Chapter 6: Heat Treatment Flashcards

1
Q

definition of heat treatment

A

combination of heating & cooling operations applied to a metal or alloy in the solid state to produce desired properties

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2
Q

general heat treatment cycle parts

A

1) heating
2) holding at temperature (soaking)
3) cooling

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3
Q

what can happen to a steel if the heating rate is too fast?

A

steel that is highly stressed can:
distort & crack

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4
Q

benefits of pre-heating

A

good for treating steels with:
variation on section thickness
low thermal conductivity

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5
Q

what is the objective of holding steel at heat-treating temperature?

A

ensure uniformity of temperature throughout its entire volume

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6
Q

factors that affect cooling rate

A

size of workpiece
quenching media used

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7
Q

what does unstable austenite mean?

A

the austenite can change into any other phase

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8
Q

how does alloying elements affect the TTT diagram? and which element does not?

A

alloying elements shift the TTT curve to the right, martensite will form at slower cooling rate

steel becomes easier to harden (less drastic quenching can be used)

cobalt does not make it easier

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9
Q

how are austenitising temperature affected by alloying elements?

A

raised by carbide formers (Cr, tungsten)
lowered by Ni, Mn

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10
Q

what does alloying elements do to Ms and Mf temperatures? and what will happen?

A

Ms and Mf temperature will be lowered

austenite will not be able to transform fully in to martensite at rtp, leading to retained austenite

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11
Q

how does complex carbide affect austenite transformation?

A

elements like (tungsten, titanium, vanadium) for very strong, stable complex carbide

which are difficult to dissolve in austenite

leading to higher austenitising temperature

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12
Q

causes of retained austenite

A

presence of large amount of alloying elements in austenite will lower Ms and Mf temperature

if Mf is below rtp, not all austenite will form into martensite

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13
Q

effects on properties by retained austenite

A

1) reduce overall hardness of steel (austenite is softer than martensite)
2) spontaneous cracking may occur (when austenite transform into martensite)
3) steel is dimensionally unstable (localised deformation may occur when RA transform into martensite)

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14
Q

ways to eliminate RA?

A

double tempering

sub-zero treatment

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15
Q

what is double tempering method to eliminate RA?

A

1st tempering:
tempers martensite
cooling from temperating temp transform some RA to martensite

2nd tempering:
temper newly formed martensite
may be repeated if needed

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16
Q

what is sub-zero treatment method to eliminate RA?

A

refrigerating the steel, temp as low as -98°C
allowed to warm up to rtp afterwards

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17
Q

how does distortion occur in heat treatment?

A

when steel is cooled, thermal contraction occurs, surface cools faster than the core

stresses are produced by the non-uniformed contraction and are relieved by plastic deformation resulting in distortion

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18
Q

how does cracking occur in heat treatment?

A

temperature below Ms, surface is martensitic and brittle, stresses generated will lead to crack

19
Q

measures to prevent distortion or cracking

A

1) ensure component is well support in furnance
2) remove stress raisers in design
3) quenching rate not too drastic
4) ensure steel is selected correctly

20
Q

martempering process

A

1) quench in salt or metal bath above temp Ms
2) hold for short time to equalise the surface and core temp
3) cool through Ms to Mf
4) temper the steel

21
Q

limitations of martemping

A

only for small components of alloy steels

22
Q

austempering process

A

1) quench in salt or metal bath above temp Ms
2) hold for sufficient time to allow isothermal transformation of austenite to bainite
3) air cool steel

23
Q

limitations of austempering

A

only for small components

24
Q

when to use what kind of heat treatment?

A

components need to withstand loading:
use through hardening

components need to withstand wear and fatigue (surface properties):
use surface hardening

25
Q

purpose of surface heat treatment

A

improve wear & fatigue resistance

26
Q

how is surface obtained?

A

diffusing an element into the steel

element must be soluble in the the steel at the temp used

27
Q

what happens when an element is diffused into the steel surface?

A
  • surface volume increase
  • residual compressive stress induced on surface
28
Q

types of surface treatment process and their description

A

1) thermochemical - surface chemistry of steel is modified by adding carbon and/or nitrogen

2) selective surface hardening process - only the surface is hardened and the core is left in pre-treated condition

29
Q

where is carburising applied and the process of it

A

applied to low carbon steel (<0.2%C)

process:
- heat steel to temp above 880°C
- quench to develop hard martensitic case (>0.8%C)

30
Q

when is gas carburising used and what is the process?

A

used for treating large quantities of small compoments

process:
heat component to carburising temp for 3-4hrs
- controlled atmosphere consists of mixture of hydrocarbon gases

2CO –> C + CO2

31
Q

advantages and disadvantages of gas carburising

A

adv:
- neat and clean
- carbon content of surface layers can be controlled accurately and easily

disadv:
- more complex furnace or equipment
- high skilled work

32
Q

when is gas nitriding used? and what is the process and reaction equation?

A

used for hardening finish-machined and heat-treated steels

process:
heat component at 500°C for 2-4 days in a furnace with ammonia gas being circulated

2NH3 = 2N + 3H2

33
Q

advtanges & disadvantages of gas nitiriding

A

adv:
- very hard case obtained
- good corrosion resistance
- clean, simple to use

disadv:
- service temp cannot exceed 500°C
- nitriding reduces toughness

34
Q

which steels are carbonitriding used for?

A

mild steel and low carbon steel

35
Q

process of carbonitriding

A
  • C & N are introduced to the surface together
  • heat steel to 830°C for up to 3hrs
  • quench in oil
36
Q

advantages of carbonitriding

A
  • N stabilises austenite (austenite transform slowly, no distortion)
  • amount of C & N can be varied
37
Q

how is selective hardening produced?

A

rapid application of heat to steel surface for austenitisation, followed by quenching

38
Q

steels used for selective hardening

A

steels with 0.3 - 0.6%C

steels are heat treated for core properties before processing

39
Q

flame hardening process

A

selected area is heating by torch to red hot and then quenched in water

40
Q

case depth from flame hardening depends on?

A

gas mixture used

burner distance - short burner distance means thicker case

41
Q

what is induction hardening?

A

selected area is to be surrounded by an inductor coil, high freq current is pass through the coil

42
Q

induction hardening process

A
  • area surrounded by inductor coil
  • high freq current pass through the coil
  • induces eddy currents, heating it up
  • surface is then quenched with water pressure jets
43
Q

pattern of heating of induction hardening depends on?

A

shape of coil
number of turns in the coil
operating frequency
power input

44
Q

what does case depth depend on in induction hardening? and what is the approximate depth?

A

current frequency
higher freq = thinner case

approx. 7-8mm can be obtained by induction hardening