Ideas for Machining Austenitic Stainless Steels

For shops machining stainless steel parts and components, there may be unrecognized opportunities to improve performance and bottom line results. Any useful idea, new or forgotten, could benefit the plant typically coping with pressures to increase productivity, reduce part costs, achieve closer tolerances, produce better finishes, machine more difficult parts, get consistent performance, run work problem-free for long periods, deliver on time, manage costs, maximize use of existing equipment, minimize downtime and enjoy satisfactory margins. 

This discussion will focus on improvements possible when machining the austenitic stainless steels, which are the most widely used stainless alloys. However, their relative value may be better understood by comparing them briefly with the four other classifications of stainless steels that are universally machined.

Ferritic stainless steels are magnetic and have lower alloy content than the austenitic grades; because of this, they possess a lower level of corrosion resistance. The ferritic family includes grades such as CarTech 430, CarTech 430F, CarTech 430FR, CarTech 434, CarTech Chrome Core® 12 alloy, Cartech Chrome Core 18-FM stainless, etc. 

If parts productivity is a major goal, use nothing less than a machinability-enhanced austenitic stainless. These parts were made from free-machining CarTech 303 and 304 stainless .  The martensitic stainless steels are magnetic and offer more strength than the austenitic alloys, but they too are less corrosion resistant. Commonly used grades in this category include CarTech 410, CarTech 416, CarTech 440C, CarTech 420, CarTech 420F, CarTech 440F-Se, CarTech TrimRite®, and others.

Precipitation hardenable stainless steels combine high strength and good corrosion resistance. Frequently used PH stainless alloys include CarTech 15Cr-5Ni stainless, CarTech Custom 630 (17Cr-4Ni) stainless, CarTech Custom 450® stainless, CarTech Custom 455® stainless and CarTech Custom 465® stainless.

The duplex stainless steels resist corrosion from specialized chemicals that cannot be handled successfully by the regular 18Cr-8Ni austenitic stainless steels. The most common duplex grades are 7-Mo® stainless and 7-Mo PLUS® stainless.

Popular Austenitics

In general, the austenitic family of stainless steels offers the best combination of corrosion resistance and mechanical properties. The most commonly used austenitics, covered by this discussion, are CarTech 303, CarTech 304 and CarTech 316 stainless.

CarTech 303, designed expressly for machining, contains 10 times more sulfur than CarTech 304 stainless. This sulfur additive will enhance the machining characteristics of the alloy, but will have a negative effect on corrosion resistance and weldability. CarTech 304, with very little sulfur, is capable of providing a smoother or brighter surface finish than CarTech 303. CarTech 304 can cause more tool wear and produce stringier chips because it lacks the sulfur additive. This can result in birdnesting around the cutting tool or part during the machining operation. The chips produced by stainless CarTech 316 can be tougher than those from CarTech 304, thus harder to break. More machine horsepower is required to cut CarTech 316 product.

What's the Objective?

Most shops want to machine it faster, better and with greater precision. But for what purpose? To reduce part costs? Increase productivity? Shrink cycle time? Improve quality? Establishing a specific objective may have a bearing on the stainless bar stock most suitable for the job. For example, an ordinary stainless steel that meets industry specifications can restrict productivity. If productivity is the goal, think of a stainless grade that is designed for higher machining rates. Consider the importance of product consistency facilitated by controlled melting and manufacturing procedures. Think of alloys formulated with higher speeds and feeds in mind.

A growing number of "lights out" machine shops have been running work around the clock with minimum or no supervision. This practice works fine so long as machine operators come to work in the morning and find a bin full of parts and not a bunch of broken tools and/or machine damage. Could such operations hope to succeed without demanding absolute consistency in the stainless bar stock being machined, as well as an extremely high level of consistent performance from their machines and cutting tools?

When the surface finish is important, the time to tell the steel supplier is before placing the order. If a shiny surface finish is needed, for instance, and fully annealed material is purchased, the shop may not be able to obtain the finish desired without incurring the unexpected time and expense of a secondary operation. The softer materials tend to smear and tear.

The importance of stating specific needs cannot be overestimated. The stainless producer, for example, can increase the hardness or refine the grain size of the stainless by varying manufacturing procedures. They can produce stock that will perform better with advanced tooling materials or coatings. But only if they know about the machine shop's specific requirements!

