Definition of Heat Treatment

Definition of heat treatment

Heat treatment is an operation or combination of operations involving heating at a specific rate, soaking at a temperature for a period of time and cooling at some specified rate. The aim is to obtain a desired microstructure to achieve certain predetermined properties (physical, mechanical, magnetic or electrical).

1 Objectives of heat treatment (heat treatment processes)

The major objectives are • to increase strength, harness and wear resistance (bulk hardening, surface hardening)

• to increase ductility and softness (tempering, recrystallization annealing)

• to increase toughness (tempering, recrystallization annealing)

• to obtain fine grain size (recrystallization annealing, full annealing, normalising)

• to remove internal stresses induced by differential deformation by cold working, non-uniform cooling from high temperature during casting and welding (stress relief annealing) 2 • to improve machineability (full annealing and normalising)

• to improve cutting properties of tool steels (hardening and tempering)

• to improve surface properties (surface hardening, corrosion resistance-stabilising treatment and high temperature resistance-precipitation hardening, surface treatment)

• to improve electrical properties (recrystallization, tempering, age hardening)

• to improve magnetic properties (hardening, phase transformation)

3 Metals

Ferrous metals Non-ferrous metals

Steels Cast Irons

Plain carbon steels Grey Iron

Low carbon steels White Iron

Medium carbon steels Malleable & Ductile Irons

High carbon steels

Low alloy steels

High alloy steels Stainless & Tool steels Steels Low Alloy High Alloy low carbon Med carbon high carbon

<0.25 wt% C 0.25-0.6 wt% C 0.6-1.4 wt% C

heat austenitic Name plain HSLA plain plain tool treatable stainless Cr,V Cr, Ni Cr, V, Additions none none none Cr, Ni, Mo Ni, Mo Mo Mo, W Example 1010 4310 1040 43 40 1095 4190 304 Hardenability 0 + + ++ ++ +++ 0 TS - 0 + ++ + ++ 0 EL + + 0 - - -- ++ Uses auto bridges crank pistons wear drills high T struc. towers shafts gears applic. saws applic. sheet press. bolts wear dies turbines vessels hammers applic. furnaces blades V. corros. resistant increasing strength, cost, decreasing ductility Based on data provided in Tables 11.1(b), 11.2(b), 11.3, and 11.4, Callister 7e. 5 How to Strengthen Metals: • Key: prevent dislocations from moving through crystal structure!!! • Finer grain boundries – can be done by recrystallizing (and cold working) • Increase dislocation density via COLD WORKING (strain hardening) • Add alloying elements to give –SOLID SOLUTION HARDENING. • Add alloying elements to give precipitates or dispersed particles – PRECIPITATION HARDENING (aka Heat Treat) • DISPERSION HARDENING– fine particles (carbon) impede dislocation movement. – Referred to as Quench Hardening, Austenitizing and Quench or simply “Heat Treat”. – Generally 3 steps: heat to austenite T, rapid quench, then temper.

The Effect of Grain Boundries:

• Dislocations pile up at GB and can’t go further – this effectively strengthens the crystal! Work Hardening

Work hardening, or strain hardening, results in an increase in the strength of a material due to plastic deformation.

Plastic deformation = adding dislocations – as dislocation density increases, they tend to “tie up” and don’t move.

Ludwik’s Equation:

Strain hardening index Solution Hardening (AKA Alloying):

= strengthening by deliberate additions of impurities (alloying elements) which act as barriers to dislocation movement. Example: addition of zinc to copper making the alloy brass (copper dissolves up to 30% zinc). Zinc atoms replace copper atoms to form random substitutional solid solution. The zinc atoms are bigger than copper and by squeezing into the copper lattice, they distort it making it harder for dislocations to move. δ ferrite: Interstitial solid solution of carbon in iron of body centred cubic crystal structure (Fig .2(a)) (δ iron ) of higher lattice parameter (2.89Å) having solubility limit of 0.09 wt% at 1495°C with respect to austenite. The stability of the phase ranges between 1394-1539°C.

