Stainless Steel Fasteners

Stainless Steel Overview

Stainless steel is used extensively throughout industry for both original equipment manufacture as well as for replacement. Stainless steel is a family of iron-based alloys containing about 10.5% chromium or more, plus other alloying elements such as nickel, manganese, molybdenum, sulfur, selenium, titanium, etc. The chromium is chiefly responsible for corrosion and heat resistance; the other alloying elements are present in stainless steel to enhance corrosion resistance and to impart certain characteristics with respect to strength and fabricability.

Stainless steel fastener materials are identified as the B8 class of alloys and are identified in the ASTM Specification A193/193M (Standard Specification for Alloy Steel and Stainless Steel Bolting Materials for High Temperature Service). The corresponding nut specification is ASTM Specification A194/194M.

GRADE 8 Fastener Materials

SYMBOL	ALLOY
B8/B8A	TYPE 304
B8C/B8AC	TYPE 307
B8M/B8MA/B8M2/B8M3	TYPE 316
B8P/B8PA	TYPE 305
B8N/B8NA	TYPE 304N
B8MN/B8MNA	TYPE 316N
B8MLCuN/B8MLCuNA	6% molybdenum alloy
B8T/B8TA	TYPE 321
B8R/B8RA	Nitronic 50
B8S/B8SA	Nitronic 60

Source: Stainless Steel Industry Data

ASTM Specification F593 (Standard Specification for Stainless Steel Bolts, Hex Cap Screws and Studs) covers the broad range of commercial ferritic, martensitic and precipitation hardened grades of stainless steel, 0.25 to 1.50 inch nominal diameter. These specifications cover the 300 series stainless steels and high manganese and high silicon austenitic grades, all of which are essentially 18-8 (18% chromium and 8% nickel) materials. Because they have similar corrosion resistance properties, these 18-8 materials are often interchanged in fastener applications.

Stainless Steel Identification

Most AISI stainless steels are identified by a system of numbers in either 200, 300, or 400 Series. Special analysis and proprietary stainless steels are identified by trade names, some of which may resemble AISI numbers. There are five primary classifications of stainless steel: Austenitic, Martensitic, Ferritic, Precipitation Hardening, and Duplex. Each identifies its metallurgical structure and reflects different characteristics with respect to corrosion resistance, fabricability and hardenability.

Austenitic Stainless Steels

Austenitic stainless steels are chromium-nickel-manganese and chromium-nickel compositions identified by 200 and 300 Series numbers. They can only be hardened by cold work and are non-magnetic in the annealed condition. Typical of the austenitic group is Type 304, which contains nominally 18% chromium and 8% nickel.

Ferritic Stainless Steels

Ferritic stainless steels are straight grade chromium magnetic steels in the 400 Series that cannot be hardened by heat treatment and only slightly hardened by cold working. Type 430 is typical of this group.

Martensitic Stainless Steels

Martensitic stainless steels are straight grade magnetic chromium 400 Series that can be hardened by heat treatment only. Type 410 is typical of this group.

Precipitation Hardening Stainless Steels

Precipitation Hardening stainless steels are hardened by a combination of a low-temperature aging treatment and cold working. UNS numbers only, e.g. Type S17400, identifies them although many are referred to in literature by proprietary trade names such as 17- 4PH. The precipitation hardening stainless steels are especially useful because fabrication can be completed in an annealed condition and uniform hardening achieved without a high-temperature treatment that may result in distortion and scaling.

Duplex Stainless Steels

Duplex stainless steels are characterized by their high yield strength, which is twice that of the annealed yield strength of typical austenitic stainless steels, like 304 and 316. Usage of both Duplex stainless steel should be limited to temperatures below 570° F as extended elevated temperature exposure can embrittle this material.

Super-Austenitic Stainless Steels

Super-Austenitic stainless materials should be given consideration in those cases where aggressive chloride environments are encountered. They have higher nickel and molybdenum contents for improved pitting and crevice corrosion resistance.