Tight Tolerances

Not too long ago, machine shops were required to meet dimensional tolerances of ±0.001" on some critical parts and components. Today, a precision standard of ±0.0005", or better, is commonplace. If one applies the downstream principle and suggestions of the machine builders, shops coping with tighter-than-ever dimensional toleranced parts should seek closer OD dimensional tolerances on the stainless bar they machine. Bar ends should be chamfered also to help feed the bar through the bar feeder and into the machine. This chamfering becomes more critical for smaller diameter bar stock and for bars run on more automated machines. Both requirements could be critical on long running jobs.

Many machine shops, especially Swiss shops, still exercise tolerance control of the bundle by hand mikeing their cold finished stainless bar stock and sorting their bars into different lots based on the size of the collets or bushings that will be used to support them. This practice can be expensive in terms of cost, time expended and quality lost.

If the bar tolerances of the bundle(s) vary, the machine may not maintain control of the machining process. Just about anything can happen, and none of it is good. If the bushing or collet is too big, the machining bar will slop around loosely, making it impossible to meet part tolerances. If the bushing is too tight, the bar could jam in the bushing. Both the bushing and bar could burn. The stock held could even seize or weld in place.

Hardness Control

Since the hardness of all austenitic stainless steels can vary depending on manufacturing, shops should routinely indicate what hardness or tensile range they want in their machining bar. Higher hardness or tensiles, most shops know, allows for a better surface finish. Hardness can also be specified by the ultimate tensile strength, which is probably a better and more accurate measure of bar hardness.

The right level of hardness can, it should be noted, provide a good combination of drillability and surface finish. Excessive hardness, on the other hand, can increase tool wear when parts are machined.

Tool Coatings

Coated carbide grade technologies enable shops to enhance cutting tools when used to machine stainless steel parts. The coatings most frequently used are made of titanium nitride (TiN), titanium aluminum nitride (TiAlN), titanium carbo nitride (TiCN) and alumina oxide (Al203).

Coatings can be applied by two processes - Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD). PVD coatings are applied to the carbide substrate at lower temperatures. This preserves edge strength and permits coating of sharp edges. PVD coatings have a relatively smooth surface that generates less frictional heat, allows lower cutting forces and resists edge build-up that can lead to insert chipping.

CVD coatings are applied to the carbide substrate at higher temperatures. This causes interdiffusion of the coating with the substrate to assure a strong bond. The CVD process also permits deposition of multi-layer coatings that can suppress both crater wear and flank wear, thereby expanding the range of tool application. CVD is currently the only coating process that can efficiently apply alumina (Al203), which permits higher cutting speeds.

Cermet grades consist of mostly carbonitride (TiCN). Cermets are used most successfully for high-speed finish machining of most stainless steels.

For more specific coating selection recommendations, refer to the section entitled "Tooling/Coatings Selection Guide" at the bottom of this article.

Better Thread Finishes 

Better thread finishes may be achieved by noting the special parameters desired on orders for stainless machining stock. A material with finer grain size can produce smoother finishes and more defined peaks when cutting or rolling threads. At the same time, the more suitable material can minimize, if not eliminate tearing when thread cutting. A higher material hardness can also achieve the same results for cutting threads.

Easier Drilling

Drill wear can be reduced substantially with good drill design. In a typical scenario, for example, a shop using a standard jobbers drill may suffer drill and work piece problems after penetrating the stainless stock. With this drill design - use of the conventional jobbers point is a common cause of drill failure - the material is mechanically pushed from the center of the drill tip to the outer cutting edge of the drill. This pushing action work hardens the material at the drill point. That, in turn, hardens the material at the drill point, causing excessive drill wear and "hard spots" in the stainless.

One solution to a problem like this one is to use a split point (providing cutting edges at the drill point) drill with a 130 to 140° included angle. Drill makers utilize the more common split point, and even have their own special designs to accomplish the cutting of material at the drill point. This design greatly reduces pushing of the material in the center of the drill, and also requires less drilling force. The shallower angle extends drill life and allows for a straighter hole.

Tool materials can improve drill life as well. Using an CarTech M42 tool steel for the drill material (this is also referred to as a cobalt drill) increases the drill life over CarTech M2 alloy, which is the more common high speed drill material. The CarTech M42 alloy is designed to exhibit higher hot hardness, thus resisting softening from the higher cutting temperatures common in machining stainless. The use of carbide drills extends life one step better, but the cost of carbide can become an issue.

