CHARACTERIZATION OF MACHINED

SAMPLES:

SS-304L & ZIRCALOY-4

PROJECT REPORT SUBMITTED BY:

KISHLAY MISHRA

SUMMER TRAINEE AFFILIATION

Under the Guidance of

Dr. G.K. Dey

Head, Materials Science Division

Summer Trainee Internship (May‐ July 2013)

Bhabha Atomic Research Centre

Trombay, Mumbai 400 085

CERTIFICATE

This is to certify that Mr. Kishlay Mishra (IIT Kanpur) B.Tech student of the

fore mentioned college have done a project on “Characterization of

machined samples : SS-304L and Zircaloy-4” as a part of Summer

Internship conducted by HRDD, BARC, Trombay, Mumbai. The work

described in this report is bonafide work carried out by the student under

the guidance of following BARC Scientific Officer.

Dr. G.K. Dey Head, Material Science Division BARC, Trombay, Mumbai.

ACKNOWLEDGEMENT

This project is an outcome of all efforts and coordination of various people to whom I shall always be grateful.

I would like to express our sincere thanks to Dr.G.K.Dey and for taking me

under his tutelage-ship, for providing constant encouragement and useful

suggestions which proved to be of immense help during the tenure of my

training.

I take this opportunity to show our gratitude to all the employees at BARC

who had contributed their efforts, substantial time and support to help with

my assigned job. I pay my obeisance to them.

I would also like to acknowledge the contribution from Prof. I.Samajdar of

Indian Institute of Technology Bombay for extending the facilities at the

National Facility for Texture and OIM for this study.

Last but not the least i would like to express my heartfelt gratitude to the

college administration for allowing me to pursue the research in this

esteemed organization.

ABSTRACT

This study aims to understand the effects of surface machining on the

microstructure in SS-304L and Zircaloy-4, and crystalline arrangement of

machined layer. To provide a holistic picture, for every property,

comparison was done between machined surface and the bulk.

In depth characterization was carried out by Optical Microscope, FEG-

SEM, TEM, EBSD and Glancing Angle-XRD studies. SEM images depict

affected layer of approximate 8 microns from the machined edge alluding to

plastic flow of material due to machining in that region. TEM results show

presence of grains of size of around 30nm in the affected layer of SS304L .

Diffraction pattern in the TEM confirms the nanocrystalline nature of the

grains. GIancing angle-XRD shows no change of phase in both the

samples of SS304L and Zircaloy 4 in the orientation chosen for the

scanned area. EBSD shows Martensitic formation in SS304L and no phase

change in Zircaloy 4.

INTRODUCTION

Every metallic material that we use, is machined in one way or another.

Machining provides a surface finish that makes the object precise in its

dimensions and adds aesthetic value. The surface finish in turn is primarily

dictated by the surface machining and grinding operation which in practice

is one of the most important stages in the fabrication process. Conclusively,

machining is a primary requirement and is inevitable. But the problem with

machining is that it changes the microstructure of surface and creates a

layer of altered material. Through a series of experiments we hope to

accomplish a detailed investigation on the precise changes in the surface

conditions resulting from closely controlled surface finishing operation and

comparing it to the bulk.

MATERIALS USED

To obtain a wider perspective, we selected two very different materials:

· AISI grade 304L Austenitic Stainless Steel with a chemical composition (in wt.%):

0.03 max C , 18.00 - 20.00 Cr, 8.00 - 12.00 Ni, 0.75 max Si, 2.00 max Mn, 0.045 max P, 0.03 max S, and balance Fe.

· Zircaloy-4 with a chemical composition of: Zr , 0.5%Sn , 0.2%Fe , 0.1%Cr. These samples were machined using a lathe machine.

PROCEDURE

Four characterisation techniques were employed-

1.) Scanning Electron Microscopy (SEM)

■ The samples were polished at the perpendicular side of the machined

surface to study microstructural changes. Abrasive polishing was

done using emery papers of different grades(SiC grits) namely; 80,

150, 320, 420, 600, 800, 1000, 1200. Area of interest was the region

near the machined face(edge) thus, special emphasis was on that

area.

■ After Diamond polishing, the samples were then viewed under an

optical microscope to get an overview of the machined layer present

in the samples.

