Untitled Quiz

Metallurgy & Materials Science

• Presented by Dr. R. Vaira Vignesh
• Assistant Professor, Amrita School of Engineering, Coimbatore
• Topic is Corrosion is a Bliss
• Presentation materials are from internet/articles/books; for classroom use only

Heat Treatment of Steels

• Combination of heating and cooling metal/alloy
• Changes micro-constituents to modify properties
• Applied to ingots, castings, semi-finished products, welds.
• Changes micro-constituent's properties: nature, size, distribution

Stress and Strain

• Non-uniform plastic deformation--manufacturing sequence.
• Includes machining, welding and grinding
• Deformation processing (hot/cold work)
• Thermal variation
• Phase transformations
• Heterogeneity of chemical or crystallographic order
• Hooke's Law: Complication, muddle, misunderstanding, obfuscation
• Load removed: Tries to recover elasticity.
• Inhibited from full recovery - Adjacent material is deformed plastically.

Residual Stresses

• Stresses present in matrix after load removal
• Thermal or Mechanical
• Positive or Negative
• Laser peening--compressive residual stresses
• Strengthens thin sections.
• Toughens brittle surfaces
• Negative effects as well.
• Invisible to manufacturers unless significant distortion.
• Affects structural integrity
• Thick walled structures prone to brittle fracture.
• Undesired stresses affect fatigue performance.
• Often a cause of premature failure of critical components.

Heat Treatment

• Combination of heating and cooling operations timed and applied to a metal or alloy in a solid state.
• Changes in microconstituents—nature, size and distribution.
• Applied to ingots, castings, semi-finished products, welded joints, and various elements of machines and instruments.

Purpose of Heat Treatment

• Relieve internal stresses
• Refine grain structure (coarse to fine)
• Augment material properties (mechanical, corrosion, tribological)

Heat Treatment of Steels

• Steel is alloy of Fe and C (0.2% to 2% weight %)
• Other alloying components: up to 5% in low alloy steels
• More alloyed steels: tool steel and stainless steel
• Wide variety of properties
• Liquid phase is always avoided in heat treating.

Quick Review

• Austenite - Solid solution of C and/or other alloying elements in γ-Fe; not stable at room temperature, unless stabilizers added.
• Ferrite - BCC iron phase; limited solubility of C (α-Fe) - BCC
• Cementite - Fe3C (6.67 wt.% C) - Complex orthorhombic
• Ledeburite - Eutectic mixture of Austenite and Cementite
• Pearlite - Alternate lamellae of Ferrite and Cementite (austenite decomposition by eutectoid reaction)

Quick Review (cont.)

• Martensite - Austenite transformation below Ms (240°C)--shear type transformation
• Bainite - Austenite transforms below temp. at which pearlite is produced; above transformation temp of Martensite
• Troosite - Radial lamellae of ferrite and cementite, tempering below 450°C
• Sorbite - Ferrite and finely divided cementite, tempering above 450°C
• A1 – 727°C (Eutectoid transformation temperature)
• A2 - 768°C (Curie temperature); Ferro --> Para-magnetic
• A3 – γ-Fe / y-Fe + α-Fe boundary
• Acm
• Ar3
• AC3

Annealing

• Generic term—heating to a temperature and holding at that temperature.
• Cooling at an appropriate rate to soften or improve properties.
• Improve mechanical or electrical properties; promote dimensional stability
• Maximum temperature = A1, A3, Acm
• Austenite is present at temps above A1
• Subcritical, inter-critical, super-critical.

Annealing Cycles

• Subcritical (Stress-Relief, Process Annealing)--(Below A1)
• Inter-critical annealing
• Above A1, but below A3
• Above A1, but below Acm
• Super-critical (Full annealing, Normalizing)--(Above A3 or Acm)

Stress-Relief Annealing

• Relieve stresses, consequence of manufacturing sequence.
• Separates stress-relief from post-weld.

Annealing (cont.)

• Minimizing internal residual stresses.
• Uniform heating.
• Cooling after specified holding time

Normalizing

• Heating to ≥55°C above A3 for hypoeutectoid compositions.
• Above Acm for compositions higher than eutectoid.
• Cooling practice through transformation for desired properties.
• Refine the grains, decrease average grain size.

Full Annealing

• Heating to 50°C above A3, for hypoeutectoid or A1, for hypereutectoid steels.
• Hold at this temp. for specified time.
• Uniform cooling to achieve the desired structure.

Carbo-Nitriding

• Modified form of gas carburizing.
• Introducing ammonia into the gas carburizing atmosphere.
• Lower temp, shorter time, shallower case than gas carburizing.

