stainless steel machining

8/6/2019 Stainless Steel Machining

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STAIN LESSSTEELFOR

M ACHIN IN G

H A N D B OOK

D E S I G N E R

StainlessSteel

TheValueOption


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Table of ContentsAcknowledgement.................................1Preface ..................................................1Introduction to Stainless Steels ..............2

Identification.......................................3Metallurgical Structure .......................3Material Selection for Corrosive

Environments .................................4Chemical Composition and

Mechanical Properties....................4General Guidelines................................4

The Machinable Family ofStainless Steels ............ ............... .......6Free-Machining Stainless Steels.........6Stainless Steels for Screw Machine

Operations......................................7Machinability vs. Cost ........................7

Passivation of Stainless Steel ............... ..8Cleaning ................................................8The Free-Machining Stainless Steels .....8

Types 303 and 303 SeStainless Steels...............................8

Type 203 Stainless Steel ............. .......8Type 430F Stainless Steel ............. .....9Type 416 Stainless Steel ............. .......9Type 420F Stainless Steel ............. .....9

The Specialty Steel Industry of NorthAmerica (SSINA) and the individual com-panies it represents have made everyeffort to ensure that the information pre-sented in this handbook is technicallycorrect. However, neither the SSINA norits member companies warrants theaccuracy of the information contained inthis handbook or its suitability for anygeneral and specific use. The SSINAassumes no liability or responsibility ofany kind in connection with the use ofthis information. The reader is advisedthat the material contained herein shouldnot be used or relied on for any specificor general applications without firstsecuring competent advice.

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ACKNOWLEDGEMENTIn reissuing this booklet, the Specialty Steel Industry of North

America (SSINA) wishes to acknowledge that the contents wereprepared by the Committee of Stainless Steel Producers, AmericanIron and Steel Institute. SSINA stainless steel producers wererepresented on this Committee. A number of member companiesof the SSINA have reviewed this data and believe it to reflect anaccurate representation of current stainless machining information

on generic grades.NOTE: Several stainless steel producers market proprietary ortrademarked grades for improved machinability. These grades are notincluded in this handbook.

PREFACEReference to stainless steel is often made in the singular sense as if

it were one material. Actually there are approximately 150 separate anddistinct compositions, each one formulated to serve specific end-useand/or manufacturing requirements.

Stainless steels are only one segment of the steel spectrum, but theyserve a multitude of applications from brightly polished consumerproducts to machinery and equipment for tough industrialenvironments. The variety of stainless steels available today provides awide variety of properties, from specially formulated alloys capable ofperforming in the most difficult environments to a selection of typesideally suited for machining or other fabrication operations.


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2

WHY UPGRADE TOSTAINLESS STEELS?

Upgrading to stainless steels rewardsboth manufacturer and user. Among themore obvious benefits are:

Corrosion and Heat Resistance Stainless steels, depending upon thecomposition, resist corrosion by manyacids, moisture, atmospheric conditions,and other aggressive environments atlow or high temperatures.

Strength Parts often can be madestronger and tougher with stainlesssteels than with mild steels or nonferrousmetals . . . including parts exposedeither to high temperatures or to hun-dreds of degrees below freezing.

Durability The combined qualitiesof corrosion resistance and strengthresult in products capable of providing alifetime of useful, trouble-free service.Accordingly, manufacturers reputationsare enhanced by products made ofstainless.

Low Maintenance Homeownersand industrial users alike prefer stainless

steels because there is no need for pro-tective coatings or other special surfacetreatments that can deteriorate ornecessitate periodic maintenance.

Appearance Whether a produc t ishighly polished or just routinely made ona screw machine to a 63 RMS as-machined finish, its surface presents anenduring bright, metallic lustre.

Fabrication Flexibility Stainlesssteels can be machined, cold formed,forged, extruded, or welded by contem-porary fabricating tools and techniques.Designers will find among the stainlesssteels, properties capable of meeting awide range of manufacturing and end-use requirements.

SELECTING A STAINLESS STEELEarly uses of stainless steels were lim-

ited to such applications as gun barrels,cutlery, and nitric acid tanks. As industrybegan to exploit the full potential of thesecorrosion resistant metals, new composi-tions were developed to accommodaterequirements for greater resistance tocorrosion, greater strength levels, differ-ent fabricating characteristics, resistanceto elevated temperature, etc.

For example, Type 304, one of themost frequently used stainless steelcompositions having application in abroad range of products from cookwareto chemical p lant equipment, has severalvariations. For greater resistance tocorrosion, especially in marine environ-ments, specifiers often select Type 316,which has a higher alloy content thanType 304. Type 305, on the other hand,has a lower work-hardening rate thanType 304 and is better suited to coldforming operations, while Type 303 is themore machinable variation of Type 304.

and it demonstrates that stainless steelsare readily machinable . . . even onhigh-volume, high-speed automaticscrew machines.

* The Specalty Steel Industry of NorthAmerica (SSINA) has a software programavailable called Life Cycle Costing ForStainless Steel. Contact the SSINA forinformation on this prog ram.

INTRODUCTION TOSTAINLESS STEELS

Stainless steels are iron-base alloyscontaining 10.5 percent or morechromium. Chromium is the alloying ele-ment that imparts to stainless steels theircorrosion-resistance qualities. It doesthis by combining with oxygen to form athin, transparent chromium-oxide pro-tective film on the metal surface(Figure1).

The chromium-oxide film is stable andprotective in normal atmospheric or mildaqueous environments, and it can beimproved by higher chromium, by nickel,molybdenum, and/or other alloying ele-

ments. Chromium improves film stability;molybdenum and chromium increaseresistance to chloride penetration; andnickel improves film resistance in strongacid environments.

