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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TABLE 1 — CORRELATION OF MATERIAL, COOLING RATE, AND HEAT TRANSFER COEFFICIENT Heat transfer coefficient, Core hardness Cooling rate, °F/s (°C/s) 20 mm diam. 30 mm diam. 40 mm diam. 55NiCrMoV6 >57 HRC 38.3 (3.5) 0.85 190 275 350 X210Cr12 >64 HRC 40.64 (4.8) 0.63 260 370 480 90MnV8 >64 HRC 42.08 (5.6) 0.54 300 430 550 42CrMo4 >54 HRC 140 (60) 0.05 >2000 >2000 >2000 42CrMo4 mod >54 HRC 38.84 (3.8) 0.80 350 460 600 16MnCr5 >300 HV 59 (15) 0.20 800 1150 1500 20MoCr4 >300 HV 73.4 (23) 0.13 1250 1800 >2000 15CrNi6 >300 HV 39.2 (4) 0.75 200 320 400 Even with the highest cooling rate possible (close to 120°F/s or 50°C/s) in high-pressure gas quenching, peak cooling in gas does not come close to that of oil in the nucleate boiling phase, where maximum values of 212° to 300°F/s (100° to 150°C/s) are possible. As these high cooling rates during nucleate boiling occur in the important phase of steel quenching (the ferrite and pearlite noses of CCT diagrams), quenching of low-hardenability steel in oil produces a pure martensitic structure. By comparison, high-pressure gas quenching produces a hardened structure containing pearlite and ferrite, despite the fact that average cooling rates of both quench systems are equal. Thus, there is a large uncertainty for gas quenching in predicting hardness and structure of quenched steel components. A procedure developed by Ipsen to predict the hardness and structure after gas quenching makes use of the necessary cooling rate in the temperature region of the pearlite and ferrite formation (i.e., between 1470° and 930°F (800° and 500°C). If the necessary cooling rate to avoid pearlite and ferrite formation is reached 19 Temperature Fig. 2 — Three cooling stages of an oil quench. Time Fig. 3 — Comparison of cooling rates of high-pressure gas quench and cold oil quench. 1800 1600 1400 Temperature, °F load in a molten salt bath, and a 40-bar hydrogen gas quench matches the average cooling rate of an agitated, highgrade fast oil quench. However, it is not accurate to conclude that quenching in 40-bar hydrogen gas produces the same metallurgical result with respect to hardness, case depth, and microstructure. The temperature and time dependence of the cooling rate is totally different in a liquid with pronounced nucleate boiling phase and pure convection cooling. Figure 4 illustrates this effect by comparing the temperature-dependent cooling rates of gas and oil quenching. HTPRO Steel grade 1200 1000 800 600 Cold oil Gas quench in cold chamber 400 200 0 0 50 100 Cooling rate, °F/s or exceeded during a given point in the quenching process, you can be sure about achieving the required results. Defining these cooling rates from a given CCT diagram leads to the question: What is the necessary heat transfer coefficient for given workpiece diameters to reach the specified cooling rate in the core of the pieces? Solving the heat conduction equation for a given problem or respective approximation formulas enables estimating the necessary heat transfer coefficient. Table 1 shows the results of such estimation. Therefore, a more useful method is empirical measurement of the heat transfer coefficient in each gas quench system, a task easily performed 150 200 using a specialized tool such as the Ipsen flux sensor. Conclusion In many cases, the first attempt to gas quench a component does not lead to reduced distortion because load and gas flow considerations are not optimized. Using existing knowledge about laminar and turbulent gas flow together with gas quenching adapted and adjusted to load configuration nearly always leads to much lower distortion compared with oil and salt bath quenching. HTPRO For more information: Aymeric Goldsteinas, product development manager, Ipsen Inc., 984 Ipsen Rd., Cherry Valley, IL 61016; 815/332-2551, aymeric.goldsteinas@, ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 55

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