Carbo-Nitriding (cont.)

• Avoids cyanide-bearing waste.
• Improves hardenability, wear resistance, case depth.
• Adds Nitrogen for austenite stabilization.

Spheroidizing

• Strength of ferrite depends on its grain size and cooling rate.
• Carbides in pearlite or spheroids affect formability of steel.
• Spheroidized steels: globular carbides in a ferritic matrix.

Spheroidizing (cont.)

• Prolonged holding at just below A1.
• Alternating heating and cooling.
• Heating to just above A1, cooling slowly or holding below A1.
• Prevent carbide reformation.

Process Annealing

• During cold working.
• Hardness of steel increases, and ductility decreases.
• Must be annealed to restore ductility.
• Restore steel's ductility; merely soften steel.
• Intermediate treatment

Quenching

• Rapid cooling from austenitizing temperature; minimizes grain boundary carbides.
• Improves ferrite distribution, control martensite in microstructure.
• Achieving toughness, hardness, minimization of residual stress, distortion and cracking.

Tempering

• Reheating previously hardened steel below the critical temp.
• Increasing ductility.
• Increasing toughness.

Martempering

• Interrupted quench from austenitizing temp.
• Hold temp. above martensite transformation to equalize temp. throughout the piece.
• Minimize distortion, cracking, and residual stress.
• Essentially primary martensitic, untempered and brittle.

Martempering (cont.)

• Quench into hot fluid medium (oil, salt, etc.)
• Holding until uniform temp.
• Avoids large temp. difference.

Austempering

• Isothermal transformation below pearlite formation and above martensite.
• Heating to a temp. in austenitizing range, usually (790 to 915 °C.)
• Quenching in a bath maintained at a constant temp (usually 260-400 °C).
• Transforming isothermally to bainite in the bath.
• Cooling to room temperature.

Austempering (cont.)

• Molten salt is preferred quenching medium.
• Eliminates problem of a vapor phase barrier.
• Increased steel toughness, ductility, and strength.
• Reduced distortion.
• Reduced machining time, stock removal, sorting, and inspection.
• Shortest overall time cycle possible

Surface Hardening of Steels

• Methods vary based on diffusion and related processes.
• Includes surface engineering, heat treatment, and surface modifications.

Carburizing

• Diffusing carbon into the surface, increasing hardness.
• Used on low-carbon steels.

Carburizing (cont.)

• High carbon content in surface due to rapid diffusion and high solubility of carbon in austenite.
• Quenched and tempered surface becomes high-carbon tempered martensite - ferritic center is soft and ductile.
• Thickness is smaller in carburized steels than flame or induction hardened steels.
• Increased carbon content at the surface gives a martensitic layer. (wear resistant)

Carburizing (cont.)

• Uses methane (or propane) for decomposition into atomic carbon and hydrogen.
• Time is 2 hours to 36 hours for 1 mm to 4 mm depth case.

Carburizing (cont.)

• Liquid carburising/cyaniding
• Salt bath (cyanide-chloride-carbonate) at 845°C to 955°C.
• Cyanide salts introduce nitrogen, improving hardness.
• Fastest carburising process for small batch sizes.

Carburizing (cont.)

• Components surrounded by a carbonaceous medium
• Process of either gas (CO->CO2) or solid carburising
• Appropriate temps (790-845C) and times (2-36 hrs)

Nitriding

• Ferritic thermochemical method for diffusing nascent nitrogen into the surface of steels.
• Diffusion process is based on solubility of N in Iron.
• At 450°C, Iron-base alloy will absorb up to 5.7% to 6.1% of Nitrogen.
• Beyond this, the surface phase is predominantly ɛ phase.
• High carbon gives more potential for ɛ phase formation.

Nitriding (cont.)

• No molecular size change, no dimensional change.
• Slight growth due to volumetric change.
• Distortion induced by surface stresses being released.

Nitriding (cont.)

• Process parameters: Furnace temp., process control, time, gas flow, gas activity control, process chamber maintenance.

Carbo-Nitriding

• Modified form of gas carburizing.
• Introducing ammonia for nitrogen addition to the carburized case.
• Nitrogen diffuses with carbon simultaneously for a shorter time.
• Shallower case.
• Similar to liquid cyaniding.

Carbo-Nitriding (cont.)

• Preferred over liquid cyaniding due to cyanide disposal problems.
• Hard, wear-resistant case, with better hardenability.
• Nitrogen increases the hardenability of steel.
• Increases retained austenite (for alloy steels).