residual stress on components of manufacturing process

8/3/2019 Residual Stress on Components of Manufacturing Process

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RESIDUAL STRESS ON COMPONENTS OF

MANUFACTURING PROCESS

V.PIRANESH*V.SUBHAN**

.* Student 2nd

year, Dept. of Mechanical Engineering, Kongu Engineering

College, Erode. [email protected]

(9597848518)

.** Student 2nd year, Dept. of Mechanical Engineering, Kongu EngineeringCollege, Erode.

[email protected](9790099442)
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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ABSTRACT

Any manufacturing process that changes the shape of a solid, or where

severe temperature gradient exists during the process, causes residual

stresses. By their very nature, process that change the shape of a solid

cause non uniform plastic deformation in the solid, which leads to residual

stress. Also, process that produce high thermal gradients in a solid often

lead to residual stress. Furthermore, process that induces localized phase

changes produce residual stresses. The residual stresses caused by

manufacturing process usually show very steep residual stress to distance

gradients. Compressive residual stress has a beneficial effect on fatigue

life, crack propagation and stress corrosion of materials. This thesis work

is to study the residual stresses on components of manufacturing (bending

and welding) processes.

INTRODUCTION

A state of stress that may exist in the bulk of the material without application of an

external load (including gravity) or other sources of stress, such as thermal gradient, is

called a residual or internal stress. Residual stresses can be classified into three kinds


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according to the distance or range over which they can be observed. The first type of

residual stress, termed macroscopic, is long-range in nature, extending over at leastseveral grains of the materials, and many more. The second kind of residual stress, often

called structural micro stress, covers a distance of one grain or a part of a grain. It can

occur between different phases and have different physical characteristics, or between

embedded particles, such as inclusions, and the matrix. The third kind of residual stressranges over several atomic distances within the grain, and is equilibrated over a small

part of the grain.

Figure1:Macrostresses ( 1),Mesostresses( 11), Microstresses( 111).

Residual stresses are a consequence of interactions among time, temperature,

deformation, and microstructure. Material or material-related characteristics that

influence the development of residual stress include Thermal conductivity, Elasticmodulus, Poissons ratio, Plasticity, Thermodynamics and kinematics of transformations,Mechanisms of transformations, and Transformation plasticity. Residual stresses can be

developed in materials and engineering components during manufacturing by differentprocesses. Some of these processes are Plastic deformation or forming, including rolling,

drawing, bending, forging, pressing, spinning, shot-penning, laser shock, welding,

brazing sprayed coating, cladding, electro deposition, machining, grinding and duringthermo chemical heat treatment including quenching, laser and plasma heat treatment,

carburizing, case hardening, ion platting and a combination of these treatments. Casting

and during cooling of a multiphase materials such as metal matrix composite.

Compressive residual stress has a benificial effect on fatigue life, crack propagation and

stress corrosion of materials; where as tensile residual stress can reduce their performancecapacity. The difference between macro and micro stresses is shown in figure


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Figure 2. Descriptions of Macro and Micro Stresses

There are three types of methods/procedures of Residual-Stress Measurement The destructive procedures of residual-stress measurement --These procedures all

are based on sectioning or removal of material to cause a redistribution of residual

stress. This is measured as a strain change.

The semi destructive methods of residual stress measurement -- These methodsare based on the same principle as the destructive based on the same principle asthe destructive methods or the perturbation of the residual-stress field by other

means.

The non destructive methods of residual --Stress measurement: these methods donot permanently disturb the residual-stress field, but directly measure the atomiclattice strain caused by the stress or measure some physical property perturbed by

the lattice strain.

NEED FOR RESIDUAL-STRESS

MEASUREMENTS

The major reasons that residual stresses are of concern are:

Failures that are suspected as being caused by fatigue, stress corrosion, corrosionfatigue, or hydrogen embrittlement.

Assessment for the continued serviceability of a component, for example, lifeassessment; this is usually focused on a concern for insevice failure.

Distortion occurring during processing of a component. Distortion of components during service.

Manufacturing processing or repair procedures induces most cases of suspected harmful

residual-stress fields, although sometimes abusive service conditions or an accident mayhave caused them. When manufacturing process or sometimes repair procedures are

judged the most likely source of the residual stresses. it is often possible to predict the

magnitude and distribution of the residual stresses.


