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FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 0 6 1 graph that while the quenchant is in contact with a very hot part it is in the vapor phase and the cooling rate be- tween concentrations of 5–15% do not seem all that sig- nificant. The same relative insignificance holds true at the lower temperature ranges. The cooling rate in the nucle- ate boiling range of 800-1200 ° F shows a much different cooling rate for the three different concentrations. At a quenchant concentration of 5% the cooling rate at 1000 ° F is 320 ° F/sec while at a 15% quenchant concentration the cooling rate is 200 ° F/sec. The critical point of understand- ing to this comparison is that a 10% change in quenchant concentration changes the cooling rate by 50% (Note, the author is using approximate values for simplicity of report- ing). Metallurgically, the range of 800-1200 ° F is also a key area in the formation of different structural outcomes such as martensite, bainite, and pearlite as shown in Fig. 2. In most hardening processes for steel the objective is to form martinsite ( M ). In order to form martinsite the cooling rate must be fast enough to transform austenite ( A ) to martensite by forcing the part temperature to stay to the left of the red line and miss the ‘knee’ of the tran- formation curve. A slower cooling rate, within the red and green boundries, translates to a mixed microstructure of martensite and bainite ( B ). Further arresting cooling rate, to the right side of the green line, yields a combination of bainite and pearlite ( P ). The cooling rate associated with the knee of the curve falls into the critical range of 800- 1200 ° F, which is the same range where the effect of quen- chant concentration changes has the biggest impact on cooling rates. It is evident in Fig. 3 that bainite does not exhibit the same hardness as martensite in eutectic and eutectoid steels. The correlation between cooling rate and hardness can be deduced from the isothermal transformation dia- gram and the structural difference in hardness shown. The transition of austenite into alternate microstruc- tures does have risks. Two common undesirable outcomes of too rapid cooling are cracking and distortion. Both of these outcomes have unbalanced stress formations as a root cause of their occurrence. Transient stresses are responsible for the great major- ity of cracking in induction hardening which occur because of volumetric expansions and contractions associated with temperatures, thermal gradients, and phase transforma- tions [1] . Using lower-than-desirable quench temperatures and concentration, higher than specified quench flow rates and pressures can initiate cracking [1] (Fig. 4). The role of quenchant concentration and resulting cooling rates is evident in temperature range for stress differentials to occur (400–1200 ° F). There is a correlation between structure formation, cracking, and distortion. The temperature range for these occurrences lies in the area Fig. 2 — Typical isothermal transformation diagram. Fig. 3 — Hardness of martensite verses bainite. Courtesy of Knife Steel Nerds. Fig. 4 — Stages of stresses. Courtesy Handbook of Induction Heating, Second Edition. 7