Fasteners are manufactured in a wide range of materials from common steel to titanium, and other exotic materials. Selecting a fastener material is important when choosing a fastener due to differences between materials in strength, corrosion resistance, cost, etc. In this article information is given about stainless steels, duplex stainless steels and other non ferrous materials to help in selecting appropriate material. When replacing fasteners, it is generally best to match what you are replacing. The information will be helpful in deciding change of material in case of failure of existing fastener due to design deficiency.
Stainless steels
All stainless steels are iron-based alloys that contain a minimum of around 10.5% chromium. The chromium in the alloy forms a self-healing protective clear oxide layer. This oxide layer gives stainless steels their corrosion resistance. The self healing nature of the oxide layer means the corrosion resistance remains intact regardless of fabrication methods. Even if the material surface is cut or damaged, it will self heal and corrosion resistance will be maintained as long as there is oxygen in the environment.
The corrosion resistance of different grades of stainless steel is different in different environments. Even a small amount of some elements in an environment can significantly alter the corrosion resistance. Chlorides in particular have an adverse effect on the corrosion resistance of stainless steels.
Although the corrosion resistance of stainless steel comes from the presence of chromium, other elements are added to enhance other properties. These elements alter the microstructure of the steel. Stainless steels are grouped based on their metallurgical microstructure. Stainless steels can be divided into three basic groups based on their crystalline structure – austenitic, ferritic, and martensitic. Duplex stainless steels have mixture of austenitic and ferritic structure.
Brief information on ferritic, martensite, austenitic and duplex stainless steels is as under.
Ferritic steels
They are moderate corrosion resistance magnetic steels. They can not be hardened by heat treatment. They have high resistance to stress corrosion cracking. They have better decorative appeal. Ferritic stainless steels include grades like 410 containing only chromium as a major alloying element (no nickel). The quantity of chromium present ranges from 10.5 to 18%. They are called F1 type stainless steels.
Martensitic steels
High carbon and lower chromium content are the distinguishing features of martensitic stainless steels when compared with ferritic stainless. They also usually don’t contain nickel. They are magnetic steels and can be heat treated to excellent strength. Martensitic steels are not as corrosion resistant as austenitic or ferritic grades, but their hardness levels are among the highest of all the stainless steels.
C1 type steels: e.g. Grade 420 (1.4021, 1.4028)
They are used in pumps, turbines, etc.
C3 type steels: e.g. Grade 431 (1.4057)
They have better resistance to corrosion than C1 type steels.
C4 type steels: e.g. Grade 430F (1.4104)
They are intended for machining, otherwise they are similar to grade C1.
Austenitic steels
Austenitic stainless steels are non-magnetic and non heat-treatable steels. They are usually annealed and cold worked. After cold working some magnetic properties may be evident particularly for A2 type steels. They have excellent corrosion and heat resistance with good mechanical properties over a wide range of temperatures. When there is a risk of inter-granular corrosion, steel grades A3 and A5 (stabilized steels) or A2L and A4L (low carbon steel – carbon content not exceeding 0.03%) are recommended.
A1 type steels: e.g. Grade 303 (1.4305)
Grade 303 is the most readily machineable of all the austenitic grades of stainless steel. The machineable nature of grade 303 is due to the presence of sulphur in the steel composition. While the sulphur improves machining, it also causes a decrease in the corrosion resistance and a slight lowering of the toughness. The corrosion resistance of grade 303 is lower than that for grade 304. Grade 303 stainless steel is not suitable for use in marine environments.
A2 type steels: e.g. Grades 304 (1.4301), 304L (1.4306) (=A2L)
They are most frequently used chromium-nickel steels. They are suitable for outdoor applications against normal corrosion and only to a limited extent in oxidizing acids. Steels of this type are not suitable for environment with chloride content, i.e. swimming pools, sea water and coastal areas. They have good weldability.