In the event that the protective (pas-sive) film is disturbed or even destroyed,it will in the presence of oxygen in theenvironment reform and continue togive maximum protection.

Selection of the proper stainless steelfrom the many types available requiresan evaluation based upon four importantcriteria. Listed in order of importance,they are:

Corrosion or Heat Resistance theprimary reason for specifying stainlesssteel. The spec ifier needs to know thenature of the environment and thedegree of corrosion or heat resistancerequired.

Mechanical Properties with partic-ular emphasis on strength, at room, ele-vated, or low temperature. Generallyspeaking, the combination of corrosionresistance and strength is the basis forselection.

Fabrication Operations and howthe product is to be made is a third-levelconsideration. This includes machining,forming, welding, etc. For parts requiringmore than one fabrication operation, themost difficult usually takes precedence.

Total Cost* To put everything intoproper perspective, a total value analy-sis is appropriate which will consider not

only material and production costs, butthe cost-saving benefits of a mainte-nance-free product having a long lifeexpectancy as well.

With respect to machining, this hand-book helps to explain the differencesbetween stainless steels and other met-als, and the d ifferences from one stain-less steel to another. It identifies thestainless steel types that were devel-oped to improve machining production,

Figure 1Effect of Chromium Content on Corrosion Rate(In Normal Atmosphere)

Other alloying elements may be added during melting, such as nickel,molybdenum, columbium, or titanium, which serve to change or enhance certainproperties or characteristics.

8

7

6

5

4

3

2

1

0 2 4 6 8 10 12 14 16

% Chromium

mpy

CorrosionRate


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IDENTIFICATIONSeveral methods are commonly used

to identify stainless steels. They are:1. Classification by metallurgical

structure austenitic, ferritic,martensitic, precipitation harden-ing, or duplex

2. The AISI numbering system namely 200, 300, and 400 seriesnumbers

3. The Unified Numbering System,

which ASTM and SAE developed toapply to all commerical metals andalloys

4. Trade names which are gener-ally used with proprietary or specialanalysis stainless steels(Note: These are not included inthis handbook)

METALLURGICAL STRUCTUREThe five categories of stainless steel

according to metallurgical structure are:1. Austenitic2. Ferritic3. Martensitic

4. Precipitation Hardening5. Duplex

Following is a characterization of eachgroup and identification of the typicalstainless steel(s) in each. Understand-ing the characteristics of each categoryis basic to a better understanding of allstainless steels.

Austenitic stainless steels are thosecontaining chromium, nickel and man-ganese or just chromium and nickel asthe principal alloying elements. They areidentified as AISI 200 series or 300series types, respectively.

The austenitic stainless steels can behardened only by cold working; heattreatment serves only to soften them. Inthe annealed condition, they are notattracted to a magnet although somemay become slightly magnetic after coldworking.

The 200 and 300 series stainlesssteels are characterized as havingexcellent corrosion resistance, unusuallygood formability, and the ability todevelop excellent strength characteris-tics by cold working. Annealed, theypossess maximum corrosion resistance,ductility, good yield and tensile strength,high impact strength, and freedom fromnotch effect.

Typical of this group is Type 304, alsowidely known as 18-8 stainless steel(which refers to 18 percent chromium, 8percent nickel). It is a general-purposestainless of which there are numerousmodifications. In these variations,

1. The chromium/nickel ratio is modi-fied to change the cold formingcharacteristics such as in Types301 and 305.

2. The carbon content is decreased toprevent carbide precipitation in

ferritic group. Consequently, whencooled rapidly from high temperature,they do harden, and in some cases totensile strengths exceeding 200,000 psi.

The martensitic 400 series types resistcorrosion in mild environments (atmo-sphere, fresh water, weak acids, etc.);they have fairly good ductility; and theyare always strongly magnetic. Typical ofthe martensitic grades is Type 410 with12 percent chromium. Some of the

martensitics have been modified toimprove machinability, such as Types416, 420F and 440F.

Precipitation Hardening. As justnoted, cold working of the austeniticsand rapid cooling from high temperatureof the martensitics are two methods forincreasing strength and hardness ofstainless steels. In precipitation harden-ing stainless steels tensile and yieldstrengths (in some cases exceeding300,000 psi) can be achieved by a low-temperature (about 900F) aging treat-ment in combination with cold working.

The precipitation hardening stainless

steels are especially useful becausefabrication can be completed in anannealed condition and then the com-ponent uniformly hardened without therequirement of cold working or thenecessity of dealing with the problemsof distortion and heavy scaling associ-ated with high temperature thermaltreatments.

Typical of the precipitation hardeninggrades, identified by UNS numbers(which also serve as AISI numbers), areTypes S13800, S15500, S17400 andS17700.

Prior to hardening, the machinabilityof the precip itation hardening stainlesssteels is about equal to or slightly lessthan Type 304 in the annealed condi-tion. For example, Type S17700 has amachinability rating of 45, in comparisonto Type 416 stainless steel at 100 per-cent. The corrosion resistance of thesesteels in the hardened condition is aboutequal to that of Type 304.

Duplex stainless steels are dualphase materials with austenite and fer-rite in a close to 50-50 balance. Typicalcharacteristics include excellentstrength and corrosion resistance andgood fabrication properties, especiallythe nitrogen containing grades.

Type 329 is a duplex stainless steelthat contains 26 percent chromium,which gives it good resistance in chlo-ride environments. The newer duplexmaterials with nitrogen (S31803 andS32550) are readily weldable, whichfrequently was a problem with Type 329.They are used extensively in oil and gasproduction and chemical processing.

