Chapter 16 – Stainless Steels

Gateway Arch in St Louis – 304 series SS F-35 Joint Strike Fighter (JSF) Lightning II, built by Lockheed Martin – airframe 17-7 PH – 600 series SS

Key Points: • Alloy steels containing at least 10% Cr are SS.

• Contain sufficient amount of Cr that they are considered high alloy.

• Corrosion resistance is imparted by the formation of a passivation layer characterized by:

– Insoluble chromium oxide film on the surface of the metal - (Cr 2 O 3 ) . – Develops when exposed to oxygen and impervious to water and air.

– Layer is too thin to be visible – Quickly reforms when damaged – Susceptible to sensitization, pitting, crevice corrosion and acidic environments.

– Passivation can be improved by adding nickel, molybdenum and vanadium.

Where Used?

• Food industry (cookware, flatware, food transport and storage tankers) due to its corrosion resistance and antibacterial properties.

• Surgical equipment • Aerospace • High end automotive, industrial, etc.

• Also: strong, tough, high operating temperatures, thermal conductivity = 1/3 that of carbon steels, difficult to machine, more expensive than carbon steel ($5/lb vs $0.5/lb)

.

Key Points:

•SS can also be classified by crystal structure (austenitic, ferritic, martensitic) •Best corrosion resistance (CR): Austenitic (25% Cr) •Middle CR : Ferritic (15% Cr), •Least CR: Martensitic (12% Cr), but strongest

Key Points: • Over 150 grades of SS available, usually categorized into 5 series containing alloys w/ similar properties.

• AISI classes for SS:

– 200 series = chromium, nickel,manganese (austenitic) – 300 series = chromium, nickel (austenitic) – 400 series = chromium only (ferritic) – 500 series = low chromium <12% (martensitic) – 600 series = Precipitation hardened series (17-7PH, 17-7 PH, 15-5PH)

200/300 Series SS (Austenitic):

• Most common SS (roughly 70% of total SS production) • • • • • • Used for flatware, cookware, architecture, automotive, etc.

0.15% C (max), 16% Cr (min) and Ni or Manganese Austenitic, High strength, best corrosion resistance. High temp capability up to 1200 F. non-magnetic, good ductility and toughness, not hardenable by heat treatment, but they can be strengthened via cold working, best corrosion resistance but most expensive, corrosive in hydrochloric acid. General use where corrosion resistance is needed.

Typical alloy 18% Cr and 10% Ni = commonly known as

18/10

stainless Also have low carbon version of Austenitic SS (316L or 304L) used to avoid corrosion problem caused by welding, L = carbon content < 0.03%

Common 300 series grades of SS: •300 Series—austenitic chromium-nickel alloys   Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304. Type 302—same corrosion resistance as 304, with slightly higher strength due to additional carbon.  Type 303— free machining version of 304 via addition of sulfur and phosphorus . Also referred to as "A1" in accordance with ISO 3506 .

[10]  Type 304—the most common grade; the classic 18/8 stainless steel. Also referred to as "A2" in accordance with ISO 3506 .

[10]  Type 304L— same as the 304 grade but contains less carbon to increase weldability. Is slightly weaker than 304.  Type 304LN—same as 304L, but also nitrogen is added to obtain a much higher yield and tensile strength than 304L.  Type 308—used as the filler metal when welding 304  Type 309—better temperature resistance than 304, also sometimes used as filler metal when welding dissimilar steels, along with inconel .  Type 316—the second most common grade (after 304); for food and compared to type 304. 316 is often used for building ISO 3506 .

[10] surgical stainless steel nuclear reprocessing uses; alloy addition of molybdenum prevents specific forms of corrosion. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion plants. 316L is an extra low carbon grade of 316, generally used in stainless steel watches and marine applications due to its high resistance to corrosion. Also referred to as "A4" in accordance with 316Ti includes titanium for heat resistance, therefore it is used in flexible chimney liners.  Type 321—similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of niobium for desensitization during welding.

