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Martensite and the Control of Retained Austenite

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Formation of martensite in fine-grained steels is probably the most common goal in heat treatment of components. The carbon content of the parent austenite phase determines whether lath (low-carbon) or plate (high-carbon) martensite, or mixtures of the two will be produced, assuming the quench rate and steel hardenability are adequate for full hardening. Lath martensite produces higher toughness and ductility, but lower strengths, while plate martensite produces much higher strength, but may be rather brittle and non-ductile.

For a given alloy content, as the carbon content of the austenite increases, the martensite start, Ms, temperature and the martensite finish, Mf, temperature will be depressed which results in incomplete conversion of austenite to martensite. When this happens retained austenite, which may be either extremely detrimental or desirable under certain conditions, is observed.  The amount of retained austenite present depends upon the amount of carbon that can be dissolved in the parent austenite phase and the magnitude of the suppression of the Ms and Mf temperatures. This paper examines the conditions under which austenite is retained and the problems associated with it presence, with detecting it and with measuring it.

When to Use a Partial Pressure in a Vacuum Furnace

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When heat treating or brazing in vacuum, the vapour pressure of the constituents in the materials being processed can be a very important consideration. 

The vapour pressure of a material is that pressure exerted at a given temperature when a material is in equilibrium with its own vapour.  Vapour pressure is a function of both the material and the temperature.  Figure 1 shows the approximate vapour pressure curves for a variety of metals and compounds.  The area to the left of each curve represents the conditions of temperature and pressure under which the material exists as a solid.  The area to the right of each curve represents those conditions under which the material exists as a gas (or vapour). BY JEFF PRITCHARD

A Vacuum Heat Treater’s Library

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Over the years, many people have asked if we could recommend good books on the subject of Vacuum Heat Treatment. The following list includes books that we have found particularly useful  with respect to the scientific and practical aspects of vacuum, heat treatment, metallurgy and material science. Enjoy..

As you can tell from the list, some books are classics which have stood the test of time, some are relative newcomers, but all share the common trait that they are used each and every day by those of us who work in the fields of vacuum, heat treatment and metallurgy. The readers are encouraged to offer suggestions as to their favorite and most useful texts.

Determining the Nodularity of Graphite in Ductile Iron Using ASTM E2567

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ASTM Committee E-4 on Metallography began a program to develop a test method for rating the nodularity of graphite in ductile iron in 1986. The initial effort centered upon using the sphericity equation to assess the shape. However, an interlaboratory study showed that the perimeter measurement varied with the magnification used. A perimeter-free shape factor based on the maximum Feret’s diameter was determined to be magnification independent and reliable.

Additionally, two types of “convex perimeters” were proposed over the ensuing years, but they were demonstrated to be highly biased towards yielding high nodularity ratings regardless of the irregularity of the particle’s shape. The study reported here compared the use of the maximum Feret’s diameter in the shape equation to the mean Feret’s diameter, 100X vs. 200X, a minimum shape factor limit for a particle to be a nodule of 0.5 vs. 0.6, and calculation of an area-based vs. a number-based % nodularity.

Eutectic Melting

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In vacuum processing, metal surfaces remain very clean and free of oxides. When these near-perfect surfaces are in contact with other surfaces, certain elements have a tendency to interact between the surfaces through solid state diffusion. Therefore, a major consideration when selecting both hearth and load fixturing materials for vacuum heat treating is the possibility of solid state diffusion between different materials in contact at high temperatures. Solid state diffusion of certain elements can cause the formation of a lower melting point alloy called a eutectic. For example, solid state diffusion between carbon and nickel can begin to occur at temperatures as low as 1165ºC (2130ºF) and cause local melting, also known as eutectic melting. BY JEFF PRITCHARD

Why Heat Treat in a Vacuum?

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The air we breathe contains a number of elements that can react with metals under the proper conditions.  Moisture, oxygen, carbon dioxide and hydrogen are present in significant amounts in our atmosphere.  Each can react to varying degrees with many different metals.  While many of these reactions occur to only a small extent at room temperature, they are often greatly accelerated in the presence of heat.  Consider the example of a piece of polished metal held over a heat source.  It will eventually turn blue or black as the elements in the atmosphere react with the hot metal.

In most cases, the heat treater tries to minimize the extent of these reactions during heat treating.  The reactions cause changes in the surface properties of the metal that may result in a heat treated component with a “skin” that is much softer (or harder) than the rest of the component.  To minimize these undesirable reactions, the source of the reactive elements, air, must be eliminated from the heat treating environment.  Sometimes this is done by replacing the air in a heat treating chamber with a non-reactive atmosphere such as nitrogen, argon or other gas mixtures.  This is often referred to as controlled atmosphere heat treating.  Another alternative is to heat treat in a bath of non-reactive molten salt.  However, these environments still contain some very low levels of residual impurities so metals heat treated in a controlled atmosphere or salt usually exhibit a small amount of discoloration. BY JEFF PRITCHARD

Fracture of a 17th Century Japanese Helmet

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There was a crack in the helmet which is not visible in this image (some associated damage can just be seen in the lower left side of the helmet visor). The crack was opened and the fracture began at a streak with mostly intergranular fracture and then propagated by cleavage as shown below.

Note the intergranular fracture in the center foreground. The walls show transgranular cleavage the propagated from the intergranular origin. Next to the fracture, we see a region of columnar grains at the surface with a small region of finer, more equiaxed grains below and the very coarse columnar grains blow that, as shown below.