■ The SS304L sample was then studied under FEG-SEM after electro-

etching with oxalic acid. The machined layer was clearly visible and

could easily be differentiated from the bulk.

2.) Glancing Angle X-Ray Diffraction

■ Another set of samples from each of the two materials were prepared

for the Glancing Angle XRD. The samples were cut in a direction

parallel to the machined face and the cut surface were polished using

emery paper and diamond polishing. The machined surface and the

finely polished bulk surface were probed separately at different

depths in the analysis.

3.) Transmission Electron Microscopy (TEM)

■ TEM sample of SS-304L was prepared by making fine cuts into the

sample and obtaining multiple specimens of dimensions 2.5 mm X 2

mm X 0.5 mm. Two such specimen were pasted together so that the

machined edges are at the center, this was done to double the

probability of getting a thin area for TEM

■ Mounting cylinder was heated and the samples were glued together

using epoxy glue. Using disc grinder the samples were then polished

on emery paper(600, 800, 1000) ■ The thinned specimen was then transferred to copper grid.

■ SS304L sample was dimpled for 6 hours and the ion milled for 1 day

to electron transparency.

■ The sample prepared was not thin enough as dimpling had been done wrong the whole process was repeated.

■ TEM was done of SS304L in BARC.

■ More advanced form of TEM, TECNAI was done in Tata Institute of

Fundamental Research along with Energy Dispersive

Spectroscopy(EDS).

4.) Electron BackScattered Diffraction pattern (EBSD)

■ SS304L and Zircaloy 4 samples were polished with emery paper of grades upto 2500.

■ Diamond polishing and electropolishing of the two samples were

followed by Orientation Imaging Microscopy.

■ No pattern was obtained. The samples were then polished using colloidal Si polishing rather than electropolishing.

■ The second OIM was again unsuccessful.

■ The samples were prepared again from the start. Finer Polishing was

done upto 4000 grade of emery paper with caution followed by

diamond polishing and electropolishing.

RESULTS AND DISCUSSION

Optical Microscope:

Fig 1. SS304L showing mechanical deformation near the machined edge

Fig.2 SS304L showing machined edge

Fig3. Zircaloy 4 showing uniform structure in the bulk and slightly deformed structure near the machined edge.

Optical microscopy was done to inspect visual differences in the bulk layer to the region near the machined edge.

FEG-SEM (SS-304L)

Fig 1. An SEM image showing the machined layer.

The above image shows the machined layer and its different structure as

compared to the bulk material. The affected layer is of the order of 8

microns.

Fig 2. Enlarged image of the machined layer.

This enlarged image of the machined layer depicts plastic flow in the

material which took place due to the machining action. In this region there

is very heavy plastic deformation.

TEM (BARC)(SS-304L)

Fig 1. A bright field TEM micrograph showing some grains.

The above bright field image shows some of the visible grains with an average size of ~30nm.

Fig 2. A dark field image showing the grains.

The above dark field image clearly depicts the grains of average size around ~30-35nm.

Fig 3. A diffracted beam image depicting the ring pattern.

The above image shows the diffraction ring pattern which is an indication of the nanocrystalline nature of machined layer.

TECNAI-TEM (TIFR)(SS-304L)

Fig 1. A single grain having a size of around ~30nm.

The above bright field image shows a single grain of size ~30nm and is distinguishable by the lattice fringes present in a single orientation within.

Fig 2. Another image showing a grain of around ~25nm.

The above image shows another grain of size ~25nm.

Fig 3. An Bright Field Image showing a number of grains.

The above bright field image shows a number of grains in the machined layer have in average size ~25 - 30 nm.

Fig 4 . A Dark Field image showing the grains.

The above Dark Field image shows some grains having an average size of ~20nm

Fig 5. A diffracted beam image depicting a ring pattern.

The above image shows the diffraction ring pattern which is an indication of the nanocrystalline nature of machined layer.

Glancing Angle XRD

SS-304L : Polished Side

SS-304L : Machined Side

It is evident from the above XRD graphs that there is no extra peak in the

machined side graph if compared to the polished graph. If there had been

any new phase formation during machining, a new peak would have been generated in the machined graph.

