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This Fluid Sealing Association Knowledge Series training presentation introduces the variety of metals used in mechanical seal construction.
▪ Atomic Structure of Metals
▪ Solidification and Microstructure
▪ Cast and Wrought Metals
▪ Alloy Designation Systems
▪ Stainless Steel
• Stainless Steels for Mechanical Seals
• Austenitic Stainless Steel
• Specialty Stainless Steel
• Duplex Stainless Steel
• Martensitic Stainless Steel
• Precipitation Hardenable Stainless Steel
• Superaustenitic Nickel Alloys
• Nickel Based Super Alloys
▪ Metallic Bushing Materials Copper Alloys
▪ Specific parts, Bellows, Springs, Screws
Atomic Structure of Metals
▪ The majority of common metals have either a Face Center Cubic structure or a Body Centered Cubic structure, although some have a Hexagonal Close Packed structure. The major differences between these structures is the arrangement of the atoms that form a "building block". The different building blocks lead to different physical properties of metals.
Atomic Structure of Metals
A Face Center Cubic Structure consists of an atom at each cube corner and an atom in the center of each cube face. A hard sphere concept can be used to describe atomic packing in each building block. These atoms are closed packed, i.e. they cannot be packed any tighter, and each atom touches its neighbor in any direction.
Atomic Structure of Metals
A Body Center Cubic Structure consists of atoms at each cube corner and one atom is at the center of the cube. Compared to a Face Center Cubic structure, this structure has more space between the atoms.
Atomic Structure of Metals
A Hexagonal Close Packed Structure is similar to a Face Centered Cubic structure in that they atoms are packed close together, however the arrangements of the atoms differs slightly resulting in the structure being less ductile.
Atomic Structure of Metals
The building blocks affect the metals physical properties. For example, bending a metal requires the planes of atoms formed by a collection of many of these building blocks to slide past one another. The different structures require different amounts of energy (force) to move therefore affecting how malleable or ductile the metal is.
Atomic Structure of Metals
Examples of metals with different structures:
▪ Face Center Cubic
Austenitic stainless steels
300 series – e.g. AISI 302, AISI 316
Non-magnetic
▪ Body Center Cubic
Ferritic stainless steels
Carbon steel and some 400 series stainless steels
Magnetic
▪ Hexagonal Close Packed
Martensitic Stainless Steels
Solidification and Microstructure
Metals and metal alloys are created by mixing and melting the ingredients together.
The molten mixture is then poured into a mold and allowed to cool. As the metal cools, solidifications starts occurring with the atoms arranging themselves into the basic building blocks (body or face centered cubic structures) which grow and get larger forming crystals (or grains). These grains keep growing until their growth is blocked by another grain and a boundary is formed.
Solidification and Microstructure
1. Numerous individual crystals start solidifying
3. Irregular grains form as crystals grow together
2. Crystals grow as metal cools
4. Solidified metal consisting of numerous grains and grain boundaries
Solidification and Microstructure
The rate of cooling will affect the size and shape of the grains that are formed. Larger grains have fewer grain boundaries which can affect the resulting physical properties of the metal.
Decreasing cooling rate
Grain structure visible in a sectioned and etched ingot
Cast and Wrought Metals
Casting
Produced by pouring molten metal in a mold to produce a desired object at or near finished shape.
Wrought
Produce a cast ingot, then mechanically work the ingot to produce sheet, plate, extruded bar, tube, etc.
This results in a smaller, more directional grain structure and improved physical properties.
Note: All wrought alloys can be cast but not all cast alloys can be produced as wrought
Cast and Wrought Metals
Comparison of Wrought and Cast Alloys
Wrought alloys are initially cast as ingots and then hot/cold worked mechanically into the desired form by rolling , extrusion, forging. The resulting difference in strength is due to the cold/hot working of wrought alloys to obtain the desired shapes since it creates a dislocation density that reduces slip of atomic planes in the grain structure.
Wrought Alloys
Cast Alloys
Grain structure: Fine Coarse
Surface Finish: Excellent Poor to good
Strength: Excellent Good
Cast and Wrought Metals
Comparison of Wrought and Cast Alloys
Alloy Designation Systems
The Unified Numbering System
The Unified Numbering System (UNS) is an alloy designation system widely accepted in North America. It consists of a prefix letter and five digits designating a material composition.
The UNS is managed jointly by the American Society for Testing and Materials (ASTM) and the Society of Automotive Engineers (SAE).
A UNS number alone does not constitute a full material specification because it establishes no requirements for material properties, heat treatment, form, or quality.
