heatbath.qxd 7/24/03 8:11 PM Page 1 THE BASICS OF MOLTEN SALT QUENCHANTS Molten salt is an ideal he wide operating temperature nonuniform hardness or cracks. This range and other characteristics of tendency increases with increasing medium for interrupted molten salts make them ideally hardenability of the steel. Causes of suited to low-distortion quench- distortion usually can be traced to un- quenching processes. ing of iron, steel, and tool steel. even or nonuniform quenching, Nitrate-based salt, the Thanks to advances in salt thermal stresses, and phase transfor- T quality, pollution abatement, and mation stresses. most widely used, material handling, salt bath These causes can be largely over- blankets the quench quenching is more efficient and eco- come by interrupting the quenching nomical than ever before. process with a salt bath. This is temperature range of 150 A properly selected salt can be used achieved by rapidly cooling the parts to 595°C (300 to 1100°F). for processes such as martempering from the austenitizing temperature to and austempering in a single bath. Ex- a point above the martensite forma- by Gajen P. Dubal* cellent thermal stability and tolerance tion or start temperature (Ms). The for contaminants make salt quenching parts are held at that temperature for a Heatbath/Park Metallurgical Corp. systems almost maintenance-free. And specified time, and then cooled (usu- Detroit, Mich. baths can provide satisfactory per- ally in air) to room temperature. The formance for many years simply by interruption in quenching consider- adding salt to replace that which is ably reduces thermal stresses and, dragged out. thereby, distortion. Processes in this The focus of this article is on nitrate- category are martempering, austem- based salts for quenching iron and pering, and their variations. steel parts, since this constitutes the For most carbon and low-alloy largest portion of molten salt steels, the temperature at which the quenching applications. Chloride- quench is interrupted is usually in the based salts used for quenching tool 175 to 370°C (350 to 700°F) range. A steels also are briefly covered. nitrate-based salt is the best choice here. A chloride-based salt is selected Controlling distortion for high-alloy and tool steels, where Quenching in water, brine, a water- the interruption temperature range based polymer solution, or oil can is 480 to 705°C (900 to 1300°F). cause distortion and in some cases, Continued Automated salt bath furnace line for heat treating high-speed steel tools includes a salt quench furnace and air-cool conveyor. Source: ASM Handbook, Vol. 4, Heat Treating: ASM Interna- * Member of the ASM Heat Treating Society tional, Materials Park, Ohio, 1991, p. 728. HEAT TREATING PROGRESS • AUGUST 2003 81 heatbath.qxd 7/24/03 8:11 PM Page 2 Cooling rate, °F/s (°C/s) Molten salt’s advantages measuring quench severity, as well as 80 (44) 160 (89) 240 (133) 320 (178) The most distinct ad- for understanding the quenching vantage of salt quenching mechanism and studying the effects 1600 is its wide operating tem- of quench variables. (871) perature range — 150 to Typical cooling and cooling rate Cooling rate 595°C (300 to 1100°F) for curves for a nitrate-based salt bath are 1200 1 (649) a typical nitrate-based shown in Fig. 1. The cooling rate salt, which is unmatched curve clearly shows that no vapor 60.5 °F/s (33.6 °C/s) 800 by any other quench phase is present and that the cooling (427) medium. rate varies uniformly with tempera- Temperature, °F (°C) 400 The quenching mecha- ture. This is in sharp contrast to oil, (204) Cooling nism also is considerably polymer, or brine quenchants. different. In most liquid The main operating variables in salt 20 40 60 80 quenchants, heat extrac- quenching processes are temperature, Cooling time, s tion occurs throughout agitation, water content, and residence Fig. 1 — Typical cooling and cooling rate curves for a nitrate- the three stages of vapor, time. based molten salt bath at 255°C (495°F). The cooling rate curve boiling, and convection. Temperature: Although primarily shows that no vapor phase is present and that the cooling rate Salt quenching has no governed by the Ms temperature of varies uniformly with temperature. No agitation or water addi- vapor phase, so problems the steel, there is some room for tion. Average cooling rate from 650 to 260°C (1200 to 500°F): with vapor phase barriers varying salt bath temperature. In gen- 33.6°C/s (60.5°F/s). Ref. 1. are avoided. Most of the eral and regardless of medium, the heat extraction is by con- lower the bath temperature, the faster vection. As a result, distortion is min- is the cooling rate. In