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NATURE OF RESIDUAL STRESSES:

Residual stresses are the inevitable consequence of thermo mechanical processingof steel. The resulting stress fields usually are nonuniform and show high stress gradients.

For example fig. 2 illustrates the residual-stress magnitudes and distributions typical of

steel with 650 Mpa yield strength. Because of the high stress gradients, tens to hundredsof residual stress measurements with resolution on the order of 1mm may be required toidentify precisely the maximum stress and its location.

The characteristically nonuniform, high stress gradient nature of residual stresses

require that either the induced stress field is well understood and predictable, or manyresidual-stress measurements must be performed on one or more components in order to

reveal the nature of the stress fields.

STRESS MEASUREMENT:

A number of procedures and methods have been applied to determine the residualstresses extant in a metallic component, usually as a result of manufacturing processing.

However stress is never the quantity measured because a stress is a quantity that isapplied to a metal and can only be measured to determine residual stress is elastic strain-

either the elastic strain resulting directly from the existing residual stress in the metal or

the elastic strain change resulting from relief of some portion, or all,of the existingresidual stress. The stress that is causing, or has caused. The strain is then calculated

using the applicable elastic constants for the metal.

STRAIN MEASUREMENT METHODSAll residual-stress determination methods measure elastic strain, not stress, and the

residual stress is calculated from the strain values. Several methods for the measurementof strain have been applied in residual-stress studies, these methods include

Mechanical gages Electrical-resistance gages Optical gages Birefringent methods Diffraction methods (X-ray & Neutron) Ultrasonic methods Magnetic methods

DESTRUCTIVE MEASUREMENT PROCEDURES

Destructive methods of residual-stress measurement are fundamentally stress-

relaxation procedures; that are the information is obtained by relaxing the residual stressin some finite-volume element of the component and measuring the resulting strain

change.

The strain change is then used, along with applicable assumptions about the nature of the

stress field, to reconstruct the original stress field. Assumptions about the nature of thestress field include the magnitudes and gradients in the stress field and whether it is


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sufficient to assume that the gradients are one-, two-, and three-dimensional. In

particular, the gradients that exist will dictate the size of the element that is to be isolatedand made stress-free; that is, the higher the stress gradient is, the higher the stress

gradient is , the smaller the finite element must be in the direction of that gradient. It must

be emphasized that the larger the element and the higher the stress gradient, the less

quantitive and more qualitative are the measurement results. Electrical-resistance strain-gage technologies are emphasized as the dominant method of strain measurement due to

their economic, procedural, and procession advantages over other methods. However,

modern XRD equipment when available has all of these advantages as well can be usedto measure the stresses existing before and after sectioning.

SEMI DESTRUCTIVE PROCEDURES

Nondestructive methods of residual-stress measurement are characterized as a methodthat is no way affect the serviceability or reduce the mechanical strength or other

properties of the component in which stresses are measured. Between the nondestructive

and destructive methods, which have a severe effect on the serviceability, strength, andproperties, are the semidestructive methods. These are methods that have a small to

negligible effect on the components in which stresses are measured, or methods in which

the components may be repaired after the measurement.

These methods that are considered semidestructive are those that requiresmall holes to be drilled or rings to be trepanned in the component or indentations to be

made in the surface. The first two methods provide quantitative data and the third only

qualitative data. These methods include

Blind hole Drilling and Ring coring Indentation methods Spot annealing

NONDESTRUCTIVE PROCEDURES

In Strain Measurement Methods all measure the change in some direction (strain) of

the component produced by the removal of a finite volume of stressed metal from thatcomponent. Thus, they measure the strain induced by removing material so as to

perturb the residual-stress field. On the other hand, nondestructive procedures measure

a dimension in the crystal lattice of the metal or some physical parameter affected bythe crystal lattice dimension. Whenever a mechanical force, resulting in stress that is

less than the yield strength, is placed on a solid metal component, that component

distorts and strains elastically. That elastic strain results in a change in the atomic lattice

dimension, and this dimension, or change, is measured by the nondestructive stress-measurement procedures. For example, the diffraction methods (X-ray & Neutron)

measure an actual crystal dimension, and this dimension can be related to the

magnitude and direction of the stress that the metal is subjected to, whether that stressis residual or applied. Some of these methods include

X-ray diffraction Neutron diffraction Ultrasonic velocity Magnetic Barkhausen noise


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RESIDUAL STRESSES RELATED TO WELDING:

