A. Why Bi Metal Screw
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The rapid development of zinc and aluminum coatings of steel sheet has extended its operational life from 10 years to approx. 20 years. Hence has brought a large demand for screw made of austenitic stainless steel. The metallurgical and mechanical properties of austenitic stainless steel, however, makes it difficult for drilling/tapping through steel sheet. Therefore, by utilizing austenitic stainless welded with steel, bi metal screw is under invention. |
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B. Criteria for Bi Metal Screw - Performance
Both the drilling/tapping function of the steel section, and, tapping function of the stainless steel section
- Corrosion Resistance
The essence of Bi Metal is to have the austenitic stainless steel applied to the base material.
Therefore, any risks raised to diminish the original corrosion-resistant function must be avoided.
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C. Current Products in the Market
- FIG 1 : Conventional S/S Screw treated by Nitriding
 Corrosion Resistance of the surface of austenitic stainless steel is degraded due to the nitride- The nitriding treating ( approx. 600 Degrees for 2 ~ 4 hours ) will cause crystal and grain boundary corrosion
- FIG 2 : Austenitic S/S welded with High Carbon Steel, then quenched to increase sectional hardness of carbon
- Higher Manufacturing Cost
 a) High Carbon is not conducive to forging ( drill point )
b) Welding heat-affected zone carries a over 500 HV hardness. It tends to induce cracking and difficult to thread
- Brittle and Frangible
After quenching, the high carbon carries as high as 550 ~ 600 HV hence become brittle and frangible
- FIG 3 : Austenitic S/S welded with Low Carbon Steel, then carburized and quenched to increase sectional hardness of carbon
 Diminishing Corrosion-Resistant Function
The method is to weld the two section first then thread and form the end section.
The austenitic stainless steel section is contaminated due to the carburization step.
Hence, the original corrosion-resistant function is distorted
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D. Bi Metal Screw Production Flow ( Method of Manufacturing Patented )
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Compose Austenitic Stainless Steel with Head : FIG 4
Austenitic Stainless section been headed with non-threaded bolt |
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Pre-form Low Carbon Steel with Shoulder and Point : FIG 5
Forge Low Carbon Steel section with a shoulder and drill point |
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Carburizng Low Carbon Steel with Shoulder and Point : FIG 6
Adjust Core Hardness A to approx. 180 HV and Surface C to approx. 200 HV |
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Weld Carbon and Stainless Section together : FIG 7
After welding, welding flash 62 and the shoulder 9 occur |
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Cut off Weld Flash and the Shoulder : FIG 8
A non-carburized hardened area having an L width near the welded interface W ~ hardness influenced by welding rises slightly but still lower than 240 HV, suitable for further processing |
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Threading and Quenching Low Carbon Steel Section : FIG 9
After Threading, quenching to increase sectional hardness of Low Carbon Steel but no contamination of Austenitic Stainless Steel |
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Advantages
 No Contamination
Because the Low Carbon Steel is carburized before welding, Austenitic Stainless Steel is not contaminated by carbon or nitrogen, hence over come the drawback of diminishing original corrosion-resistant properties - No Cracking
The carburized shoulder is cut after weld hence providing a proper non-carburized area with low hardness less than 240 HV, it will not include cleavage and cracking and result in easier threading
- No Brittleness and Frangibility
Because the Low Carbon Steel is only carburized such will not increase brittleness and frangibility to the screw
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6.3X29-100
Bi-metal screw before surfacing |
6.3X25-100
#1 single thread for 1+lmm laps of sheet |
5.3X27-100
#2 low profile , single thread for light section
3mm steel |
5.5X 26 - 100
#2 single thread for light section 3mm steel |
5.5X28 -100
#3 single thread for medium section 6mm steel |
6.3X29-100
#3 single thread for medium section 6mm steel |
5.5/6.3X60-300
#3 Hi-Lo thread for medium section 6mm steel - Low Profile |
5.5/6.3X60-300
#3 Hi-Lo thread for medium section 6mm steel |
5.5X45-100
#5 single thread for heavy section 12mm steel |
5.5/6.3X70-310
#5 Hi-Lo thread for heavy section 12mm steel |
6.3X110-210
#5 single thread with wing for fixing fibre cement on to 12mm metal support |
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