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Hardening

Hardening to promote wear resistance is accomplished by a variety of means. Here, we will discuss aspects of heat treating, nitriding, and carburizing. All are recognized for their comparatively thin films.

Heat treatment comprises myriad processes, each with a specific task. They include: relief of residual stresses, creating ease in machining, altering microstructure for improved properties, and to raise hardness.

Strains often build up, distortions or imperfections, too. That is why annealing is often required. This is a process by which a work piece is held at a specific temperature, for a prescribed period, then gradually cooled at a predetermined rate. The result is a clearer, stronger, more uniform material.

Stress relief is a strength-increasing process called normalizing, where residual stresses are removed that may have been formed by differential cooling rates within the work piece during forming, or from shot cleaning, grinding, or other processing.

Quenching and tempering create the ideal combination of ductility and strength. The liquid medium can be oil, water, brine, or a water-soluble polymer. The result is less retained austenite and a more desirable martensitic, homogeneous microstructure with fewer residual stresses.

Hardening techniques include the nitriding process. Here, ferrous alloys, usually of special composition, under specific time-temperature conditions, are in contact with a nitrogenous material to produce case depth hardness through absorption of the nitrogen without quenching.

For nitriding steel, the best alloying elements include aluminum, chromium, and molybdenum. Aluminum is the strongest nitride former. Nitriding stainless steel can yield hardness between 64-70 Rockwell C, though corrosion resistance will be reduced.

Alloys used to form nitrides are known as nitroalloys. Several types are available with ranges in composition as follows: aluminum (0.85-11.2%), carbon (0.20-0.45%), chromium (0-1.8%), molybdenum (0.15-1.00%), manganese (0.4-0.7%), silicon (0.2-0.4%). The result is improved surface hardness, wear resistance, fatigue life and, to some extent, corrosion resistance (except for stainless steel).

Surface distortion risks are minimal, but the process may require considerable time, especially for deep cases with gas nitriding. Generally, real dimensional growth does not exceed 0.001 to 0.002 inch.

Case depths to 0.030 inch are attainable, as well as hardness 50-65 Rockwell C, depending on steel. Tempering temperatures should be a minimum of 50 degrees Fahrenheit above the nitride temperature.

Major methods include liquid, gas, and plasma.Liquid or salt bath nitriding requires use of molten salt baths. Average temperatures are between 950 and 1075 degrees Fahrenheit. Here, the source for nitrogen comes from salts containing cyanides or cyanates. Advantages include minimal distortion, as well as the capability of hardening plain carbon steels.

For ion nitriding (also known as plasma nitriding), the vacuum chamber is the anode, and the work load (isolated from the chamber) is the cathode. After a controlled amount of gas is introduced, a direct current potential is established, where positively charged particles of nitrogen bombard and diffuse into the metal surfaces, combining with the alloying elements.

Advantages to this process include the range of metals, including stainless steels, and different cases produced. Processing times are shorter, the method more efficient and environmentally friendly.Carbo nitriding is similar to cyaniding, except that carbon and nitrogen are absorbed simultaneously by heating in a gaseous atmosphere.

Temperatures between 1450 and 1650 degrees Fahrenheit are common, and, unless there is risk in distortion, quenching is normally performed to reduce risk of loss in impact strength or becoming too brittle.Carburizing improves hardness by increasing the carbon content of exposed steel surfaces.

This is accomplished by heating the steel above its upper critical temperature with an appropriate carbonaceous material. Outcome is based on alloying elements in the steel and in equilibrium.

If the atmosphere has a higher carbon potential than the steel, the steel will absorb carbon. If the carbon potential of the atmosphere is lower, than the steel may yield carbon to the atmosphere.

All of these hardening techniques are designed to reduce surface damage. Whether it is material loss or deterioration in performance you experience, it’s important to understand that structural changes, plastic deformation, surface cracking, loss or gain of material, or material degradation by chemical reaction, are all dependent on hardening.

It is these hardening processes that, when used with coatings, can produce true synergism for preventing or reducing wear. Whether it’s abrasive wear, adhesive wear, erosive wear, polishing wear, or fretting wear.

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