Free-Machining of Stainless Steels

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Introduction to Free-Machining Stainless Steels

Stainless steels are renowned for their corrosion resistance, attributed to a complex oxide protective film formed primarily by chromium and nickel on the alloy surface. However, these materials typically present significant machining challenges compared to carbon steels and other alloys. To address these challenges, manufacturers have developed free-machining grades through careful compositional modifications. These modifications, while improving machinability, require careful consideration of their impact on the material's fundamental properties.

Understanding Compositional Modifications and Their Effects

One key approach to improving stainless steel machinability involves adding elements such as sulfur, selenium, lead, and copper. These alloying elements effectively reduce friction forces and chip welding tendencies during machining operations. However, it's crucial to note that these additions can affect both functional properties (corrosion resistance, transverse ductility) and technological characteristics (machinability, weldability) of the resulting free-machining stainless steel.

Evaluation of Machining Performance

Machine operators typically focus on different aspects of performance, with some prioritizing cutting speed and others emphasizing tool life at medium cutting speeds. A comprehensive evaluation of machinability considers three primary factors:

• Cutting speed capabilities
• Tool life expectancy
• Surface finish quality

Free-Machining Grades and Their Characteristics

Sulfur plays a particularly important role in free-machining grades by forming inclusions that reduce friction forces and improve chip breakage. This improvement is clearly demonstrated in various grades:

Figure 1: Comparative Machinability of Frequently Used Stainless Steels and Their Free-Machining Counterparts Ratings in %

Austenitic Stainless Steel Variations

The development of Grade 303 represents a significant advancement in machining austenitic stainless steels. While maintaining similar physical and mechanical properties to Type 304, Grade 303 offers 25-30% faster machining speeds. Type 303 Se, containing selenium instead of sulfur, provides superior surface finish and better cold working capabilities for operations like staking, swaging, spinning, and thread rolling.

Alternative Free-Machining Grades

The industry also offers several other free-machining alternatives:

• Type 203: An austenitic grade modified from the 200 series, containing manganese, nickel, and copper
• Type 430F: Enhanced machining characteristics while maintaining Type 430 properties
• Types 416 and 420F: Free-machining alternatives to Types 410 and 420 respectively

Table 1. Common free machining stainless grade steels

AISI Standard Designation	EURONORM EN 10.088-3	UGITECH Commercial Designation	C	Si	Mn	Ni	Cr	Mo	Cu	S	P	Specific Features
304 Cu	1.4567	UGIMA® 304 L Cu	0.03	1.0	2.0	9.5	18.5	-	2.0	0.02	0.045	The best compromise between corrosion resistance and machinability. Well suited for deep drilling and free cutting (limited chipping).
303	1.4305	UGIMA® 303	0.1	1.0	2.0	9.0	18.0	-	0.3	0.3	0.045	The reference in machinability. Austentic steel appropriate for free machining operations.
316L	1.4404	UGIMA® 4435	0.03	1.0	2.0	12.5	17.0	2.5	-	0.02	0.045	Excellent machinability. Austenitic steel appropriate for free machining grades of ≤0.03% carbon content ensuring increased resistance to corrosion.
430 F	1.4105	UGITECH 430 F	0.07	1.0	1.0	-	16.5	-	0.25	0.25	0.045	The alliance of magnetic properties with improved machinability.
416	1.4005	UGITECH 416	0.12	1.0	1.5	-	13.0	-	-	0.35	0.045	Martensitic stainless steel recommended for rough machining conditions (free machining).
420	1.4021	UGITECH 420	0.25	1.0	1.5	-	13.0	-	-	0.03	0.045	A combination of good machinability and high mechanical properties (corrosion resistance limited).
420 F	1.4029	UGITECH 420 F	0.25	1.0	1.5	-	13.0	-	-	0.25	0.045	The alliance of machinability and mechanical properties for special purposes, such as workability in very hard or rough environments with low corrosion risk.