Ever wonder what influences the heating (and cooling) of a component part or workload during heat treatment? Ever ask yourself why we care so very much about the influence of temperature and time, ramp rates and when to begin soaking a load?

Ever wonder what influences the heating (and cooling) of a component part or workload during heat treatment? Ever ask yourself why we care so very much about the influence of temperature and time, ramp rates and when to begin soaking a load? The time has come to answer these questions. Let’s learn more.

Under ideal heating conditions, temperature is uniformly applied to the entire surface of each part within a workload; in the same manner and at the same time; under the same atmosphere conditions; and in equipment perfectly suited to the nature of the process. In the real world this never happens.

The manner in which heat is applied to the surface of a workpiece is highly dependent on loading. The design/method of operating the furnace plays a significant role as does the influence of process variables such as time and rate of heating (or cooling), which are as equally important as temperature.

One of our goals in heating is to reach the process temperature, which is determined by the material, the properties (physical, mechanical, metallurgical) we are trying to achieve, and the very nature of the process we are running. We must also make a distinction between the temperature of the chamber (or bath) in which the part or workload is placed and the temperature of the part itself, which is affected by the exposure time and rate of heat absorption. Another factor to take into account is the (required and actual) temperature uniformity of the chamber in which the heating occurs, which is why so much emphasis is placed on checking and rechecking this variable.

By comparison, time is a variable factor influenced by the method of heat transfer to and from the surface of the part. This includes such items as loading and exposure (i.e. the relationship of one part to another and to the heat source); the difference between the part temperature and the chamber temperature (which is why in some instances heat heads are not allowed); emissivity; heat-transfer methods (radiation, convection, conduction); the geometry/shape and mass of the part (which is why we care about the ruling or maximum part cross section, thin/thick sections and radical change of section); and a variety of other factors (Fig. 1). This is why we often hear the phrase “provided the part can absorb the heat energy at the rate delivered.” All of the above factors affect the rate at which heating or cooling should take place, regardless of how fast heat could be transferred.  

The Influence of Loading

Of all the variables affecting heating, loading is the one most often taken for granted. Non-uniformity of results, even from the same heat of steel, can be introduced if different parts of the workload are unequally exposed to heat (or cooling). Hence, it is not merely time and temperature but a time-temperature-mass-surface relationship that must be factored into the heating time.

Component parts come in all shapes and sizes. To meet this demand, standard and custom furnaces have been designed to accommodate the many workload configurations. Loading arrangements fall into two general classes: weight-limited and volume-limited. In either case, when loading parts in furnace baskets or onto racks, the goal is often to maximize loading efficiency. As heat treaters, however, we must also be concerned with proper part spacing (i.e. how parts are situated within the load for optimal heat transfer, atmosphere circulation, temperature uniformity and heat extraction during quenching so as to minimize dimensional variation).

How parts are loaded is very much a function of the style of furnace being used. Final spacing is dictated by concerns for heating, soaking, atmosphere flow, the volume and type of quench media (brine, water, polymer, oil, salt, air), and gross load weight. A number of “rules of thumb” used in the industry can help, but determining the proper spacing around parts is critical and is often best done by trial and error. In general, the gap around a part should be no less than 25% of its envelope diameter and no greater than 75%. Again, one must factor in both process and equipment variables, including the type and construction of the quench tank, nature of the process and part mass to name a few. Despite the almost limitless choices, some common-sense rules apply.[6]

Other factors include: using batch or continuous processes, having skilled (i.e. trained) operators, avoiding human error, the manner in which heat is applied, control of process and equipment variables, and understanding/properly adjusting automatic controls (e.g., instrumentation, sensors, valves, etc.).

Critical Temperatures

In heat treatment, we attempt to influence (dare I say control?) the microstructure of a material in part by the method of heating and/or cooling. This includes the microstructure, distribution of microconstituents, grain size, mechanical properties and the mechanisms by which they are brought about. The critical temperature is precisely that temperature where a phase change occurs in a metal during heating or cooling (i.e. the temperature at which we reach a critical point on the phase-transformation diagram for that material).

For example, on the iron-iron carbide phase diagram, critical temperatures (critical points) are denoted by the letter “A” (for “arrest”) followed by either the letter “c” (for the French word “chauffage,” meaning heating) or “r” (an abbreviation for the French word “refroidissement,” meaning cooling). These signs, Ac or Ar, are followed by subscripts 1, 2 or 3, which indicate the particular point being referenced. Thus, Ac₁ refers to the temperature at which austenite begins to form during heating, while Ar₁ refers to the temperature at which transformation of austenite to ferrite (or to ferrite plus cementite) is complete during cooling. Similarly, Ac₃ refers to the temperature at which the transformation of ferrite to austenite is completed during heating, and Ar₃ refers to the temperature at which austenite begins to transform to ferrite during cooling.

Varying the rate of heating will have an effect on the rate of transformation and dissolution of microstructural constituents. In general, the temperature of transformation increases as the rate of heating increases.

In the case of O1 tool steel (Fig. 2), austenite begins to form on heating to approximately 730°C (1350°F). As we continue to heat, we see that the rise of temperature slows (as heat is absorbed during this transformation) until we reach approximately 760°C (1400°F), where the transformation to austenite is complete and the temperature rise resumes its original rate. After soaking for a predetermined time at the peak temperature, which is of our choosing, the steel is withdrawn from heat and begins cooling. Austenite begins transforming back into ferrite at about 700°C (1295°F), and the rate of cooling slows (as heat is being released during transformation). When the transformation is complete around 670°C (1240°F), the original cooling rate resumes.

Some Effects of Alloying Elements[5]

In steel heat treatment, alloying elements produce a variety of mechanical properties and microstructures. One group of elements (e.g., Ni, Co, Mn, Pt, Pd, Rh and Ir) lowers the Ac₃ temperature and broadens the austenite-phase region. Another group lowers the Ac₃ temperature. In this group, elements such as N, C, Cu, Zn, Au and Re first broaden then cause the austenite-phase region to vanish. Other elements elevate the Ac₃ temperature, narrow and then close the austenite-phase region (e.g., Cr, Mo, W, Si, Ti, Al and Be). Elements such as Zr, Ta, Nb and Ce are responsible for producing other phases before the austenite field closes.

Final Thoughts

To understand heating of a material, one must carefully consider all of the factors that can influence such a seemingly simple task. Hopefully, we now have a greater appreciation for this important step in the process. IH