salt bath 100 (56) imized and the hardness achieved is quenching, the effect of this variable more uniform and consistent. is marginal, but it still can be opti- 80 mized. The recommended practice is (44) Other important advantages of salt quenching include: to start at the lowest temperature and 60 • Quench severity can be controlled incrementally increase it until the best (33) by varying the temperature, agitation, combination of hardness and distor- 40 tion is obtained. (22) and water content of the salt bath. Cooling rate, °F/s (°C/s) • Productivity in salt quenching is In some cases, salt bath temperature 20 high because parts rapidly attain tem- can be manipulated to achieve re- (11) perature equalization. markable results. For example, large sections can be martempered by first 50 (25) 100 (51) 150 (76) • Due to excellent thermal and Flow rate, ft/min (cm/s) chemical stability, salt baths can pro- quenching in water or brine for a short vide consistent quenching perform- time and then transferring them to a Fig. 2 — Increasing agitation results in con- ance for many years. Ordinarily, the salt bath. This helps achieve higher siderable increase in quench severity (cooling hardness to a greater depth. Similarly, rate). Temperature of the low-melting-point only maintenance required is to re- salt bath: 175°C (350°F). The cooling rate place salt that is dragged out. low-hardenability steels can be plotted on the vertical axis is the average of the • Salt can be easily washed off with austempered by using two salt baths, rates determined at every 55.5°C (100°F) in- water and recovered for reuse. the first bath at a lower temperature terval between 650 and 260°C (1200 and Choosing to recover salt eliminates than the second.2 With such manipu- 500°F). Ref. 1. disposal and reduces operating costs. lations, cooling curves satisfy a key re- • The same salt bath can be used for quirement of quenching: avoiding the tempering. pearlite nose of the TTT (time-tem- Limitations: Salt has to be used perature-transformation) diagram. above its melting point of about 150°C In most production setups, heat (300°F), and, because of its oxidizing input to the salt from the work ex- nature, combustible or incompatible ceeds heat losses by radiation, neces- materials should definitely be kept out sitating a cooling system to maintain of the salt bath to avoid the possibility bath temperature within the required of violent reactions. range. Solutions include external Salt used to present safety and en- cooling fins and/or a heat exchanger vironmental problems, but technology inside the bath. for dealing with them is now well de- Agitation: The effect of bath agita- veloped and they are no longer con- tion is greater than that of tempera- sidered deterrents to its use. ture. Increasing agitation results in a considerable increase in quench Quench process variables severity, as shown in Fig. 2.1 The Quench severity is a measure of the cooling rate plotted on the vertical axis ability of a quenchant or quenching is the average of the cooling rates de- system to extract heat from a test spec- termined at every 55.5°C (100°F) in- imen, part, or workload. Among the terval between 650 and 260°C (1200 various methods, cooling curve and 500°F). This temperature range is analysis is the most useful tool for chosen because it spans the pearlite 82 HEAT TREATING PROGRESS • AUGUST 2003 heatbath.qxd 7/24/03 8:12 PM Page 3 nose of the TTT diagram for most tortion. Water content of the bath typ- peratures up to 900°C (1650°F). steels. ically varies from 2 to 3% at 150°C Quenching from a higher temperature Agitation can be provided by pro- (300°F) to about 0.5% at 315°C (600°F). is not recommended unless parts are peller-type agitators or by a pump Safety considerations dictate that small and well separated. Even then, with a draft tube. As many as four water is added only to a bath having parts should first pass through a neu- propellers with single or dual im- sufficient agitation — never to a still tral salt bath maintained at a lower pellers may be required, depending bath — and it should be added very temperature. A chloride-based salt is on bath size. If a pump is used, salt slowly, as a fine stream or spray, di- recommended when quenching from flow from bottom to top is preferable. rectly into the vortex created by agita- high austenitizing temperatures like Agitation should not be provided by tion. Instead of fresh water, salt solu- 980 to 1315°C (1800 to 2400°F), as are bubbling air into the salt bath, because tion from the washing operation or required for tool steels. it is inefficient and will cause undesir- low-pressure steam also can be used.