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What sets stainless steel apart from normal steel is the inclusion of chromium in its alloys. All stainless steel chemical compositions contain at least 10.5% chromium. The inclusion of chromium makes these steels more corrosion resistant. The different grades of this material have various alloying elements that serve to further improve corrosion resistance, heat treatability, and machinability. It should be noted that heat treatment can significantly affect the metal’s mechanical properties.
Stainless steels can be classified based on their crystalline structure. This includes Austenitic, ferritic, martensitic, and duplex:
Stainless Steel 15-5 is a precipitation-hardened (PH) metal. The process gives it excellent toughness, strength, and corrosion resistance. Mechanical properties are enhanced by low-temperature heat treatment, making this material ideal for aerospace and nuclear applications.
Stainless Steel 15-5 Properties
| Tensile Strength, Yield (MPa) | Fatigue Strength (MPa) | Elongation at Break (%) | Hardness (Brinell) | Density (g/cm^3) |
|---|---|---|---|---|
| 1280 | 78 | 11 | 389 | 7.82 |
This precipitation-hardened (PH) steel grade has better corrosion-resistant properties at high temperatures when compared to 15-5 stainless steel. It achieves this increased corrosion resistance by sacrificing mechanical strength. This is also one of the more widely used grades of PH stainless steel. Applications include chemical processing parts and gas turbines.
Stainless Steel 17-4 Properties
| Tensile Strength, Yield (MPa) | Fatigue Strength (MPa) | Elongation at Break (%) | Hardness (Brinell) | Density (g/cm^3) |
|---|---|---|---|---|
| 1378 | 77.4 | 7 | 418 | 7.82 |
This grade of stainless steel has an austenitic crystal structure and is one of the most widely used grades. 18-8 is often referred to as 304 stainless steel or SS304, and FlagShip quotes 18-8 as SS304, but the two have slight differences in some alloying elements. 18-8 has good corrosion resistance characteristics and is regularly used to create fasteners and pressure piping.
Stainless Steel 18-8 Properties
| Tensile Strength, Yield (MPa) | Fatigue Strength (MPa) | Elongation at Break (%) | Hardness (Brinell) | Density (g/cm^3) |
|---|---|---|---|---|
| 214 | 78 | 69 | 122 | 8.1 |
This grade of austenitic stainless steel was formulated to be easier to machine than SS304 by including sulfur among the alloying elements. This addition, however, makes the alloy less corrosion resistant than SS304. It is ideal for items that require heavy machining like gears and shafts.
Stainless Steel 303 Properties
| Tensile Strength, Yield (MPa) | Fatigue Strength (MPa) | Elongation at Break (%) | Hardness (Brinell) | Density (g/cm^3) |
|---|---|---|---|---|
| 240 | 77.5 | 51 | 161 | 8.1 |
This grade of austenitic stainless steel has good corrosion resistance properties and is used extensively for fasteners. It is often seen as a low-cost alternative to SS316 while not having the same corrosion resistance. This alloy is very similar to grade 18-8 stainless steel as it has the same amount of chrome and nickel, however, it has improved strength due to a higher level of carbon in the alloy.
Stainless Steel 304 Properties
| Tensile Strength, Yield (MPa) | Fatigue Strength (MPa) | Elongation at Break (%) | Hardness (Brinell) | Density (g/cm^3) |
|---|---|---|---|---|
| 216 | 78 | 69 | <122/td> | 8.2 |
This austenitic grade of stainless steel contains molybdenum which gives it excellent corrosion resistance. In addition to this, it is highly formable and weldable. Applications include chemical tanks and boat fittings. The low carbon version, 316L, is more resistant to chlorides than the basic formulation.
Stainless Steel 316 Properties
| Tensile Strength, Yield (MPa) | Fatigue Strength (MPa) | Elongation at Break (%) | Hardness (Brinell) | Density (g/cm^3) |
|---|---|---|---|---|
| 206 | 75 | 40 | 188 | 8.06 |
Stainless steel 440C has the highest carbon content of the 400 series. This means that 440C has only mild corrosion resistance. However, it has excellent hardness characteristics (which can be further increased with heat treatment) and mechanical strength. Typical applications include bearing housings and surgical instruments.
Stainless Steel 440C Properties
| Tensile Strength, Yield (MPa) | Fatigue Strength (MPa) | Elongation at Break (%) | Hardness (Brinell) | Density (g/cm^3) |
|---|---|---|---|---|
| 446 | 83.5 | 14.2 | 226 | 7.88 |
Stainless steel 410 is the most general-purpose steel among the 400 series. It has a low carbon content that gives it improved corrosion resistance. Like other martensitic steels, 410 can be hardened to achieve impressive mechanical strength. 410 stainless steel is typically used for cutlery, fasteners, and machinery parts.
Stainless Steel 410 Properties
| Tensile Strength, Yield (MPa) | Fatigue Strength (MPa) | Elongation at Break (%) | Hardness (Brinell) | Density (g/cm^3) |
|---|---|---|---|---|
| 311 | 75 | <24.8/td> | 148 | 7.78 |
As machined (standard): This standard finish leaves the part in its original state after machining. FlagShip can achieve 125 Ra or better. Finishing passes can be performed to further refine the surface roughness. This is the most common surface finish for stainless steel.
Powder coating: Powder coating creates an excellent and robust surface finish. Stainless steel is corrosion resistant to a wide array of chemicals, but some, like chlorides, still pose a challenge. Powder coating can be used to improve its anti-corrosion properties in these situations.
Bead blasting: This process creates a smooth and uniform matte finish by lightly blasting the surface. This surface finish is generally applied for cosmetic purposes.
