Machining
Summary
MACHINING STAINLESS STEELS is a complex operation. Not only does a shop need the correct supporting equipment and supplies, a better understanding of the metal itself is ad- vantageous. Technology in the production of a more machinable stainless steel is advancing. The incorporation of complex oxides has led to the development of materials that allow higher machining speeds and increased productivities, both of which are reducing machining costs and keeping shops competitive.
Introduction
Stainless steel forgings, castings, plate, and long products all are frequently machined. This fundamentally involves the removal of a layer of material from the workpiece with a cutting tool one or multiple times until a finished or semifinished part is produced. Machining, in it- self, is a complex topic with many variables. Rather than attempt to understand all aspects of machining, it is helpful to consider a material’s
machinability, that is, its ability to be machined
and the factors that affect its ability to be ma- chined. In Fig. 1, a macroview shows how the machinability of a material is influenced by the interaction of humans, machine, methods, mate- rial, and management. Some of the variables can affect the appearance of the material, while others affect the performance of the piece, mak- ing machining an art as well as a science. Opti- mum machinability is obtained when each of these sectors come together, providing the best possible conditions for efficient machining. Any change in one of these sectors can change the behavior or efficiency of a machining job.
From a more focused viewpoint, the machin- ability of a material is further described by:
1. Consistency: Does the material machinabil- ity stay the same when bundles are changed? 2. Tool life/wear: How long does the tool last in the machining operation? This could be minutes, hours, shifts, or days.
3. Productivity: How many parts were made in an hour, shift, or day?
4. Cost per part: What is the cost of the final geometry?
5. Cycle time: How fast can a part be com- pleted?
6. Surface finish: How smooth or shiny is the part?
7. Chip control: Are the chips manageable? 8. Maximum cutting speed: How fast can the
part be cut without affecting tool life? 9. Maintaining tolerances: How long can the
machining operation continue before ad- justments are made?
10. Minimal operator intervention: Does the operator need to constantly adjust setup?
This list is somewhat empirical or job re- lated, but it provides guidelines for defining a job since cutting conditions can be very differ- ent for each material and part. For example, if the surface finish of a part is very important, it may be necessary for chip control and tool life to be sacrificed. Clearly, machining involves much more than simply cutting a piece of metal.
Machining is a very empirically mature sub- ject. The recommended feed rate, depth of cut, tool material, and cutting fluid for a given mate- rial/material condition (thermomechanical his- tory) can be found in readily available pub- lished tables. Books such as the ASM
Handbooks; Machinery’s Handbook, published
by Industrial Press; Marks’ Standard Hand-
book for Mechanical Engineers, published by
McGraw-Hill Book Company; or the Machin-
ing Data Handbook, 3rd edition, by the
Machinability Data center at the Institute of Ad- vanced Manufacturing Services (IAMS; for- merly known as Metcut Research Associates Inc.) in Cincinnati, OH; include much of the data used in industry today. Material manufac- turers are also a source of valuable machining data. A typical guide from ASM is shown as Table 1 (Ref 1).
Rather than simply reproducing data, the focus of this chapter is the metallurgical factors governing the machinability of stainless steels. Most of the information regards machining stainless bar products; however, many of the concepts could be applied to forgings as well as castings.
Physical and Mechanical Properties The machinability of stainless steels is very difficult to characterize in definitive terms be- cause of the broad nature of these materials. A ferritic stainless steel, such as type 430, will machine very differently from the martensitic. In some sense, this is like comparing brass to carbon steel. Both type 410 and type 430 are stainless steels, but the chemistry and structural differences create diversity in machining char- acteristics.
The machinability of stainless steels can be thought of as a function of the steel’s chemistry, cleanliness, structure, processing history, and the cross-section size of the stock, with no one factor more important than another:
Each variable contributes uniquely to machin- ability. Machine shops and users generally have very little influence on these material variables. Because no two mills are exactly identical, there will be differences in machinability of a steel grade provided by different mill suppliers. How- ever, having an understanding of how these vari- ables contribute to machinability is invaluable. Armed with an understanding of the material and how it is made, one can determine the tooling, coolant, and setup of the machining job.
Let us take a closer look at these variables. Chemistry
The role of chemistry is to define not only the different grades of stainless steel (ferritic, martensitic, etc.), but also how the grade is chemically balanced within the specific grade; for example, the amount of carbon in a martensitic stainless can change tool wear char- acteristics, or a change in nickel content within specification limits can alter the stringiness of a chip. Combined, both will be the basis the mate- rial’s machinability.
Each of the elements used to produce stain- less steels will contribute some general machin- ing attributes. The effects of the elements as de- scribed next are general, and slight deviations may be encountered depending on the stainless grade. However, for the more common stainless grades used today, these effects of these alloy- ing elements are fairly accurate.
