Turning Stainless Made Painless

Turning more stainless steel but enjoying it less? Then try using the machining parameters and insert grades discussed below. You’ll probably be pleased with the results.

Stainless steel represents about 24% of the metal turned today. That percentage has been rising and will continue to rise, spurred by the growth of the chemical, oil, food-processing, and power industries, all of which crave stainless steel for its ability to withstand hostile, corrosive environments.

However, stainless steels present challenges of their own to machinists. Their machinability characteristics lead to more friction, heat, and chip-control problems than the characteristics of most traditional steels and cast irons do. Luckily, inserts designed specifically for stainless steels are available to make turning stainless easier. These new inserts provide higher positive rakes, improved coatings, and better matches for heavy roughing, medium roughing, and finishing operations. With the right tooling, a machinist can effectively apply proven turning principles and techniques to facilitate stainless-steel turning and manage the most difficult cuts.

Stainless Characteristics

There are three basic types of stainless steel - ferritic, martensitic, and austenitic - each presenting its own set of machining challenges. And for each type there are three basic issues to consider: cutting forces and the material’s workhardening and chipforming properties (Table 1).

Ferritic stainless steels, which contain 10.5% to 27% chromium and 0.013% carbon, are nonhardenable but 50% stronger than traditional steels. Because ferritic grades machine like traditional steels, an insert with a chipbreaker designed for general steel turning will suffice.

Martensitic stainless steels, which contain 11.5% to 18% chromium and more than 0.013% carbon, are heat-treated for high strength and toughness. They, too, machine like steels but with much higher cutting forces. Hardness varies within martensitic steels, causing extreme variation in cutting forces. Therefore, an insert with a very strong cutting edge is required.

Austenitic stainless steels are the most difficult to machine. They contain 16% chromium with varying percentages of nickel, molybdenum, and magnesium to build up an austenitic structure. Austenitic stainless steels are used when corrosion resistance and toughness are paramount.

Table 1 Machining characteristics of ferritic, martensitic, and austenitic.

Austenitic stainless steel has much lower thermal conductivity and higher ductility than other stainless steels and conventional steels. Therefore, more energy is needed to form a chip, thus generating more heat. And because the material doesn’t dissipate heat well, the workpiece, especially the cutting area, gets hotter. The resultant higher cutting temperatures increase the tendency for tool wear and plastic deformation, especially when interruptions are encountered.

To battle these high temperatures, in most austenitic stainless-steel turning applications, flood coolant is required to cool down the work area and help chips flow over the tool edge, preventing the chipwelding that can cause built-up edge (BUE). Water-soluble coolant is the most commonly used coolant for these applications, though shops typically use whatever coolant they’ve chosen for the rest of their machining operations. Dry turning of this metal is recommended in some cases when interruptions are involved to prevent thermal cracking of the inserts.

These steels also tend to produce a segmented chip rather than a smooth flowing chip, resulting in large variations in cutting forces.

Nickel and molybdenum, added to austenitic stainless steels to improve corrosion resistance and tensile strength, often work against machinability, because they deform plastically. This plastic deformation subjects inserts to extreme friction, high cutting forces, and high temperatures, causing the insert to smear and the component surfaces to workharden.

Austenitic stainless steel features a deformation-hardened layer on incoming mill stock that is considerably thicker than that found in traditional steel. The inside of the material may be only half as hard as the surface, but it’s at the surface where the cutting takes place.

To overcome these workhardened surfaces, make deeper cuts and set feed rates that ensure entry of the cutting edge past the hardened zone, which ranges from 0.003" to 0.005" deep.

When turning any of these stainless-steel varieties, observe these rules of thumb, which are based on experience in thousands of stainless-steel turning applications:

1. Use inserts designed specifically for stainless-steel turning.
2. Match the insert to the application to achieve better finishes and longer tool life.
3. Learn to visually diagnose and remedy symptoms of premature edge failure.
4. Make sure the shims are in good shape.

Insert Improvements

New developments in inserts for stainless-steel turning include improved chipbreaker geometries teamed with specialized insert geometries, stronger substrates, and more adherent coatings.

Various types of very hard, continuous chips form in most stainless-steel workpieces from excessively hard cutting action. To break up the continuous chips into smaller, more manageable sizes, select an insert with a high positive rake and aggressive chipbreaker design. A higher positive rake translates into freer cutting, longer edge life, more continuous cutting, smoother chip flow, lower cutting temperatures, and less deformation-hardening of the material.