Table I lists the standard andenhanced machinability types of stain-less steel.

weldments such as in Types 304Land 316L.

3. Columbium or titanium can beadded to stabilize the structure forservice at high temperature such asin Types 347 and 321. They alsoserve to prevent carbide precipita-tion during welding.

4. Molybdenum is added or thechromium and nickel contentsincreased to improve corrosion or

oxidation resistance such as inTypes 316 and 317, with molybde-num, and Types 309 and 310 withhigher alloy content.

5. Sulfur is adjusted to improvemachining characteristics such asin Type 303. (The use of seleniumto enhance machining characteris-tics has, for the most part, beendiscontinued. Type 303 Se, how-ever, is still produced.)

6. Calcium additions are sometimesused to improve machinability,particularly deep hole drillingcharacteristics.

7. Nitrogen content is increased toenhance strength characteristicssuch as in Types 304N and 316N.

8. The 200 series stainless steels,namely Types 201, 202, 203, and205, are counterparts to the 300series Types 301, 302, 303, and305. In the 200 series manganesereplaces some of the nickel.

Ferritic stainless steels are straight-chromium types identified by 400 seriesnumbers. The low carbon-to-chromiumratio in the ferritics eliminates effects ofthermal transformation, which simplymeans that they are not hardenable byheat treatment. (They can be hardenedslightly by cold working, however.)

The ferritic 400 series stainless steelsare strongly attracted to a magnet, havegood ductility, and have good resis-tance to corrosion and oxidation. Thecorrosion resistance of ferritic stainlesssteels is improved by increasedchromium and molybdenum contents,while ductility, toughness, and weldabil-ity are improved by reducing carbonand nitrogen contents. The base com-position is Type 430, with nominally 17percent chromium. The free-machiningferritic is Type 430F.

There are several new ferritic stainlesssteels with improved characteristics,which can be classified as follows; thosewith about 18 percent chromium havingcorrosion resistance similar to Type 304,such as Type 444, and those with morethan 18 percent chromium with resis-tance to corrosion comparable or supe-rior to Type 316 in many media.

Martensitic stainless steels are alsostraight-chromium types bearing AISI400 series numbers, but having a car-bon-to-chromium ratio higher than the


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4

MATERIAL SELECTION FORCORROSIVE ENVIRONMENTS

Many variables characterize a corro-sive environment i.e., chemicals andtheir concentration, atmospheric condi-tions, temperature, time, flow rate, etc.

so it is difficult to say which stainlesssteel to use without knowing the exactnature of the environment. However,there are guidelines.

One of the three most widely used

stainless steels (Type 304, 430, or 410)is a good starting point in the selectionprocess, because these types are themost readily available.

Type 304 serves a wide range ofapplic ations. It withstands ordinary rust-ing in architecture, it is strongly resistantin food-processing environments(except possibly for high-temperatureconditions involving high acid and chlo-ride contents), it resists organic chemi-cals, dyestuffs, and a wide variety ofinorganic chemicals. Type 304 resistsnitric acid well and sulfuric acids atmoderate temperature and concentra-tions. It is used extensively for storage ofliquified gases, equipment for use atcryogenic temperatures, appliances andother consumer products, kitchenequipment, hospital equipment, trans-portation, and waste-water treatment.

Type 316 contains slightly more nickelthan Type 304 and 2-3 percent molyb-denum, giving it better resistance tocorrosion than Type 304, especially inchloride environments that tend tocause pitting. Type 316 was developedfor use in sulfite pulp mills because itresists sulfuric acid compounds. Its usehas been broadened, however, tohandling many chemicals in theprocess industries.

Type 317 contains 3-4 percent molyb-denum and more chromium than Type316 for even better resistance to pitting.

Type 430 has lower alloy content thanType 304 and is used for highly polished

the chip/tool interface, cause pressurewelding of portions of the chip to thetool. In addition, the low thermal conduc-tivity of stainless steels contributes to acontinuing heat buildup.

The difficulties involved in the tradi-tional machining of stainless steels canbe minimized by observing the followingpoints:

Because more power is generallyrequired to machine stainless steels,

equipment should be used only upto about 75% of the rating for car-bon steels

To avoid chatter, tooling and fixturesmust be as rigid as possible. Over-hang or p rotrusion of either theworkpiece or the tool must be mini-mized. This applies to turning tools,drills, reamers, and so on

To avoid glazed, work-hardenedsurfaces, particularly with austeniticalloys, a positive feed must be main-tained. In some cases, increasingthe feed and reducing the speedmay be necessary. Dwelling, inter-

rupted cuts, or a succession of thincuts should be avoided Lower cutting speeds may be nec-

essary, particularly for nonfree-machining austenitic alloys, precipi-tation-hardenable stainless steels, orhigher-hardness martensitic alloys.Excessive cutting speeds result intool wear or tool failure and shut-down for tool regrinding or replace-ment. Slower speeds with longertool life are often the answer tohigher output and lower costs

Tools, both high-speed steel andcarbides, must be kept sharp, with afine finish to minimize friction withthe chip. A sharp cutting edge pro-duces the best surface finish andprovides the longest tool life. To pro-duce the best cutting edge on high-speed steel tools, 60-grit roughingshould be followed by 120- and 150-grit finishing. Honing produces aneven finer finish

Cutting fluids must be selected ormodified to provide proper lubrica-tion and heat removal. Fluids mustbe carefully directed to the cuttingarea at a sufficient flow rate to pre-vent overheating

For additional information contact the

companies listed as bar producers onthe back cover.

