Welding InspectionHeat Treatments

Course Reference WIS 5

Heat Treatment of SteelsHeat Treatment of SteelsAll 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.

Heat TreatmentsHeat TreatmentsThe relevant variables for heat treatment process, which must be carefully controlled are as follows.

The heating rate

Temperature attained

The time at the attained temperature (soak time)

Cooling rate

Heat Treatment of SteelsHeat Treatment of Steels

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

Heat TreatmentsHeat TreatmentsAnnealingAnnealing

TemperatureTemperature: 920oC 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.

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.

TemperingTemperingTemperature:Temperature: 250 to 680oC 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

Heat TreatmentsHeat Treatments

Stress ReliefStress Relief

TemperatureTemperature: 550 to 680oC 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

Heat TreatmentsHeat Treatments

Pre HeatPre Heat

We can preheat metals and alloys when welding for a number 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 cooling

2. 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.

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)

Pre HeatPre Heat

Pre Heat Comparison ChartPre Heat Comparison Chart

Heat input

Com

bine

d m

ater

ial t

hick

ness

A B C D E0.43 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

00.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Section Ref 14Section Ref 14

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

Pre HeatPre Heat

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 inspector to be confident that the required temperature has been reached

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

If a gas flame is being used for preheat application the temperature should be taken form the opposite side to the heat source

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

Methods of Measuring Pre HeatMethods of Measuring Pre Heat

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

QuestionsQuestions

QU 2. What is the maximum recommended heat treatment temperature for steel weldments?, state which heat treatments may be considered when maximum toughness is required

QU 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 heat treatment process

QU 5. Which heat treatment process is required when maximum ductility is required for example for extensive cold working operations.