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Duplex Stainless Steels are growing at a high rate. It is important to understand how they differ from the more familiar stainless steels and how these differences affect the fabrication methods and parameters
Sulphur, selenium and calcium treated improved machinability stainless steels are compared. European steel numbers for these grades are listed and include 1.4005 1.4006 1.4021 1.4028 1.4029 1.4031 1.4057 1.4112 1.4125 1.4542 1.4305 1.4307 1.4541 1.4401 and 1.4404. The former BS grades covered by this article include 416S21 416S29 416S37 416S41 441S29 441S49 303S21 303S41 325S21 and 326S36 Grade types 440B 440C and 17/4PH 17/4 17-4 are also mentioned. The short-comings of these types of steels, compared to the un-treated types is outlined.
Topics discussed include machine and tooling rigidity, selection of tool materials (high speed steels or HSS and cemented carbides ), tool geometry and sharpness and lubrication and cooling (mineral and emulsifiable oils).
Stainless steels are often regarded as ‘difficult to machine’ and classed a single group of steels, based on experience with the most common austenitic types. Machining these steels using feeds, speeds and depth of cut parameters for more conventional steels can result in excessive tool wear or breakage. Although cold drawn bar can have better surface finish and accuracy of tolerances than ‘black’ bar, the relative ease of machining of the fully annealed ‘black’ bar may make this a better overall choice for the machinist or engineer. The machinability of the other stainless steel ‘families’ ie ferritic, duplex, martensitic and precipitation hardening is however different. A machiniability index for grades such as 303 (1.4305) 304 (1.4301) 430 (1.4016) 2205 (1.4462) 416 (1.4029, 1.4005) 410 (1.4006) 440C (1.4125) within these families is shown.
Drill and reamer tool angles are shown along with tables of suggested feeds and speeds for drilling and reaming a range of stainless steels, using either high speed steel (HSS) or carbide tipped tools. The particular problems drilling small diameter holes in stainless steels are highlighted. The data is taken from the BSSA Stainless Steel Specialist Course, training note No.9 ‘Machining Stainless Steels’
Tables of suggested feeds, speeds and depth of cut for single point, form tool and cut-off (parting-off) turning are shown. Tool profiles are shown and discussed along with suggested relief angles for these types of lathe turning operations. The data is taken from the BSSA Stainless Steel Specialist Course, training note No.9 ‘Machining Stainless Steels’