21 November 2018

Nicolas’ Technical corner

Rabourdin offers customers various kinds of heat treatment to improve the durability of a machined part. Rabourdin has their own in-house nitriding line called GR nitriding. This physical-chemical surface heat treatment in a salt bath increases the hardness of steel, stainless steel and cast iron. Instead of applying an additional coating, a diffusion area is created in the actual surface layer.

This process is used to improve the metal’s resistance to wear, abrasion, fatigue, friction and corrosion (and maintaining this property on stainless steels).

The GR nitriding line will accept parts up to:

  • Max. length: 1350 mm
  • Max. diameter: 500 mm
  • Max. weight: 30 kg – beyond this, ask us.

A dedicated in-house team is available to address all of your requirements for standardised, GR and other nitriding.

A few words on GR nitriding

The GR nitriding process is recommended for every metal part that suffers from fatigue and that needs to resist wear and corrosion.

For example: Mould parts, cylinder rods, piston rods, nuts and bolts, mechanical assemblies, articulation systems and many more, for cars, cycles, agricultural machinery, public works equipment, mining, quarry and drilling applications, and many more.

The parts to be treated are nitrided in a salt bath at a standardised temperature. The duration too is standardised to ensure homogeneous process results over the entire manufacturing process as well as for the services performed on customer parts processed on a subcontracting basis. This treatment ensures that the following standard characteristics are achieved.

  • Nitrided layer thickness: 0.03 to 0.05 mm.
  • Superficial hardness: >= 950 HV0.3.

The GR nitriding treatment is performed after all of the machining is completed (and therefore on finished parts) and after heat treatment (with tempering at 530°C at least to avoid any risks of lowered hardness and deformation).

  1. Lesser size modifications:

Given that the GR nitriding treatment does not produce any structural transformations in the base metal and that the heat stresses remain very low thanks to the slow heating and cooling process, there is no expectation of any significant weight changes. Only compression stresses that occur in the superficial layer following the formation of nitrides or carbides induce slight weight changes that nevertheless remain within a range of 3 to 9 micrometres in size (depending on the material) and are limited only to this superficial layer. Special geometries (like rings) may result in greater deformations – as the circumference of the part becomes greater: the inside and outside diameter increase.

  1. Good surface hardness:

The GR nitriding treatment significantly increases the surface hardness, with the maximum measured hardness exceeding 950 HV0.3. Materials containing elements that form nitrides (e.g. aluminium, chromium, molybdenum, vanadium) have a higher nitriding hardness.

  1. High wear resistance:

Due to the connection between the iron and the alloy elements in the surface, the connection zone gains a non metallic characteristic. Hence, during the slippage process a metal-non metal coupling occurs, stopping any welding (even cold welding) thereby increasing wear resistance.

  1. Good rigidity:

Due to the considerable increase in the surface hardness during the treatment, greater shape rigidity is obtained. The result is a greater resistance to flexing or deformation.

  1. Corrosion resistance:

Due to the connection area that is rich in nitrogen, the surface of the material benefits from a degree of protection against corrosion.

  1. A greater load bearing capacity:

The nitride layers are highly tensile, even after loading (compression tests) exceeding the elasticity limit (the elastic/plastic zone limit) towards the plastic area (with permanent deformation), no damage is notable on the surface, under conditions of a slight variation in shape. Treated parts may be heated to 230° C-2 h. to achieve the greatest possible tenacity.

  1. Other properties:

The slippage properties of the wear resistance are maintained for temperature ranging up to 500°C or higher, but the properties are only valid over the short term. Nitriding reduces heat propagation in the part core, given that the connection area is a lesser conductor than the base material, it does not heat as quickly as without any GR nitriding treatment. Deep and narrow bores as well as inner contours can also be nitrided.