Characteristics of Residual StressThey are elastic in Nature

They are self equilibrating

Any disturbance like removal of material or introduction of thermal load will alter the equilibrium

Can not be eliminated but only be minimized

INFLUENCE OF RESIDUAL STRESSESBrittle FractureFatigue FailureStress Corrosion CrackingDimensional instability

INFLUENCES OF RESIDUAL STRESSESMinimize load carrying capacityResidual stress can make a defect a fracturePresence of compressive residual stress can cause buckling in thin structural members

METHODS OF MINIMIZING RESIDUAL STRESSESStress relieving AnnealingVibration stress relivingLocal Post weld heat treatmentOver stressingHammer PeeningNeedle pin Peening

Methods of Heat treatment Out Gassing or Hydrogen Back out Pre heating Post Heating Post Weld Heat Treatment

Out Gassing or Hydrogen Back outThe objective of this heat treatment is to raise the temperature of the steel and hold it to allow the hydrogen to diffuse out. The hold time varies depends upon the material and section thickness with temperature upto 450C and the time between one and 48 hours .

Pre heatingThe objective is to Reduce the rate of cooling Allow the Hydrogen to diffuse away from the weld at a faster rate Pre heating also drives away the moisture from the weld areas

Pre heatingThe objective is to Reducing the residual stresses already presentPreheating also makes it easier to weld materials having high thermal conductivity like copper and Aluminum by compensating for the heat that dissipates rapidly. Uniform preheating reduces the distortion due to welding.

Post HeatingThis is an extension of preheatPost heating is performed after welding

Post Weld Heat TreatmentIt consists of heating the weldments at a controlled rate to temperature range below the transformation temperature (AC1) and holding it at the temperature for certain time and cooling it down at a controlled rate. The lower and upper critical temperature indicate where the crystal structure of steel begins and finally completes a change from BCC structure to FCC structure.

Post weld Heat TreatmentThe objective of PWHT: Relieve the residual stresses. Improves ductility &toughness and reduce HAZ hardness Improving the machinability and dimensional stability after machining

Post weld Heat TreatmentThe objective of PWHT: diffusion of hydrogen from the joint and reduce the susceptibility to cracking. It is usually applied to higher strength carbon-Mn steels and low alloy steel

Post weld Heat TreatmentThe PWHT is usually carried-out by heating the whole assembly in a furnace or a oven. Where the assembly exceeds the size of available furnace or for welds done in field fabrication, localized heat treatment will be carried-out

Post weld Heat TreatmentDepending upon the welding condition and the knowledge of the earlier failures

Post weld Heat Treatment PWHT can have both beneficial and detrimental effects.

Post weld Heat TreatmentBenefits :Relaxation of residual stressesAdequate hydrogen removalImproved mechanical properties

Post weld Heat TreatmentDetrimental effects

Excessive or in appropriate PWHT temperature and long holding time can adversely affect properties

The influence/effect of PWHT depends upon the composition and prior mechanical processing of the base metal.

Methods of Post weld Heat TreatmentUsing permanent furnaces

Local Heat treatmentBy Induction HeatingBy Resistance Heating

Permanent FurnacesAdvantages: The temperature distribution will normally meet the requirements of current fabrication, codes or customer specificationsUniform heating of complete structure avoids inducing thermal stress as is the case with local heat treatment.

Permanent FurnacesAdvantages: Less labour utilization Cost effective and economical method and one time capital investment Minimizes delays to the production programme

Local Post weld Heat Treatment (LPWHT)

If the final fabrication is too large If the component has to be erected at site where there are no furnace available During erection of pipe works on engineering construction sites, repair work, complex configurationsNecessity for Local PWHTThis is carried-out

Control of heat loss during heating

During PWHT heat losses simultaneously taking place due to Conduction through the pipe wall on either sideConvection by air movement inside the pipeRadiation losses Precautions to be taken

Precautions to be takenControl of heat loss during heating

To make most efficient use of the heating source and to avoid excessive cooling rates weld joint and the adjacent pipe is covered with insulationIt is better to hold the insulation and the heating elements with steel band

Control of heat loss during heatingTo avoid air circulation inside the pipe which can cause cooling of the inside wall all openings in the pipe must be closed with dummy covers before starting of the cycle.Precautions to be taken

Precautions to be takenMeasurement of temperature

During the heat treatment cycle, the temperature is measured and recorded with the help of thermocouples and recorders Normally chromel-Alumel thermocouples are widely used.

Precautions to be takenMeasurement of temperature Chromel wire is non magnetic and connected to the positive terminal of the powers source. The alumel wire is magnetic and connected to the negative terminal.

