processes and products review

Conventional processes/ products reviews




Contaminated screws from conventional prior art I&II:




How to check out the contaminated screws? Here is an easy way:


Immersing bi-metal screws into the 63% Nitric Acid. The contaminated screws will appear like this:




The characterisics of stainless steel fasteners


The characteristics of bi-metal fasteners


  • The tapping and drilling parts of bi-metal screws use hardenable steel such as AIS11035 Medium carbon steel (0.35~0.5%C) or AIS11022(Carburized steel of Core: 0.15~0.25%C, Surface: 0.8%C) and the shank/head is made ofAIS1304/316 Austenitic stainless steel. Both of them were combined together by welding.

  • The tapping and drilling part of hardenable steel is only hardened by induction heating. It can come out with hardness up to 600~700HV to highly increase the drilling capacity and working efficiency.

  • The Austenitic stainless steel part is not under any heating process so it can keep the outstanding anti-corrosive function and toughness of low temperature.



The characteristics of AISI 410 one piece drilling screws


  • AISI 410 drilling screws which is produced by Martensitic grades stainless steel that could be strengthened and hardened by heat treatment (quenching and tempering).

  • AISI410 has higher carbon contains (0.15%max,Cr12%) and therefore higher strength /hardness, but have reduced resistance to corrosion.

  • Due to poorer corrosion resistance and full sectional hardening (it is too brittle to use in low temperature environment), they only be suggested to use in low anti-corrosive purpose .



The characteristics ofAISI 316/304 one piece drilling screws


  • AISI 316 (C:0.08% max、 Cr16-18%、 Ni10-14%、 Mo2-3%) and AISI 304 (C:0.08% max、 Cr18-20%、 Ni8-12%) austenitic grades stainless steel drilling screws which can only be hardened by cold-working or Nitriding.

  • AISI 316/304 are not heat treatable and give up strength for corrosion resistance, therefore, the hardness of drilling screws is insufficient to penetrate heavy gage steel, they are only restricted to use in the thin steel plate less than 3mm, or aluminum or wood.



The demerits of cold-work-hardening austenitic stainless steel screws


  • 1.The yield and tensile strengths (hardness increase with increasing decreases with increasing amount of cold-work (fig1)

  • 2. The yield and tensile strengths increase with decreasing temperatures for cold-worked type austenitic stainless steel and will reduce toughness sharply (fig2)




The demerits of Nitriding for austenitic stainless steel screws


  • Surface preparation prior to Nitriding is ordeal (acid pickling、cyanide salt bath) where stubborn passive films (Cr203) are encountered on screw which will reduce the yield rate of nitriding process.
  • Surface hardness up to Hv1200 can be obtained from Nitriding treatment , the great hardness gradient between base metal and hard coating will result in the brittleness of Nitriding layer (also deteriorate the drilling capacity of screw).
  • The precipitation of finely dispersed chromium nitrides tends to deplete the surrounding matrix , therefore the corrosion resistance will be deteriorated after Nitriding procedures.





Comparison between bi-metal fasteners and one piece stainless steel drilling screws