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Surface Treatments: Surface Hardening

Equipment & Coatings

Alfredo Valarezo

Surface Treatments

• Thermochemical treatments (Carburizing, nitriding, carbonitriding, chromizing)

• Electrochemical treatments (hard chrome, cadmium, nickel)

• Thermomechanical treatments (thermal Spray)

• Mechanical treatments (shot peening, blasting)

Clasificación en consideración de la naturaleza del depósito (por difusión atómica o de iones, o por

adición de material en la superficie) y el espesor del recubrimiento.

Ingeniería de Superficies: el termorociado Clasificación de los Procesos en Ingeniería de Superficies

Ingeniería de Superficies: el termorociado

Dureza de varios materiales y tratamientos superficiales

©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Surface Hardening

Thermochemical treatments applied to steels in which the composition of the part surface is altered by adding various elements

• Often called case hardening

• Most common treatments are carburizing, nitriding, and carbonitriding

• Commonly applied to low carbon steel parts to achieve a hard, wear-resistant outer shell while retaining a tough inner core


Carburizing

Heating a part of low carbon steel in a carbon-rich environment so that C is diffused into surface

• In effect the surface is converted to a high carbon steel, capable of higher hardness than the low-C core – Carburizing followed by quenching produces a case hardness of

around HRC = 60

– Internal regions are low-C steel, with low hardenability, so it is unaffected by quench and remains relatively tough and ductile

• Most common surface hardening treatment

Carburizing in Salt Bath

Salt Bath Furnace

NaCN + O2 → 2NaCNO

2NaCNO + O2 → Na2CO3 +CO + N2

2CO → CO2 + C

Sodium Cyanide Salt

(Sal de Cianuro de sodio)

Carburizing by Pack

Carburizing Steel 1020


Nitriding

Treatment in which nitrogen is diffused into surface of special alloy steels to produce a thin hard casing without quenching

• Carried out at around 500C (950F)

• To be most effective, steel must have alloying ingredients such as aluminum (AlN) or chromium (CrN) to form nitride compounds that precipitate as very fine particles in the casing to harden the steel

• Hardness up to HRC 70

Carburizing Steel 1015 (Temp. 550C)


Chromizing/Boronizing

• Requires higher temperatures and longer treatment times than the preceding hardening treatments

• Usually applied to low carbon steels

• Casing is not only hard and wear resistant; it is also heat and corrosion resistant


Furnaces for Heat Treatment

• Fuel-fired furnaces – Normally direct-fired - the work is exposed

directly to combustion products

– Fuels: natural gas or propane and fuel oils that can be atomized

• Electric furnaces – Electric resistance for heating

– Cleaner, quieter, and more uniform heating

– More expensive to purchase and operate


Batch vs. Continuous Furnaces

• Batch furnaces – Heating system in an insulated chamber, with a door for

loading and unloading

– Production in batches

• Continuous furnaces – Generally for higher production rates

– Mechanisms for transporting work through furnace include rotating hearths and straight-through conveyors


Other Furnace Types

• Atmospheric control furnaces – Desirable in conventional heat treatment to avoid

excessive oxidation or decarburization

– Include C and/or N rich environments for diffusion into work surface

• Vacuum furnaces – Radiant energy is used to heat the parts

– Disadvantage: time needed each cycle to draw vacuum


Selective Surface Hardening Methods

• These methods heat only the work surface, or local areas of the work surface

• They differ from surface hardening methods in that no chemical changes occur

• Methods include: – Flame hardening – Induction hardening – High-frequency resistance heating – Electron beam heating – Laser beam heating


Flame Hardening

Heating of work surface by one or more torches followed by rapid quenching

• Applied to carbon and alloy steels, tool steels, and cast irons

• Fuels include acetylene (C2H2), propane (C3H8), and other gases

• Lends itself to high production as well as big components such as large gears that exceed the size capacity of furnaces


Induction Heating

Application of electromagnetically induced energy supplied by an induction coil to an electrically conductive workpart

• Widely used for brazing, soldering, adhesive curing, and various heat treatments

• When used for steel hardening treatments, quenching follows heating

• Cycle times are short, so process lends itself to high production


Figure 27.7 Typical induction heating setup. High frequency alternating current in a coil induces current in the workpart to effect heating.

Induction Heating


High-frequency (HF) Resistance Heating

Used to harden specific areas of steel work surfaces by application of localized resistance heating at high frequency (400 kHz typical)

• Contacts are attached to workpart at outer edges of the area

• When HF current is applied, region under conductor is heated quickly to high temperature - heating to austenite range typically takes less than a second

• When power is turned off, area is quenched by heat transfer to the surrounding metal


Figure 27.8 Typical setup for high-frequency resistance heating.