Another solution is to use a softer stainless steel. The chemistry of some stainless grades offers a lower work hardening rate than can be normally expected with conventional austenitic stainless steels. Such alloys offer significant potential for easier cutting and lower drill wear.

Surface Finish

The traditional method of getting a better surface finish by increasing speeds and reducing feeds still applies for screw machines. With multiple-spindle machines, however, increasing spindle speeds could be tricky. It is easy to forget that increasing the speed for just one operation will adversely affect everything else. In such event, failure to achieve a good surface finish could be the least of the problems caused.  

Higher hardness in a stainless steel can also improve surface finish, but it tends to reduce the life of tools made from conventional tool steels. This obstacle may be overcome by the use of alternate tooling materials such as carbide, ceramics or powder super high speed steels such as CarTech Micro-Melt® Maxamet® alloy. The harder materials will produce more heat in the cutting zone, thus breaking down the tooling components. The use of high pressure coolant systems and/or coolants with more cooling ability can also help.

Better Tolerances

The achievement of close and better tolerances on parts not only starts with better starting material tolerances, but also with stable material. Thin section parts or parts with multiple contrasting features can move due to stresses in the material. If distortion is an obstacle, think about stress relieving.

There are cases where the 300 series austenitic stainless steels should be stress relieved or annealed, especially when the shop is machining thin sections. It is important to keep stress relieving at temperatures below 900°F to preserve essential mechanical properties and corrosion resistance. Stress relieving above 900°F causes carbide precipitation, which has an adverse effect on corrosion resistance. Carbide precipitation is a phenomenon where the carbon in the material ties up the chromium. This formulation equals rust!

Form A Partnership

No matter how skilled and well equipped they are, most machine shops can use all the outside help they can get to prosper in today's competitive environment. In general, the more demanding and economically sensitive the application, the more it may pay to bond with the machining bar supplier. What could be more cost- and quality-effective than combining the shop's machining expertise with the steel producer's knowledge of the alloy metallurgy and product application experience?

It behooves the shop to really know its stainless machining bar manufacturer. Does it provide technical service? Shop-floor assistance with the machining application? Advice on alloy selection and fabrication? What is the producer's record for making bar stock with minimum or no variation from heat to heat, order to order, bar to bar? How much does the manufacturer invest in research and development, and in capital/process improvements?

Also, how freely has the shop shared information with its stainless supplier? Has the supplier been told about the specific application requirements? The reasons for desired conditions such as finish and hardness? The special challenges or problems experienced or anticipated by the shop? This may not be the time for true confessions, but it certainly should be considered the time for mutual trust and confidence.

Tooling/Coatings Selection Guide

The following turning selection system from Kennametal can serve as a guide in selecting tooling materials and coatings for machining stainless steels. Kennametal qualifies its recommendations with the claim that its PVD coating process enables it to coat sharp-edged and high positive-rake inserts for cleaner cutting action and lower cutting forces.

For finish machining, lighter depth of cuts and slower feedrates, consider carbide KC5010 and cermet KT315 turning grade featuring a new PVD TiCN coating. Operating speeds for the KT315 grade are 500 to 1,000 sfm.

For semi-finishing most stainless steels, consider KC5010, a carbide substrate grade with a PVD TiAIN coating over a very deformation resistant unalloyed substrate to provide excellent resistance to heat and wear. Operating speeds for KC5010 are 375 to 825 sfm.

For medium-to-roughing machining, consider KC9225 grade.It has a cobalt-enriched substrate that combines toughness and wear resistance. Its polished, multi-layer K-MTCVD coating provides extended tool life at higher cutting speeds (350-900 sfm).

For tough applications at moderate speeds and medium-to-high feed rates, consider KC9240, a CVD/PVD coated grade. This grade offers an excellent combination of toughness, built-up-edge resistance and wear resistance for difficult-to-machine stainless steels.

For tough, cast stainless steel machining applications, KC9245 is a candidate material. This is an engineered, K-MTCVD coated carbide grade. Its substrate withstands heavy interruption, while its polished surface resists build-up even at slow speeds. In addition, its wear-resistant coating resists the micro-chipping common when machining austenitic stainless steels.

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By Robert S. Drab and Humberto L. Raposo
Carpenter Technology Corporation
Reading, PA