Iron is the base element in a stainless steel. It
is a soft, gummy material that has high work- hardening characteristics. Iron is characterized by surface finishes that are difficult to obtain and chips that are stringy, and it has a high ten- dency toward tool built-up edge (BUE).
Chromium strengthens and reduces ductility
of stainless steel. Machine and tool setup re- quire more rigidity. Chromium allows chips to begin breaking.
Carbon content increases strengthen stainless
steels and promote carbide formation. Low car- bon levels, typical in ferritic stainless steels, do not help machinability much. Increasing amounts of carbon to greater than 0.08% will aid in chip breakability and reduced BUE in these grades. However, as carbon content increases,
Machinability of =
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the amount of carbide increases, the structure changes to martensitic, and the wear on tools increases.
Nickel increases the toughness and ductility
of stainless and reduces the work hardening rate. Nickel also increases elevated temperature mechanical properties. This causes chips to be more difficult to break. Nickel will have a ten- dency toward increased BUE; however, better tool life will generally result.
Sulfur reduces mechanical and corrosion
properties and can be a cause of hot cracking in the resulfurized grades. It is best known as a free-machining contributor that promotes better tool life and greater machining speeds.
Manganese is generally added to combine
with sulfur to form manganese sulfide (MnS), which acts as a self-lubricant and improves machinability. In high-manganese grades, such
as duplex and 200 series alloys, manganese has the same relative effect as nickel when used in greater amounts, as, for instance, in the 200 se- ries stainless steels.
Molybdenum increases the strength and ele-
vated temperature mechanical properties. This increase in hot hardness and strength means more energy will be needed to cut the material, thus creating hotter cutting conditions. While the molybdenum helps in chip breakability, it will require more rigid setups and will reduce tool life.
Copper improves ductility and reduces the
strain-hardening or work-hardening rate (with the exception of participation-hardening alloys, for which copper is used as the precipitant). Chips can be more difficult to break, which in- creases the tendency of BUE and promotes bet- ter tool life.
Material Hardness, HB Condition Depth of cut(a), in. Speed, fpm Feed, ipr Tool material
AISI Brazed Indexable
Feed, ipr Tool material grade Speed, fpm Feed, ipr Tool material grade Ferritic steels 405, 409, 429, 430, 434, 436, 442, 446(c) 135-185 Annealed 0.040 150(235) 0.007 M2, M3 575 650 0.007 C-7 850 0.007 CC-7 0.150 120(190) 0.015 M2, M3 450 500 0.015 C-6 650 0.015 CC-6 0.300 95(150) 0.020 M2, M3 350 400 0.030 C-6 525 0.020 CC-6 0.625 75(115) 0.030 M2, M3 275 310 0.040 C-6 . . . . . . . . .
Austenitic and duplex steels
201, 202, 301, 302, 302B, 304, 304L, 305, 308, 309, 309S, 310, 310S, 314, 316, 316L, 317, 321, 330, 347, 348, 384, 385(c) 135-185 Annealed 0.040 95 0.007 M2, M3 325 375 0.007 C-3 500 0.007 CC-3 0.150 75 0.015 M2, M3 300 325 0.015 C-3 425 0.015 CC-3 0.300 60 0.020 M2, M3 225 250 0.020 C-2 325 0.020 CC-2 0.625 45 0.030 M2, M3 175 200 0.030 C-2 . . . . . . . . . 225-275 Cold drawn or duplex annealed 0.040 80 0.007 T15, M42(b) 300 325 0.007 C-3 425 0.007 CC-3 0.150 65 0.015 T15, M42(b) 250 275 0.015 C-3 350 0.015 CC-3 0.300 50 0.020 T15, M42(b) 290 215 0.020 C-2 275 0.020 CC-2 0.625 40 0.030 T15, M42(b) 140 165 0.030 C-2 . . . . . . . . . 2205, 2507 295-310 Annealed
Martensitic and PH steels
403, 410, 420, 422, 501, 502(c) 135-175 Annealed 0.040 155 0.007 M2, M3 475 620 0.007 C-7 800 0.007 CC-7 0.150 125 0.015 M2, M3 400 480 0.015 C-6 625 0.015 CC-6 0.300 100 0.020 M2, M3 320 380 0.030 C-6 500 0.020 CC-6 0.625 80 0.030 M2, M3 240 300 0.040 C-6 . . . . . . . . . 175-225 Annealed 0.040 145 0.007 M2, M3 460 570 0.007 C-7 850 0.007 CC-7 0.150 115 0.015 M2, M3 385 450 0.015 C-6 550 0.015 CC-6 0.300 90 0.020 M2, M3 300 350 0.030 C-6 450 0.020 CC-6 0.625 70 0.030 M2, M3 235 265 0.040 C-6 . . . . . . . . . 275-325 Quenched and tempered 0.040 95 0.007 T15, M42(b) 360 465 0.007 C-7 700 0.007 CC-7 0.150 75 0.015 T15, M42(b) 280 360 0.015 C-6 450 0.015 CC-6 0.300 60 0.020 T15, M42(b) 225 280 0.020 C-6 375 0.020 CC-6 375-425 Quenched and tempered 0.040 65 0.007 T15, M42(b) 290 320 0.007 C-7 475 0.007 CC-7 0.150 50 0.015 T15, M42(b) 225 250 0.015 C-6 300 0.015 CC-6 0.300 40 0.020 T15, M42(b) 180 200 0.020 C-6 250 0.020 CC-6
High-speed steel tool
Speed, fpm
Coated Uncoated
Table 1 Machining setup recommendations for turning wrought stainless steels
PH, precipitation-hardenable. Source: Ref 1
Nitrogen strengthens stainless steels. It aids
in chip breakability and reduces BUE but in- creases tool wear.