Alloy Designation Systems
Alloy Casting Institute (ACI)
Managed by the Steel Founders’ Society of America, the ACI alloy designation system uses a series of letters and numbers to describe the composition of a cast alloy.
Example: CF8M (Cast 316 stainless steel)
C Cast corrosion resistant alloy
F 19-23% Cr and 9-12% Ni
8 0.08% C maximum M Alloy contains molybdenum
Alloy Designation Systems
American Society for Testing and Materials (ASTM)
ASTM’s designation system for metals consists of a letter (A for ferrous materials) followed by an arbitrary sequentially assigned number. These designations often apply to specific products. An ASTM standards created using rationalized SI units have a suffix letter M.
Example - ASTM A 582/A 582M-95b (2000)
A - Describes a ferrous metal, but does not sub classify it as cast iron, carbon steel, alloy steel, tool steel, or stainless steel
582 - Is a sequential number without any relationship to the metal’s properties
M - Indicates that the standard is written in rationalized SI units, hence together 582/A582M includes both US customary and SI units
95 - Indicates the year of adoption or last revision and a letter b following the year indicates the third revision of the standard in 1995 (2000) - Indicates the year of last re-approval
Alloy Designation Systems
American Iron and Steel Institute (AISI)
AISI designation system for stainless steels consists of a 3 digit number which can be followed by a modifying letter indicating alloy variations of the original alloy.
Example: AISI 316L
Austenitic - 2
Austenitic - 3
Martensitic and Ferritic - 4
Sequential number without any relationship to the metal’s properties Alloy Modifier
Example of Alternate Alloy Designations For 316SS
Standards Organization Designation
Standards Organization Designation
EN steel number 1.4401 1.4436
DIN
EN steel name
X5CrNiMo17-12-2
X5CrNiMo18-14-3
SAE grade 316
UNS S31600
BS 970
X5CrNiMo17 12 2
X5CrNiMo17 13 3
X5CrNiMo 19 11
X5CrNiMo 18 11
316S 29
316S 31
316S 33
En58J UNI
X5CrNiMo17 12
X5CrNiMo17 13
X8CrNiMo17 13
JIS SUS 316 SUS316TP
Alloy Designation Systems
Trade Names
Trade names are often used to identify propriety alloys produced by a manufacturer.
In some instances, these alloy are also available from other manufacturers under a generic name (e.g. Alloy 20, Alloy C-276,…)
Stainless Steel
What is a Stainless steel?
Definition: Iron based alloy with >12% Chromium
Classes
Examples
Austenitic 302, 304, 316, 317, Alloy 20
Ferritic Generally not used for Mechanical Seals
Duplex CD4MCuN, 2205
Martensitic 410, 416, 440, CA6NM
Precipitation Hardenable 17-4 PH, AM 350
Why are stainless steels corrosion resistant?
Corrosion resistance is obtained by a passive oxide layer on metal surface (Chrome oxide). Alloying elements can improve physical properties and corrosion resistance:
Stainless Steel
Stainless Steel Alloying Elements and Their Purpose
Chromium Oxidation Resistance
Nickel Austenite former - Increases resistance to mineral acids. Produces tightly adhering high temperature oxides
Molybdenum Increases resistance to chlorides
Copper Provides resistance to sulfuric acid. Precipitation hardener together with titanium and aluminum
Manganese Austenite former - Combines with sulfur. Increases the solubility of nitrogen
Sulfur Austenite former - Improves resistance to chlorides. Improves weldability of certain austenitic stainless steels. Improves the machinability of certain austenitic stainless steels
Titanium Stabilizes carbides to prevent formation of chromium carbide. Precipitation hardener
General resistance superior to 316, 317 and duplexes
Resistant to Sulfuric acid, organic acids, hydroxides, non-halogenated organics
Pitting and crevice corrosion resistance
Inferior to super duplex stainless steels
Specialty Stainless Steel
Alloy 20 Specifications
UNS 08020
ASTM B463, B473
*maximum unless otherwise indicated
Duplex Stainless Steel
Duplex Stainless Steels
Microstructure
40-60% ferrite, balance austenite
Corrosion resistance
Between 317 and Alloy 20
Good pitting and crevice resistance
Improved resistance to Acid-chloride, seawater
Ferrite improves resistance to acid-chloride stress corrosion cracking
Mechanical properties
About 2x stronger than AISI 300 series (austenitic) stainless steel
Increased hardness over austenitic stainless steel
Non-hardenable by heat treatment, easy to weld
Duplex Stainless Steel
Duplex Stainless Steels
*maximum unless otherwise indicated
Martensitic Stainless Steel
Martensitic Stainless Steel
Microstructure
Martensite, Strongly Magnetic
Heat treatable to high hardness levels
Hard to impossible to weld
As quenched structure is unstable (highly stressed)
Requires a tempering heat treatment to relieve the stresses
Corrosion resistance
Generally poor compared to other stainless steels.