Also refer to the American Society forMetals (ASM) pub lication: Machining of Stainless Steel.

trim applications in mild atmospheres. Itis also used in nitric acid and foodprocessing.

Type 410 has the lowest alloy contentof the three general-purpose stainlesssteels and is selected for highly stressedparts needing the combination ofstrength and corrosion resistance, suchas fasteners. Type 410 resists corrosionin mild atmospheres, steam, and manymild chemical environments.

Note: The Specialty Steel Industry ofNorth America (SSINA) has a handbookon the Selection and Use of Stainless Steel with details on corrosion resis-tance, chemical and mechanical proper-ties, high and low temperature proper-ties and other information. Contact theSSINA for information on how to obtain acopy of this handbook.

CHEMICAL COMPOSITION ANDMECHANICAL PROPERTIES

Table II lists the chemical compositionand the mechanical properties of thestandard enhanced machinability types.

A complete listing of all types is avail-able in the SSINA Handbook Selection and Use of Stainless Steel.

Table III shows the physical proper-ties of the standard and enhancedmachinability types.

GENERAL GUIDELINESThe characteristics of stainless steels

that have a large influence on machin-ability include:

Relatively high tensile strength High work-hardening rate, particu-

larly for the austenitic alloys High ductilityThese factors explain the tendency of

the material to form a built-up edge onthe tool during traditional machiningoperations. The chips removed inmachining exert high pressures on thenose of the tool; these pressures, whencombined with the high temperature at

Table ITABLE 1 LISTS THE STANDARD AND ENHANCED

MACHINABILITY TYPES OF STAINLESS STEELMetallurgical Standard Enhanced Machinability

Type Type UNS No. Type UNS No.Austenitic 304 S30400 303 S30300

304 S30400 303Se S30323 203 S20300304 S30400 304Ca 316 S31600 316F S31620

Ferritic 430 S43000 430F S43020

Martensitic 410 S41600 416 S41600420 S42000 420F S42020440C S44004 440F S44020


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Table IISTAINLESS STEELS CHEMICAL COMPOSITIONS AND MECHANICAL PROPERTIES

Mechanical PropertiesChemical Analysis % (Max. unless noted otherwise) (Annealed bar unless noted otherwise)

Yield ElongationAISI Tensile Strength in 2" HardnessType Strength (0.2% offset) (50.80mm) (Rockwell)

(UNS No.) C Mn P S Si Cr Ni Mo Other ksi, min. ksi, min. %, min. max.Austenitic203 0.08 5.00/ 0.040 0.18/ 1.00 16.00/ 05.00/ 0.50 1.75/ 75 30 40 262(S20300) 6.50 0.35 19.00 06.50 2.25 Cu (Brinell)

303 0.15 2.00 0.20 0.15 1.00 17.00/ 08.00/ 75 30 35 262(S30300) (min) 19.00 10.00 (Brinell)303Se 0.15 2.00 0.20 0.060 1.00 17.00/ 08.00/ 0.15 75 30 35 262(S30323) 19.00 10.00 (min) Se (Brinell)304 0.08 2.00 0.045 0.030 0.75 18.00/ 08.00/ 0.10 N 75 30 40(S30400) 20.00 10.50 (125) a (100) a (10) a

316 0.08 2.00 0.045 0.030 0.75 16.00/ 10.00/ 2.00/ 0.10 N 75 30 40(S31600) 18.00 14.00 3.00 (125) a (100) a (12) a

316H 0.04/ 2.00 0.045 0.030 0.75 16.00/ 10.00/ 2.00/ 75 30 40(S31609) 0.10 18.00 14.00 3.00316F 0.08 2.00 0.20 0.10 1.00 16.00/ 10.00/ 1.75/ 75 30 40 262(S31620) (min) 18.00 14.00 2.50 (Brinell)Ferritic430 0.12 1.00 0.040 0.030 1.00 16.00/ 0.75 70 40 20(S43000) 18.00430F 0.12 1.25 0.060 0.15 1.00 16.00/ 70 40 20 262

(S43020) (min) 18.00 (Brinell)Martensitic416 0.15 1.25 0.060 0.15 1.00 12.00/ 75 40 22 262(S41600) (min) 14.00 (Brinell)420 Over 1.00 0.040 0.030 1.00 12.00/ 75 40 25 241(S42000) 0.15 14.00 (Brinell)420F Over 1.25 0.060 0.15 1.00 12.00/ 75 40 22 262440C 0.95 1.25 0.40 0.030 1.00 16.00/ 110 65 14 262(S44004) (min) (min) 18.00 (Brinell)440F 0.95 1.25 0.040 0.10 1.00 16.00/ 110 65 14 262(S44020) (min) (min) 18.00 (Brinell)

Notes: Data are for information only and should not be used for design purposes. For design and sp ecification, refer to appropriate ASTM specifications. Data were obtainedfrom various sources, includ ing AISI Steel Products Manuals, ASTM specifications, and individual company literature.

a = Cold finished, for sizes up to 3 4 in. inclusive, A276 Condition B.

