Presentation on PWHT & Stress Relieving

7/30/2019 Presentation on PWHT & Stress Relieving

http://slidepdf.com/reader/full/presentation-on-pwht-stress-relieving 1/23



What are Residual stresses :

• Developed in the weldment mainly due to the restraint provided to

the shrinkage strains.

• In thin materials, the shrinkage strains are accommodated by

distortion of the plates.

• However in thicker sections and in thin sections clamped externally,the shrinkage strains manifest themselves in the form of RESIDUAL

STRESSES.

• Since welding is local melting, the local strains are unable to distort

thick sections of materials and therefore residual stresses are created.

• These residual stresses can reach a peak value close to the yield point

of the material. Thus in steels strengthened by alloying elements, the

magnitude of residual stresses are also high.



Heat treatment of welds

Preheating :

Heating of the parent metal around the area to be welded before welding and

holding the temperature until completion of welding.

Purpose :

1. To minimize the risk of cracking in the weld and heat affected zone of steels.

- by reducing the rate of cooling thereby reducing the formation of hard

martensite.

- by allowing the hydrogen to diffuse away from the weld at a faster rate.

- by reducing the residual stresses induced in the weld region2. Makes it easier to weld metals having high thermal conductivity like Cu and Al reducing

heat transfer rate.

3. Reduces the distortion due to welding



Preheating :

Method :

By

• Electric Resistance

• By Induction Heaters

• By Oxy-acetylene or Oxy-Propane gas mixture with neutral flame.

6 times thickness on both sides of the joint (min. 100 mm.)

Preheat to 50 deg.C. if ambient is <10 deg.C.



Post heating :

Heat applied to the joint immediately after completion of welding before the joint

cools down to ambient temperature.

Purpose :

To ensure a uniform temp. over the whole joint while cooling and allow hydrogen

diffusion out of the weldment.



Post-Weld heat treatment :

• After completion of welding.

• Heating the weldment at a controlled rate to temperature range specified.• Holding it at the temperature for certain time

• Cooling it down at a controlled rate.

Mechanism :

By heating the weld zone, the yield strength of the weld zone is lowered andrealization of the residual stresses occur by plastic flow combined with asmall but real creep effect.

•Carried out in furnaces , or by resistance or induction equipment.

•Temperature measurement by thermocouples fixed near the joint.• Recording by temp. recorder instrument.•Thermocouples minimum

1 upto 6” dia,2 up to 10” dia,3 up to 12” dia and above.



Purpose :

To achieve :

1. Tempering of weld and HAZ which improve ductility and

toughness and reduce HAZ hardness.

2. Relieving the residual stresses to safe levels reducing the

chances of premature failure by brittle fracture, fatigue or

stress corrosion.

3. Improving the machinability and dimensional stability after

machining.



FOR CS PIPING:

Pre Heat : 50 deg.c if ambient is less than 5 deg.c100 to 150 deg.c if t is >25 mm

Post Heat: 250 deg.c if t is >30 mm for 30 minutes

PWHT:

Required only when thk. is over 19mm.

Shall be heated at a rate of 200 to 350 deg.c/hr.

Shall be held at soaking temp. of 600 to 630 deg.c

for 1 hr. for every 25 mm thk. .( min 1 hr.)

Cooled down to 400 deg.c at controlled rate.

Then further cooled down under insulation.



FOR C, 0.5 Mo PIPING:

Pre Heat : 50 deg.c if ambient is less than 5 deg.c100 to 150 deg.c if thk is >25 mm

Post Heat: 250 deg.c if thk is >25 mm for 30 minutes

PWHT:

Mandatory when thk. is over 19mm.


Shall be held at soaking temp. of 630 to 660 deg.cfor 1 hr. for every 25 mm thk.( min 1 hr.)





FOR Cr Mo AS PIPING:

Pre Heat : Required for all thickness.

200 deg.c for Cr < 2%250 deg.c for Cr > 2%

Post Heat: 300 to 350 deg.c for 30 minutes for

t > 20mm if Cr = 2%

t > 12mm if Cr = 2 to 6%t > 6 mm if Cr > 6%

PWHT:

Mandatory for thk. > 13mm.



for 1 hr. for every 25 mm thk.(min 2 hrs.)

Cooled down to 400 deg.c at controlled rate of 150 to 200 deg.c/hr.

The hardness in HAZ shall be 225 BHN for Cr<2%

240 BHN for Cr>2%



FOR LTCS PIPING:

Pre Heat : 50 deg.c if ambient is less than 5 deg.c

100 to 150 deg.c if t is >25 mm

Post Heat: 250 deg.c if t is >30 mm for 30 minutes

PWHT:

Required only when

Design temp.(Deg. C) Wall thk.

-29 to – 35 t > 15 mm

-35 and below t > 10 mm



for 1 hr. for every 25 mm thk.( min 1 hr.)





