G.S.Pisarenko Institute for Problems of strength National Academy of Sciences, Ukraine

G. V. Stepanov

Decrease of residual stresses in structure elements using localized thermal treatment

Kiev-2007

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Main items of presentation

• Introduction

• Examples of localized thermal treatment of structural elements -  local thermal treatment of tube elements, formation of residual stresses,

decrease of residual stresses; - thermal treatment of header- steam generator connector weldment;

optimization of thermal treatment procedures; - thermomechanical treatment of header- steam generator connector

weldment for decrease of tensile residual stresses, stresses in operation or plastic increment per loading cycle ;

- transition heating as a procedure for decrease of stress intensity factor in thick tube with edge crack

• Summary   

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Level and distribution of residual stresses in the elements of constructions significantly influence their further deformation and damage. Change of such stresses in many cases is an actual problem.

In the majority of cases undesirable residual stresses in structural components caused by prior treatment. For example, residual welding stresses are reduced using thermal treatment (TT), including heating, holding at elevated temperature for stress relaxation, and further cooling.

Uniform heating of the structural component and its holding at elevated temperature lower initial residual stresses and do not induce additional stresses on cooling in case of stable microstructure. But local heating of the structure in some cases results in plastic strains both on heating and cooling, which should be taken into consideration in the development of thermal treatment conditions.

The additional factor, which influences the stress-strained state (SSS) of the element of construction is the transient mechanical and thermal loading.

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Introduction

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Local heat treatment of the circular weld in the pipe

Local heating of the tube element on narrow circular surface to elevated temperature above some critical value (with or without holding at such temperature) initiates plastic flow which results in formation of residual stresses after cooling.

The same heat treatment of the tube with the circular weld can be used for decrease or redistribution of residual stresses.

Such heat treatment is widely used for decrease of residual stresses in welded units. The effect of treatment depend on maximum temperature, time of heating, holding or cooling and location of heated areas.

The results obtained by FEM illustrate these effects of heat treatment

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Local heat treatment of the circular weld in the pipe

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Local heat treatment of the circular weld in the tube

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Fig. 1.1. Schema of heating the section of the pipe 

As the illustration is presented the change in the residual stress distribution, caused by thermal treatment (TT-1) using heating to 6000C during 1000s and the subsequent slow cooling during 9000s in the region 1 of steel tube with a diameter of 1000 x 50 mm, 1300mm

the length (Fig.1.2, curves 1 and 2)

Reduction in initial residual stresses (from welding or stresses induced by local heat treatment) is obtained by TT -2 (heat treatment of the annular regions of pipe 2, on some distance from the region 1 of

initial residual stresses .

It was assumed that inelastic deformation of the material was determined by the isotropic strain hardening equation accounting for the linear temperature effect :

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Local heat treatment of the circular weld in the tube

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-200

-100

0

100

200

300

400

0 0.2 0.4 0.6y, м

, MPa

2

1

3

4

Fig.1.2. Distribution generating on the internal surface of the pipe of axial (curves 1, 2) and circumferential (curves 3,4) residual stresses before (curves 1, 3) and after (curves 2, 4) heat treatment TT-2 

E=200(150) ГПа=0,3; Cv=3,5 МДж/м3; k=40Дж/(м.К.с); =1,25.10-5 К-1; =30 Дж/(м.К.с); Y=400(250) МПа.

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Local heat treatment of the circular weld in the tube

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-400

-200

0

200

400

600

0 5000 10000 15000 20000t , sec

, МПа

Fig.1.3. Change in the time of axial (curve 1) and circumferential (curve 2) stresses at point M on internal generating line of the pipe.

1

2

0

100

200

300

400

500

600

0 5000 10000 15000 20000t , sec

T , 0CFig.1.4. Change in the time of temperature at point M of pipe at the stage of the formation of residual stresses in the region 1 (by TT-1) and the subsequent heat treatment by heating in the region 2 (by TT-2)

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Local heat treatment of the circular weld in the tube

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Dynamic loading, high strain rate #6

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The redistribution of residual stresses in the region 1 after TT-2 is characterized by the lowered (almost in two times) level of maximum residual tensile stresses on the internal surface of the pipe. Plastic deformation is limited by the short period of the action of the elevated temperature (less than 1000 s in the our calculations).