Table IIIPHYSICAL PROPERTIES OF STAINLESS STEELS

Modulus Specificof Electrical Specific Thermal Magnetic

Stainless Elasticity Resistance Heat Conductivity Mean Coefficient of Perme- AnnealingSteel Density psi at 68F Btu/Lb/F BTU/ft/hr/F Thermal Expansion, in/in/F x 10- 6 abi lity Tempera-Type Lb/Cu. In. x 10 6 Microhm-Cm 32-212F (68-212F) 32-212F 32-600F 32-1000F 32-1200F 32-1600F max. ture

Austenitic203 0.29 28.0 74 9.5 9.4 11.6 1.02 1850-2050303 0.29 28.0 a 72 0.12 9.4 9.6 9.9 10.2 10.4 (1.02) 1850-2050303Se 0.29 28.0 a 72 0.12 9.4 9.6 9.9 10.2 10.4 (1.02) 1850-2050304 0.29 28.0 a 72 0.12 9.4 9.6 9.9 10.2 10.4 11.0 1.02 1850-2050316 0.29 28.0 a 74 0.12 9.4 8.9 9.0 09.7 10.3 (11.1) 1.02 1850-2050

(32-1500F)316H 0.29 28.0 a 74 0.12 9.4 8.9 9.0 09.7 10.3 (11.1) 1.02 1850-2050

(32-1500F)

316F 0.29 29.0 74 0.116 8.3 9.2 9.7 10.1 2000 a

Ferritic430 0.28 29.0 60 0.11 13.8 5.8 6.1 06.3 06.6 6.9 1250-1400

(32-1500F)430F 0.28 29.0 60 0.11 15.1 5.8 6.1 06.3 06.6 6.9 1250-1400

Martensitic416 0.28 29.0 57 0.11 14.4 5.5 6.1 06.4 06.5 1500-1650

1200-1400420 0.28 29.0 55 0.11 14.4 5.7 6.0 06.5 1550-1650

1350-1450420F 0.28 29.0 55 0.11 14.5 5.7 1550-1650

1350-1450440C 0.28 29.0 60 0.11 14.0 5.6 1550-1650440F 0.28 29.0 60 0.11 14.0 5.6 1350-1450

a = Cool rapid ly from these annealing temperatures.b = Full annealing cool slowly.c = Process annealing.


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6

THE MACHINABLE FAMILY OFSTAINLESS STEELS

What is Machinability?Machine shop operators have differ-

ing opinions as to what machinabilityreally is. Some are interested only in thespeed at which a material can be cut,others consider tool life at a reasonablespeed to be most important, while oth-ers rate machinability on the surface fin-

ish produced. Obviously, all factors areimportant cutting speed, tool life, andsurface finish and all are consideredin rating the machinability of a metal.

The machinability of stainless steels issubstantially different from that of car-bon or alloy steels and other metals, asillustrated in the chart, ComparativeMachinability of Common Metals (Fig-ure 2). In varying degree, most standardstainless steels are somewhat moredifficult to machine. That is why thereare enhanced or free-machiningstainless steel types.

In fact, stainless steels are routinelymachined on high-production equip-ment. The best way to get maximummachinability, wherever end-use condi-tions permit, is to specify a free-machin-ing stainless steel.

The 400 series stainless steels are theeasiest to machine, but they do producea stringy chip that can slow productivity.The 200 and 300 Series, on the otherhand, are characterized as being themost difficult to machine, primarilybecause of their gumminess and, sec-ondarily, because of their propensity towork harden at a very rapid rate. How-ever, the difficulty is not so great as tobe a deterrent to selecting a stainlesssteel for a machined part.

Free-Machining Stainless SteelsCertain alloying elements in stainless

steels, such as sulfur, selenium, lead,copper, aluminum, calcium, or phospho-rus can be added or adjusted duringmelting to alter the machining character-istics. These alloying elements serve toreduce the friction between the work-piece and the tool thereby minimizingthe tendency of the chip to weld to the

tool. Also, sulfur forms inclusions thatreduce the friction forces and transverseductility of the chips, causing them tobreak off more readily. The improvementin machinability in the free-machiningstainless steels namely Types 303,303 Se, 203, 430F, 416, and 420F isclearly evident in the chart, Compara-tive Machinability of Frequently UsedStainless Steels (Figure 3).

Figure 2Comparative Machinability of Common Metals

Figure 3Comparative Machinability of Frequently Used Stainless Steelsand Their Free-Machining Counterparts

Ratings in %

AISITYPE

0 10 20 30 40 50 60 70 80 90 100

420

416

410

430F

430

203

303

304/316

0 10 20 30 40 50 60 70 80 90 100

AverageSource: National ScrewMachine Products Association

MaximumIndicates highest ratingavailable using High SpeedSteel Tools with suitablecutting fluid.Source: Steel Industry Sources

% based on 100% for AISI Type 416 free-machining Stainless Steel

420F

440C

440F

Ferritic Stainless Steels

100% Type 416 Stainless Steel

% 50 100 150 200

Martensitic Stainless Steels

Austenitic Stainless Steels

Free-Machining Steels

Carbon Steel

Alloy SteelCast Iron

Tool Steel

High-Temperature Alloys

Copper Alloys

Aluminum Alloys

% 50 100 150 200


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Should a d esigner select Type 304 onthe basis of corrosion resistance andstrength but recognizes a need for thebest possible machining rate, he mayelect to use free-machining Type 303. Thechromium, nickel, and sulfur contents ofType 303 are slightly d ifferent than that ofType 304, but its physical and mechani-cal properties are quite similar. Type 303can be machined at speeds about 25-30percent faster than Type 304.

Type 303 Se is a free-machining vari-ation of Type 304 that c ontains seleniuminstead of sulfur. Type 303 Se is a betterchoice than Type 303 when a bettermachined surface finish is required orwhen cold working may be involved,such as staking, swaging, spinning, orsevere thread rolling, in addition tomachining.

Type 203 is an austenitic free-machin-ing grade which is a modification of the200 series stainless steels. Although itcontains significant additions of man-ganese like the 200 series it also containsnickel and copper.

There are also free-machining alterna-tives to consider in the 400 seriesstainless steels. For instance, if end-useconditions call for Type 430 stainless,the specifier might selec t Type 430F.The composition of Type 430F isadjusted to enhance the machiningcharacteristics while preserving to theextent possible the qualities of Type 430.