Equivalent to AISI 304
Martensitic Stainless Steel
Martensitic Stainless Steel Composition in
1. Sulfur 0.15
Precipitation Hardenable Stainless Steel
Precipitation Hardenable Stainless Steels
Microstructure
Typically martensitic, but some special grades are austenitic
Magnetic
Relatively soft and ductile in the solution-annealed state
Extremely high strength after precipitation heat treatment
Corrosion resistance
Generally poor compared to other stainless steels.
Equivalent to AISI 304
Mechanical properties
The strengthening mechanism comes from the formation of submicroscopic precipitates, which are compounds of aluminum, copper, titanium, or molybdenum. These precipitates provide resistance to strain exerted on the structure
Precipitation Hardenable Stainless Steel
Precipitation Hardenable Stainless Steels
Composition in
*maximum unless otherwise indicated
1. Mo 2.8%, N 0.10%
Superaustenitic Nickel Alloys
Superaustenitic Nickel Alloys
Microstructure
Austenitic
Non-magnetic
Work hardenable, non-hardenable by heat treatment, easy to weld
Corrosion resistance
Excellent corrosion resistance to oxidizing environments, pitting and crevice corrosion.
Superaustenitic Nickel Alloys
Superaustenitic Nickel Alloys
unless otherwise indicated
Nickel Based Super Alloys
Nickel Based Super Alloys
Microstructure
Austenitic
Non-magnetic
Precipitation hardenable, Excellent weldability
Exceptionally high yield, tensile and creep-rupture properties at elevated temperatures
Corrosion resistance
Excellent corrosion resistance to many inorganic and organic, other than strongly oxidizing, compounds throughout wide ranges of acidity and alkalinity. Good pitting and stress-corrosion cracking resistance.
Nickel Based Super Alloys
Nickel Based Super Alloys
*maximum unless otherwise indicated
1. Mn 0.35%, Si 0.35%, P 0.015%, S 0.015%, B 0.006%, Cu 0.3%, Fe bal
Nickel Super Alloys: Alloy 718
▪ Specifications
▪ Cast –
19 Cr, 3 Mo, 53 Ni, Bal Fe
▪ Wrought – ASTM B637, B670, NACE MR-01-75
▪ UNS N07718
▪ Corrosion resistance
▪ Pitting and crevice resistance better than duplexes
▪ Excellent weldability
▪ No post weld heat treatment required
▪ Good high temperature oxidation resistance
Metallic Bushing Materials: Copper Alloys
Four Primary Types
▪ Coppers – essentially pure
▪ Cupronickels – copper alloyed with nickel
▪ Brasses – copper alloyed with zinc
▪ Bronzes – copper normally alloyed with tin
* May also be alloyed with Al, Al and Ni, P and Si
Metallic Bushing Materials
One of the functions of a bushing in a mechanical seal is to prevent contact between rotating parts of the seal with stationary parts of the seal which may create damage and the possibility of sparks.
Copper alloys that contain lead are the most popular bushing materials as the lead content helps with lubricity and reduces the tendency to gall when contact with a rotating part is made. Common materials include:
• Leaded Tin Bronze
• Leaded Red Brass
Leaded Tin Bronze & Leaded Red Brass
Leaded Tin Bronze
Leaded Red Brass
Composition in weight %*
Function Specific Materials
Metal bellows material
▪ AM 350
▪ Good physical properties
▪ Moderate corrosion resistance
▪ 316 SS
▪ Good corrosion resistance
▪ Moderate physical properties
▪ Alloy 20
▪ Can be work hardened
▪ Broad corrosion and chemical resistance
▪ Alloy C276
▪ Excellent chemical resistance
▪ Alloy 718
▪ High temperature strength
▪ Alloy 42 – End fittings for high temperature service
▪ Low coefficient of thermal expansion
Function Specific Materials
Springs Material
▪ 316
▪ Used in single coil spring designs
▪ Alloy C276
▪ Resistance to chloride stress corrosion
▪ ElgiloyTM
▪ Cobalt, Chromium Nickel Alloy
▪ High strength
Function Specific Materials
Set screws
▪ Hardened steel
▪ Coated/uncoated
▪ Heat treatment
▪ Stainless steel (300, 400 series)
▪ Specialty (Monel/Alloy C276/Alloy 20)
▪ Duplex Stainless
▪ Forged Screws with rolled threads will exhibit better physical properties than machined ones