A change in the residual stresses caused by TT-2 depends on the distance of the heating area from the placement of initial residual stresses , width of heating area and level of temperature in TT-2

Analogous effect (plastic deformation) causes the pulse loading (by high explosion or by impulse electromagnetic field) over the annular surfaces of pipe at some distance from weld. In this case the development of inelastic deformation is the result of a short-time pressure, heating on the annular surface of pipe and its inertial radial motion.

 

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Local heat treatment of the circular weld joint in tubes

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Heat treatment of the weldment

Fig. 2.1. Three-dimensional calculation scheme of the steam generator.

Fig. 2.2. Scheme of the weldment and mounting of heating elements: (1) steam generator vessel with thermalinsulation; (2) connector; (3) pocket; (4) thermal insulation; (5) weld joint; (6) heating elements; (7) header;(8) thermo insulating plugs.

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The header – PGV-1000 steam generator connector weldment (Fig. 2.1) in

WWER nuclear power reactors according to the calculations is one

of the most loaded. Therefore the decrease of the residual stresses in it is

especially urgent.

 

The mechanism of the development of residual stresses in the weldment is

analogous to that in the pipe with the circular weld. The local heat

treatment of the weldment after it repair for decrease of residual welding

stresses creates after cooling high level of residual tensile stresses in

the "pocket" near its bottom  

Calculations of the stress-strain state of the weldment illustrate the effects

of local thermal treatment parameters (local heating, holding, and cooling) on the residual stress-strain state of this structure.

 

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Heat treatment of the weldment

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According to calculations resulting stress state of the weldment after heat treatment is characterized by the significant level of the residual stresses, initiated by cooling after holding at elevated temperatures.

The distribution of axial surface on the pocket surface has a maximum in the region of fillet at the height of approximately 20 mm from the bottom of pocket (Fig. 2.3, a dependence 1).

Selection of optimal parameters of heat treatment can essentially decrease maximum of residual stresses as illustrated by dependences 2 and 3 on Fig. 2.3)

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Heat treatment of the weldment

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Fig. 2.3. Distribution of residual circumferential (z) stresses on the pocket surface on the connector side (two-dimensional calculation results): (1) “reference” thermal treatment; (2) thermal treatment alternative with the boundary conditions being convection in the pocket; (3) thermal treatment alternative with additional heating of the thickened portion of the connector up to 500 0C..

0.00E+00

5.00E+07

1.00E+08

1.50E+08

2.00E+08

0 0.02 0.04 0.06 0.08 0.1 0.12

distance from pocket bottom, m

stre

ss, P

a 1

2

3

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Heat treatment of the weldment

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Conclusions.

Results of modeling of heat treatment process clearly show the influence of process parameters on residual stress-strain state in the weldment.

Analysis of 2-D modeling results demonstrated that a considerable reduction in the level of residual stresses induced by thermal treatment could be achieved by optimal choice of heat treatment parameters. The results 3-D modeling confirm this resume and are used in practice.

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Heat treatment of the weldment

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Thermomechanical treatment of weldment

Additional reduction in the residual stresses in the repaired the header –

PGV-1000 steam generator connector, formed after its heat treatment, can be

obtained by its additional thermomechanical treatment (loading the

generator SG housing by pressure, local heating of the connector and cooling)

which creates compressive stresses in the region of fillet in the pocket.

The results of numerical simulation of one variant of thermomechanical

treatment for creation of residual compressive stresses near the pocket bottom

are presented below. Basic task of the given calculations is estimation of

thermomechanical treatment effect on residual stress-strain state in the

weldment.

The simplified axisymmetric FE model (Fig. 3.1) of the connector was used

in the calculations.

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Fig.3.1. Two-dimensional model of the weldment used for simulation of thermomechanical treatment

All internal surfaces of steam generator shell are loaded with pressure, the action of the remote part of shell it is simulated by normal stress in its wall. Radius of spherical shell is accepted equal to the doubled housing radius to account for different curvature of housing in axial and circular direction.

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Thermomechanical treatment of weldment

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Fig.3.2. Stress distribution along the surface in the pocket after loading of the steam generator vessel by pressure of 6,0 MPa (1-st stage of treatment before heating).