Another important factor in an insert’s geometry is its nose radius. Be sure to select an insert with the correct nose radius for a given application, whether it is heavy roughing, medium roughing, or finishing. An excessively large nose radius can cause vibration and generate more heat. A nose radius that’s too small may be too weak and may break, resulting in a poor surface finish and inferior chipbreaking. Generally, a nose radius measuring 3/64" or larger is recommended for heavy roughing, while a 1/32" radius is recommended for medium roughing and finishing, at least as a starting point.

An insert with a geometry designed specifically for stainless steel should also possess a similarly designed chipbreaker. In the past, chipbreakers were made only for traditional steels, and users had to make compromises with machining parameters to turn stainless steels. Today, chipbreakers designed specifically for stainless steels are available. These chipbreakers feature small edge rounding, a high angle, and a positive rake. This design lowers cutting forces and, therefore, workhardening. While having a high angle and positive rake, the chipbreaker must also be strong - two requirements that seem to be mutually exclusive. However, these requirements can be met by putting a reinforced positive land on the insert geometry. This provides edge strength while keeping the rake positive. The chipbreaker must be wide and deep enough to lift the chip away from the rake surface and give the chip room to curl and break.

		Material		CMC Number	Brinell Hardness	Coromant Grades
						1025			2015			2025			2035
						Feed fn in/rev
						.004	.008	.012	.008	.016	.024	.008	.016	.024	.008	.016	.024
						Cutting speed vc ft/min
		Ferritic/Martensitic	Free Machining steel	05.10	200	1246	886	640	1017	853	771	984	820	722	525	443	394
			Non-Hardened	05.11	200	918	705	558	787	672	607	754	640	558	410	344	295
			PH-Hardened	05.12	330	525	492	476	328	262	213	295	213	164	246	180	131
			Hardened	05.13	330	689	640	640	410	328	279	344	230	164	213	148	115
		Austenitic	Free-Machining Steel	05.20	180	1476	1099	836	1164	918	705	968	722	541	574	508	459
			Austenitic	05.21	180	1017	771	607	820	640	508	672	508	377	394	361	328
			PH-Hardened	05.22	330	607	574	558	377	295	246	328	230	180	279	197	148
			Super Austenitic	05.23	200	705	689	656	558	443	344	443	328	246	279	246	213
		Austenitic-Ferritic	Non-Weldable >=0.05% C	05.51	230	918	705	558	656	558	492	689	525	394	377	312	279
		(Duplex)	Weldable, < 0.05 % C	05.52	260	623	541	492	492	426	377	426	377	344	476	312	197
		Ferritic/Martensitic	Non-Hardened	15.11	200	--	--	--	705	574	525	672	541	492	361	295	262
			Hardened	15.12	330	--	--	--	295	213	180	246	164	131	213	148	115
				15.13	330	--	--	--	361	279	246	295	197	148	180	131	98
		Austenitic	Austenitic	15.21	180	--	--	--	754	574	459	623	459	361	377	312	279
			PH-Hardened	15.22	330	--	--	--	295	213	180	279	180	148	213	148	115
				15.23	200	--	--	--	361	377	312	377	295	230	246	197	180
		Austenitic-Ferritic	Non-Weldable, >=0.05% C	15.51	230	--	--	--	607	492	443	508	394	312	344	279	246
		(Duplex)	Weldable, <0.05% C	15.52	260	--	--	--	443	361	328	410	344	328	426	279	180

Figure 1: Feed and speed recommendations for turning various types of stainless steel.

It is very important to choose the correct grade to suit a given application. For high-speed turning of stainless steels, a cobalt-enriched M-15 carbide grade should be used. For best results, the grade should be multicoated via medium-temperature chemical vapor deposition (MTCVD) with layers of titanium carbonitride (TiCN), aluminum oxide (Al₂O₃), and titanium nitride (TiN). The Al₂O₃ acts as a chemical barrier that prevents chemical diffusion of cobalt between the tool and the chip, which happens because carbide has an affinity for stainless steel at the high temperatures generated at high speeds. For turning at medium speeds, an M-25 grade is recommended because of its ability to handle high temperatures and varying cutting forces. For low-speed stainless-steel turning, select an M-35 grade that is straight (non-cobalt-enriched). In cobalt-enriched grades, the cobalt is drawn to the surface of the insert, so that the surface is tougher than the inner part of the insert. In a straight grade, the cobalt, and hence the toughness, is evenly distributed throughout the insert. A CVD multicoating of TiN and TiCN is recommended, though here Al₂O₃ is not needed.

