Mechanical Property Estimation of Similar Weld using Ball Indentation Technique

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Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 1095-1100

Published Online November 2012 (http://www.SciRP.org/journal/jmmce)

Mechanical Property Estimation of Similar Weld using Ball

Indentation Technique

Harshit Kumar Khandelwal1*, Kamal Sharma2, Rahul Chhibber3

1Homi Bhabha National Institute, Mumbai, India

2Reactor Safety Division, Bhabha Atomic Research Centre, Mumbai, India

3Indian Institute of Technology Rajasthan, Jodhpur, India

Email: *[email protected]

Received July 4, 2012; revised August 10, 2012; accepted August 24, 2012

ABSTRACT

A weld joint is composed of three principal zones viz., base metal, Heat Affected Zone (HAZ), and weld zone. Thus,

the variation in mechanical behavior exists not only among these zones, but also from point to point in each individual

zone. Being destructive in nature, the conventional method of mechanical testing cannot successfully used to estimate

the variation in the mechanical behavior at different zones of the weld joint. Moreover, the conventional method of me-

chanical testing cannot characterize the material using small amount of material. In this respect, Ball Indentation (BI)

methodology was considered to be useful approach, since it can characterize the mechanical properties of a material

using very small amount of material in non destructive manner. The present work is an attempt to characterize the

variation in the mechanical properties among each zone (global variation), and from point to point in each zone (local

variation) of the similar weld joint used in nuclear application using BI approach. For this purpose, the similar weld

joint of two SS-304 LN pipe lines was investigated using BI approach.

Keywords: Ball Indentation; In Situ; Similar Weld Joint; Flow Properties; Pressurized Water Reactor; Reactor Pressure

Vessel

1. Introduction

1.1. Similar Weld Joint (Weld Joint of SS 304

LN Pipelines)

Pressurized Water Reactor (PWR) is the most widely

used reactor type in the world, in which enriched (about

3.2% U-235) uranium dioxide is utilized as the fuel in

zirconium alloy cans. The fuel is arranged in the arrays

of fuel pins and interspersed with the movable control

rods. The fuel is held in a steel vessel through which wa-

ter at high pressure (at 16 MPa, to suppress boiling) is

pumped to reactor vessel via cold lag pipeline to act as

both a coolant and a moderator. After absorbing the heat

from the core of reactor, high-pressure water is passed to

a steam generator, via hot lag pipeline which raises

steam in the usual way. Both hot lag and cold lag pipe

lines are made of SS 304 LN material and have the simi-

lar weld joint. The chemical composition of SS 304 LN

which was used for the present investigation is shown in

Table 1.

2. Ball Indentation Approach

Ball Indentation (BI) is a non destructive approach to

Table 1. Chemical composition of SS 304 LN material used

for investigation.

C MnNiSiS P Cr N Fe

0.032 8-1210.030.04 18-20 0.1 Balance

determine mechanical behavior of the materials. More

over it utilizes very small amount of material during me-

chanical behavior examination. This technique can be

utilized for the situations where in situ investigation of

mechanical behavior is required. In order to find out the

mechanical properties using BI testing, many theories

and models have been proposed. Tabor [1] proposed an

empirical relationship, which correlates the plastic strain

corresponding to uniaxial loading with plastic indenta-

tion strain corresponding to indentation loading using

spherical indenter. Mayer [2] was the first one, who cor-

related the yield strength of material with the impression

diameter and the mean pressure during indentation load-

ing. Haggag et al. [3-8] developed BI setup to predict the

mechanical behavior of the materials. Later this setup

was utilized for the investigation of flow and fracture be-

havior of different materials by several researchers [9-11].

There exists a good agreement between BI generated

*Corresponding author.

H. K. KHANDELWAL ET AL.

1096

flow and fracture properties with those generated through

conventional mechanical testing. H. K. Khandelwal et al.

[12] characterized the flow behavior of reactor pressure

vessel steel SA 508, pressure tube material Zr-2.5Nb and

other nuclear component materials using BI approach.

Kamal Sharma et al. [13] used numerical simulation with

finite element and artificial neural network methodology

to estimate the mechanical properties using BI approach.

Kamal Sharma et al. [14] also applied BI methodology to

estimate the degradation of Zr-2.5Nb alloy. G. Das et al.

[15] utilized BI approach for the study of the effect of

cold work on tensile flow behavior of SS 316 steel. Mok

[16,17] and Duffy [17] worked on BI approach with va-

rious strain rates. Mathew et al. [18] investigated the aging

effect on the mechanical behavior of alloy 625 through

BI approach.

