ENTC 207 stainless steel

Thermal expansion?

ASS = 50% greater than CS FSS=MSS=CS

How does nickel effect the ability to work harden?

Above 9%, the work hardability decreases.

Nickel?

Allows higher CR without compromising strength

What is the difference in tempering SS vs Alloy steel?

Alloy steels are tempered to reduce carbon levels of the martensite. In SS, Carbon is already low in the martensite, so the toughening mechanism is simply removel of the thermally induced stresses.

446

Ferritic. 23 - 30%. Chromium much high for improved scaling resistance. Good resistance to HIGH temperature oxidation and corrosion. X-ray tube bases Boiler baffles Oil burner components Annealing boxes Industrial mufflers Kiln linings Glass molds Furnace parts

worst toughness-inhibiting effects

The worst toughness-inhibiting effects come from interstitial elements to grain boundaries: oxygen, carbon, and nitrogen.

Does sensitization happen with all stainless steels?

...

Tensile strength of SS

28-29 e6 psi vs 30e6 psi for CS. Means more elastic deflection.

Percent elongation, highest grade

302,301,304,303

Highest impact strength grade

302,301,304,316,310

Highest cont use temp steels?

310 and 446. Nickel types preferred for high temp.

Which type are oil hardened?

403,410,414,416 Others are air hardened

Highest tensile strength grade

440c, 301, 420

highest yield strength grade

440c. 446

stress relieving

650-850F if you don't want sensization, but not really effective. 1600F is effective, but must be followed by a rapid quench. Used on highly machined parts.

Electrical resistivity.

6x that of CS.

Hydrochloric acid

Attacks most types.

316L

Austenitic. C lower for welding application. With the addition of molybdenum, the steel is popular for use in severe corrosion environments

304L

Austenitic. C lower for welding application. the steel can be welded without the resulting issue of carbon precipitation This steel Grade is found in a variety of commercial sectors, particularly in the chemical industry.

304

Austenitic. C lower to avoid carbide precipitation. Grade 304 is the standard "18/8" stainless; it is the most versatile and most widely used stainless steel, available in a wider range of products, forms and finishes than any other. It has excellent forming and welding characteristics. Good oxidation resistance in intermittent service to 870°C and in continuous service to 925°C. Continuous use of 304 in the 425-860°C range is not recommended if subsequent aqueous corrosion resistance is important • Food processing equipment, particularly in beer brewing, milk processing & wine making. • Kitchen benches, sinks, troughs, equipment and appliances • Architectural panelling, railings & trim • Chemical containers, including for transport • Heat Exchangers • Woven or welded screens for mining, quarrying & water filtration • Threaded fasteners • Springs

316

Austenitic. Mo added for corrosion resistance. This is particularly apparent for pitting and crevice corrosion in chloride environments. Excellent corrosion resistance when exposed to a range of corrosive environments and media. It is usually regarded as "marine grade" stainless steel but is 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. · Food processing equipment · Brewery, dairy, and pharmaceutical production equipment · Chemical and petrochemical equipment · Laboratory benches & equipment · Coastal architectural panelling · Coastal balustrading · Boat fittings · Chemical transportation containers · Heat exchangers · Mining screens · Nuts and bolts · Springs, nuts, bolts and screws · Medical implants · Sinks and splashbacks · Saucepans · Cutlery and flatware · Sanitaryware and troughs · Tubing

303

Austenitic. S added for eaiser machining. The sulphur addition which is responsible for the improved machining and galling characteristics. Good resistance to mildly corrosive atmospheres, but significantly less than Grade 304 due to the sulphur addition; the sulphide inclusions act as pit initiation sites. Grade 303 should not be exposed to marine or other similar environments, as these will result in rapid pitting corrosion. • Nuts and Bolts • Bushings • Shafts • Aircraft Fittings • Electrical Switchgear Components • Gears

Passivating.

Basically, a less drastic version of pickeling. Will not remove base metal or OXIDE layer. Use for cleaning free iron mostly.

Flame cutting?

Do not do, b/c refractory oxides use plasma instead.

430

Ferritic. Basic type. 17% Chromium. 430 has good resistance to a wide variety of corrosive media including nitric acid and some organic acids. Stress corrosion cracking resistance of Grade 430 is very high, as it is for all ferritic grades. • Linings for dish washers • Refrigerator cabinet panels • Automotive trim • Lashing Wire • Element Supports • Stove trim rings • Fasteners • Chimney Liners

Free iron?