When turning austenitic stainless steels, a strong grade of carbide, designed specifically for austenitic stainless steel, must be used. It must be tough enough to withstand the varying cutting forces, which mimic those of an interrupted cut even in straight cuts. MTCVD coatings are usually employed, because they adhere better to the substrate than CVD coatings, which tend to flake when cutting these steels. MTCVD coatings also reduce friction and BUE. For austenitic stainless steels, a cobalt-enriched grade is desirable. The grade’s greater surface toughness reduces chipping and the pull-out effects of BUE.

Parameters

In rough turning, the maximum metal-removal rate is obtained with a combination of high feed and moderate cutting speed. Machine power is sometimes the limiting factor, and in such cases the cutting speed should be lowered accordingly. Generally, when machining stainless steel, it’s better to maintain a heavy feed and depth of cut (DOC) - to get under the hardened workpiece surface - and a low cutting speed. Trying a light feed, shallow DOC, and high speed will introduce more heat and poor chipbreaking to the process.

In finishing, setting the feed at an ipr no higher than one-third of the nose radius will provide a good surface finish and dimensional accuracy.

Figure 1 lists recommended starting speeds and feeds for various stainless-steel-turning applications. These rec ommendations are conservative. Don’t be afraid to push the tooling to the limit to maximize productivity.

To minimize unacceptable tool wear, push the insert to its DOC and feed limits (determined by available power and component design) for 15 minutes, and then index the edge. This 15-minute rule of thumb helps yield more parts per edge with fewer indexing stoppages. It makes no sense economically to push the edge beyond that time. Productivity should always be the primary concern, not getting every last second of edge life out of a tool to save a few cents.

Employing appropriate machining parameters helps minimize tool wear. But it doesn’t prevent wear. Remember, tool wear is not an inherently negative process. Tools will always wear. It is not if, but when, how much, and in what manner tools wear. When a cutting edge has performed a considerable amount of metalcutting within a reasonable time, wear is quite acceptable. It is a problem when premature breakdown or tool fracture occurs, causing excessive stoppages for edge changes and, in some cases, workpiece damage.

Figure 2: Principal tool-wear characteristics in stainless-steel turning: (1) flank wear, (2) edge chipping, (3) notching, (4) BUE, (5) flaking, (6) crater wear, (7) plastic-deformation-based wear, and (8) thermal cracking.

Prevent the Avoidable

Some edge-wear patterns are normal, while others are symptomatic of improper insert application, incorrect machine settings, or inappropriate turning techniques. The latter patterns are signs of trouble ahead. It’s important to diagnose and correct these early warning signs and apply some innovative cutting techniques to minimize future tool troubles.

Tool-failure mechanisms that come into play in stainless-steel turning are similar to those found in steel-turning operations. The differences can be seen in the degree of wear (Figure 2). Here is a brief overview of the most common wear patterns and remedial actions to take to correct them early (Table 2).

Flank wear should generally be viewed as normal. It occurs at the flank or clearance face of the cutting edge along the length of engagement with the workpiece. Its occurrence can be positive in that it makes the edge sharper. However, after a certain amount of wear, continued friction against the machined surface abrades the tool and diminishes edge performance.

The solution to excessive or premature flank wear usually is to slow down the cutting speed. Another remedy is to use multicoated inserts or inserts coated with Al₂O₃ rather than just TiN.

Edge chipping, or breakage of the edge line, can occur when an edge has reached the end of its useful life or when the tool must execute an interrupted cut. If the edge is simply worn, you should consider indexing and changing inserts more frequently. The remedies in the case of an interrupted cut are to reduce feed rate; change the tool’s approach angle to ensure stability; and/or select a tougher, cobalt-enriched insert grade. Often, the answer is a combination of these remedies.

Notching is a type of wear in which the carbide actually breaks down, leaving a void on the cutting edge. Notch wear on the insert’s leading edge is caused by workhardening of the stainless-steel surface. During turning, the DOC line is machining harder material than the underlying nose radius of the insert, creating a notch. On the insert’s trailing edge, notching is caused by oxidation where air and coolant come in contact with the tool at high temperatures. Trailing-edge notch wear is less common than leading-edge notching in stainless-steel turning.

Notch wear is the culprit behind burr formation, which occurs when the cutting edge begins pushing the material instead of cutting it. As the notch becomes more pronounced and starts breaking down the cutting edge, the cutting forces at the DOC line rise, causing the edge to push rather than shear the chip. Along with a sharp, strong edge, larger lead angles also can help prevent this from happening.

To prevent notch wear, the remedies cited for flank wear are usually effective.

BUE is related to temperature and cutting speed. Under high temperature and pressure, the stainless-steel chips become gummy and tend to smear and stick to the insert flank. The workpiece material welds onto areas of the edge where the substrate is exposed. The BUE is torn off repeatedly, each time taking with it a chunk of the insert, leading to chipping.

