Stainless Steel
Stainless Steel Types
Stainless steels are iron based alloys containing 10% or more chromium. Chromium provides the corrosion resistant properties by the creation of a passive chromium oxide layer. Stainless steels are broken down into the following categories.
Austenitic Stainless Steel - ASME P-Number 8
Duplex Stainless Steel - ASME P-Number 10H
Ferritic Stainless Steel - ASME P-Number 7
Martensitic Stainless Steel - ASME P-Number 6
Precipitation Hardening (PH) Stainless Steel
| Type | ASME P-Number | Grain Structure | Chromium | Nickel |
|---|---|---|---|---|
| Austenitic | P-No. 8 | FCC - Face Centered Cubic | 16% to 26% | 10% to 22% |
| Duplex | P-No. 10H | 50% FCC & 50% BCC | 20% to 30% | 3% to 8% |
| Ferritic | P-No. 7 | BCC - Body Centered Cubic | 10% to 30% | 0.5% to 1.0% |
| Martensitic | P-No. 6 | BCC - Body Centered Cubic | > 12% | 0% - 1.0% |
Austenitic Stainless (P-No. 8)
Austenitic stainless steels are a group of alloys known for their excellent corrosion resistance and oxidation resistance. Austenitic stainless steel derives its name from Austenite which is a non-magnetic phase with a face-centered cubic (FCC) grain structure.
Some austenitic stainless steels such as 316 contain small amounts of ferrite which improve their corrosion characteristics.
| ASTM Type or Grade | Composition (wt%) | Microstructure |
|---|---|---|
| 304 | 18-20% Cr, 8-11% Ni | Austenite |
| 316 | 16-18% Cr, 11-14% Ni | Austenite + 3-10% Ferrite |
| 310 | 24-26% Cr, 19-22% Ni | 100% Austenite |
| 321 | 17-19% Cr, 9-12% Ni | Austenite + 4-12% Ferrite |
Austenitic Stainless Steel Base Metal Examples
SA-213 TP304, SA-213 TP347, SA-240 316, SA-351 CF8M, SA-965 F321
Austenitic Stainless Steel Filler Metal Examples
SFA 5.9 ER410, ER410NiMo
Duplex Stainless (P-No. 10H)
Duplex is approximately equal amounts of Austenite and Ferrite. Duplex stainless steel provides the corrosion resistance of austenitic stainless steels and the strength of ferritic stainless steels. Duplex stainless steels will outperform 304 and 316 stainless grades against chloride stress corrosion and chloride pitting corrosion.
Duplex stainless steel is categorized as an ASME P-Number 10H
Duplex is categorized into the following groups
Lean Duplex (PREN < 30)
S32101, S32202, S32304, S32003, S82011, and S82441
Standard Duplex (PREN 30-40)
S31803, S32205, J92205, and J93372
Super Duplex (PREN 40-48)
S32520, S32550, S32750, S32760, S32906, J93380, and J93404
Hyper Duplex (PREN 48-55)
S32707, and S33207
Duplex Stainless Steel Filler Metal Examples
SFA 5.9 ER2209, ER2553, ER2594
PREN - Pitting Resistance Equivalent Number
PREN provides a measurement scale for determining a materials relative ability to resist pitting corrosion. PREN cannot predict if a specific grade of material will be suitable for a given application, but it helps to rank and compare various materials regarding their relative measure of pitting corrosion resistance.
Pitting corrosion occurs on metals with protective films or oxides, such as stainless steel with a passive chromium oxide layer. Pitting corrosion involves small localized attacks that break through the oxide layer creating pitting in the surface layer of the metal. Common causes of pitting corrosion include exposure to chlorides, bromides, fluorides, iodides, and sulfides.
PREN Calculation
Formula 1 (duplex alloys without tungsten)
PREN 1 = %Cr + (3.3 X %Mo) + (16 X %N)
Formula 2 (adjusts for tungsten in super duplex)
PREN 2 = %Cr + 3.3 X (%Mo + 0.5%W) + 16 X %N
Ferritic Stainless (P-No. 7)
Ferritic stainless steel is known for its high chromium content, typically ranging from 10.5% to 30%. Ferritic stainless steels are sometimes considered a straight chromium alloy.
Ferritic stainless steel offers remarkable resistance to corrosion in oxidizing environments. Ferritic stainless steel is resistant to Stress Corrosion Cracking (SCC), which makes it suitable for applications where high chloride exposure is a concern.
Ferritic stainless steel has a body-centered cubic (BCC) grain structure, which gives it magnetic properties.
Ferritic stainless steel is categorized as an ASME P-Number 7
Ferritic Stainless Steel Base Metal Examples
SA-182 F430, SA-240 405, SA-240 409, SA-240 410S, SA-268 TP405, SA-268 TP409
Ferritic Stainless Steel Filler Metal Examples
SFA 5.9 ER409, ER409Nb, ER430
Martensitic Stainless (P-No. 6)
Martensitic stainless steels are named after their distinctive crystal structure known as martensite. Martensitic stainless steels contain at least 12% chromium, as well as nickel, molybdenum, and carbon. The carbon content varies, ranging from 0.10% up to 1.2%.
Martensitic stainless steel can be hardened through aging and heat treatment, and is magnetic due to its body-centered cubic (BCC) grain structure.
Applications for martensitic stainless often include pumps, valves and shafts in the 0.4% carbon range. Above 0.4% martensitic stainless has improved wear resistance and can be used in cutlery, surgical blades, plastic injection molds, and nozzles.
Martensitic Stainless Steel Base Metal Examples
SA-182 F429, SA-182 F6a Cl. 2, SA-240 410, SA-268 TP410, SA-479 403 & 410, SA-731 S41500
Martensitic Stainless Steel Filler Metal Examples
SFA 5.9 ER410, ER410NiMo
Precipitation Hardening Stainless
PH stainless steels are Fe-Cr-Ni alloys with additional elements like copper, molybdenum, niobium, titanium, and aluminum.
Precipitation hardening steels (PH) are a group of corrosion resistant alloys that derive their characteristic advantage through a heat treatment precipitation hardening process.
PH steels undergo ageing heat treatment in the range of 900°F - 1150°F (480°C - 620°C).
PH steels achieve impressive strength levels, up to 120 ksi to 246 ksi in some cases, surpassing martensitic stainless steels, while maintaining corrosion resistance similar to Type 304.
Applications include aerospace and marine components, fuel tanks, landing gear covers, pump parts, shafts, bolts, saws, knives, and flexible bellows-type joints.