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WI 3.1

M.Rogers

Heat Treatments Heat Treatments




WI 3.1

M.Rogers





WI 3.1

M.Rogers

All heat treatments are basically cycles of three elements

§ Heating

§ Holding (soaking)

§ Cooling

Post weld heat treatments are used to change the properties of

the weld metal, controlling the formation of structures

Pre heat treatments are used basically to increase weldabilty,

control expansion and contraction forces during welding.





WI 3.1

M.Rogers

Heat Treatments

Welding InspectionWelding Inspection

Course notes section reference 18




WI 3.1

M.Rogers

The Iron Carbon DiagramThe Iron Carbon Diagram

0.2 0.4

AUSTENITE

FERRITE +

CEMENTITE

AUSTENITE +

CEMENTITE

0.8

T E M P

o C

CARBON %

723oC

910oC Upper Critical Point

Lower Critical Point

Body Centred Cubic

FERRITE

a Alpha

Face Centred Cubic

AUSTENITE

? Gamma




WI 3.1

M.Rogers

The basic heat treatments are:

§ Annealing

§ Hardening

§ Stress relieving

§ Normalising

§ Tempering

§ Pre-heating

The method of applying the heat to the material may vary from

procedure to procedure. e.g flame, electrical resistance and

furnaces



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WI 3.1

M.Rogers

AnnealingAnnealing

§§

TemperatureTemperature: 920o

C hold for sufficient time§§ CoolingCooling: Furnace cool to 650oC then cool down in air

§§ ResultResult: Produces a very soft, low hardness material

suitable for cold working or machining operations.

Decreases toughness and lowers yield stress

NormalisingNormalising

§§ Temperature:Temperature: 920oC hold for sufficient time

§ Cooling: Slow cool in air

§ Result: Relieves internal stresses improves mechanical

properties and increases toughness, grain refinement.




WI 3.1

M.Rogers

§ The heating rate

§ Temperature attained

§ The time at the attained

temperature (soak time)

§ Cooling rate

The relevant variables for heat treatment process, which

must be carefully controlled are as follows.




WI 3.1

M.Rogers

Stress ReliefStress Relief

§§

TemperatureTemperature: 550 to 680o

C hold for sufficient time§§ CoolingCooling: Slow cool in air.

§§ ResultResult: Relieves residual stresses improves mechanical

properties and increases toughness, may also be used to

reduce hydrogen levels

Post Hydrogen ReleasePost Hydrogen Release

§§ Temperature:Temperature: Approximately 250oC hold up to 10 hours

§§ CoolingCooling: Slow cool in air

§§ ResultResult: Relieves residual hydrogen




WI 3.1

M.Rogers

PreheatPreheat

We can preheat metals and alloys when welding for anumber of reasons. Primarily we use most pre-heats to

achieve one or more of the following:

1. To control the structure of the weld metal and HAZ on

cooling2. To improve the diffusion of gas molecules through an

atomic structure.

3. To control the effects of expansion and contraction.

Preheat controls the formation of un-desirable microstructures that

are produced from rapid cooling of certain types of steels.

Martensite is an undesirable grain structure very hard and brittle it

is produced by rapid cooling form the austenite region.




WI 3.1

M.Rogers

Preheat temperatures are arrived by taking into

consideration the following:

§The heat input

§The carbon equivalent (CE )

§The combined material thickness

§The hydrogen scale required (A, B, C, D, E)




WI 3.1

M.Rogers

Quench HardenQuench Harden

§§ TemperatureTemperature: 920oC hold for sufficient time

§§ CoolingCooling: Fast cool, quench in water, oil.

§§ ResultResult: Hardens carbon steels. After this

treatment the material is very hard and highly

stressed. The material is brittle but with high

tensile strength.

TemperingTempering

§§ Temperature:Temperature: 250 to 680oC hold for sufficient

time

§§ CoolingCooling: Slow cool in air

§§ ResultResult: Relieves residual stresses improvesmechanical properties and increases

toughness, may also be used to reducehydrogen levels




WI 3.1

M.Rogers

Heat input

C o m b

i n e d m a t e r i a l t h i c k n e s s

A B C D E

0.43 0.45 0.47 0.53 0.55

175 150 125 100 75 50 20 0

200

180

160

140

120

100

80

60

40

20

0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Preheat Comparison Chart Preheat Comparison Chart




WI 3.1

M.Rogers

§ Slows down the cooling rate, which reduces the risk of

hardening

§ Allows absorbed hydrogen a better opportunity of

diffusing out, thereby reducing the risk of cracking

§ Removes moisture from the material being welded

§ Improves overall fusion characteristics

§ Lowers stresses between the weld metal and parent

material by ensuring a more uniform expansion and

contraction

Advantages of preheat




WI 3.1

M.Rogers

§ Temperature indicating crayons (Tempil sticks®)

§ Thermocouples or touch pyrometers

§ At intervals along of around the joint to be welded

§ The number of measurements taken must allow the inspectorto be confident that the required temperature has been

reached

§ In certain cases the preheat must be maintained a certaindistance back from the joint faces

§ If a gas flame is being used for preheat application thetemperature should be taken form the opposite side to theheat source

§ If this is not possible time must be allowed before taking thepreheat temperature e.g 2 mins for 25mm thickness

Methods of Measuring Preheat Methods of Measuring Preheat



WI 3.1

M.Rogers


§The heat input Increased

Pre-heat Requirement

Reduced

§Carbon Equivalent Increased Increased

§Combined material thickness Increased Increased

§Hydrogen content Increased Increased

M R



WI 3.1

M.Rogers

Any QuestionsAny Questions

M R


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WI 3.1

M.Rogers

QU 1. How can the levels of hardness be controlled in the

HAZ?

QU 2. What is the maximum recommended heat treatment

temperature for steel weldments?, state which heat

treatments may be considered when maximum toughness

is requiredQU 3. What are the four main considerations for determining

preheat temperatures, and as a welding inspector which

factors require inspection when applying pre-heat to a

carbon steel joint to be welded?

QU 4. What factors need to be checked/controlled during a heattreatment process

QU 5. Which heat treatment process is required when maximum

ductility is required for example for extensive cold working

operations.

QuestionsQuestions

Heat Treatments

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