Fig.2(a): Crystal structure of ferrite

This is not stable at room temperature in plain carbon steel. However it can be present at room temperature in alloy steel specially duplex stainless steel.

γ phase or austenite: Interstitial solid solution of carbon in iron of face centred cubic crystal structure (Fig.3(a)) having solubility limit of 2.11 wt% at 1147°C with respect to cementite. The stability of the phase ranges between 727-1495°C and solubility ranges 0-0.77 wt%C with respect to alpha ferrite and 0.77-2.11 wt% C with respect to cementite, at 0 wt%C the stability ranges from 910-1394°C.

Fig.3(a ): Crystal structure of austenite is shown at right side. 11 α-ferrite: Interstitial solid solution of carbon in iron of body centred cubic crystal structure (α iron )(same as Fig. 2(a)) having solubility limit of 0.0218 wt % C at 727°C with respect to austenite. The stability of the phase ranges between low temperatures to 910°C, and solubility ranges 0.00005 wt % C at room temperature to 0.0218 wt%C at 727°C with respect to cementite. There are two morphologies can be observed under equilibrium transformation or in low under undercooling condition in low carbon plain carbon steels. These are intergranular allotriomorphs (α)(Fig. 4-7) or intragranular

idiomorphs(αI) (Fig. 4, Fig. 8)

12 Fe3C or cementite:

Interstitial intermetallic compound of C & Fe with a carbon content of 6.67 wt% and orthorhombic structure consisting of 12 iron atoms and 4 carbon atoms in the unit cell. Stability of the phase ranges from low temperatures to 1227°C

Fig.9(a): Orthorhombic crystal structure of cementite. The purple atoms represent carbon. Each carbon atom is surronded by eight iron atoms. Each iron atom is connected to three carbon atoms.

13 Limitations of equilibrium phase diagram

Fe-Fe3C equilibrium/metastable phase diagram

Stability of the phases under equilibrium condition only.

It does not give any information about other metastable phases. i.e. bainite, martensite

It does not indicate the possibilities of suppression of proeutectoid phase separation.

No information about kinetics

No information about size

No information on properties.

14 Hardenability

• This is dependent upon the chemical composition of the steel alloy. • The addition of Nickel, Chromium and Molybdenum will slow the transformation to other phases and allow more martensite to form. • Most heat treatable steels are alloys rather than plain carbon steels. Heat Treatment of Steels

• Steel = 0.06% to 1.0% carbon • Must have a carbon content of at least .6% (ideally) to heat treat. • Must heat to austenitic temperature range. • Must rapid quench to prevent formation of equilibrium products. • Basically crystal structure changes from BCC to FCC at high Temp. • The FCC can hold more carbon in solution and on rapid cooling the crystal structure wants to return to its BCC structure. It cannot due to trapped carbon atoms. The net result is a distorted crystal structure called body centered tetragonal called martensite.

Heat Treatments 800 Austenite (stable)

TE T(°C) A a)b)AnnealingQuenching P 600 c) Tempered Martensite B 400 A

Adapted from Fig. 10.22, Callister 7e.

0% 200 M + A 50% M + A 90% b) a)

-1 3 5 c) 10 10 10 10 time (s) 18 Direct Hardening – Austenitizing and quench: • Austenitizing – again taking a steel with .6% carbon or greater and heating to the austenite region. • Rapid quench to trap the carbon in the crystal structure – called martensite (BCT) • Quench requirements determined from isothermal transformation diagram (IT diagram). • Get “Through” Hardness!!!

Quenching:

• Depending on how fast steel must be quenched (from IT diagram), the heat treater will determine type of quenching required: – Water (most severe) – Oil – Molten Salt – Gas/ Air (least severe) – Many phases in between!!! Ex: add water/polymer to water reduces quench time! Adding 10% sodium hydroxide or salt will have twice the cooling rate!

