processes and products review|Bi-Metal Screw Manufacturer|Metalink Bi-Metal Screw

processes and products review

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

The tapping and drilling parts of bi-metal screws use hardenable steel such as AISI 1035 Medium carbon steel (0.35~0.5%C) or AISI 1022 (Carburized steel of Core: 0.15~0.25%C, Surface: 0.8%C) and the shank/head is made of AISI 304/316 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.

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

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 gauge steel, they are only restricted to use in the thin steel plate less than 3mm, or aluminum or wood.

1.The yield and tensile strengths (hardness increase with increasing decreases with increasing amount of cold-work (Fig. 1)
2. The yield and tensile strengths increase with decreasing temperatures for cold-worked type Austenitic stainless steel and will reduce toughness sharply (Fig. 2)

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.