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Heat treatments

Murex offers you the following procedures for hardening the surfaces of screws:
  gasnitriding
  plasmanitriding
  through-hardening



Gasnitriding
During gasnitriding ammonia gas is used as nitrogen carrier. The ammonia gas is undone in its atomic components in the nitriding-oven with over 500°C at the surface of the workpiece. By diffusion the nitrogen penetrates into the steel surface and combines with the metals, which the alloy consists of, to so-called special nitrides (for example to chrome nitrides). These give the nitriding coat the highest attainable hardness which is possible for steels (up to 1200 HV).

advantages:
  high fatigue endurance limit
  nitride-hardened elements are very distortion low
  The hardness of the nitriding coat remains when heating up to 500°C.
  Nitriding hardening results in an extremely hard, but also wear resistant and slidable outer coat.

disadvantages:
  The low clasping of the nitriding coat with the basic material can, under high surface pressure, lead to chipping off the hardness coat.
  Small penetration depth (approx. 0,5 mm to 0.6mm)
  long treatment times in some cases



Plasmanitriding
During plasma nitriding the workpieces which can be treated are brought into vacuum retorts, which are fed with a nitrogen water mixture as treatment gas. Between the cathodically switched construction units and the anodically switched retort a plasma is ignited by creation of DC voltage. The ionized nitrogen molecules hit the workpieces
with high kinetic energy and diffuse into the surface under formation of special nitrides - same as during gas nitriding. Due to the high kinetic energy of the coiled nitrogen molecules, nitriding of stainless steel is possible too with this procedure, since the passive chrome oxide coating can be destroyed.

advantages:
  no embrittlement of the core material
  wear resistant surface
  high speed
  high temperature resistance
  minimum delay
  good sliding qualities
  accurate partial surface treatment by cover with paste
  also non-ferrous metals can be plasma-nitrided

disadvantages:
  Small depth of hardening (max. 0.25 mm with the current screw materials)
  Sensitivity during shock loading



Through-hardening (tool steels, resp. powder metallurgy steel)
Hardening consists of several processing steps. First the workpiece
is warmed up to  hardening temarature in the hardening oven and held on temperature for some time.  Afterwards it is quenched, which means dipping it into water or oil. Thus the structure of the steel changes, it becomes very hard. In order to reduce break sensitivity, which is connected to the high hardness, to a permissible amount, our screws are started and paged out after the hardness process.

advantages:
  Approximately the same structure and constant hardness over the whole workpiece cross section (and not only hard surface, like when nitriding)

disadvantages:
  Delay and length variation
  Partial starting of the shank range is mostly necessary because of danger of fracture.
  Max. length of the elements: 2850 mm
  through-hardened screws cannot be regenerated later (not weldable)