Advanced Materials & Processes

NOV-DEC 2013

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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Page 56 of 92

REDEFINING QUENCHING TECHNOLOGY HTPRO GAS QUENCHING IS THE SAFEST, MOST ENVIRONMENTALLY FRIENDLY OF QUENCHING SYSTEMS AND PRODUCES THE LEAST AMOUNT OF DISTORTION. Aymeric Goldsteinas* and Jake Hamid* Ipsen Inc., Cherry Valley, Ill. Gas quenching has several benefits compared with other quench systems. It is the only dry quenching that exists and, therefore, it eliminates all environmental or safety problems connected with liquid quenching. This article examines why defining gas quenching in bar pressure no longer applies, why a new definition is needed, and how this definition enables a better understanding of which steel and what cross section can be hardened via gas quenching. crostructure transforms into the harder martensite phase. The cooling rate must be fast enough to minimize formation of softer bainite and pearlite phases, which negatively impact mechanical properties. The key to accomplishing the transformation is uniform heat removal from the part surface. Figure 1 shows a representative continuous cooling curve for a ferrous alloy. Quenching oils have a range of quenching severity depending on their physical properties, especially viscosity. Oil, like water, exhibits a pronounced vapor phase upon quenching followed by a nucleate boiling phase with a very rapid heat transfer in the temperature range of 570° to 1110°F (300° to 600°C). The three cooling stages of an oil quench are shown in Fig. 2. A hardened core or hardened surface for metals is accomplished by heating to a sufficiently high temperature and rapid cooling (quenching) to room temperature. Quenching uniformity is of the utmost importance, which requires addressing quench system inadequacies that may be detrimental to process results. Dry gas quenching meets industry needs more efficiently than liquid quenching. These stages might not occur at all part locations at the same time. During the oil-quench nucleate boiling phase, extremely high instantaneous heat transfer coefficients can be achieved. This is an advantage in the temperature range where pearlitic transformation occurs, an advantage not shared by gas quenching. However, with the breakdown of the vapor phase at the onset of boiling, the so-called Leidenfrost effect occurs. State-of-the-art quenching Heat treating ferrous metal parts involves heating to a temperature above the upper critical temperature (Ac3) into the austenite region of the phase diagram, which depends on alloy composition. Parts are rapidly cooled by a quenching fluid or gas, so that the mi- Eutectoid temperature 700 Austenite Temperature, °C Pearlite 4 600 1000 500 50% 800 Pearlite + Bainite 400 900 Austenite 3 2 300 Bainite 600 500 Ms 200 M50 1 M90 Martensite and austenite 100 0 700 Martensite .01 1 10 2 3 100 Time, s 400 Bainite and martensite Fine pearlite 10,000 4 100,000 Fig. 1 — Continuous cooling curve for a ferrous alloy. *Member of ASM International and ASM Heat Treating Society 54 ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 300 1,000,000 Temperature, K 18 The result is a totally nonuniform heat transfer rate on various surfaces of different parts, which depends on a number of variables and factors. This uneven transitory step creates huge temperature differentials, and is the major factor in distortion when quenching in these media. Gas quenching is a pure, convective type single-phase quench. Gas species, pressure, and velocity are the main controlling factors. Gas-quench cells are equipped with powerful fans capable of injecting gases (typically up to 20-bar positive pressure) in conjunction with heat exchangers using chilled water to quickly remove heat from the quenching gases. The most common quenching media is high-pressure nitrogen gas. A major benefit of the more uniform cooling rate of gas quenching is lower part distortion. High-pressure gas quenching can sometimes eliminate the need for post-heat treatment straightening or clamp-tempering operations, reduce grind stock allowances and hard machining, and replace more costly processes, such as press quenching. Comparison of oil and gas quench rates Gas quenching intensity can be adjusted to match the cooling rate of liquid quenching as shown below. Heat transfer coefficient Liquid (), W/m2K medium 500 1000 2000 3000 Gas Salt 10-bar N Oil (unagitated) 20-bar He Oil (agitated) 40-bar H Water 100-bar H The mean heat transfer coefficient over total cooling in quenching a full load in a high-volume gas flow from 1650° to 212°F (900° to 100°C) can be matched to liquid quenching. Thus, the overall cooling rate from start to finish for quenching in 10-bar nitrogen gas (gas velocity = 10 m/s) compares to quenching a full

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