Quenching Media

Four commonly used quenching media: • Brine – the fastest cooling rate • Water – moderate cooling rate • Oil – slowest cooling rate • Gas – used in automatic furnaces, usually liquid nitrogen, can be very fast cooling.

Too rapid cooling can cause cracking in complex and heavy sections. Hardening Temperatures

• The temperatures for hardening depend on the carbon content. • Plain carbon steels below 0.4% will not harden by heat treatment. • The temperature decreases from approx 820 deg C as carbon content increases from 0.4% up to 0.8%, where temperature is approx 780 deg C. • Above 0.8% the temperature remains constant at 780 deg C. Hardenability

• 1. Austenitic Grain size. The Pearlite will have an easier time forming if there is a lot of g.b. area. Hence, having a large austenitic grain size improves hardenability. • 2. Adding alloys of various kinds. This impedes the g  P reaction. TTT diagram of a molybdenum steel 0.4C 0.2Mo

After Adding 2.0% Mo Jominy Test for Hardenability

• Hardenability not the same as hardness! • Ability to form martensiteHardenability --Steels • Jominy end quench test to measure hardenability.

Adapted from Fig. 11.11, flat ground Callister 7e. (Fig. 11.11 specimen adapted from A.G. Guy, Essentials of Materials (heated to g Science, McGraw-Hill Book phase field) Rockwell C Company, New York, 1978.) 24°C water hardness tests

• Hardness versus distance from the quenched end.

Adapted from Fig. 11.12,

Callister 7e. Hardness, HRC Hardness, Distance from quenched end 26 Why Hardness Changes W/Position

• The cooling rate varies with position.

20 distance from quenched end (in) Hardness, HRC Hardness, 0 1 2 3 T(°C) 0% 600 100% Adapted from Fig. 11.13, Callister 7e. (Fig. 11.13 adapted from H. Boyer (Ed.) 400 Atlas of Isothermal Transformation and Cooling Transformation Diagrams, M(start) American Society for Metals, 1977, p. 200 376.) A  M

0 M(finish)

0.1 1 10 100 1000 Time (s) 27 Factors Which Improve Hardenability

After Adding 2.0% Mo Quenching Medium & Geometry • Effect of quenching medium:

Medium Severity of Quench Hardness air low low oil moderate moderate water high high • Effect of geometry: When surface-to-volume ratio increases: --cooling rate increases --hardness increases

Position Cooling rate Hardness center low low surface high high

29 Chapter 14 — Heat Treatment of Steels

The outer layers of the component transform first and are placed in tension, whereas the inner layers transform last and are placed in compression. With severe quenching, this leads to quench cracking. Chapter 14 — Heat Treatment of Steels

At any tempering temperature, a specific maximum amount of residual stress can be removed up to a maximum value. Tempering The brittleness of martensite makes hardened steels unsuitable for most applications. This requires the steel to be tempered by re- heating to a lower temperature to reduce the hardness and improve the toughness. This treatment converts some of the martensite to another structure called bainite. Tempering Temperatures

10.4 Direct Hardening - Selective Hardening : • Same requirements as austenitizing: – Must have sufficient carbon levels (>0.4%) – Heat to austenite region and quench • Why do? – When only desire a select region to be hardened: Knives, gears, etc. – Object to big to heat in furnace! Large casting w/ wear surface • Types: – Flame hardening, induction hardening, laser beam hardening Flame Hardening: Diffusion Hardening:

• Most Common Types: – Carburizing – Nitriding – Carbonitriding – Cyaniding Diffusion Hardening (aka Case Hardening):

• Why do? – Carbon content to low to through harden with previous processes. – Desire hardness only in select area – More controlled versus flame hardening and induction hardening. – Can get VERY hard local areas (i.e. HRC of 60 or greater) – Interstitial diffusion when tiny solute atoms diffuce into spaces of host atoms – Substitiutional diffusion when diffusion atoms to big to occupy interstitial sites – then must occupy vacancies

Diffusion Hardening:

• Requirements: – High temp (> 900 F) – Host metal must have low concentration of the diffusing species – Must be atomic suitability between diffusing species and host metal Diffusion Hardening - Carburizing:

• Pack carburizing most common: – Part surrounded by charcoal treated with activating chemical – then heated to austenite temperature. – Charcoal forms CO2 gas which reacts with excess carbon in charcoal to form CO. – CO reacts with low-carbon steel surface to form atomic carbon – The atomic carbon diffuses into the surface – Must then be quenched to get hardness!