Stainless Steel Characteristics

This guideline can be helpful in narrowing down the choice of materials for any given corrosive environment. The final determination, however, should be based on tests conducted under actual working conditions.

Category	Type	Corrosion Resistance
Austentic	316	Superior
	304	Excellent
Duplex	2205	Superior
Precipitation
Hardened	17-4	Excellent
Ferritic	430	Good
Martensitic	410	Fair

Stainless Steel Chemical Composition Limits Chart

Stainless Steel Corrosion Resistance

Generally alloys with greater than 12% chromium in their composition will not rust. The martensitic grades of material (such as Type 400) offer marginal or lower corrosion resistance than the 300 series stainless steel. Type 410 performs well in mild atmospheres, fresh water, and mine water, steam, carbonic acid, crude oil, gasoline, blood, alcohol, ammonia, mercury, soap, and sugar solutions. It also has good scaling and oxidation resistance up to 1000°F (649C). Type 416 is a free-machining variation of Type 410 and has similar characteristics.

If an application calls for a material with corrosion resistant properties better than that of Type 304, Type 316 is the next logical solution. Type 316 stainless steel is a higher alloyed material containing 2-3% molybdenum, which provides improved pitting and crevice corrosion resistant properties, especially in environments containing chlorides. It has wide application in pulp, paper mills and water treatment plants. It is also widely used in phosphoric and acetic acids that tend to cause pitting corrosion in the 18-8 types. In more aggressive pitting environments, those materials with higher levels of molybdenum should be considered.

Stainless steels do not need any form of protective coating for resistance to corrosion, in contrast with plain steel and some nonferrous fastener materials. While plated or galvanized steel fasteners are adequate where corrosive conditions are not severe, many users consider the extra cost of stainless steel fasteners as insurance against possible failure or diminished visual appeal. When the cost of failure is considered, in conjunction with the ease in which damage can occur to a protective coating, it makes good sense to specify a fastener made of a material that is inherently corrosion resistant. A small discontinuity in a plated surface can lead to corrosive failure. Such discontinuities result from wrench or driver damage, poor plating practices, or simply from the turning action of thread against thread. Furthermore, while the cost of stainless steel fasteners may be more than plated fasteners, the overall cost of the finished product will generally be affected only by an insignificant amount. Plated coatings are sometimes applied to stainless steels for purposes of altering appearance. For instance, the user may want a black or a chrome plated fastener to match the surfaces being joined. Such requirements can be accommodated in stainless steel.

Stainless Steel Strength

Nickel is a primary austenite former element that directly benefits the workability characteristics for these alloys. Carbon and nitrogen directly impact the strength of these alloys. The nitrogen variant of these alloys will offset the loss of mechanical properties of low carbon grades of austenitic stainless steels. Alternatively, the addition of aluminum, titanium and/or columbium to the austenitic stainless steel chemistry can significantly increase the mechanical properties for these materials through heat treatment. Mixed structures (ferrite/austenite), typical of the duplex stainless steels, will also provide higher strength characteristics than those of the fully austenitic grades, while retaining excellent corrosion properties.

Stainless Steel Machinability

The addition of sulfur and selenium to the austenitic grades of material improves the machinability of these alloys. These elements, in combination with chromium and manganese form stringer like inclusions in the structure, which allow better chip removal. For instance, Type 303 has a considerably higher sulfur content that enhances machining characteristics. This property might be very beneficial in the production of large bolts or where small production runs or specials are needed. However, it should be recognized that while the 18-8 stainless steels are considered to have similar corrosion resistance to one another, the sulfide stringers in Type 303 can result in end grain attack at cut ends, especially when exposed to water or some chemical solutions. Accordingly, an engineer or designer should specify Type 304 when it is known that Type 303 is not suitable for the application. Type 416-ferritic stainless steel also has sulfur in its composition in order to provide improved machinability for the martensitic grades of stainless steel.

