prior heat treatment and by the properties to be developed by tempering. Process Control Variations in hardness after tempering are most frequently the result of differences in prior microstructure, as discussed previously. When prior microstructure is the same, the control of temperature is the most important factor in the control of the tempering process. In general, the control of tempering temperature to within ±13 °C (15 °F) is adequate and is within the practical limits of most furnace and molten-bath equipment. Temperature variations are seldom permitted to exceed ±6 °C (±10 °F) unless mechanical property requirements are correspondingly broad. Examples of the range of variation in hardness obtained after tempering for a variety of wrought and cast steel parts are presented in Fig. 14, 15, and 16. [...]... to 150 °C (200 to 30 0 °F)(a) 150 to 230 °C (30 0 to 450 °F) Flash point (min), °C ( °F) 210 (410) 275 (525) Fire point (min), °C ( °F) 245 (470) 31 0 (595) 23 5-5 75 Viscosity, sus, at: 38 °C (100 °F) 100 °C (210 °F) 50. 5-5 1 11 8-1 22 150 °C (30 0 °F) 36 . 5 -3 7.5 5 1-5 2 175 °C (35 0 °F) 4 2-4 3 205 °C (400 °F) 3 8 -3 9 230 °C (450 °F) 3 5 -3 6 Viscosity index (min) 95 95 Acid number 0.00 0.00 Fatty-oil content None... the heating time is comparatively long to help provide uniform heating throughout the part To meet production requirements, length of the inductor can be increased, or more than one bar can be processed at a time Table 4 Approximate power density required for tempering Frequency(a), Hz Input(b) W/mm2 kW/in.2 15 0-4 25 °C (30 0-8 00 °F) 42 5-7 05 °C (80 0-1 30 0 °F) 15 0-4 25 °C (30 0-8 00 °F) 42 5-7 05 °C (80 0-1 30 0... crack as a result of part configuration or surface defects These steels include 1040 , 1050, 1141, 1144, 4047 , 4 132 , 4140, 4640, 8 632 , 8740, and 9840 Some steels, such as 1020, 1 038 , 1 132 , 4 130 , 5 130 , and 8 630 , are not sensitive Before being tempered, parts should be quenched to room temperature to ensure the transformation of most of the austenite to martensite and to achieve maximum as-quenched hardness... of 534 to 6 53 HRB and tempered 1 h at 475 °C (890 °F) in continuous roller-hearth furnaces Data represent a 2-month production period (d) Forged 1046 steel heated to 830 °C (1525 °F), and quenched in caustic Forgings were heated in a continuous belt-type furnace and individually dump quenched in agitated caustic Forgings weighed 9 toll kg (20 to 24 lb) each; maximum section, 38 mm (1 1 in.) (e) As-quenched... 5 mm ( 3 in.), using vigorous agitation of the martempering medium In addition, 16 thousands of gray cast iron parts are martempered on a routine basis The grades of steel that are commonly martempered to full hardness include 1090, 4 130 , 4140, 4150, 434 0, 30 0M ( 434 0M), 4640, 5140, 6150, 8 630 , 8640, 8740, 8745, SAE 1141, and SAE 52100 Carburizing grades such as 33 12, 4620, 5120, 8620, and 931 0 also... production tempering (a) Valve bonnets, 75 mm (3 in.) in diameter, made of 4140 steel from one mill heat Parts were heated at 870 °C (1600 °F), oil quenched and tempered for 2 h at 605 °C (1125 °F) to a hardness specification of 255 to 30 2 HB (b) Valve segments made of 4140 steel from one mill heat Section thickness varied from 13 to 25 mm ( 1 to 1 2 in.) Parts were heated at 870 °C (1600 °F), oil quenched... to 37 0 °C (39 0 to 700 °F) should be avoided Fig 23 Room-temperature Charpy V-notch impact energy versus tempering temperature for 4 130 , 4140, and 4150 steels austenitized at 900 °C (1650 °F) and tempered 1 h at temperatures shown Source: Ref 21 References cited in this section 17 J.P Materkowski and G Krauss, Tempered Martensite Embrittlement in SAE 434 0 Steel, Metall Trans A, Vol 10A, 1979, p 164 3- 1 651... locally by gas burners that heated the steel to 815 °C (1500 °F) Tools were oil quenched and tempered at 30 5 to 32 5 °C (585 to 615 °F) for 10 min in electrically heated recirculating-air furnace to a desired hardness range of 42 to 48 HRC Data were recorded during a 6-month period and represent forgings from 12 mill heats (c) Plate sections, 19 to 22 mm ( 3 7 to in.) thick, of 1045 steel were water quenched... more than 0 .35 % C, it is recommended that the parts be transferred to tempering furnaces before they cool to below 100 to 150 °C (212 to 30 0 °F) Alternately, many heat- treating operations use quenching oil for the tempering operation (martempering) or to avoid cooling below 125 °C (255 °F) Steels that are known to be sensitive to this type of cracking include 1060, 1090, 134 0, 40 63, 4150, 434 0, 52100,... Grades Some carbon steels higher in manganese content such as 1041 and 1141 can be successfully martempered in thin sections Low-alloy steels that have limited applications for successful martempering are listed (the lower-carbon grades are carburized before martempering): 133 0 to 134 5, 4012 to 4042 , 4118 to 4 137 , 4422 and 4427, 4520, 5015 and 5046 , 6118 and 6120, 8115 Most of these alloy steels are suitable . . . 29 0-5 95 55 0-1 100 650 1200 3 . . . . . . . . . 4 5-5 5 . . . 4 5-5 5 . . . . . . 62 0-9 25 115 0-1 700 935 1720 4 . . . . . . . . . . . . 1 5-2 5 2 0 -3 2 5 0-6 0 . . . 59 5-9 00 110 0-1 650 940. 15 0-4 25 °C (30 0-8 00 °F) 42 5-7 05 °C (80 0-1 30 0 °F) 15 0-4 25 °C (30 0-8 00 °F) 42 5-7 05 °C (80 0-1 30 0 °F) 60 0.10 0.24 0.06 0.15 180 0.08 0.22 0.05 0.14 1,000 0.06 0.19 0 .04 0.12 3, 000. NaNO 2 NaNO 3 KNO 3 Na 2 CO 3 NaCl KCl BaCl 2 CaCl 2 °C °F °C °F 1 3 7-5 0 0-1 0 5 0-6 0 . . . . . . . . . . . . . . . 16 5-5 95 32 5-1 100 635 1175 2 . . . 4 5-5 7 4 5-5 7 . . . .