Classification of weld joint discontinuities :

Welding process related : 1. Misalignment

2. Undercut

3. Concavity or Convexity.

4. Burn-through

5. Incomplete penetration

6. Lack of fusion

7. Shrinkage

8. Arc strikes

9. Slag inclusions10.Tungsten inclusions

11.Spatter

12.Porosity

13.Excess penetration / Overlap

Metallurgical related :

1. Cracks or fissures

a. Hot cracks

b. Cold or delayed cracks

c. Reheat, Stress-relief cracking

d. Lamellar tearing

e. Hydrogen induced cracking



Hot Cracking :

In general, the common factors that promote cracking are .,

1. Microstructure

2. Chemical composition

3. Rate of cooling

• Cracks initiate in a solidifying metal under the influence of low meltingconstituents like S, P,B and Se are termed as hot cracks.

• Occurs in weldment during solidification before the joint is fully

formed.

• Presence of Mn in excess quantities in base metal can prevent Hot

Cracking.



Cold Cracking :

•Cracks that occur in weldments after completion of solidification of steel are.

Observed more predominantly in Low Alloy Steels.

•Cracks initiate in a weldment under the combined influence of microstructure,

hydrogen content and residual stresses are termed as cold cracks.

•Also known by several names such as ‘Delayed cracking’, ‘HAZ cracking’.

• Cold cracks are observed in the HAZ of the weldment.

•Can be prevented by

•Pre heating•Suitable welding technique

•Slow cooling of weldment ( Post Heating)

•Reducing external restraint on the weld joint.



Stress relief cracking :

Also known as ‘Reheat cracking’ is observed in creep resistant steels

containing high Molybdenum and Vanadium.

Crack appears during the stress-relief treatment given to the weldment and

is found in the HAZ.

Cracking is aggravated by the presence of high residual stresses, high

stress concentration due to notches introduced by welding and high

restraints in the weldments.

The only solution to mitigate this is

•Post heating & maintaining inter-pass temperature.



Lamellar Tearing:

• Non metallic inclusions like sulphides, silicates etc. present in the ingots

convert into lamellar Inclusions parallel to the surface of the plate while

rolling.

• During welding these defects generate cracks.

• Proper joint design can reduce lamellar tearing.

• Basically a defect in the manufacturing process and can not be mitigated.

The only solution is Ultrasonic testing before taking up the material for

further processing.



Hydrogen induced cracking:

• Occurs under the bead or HAZ caused by dissolution of Hydrogen

during welding.

• During welding, at high temperatures, Hydrogen – predominantly from

electrode coating dissolves into the weld metal diffuse into the HAZ and

base metal.

• During the cooling state of the weldment, the Hydrogen tries to escape.

• With further fall in temperature accumulated hydrogen returning to its

molecular state, thereby exerting enormous pressure. This leads to failure by cracking.

• Low hydrogen electrodes, proper preheating, post heating can eliminate

hydrogen cracking.



CARBON EQUIVALENT :

CE = C%+ Mn % + Cr% + Mo%+V% + Ni%+Cu%

6 5 15

The CE shall preferably be less than 0.42 for obtaining a

crack free weld joint.



HEAT TREATMENT

Process adopted to improve one or more properties as required by

the service conditions of the steel structures, machine

components, equipment, pipe lines etc.

To relieve stresses and improve machinability.

To improve mechanical properties like strength, hardness,

ductility, shock resistance etc.

To modify the structure of the material to improve elastic and

magnetic properties.

To improve resistance to heat, corrosion and wear.

To change the grain size.

To stabilize the structure against dimensional changes.



ANNEALING:

Heating to temperatures below re-crystallization temp. ( 760 to 970 deg.c.

depending on alloying element), holding for long enough time to enable the

internal changes to take place, and finally cooling slowly at a suitable rate in the

furnace.

•Reduces hardness

•Improves machinability

•Increases ductility

•Produces desired microstructure to obtain desired mechanical and physical

properties.



NORMALIZING:

Heating up to a temperature above re-crystallization temp., keeping there for 15

minutes and cooling in open air.

Improves machinability.

Improves tensile strength.

Removes strains caused by cold working processes like

hammering, rolling and bending.

HARDENING:

Heating to a temperature above re-crystallization temp., allowed to remain for a

stipulate period and then cooling rapidly to room temperature by quenching. It

Increases hardness

Raises strength

Increases wear resistance

But makes the metal brittle



TEMPERING:

When a steel has been hardened, it is very hard and brittle with high residual

stresses. By tempering the material gets softened and improves toughness along

with ductility. This process involves reheating to temp. below critical temp. for a

certain period of time and then cooling in still air.

CASE HARDENING:

It is a surface heat treatment process that develops hard, wear resistant surface

with a soft tough core. There are 5 methods

Nitriding : - By exposing to NH3 gas,

- leads to formation of nitrides of Al, Cr, Mo.

Carburising : - Absorption of C on on exposure to CO.

Cyaniding : - Generally Sodium Cyanide bath is used.

- Carbon and Nitrogen is absorbed using Cyanides.

Induction Hardening: - Heating with High frequency eddy currents.

Flame hardening: - By oxy-acetylent flame.



Thank You

Documents

Presentation on PWHT & Stress Relieving