Grade 304 is the most versatile and widely used stainless steel. It is still sometimes referred to by its old name 18/8 which is derived from the nominal composition of 304, 18% chromium and 8% nickel.
A3 type steels: e.g. Grades 321 (1.4541) and 347 (1.4550)
Stabilized stainless steels with properties similar to A2.
A4 type steels: e.g. Grades 316 (1.4401, 1.4436), 316L (1.4435) (=A4L)
They are chromium-nickel steels which are molybdenum alloyed and give a considerably better resistance to corrosion than A1, A2 and A3 types. They are known as acid proof steels. This steel grade was developed for boiling sulfuric acid (thus given the name “acid proofâ€) and is to a certain extent also suitable in an environment with chloride content. A4 steel is frequently used by the marine industry. But they are not resistant to warm sea water. Warm chloride environments can cause pitting and crevice corrosion. Grade 316 is also subject to stress corrosion cracking above around 60°C. The austenitic structure of grade 316 stainless steel gives them excellent toughness, even at cryogenic temperatures. They have good weldability.
Grade 316L, the low carbon version of 316 stainless steel, is immune to grain boundary carbide precipitation (sensitization). They can be used for seawater.
A5 type steels: e.g. Grade 316Ti (1.4571)
They have excellent resistance to corrosion. They are stabilized “acid proof steels†with properties similar to A4. Grade 316Ti contains a small amount of titanium (around 0.5 %.) The titanium atoms stabilize the structure of the 316 at temperatures over 800°C. This prevents carbide precipitation at the grain boundaries and protects the metal from corrosion.
Duplex stainless steels
Duplex stainless steel is used for corrosion resistance applications. Duplex alloys were originally created to overcome corrosion problems caused by chloride bearing cooling waters and other aggressive chemical process fluids. They are called duplex because of their mixed microstructure with about equal proportions of ferrite and austenite steel. Duplex stainless steels are a family of grades, which range in corrosion performance depending on their alloy content. The term "Super-Duplex" was first used in the 1980′s to denote highly alloyed, high-performance duplex steel with a pitting resistance. Duplex stainless steel is better than the 300 series stainless steel (Austenitic Steels) for nearly every type of corrosion. Its yield strength is double that of the 300 series stainless steel.
Use of Duplex stainless steel has following benefits.
- High strength.
- High resistance to pitting and crevice corrosion.
- High resistance to stress corrosion cracking, corrosion fatigue and erosion.
- Good sulfide stress corrosion resistance.
- Low thermal expansion and higher heat conductivity than austenitic steels.
- Good workability and weldability.
- High energy absorption.
Today, modern duplex stainless steel can be divided into four groups as under:
- Lean Duplex such as 2304, which contains no deliberate Mo addition.
- 2205, the work-horse grade accounting for more than 80% of duplex usage.
- 25 Cr Duplex such as Alloy 255 and DP-3.
- Super Duplex; with 25-26 Cr and increased Mo and N compared with 25 Cr grades, including grades such as 2507, Zeron 100, UR 52N+, and DP-3W.
Composition of various grades of Duplex Stainless Steelsa is as under:
The table lists the duplex stainless steels covered in ASTM specifications for plate, sheet, and bar products.