It might also be possible that if at all a new phase has formed in the

machined layer, its quantity is too small to detect or its peak might also lie

beyond the range of orientation(2theta value) taken here.

Zircaloy-4 : Polished Side

Zircaloy-4 : Machined Side

Comparing the two XRD graphs there is again no new peak formation in the machined side graph.

EBSD

SS-304L :

Details Of the process: ● Operator: support ● Calibration: 0.596700 0.676900 0.601000 ● Working Distance: 15.000000

● Number of points: 358358 ● Number of good points: 358350

● Dimensions: ● X Min: 0.00 microns ● X Max: 250.00 microns ● Y Min: 0.00 microns ● Y Max: 309.60 microns ● Step: 0.50 microns

● Average Confidence Index: 0.28 ● Average Image Quality: 810.55 ● Average Fit [degrees]: 1.50

● Phases:

•Iron - Gamma

•Iron - Alpha

Phase Map:

Fig 1. An EBSD Phase Map image of SS-304L.

In the above phase map, the austenite phase is represented by red color

and the martensite is represented by green color. Austenite phase

dominates the mid and bottom part (81.3%).

In the top region (Machined Layer), the grain size is extremely small and

also has a good amount of martensite, denoted by green regions among

the red regions(18.7%).

Zircaloy-4:

Details of the process: ● Operator: support ● Calibration: 0.546719 0.770934 0.679779 ● Working Distance: 16.000000

● Number of points: 139653 ● Number of good points: 139653

● Dimensions:

X Min: 0.00 microns

X Max: 132.50 microns Y Min: 0.00 microns

Y Max: 227.33 microns Step: 0.50 microns

● Average Confidence Index: 0.49 ● Average Image Quality: 694.60 ● Average Fit [degrees]: 0.88 ● Phases: Zirconium (Alpha)

Fig 2. An IPF image of Ziracloy-4.

From the above image, it can be corroborated that there is no appreciable

change in the microstructure of Zircaloy-4 due to machining (top region).

No phase change has occurred due to machining.

CONCLUSION

1. Inferring from Optical Microscopy -Machined edge region showed deformation and non uniformity in its structure in contrast to the bulk region. 2. From SEM micrographs we can conclude that when SS304L undergoes

machining, its microstructure change upto a distance of 8 microns from its

edge. Large amount of plastic deformation takes place in that region

thereby affecting its properties.

3. From TEM micrographs, Diffraction pattern assured a nanocrystalline structure near the edges of the order of 30nm (average grain size).

4. From EBSD martensite phase was detected at 18.7 % of the total area 250 micron X 309.6 micron scanned near the machined edge of SS304L.

Fine Martensitic grains formed primarily near the machined edge to a

distance of 80 microns from the edge. This clearly states that martensite

formation took place due to machining.

5. In SS304L deformed layer extended from the edge to a distance of 8

microns whereas phase change occurred upto a distance of 80 microns.

Thus the deformed layer affected the microstructure of the specimen well

beyond its boundaries.

6. From Glancing angle XRD no different phase was recorded in SS304L

and Zircaloy 4. On the contrary EBSD showed martensitic formation thus,

we can conclude that martensite traces formed might not have been

detected due to constraints of the orientation (2theta) values chosen. When

calculated theoretically martensitic peak comes at 2theta value of 95

degrees as opposed to the 2theta range taken in the experiment 10-90

degrees

REFERENCES

1 . Microstructural changes in AISI 304L stainless steel due to surface

machining. By S. Ghosh, V.Kain , Journal Of Nuclear Material 403 (2010)

62-67.

2. Swati Ghosh, Vivekanand Kain, Materials Science and Engineering A 527 (2010) 679.

3. Microstructure of warm worked zircalloy-4. By H. Paul, M. Darrieulat, N. Vanderesse, L. Litynska, M. Miszczyk 4. Experimental Evaluation of the Rolling Reduction and Heat-Treatment

Effects on the Texture and Creep Behavior of a Zircaloy-4 Sheet. By Yoon-

soo Lim, Hyun-Gil Kim and Yong-Hwan Jeong

5. Characterization Of Thin Films by Glancing Incidence X-Ray Diffraction. By R.D. Tarey, R.S. Rastogi and K.L. Chopra