Allowing BUE to grow without intervening causes premature edge breakdown and, in some instances, catastrophic insert failure. Fortunately, the temperature and cutting-speed ranges where BUE occurs are well defined and can be avoided. Much of modern stainless-steel turning takes place above the ranges where BUE occurs. And many modern insert grades are highly resistant to BUE if used properly.

Increasing cutting speed usually reduces BUE. If this isn’t possible because of machine restrictions or part specifications, a tougher grade, such as M-35, with a higher positive rake can alleviate BUE.

Flaking involves coating damage caused by an inferior coating, poor adhesion of the coating to the substrate, or plastic deformation of the workpiece. The remedy is to switch to inserts with a more tenacious coating, such as an MTCVD coating.

Crater wear occurs where the chip contacts the tool and is caused by cobalt diffusion between the tool and the chip and abrasion wear on the insert. It weakens the edge. The first recourse is to reduce speed to lower the cutting temperature. Second, reduce feed rate. And it’s a good idea to switch to an MTCVD Al₂O₃-coated insert with a positive geometry, which resists crater wear.

Thermal cracking is mainly a fatigue-wear phenomenon caused by rapid changes in cutting-zone temperatures and interrupted turning. To avoid it, turn up the coolant flow and select a tougher carbide grade with high resistance to thermal shock or, in some cases, turn off the coolant supply to keep the temperature level even.

Plastic deformation of the stainless-steel workpiece can contribute to notching, edge chipping, and crater wear. This problem often results from higher cutting forces and temperatures encountered when turning stainless steels, especially austenitic stainless steels, which contain nickel and molybdenum. For a tool material that can stand up to these conditions without causing plastic deformation, select an Al₂O₃-coated, wear-resistant grade. Al₂O₃ has high hot hardness and resists the cratering encountered at high speeds where plastic deformation occurs.

Table 2: Recommended remedies for the most common types of insert wear. XX = best alternative remedy; X = possible remedy.

Another weapon against improper insert wear is the shim that protects it. A proper shim in good condition is essential for turning stainless steel. It shields both the insert and toolholder from damage caused by high cutting forces. The varying cutting forces in stainless-steel turning create high pulsating pressures on the insert seat and the shim, which act as shock absorbers for the insert and workpiece. Check the condition of the shim every time you change an insert, and, when in doubt, replace it. When using a two-sided insert, watch for a shim face that has an imprint of the insert embossed on it. Such a shim must be replaced. If a shim loses its flatness, its ability to properly support the insert will suffer.

Difficult Cuts

Some new application techniques have been developed to facilitate troublesome cuts such as approaches to shoulders, uneven entering angles, and face turning.

Turning against shoulders. The approach to a shoulder presents a severe change of condition for a turning tool and can be highly stressful on the cutting edge. Chip jamming and hammering also can occur. There are several ways to overcome the risks associated with turning against a shoulder. To overcome chip jamming and hammering, increase the cutting speed when the insert is 0.040" from the shoulder. Or turn the shoulder through a number of axial cuts, forming a step-like profile that can then be finished by two radial cuts. To improve chip control and reduce cutting-edge stress, direct the tool radially in rather than feeding radially out just before the shoulder.

Uneven entering angles. When initial cuts have an uneven, acute start, the solution may be to use an extra insert for the starting process. The tool can have an entering angle of about 45° and can be used to premachine rough edges. It provides an advantageous approach for the main tool. A lower feed rate to engage into the cut is another method.

Facing toward center. Face turning is common in stainless steel and may involve facing either to the center of the workpiece or to a hole. When facing to a center, the cutting speed approaches zero at the center, upsetting the relationship between speed and feed. Higher spindle speeds in modern CNC lathes can compensate for small diameters, but only up to a point. Close to the center, the tool begins to push the workpiece material rather than cut it. Lower surface speeds also have the added disadvantage of causing smearing of the cutting edge and BUE. The best solution is to let a drill cope with the material in the center first, then follow with the face-turning operation. If there is no center hole, reduce the feed rate when the facing diameter becomes 0.400". For example, a feed of 0.010 ipr can be reduced to 0.002 ipr.

Remember, stainless steel, though difficult to turn, is still a very manageable material. Heat is the main enemy, but new inserts and properly applied turning techniques can save the day. Push the tools to the limit and follow the rules of thumb, and you’ll be piling on-spec stainless-steel parts in the bin.

About the Author

Mike Castner is product specialist, turning inserts, with Sandvik Coromant Co., Fair Lawn, NJ.