BI approach is based on multiple indentations at a par-

ticular location on the component using a spherical in-

denter in order to determine mechanical behavior at that

location. During multiple indentations, work piece is sub-

jected to several loading and unloading cycles at the in-

dentation location and data from each of these cycles

yield a point on the flow curve. In order to carry out BI

testing, a computer controlled BI setup is mounted over

the component whose mechanical properties to be inves-

tigated. The computer allows the indenter of BI machine

to touch the work piece surface. During indentation, se-

veral loading and unloading cycles perform at a particu-

lar location on the work piece surface. The indentation

prfile during each loading and unloading cycle of BI tes-

ting is shown in Figure 1. Figure 2 shows the cyclic

nature of load vs displacement during BI testing. Total

depth of indentation (ht), plastic depth of indentation (hp),

and applied indentation load (P) are measured during each

loading cycle. The cycles of indentation continues up to

the point when the total depth of indentation reaches the

value corresponding to the half of ball indenter diameter.

For each loading and unloading cycle, the value of

plastic indentation diameter (dp) and total indentation

diameter (dt) from ht and hp, can be find using Equation

(1) and Equation (2) respectively. In Equation (1), the

value of dp can be calculated using regression analysis.

Figure 1. Indentation geometry during BI testing [18].

Figure 2. Load vs Displacement profile during BI Testing.



322

12 PPP

0.25d

d 2.7350.25d

EE hh









 





























(1)





dh 

h (2)

Tabor [1] correlated the indentation hardness and strain

corresponding to a spherical indenter with plastic strain

during uniaxial loading. He established the relation

(Equation 1.3) between true plastic strain (εp) corre-

sponding to uniaxial tensile test and the indentation strain

(dp/D) corresponding to ball indentation test [1].

0.2 D

 (3)

In Equation (3), D and dp are ball indenter diameter

and plastic indentation diameter respectively. The plastic

depth (hp) corresponding to a cycle, during BI testing is

converted to corresponding plastic indentation diameter

dp, using Equation (1). The maximum strain that can be

measured during BI test is 20%, when dp = D. In the pre-

sent work, maximum strain value lies between 10 to 12 %.

P is the maximum measured load corresponding to a cy-

cle in BI loading. Under load P, the mean normal pres-

sure (Sm) acting on the material is given by 4P/πdp

Equation (4) and Equation (5) correlates the uniaxial

flow stress (σ) with the mean normal pressure.





 (4)

πd











 (5)

In Equation (4) and Equation (5), δ is constraint factor

which can be found out using Equation (6).

2.87 m







 (6)

The parameter αm in Equation (6) depends mainly on

H. K. KHANDELWAL ET AL. 1097

the strain rate sensitivity and work hardening characteris-

tic of the material being tested. αm varies from 0.9 to 1.25

for different materials. αm has the value of 1.0 for low

strain rate sensitive materials [20].

The flow portion of the true stress vs true plastic strain

curves can be represented using power law relationship

as given in Equation (7) [20].







 (7)

In Equation (7) n, K, σ, and εp represent strain harden-

ing exponent, strength coefficient, true stress and true

plastic strain respectively. The values of strength coeffi-

cient (K) and strain hardening exponent (n) can be de-

termined using regression analysis of the data fitted to

Equation (7). For the case when



p = n, Equation (8) pro-

vides the expression for true ultimate tensile strength-

[20].

TrueUTS





 (8)

Thus engineering ultimate tensile strength is given

by Equation (9),

UTS











(9)

Yield strength of specimen is not equal to the value of

stress corresponding to first cycle during BI testing, since

the minimum attainable ball indentation strains for a no-

minal size indenter are about 20 times larger than the

strain corresponding to yield stress. Hence, the strain cor-

responding to yield stress is too small to measure directly

from indentation test. Therefore, a different approach is

adopted for the estimation of yield strength of the ma-

terial [19].

Mayer [2] was the first one who established an em-

pirical relationship between the mean pressure and the

impression diameter during indentation (Equation 1.10).

Data points from all loading cycles (maximum value of

dt/D = 1.0) are fit by regression analysis to the relation-

ship of Equation (10).









 (10)

In Equation (10), m and A are Meyer’s exponent (m)

and yield parameter respectively. Parameter “m” gener-

ally has a value between 2 to 2.5. The value of dt can be

determined from total depth of indentation using the

Equation (2).

The yield strength (σy) is proportional to the Meyer’s

hardness (4P/pd2), where d is the final impression di-

ameter, and is given by Equation (11).



 (11)

In Equation (11), parameter βm is a constant for a

given class of material. The value of βm for each class of

material is determined from the yield strength obtained

using standard tensile tests.

3. Experimentation

The mechanical behavior of similar weld joint was char-

acterized using a weld joint of two SS 304 LN pipelines.

This weld joint comprised of three principal zones viz.,

two zones of SS 304 LN base metal and a weld zone,

lying in between the two base metal zones. This investi-

gation was carried out in order to study the gradients in

the mechanical behavior of subzones of the similar weld

joint viz., Weld, HAZ and base metal zone. In order to

study the gradients in mechanical behavior at these sub-

zones, BI experiments were performed along the three

principal zones of weld joint. The BI testing was per-

formed using load control condition. For this purpose, BI

machine was installed over the weld jointed pipelines

and BI experiments were performed at these zones of the

weld joint at 0.5 KN/min. loading rate. The maximum

load during each of the experiments was kept 1.5 KN.