High-frequency Resistance Heating

Surface Treatments

• Thermochemical treatments (Carburizing, nitriding, carbonitriding, chromizing)

• Electrochemical treatments (hard chrome, cadmium, nickel)

• Thermomechanical treatments (thermal Spray)

• Mechanical treatments (shot peening, blasting)

What is Thermal Spray?

Thermal Spray Processes

σ

z

σ

+

z

+

Plastically

worked zone

-

Z2

Z1

LOW VELOCITY Molten particle

More decomposition

HIGH VELOCITY Semi - molten

particle

Less Decomposition

The science behind thermal spray P

arti

cle

Sta

te

•Particle Velocity

•Kinetic Energy

•Fluid dynamics

•Particle Temperature

•Oxidation/decomposition

•Rapid cooling/phase transformations

Loca

l De

po

siti

on

Te

mp

era

ture

•Intersplat bonding strength

•Wetting/ adsorbate-condensates

•Residual Stress (quenching-peening)

Mat

eri

als

intr

insi

c P

rop

ert

ies •Substrate-splat – adhesion

interface

•Splat-splat-intersplat interfaces

•Interpass interfaces

•Elastic –plastic behavior

•High strain – strain rates

•Heat transfer

Tension Compression

Residual Stresses

Industries that demand Thermal Spray…

• Defense and Aerospace: Typical Applications – Landing Gear

– Hydraulic Shafts

– Gas and Aeroturbines

– Part reclamation

– Antiskid Plattforms

– Worn out components, in general

26

http://www.asetsdefense.org/ http://www.istockanalyst.com/article/viewarticle/articleid/1725016 http://alphamar-imw.com

Cyllinder Bore

Crankshaft Repair by HVOF

Propuesta de Partes a Reparar: Eje Trasero Camiones

Recubrimiento

Preparacion Superficial

Propuesta de Partes a Reparar: Florero de la Transmisión

Recubrimiento

Propuesta de Partes a Reparar: Secciones de Manifold

Recubrimiento

Propuesta de Partes a Reparar: Asientos y Pines

Recubrimiento

Propuesta de Partes a Reparar a FUTURO: Carcazas

Recubrimiento

ELECTRODEPOSITION Electroplating

ELECTROPLATING

• Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode.

• The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal.

ELECTROPLATING

• The anode and cathode are both connected to an external supply of direct current

• The anode is the positive terminal, and the cathode is the negative.

• The metal at the anode is oxidized from the zero valence state to form cations with a positive charge.

• These cations associate with the anions in the solution.

• The cations are reduced at the cathode to deposit in the metallic, zero valence state.

• The plating is most commonly a single metallic element, not an alloy. However, some alloys can be electrodeposited, notably brass and solder.

Ni - ELECTROPLATING

• Nickel Electroplating is a process in which a coat of nickel metal is deposited over another metal for inducing certain superior properties.

• The process is carried out in an electrolytic solution, and is widely used in the electronics and chemical industries.

• Some of the features of nickel coating that is deposited on the metal surface are as follows:

– Decorative appearance – Corrosion protection – Wear resistance – Low coefficient of friction. – Ferromagnetism – Controllable internal mechanical

stresses

Surface Engineering Techniques

• The techniques covered may be divided broadly into three categories:

• Techniques to prepare a surface for subsequent treatment (e.g., cleaning and descaling)

• Techniques to cover a surface with a material of different composition or structure (e.g., plating, painting, and coating)

• Techniques to modify an existing surface topographically, chemically, or microstructurally to enhance its properties (e.g., glazing, abrasive finishing, and ion implantation)

Hydrogen Evolution and Cathode Efficiency

• The discharge of nickel ions is not the only reaction that can occur at the cathode; a small percentage of the current is consumed in the discharge of hydrogen ions from water.

• This reduces the cathode efficiency for nickel deposition from 100 per cent to 92 to 97 per cent depending on the nature of the electrolyte.

• The discharged hydrogen atoms form bubbles of hydrogen gas at the cathode surface.

Deposition Rate

If the plating process is operated at 5 A/dm2, for example, it takes about 20 minutes to deposit a nickel coating with an average thickness of 20 um.

Faraday's Law for Nickel

• W = (I*t*A)/(n*F)

• where: • W = weight of plated metal in grams. • I = current in coulombs per second. • t = time in seconds. • A = atomic weight of the metal in

grams per mole. • n = valence of the dissolved metal in

solution in equivalents per mole. • F = Faraday's constant in coulombs

per equivalent. F = 96,485.309 coulombs/equivalent.

Nickel Plating Solutions

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