Introduction to the methods thermal annealing PWHT (Post Weld Heat Treatment)

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Welding

Wednesday, April 6, 2016

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Introduction to the methods thermal annealing PWHT (Post Weld Heat Treatment)

Post weld heat treatments
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thermal annealing method using inductor

thermal annealing equipment by inductor

I. The purpose:

- Improved mechanical properties of materials

- Changing of the material microstructure

- Reduced residual stress

- Changes in chemical composition of the material

PWHT is used to change the properties of the weld metal, controlling the formation of crystal structures.

II. PWHT method:

- PWHT by kiln.
- Mayso electronic / electric heating cushion

- electromagnetic induction
III. The advantages and disadvantages:

Post weld heat treatments

- Advantage :

  Capable of easily changed temperature

  Capable of keeping constantly maintaining heat

- Disadvantages :

The components may burn out or arcing in the heating system.

HF local heat treatment

- Advantage:

  High speed heating

  Capable of heating a narrow location

- Disadvantages :

  High cost

  Large equipment, difficult to move.

PWHT general control shall ensure that:

- Equipment as prescribed

- Temperature control equipment in good condition

- Process according to regulations, being used for example :

• Application Method
• The rate of heating and cooling rate
• Maximum temperature
• Measured temperature soaking time (and calibration)

- DOCUMENTS AND RECORDS

Heat treatment Cycle

· Requirements:

- Control heating rate to avoid thermal expansion uneven

- Controlling time temperatures dipping to balance.

- Controlling the temperature gradient - NO flames directly collide!

- Control of flue gas to reduce the rate.

- Control the speed of cooling to prevent formation of brittle structures.

=> Many metals have been heat treated before and after welding.

The function of the test is to ensure that the post weld given exactly matching the specification or in the details provided.

The heat treatment types available:

· Preheat

· Anneal

· Normalising

· Quench hardening

· Temperature

· Reduce stress

• The question arises:

The main reason to implement PWHT (in steel welds) What?

Answer:

To reduce residual stresses

Sub-questions:

benefits to reduce residual stress is what?

Additional replies:

To improve the resistance of the weld brittle destruction.

Post weld heat treatment

IV. procedure PWHT, basic requirements:

The maximum ratio is usually baked between 300 or 400 ° C

- High temperature should be avoided. gradients that can cause distortion / cracking

- The percentage depends on the thickness baked but typically up to ~ 200 ° C / h

Temperature 'Soak' depends on the type of steel - often prescribed by the standard (~ 550 ~ 750 ° C)

- Time 'Soak' minimum need to be sure, and the whole object / thickness reaches a specified temperature.

- Standards often define 1h each involving a maximum of 25mm. overall thickness.

- The percentage of the maximum cooling down 400 or 300 usually ° C - for the same reasons as thermal control rate.