Metallurgy

Credits: VP Sastry, Reliance

· Steel: carbon content is less than 2.1% by weight.

· Ultimate tensile strength (UTS), often shortened to tensile strength is the maximum stress that a material can withstand while being stretched or pulled before necking.

highest point of the stress-strain curve.

· Yield strength: till it is plastic

· Allowable Stress: It is calculated as UTS/3.5 or Yield Stress/1.5 whichever is less.

· Ductility: wire

Malleability: sheet

· Toughness is the ability to absorb energy up to fracture (energy per unit volume of material).

Approximated by the area under the stress-strain curve.

· The temperature at which a ductile material transition to brittle behavior is know

· as the Ductile to Brittle Transition Temperature

· Hardness is a measure of a material’s resistance to localized plastic deformation (a small dent or scratch).

· Fatigue limit or endurance limit is the amplitude (or range) of cyclic stress that material can sustain without causing fatigue failure.

AISI - SAE Classification System

AISI X X X X

· American Iron and Steel Institute (AISI)

1st number is the major alloying element

2nd number designates the subgroup alloying element or the relative per cent of primary alloying element.

Last two numbers approximate amount of carbon (expresses in 0.01%)

Carbon Steels

10xx Plain Carbon

11xx Resulfurized

12xx Resulfurized and rephosphorized

Manganese steels

13xx Mn 1.75

Nickel steels

23xx Ni 3.5

25xx Ni 5.0

Nickel Chromium Steels

31xx Ni 1.25 Cr 0.65-0.80

32xx Ni 1.75 Cr 1.07

33xx Ni 3.50 Cr 1.50-1.57

34xx Ni 3.00 Cr 0.77

Chromium Molybdenum steels

41xx Cr 0.50-0.95 Mo 0.12-0.30

· “Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, molybdenum, nickel, niobium, titanium, tungsten, vanadium or zirconium. OR When the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60”.

Plain carbon steels (less than 2% carbon and trace amounts of other residual elements)

Depending upon the carbon content the carbon steel is classified into following:

-- Low Carbon Steels : Easy to fabricate, weld and form. But has low high temperature strength and hardness. Used for making pressure vessels where design temperature is less than 350°C.

(less than 0.15% carbon)

-- Medium Carbon Steels : Balances ductility and strength and has good wear resistance; used for large forged parts, springs and automotive components.

(0.16% to 0.59%)

-- High Carbon Steels : High strength and hardness; but brittle ; used for making (non-industrial-purpose) knives, axles or punches.

addition of carbon generally makes the metal more difficult to weld.

(0.6% to 0.95%)

· High Strength Low Alloy Steels (HSLA): With medium to high carbon levels, low-alloy steel is difficult to weld. Lowering the carbon content to the range of 0.10% to 0.30%, along with some reduction in alloying elements, increases the weld ability and formability of the steel while maintaining its strength.

used in cars, trucks, cranes, bridges, roller coasters and other structures designed to handle large amounts of stress or need a good strength-to-weight ratio.

· Low Temperature Carbon Steels: use in low temperature equipment and especially for welded pressure vessels. Minimum temperature upto which plain carbon steels can be used is -29°C. Whereas LTCS can be used up to -45°C. comes from normalizing heat treatment given to carbon steels.

For further lower temperature usage Stainless steel having minimum design temperature -180°C finds application.

Stainless Steels

· Contain at least 10.5% Chromium

· Design Temperature : -180°C to 800°C

Manganese:

· beneficial to surface quality

· strength and hardness, but less than carbon

· decreases ductility and Weldability, but less than carbon

Phosphorus:

· increases strength and hardness and decreases ductility and notch impact toughness of steel

Sulfur:

· decreases ductility and notch impact toughness

· Weldability decreases

Silicon:

· generally detrimental to surface quality.

Copper:

· detrimental to surface quality

· beneficial to atmospheric corrosion resistance

Lead:

· added to carbon and alloy steels by means of mechanical dispersion during pouring to improve the machinability.

Boron:

· improve hardenability

Chromium:

· increase corrosion resistance and oxidation resistance, to increase hardenability, or to improve high-temperature strength

· frequently used with a toughening element such as nickel to produce superior mechanical properties

· strong carbide former

Nickel:

· strengthener.

· does not form carbides in steel.

· increases the hardenability and impact strength of steels

Molybdenum:

· hardenability

· may produce secondary hardening during the tempering of quenched steels.

· enhances the creep strength of low-alloy steels at elevated temperatures

Aluminum:

· controlling grain growth prior to quenching.

Zirconium:

· sulfide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending.

Titanium:

· retard grain growth and thus improve toughness.

Vanadium:

· increases the yield strength and the tensile strength

Damage Mechanisms

· Brittle Fracture: sudden rapid fracture under stress

· Creep: high temperatures, metal components can slowly & continuously deform under load below the yield stress. This time dependent deformation of stressed components which can eventually lead to rupture,

· Thermal Fatigue: variations in temperature. Damage is in the form of cracking, wherever relative movement or differential expansion is constrained

· Mechanical Fatigue: progressive and localized structural damage that occurs when a material is subjected to cyclic loading.

component is exposed to cyclical stresses for an extended period, of resulting in sudden, unexpected failure. well below yield strength of the material.

· Erosion / Erosion – Corrosion: result of relative movement between solids, liquids, vapor or any combination thereof.

· Cavitation: form of erosion caused by formation & instantaneous collapse of innumerable tiny vapor bubbles.

collapsing bubbles exert severe localized impact forces that result in metal loss

· Corrosion under Insulation (CUI): resulting from water trapped under insulation or fireproofing.

· Galvanic Corrosion: occur at the junction of dissimilar metals when they are joined together in a suitable electrolyte, such as a moist or aqueous environment or soils containing moisture.

more noble material (cathode) is protected by sacrificial corrosion of the more active material (anode). The anode corrodes at a higher rate than it would if it were not connected to the cathode.

· Carbon Dioxide (CO2) corrosion: CO2 dissolves in water to form carbonic acid (H2CO3). The acid may lower the pH and sufficient quantities may promote general corrosion and/or pitting corrosion of carbon steel.

· Microbiologically Induced Corrosion (MIC): caused by living organisms such as bacteria, algae or fungi.

· Caustic Corrosion: Localized corrosion due to the concentration of caustic or alkaline salts that usually occurs under evaporative or high heat transfer conditions

· Sulfidation: resulting from their reaction with sulphur compounds in high temperature environments. Hydrogen accelerates corrosion. above 260°C.

· Chloride Stress Corrosion Cracking: Surface initiated cracks caused by environmental cracking of 300 series SS & some Nickel based alloys under the combined action of tensile stress, temperature and an aqueous chloride environment. The presence of dissolved oxygen increases propensity for cracking.

· Hydrogen blistering: results from hydrogen atoms that form during the sulfide corrosion process on the surface of the steel, that diffuse into the steel, and collect at a discontinuity in the steel such as an inclusion or lamination. The hydrogen atoms combine to form hydrogen molecules that are too large to diffuse out and the pressure builds to the point where local deformation occurs, forming a blister.