In the heat treating of materials, particularly metals, it is well known that the path of time and temperature followed in the cooling or "quenching" of the material after a heat treatment is critical in optimizing the ultimate working properties of the material. For example, FIG. 1 shows an isothermal transformation diagram showing the various phases possible in quenching of a steel. The diagram itself has, temperature increasing linearly on the vertical axis and time increasing In a logarithmic manner along the horizontal axis. The specific diagram is constructed for a given isothermal quench bath under known agitation conditions. The material to be quenched starts off at an initial temperature 100 which is substantially higher than the martensite start temperature M.sub.s, which is shown as line 102 in FIG. 1. Below this line, the material exists solely in the martensite ("M") phase. At the initial temperature 100, the material is essentially all in the austenite ("A") phase. The material may be converted in to a material which is all martensite through the quench process through a variety of routes. In one route, indicated as the "fast" quench, and identified in FIG. 1 as curve 104, the material is quickly dropped in temperature through a water or brine-based quench medium. In another pathway shown as curve 106, a "slow" or "slack" quench takes the material eventually to a martensite composition, but through a path which involves phase conversions which include ferrite and cemetite inclusions. This type of quench would be accomplished through a hot oil based quench medium. A more ideal quench pathway, shown as curve 108, would avoid the passage through the phase envelopes where ferrite ("F") and cementite ("C") phases accompany the austenite phase. These phase envelopes are defined by phase transition lines such as 110 and 112. In fact, it will be recognized that line 102 is itself a phase transition line. Although not shown in the particular phase diagram, it will be recognized by those of skill in this art that the phase diagram is not completely disclosed and that the area above line 102, there is also a potential for pearlite ("P") and bainite ("B") phases, although these are not shown in the present diagram.
In many situations in the prior art, the quench pathway would be effectively determined once the initial temperature of the particular body and the initial conditions of the quench tank and medium were set. This is because traditional quench media such as brine, water, polymer/water mixtures, oils, molten salts (such as marquenching or ausquenching), fluidized beds of particles, inert gas/air blast and still air, all have fixed cooling curves at a given bath concentration (or pressure), bath temperature and gill agitation rate. As a consequence, the quench practice known in the prior art has always been a compromise, with the treater trying to "fit" a fixed quenchant cooling curve or heat extraction index to either an isothermal transformation ("I-T") diagram of the type shown as FIG. 1 or a continuous cooling transformation ("CCT") curve of the specific material being quenched. The isothermal transformation diagram is so named because it depicts the cooling characteristics of a material in a quenchant which is maintained at an isothermal, or constant temperature, condition. The generation of a particular curve is dependent upon several factors, including the temperature of the quench medium, the geometry of the quench tank, the agitation rate in the tank and the inherent ability of the quench medium to extract heat from the workpiece.
There are known procedures in the prior art to vary the quench rate of a part formed from a heat-treatable material during the quench cycle. For example, an interruption of quenching, also known as "time quenching", involves the use of an initial rapid cooling step to achieve high hardness and then physically removing the part or material from the first quench bath and inserting it into a second quench bath containing a second quench medium, typically a less severe medium. This presents two problems. The first is the problem of knowing exactly when to interrupt the first quench. If interrupted too soon, the part receives a "slack quench" and does not achieve and full hardness is not achieved. If interrupted too late, the outside surface cools and transforms while the core or interior of the part is still too hot (above the martensite start temperature M.sub.s) and the resultant phase discontinuity can result in increased distortion and possibly cracking. The second problem is the severe discontinuity in quench rate caused by ceasing the first quench cycle and exposing the part or material to air, one of the least severe possible quench media, until it can be immersed into a second quench medium.
Another known method of varying the quench rate is to vary the rate of agitation of a fixed quench medium. The problem in this latter situation is one of control. A good heat treater knows that quenching thinner parts of a material in a region of low agitation and quenching thicker parts in a region of high agitation can result in reduced distortion while maximizing hardness of the part. Insertion of temperature probes in the core of the part or doing pretreatment computer modeling allows one to vary the agitation rate. However, no amount of variance in the agitation rate can escape the inherent limitation of the ability of a given quenchant to absorb heat from the material.