Therefore, the composition is adjusted to enhance the machinability, but without changing other basic qualities. Some machine operators are interested only in the cutting speed, others in the tool life at a medium cutting speed. However, comprehensive evaluation of machinability is made using the following factors: cutting speed, tool life and surface finish.
Additionally, sulfur forms inclusions that reduce the friction forces and transverse ductility of the chips, causing them to break off more readily. The improvement in machinability in the free-machining stainless steels namely Types 303, 303 Se, 203, 430F, 416, and 420F is clearly evident in the chart, "Comparative Machinability of Frequently Used Stainless Steels" (Figure 1).
Figure 1: Comparative Machinability of Frequently Used Stainless Steels and Their Free-Machining Counterparts Ratings in %
The free-machining grades have significantly lower corrosion resistance than their non-free machining equivalents because of the presence of these non-metallic inclusions; these grades are particularly prone to pitting corrosion attack and must not be used in aggressive environments such as for marine exposure. Free-machining grades containing high sulfur levels also have reduced ductility, so cannot be bent around a tight radius nor cold forged. Because of the sulfur additions these grades are very difficult to weld, so again would not be chosen for welded fabrication.
Austenitic stainless steels are generally regarded as being difficult to machine, and this has led to the development of the free-machining Grade 303. The chromium, nickel, and sulfur contents of Type 303 are slightly different than that of Type 304, but its physical and mechanical properties are quite similar. Type 303 can be machined at speeds about 25-30 percent faster than Type 304.
Type 303 Se is a free-machining vari¬ation of Type 304 that contains selenium instead of sulfur. Type 303 Se is a better choice than Type 303 when a better machined surface finish is required or when cold working may be involved, such as staking, swaging, spinning, or severe thread rolling, in addition to machining.
Type 203 is an austenitic free-machin¬ing grade which is a modification of the 200 series stainless steels. Although it contains significant additions of man¬ganese like the 200 series it also contains nickel and copper. There are also free-machining alterna¬tives to consider in the 400 series stainless steels. For instance, if end-use conditions call for Type 430 stainless, the specifier might select Type 430F.
The composition of Type 430F is adjusted to enhance the machining characteristics while preserving as much as possible the qualities of Type 430. The free-machining variation of Type 410 is Type 416, and for Type 420, the specifier should consider Type 420F. Table 1 compares common grades of free-machining stainless steel grades.
Финские стали Outokumpu Prodec
Of the stainless steel grades, the free-machining grades are the most similar to carbon steel. These grades often have an increased sulfur or selenium content to facilitate chip breaking. Outokumpu’s Prodec grades, which are still within the specification of standard 4307 or 4404, are machinability enhanced by a special metallurgical treatment. These grades have up to 40% better machinability compared to standard grades.
Among the duplex grades LDX 2101 stands out as a grade with good machinability and is in the same range as the Prodec grades when using cemented carbide tools.
When machining stainless steels it is important to use tools and inserts that are designed for stainless steel, and the tool supplier’s recommendations should be followed in order to have optimal chip length and increased tool life. Outokumpu also provides starting recommendations for both High Speed Steel (HSS) tools and Cemented Carbide (CC) tools in the machining guidelines presented for a number of stainless steel grades.
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