Why Are Material Grades and Condition Important When Tapping Stainless Steel?

Why is it critical that we know the grade and condition of the material you are tapping? In the development process of our SuperTuf material specific tap line we determined the machining characteristics of the various grades of stainless were best tapped with tools with varying flute shapes, chamfer design and relief angles.  Stainless steel materials (free machining) can be effectively threaded with Titanium Nitride coated taps with improved surface finish and extended tap life.

Let’s start with an example: 304 stainless is a good grade for both corrosion resistance and strength but if 303 is satisfactory,

supertuf tap

the machining rate will be 25-30% better than 304. The chromium, nickel and sulfur contents are slightly different but physical and mechanical properties are very similar. Then consider 303 SE which is a free machining variation containing selenium instead of sulfur and produces a better threaded surface. Stainless steels are classified by metallurgical structure:

Austenitic are those containing chromium, nickel and sometimes manganese. They can only be hardened by cold working; heat treatment only causes them to soften. Both series 200 and 300 are examples of Austenitic Stainless Steels.

Ferritic 400 series stainless steels are not hardenable by heat treating.  They are ductile and have good resistance to corrosion and oxidation. The machinability rating is about half of 300 series free machining series.

Martensitic 400 series is a straight chrome type that can be heat treated. Types 416 and 420F have been modified to improve machinability. Most compositions machine similar to Ferritic 400 series steels.

Precipitation Hardening stainless steels are often selected because machining can be completed in the annealed condition and then hardened at 900 degrees to yield strength of 300,000 PSI.  Machinability is similar to the corresponding martensitic grade.

Duplex stainless steels provide excellent strength and good machinability. They are a two-phase material with austenite and ferrite in close balance.

SuperTuf Tap Offerings for Stainless Steel & Difficult Materials

supertuf hand tap

SuperTuf SS and SuperTuf SS-DA are designed for free machining Austenitic and Ferritic Stainless Steels. SuperTuf DM with TiCN coating are designed to perform well in the tougher Duplex and Precipitation Hardening heat treated stainless steel materials.