Titanium promotes carbide formation and in-
creases tool wear.
Niobium promotes carbide formation and in-
creases tool wear.
The production of stainless steels is identi- fied by industry specifications, such as AISI, UNS, EN, JIS, etc. These specifications are all defined with fairly broad elemental chemical compositions. For example, an AISI 304 has a 2 wt% window for the nickel content; that is, this grade can have a nickel level of 8 to 10%. A type 304 with 8% nickel can have different machinability characteristics from a type 304 with 10% nickel. This 2% difference alters chip morphology and surface finish capability. Since today’s mill technology can meet very tight ele- mental targets within the grade specification, how the mill balances the grade’s chemistry will provide the foundation of its machinability characteristics.
Cleanliness
The cleanliness of steel is determined by the amount and type of inclusions it contains. Vac- uum and argon oxygen decarburization (AOD) melting and refining along with proper steel- making techniques can reduce the inclusions to negligible levels. It is beneficial to machinabil- ity to avoid hard inclusions. However, certain inclusions are plastic and act as solid-state lu- bricants and chip breakers and prevent adhesion of the material to the tool. The beneficial effect of controlled inclusions is discussed in this chapter.
Structure
Material structure consists of both the phases that are present and the microstructure of those phases. Each type of stainless steel belongs to a larger family, which is characterized by a single predominant phase or a combination of two. These are ferritic, austenitic, martensitic, pre- cipitation hardening, and duplex (see the chap- ters on stainless steels, Chapters 6 to 10, in this Volume). Their machining characteristics are described in the next section. The microstruc- ture of a given alloy is independent of the grade type and composition and is mainly influenced by grain size. Grain size is not normally speci- fied or reported on certifications; however, mills
measure and control it to varying degrees. The material’s grain size results from the thermal and mechanical history during manufacturing and from the mill’s equipment capability and practices.
The grain size of a particular product can dra- matically change its machinability. It is entirely possible for the grain size difference between two lots of material to be large enough to pre- vent both lots from being effectively machined with the same setup, requiring adjustments in the machining setup to remedy the situation. Finer grain sizes strengthen the stainless steel, cause hotter cutting conditions, and have a higher tendency of BUE. On the brighter side, finer grain sizes yield better surface finishes and smoother roll thread crests.
Process
The type of equipment used by the stainless manufacturer, the manufacturing sequence, and the practices employed by the mill can affect machinability as well as mechanical properties, but more important, processing affects how consistently the material can be machined. The melt type, hot rolling parameters, thermal treat- ments, cold-finishing parameters, and sequence of these operations can affect how consistently a material machines. Many times, the culprit is equipment operational procedures or practices that can vary one day to the next. Equipment types can also play a role in machinability. For example, machinability can vary when the same-size material is drawn across two different draw benches using different pulling mecha- nisms and two different straightening mecha- nisms. Whether the material is continuously an- nealed or batch annealed can cause different strain distributions across the material cross section as well as material strength differences. Various annealing lines vary in time/tempera- ture profiles and therefore result in different grain size and mechanical properties.
With all this in mind, manufacturing consis- tency can be a great asset in machinability. A machine shop can adjust when a material is con- sistently bad, but it is very difficult when one lot is easy to machine followed by a lot that is tough to machine, while a third bundle performs differently from the first two. Mills that promote machining consistency pride themselves by practicing manufacturing consistency. Toler- ance variation will be tighter and machining costs will be lower with their products.
Cross-Section Size
Mill processing equipment dictates a manu- facturing route based on size. Smaller diameters are cold drawn, while larger diameters are straightened/cut/turned, yielding a softer prod- uct. This can have an impact on machining per- formance despite all other factors being the same. Cold finishing of stainless steels can be accomplished via a couple of general manufac- turing routes.