Stainless Steel Mechanical Requirements Chart

Stainless Steel Appearance

Type 304 and 316 fasteners are used extensively in architectural applications for both their visual appeal and structural and corrosion properties. Some applications do not require the high degree of corrosion resistance offered by 18-8 stainless steels. In these cases, the user can consider types that may be lower in cost. Type 430 stainless steel contains about 18% chromium, but no nickel. Although it has lower corrosion-resistance properties than 18-8, it has wide application for decorative trim because when it is buffed it closely resembles a material that has been chromium plated.

Stainless Steel Mechanical & Physical Properties

ASTM Specifications A193/193M, A194/194M, F593 and F594 F467/467M, F468/468M, F738M, F836M, F837/837M, F879/879M and F880/880M provide the reference base for non-ferrous material selection and specification purposes.

Stainless Steel Tensile & Yield Strength

The most widely associated mechanical property associated with standard threaded fasteners is tensile strength. Tensile strength is the maximum tension-applied load the fastener can support prior to or coinciding with its fracture.

Yield strength is the load that is carried at the point where a fastener permanently deforms. When subjected to enough force, steel will begin to stretch. If the amount of force is low enough, the steel will elastically return to its original shape when the force is removed. At the yield point, the force becomes strong enough that the steel will stretch and not return to its original shape. This amount of force is the yield strength.

Yield strength is the load that is carried at the point where a fastener permanently deforms. When subjected to enough force, steel will begin to stretch. If the amount of force is low enough, the steel will elastically return to its original shape when the force is removed. At the yield point, the force becomes strong enough that the steel will stretch and not return to its original shape. This amount of force is the yield strength.

The ASTM specification distinguishes these finished conditions as: CLASS 1 - Carbide solution annealed CLASS 2 - Carbide solution annealed and strain hardened.

The different conditions are designated by a letter; A - Solution annealed SH - Strain hardened H- Hardened HT - Hardened and tempered AH - Age hardened.

Stainless Steel Strength Values

The charts below show a relative comparison of strength values between stainless steels and other corrosion resistant fastener materials and strength-to-weight ratios. In applications where weight is an important consideration users look to strength-to-weight ratios for an indication of the most efficient material to use. The strength-to-weight ratio is defined as the ratio of tensile strength to density. Of particular interest is the similarity between Type 410 stainless steel and aluminum, and the fact that Type 410 has a higher strength-to-weight ratio than aluminum 2024-T4.

Fastener Tensile Strength Comparison Chart

Stainless Steel Thread Strength

The best quality thread is achieved by thread rolling. The plastic deformation—or cold working— involved in rolling threads results in; (1) more accurate and uniform thread dimensions, giving a better fit between threaded parts and fewer concentrated loads at points of misfit; (2) smoother thread surfaces and fewer scratches to initiate cracks or galling; and (3) higher yield, tensile, and shear properties to better withstand service loads.

Stainless Steel Tensile Strength Areas Chart

Stainless Steel Shear Strength

A pushing or pulling force at 90 degrees from the axis of a part causes shear. Thus, a rivet used as a pulley axle will shear if the load on the pulley exceeds the shear value of the rivet. Shear strength is defined as the load in pounds to cause rupture, divided by the cross sectional area in square inches of the part along the rupture plane. The allowable shear stress for bolts with no threads in the shear plane was taken as 60% of the minimum tensile strength divided by a safety factor of 3.0. This allowable shear stress provides a minimum safety factor of about 1.2 against shear yielding of the bolt material. When threads are included in the shear plane, 70% of the nominal allowable shear stress is used.

Allowable Shear Stress Chart of Stainless Steel Bolts

Stainless Steel Torque

The chart below offers some suggested maximum torque values for stainless steel fasteners. This table is a guide based on industry tests that provide maximum clamping values with minimum risk of seizing. The values are based on fasteners that are dry—free of any lubricant—and wiped clean of chips and foreign matter.

