Isothermal Hardening

The cooling power (or quenching severity) of molten salt is increased by agitation. When salt bath fur-naces are used for martempering and austempering, two processes that will be described in this article, they are equipped with pumps or propellers to create the required turbulence.

In salt baths, heating is even be-cause a uniform temperature is main-tained throughout the molten salt. In other types of heat treating furnaces, uneven distribution of heat causes problems. Some areas of the work heat up more rapidly than other areas, causing distortion.

Salt baths also minimize distortion by preheating the work automatically. As a cold part enters the bath, it be-comes enveloped in a cocoon of frozen salt. Although this cocoon melts in about I minute, it acts as a protective shield until the work has been safely preheated.

Isothermal Treatment. Today, salt baths are used for aluminizing, an-nealing, brazing, carburizing, case hardening, cleaning, descaling, draw-ing, heating of forgings, neutral hardening, nitriding, precipitation harden-ing and tempering.

And there’s still another applica-tion—isothermal treatment, one of in-dustry’s most advanced heat treating techniques. By using the isothermal process in austempering and martern-pering, it’s possible to hold critical dimensions on hardened steel parts.

In order to explain austempering and martempering fully, it’s necessary to make reference to S-curves, Figures 2 and 3. In heat treatment the micro-structure of the workpiece is trans-formed to obtain desired properties. Essentially, an S-curve comprises two curves, one indicating the beginning of transformation, the other the end of transformation. S-curves are impor-tant in determining the rate of cooling required to avoid premature trans-formation, the time required to obtain a desired structure and the tempera-hire at which the structure is actually obtained.

The shape of the S-curve depends on the chemistry of the steel, temper-ature and time, grain size and cooling rate. All these factors affect the dis-placement of the curve and the start of transformation, which is called the Ms point. The general effect of carbon and most alloying elements is to move
the curve to the right, which may permit a milder quench or slower cool-ing rate than is necessary for a low-carbon or nonalloy steel to achieve an equivalent microstructure. It also low-ers the temperature at which certain microstructures such as martensite be-gin to form.

Prior to the introduction of the S-curve in the early 1930’s, the iron-carbon equilibrium diagram was the only tool available for explaining the phenomenon of hardening. It did not show the effects of heating time or cooling rates. Because it includes time as a factor, the S-curve represents a three-dimensional approach to under-standing heat treating problems.

The isothermal process involves heating steel or cast iron metal to cause its constituents to go into solu-tion (austenitizing) , and then quenching it at or above its critical cooling rate so the material reaches the isothermal quench temperature with no undesirable higher tempera-ture transformation properties.

The quench temperature is indicat-ed by the S-curve for the particular steel being treated. For example, when steel is heated to 1500 F or so, carbide in the iron dissolves to form austenite. If the austenite in steel with 0.80 percent carbon (eutectoid steel) is cooled to 1200 F and held at that temperature long enough, it trans-forms entirely to a soft pearlite micro-structure. If however, the cooling rate quickly reduces the austenite to 600 F, pearlite is avoided. Bainite, a much harder microstructure, results. And, if the cooling rate is sufficiently rapid, formation of both pearlite and bainite is bypassed. The microstruc-ture then becomes martensite, the hardest structure that can be pro-duced. When the temperature of the austenite approaches 400 F, marten-site begins to form, reaching 90 per-cent or more at 200 F.

The rate at which a workpiece cools is determined by the quenching tem-perature, degree of agitation and size of the workpiece. Heat is abstracted from the piece by cooling the surface. Quenching severity is a measure of how fast heat is removed.

Many quenchants have been and are being used with varying degrees of success: water, brine solutions, oil, air (in special cases) , sodium hy-droxide solutions and molten salt.

In austempering and martempering, molten salt has proved its worth as a quenchant from several standpoints. Distortion is minimal, permitting parts to be machined before hardening in many instances. Parts are endowed with higher, more uniform hardness, and greater toughness and ductility.

Martempering and Austempering. Both martempering and austempering produce high strength. In martemper-ing, rapid cooling is interrupted just above the martensitic transformation temperature, which varies according to the steel’s composition. The work is held in a constant-temperature bath until this temperature is equalized throughout the piece. Then it is cooled to room temperature and tem-pered in the usual manner. No bainite is allowed to form. Maximum hardness is the final result.

In austempering, the piece is quenched in a fixed-temperature bath and held at this temperature (500 to 750 F, depending on the steel) until the austenite completely transforms to bainite and the hardening transfor-mation is complete. This process in-volves less total time (no additional tempering is needed) and the result-ing bainite structure has a higher level of toughness for a given hardness.

The 5-curve for austempering, Fig-ure 3, reveals the similarities between martempering and austempering. Here too, the steel or cast iron ma-terial must be austenitized and then quenched in a salt bath at a tempera-ture above the Ms point. In austem-pering, the part must be held just above the Ms point at a constant tem-perature until austenite completely transforms to bainite.