The free-machining variation of Type410 is Type 416, and for Type 420, thespecifier should consider Type 420F.

Specifiers should recog nize that thealloying elements used to improve free-machining characteristics adverselyaffect corrosion resistance, transverseductility, and other qualities, such asweldability. Free-machining stainlesssteels are normally not used if welding,cold forming or hot forging are to bedone. However, the free-machininggrades are often specified after evalua-tion as to their suitability for the intendedcorrosive environment in which they will

Stainless Steels forScrew Machine Operations

Higher production rates are not theonly benefits derived from the free-machining stainless steels. As shopsbecome better acquainted with machin-ing stainless steels they acquire confi-dence to use screw machines forincreased productivity. This can be par-ticularly important because many partsare being machined at higher cost thanwould be required to produce thosesame parts on automatic screwmachines.

Low productivity can exist when theproduction department underestimatesthe practicality and feasibility of stainlesssteels for screw machine production.This situation will change as theybecome aware of the extreme versatilityof the several types of automatic sc rewmachines in use, and of the extremeversatility in stainless steel selection.

If the part meets the following sixcriteria, screw machining should beseriously considered for stainless steel:

1. The part to be produced should bein lots of 1,000 or more. This ismore a function of time than num-ber, however, and production runsshould be at least as long as theset-up time.

2. Tolerances, in general, should be inthe range of .001 inch.

3. The part has to be made from rod,bar, tubing, or hollow bar.

4. Turned or formed finishes in therange of 16 to 125 RMS should beacceptable.

5. Overall diameter of cross-sectiondimension should be between 1 64inch and 8 inches.

6. Overall length of machined part isusually less than 10 stockdiameters.

Machinability vs. CostMany factors influence the ratio of

machinability to cost of screw machineproducts, including the choice of materi-als. Experience has clearly establishedthat the cheapest material does notalways yield the lowest cost. Metalshaving low machinability ratings requiremore frequent tool changing, longerdowntime, and greater difficulty in main-taining good finish. This is particularly

true when stainless steels are involved;machining costs tend to increase asmachinability decreases. Accordingly, astainless steel with improved machiningcharacteristics is to be preferred. Even ifthere is a slight premium in the cost ofsuch bar stock, it is generally morethan offset by the resultant ease ofmachining.

When material specifications permit, itis to everyones advantage to give thescrew machine shop the opportunity touse a free-machining material.

serve. When they are spec ified, how-ever, significantly higher productionrates can be achieved.

There are other alternatives specifiersshould consider, especially if there isconcern over the use of a free-machin-ing stainless steel. For example, compo-sition of a stainless steel is often alteredslightly by the producer to enhance cer-tain fabrication characteristics, includingmachinability, but without changing

other basic qualities. While the improve-ment may not be as great as with atrue free-machining type, there isimprovement.

Also, producers of stainless steel barcan provide a product that is in the bestcondition for machining. However, asexperienced machinists know, the bestcondition for turning may not be the bestcondition for drilling, so specifiers areencouraged to discuss the machiningapplication with a producer.

Other considerations for reducedmachining are forgings or tubular prod-ucts. Seamless or welded mechanical

tubing is readily available in a broadrange of compositions (carbon, alloy,and stainless steels), sizes and wallthicknesses and shapes. Mechanicaltubing for machining is often referred toas hollow bar.

Hollow bar is espec ially attractive forprojects that require 30 percent or moremetal removal from the center. Not onlydoes hollow bar generally cost less perfoot than solid bar of equal d iameter(less cost because less weight), but itwill require less machining, produce lessscrap, improve cycle time, and allowmore parts to be produced per hour.

Mechanical tubing is usually availablefrom 1-inch diameter up to a 5-inchdiameter. Check with suppliers for avail-ability of sizes and shapes.

Rod and wire in coiled form are alsoavailable for coil-fed screw machines.Table IV describes the conditions andfinishes in which stainless steel bars arenormally available.

Table IVCONDITIONS AND FINISHES FOR BAR

Conditions Surface Finishes*1. Hot worked only (a) Scale not removed (excluding spot conditioning)

(b) Rough turned**(c) Pickled or blast cleaned and pickled

2. Annealed or other- (a) Scale not removed (excluding spot conditioning)wise heat treated (b) Rough turned

(c) Pickled or blast cleaned and pickled(d) Cold drawn or cold rolled(e) Ground(f ) Polished

3. Annealed and cold (d) Cold drawn or cold rolledworked to high (e) Groundtensile strength*** (f) Polished

* Surface finishes (b), (e) and (f ) are applicable to round bars only.** Bars of the 4XX series stainless steels which are highly hardenable, such as Types 414, 420,

420F, 431, 440A, 440B and 440C, are annealed before rough turning. Other hardenablegrades, such as Types 403, 410, 416 and 416Se, may also require annealing depending ontheir composition and size.

*** Produced in Types 302, 303Se, 304 and 316.


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8

GOOD SHOP PRACTICESOf course, a good deal of success in

machining and reducing productioncosts rests with the machine shop. Hereare a few guidelines that may be usefulwhen working with stainless steels:

1. Machine tools should be rigid,modern, and as much overpow-ered as possible. Best practice isto use the machine up to about 75percent of its rated capacity.

2. The work-piece and tool should beheld rigid. Tool overhang should beminimized and extra support usedwhen necessary.

3. Tools, either high speed tool steelsor carbide, should be kept sharp,preferably being sharpened at reg-ular intervals rather than only whennecessary.