Some production lines are equipped with assembly tools that can be adjusted to specific torque values. Where torque tools are not available, there are guidelines to follow for these circumstances:

1. Tighten the nut finger tight—about one foot-pound of torque or less. 2. Tighten the nut one additional turn, 360 degrees, for proper torque. This is an arbitrary figure that applies primarily to 300 Series fasteners. For hardened and tempered 400 Series fasteners, they may be too high. In any event, a trial test should be conducted with a torque wrench for best results. In service at elevated temperature, the buildup of oxides or scale on fastener surfaces may “fuse” threaded surfaces together. Regular loosening and re-tightening can prevent this from happening. If a lubricant is going to be used, tests should be conducted to determine torque requirements and to evaluate the compatibility of the lubricant to the environment—such as high temperature. Among the popular lubricants are those that contain substantial amounts of molybdenum disulfide, graphite, mica, talc, copper or zinc fines, or zinc oxide. However, the zinc- and copper-bearing anti-seize lubricants are not recommended for use with stainless steel.

Galling & Seizing

Galling and seizing can be encountered with fasteners made of any material including stainless steels. One of the common causes for galling is mismatched threads, or threads that are not uniform from shank or shoulder to point. Fasteners made in accordance with nationally recognized standards, such as those published by the American National Standards Institute, Inc., (ANSI), will assure that nuts and bolts are uniformly threaded. Reasonable care should be exercised in the handling of fasteners to keep threads clean and free of dirt, especially coarse grime and sand. If threads are tightened down on sand, the chance of galling or seizing—in any fastener material—increases significantly.

Selection of Fastener Materials

A common practice in industry is to use fasteners made of metals or alloys that are more corrosion resistant than the materials they join. The austenitic stainless steels, with more than 12% chromium in their chemistry, provide corrosion resistance to a wide variety of environments and especially in low chloride waters. Stainless steel is invariably more noble (cathodic) than the structural members they join and are therefore protected by them. The “stainless” characteristics of these materials make them ideal fasteners for many architectural applications and suitable for atmospheric (indoor and outdoor) services.

Aqueous Corrosion

Corrosion is the wearing away or alteration of a metal either by direct chemical attack or by electrochemical reaction. Corrosion will lead to a weakened or impaired structural system, which will result in downtime, replacement and repairs. Overall corrosion loss, reflected by weight loss, is the most common form of attack. More serious attack can often be seen in the form of localized pitting and pitting attack. This is especially prevalent in chloride and acid chloride types of environments. If the environment cannot be controlled, modified or changed, then materials with higher corrosion resistance may have to be considered.

Galvanic Corrosion

Galvanic corrosion occurs when dissimilar metals are in contact in the presence of an electrolyte, which may be nothing more than a wet industrial atmosphere. When two different metals are in contact with one another, in the presence of a liquid, a battery cell is created, allowing current to flow with corrosion occurring at the anodic component of the cell.

The chart below provides a guide to the relative anodic and cathodic relationships of metals to one another when exposed in seawater, which is known as the galvanic series of metals and alloys. The further apart the combination of alloys, the greater is the corrosion attack at the anodic component. It is well recognized that magnesium, zinc and aluminum anodes, when attached to a steel hull of a ship, will corrode preferentially, thereby protecting the structural integrity of the cathodic components. By comparison, no serious galvanic action will result from the coupling of metals with the same group (stainless to stainless) or to near alloys in the galvanic series (stainless to copper-nickel).

Stainless Steel Physical Properties

The austenitic group of stainless steels has low magnetic permeability when in the annealed condition; a magnet will not attract them. Some of the austenitic materials, however, are weakly attracted by a magnet after severe cold working.

The straight-chromium, 400 Series stainless steels are always magnetic. The degree of magnetic permeability is affected by chemical composition and heat treatment. For highest initial permeability, the carbon content should be kept low; Types 416 and 430 should be fully annealed for the best magnetic behavior. During annealing, a dry hydrogen atmosphere should be used to keep surfaces bright and free of contamination, such as carbon or nitrogen, which can decrease permeability. Chemical cleaning, which removes iron particles from the surface, may also improve permeability.

Stainless Steel Fastener Markings

Stainless Steel Fastener Markings Chart 1

Stainless Steel Fastener Markings Chart 2

Stainless Steel Fastener Markings Chart 3

Stainless Steel Fastener Markings Chart 4

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