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NITIN FORGINGS & STEEL INDUSTRIES

Better Steel Through Quality & Innovation ISO 9001:2000 Certified Company
 
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Heat Treatment Hints

Annealing

Annealing is carried out in order to soften the material and at the same time to relieve internal stresses with refining of the grain which improves toughness. "True" or "full" annealing which should always be employed with Alloy, Tool Steels, involves heating steel to a temperature above the upper critical point followed by slow colling , i.e., furnace cooling or its equivalent. Exceptions are in certain of the nickel-chromium-molydenum qualities which should be subritically annealed at 640° C.Work should be charged into a furnace standing at not more than 200° C then heated steadly to the annealing temperature. Fully – machined tools should be packed during annealing to avoid surface decarbrisation.

Stabilising

As an alternative to full annealing heavily – machined tool steel parts may be stress relieved before hardening in thiscase the steel is charged into a furnace standing at not more than 200° C and heated steadily to the stabilizing temperature of approximately 650° C., followed by furnace cooling.The relieving of heavy machining stresses may be accompanied by slight distortion.The components should therefore have a slight machining tolerance left on for removal by light cut ( before hardening and tempering, if it cannot be done after ).

Hardening

Before hardening, an adequate amount of metal should be removed from the surface of all forgings and black bar material.
In any hardening operation the heating of the steel and its subsequent cooling are equally important. Components of complex shape, or which the hardening temperature exceeds 970° C, should be pre-heated to 550°/650° C (850° C in the case of high-speed steels).

The steel is heated steadily to the hardening temperature, which differs according to the composition of the steel and at which the greater part of the carbide is dissolved in a uniform solid solution know as austenite.

Typical soaking times per inch thickness of rulling section, at hardening temperatures are (for furnace hardening) 15 minutes for Corbon tool Steels and 20 minutes for other types except High-speed steels. When liquid baths, the normal hardening temperatujres should be reduced by 10°/15° C and the soaking times reduced by 30/50 o/o .

High speed steel tools, on attaining the hardening temperature, should be with drawn from the high-temperature furnance or bath. Tests on appropriate trail pieces are advised to determine the time required to reach the hardening temperature, if this exceeds 20 minutes a second preheating treatment at 1100°/1120° C is recommended.

As soon as the soaking period is complete the steels are guarented for water and when the Quantity changes to a very hard structure , normally in which the carbide a uniformaly dipressed.

The degree of hardening increases with increased agitation of the quench bath and with decrease in temperature of the quenching medium. A suitable range for water, and for brines containing up to 10% salt, is 20°/30° C. With oil on other hand there is little effect on cooling rates, even when the bath temperature ranges from 20°/60° C. In addition oil gives greater uniformity in structure and hardness.

The Temperature of quenching baths should never be allowed to drop below 20° C. Work should be quenched down to, but not below a temperature at which it may be comfortably handled, following which it should be tempered immediately.

Tempering

For some applications such as cutting tools, a martensitic structure is satisfactory, but in most cases the steel must be tempered to discompose the hard brittle as quenched constituent into a softer and tougher product. Thus the ductility is increased but the hardness and strength are reduced. The effect being greater, the higher the tempering temperature.

A further purpose served by tempering is to relieve stresses set up during the hardening operations.

Tools, especially of carbon tool steel, either of a complex shape or requiring high tempering treatment may conveniently be charged into a furnace standing at not less than 150° C, and heated steadily to the appropriate tempering temperature.

Time at temperature naturally depends upon the size and nature of the work, but 30 minutes will suffice for most small pieces where as larger parts will require longer periods in proportion. This normal method of cooling is in still air.

Certain manganese and nickel-chromium steels when tempered between about 250° C and 450° C, shows a loss in impact value. In the heat treatment of certain steels, such as high speed steels and certain hot-work qualities, the maximum hardness is not developed on quenching, and a double tempering treatment (also called secondary hardening) is required.

Surface Hardening

The packing of tools in spent charcoal during annealing or hardening is not a carburizing process.

A hard-wear-resistant case may be conferred by the introduction of carbon (as in carburizing, nitrogen(as in nitriding or both (as in cyanding).

In carburizing a carbon content of about 0.1/0.2% is usual in the steel to be treated. Alloy additions made to strengthen the heat-treated core and to provide a stronger support for the case, also generally improve the carbon diffusion Chromium Steels show more rapid, and nikel steels less rapid carburization that carbon steels. Normal times are about 6-8 hours (half that for gas carburizing) at 930 C, which will give a case depth of 0.040/0.060".

The "single quench" treatment does not give optimum properities to either the case or core, but is a compromise it is useful when distortion must be kept to a minimum.

Cyaniding is carried out at the hardening temperature for the steel giving a shallower but generally harder case than carburizing.

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