0

50

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150

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300

0 20 40 60 80mm

MP

aSY

SZ

S1(и)

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Thermomechanical treatment of weldment

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Fig.3.3. Distribution of stresses and temperature on the generatrix in the pocket after the 2-nd stage of treatment (heating on annular region at 6 MPa pressure in the second circuit).

0

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0 20 40 60 80

SY

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S1

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120

160

0 20 40 60 80

TEMP

Fig. 3.5. Comparison of the first principal stress before and after thermal treatment at a

pressure 6 MPa in the secondary circuit

Fig. 3.4. . Distribution of stresses on the generatrix in the pocket after the 3-rd stage of treatment (cooling after heating at 6 MPa pressure of in the second circuit )

-300

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Thermomechanical treatment of weldment

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Fig. 3.6. Distribution of stresses along the generating line in the pocket after 3-rd stage (heating of annular region at 6MPa pressure in the secondary circuit, cooling with the retention of pressure, complete unloading).

-400

-300

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0 20 40 60 80

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Thermomechanical treatment of weldment

Compression stresses in the area of damage !!!

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Conclusions.

It follows from the analysis of the simulation results, executed with the use of the 2-dimensional model of weldment, that its thermomechanical treatment (including the heating its thickened part at a sufficient pressure in the SG housing) creates after complete unloading the field of the residual compressive stresses in the region of fillet in the pocket.

Value of the residual compressive stresses and their distribution depend on regime of thermomechanical treatment (pressure, maximum heating temperature, and its duration) and disposition of heating areas and arias of cooling.

The question about the applicability of thermomechanical treatment of weldment should be validated using 3-D model of weldment.

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Thermomechanical treatment of weldment

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Transient heating of tick tube with edge crack

The nonstationary nature of load influences on SSS in the structure elements. An example of such loading is appears the regime of load, caused by the rapid cooling of the internal surface of the thick-walled shell (so-called thermo-shock), which influence critical stress intensity factor in the crack tip.

The rapid heating of the external surface of the thick-walled shell has

analogous influence on SSS near the edge crack tip.

The rough estimation of SSS change near the edge crack tip on the internal

surface of thick-walled pipe during the rapid heating its outer surface is

presented below.

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Fig.4.1. Schema of heating of tube with the axial crack)

Calculations are carried out for plane strain state of thick-walled pipe (inside diameter of 4 m, the thickness of wall is 0.2 m)with the edge crack of 25 mm depth .

Material of tube – steel,

the modulus of elasticity - 200 GPa,

yield stress - 400 MPa,

strain hardening modulus - 400 MPa;

the coefficient of

thermal conductivity - 80 W/(m.K),

heat capacity - 450 ( J / (kg* K)).

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Transient heating of tick tube with edge crack

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0.0

0.1

0.2

0.3

0.4

1.8 1.85 1.9 1.95 2r , м

py , %

-300

-200

-100

0

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300

1.8 1.85 1.9 1.95 2r , м

y , МРа

Fig. 4.3. Distribution in the radial direction of circumferential stress and plastic strain in the plane of the edge crack (of 25 mm depth) after heating during 4000 s (bright curves) and next cooling during 12000 s, (dark curves).

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Transient heating of tick tube with edge crack

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Conclusions.

Transient heating of the external surface of the thick-walled cylindrical tube to the temperature that is higher determined level (sufficient for the initiation of inelastic deformation forms in the tube after cooling) creates the field of the residual compression stresses on itsinternal surface and of tensile stresses ion on the external surface.

Plastic flow in the area near the edge crack tip in the period of tensile stresses in this area leads to the crack blunting and strain hardening. As a result compressive stresses in the area of crack tip is created after cooling.

By corresponding change in the heating rate on the external surface of the tube and cooling on its internal surface required residual stress change in the region of the crack tip can be provided .

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Transient heating of tick tube with edge crack

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Summary

Thermomechanical treatment of structures, based at the local transient heating/cooling coupled with mechanical loading is a flexible technological process, ensuring assigned change in the fields of the residual stresses .

The inertia and elastic wave effects in the structures during short-time processes of loading (by explosion, by pulse electromagnetic fields) have essential influence on the field of residual stresses;

The transient heating of the thick-walled structure elements with the defects can be used for the creation of compression stresses in the region the of crack tip for increase of fracture toughness of elements/