Welding is the most common cause of significant residual stresses. The cooler parent

metal restrains contractions of the weld metal up on cooling, leading inevitably large

residual stresses.Moreover, phase and volumetric changes at the microscopic level also contribute to theresidual stress phenomenon during welding. Being able to predict and model residual

stresses in different weldment configuration is important in accessing the possibility of

failure. Modeling of residual stresses is not a simple task; there are many variablesinvolved; weld geometry, temperature, time, thickness, joint restraint, welding process,

heat input, and deposition

Figure 3: (a) schematic representations of a single run butt weld and associated (b)

temperature and (c) stress changes (ASM Handbook Volume 6, 1983)

-area and there is an abundance of non-uniform temperature profiles (ASM Handbook

Volume 6, 1983).

In describing Figure 3(a) the weld is shown by the shaded area. The molten weld poolregion (current position of the arc) is shown by the origin, 0. Figure 3(b) shows thetemperature profiles along different sections with Section B-B bisecting the melted

region and Section C-C being at a close distance from the weld pool in the solid weld

metal. As expected the greatest temperature gradient is at the weld pool as shown in

Section B-B. Figure-3(c)shows the residual stresses as a result of welding. Section A-Aahead of the weld bead on the parent metal shows no residual stresses. In the melted

region (Section B-B) there are thermal stresses present but they are close to zero because

the molten metal cannot support any loads. In regions away from the arc (transverse to


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the direction of welding) where cooling is occurring, the stresses are larger due to the

lower temperature and restrained contraction. The maximum magnitudes of compressivestresses and tensile stresses occur at Section D-D where the tensile stresses peak in the

cooled weld metal and compressive stresses peak in the surrounding parent metal. This is

more clearly shown in Figure-4, where the distribution of stresses in a but-welded joint

can be seen. Note that in the middle of the weld the residual tensile stresses present arevery close to the yield point of the parent material (Gourd , 1991).

Figure 4: Distribution of stresses in a single pass butt weld (Gourd , 1991).Since residual stresses exist without external forces, the resultant force and resultant

moment produced by the residual stresses must disappear:

On any plane section where dA is area

Where dM is resultant moment (ASM Handbook Volume 6, 1983)

The residual stress pattern as indicated in Figure 4 occurs in materials of moderately

low thicknesses. According to Lancaster (1980), in thick plate there is a contraction stressat right angles to the plate surface and consequently the stress field may intensify

progressively as the joint is built up with weld runs. Figure shows residual stresses

measured before a short (15 minutes) and long (40 hours) PWHT, measured by remanent

magnetisation, along the centre line of a submerged arc weld in a 165 mm thick plate ofMn-Mo steel (650 mm in length). The curves in Figure 2.5 represent residual stresses in

three directions that is, stress in the longitudinal direction, transverse direction and the

short transverse direction (perpendicular to the root face). The figure shows that residualstresses can either be compressive or tensile along the thickness of the plate, and that long

PWHT times result in the redistribution of residual stresses. If it is assumed the three

residual stresses measured represent principal stresses, the effective residual stress is:


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Where T is transverse, L is longitudinal, and ST is short transverse (perpendicular to the

root face)A correlation between the toughness of a weldment and the presence of residual stresses

has been found experimentally and this is discussed in Section Fracture toughness. From

a micro structural perspective, if austenite transforms to martensite at low temperatures

the volume increase will be substantial because of the difference with falling temperaturein thermal contraction curves of the face centered cubic and body centered cubic phases.

This will lead to a reduction of localized residual stresses in the weldment if the weld

metal (WM) transforms to martensite. However, this effect is unlikely because the lowcarbon WM transforms at higher temperatures to acicular ferrite, and may only be

relevant to the heat affected zone (HAZ) of welds, which is a relatively small volume of

the weldment.

Figure 5: Distribution of residual stresses along centre-line of narrow gap single veesubmerged arc weld in 165 mm thick Mn-Mo steel plate (a) after 15 minutes of PWHT at

600C and (b) after 40 hours of PWHT at 600 C (Suzuki et al, 1978, pp 87-112).

STRESS RELIEVING TO REDUCE RESIDUALSTRESS

The most common method of stress relief is by heat treatment. Other methods of stress

relief are typically mechanical. These include peening, vibrational techniques and

manipulation of hydrostatic testing (overloading methods). Another alternate techniquethat can be used is temper beading. Temper beading manipulates bead placement and size


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to reduce residual stresses and it is mainly used in the United States of America, but it is

gradually becoming more frequently used in Australia.