Free iron scrubbed onto the surface with tools for example, can cause the SS to rust.

Corrosion resistance of high carbon steel vs lower?

H<L

440C

Highest C for wear resistance. Grade 440C is capable of attaining, after heat treatment, the highest strength, hardness and wear resistance of all the stainless alloys. Its very high carbon content is responsible for these characteristics, which make 440C particularly suited to such applications as ball bearings and valve parts. Corrosion resistance is lower than the common austenitic grades, Good resistance to the atmosphere, fresh water, foods, alkalies and mild acids. Best resistance in the hardened and tempered and passivated condition. • Rolling element bearings • Valve seats • High quality knife blades • Surgical instruments • Chisels.

letter for stablized SS

L.

Forming of ASS vs FSS vs CS?

Most ASS have high elongation. FSS are not as formable as drawing quality CS.

Fusion welding rules?

MSS- NO, martensite forms when welding. Might crack. FSS- Yes, but grain growth occurs, this makes it more brittle, and sigma embirrtlement also possible. ASS-YES, but might devlope sensitization.

420

Martensitic. Carbon added for higher cutting purposes. It has good ductility in the annealed condition but is capable of being hardened up to Rockwell Hardness 50HRC, the highest hardness of the 12 per cent chromium grades. In the annealed condition this grade is relatively easily machined, but if hardened to above 30HRC machining becomes more difficult. Free machining grade 416 is a very readily machined alternative. Typical applications include: • Cutlery • Knife Blades • Surgical Instruments • Needle Valves • Shear Blades

416

Martensitic. Similar to 403, but with sulfer. Sulfer added for eaiser machining. Grade 416 has the highest machinability of any stainless steel, at about 85% of that of a free-machining carbon steel. • Valve Parts • Pump Shafts • Automatic Screw Machined Parts • Motor Shafts • Washing Machine Components • Bolts and Nuts • Studs • Gears Useful resistance to dry atmospheres, fresh water and mild alkalies and acids, but less resistant than the equivalent non-free-machining grades. Less corrosion resistant than the austenitic grades and also less than 17% chromium ferritic alloys such as Grade 430. High sulphur content free machining grades such as 416 are totally unsuitable for marine or other chloride exposure.

403

Martinsitic, 12% Cr. About 0.15% carbon. corrosion resistant, heat resistant, oxidation resistant up to 1400°F. Grade 403 stainless steel is used in turbine parts and compressor blades. ATI 403™ alloy possesses a high degree of resistance to atmospheric corrosion because of its ability to form a tightly adherent oxide film, which protects it from further attack. Maximum corrosion resistance is obtained by hardening and polishing. ATI 403™ alloy is not recommended for use in severely corrosive environments.

High-Temperature Embrittlement

Medium- and high-chromium ferritic alloys containing moderate amounts of carbon and/or nitrogen develop high-temperature brittleness if cooled slowly from above 950°C (1740°F). The mechanism is similar to that of sensitization and leads to severe intergranular corrosion

Pickeling?

Or acid rescaling, is used to remove heavy oxide films produced by heat treating, welding, hot forming, and other high temp processes. It is also used to remove rust spots. (CONS)It removes a small portion of the base metal, and often leaves a rough surface. Sulferic acid or nitrichydroflouric acid.

Conductivity

Poor conductor of heat and electricity. 1/2 of CS.

high-temperature embrittlement(also called sigma phase enbrittlement)

Prolonged exposure in the temperature range of 565-925°C (1050-1700°F) results in chromium depletion from the grain boundaries, making them susceptible to intergranular corrosion. Rapid cooling of unstabilized alloys causes carbon and nitrogen to pre-cipitate within grains. This severely embrittles the material and does not avoid sensitization.

sigma-phase embrittlement cont...

Prolonged exposure in the temperature range of 565-925°C (1050-1700°F) results in chromium depletion from the grain boundaries, making them susceptible to intergranular corrosion. The most rapid sigma-phase formation occurs in the range of 700-900°C (1290-1650°F). Alloy elements such as molybdenum, titanium and silicon promote the formation of sigma phase, while nitrogen and carbon reduce its tendency to form.

Moly?

Reduces pitting.

sigma-phase embrittlement cont............