The arrangement of installed BI setup over similar weld

joint of two SS 304-LN pipelines is shown in Figure 3.

The indent points generated after BI experiments on

similar weld joint are shown in Figure 4.

SS 304 Pipe

Weld Zone BI Setup

Figure 3. BI setup installed on weld jointed SS 304 LN pipe-

lines.

Figure 4. Final indentation points after BI experiments.

H. K. KHANDELWAL ET AL.

1098

4. Results

4.1. Comparison of Different Zones of Similar

Weld Joint

4.1.1. Load vs Displacement Curves

The three subzones of similar weld joint showed vari-

ation in slopes of BI generated load vs displacement

curves. Here, heat affected zone shows higher slope of

load vs displacement curve in comparison to the base

metal zone. Weld zone showed the least slope among the

three subzones. The comparison of load vs displacement

curves at each of the three zones of similar weld joint (i.e .

SS 304 LN, HAZ of SS 304 LN and Weld) is shown in

Figure 5.

4.1.2. Flow Behavior

Each of the three subzones shows clear variation in the

flow properties. Figure 6 shows the true stress vs true

plastic strain patterns for each of these subzones.

4.2. Global Comparison

After carried out the analysis of BI generated true stress

vs true plastic strain curves for three zones of similar

Figure 5. Comparison of load vs displacement curves at

three zones of similar weld joint.

Figure 6. Comparison of true stress vs true plastic strain

curves at three zones of similar weld joint.

weld joint, mechanical properties (strength coefficient,

strain hardening exponent, yield strength and ultimate

tensile strength) were determined. The average strength

coefficient at heat affected zone was found to be higher

than base metal zone. The highest average strength coef-

ficient was found in base metal zone and least average

strength coefficient was found in the weld zone. The

highest value of strain hardening exponent was found in

base metal while least was for heat affected zone. The

average values of yield strength and ultimate tensile

strength of heat affected zones was higher than the re-

spective base metal zones. The highest yield strength and

ultimate tensile strength was found in case of heat af-

fected zone. However, the difference between average

ultimate tensile strength for SS 304-LN base metal and

its heat affected zone was not significant. The base metal

shows least yield strength and weld zone shows least

value of ultimate tensile strength was found in weld zone.

Table 2 shows the local variation of mean values and

standard deviation values of all the BI experiments at

three zones of similar weld joint.

4.3. Local Variation of BI Generated YS & UTS

along Similar Weld Joint

The local variation shows that the heat affected zone had

higher yield strength than its base metal zone. This pat-

tern was also present in case of bimetallic weld joint (for

SA 508 class-2 and SS 304-LN) tested in both loading

rates viz. 0.5 KN/min. and 1.5 KN/min [12]. The ulti-

mate tensile strength for both base metal and its heat af-

fected zone was found to be almost similar. The local

variations of yield strength and ultimate tensile strength

along the three zones of similar weld joint of SS 304-LN

pipelines are shown in Figures 7 and 8 respectively.

5. Conclusions

After carried out the mechanical behavior investigation

Table 2. Comparison of mean and standard deviation va-

lues of BI generated mechanical properties at three zones of

similar weld joint.

Position → SS 304

Weld Zone SS 304 HAZ SS 304 Base

Metal Zone

Mechanical

Properties Mean SD Mean SD Mean SD

Strength

Coefficient (MPa)1455.454 1529 306.8 1592142

Strain Hardening

Exponent 0.260.020.24 0.07 0.28 0.03

Yield Strength

(MPa) 556.815.4747.1 68.9 338.0 6.50

UTS (MPa) 785.089.68843.0 85.2 842.735.34

H. K. KHANDELWAL ET AL. 1099

Figure 7. Variation of yield strength at three zones of simi-

lar weld joint.

Figure 8. Variation of BI generated ultimate tensile strength

at different zones of similar weld joint.

using BI approach in different sub zones of similar weld

joint (SS 304 LN base metal, HAZ, and weld zone) fol-

lowing points were concluded:

 Yield strength of the heat affected zone SS 304-LN

was found to be higher than that of respective base

metal and weld zone.

 Heat affected zone and base metal zone shows almost

similar Ultimate tensile strength. However it was

higher than that of weld zone.

 Each subzone of weld joint shows significant differ-

rence in mechanical behavior with respect to each

other. Moreover, the variation of mechanical proper-

ties was found to be negligible throughout within

some zones.

The BI approach was found to be extremely useful in

the situations where it is required to conduct non destruc-

tive mechanical behavior examination, in situ inspection,

and structural integrity assessment of structural compo-

nents. Moreover, this approach is useful in the situations

where less amount of material is available for mechanical

behavior evaluation.

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