HEAT TREATMENT AS A FORM OF STRESS

RELIEF

Thermal stress relieving involves heating a component to a temperature at which thematerial yield stress has fallen, allowing creep to take effect (Lloyd, 2000). Large

residual stresses are no longer supported and, if temperatures are high enough, the stress

distribution will become more uniform across the component. Such heat treatment maylead to tempering or ageing effects and alterations to the microstructure depending on the

material and combination of temperature and time. In C-Mn steels stress relief heat

treatment is beneficial in improving fracture toughness of the HAZ, allowing service atlower temperature. In but welds of plate the general rule for satisfactory relief of residual

stresses is that uniform stress relief heat treatment must be applied over a bandwidth that

is twice the length of the weld (see Figure 6) (Papazoglou, 1981). Figure also shows the

heat bandwidth for circumferential but welds.

Figure 6: Bandwidths for local stress relieving heat treatments in butt-welded (a) plateand (b) pipe.

In heat treatment it is important to be able to achieve the correct temperature and

temperature control within specified limits. Uniform heating and cooling rate must beobtained through the heaviest section to be treated, especially where the geometry is

complex and the thickness is variable.

Stress relieving heat treatments are generally avoided unless stipulated

as mandatory by Codes and Standards, due to the costs involved and potential

consequences of incorrect PWHT procedure.

MECHANICAL METHODS OF STRESS RELIEF

(OVERLOADING, VIBRATIONAL AND PEENING)

Residual stresses can also be reduced by mechanical treatment without the need for heat

treatment. Mechanical methods do not refine the metallurgical microstructure of the


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weldment but instead work on the principle of causing localized yielding with the

combination of applied and residual stresses.

OVERLOADING

Overloading techniques involve the relaxation of stresses by permanent yielding via thehydrostatic test or the warm pressure test. Overloading techniques combine loadinggenerated by external pressure together with the presence of residual stresses. A single

overloading above the yield stress results in a decrease of any residual stresses. The

overloading technique generates compressive residual stresses around existing defects,with a beneficial effect upon brittle fracture (Nichols, 1968; Okamata et al, 1990, pp199-

203). During the increase of pressure, external loads are added to the existing residual

stresses causing localized plastic deformations. As the pressure is released, the elastic

retention produces residual stresses that play the favorable role of prestressing(International Institute of Welding, 1987). The hydrostatic test is a mandatory test for

pressure vessels conforming to AS1210 or AS4458. Limitations to the suitability of the

process for stress relief include:

The question of compensating pads and attachments receiving adequate stressrelief.

Reaching the required stress levels for conservatively designed road tankers. (Epselis, 1996)

VIBRATIONAL TECHNIQUES:

Vibrational methods of stress relief involve inducing one or more resonant or sub-

resonant states in a welded structure using suitable force exciters, resulting in elasticstraining of the treated surface (International Institute of Welding, 1987; Epselis, 1996).

The success of vibratory stress relief depends on the type, size and complexity of the

structure, where a balanced stress state is more important than the reduction of residualstresses.

Disadvantages of vibrational stress relief include (Epselis, 1996):

Softening of the hardened HAZ does not occur. It is not recommended where brittle fractures a serious risk. No favorable metallurgical changes take place as it a mechanical process. It offers no advantages over overloading techniques such as hydrostatic testing.

SHOT PEENING

Shot peening is a surface cold working process that is used to minimize the potential forfatigue, stress corrosion cracking and other modes of failure. Peening works on the

principle of introducing residual compressive stress in the surface layer by bombarding it

with small high velocity spherical media called shot (Diepart, 1992, pp 517-530). It iswell known that cracks will not initiate or propagate in compressively stressed zones.

Since numerous failures are initiated at the surface of components, compressive stresses


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induced by shot peening can considerably enhance the life of a component. Diepart

(1992, pp 517-530) concludes that shot peening has made considerable advancementsover the years but further work is needed in the area of shot media, shot velocity

measurements, engineering models and non-destructive tests to measure residual stress

profiles to avoid such pitfalls as irrelevant applications and inappropriate and incorrect

peening methods.

