There are two main types of Stainless Steel Self-Drilling Screws – Self Drilling Screws for Timber which often have a Type 17 style cutting flute:

And Self Drilling Screw for Metal often called a Stainless Steel Tek Screw:

Here we are talking about the Self Drilling Screw for Metal often called a Stainless Steel Tek Screw.

These are screws designed to self-drill into metal.  There are several material types available and they each have their strengths and weaknesses.  Getting the material right for your application is critical.  Some Stainless Steel Grades have excellent corrosion resistance (304 and 316) but are relatively soft and will not self-drill through much more than the skin of a milk pudding!  Other grades like 410 are heat treated and have excellent hardness – but anywhere near salt-water (or any sort of coastal atmosphere) and you will see some tea-staining pretty quick!  Some grades such as Hardtec and Marutex Stainless Steel feature the benefits of both (hardness and excellent corrosion resistance) – but are only available in Hex Washer Face Head style.  Getting the right material grade and screw type for your application is critical.

The Self Drilling Screw for Metal is generally available in 5 material types within the Stainless Steel range, they are:

Bi-Metal Self Drilling Screws – these are not completely Stainless, the Bi-Metal screw generally has a 304 Stainless Steel body with a Carbon Steel tip.  Because of the Carbon Steel tip they are very hard and good for drilling but they are not 100% Stainless Steel.  They have a 500hrs Salt Spray Protective Ruspert Coating on them to increase corrosion resistance somewhat.

410 Stainless Steel Self Drilling Screws – These are completely Stainless Steel.  410 is a Martensitic Stainless Steel and so can be hardened by heat treatment (which Austenitic types 304 and 316 cannot be).  For this reason it has excellent drilling properties – however 410 has lower corrosion resistance than grade 304 and should not be used in highly or even moderately corrosive environments or tea-staining will result.

304 and 316 Stainless Steel Self Drilling Screws – Because the 300 series of Stainless Steels are of an Austenitic crystal structure – hardening by heat treatment is not possible.  These screws provide excellent corrosion resistance but are really only good for self-drilling through thin aluminium due to the softness of the material.

Hardtec Stainless Steel – Hardtec is a proprietary stainless grade that has been developed to combine the high corrosion resistance of Austenitic grades like 304 and 316 and has the hardness of heat treated Martensitic grades like 410 stainless steel. It's hardened by heat treatment and so has excellent drilling performance into metals like grade 410 but unlike grade 410, these grades have Molybdenum added to them and have corrosion resistant properties in line with the 300 series Stainless Steel grades.  Salt Spray tests show corrosion equal or better to that of 304 Stainless Steel and not far behind 316 Stainless Steel.  They are an excellent screw!

For more information, see Hardtec Stainless Steel - Technical Information.