4. A good lubricant is required, suchas compounded sulfur-chlorinatedmineral oil plus fatty oil for high-speed tools, and heavy duty emul-sifiable oil for carbide tools.

5. Positive cuts are necessary. Care

should be exercised to avoiddwelling so as not to work hardenthe material, especially theaustenitic (300 Series) stainlesssteels.

PASSIVATION OFSTAINLESS STEEL

On the surface of stainless steelsthere is an extremely thin transparentfilm. Nevertheless, it is tenacious, uni-form, stable and passive. It imparts tothe surface the property of passivity,normally associated with noble or inertmetals and it is to this passive film that

stainless steels owe their superior corro-sion resistance.The film will form spontaneously, or

repair itself if damaged, both in air dueto the presence of oxygen, or whenimmersed in solutions, provided there issufficient oxygen or oxidizing elementspresent. The basic passivation treatmentfor stainless steel is exposure of a cleansurface to air. However, there is muchpractical evidence which shows thatpassivity, and therefore corrosion resis-tance, is enhanced if the passive film isformed by the action of oxidizing acidsolutions. Nitric acid is such an oxidizingacid, and is always used for passivationtreatments. Nitric acid does not corrodestainless steel, does not alter criticallydimensioned parts and will not removeheat tint, embedded iron or otherembedded surface contamination. Nitricacid passivation is most useful inenhancing the corrosion resistance offreshly machined surfaces.

The standard nitric acid passivatingsolution is made up and used as follows:

10 to 15 percent by volume of nitricacid (HNO 3) in water. Quickest andbest passivation results if used at150F for the austenitic (300 series)

stainless steels, and 120F for theferritic and martensitic (400 series)plain chromium stainless steels. Theimmersion time is approximately30 minutes, followed by thoroughwater washing.

CLEANINGChemical cleaning is also important

for the removal of free-iron from themachined surface. During machining,

the stainless steel part can pick upminute particles of iron, which if notremoved can form rust spots on the sur-face. These rust areas, which result fromthe cutting tool not the stainless steelitself can be prevented by immersingthe finished part in a chemical solution.

The primary method of cleaning sur-faces contaminated with embedded ironis nitric-HF pickling in 10% nitric 2% HFeither warm or at ambient temperature.

For the stainless steel part to achievemaximum corrosion resistance, it shouldbe cleaned to remove grease, oil, andfingerprints from the surface. The aver-

age shop degreasing solution is usuallysufficient. Passivation in a 25-40% nitricacid solution is suggested for machinedparts to remove microscopic particles oftooling, and to prevent staining.

THE FREE-MACHININGSTAINLESS STEELS

DescriptionTypes 303 and 303 Se stainless steels

are the free-machining variations ofType 304 (austenitic 18Cr-8Ni) thatare particularly well suited for screwmachining operations. Their greatestbenefit is higher productivity resultingfrom longer tool life and higher cuttingspeeds in comparison to Type 304.

Type 303 has wide application forshafting, valve bodies, valves, valvetrim, fittings, etc. This stainless steel hasdesirable nongalling properties thatmake disassembly of parts easy andhelp to prevent scratching or galling inmoving parts.

Type 303 Se has applications similar toType 303 except that it has slightly bettercorrosion resistance than Type 303 andbetter formability for applications involv-ing hot or cold working operations.

Machining CharacteristicsTypes 303 and 303 Se stainless steels

machine easily with a brittle chip. Inturning operations they can be used atspeeds of 102-130 surface feet perminute. Moderate cold workingincreases the machinability. Grindingand polishing operations can be verysatisfactorily performed. In comparisonto Type 416, their machinability ratingaverage is 75 percent. Much higher

speeds are possible if carbide tooling isused.

Corrosion ResistanceTypes 303 and 303 Se stainless steels

resist rusting from all normal atmosphericsources and are used in connection withsterilizing solutions, most of the organicchemicals and dyestuffs, and a wide vari-ety of inorganic chemicals. They resistnitric acid well, the halogen acids poorly,and the sulfuric acids moderately.

For optimum corrosion resistance, allparts made of Types 303 or 303 Seshould be entirely free from scale andforeign particles, such as iron particlespicked up from tooling. It is suggestedthat after machining, all parts becleaned and passivated. Also, if duringfabrication, components are heated andcooled in the range of 800-1650F, a cor-rective thermal treatment is suggestedto avoid chances of intergranular corro-sion. Such a treatment consists ofheating to about 1900F followed byquenching in water.

DescriptionType 203 is an austenitic free-machin-

ing stainless steel containing higher lev-els of manganese in order to obtainmaximum machining speeds. The grade

is particularly suited for high productionhigh volume automatic screw machinework. Machining speeds are somewhathigher than for Type 303.

Machining CharacteristicsType 203 stainless steel machines

with chip characteristics similar to Type303 but requires slightly higher machin-ing speeds. Typical turning speeds are125 to 155 surface feet per minute forhigh speed tools. Higher speeds arepossible with carbide tooling. Themachinability rating for this steel is about85%.