Precautions to be taken The thermocouple wires are short. Hence compensating cables are used Do not interchange the electric terminals Fixing the thermocouple on the spot is very important as far as accuracy is concerned Use capacitor discharge unit for fixing thermocouplesMeasurement of temperature

Precautions to be taken During PWHT, the materials strength is considerably low. With heavy wall piping, the weight of the long span will cause permanent deformation if it is unsupported Care should be taken to ensure that no load including the self weight of the pipe is coming on the weld during heat treatment. Support of the pipe during heating

Induction HeatingPrincipleInduction heating is produced by an alternating low voltage, high current, applied to coils wrapped around the pipe. The magnetic field associated with the current passing through the coil, penetrate the pipe.A current is induced in the pipe & resistance to the induced current heats the pipe.Local Post weld Heat treatment

Induction HeatingAdvantages High rate of heating is possible Temperature can be controlled within narrow range Localized over heating is not produced The coils have longer life and so theydo not fail during heat treatmentLocal Post weld Heat treatment

Electrical Resistance HeatingDue to heavy cost of the induction hating heating, PWHT users now-a-days planning to switch over to electrical resistance heating which is costing very less as compared to induction heatingLocal Post weld Heat treatment

Electrical Resistance HeatingAdvantages Continuous uniform heating can be maintained Temperature can be adjusted quickly and accurately Heating and welding can go on simultaneously Differential heat input can be given Highly economicalLocal Post weld Heat treatment

Electrical Resistance HeatingDis-advantagesElement burn out may take place causing interruptionThe resistance heating element can get shorted with the pipe causing stray arc.The open loop equipment operates at high voltageLocal Post weld Heat treatment

Electrical Resistance HeatingTypes of heating elementsNichrome with ceramic beadsFlexible ceramic pads

Terminologies used in Local PWHT Soak Band Heat Band Soaking Temperature Soaking Time

WSBHBGCBW Maximum width of weld thickness , T - ThicknessTSB Soaking Band ( Weld and HAZ)HB Heating Band ( Weld, HAZ and Part of PM under heat source)GCB Gradient Control Band

Definitions

Terminologies used in Local PWHTSoak BandThe soak band consists if the weld metal, HAZ and a portion of the base metal adjacent to the weld being heatedThe soak band is sized to to ensure that the required volume of the metal achieves the desired temperature

Terminologies used in PWHTHeated BandThe heated band consists of the surface area (soak band plus portion of base metal) over which the heat source is applied to achieve the required temperature in the soak band and limit induced stresses in the vicinity of the weld.

Terminologies used in PWHTGradient Control BandThe gradient control band consists of the surface area over which insulation and heat sources are placed It encompass the soak band , heated band and sufficient adjacent base metal

Terminologies used in PWHTGradient Control Band serves two functions It minimizes heat losses in the heated band It ensures the axial temperature gradient within the heated band is not exceeded

Temperature gradientsA minimum axial temperature gradient temperature at a distance of 2Rt from the edge of the soak band be no less than one half of the temperature at the edge of the soak band during heating, hold and cooling

A close through thickness gradient are required to be met in newly introduced materials like X-20 and P91 steels to prevent crackingClose contact between the pipe and the heating coil is necessary to obtain close thermal gradient between OD and ID Temperature gradientsThrough thickness gradient

Parameters used in PWHTHeating RateCooling Rate Soaking Temperature Soaking Time

Parameters used in PWHTHeating Rate The rate of heating during PWHT can affect the temperature difference between the outside and inside surfaces

Parameters used in PWHTHold Temperature and Time PWHT hold temperature and time requirements are based on the type material and the thickness

Parameters used in PWHTCooling Rate The cooling rate may maintained as that of the heating rate

Case study

Experiments carried-out at WRIPipes used for trials

Material : Carbon steelOD:458 mmID:339 mmThickness:57 mmLength:2.7 mts.

Experiments carried-out at WRIPipes used for trials

Material : P91 steelOD:325 mmID:285 mmThickness:20 mmLength:660 mm

Experiments carried-out at WRIPipes used for trials

Material : P22 steelOD:253.6 mmThickness:47 mmLength:1.6 mts.

Experiments carried-out at WRITrials were carried-out with single coils and split coil methodsTrials were carried-out with single coils and split coil methods

Evaluation of Existing Practice

Experiments carried-out at WRIIn P91 the hardness measurements were carried-out using portable EUCOTIP unitInitial Hardness measured : 197 HvFinal Hardness measured : 173 HvObservation

Experiments carried-out at WRIResidual Stress Evaluation (in MPa)

Magnitude of residual stress before PWHT : -234

Magnitude of residual stress After PWHT : -73Observation

Experiments carried-out at WRIMicro structural Evaluation

Microstructure was taken using replica and electrolysis method

The grain size was changed from coarser to finer after PWHTObservation

Experiments carried-out at WRIA temperature gradient of 30 can be achieved with a lot of care and a maximum of 50 with inadequate care.Trials with split coil appears to give better resultsConclusion