Sigma phase is an intermetallic compound consisting of chromium and iron, which is hard, brittle and non-magnetic. Pure sigma forms between 42% and 50% chromium and is one of the equilibrium phases in the iron-chromium phase diagram (Fig. 2). A duplex structure (sigma and alpha phases) has been found to form in alloys with as little as 20% chromium and as much as 70% chromium when exposed to the critical temperature range noted above. At chromium contents of less than 20%, sigma phase is difficult to form, but the presence of molybdenum, silicon, manganese or nickel have a tendency to shift the lower limit down. Sigma typically nucleates in the austenite-ferrite grain boundaries and grows into the adjacent ferrite.

Sensitization

Since carbon diffuses interstitially much more rapidly than chromium can substitutionally, chromium is combined in situ, especially along grain boundaries, which are fast-diffusion paths. This locally depletes chromium, and the alloy is sensitized. This can be eliminated by a sufficiently long homogenization anneal at a low enough temperature that carbon and nitro- gen have very little solubility.

Stabilizing

Stabilizing removes the interstitial carbon and nitrogen, along with oxygen and sulfur, from solution. This does not produce a major soften- ing, however, because the precipitate itself has a hardening effect

Define stainless steel

Stainless steels are steels with at least 10% chromium that exhibit passivity in oxidizing environments.

Prevent sensitzation?

Stainless steels can be stabilized against this behavior by 1) addition of titanium, niobium, or tantalum, which form titanium carbide, niobium carbide and tantalum carbide preferentially to chromium carbide, 2) lowering the content of carbon in the steel and in case of welding also in the filler metal under 0.02%, or 3) heating the entire part above 1000 °C and quenching it in water, leading to dissolution of the chromium carbide in the grains and then preventing its precipitation.

stress corrosion cracking

They can experience stress corrosion cracking (SCC) if used in an environment to which they have insufficient corrosion resistance.

sigma-phase embrittlement

This type of embrittlement has been shown to cause severe loss of ductility, toughness, and corrosion resistance resulting in cracking (Fig. 1) and failure of components

What does it mean to temper martensitic SS?

Transform retained austenite.

T/F SS perform best in oxidizing enviroments?

True

T/F? Most SS alloys have higher yield strenths than CS

True.

They are susceptible to galvanic corrosion on a microscale when their structure contains ..............?

Two phases. (ferrite and austenite maybe).

475°C (885°F) Embrittlement

it can occur during slow cooling from an elevated temperature as well as during elevated-temperature service. For alloys containing 18% Cr, the onset of embrittlement is fast enough to require rapid cooling from the annealing temperature to extend below 400°C (750°F) in order to ensure optimal ductility. This embrittlement phenomenon is believed to be due to the formation of a submicroscopic, coherent precipitate that is induced by the presence of a solubility gap below approximately 550°C (1020°F) in a chromium range where sigma phase forms at higher temperature

intergranular corrosion

precipitation of chromium carbide at the grain boundaries, resulting in the formation of chromium-depleted zones adjacent to the grain boundaries (this process is called sensitization) These zones also act as local galvanic couples, causing local galvanic corrosion. This condition happens when the material is heated to temperature around 700 °C for too long time, and often happens during welding or an improper heat treatment. When zones of such material form due to welding, the resulting corrosion is termed weld decay. Stainless steels can be stabilized against this behavior by addition of titanium, niobium, or tantalum, which form titanium carbide, niobium carbide and tantalum carbide preferentially to chromium carbide, by lowering the content of carbon in the steel and in case of welding also in the filler metal under 0.02%, or by heating the entire part above 1000 °C and quenching it in water, leading to dissolution of the chromium carbide in the grains and then preventing its precipitation. Another possibility is to keep the welded parts thin enough so that, upon cooling, the metal dissipates heat too quickly for chromium carbide to precipitate.

Nickel

promotes austenite. austenite can be the strucutre at room temperature.

intergranular corrosion F.S.S

s due to chromium depletion, caused by precipitation of chromium carbides and nitrides at grain boundaries. Because of the lower solubility for carbon and nitrogen and higher diffusion rates in ferrite, the synthesized zones of welds in ferritic steels are in the weld and adjacent to the weld. To eliminate the intergranular corrosion, it is necessary either to reduce carbon to very low levels, or to add titanium and columbium to tie up the carbon and nitrogen.

stress-corrosion cracking F.S.S

the most obvious advantage of the ferritic stainless steels. Ferritic steels resist chloride and caustic stress corrosion cracking very well. Nickel and copper residuals lower resistance of these steels to stress corosion.