| UNS Number Duplex Grades | Typeb | C | Mn | P | S | Si | Cr | Ni | Mo | N | Cu | Other |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| S31200 | … | 0.030 | 2.00 | 0.045 | 0.030 | 1.00 | 24.0-26.0 | 5.5-6.5 | 1.20-2.00 | 0.14-0.20 | … | … |
| S31260 | … | 0.03 | 1.00 | 0.030 | 0.030 | 0.75 | 24.0-26.0 | 5.5-7.5 | 2.5-3.5 | 0.10-0.20 | 0.20-0.80 | W0.10-0.20 |
| S31803 | … | 0.030 | 2.00 | 0.030 | 0.020 | 1.00 | 21.0-23.0 | 4.5-6.5 | 2.5-3.5 | 0.08-0.20 | … | … |
| S32001 | … | 0.030 | 4.0-6.0 | 0.040 | 0.030 | 1.00 | 22.0-23.0 | 1.00-3.00 | 0.60 | 0.05-0.17 | 1.00 | … |
| S32205 | 2205 | 0.030 | 2.00 | 0.030 | 0.020 | 1.00 | 19.5-21.5 | 4.5-6.5 | 3.0-3.5 | 0.14-0.20 | … | … |
| S32304 | 2304 | 0.030 | 2.50 | 0.040 | 0.030 | 1.00 | 21.5-24.5 | 3.0-5.5 | 0.05-0.60 | 0.05-0.20 | 0.05-0.60 | … |
| S32520 | … | 0.030 | 1.50 | 0.035 | 0.020 | 0.80 | 24.0-26.0 | 5.5-8.0 | 3.0-4.0 | 0.20-0.35 | 0.50-2.00 | … |
| S32550 | 255 | 0.04 | 1.50 | 0.040 | 0.030 | 1.00 | 24.0-27.0 | 4.5-6.5 | 2.9-3.9 | 0.10-0.25 | 1.5-2.5 | … |
| S32750 | 2507 | 0.030 | 1.20 | 0.035 | 0.020 | 0.80 | 24.0-26.0 | 6.0-8.0 | 3.0-5.0 | 0.24-0.32 | 0.50 | … |
| S32760 | … | 0.030 | 1.00 | 0.030 | 0.010 | 1.00 | 24.0-26.0 | 6.0-8.0 | 3.0-4.0 | 0.20-0.30 | 0.50-1.00 | c |
| S32900 | 329 d | 0.06 | 1.00 | 0.040 | 0.030 | 0.75 | 23.0-28.0 | 2.5-5.0 | 1.0-2.0 | … | … | … |
| S32950 | … | 0.03 | 2.00 | 0.035 | … | … | … | … | … | … | … | … |
a – Weight percent, maximum unless otherwise noted.
b – Unless otherwise indicated, a common name, not a trademark, widely used, not associated
    with any one producer, as listed in ASTM A 240.
c – W 0.50-1.00; Cr+3.3Mo+16N=40 min.
d – AISI designation.
The information on Duplex Stainless Steel is reproduced from internet site of Ocean International Supplies, Inc. For more information please refer to their website – http://www.oceanint.com
Non ferrous Alloys
Some commonly used alloys are as under.
Inconel
They are high nickel and high chromium content alloys. They are resistance to oxidizing and reducing environments. They have good corrosion resistance at elevated temperatures (up to 2150°F). Commonly used types are alloy 600, 625 and 718.
Monel
Monel is a high tensile strength nickel-copper alloy. It is highly resistant to atmospheric corrosion, salt water, various acid and alkaline solutions. Alloy 400 can be magnetic. Alloy K-500 has added aluminum and titanium for age hardening. Alloy K-500 is totally nonmagnetic and spark resistant. Alloy 400 and K-500 are used for marine engineering, chemical and hydrocarbon processing equipments.
Hastelloy C-276
Contain approximately 60% nickel, 16% molybdenum, 15% chromium. They resist corrosion in acidic environments and have high strength at high temperatures. Typical applications include equipment components in chemical and petrochemical, pulp and paper, flue gas desulfurization (FGD) scrubbers, pharmaceutical and food processing. B-2, C-4, C-22 and G-30 are other commonly used hastelloy grades.
Titanium
They have very high strength-to-weight ratio. They are 43 % lighter than stainless steels. They have good corrosion and fatigue resistance. They are used as aircraft and aerospace material and for sea water application.
Brass
Contain 65% copper and 35% zinc. They have good strength, corrosion resistance and workability.
Note:
Monel and Inconel are registered trademarks of Inco Alloys International.
Hastelloy is registered trademark of Haynes International.
