The increased use of metalformed and
plastic products has created more attention to the surface treatments
available for tools and machinery components. The increased life
provided by lonitriding allows users to take advantage of significant
savings during the manufacturing processes.
This paper presents some considerations which can optimize the ability
of the tool surface to take maximum advantage of this surface hardening
treatment.
The surface of a tool must have a geometrical correct shape without any
grinding cracks, remelt layer from EDM-ing and free of scratches, nicks,
dents, pores, orange peels or pitting. The same factors influencing the
polishability of a tool can affect the lonitriding results:
- Tool steel quality: cleanliness
(slag inclusions, porosity, oxides, entrapments, etc.), particles or
areas with different properties or hardnesses.
- Heat treatment: soft spots, surface
oxidation, decarburization, alloy depletion, etc.
- Polishing technique: smearing,
overlapping, overpolishing (orange peel, pitting, overheating,
etc.).
- Surface contamination: cutting
fluids, drawing compounds, solid films and embedded particles from
worn cutters and abrasives blended with metallic and non- metallic
powders, binders, mold releases, oils, solvents, waxes and rust
preventives.
The lonitriding process produces hardened
case depths of .010" to .025" (.25mm to .60mm) on most mold and tool
steels with a surface hardness of 60-65 HRC and higher. Due to the
ceramic nature of the nitrides, the wear and corrosion resistance,
antigalling and antiscoring properties as well as many others (fatigue
life, compressive strength, etc.) are greatly improved.
lonitriding is a clean and environmentally safe process performed under
vacuum in a controlled atmosphere at relatively low temperatures (750°
to 1040°F or 400° to 560°C) with no quenching required. The process
itself does not produce any stresses or dimensional changes, unless they
were introduced from the previous operations (heat treatment, machining,
polishing). The stress relieving effect which occurs during lonitriding
would allow these stresses to be released.
Unknown surface contaminants may act like masking agents and they could
affect the ability of the nitrogen ions to penetrate and diffuse into
the steel.
Many surface contaminants must be removed prior to lonitriding since
they will burn or decompose at processing temperature and may leave
marks or solid residues on the surface.
The increased use of synthetic cutting fluids and rust preventives
containing organic and/or inorganic chemicals (polymers, waxes,
lacquers, metallic soaps, dry lubricants based on graphite, oxides or
sulfates) are very difficult to be removed if allowed to dry on the
surface after machining.
Sun Steel Treating Inc. is committed to work with each customer and
application to identify and remove detrimental contaminants. Most
contaminants can be removed by one or a combination of the following
processes:
- Glass beading and/or vapor blasting.
They also help in reducing lapping and smearing effects prior to
lonitriding (mechanical polishing may promote blending of organic
and inorganic particles in an adherent surface layer). After
lonitriding. the surface can be further polished to the desired
finish, knowing that the higher the surface hardness the better the
polishability.
- Stress relieving at 950° to 1000°F
(510° to 540°C), followed by glass beading or polishing.
- Depending on the contaminant
chemistry, many can be removed through certain specific cleaning
procedures.
- Clean and wash the parts immediately
after the final machining/polishing and protect the surface with a
water displacing oil (like WD40).
A good finish grind or surface finish of no less than 12 microns or up
to 400-600 paper, prior to lonitriding is advisable. We prefer a lapping
or polishing operation not be performed before the process (especially
for parts with a hardness less than 35 RC). After lonitriding it is easy
to achieve the desired finish by conventional methods.
Also, we would like to be informed about any surface defects revealed
after final machining or during polishing and the method used to correct
the problem (removal by deeper grinding/polishing, torching and pinning,
micro welding, etc.). This information is needed for correct processing
since these spots, laps or smeared films do not have the same
metallurgical strength as the original material, and they may reopen
during lonitriding.
These surface preparation methods and recommendations will assure the
highest surface quality after the lonitriding process.
Our commitment to quality and service has earned us accreditation by NSF
International Strategic Registrations to ISO 9001:2000.
For more information and technical assistance please contact Sun Steel
Treating Inc., Technical Services.
* lonitriding is a surface hardening
process for metals.
** George Idriceanu, Director of Research and Development, and Process
Engineering, Sun Steel Treating Inc. |
