Decarburization (usually unintentional) is the opposite of carburization. It is the loss of carbon from the surface of a tool steel as a result of heating at a high temperature in a medium or atmosphere that reacts with carbon. Decarb is always a problem and is present on all hot rolled, cold drawn, and forged tool steel bars. There are two distinct decarburization surface conditions that should be of concern:

1. The thin layer of decarburized material, often called bark, that appears on the surface of tool steel (hot rolled, forged and cold drawn) supplied by the steel manufacturer.

2. The decarb surface that sometimes appears on finished tools and dies as a result of improper heat treatment.

At the mill during heating for cogging, rolling , forging and annealing, it is virtually impossible to prevent a certain amount of carbon being oxidized from the outer skin of the bar. A typical decarburization condition of a 1" x 3", 1 percent carbon tool steel bar would show that to a depth of approximately .005", the outer skin has lost practically all its carbon. The next .020 to .025" would form a gradation zone with the carbon gradually increasing until it reaches the normal 1% carbon analysis for the rest of the bar.

This condition is known as surface decarburization, or bark, and it is very important to understand it. Depleted of carbon, the out surface will not transform to martensite upon subsequent hardening and the steel will be left with a soft skin. For most tool applications this effect of decarburization is extremely harmful. Since decarb is a surface phenomenon and in many tool configurations the surface layers are the most highly stressed in service, tool steel fatigue failures usually originate in the decarburized layer. Decarburization also causes a decrease in tensile strength which is reflected in a reduced endurance limit, since the two values are proportional. It is relatively easy for the man in the shop to detect this thin layer of decarburization material on the surfaces of hot rolled bars or forgings. The bulk of all tool steel sold is the form of such "hot rolled" bars which go directly from mill inspection to the warehouses.

A substantial quantity of tool steel is also purchased in cold drawn bars for the manufacture of drills, taps, and similar tools. Contrary to popular opinion, cold drawn bars exhibit almost as much decarburized surface as hot rolled bars, although it may be overlooked in the shop because the finish is brighter and cleaner. Cold drawing is primarily a stretching operation. The tools steel bars are first pickled, then drawn through a die to elongate the grains of steel and render the outside of the bars smooth and accurate to size. Except for the small amount of metal eaten away by the pickling acid, actually no material is removed from the surface.

Of course, the appearance of any bar surface alone mat not indicate the presence or absence of decarburization, although a scaly surface should always serve as a danger signal. The degree to which a bar may be decarburized depends upon the chemical analysis of the steel. Straight-carbon-tool steels are least subject to decarburization, while shock resisting tool steels are most prone to this formation. Hot work tool steels are generally considered fair, and the majority of the other tool steels exhibit good overall resistance to decarb. Sometimes there will be no skin of total decarburization at all, but rather a surface zone which is simply lower in carbon than the balance of the bar. Any amount of decarb on the surface of tool steel, however, is undesirable and likely to cause trouble. Decarburization is one of the main reasons why steel manufacturers recommend removing a certain amount of metal from the surface of the bar when making a tool. If tool steel is required free from surface decarburization, it must be ground or machined.

Despite the many warnings by tool steel manufacturers about the dangers in not removing bar bark, there are still some toolmakers who fail to remove decarb before starting their work. There are generally 2 basic reasons given for this poor practice:

1. Savings in machining time.

2. Starting with a bar of the same size as the finished die because it is the only size at hand.

Although a few minutes of machining time might be saved by not removing the bar bark, invariably the entire tool or die is lost. Other toolmakers only go halfway. They remove decarb from some or most of the bar's sides but fail to grind those sides that do not require full hardness. This is a bad practice that invites trouble. Failure to remove decarb in equal amounts from all sides of the tool steel bar causes cracking and excessive warpage during the heat treat cycle.

From a heat treating standpoint when decarb is not removed from hot rolled bars, the difference in carbon between the surface and the inside of the bar is usually sufficient to cause cracking or warping in hardening - no matter what the grade of too steel. To get full value and service life from a tool or die, it is necessary to have the full carbon content right out to the surface. Prevention of subsequent decarburization of the machined tool and die is equally as important as first removing the decarburized layer of bar bark.