TECHNOLOGY
What is "Soak" time?
- Is soak time necessary to produce the required properties; complete relief of internal stresses or full transformation to tempered martensite?
- Is soak time a poka-yoke (mistake-proofing) to compensate for poor oven design or the myriad of loading/processing arrangements?
- Is soak time muda (waste)? Is more time truly better than less time?
Necessary? - No
It is well understood and accepted that Stress Relieving and Tempering are a function of time and temperature.
The Hollomon/Jaffe1 tempering parameter is a correlation of change in hardness of martensite in steels as a function of tempering temperature and time. This correlation was developed because the change in hardness of martensite is directly affected by carbide growth or coalescence and is a function of time at temperature.
1: J H Holloman and L D Jaffe, "Time-Temperature Relations In Tempering of Steel" AIME, vol 162, 1945, pp. 223-249.
A 25°F increase in temperature will produce a 1 HRC drop in hardness.
A time factor of 10x will produce an approximate 2 HRC drop in hardness.
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The Larson-Miller2 curve shows this time and temperature relationship as a function of Thermal Effect (TE). As time decreases, temperature must increase to achieve equivalent results. With increased temperature the process window (time) narrows or decreases, demonstrating the need for precision. |
2: F.R. Larson and J. Miller, "A Time-Temperature Relationship for Rupture and Creep Stresses," Trans ASME, July 1952, pp. 765-775. |
Poka-Yoke? - Yes
An oven is a heat transfer machine, yet in current practice ovens are standardized around a heat or temperature uniformity procedure. There is no true standard procedure for heat transfer measurement. Soaking at temperature is merely the mistake-proofing method for making heat transfer uniform.
Conventional continuous conveyor furnace with parts loaded in baskets.
PYRO continuous conveyor furnace with parts loaded directly on a belt.
Muda? - Yes
As demonstrated in the above example which delivered equivalent results, soak time is truly a Muda or waste. Maximizing productivity and quality requires precision. Unlike conventional stress relieving and tempering furnaces, Pyro furnaces are Precision Heat Transfer Machines. Not only in Precision of temperature uniformity, but Precision in repeatability of heat transfer rates. Furnace design is the key factor in allowing Pyro to operate with reliable repeatability further down the Larson Miller curve. As we like to say, Pyro is sandwiched between two known technologies.
Pyromaitre Inc. - 1081 Chemin Industriel - St.Nicolas, Quebec - G7A 1B3 - Canada
1 (800) 231-PYRO
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