TEMPER BEADING AS A METHOD OF STRESS

RELIEF

Temper beading is another technique that can be used as a form of stress relief. In effect,

the strategic sequencing and placement of the weld beads provides localized PWHT of

preceding passes, thus achieving substantial tempering of the total weldment. Preheating

and/or maintenance of interpass temperature during multi-pass welding also provide aform of dynamic or auto-PWHT.

METHODS FOR MEASURING RESIDUAL

STRESSES

A number of different techniques can be used to measure residual stresses in metals and

weldments. The American Welding Institute classifies these techniques into three groups

as shown in Table 2.2. The techniques for measuring residual stresses are classified intostress relaxation, x-ray diffraction/neutron scattering and cracking categories (AWS,

2000).


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Table1: Classification techniques for measuring residual stresses (AWS, 2000)

STRESS RELAXATION TECHNIQUES

In stress relaxation techniques residual stresses are measured by measuring the

elastic strain release with the use of electric or mechanical strain gauges. The residual

stresses are released by cutting the specimen into pieces or by removing a section fromthe material and hence the strain gauge is used for measuring the strain release. Thistechnique can be successfully used on plates, cylinders or tubes (AWS, 2000).

Additionally, strain release during stress relaxation can be measured using a grid

system, brittle coating or photo elastic coatings. These techniques are used for measuringsurface residual stresses in weldments as they provide reliable quantitative data. These

techniques are based on the fact that strains taking place during unloading are elastic even

though the material has undergone plastic deformation.

Therefore it is possible to determine residual stresses without knowledge of the

history of the material. A common technique for measuring residual stresses by stress

relaxation is the hole drilling technique (AWS, 2000; Lloyd, 2000). This method was firstproposed by Mathar and further developed by Soete (1949, pp354-364). In this technique,

a small circular hole is drilled in a plate (which may or may not be a weldment)

containing residual stresses. Those stresses in areas outside the hole are partially relaxed

by drilling of the hole. It is possible to determine residual stresses that exist outside thedrill hole (Soete, 1949, pp354-364). Figure-7shows that a common way to measure

residual stresses is to place strain gauges at 120 from each other and drill a hole in the

centre. The magnitude and direction of the principal stresses are determined bycalculating the strain changes at the three gauges.

Figure-7: Hole drilling technique (AWS, 2000)

Limitations of the Hole drilling method include (Ruud, 1981, pp35-40):

destructive method of measuring residual stresses; holes must be at least eight times the diameter apart; accessibility to drill holes; thickness of the specimen has to be at least four times the hole diameter; and


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areas where stress exceed 0.3 times the yield stress may give erroneous resultsdue to

local plastic strain during metal removal (drilling).

X-RAY DIFFRACTION AND NEUTRON

SCATTERING TECHNIQUES

Diffraction techniques rely on measuring the elastic strains in metals throughlattice parameter variations. As the lattice parameter of a metal in the unstressed state is

known, elastic strain in the metal/component can be determined. The technique is based

on movement of the detector (x-ray or neutron) over a range of angles to measure theangles of diffraction, ( ) which satisfy the Bragg condition of constructive interference.

When the Bragg condition is satisfied, sharp increases in the scattered intensity

are observed (Lloyd, 2000). The precise determination of ( ) yields d (interplanar

spacing) directly. In the presence of residual stresses the measured interplanar spacing

will be changed by an amount, . This change provides an internal strain gauge andhence the strains can be related to residual stresses using the elastic constant (Lloyd,

2000).

OBSERVING CRACKS TO MEASURE RESIDUAL

STRESSES

Cracks induced by hydrogen or stress corrosion may provide qualitative data on residual

stresses in complex structural components that have complicated residual stress

distributions. The crack pattern in Figure shows there are major tensile residual stresses

present in that weld.

Figure-8: Crack pattern implying the presence of tensile residual stresses (AWS, 2000).

CONCLUSION


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These techniques are used for measuring surface residual stresses in weldments as

they provide reliable quantitative data. These techniques are based on the fact that strainstaking place during unloading are elastic even though the material has undergone plastic

deformation. Therefore it is possible to determine residual stresses without knowledge of

the history of the material. A common technique for measuring residual stresses by stress

relaxation is the hole drilling technique. This method was first proposed by Mathar andfurther developed by Soete. In this technique, a small circular hole is drilled in a plate

(which may or may not be a weldment) containing residual stresses. Those stresses in

areas outside the hole are partially relaxed by drilling of the hole.

residual stress on components of manufacturing process

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