Case-hardening,

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Case-hardening is a collection of processes used to increase hardness to a value or a range higher than that found or achievable in the material bulk, locally on the surface of metal parts and for a limited depth underneath.

Higher hardness is generally associated with improved wear resistance and possibly with increased fatigue resistance if residual compressive stresses are present on the surface.

Usually the process is referred to steel. Low carbon or mild steel are not hardenable by heat treatment. Case-hardening can be performed in two stages. The chemical composition of the surface and near surface layers is modified by introducing there Carbon or other elements by a suitable Thermally activated diffusion process.

After diffusion of the added elements, the material of surface modified composition is now responsive to regular heat treatment.
Carburizing is the process of adding Carbon, either by pack-carburizing in bulk by immersing the parts in a volume of solid powders, usually of proprietary composition, in a furnace at high temperature, or in a gaseous atmosphere furnace.

Carburizing management is achieved by controlling carbon potential (through gas composition and dew point) and by checking the exposure time at temperature to diffuse carbon to the required depth. Recently a (partial) vacuum process was developed providing shorter time and other economic advantages.

Carbonitriding is a Case-hardening process that is applied in a molten salt bath or in a specially prepared atmosphere (this last process being called sometimes nitro carburizing).

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Gas Nitriding is performed in a gas atmosphere of dissociated anhydrous Ammonia on steels including elements like Aluminum or Molybdenum that readily form nitrides and also Chromium and Vanadium. This process follows complete hardening and tempering and is performed at relatively low temperature: it is not followed by other heat treatments. It has therefore special advantages, in that it introduces less deformation.

Medium or high Carbon steel is usually supplied in normalized condition that permits machining. Case-hardening can be applied by heating only the required surfaces either by flame or by induction and by immediate quenching, followed by tempering as required. Here the composition is not modified, only the microstructure is transformed to tempered martensite that exhibits high hardness.

What has Case-hardening to do with welding? Not much, in principle. It is however very important to know if Case-hardening is present in the implement one may wish to weld. It may also be useful to know how to integrate Case-hardening in a welded item.

It is not advisable to weld Case hardened material. The hardened layers should be removed by grinding or by other methods. The thickness to remove should be at least twice the thickness of the case.

After repair welding, it may be necessary to repeat Case-hardening, depending on the application, especially if the implement has surfaces subject to wear. Possibly a different process like Hardfacing or Thermal Spraying could be used for the same purpose.

If you weld on hardened surfaces you risk getting cracks. In fact it would be the first thing to investigate in case of weld failure. How can you know if there is Case-hardening on the part? In case of doubt start testing with a file: if you cannot indent the surface, it is probably hard.

Of course it would be better to test the hardness by more sensitive methods, i.e. by micro indentation, but the conclusion is the same. Even if cracking is avoided the original improved hardness is compromised after welding.

Case-hardening includes much theory and a lot of experience to be applied successfully. For a welder it is enough to know that it is not compatible with welding and that proper solutions, to have those processes work together, should be investigated by expert people knowledgeable in both disciplines.

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