Diffusion Hardening - Nitriding:

• Nitrogen diffused into surface being treated. Nitrogen reacts with steel to form very hard iron and alloy nitrogen compounds. • Process does not require quenching – big advantage. • The case can include a white layer which can be brittle – disadvantage • More expensive than carburizing Source of nitrogen

Reduction process: 2NH3 2N + 3H2 Chapter 14 — Heat Treatment of Steels

In conventional nitriding processes, the surface consists of a very thin, white layer and a deeper diffusion zone. Chapter 14 — Heat Treatment of Steels

Induction hardening is dependent on the frequency of the current and the shape of coils used to produce the current. Chapter 14 — Heat Treatment of Steels

During induction hardening, the component is immediately quenched using special designs that incorporate quenching into the equipment. Thermal Processing of Metals Annealing: Heat to Tanneal, then cool slowly.

• Stress Relief: Reduce • Spheroidize (steels): stress caused by: Make very soft steels for -plastic deformation good machining. Heat just

-nonuniform cooling below TE & hold for

-phase transform. 15-25 h.

• Full Anneal (steels): Types of Make soft steels for Annealing good forming by heating to get g, then cool in furnace to get coarse P. • Process Anneal: Negate effect of • Normalize (steels): cold working by Deform steel with large (recovery/ grains, then normalize recrystallization) to make grains small.

Based on discussion in Section 11.7, Callister 7e. 47 Softening and Conditioning - Annealing

• What does it do? 1. Reduce hardness 2. Remove residual stress (stress relief) 3. Improve toughness 4. Restore ductility 5. Refine grain size

Softening and Conditioning - Annealing

• Process Steps: 1. Heat material into the asutenite region (i.e. above 1600F) – rule of thumb: hold steel for one hour for each one inch of thickness 2. Slowly furnace cool the steel – DO NOT QUENCH 3. Key slow cooling allows the C to precipitate out so resulting structure is coarse pearlite with excess ferrite 4. After annealing steel is quite soft and ductile

Annealing versus Austenitizing:

• End result: One softens and the other hardens! • Both involve heating steel to austenite region. • Only difference is cooling time: – If fast (quenched) C is looked into the structure = martensite (BCT) = HARD – If slow C precipates out leading to coarse pearlite (with excess cementite of ferrite) = SOFT Normalising

• The main purpose of normalising is to obtain a structure that is uniform throughout the work piece and is free from any ‘locked up’ stresses. • Similar to annealing, but the cooling rate is accelerated by taking the work piece from the furnace and allowing it to cool in free air. • This more rapid cooling results in a finer grain structure which in turn leads to improved physical properties and improved finishes when machining. Normalising

1. Heat to Upper Critical Temperature, at which point the structure is all Austenite 2. Cool slowly in air. 3. Structure will now be fine equi-axed pearlite. 4. Used to restore the ductility of cold or hot worked materials whilst retaining other properties.

Chapter 14 — Heat Treatment of Steels

The key differences between annealing and normalizing are the holding temperatures and the rates of cooling. Spheroidizing

• Strangely, sometimes we would like the steel to be just as soft and ductile as absolutely possible. • Why, do you think? • Pearlite is not the lowest energy arrangement possible between ferrite and cementite. If heated to just below the eutectoid temperature, and left for an extended time, the pearlite layers break down, and spherical clumps of cementite are found. • These spherical clumps are hundreds or even thousands of times larger that those in TM, and spaced much further apart.  Softest, most duct. http://info.lu.farmingdale.edu/depts/met/met 205/ANNEALING.html