TYPES 303 AND 303 SeSTAINLESS STEELS

ChemicalComposition,Percent 303 303 SeCarbon . . . . . . . 0.15 Max. 0.15 Max.Manganese . . . . 2.00 Max. 2.00 Max.Phosphorus . . . 0.20 Max. 0.20 Max.Sulfur . . . . . . . . . 0.15 Min. 0.060 Max.Silicon . . . . . . . . 1.00 Max. 1.00 Max.Chromium . . . . . 17.00/19.00 17.00/19.00Nickel . . . . . . . . 8.00/10.00 8.00/10.00Selenium . . . . . . 0.15 Min.Molybdenum . . . 0.60 Max. (Optional)

TYPE 203 STAINLESS STEELChemicalComposition,Percent 203Carbon . . . . . . . . . 0.8 Max.Manganese . . . . . . 5.00/6.50Phosphorus . . . . . . .04 Max.Sulfur . . . . . . . . . . . .18/.35Silicon . . . . . . . . . . 1.00 Max.Chromium . . . . . . . 16.00/18.00Nickel . . . . . . . . . . 5.00/6.50Molybdenum . . . . . .50 Max.Copper . . . . . . . . . 1.75/2.25


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Corrosion ResistanceThe corrosion resistance of this grade

is very similar to Type 303. It resists awide variety of organic and inorganiccompounds.

DescriptionType 430F stainless steel is sug-

gested for faster cutting and reducedcosts when making machined parts froma 16.00/18.00 percent straight-chromium stainless steel. Type 430Fdoes not harden by heat treatment. It isused for parts requiring good corrosionresistance, such as solenoid valves, air-craft parts, gears, etc. Type 430F is notusually recommended for vessels con-taining gases or liquids under high pres-sure.

Machining CharacteristicsType 430F machines in turning opera-

tions at speeds of 124-155 surface feetper minute or at ab out the same as ASE1120, 1030, etc.

Corrosion ResistanceType 430F stainless steel is used to

combat corrosion from atmosphere,

fresh water, nitric acid, dairy products,etc. Parts must be entirely free fromscale and foreign particles. As a finaltreatment, after the scale has beenremoved or after machining, all partsshould be passivated.

DescriptionType 416 is the most readily machin-

able of all stainless steels, and it is par-ticularly well suited for good productivityon automatic screw machining opera-tions because of the longer tool life thatresults. The uses for Type 416 areextensive and include fittings, gears,housings, lead screws, shafts, valvebodies, valve stems, and valve trim. Infact, this type is ideal for parts requiringconsiderable machining work. Its lowfrictional properties minimize galling inservice. Threaded sections work freelywithout seizing, and disassembly is par-ticularly easy. Pump shafts and valvestems work more smoothly in packing,and many metal-to-metal contacts with-stand more pressure because of theiranti-seizing characteristics.

Machining CharacteristicsType 416 stainless steel cuts very

freely because of the sulfur content. Inautomatic screw machines Type 416machines at about 165 surface feet perminute.

Corrosion ResistanceWhile not as corrosion resistant as an

austenitic (300 series type) or Type430F, Type 416 resists atmosphericenvironments, fresh water, mine water,steam, carbonic acid, gasoline, crudeoil, blood, perspiration, alcohol, ammo-nia, soap, sugar solutions, etc. A highfinish is helpful in providing optimumresistance to corrosion. It also resistsscaling at elevated temperatures andcan be used for continuous service upto about 1200F. (Maximum corrosionresistance is achieved in the heattreated condition.)

DescriptionType 420F stainless steel is easy to

machine, grind, and polish, and has cer-tain anti-galling or nonseizing propertiesin service. It is used for parts made onautomatic screw machines, such asvalve trim, pump shafts, needle valves,ball check valves, gears, cams, pivots,etc. This free-machining hardenablesteel is used mainly for machined partsrequiring high hardness and good cor-rosion resistance.

Machining CharacteristicsFor automatic screw machines, Type

420F stainless steel machines like SAE2315 and 2340. In single point turningoperations employing heavy duty equip-ment, speeds of 90-110 surface feet perminute and feeds of 0.0008-0.0020 inchare suggested.

Corrosion ResistanceSince Type 420F stainless steel

should always be used in the hardenedcondition for optimum corrosion resis-tance, surfaces must be free of all scale,which is achieved by pickling, or grind-ing and polishing. If pickled after hard-ening, the parts should be thoroughlybaked at 250-300F for at least one hourto remove acid brittleness.

In the hardened condition, Type 420Fstainless steel will resist corrosion fromatmosphere, fresh water, mine water,steam, carbonic acid, crude oil, gaso-line, blood, perspiration, alcohol, ammo-nia, mercury, sterilizing solutions, soap,etc. Passivation after machining isrecommended. (It should be noted thatType 420F is not as resistant to corro-sion as an austenitic g rade or Type430F.)

TYPE 430F STAINLESS STEELChemicalComposition,Percent 430FCarbon . . . . . . . . . 0.12 Max.Manganese . . . . . . 1.25 Max.Phosphorus . . . . . . 0.060 Max.Sulfur . . . . . . . . . . . 0.15 Min.Silicon . . . . . . . . . . 1.00 Max.Chromium . . . . . . . 16.00/18.00Selenium . . . . . . . .Molybdenum . . . . . 0.60 Max. (Optional)

TYPE 416 STAINLESS STEELChemicalComposition,Percent 416Carbon . . . . . . . . . 0.15 Max.Manganese . . . . . . 1.25 Max.Phosphorus . . . . . . 0.060 Max.Sulfur . . . . . . . . . . . 0.15 Min.Silicon . . . . . . . . . . 1.00 Max.Chromium . . . . . . . 12.00/14.00Selenium . . . . . . . .

Molybdenum . . . . . 0.60 Max. (Optional)

TYPE 420F STAINLESS STEELChemicalComposition,Percent 420FCarbon . . . . . . . . . 0.15 Min.Manganese . . . . . . 1.25 Max.Phosphorus . . . . . . 0.060 Max.Sulfur . . . . . . . . . . . 0.15 Min.Silicon . . . . . . . . . . 1.00 Max.Chromium . . . . . . . 12.00/14.00Molybdenum . . . . . 0.60 Max.

stainless steel machining

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