400 Series SS (Ferritic):

• Ferritic, Automotive trim, chemical processing, blades, knives, springs, ball bearings, surgical instruments. Can be heat treated!

• Contain between 10.5% and 27% Cr, little Ni and usually molybdenum.

– Common grades: 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni • Magnetic (high in Fe content) and may rust due to iron content.

• Lower strength vs 300 series austenitic grades • Cheap

Common 400 series grades of SS: •400 common alloys  Type 405— ferritic for welding applications  Type 408—heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.  Type 409—cheapest type; used for automobile exhausts ; ferritic (iron/chromium only).   Type 410—martensitic (high-strength iron/chromium). Wear-resistant, but less corrosion resistant. Type 416—easy to machine due to additional sulfur     Type 420—Cutlery Grade martensitic; similar to the Brearley's original rustless steel. Excellent polishability. Type 430—decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced temperature and corrosion resistance. Type 440—a higher grade of cutlery steel, with more carbon, allowing for much better edge retention when properly heat-treated. It can be hardened to approximately 440A, except for diving or other salt-water applications. Type 446—For elevated temperature service Rockwell 58 hardness, making it one of the hardest stainless steels. Due to its toughness and relatively low cost, most display-only and replica swords or knives are made of 440 stainless. Also known as razor blade steel. Available in four grades: 440A, 440B, 440C, and the uncommon 440F (free machinable). 440A, having the least amount of carbon in it, is the most stain-resistant; 440C, having the most, is the strongest and is usually considered more desirable in knifemaking than

500 Series SS (Martensitic):

• Not as corrosion resistant as the other classes but extremely strong and tough as well as machineable and can be hardened via heat treat.

• High strength structural applications (Su up to 300 ksi) – nuclear plants, ships, steel turbine blades, tools, etc.

• Magnetic

600 Series Precipitation Hardening Martensitic SS:

• Have corrosion resistance comporable to 300 series austentic grades but can be precipitation hardened for increased strength!

• Key: High strength + corrosion resistance BOTH.

• Why? Aerospace industry – defense budgets determined 2% of GDP spent dealing with corrosion so developed high strength corrosion resistant steel to replace alloy steels.

• • Lockheed-Martin Joint Striker Fighter – 1 st SS for entire airframe.

Common Grades: – 630 grade = 17-4 PH (17% Cr, 4% Ni), – 17-4 PH, – 15-5 PH aircraft to use PH

High End Stainless (Nuclear Industry)

NITRONIC 50: Tradenames

Nitronic 50, XM-19

General Properties

Density Price

Composition overview

0.28

3.06

Composition (summary)

Fe/22Cr/13Ni/5Mn/2.5Mo

Base Fe (Iron)

Composition detail (metals, ceramics and glasses)

Al (aluminum) 0 B (boron) 8e-4 C (carbon) Cr (chromium) Cu (copper) Fe (iron) Mn (manganese) Mo (molybdenum) N (nitrogen) Nb (niobium) Ni (nickel) P (phosphorus) S (sulfur) Si (silicon) Sn (tin) Ta (tantalum) V (vanadium) W (tungsten)

Mechanical properties

Young's modulus Flexural modulus Shear modulus Bulk modulus Poisson's ratio Shape factor * Yield strength (elastic limit)

Tensile strength

Elongation 0 0 0 0 0.1

0 0 20.5

0 50.8

4 1.5

0.2

0.1

11.5

0 27.8

27.8

10.4

18.9

0.255

1 166

182

7 -

-

0.286

3.36

0.02

0.0025

0.06

23.5

0.75

62.1

6 3 0.4

0.3

13.5

0.04

0.03

1 0.03

0.1

0.3

0.15

28.6

28.6

10.8

25.9

0.316

202

223

23 lb/in^3 USD/lb % % % % % % % % % % % % % % % % % % 10^6 psi 10^6 psi 10^6 psi 10^6 psi ksi

ksi

% strain Also, super alloys (Inconel), NiCr