The first is by cold drawing to bar, and the sec- ond is simply a straightening-turning operation. The mechanical properties of the straightened- turned bars will be softer than the bars made by cold drawing. Typically, sizes greater than 1 in. (25 mm) are annealed/turned and straightened, with virtually no strain in the product.
Machinability of the Stainless Steel Families
Comparing the machinability of stainless steels with other materials such as carbon steels, brass, or aluminum, there are some striking dif- ferences. In general, stainless steels have: 1. Low thermal conductivity
2. High work-hardening rates 3. High tensile strengths 4. High toughness 5. High ductility
6. Large spreads between the yield and tensile strengths
Each stainless steel family (ferritic, marten- sitic, etc.) brings its own general set of machin- ing rules. This is mainly due to the chemistry of these families and its resultant effect on the physical and mechanical properties. A general description of the machining behavior is pro- vided next. One must keep in mind that these are general characteristics. Further alloying of these families, such as with a sulfur addition, can re- sult in a radical change in machining behavior. Ferritic
Ferritic stainless steels are the most basic stainless steels and are part of the 400 series grades. Their basic chemical composition is iron and chromium. These grades generally ex- hibit lower strengths, more ductility and soft- ness, and a close yield-to-tensile ratio. These grades will have a high tendency to BUE, chips
will tend be stringy but can be broken through aggressive chip breaking, and surface finishes will be somewhat of a challenge. Ferritics are the easiest of the stainless steels to machine, do not require much horsepower, have a low work- hardening rate and better tool wear, and will generally have higher speed and feed capabili- ties than other stainless families.
Martensitic
Martensitic stainless are also very basic straight chromium stainless steels, 400 series stainless grades, and are similar to the ferritic grades. The difference is that the martensitic grades have much higher carbon levels, which further strengthen the materials and allow these materials to be hardenable by heat treatment. These grades will have higher carbide levels, which will lead to higher tool wear. This is es- pecially true if the material is being machined in the hardened condition. The higher strengths will require more horsepower to cut and will need more rigid setup than ferritic steels. The work-hardening rate of martensitic stainless is lower than for ferritic stainless. Martensitic stainless also has a small yield-to-tensile ratio, making chips easier to break.
Austenitic
The austenitic grades, the 300 series stainless grades, are more difficult to machine than the ferritic and martensitic families. Austenitic stainless steels are more highly alloyed and are more prone to higher work-hardening rates. This leads to the need for higher horsepower and more rigid setups. These grades are very prone to BUE and hence are prone to poorer surface finishes and tend to tear. The yield-to-tensile ra- tios of austenitic stainless steel is very large, making chips hard to break. Chips in this family of alloys tend to be long and stringy. The higher strength and higher ductility of these grades also tend to increase cutting temperatures, necessitat- ing tooling with higher heat resistance.
Precipitation Hardening
Precipitation hardening stainless steels are characterized by higher strength and toughness. The solution-annealed hardness of AISI 630, for instance, is HRC 36 versus HRC 23 for a 304. Higher horsepower requirements, high tendency to BUE, higher tool wear, and difficulty break- ing chips are familiar scenarios for this class of
stainless. Except for alloy A-286, the precipita- tion-hardenable (PH) grades are all martensitic alloys and can be treated as such for machining purposes.
Duplex
The duplex is a unique class of stainless char- acterized by a dual-phase structure. Duplex al- loys have a structure that is roughly a 50% mix of austenite and ferrite; thus, two hardness ma- terials with different hardnesses coexist side by side. The tool will alternate cutting between soft and hard grains of the duplex structure, leading to an automatic tendency to initiate chatter in the cutting system. Strength levels of duplex al- loys are quite a bit higher than austenitic grades. Between the duplex structure and high-strength levels, high horsepower is necessary, and highly rigid setups are required. Some work at Ugitech found that to effectively machine these grades, highly alloyed carbide tooling with high hard- ness and high heat resistance, such as the C7/C8-type carbides, should be used.
Super Stainless Steels
Super stainless steels are today’s highly spe- cialized stainless grades. These grades, like the duplex alloys, are being developed to increase corrosion performance parameters to meet some of today’s increasing performance require- ments. These alloys are more highly alloyed than the duplex materials. Strength levels are higher and toughness is greater, driving machin- ability downward.
Role of Inclusions
Metallurgists have long known that the pres- ence of a soft second phase dispersed in the ma- trix of a parent metal can improve its machin- ability. These particles provide a solid-state lubricant between the chip and tool or a discon- tinuity in the material to aid in chip breaking. The challenge to the alloy designer has been to develop second phases that produce these bene-