Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.5, pp.455-461, 2011 Printed in the USA. All rights reserved
Synthesis and Characterization of Ultrafine Grained 304 Stainless Steel
through Machining
V. Senthilkumara*, K. Leninb
a Department of Production Engineering, National Institute of Technology, Trichy, India
b Department of Mechanical Engineering, Jayaram College of Engineering and Technology,
Trichy, India
* Corresponding Author:
The microstructures of 304 stainless steel chips created by plane strain machining at ambient
temperature have been analyzed using scanning electron microscopy (SEM) and the crystallite
size of the ultrafine chips were analyzed using X-Ray Diffraction Analysis. The strain imposed in
the chips was varied by changing the tool rake angle. An attempt is made in the present
investigation to correlate the plastic strain, strain rate with the grain size of the stainless steel.
Keywords: Plane strain machining; Nanocrystalline; strain rate.
Nanostructured materials, containing submicrometer sized grains, have novel attributes not
typically found in conventional materials [1]. Methods to make nanostructured metals and alloys
directly in bulk form have relied on the use of very large strain deformation or severe plastic
deformation (SPD) to achieve microstructure refinement. High-strength metals and alloys are
difficult to process by SPD methods [2]. Equal channel angular pressing, high-pressure torsion,
rolling and wire drawing have been routinely employed as severe plastic deformation (SPD)
processes for introducing large strains. Typically, these processes involve multiple passes of
deformation with changes in orientation of deformation between passes. Microstructure
refinement in bulk materials can also be realized by the process of chip formation as in plane
strain machining [3, 4]. Machining parameters such as rake angle, depth of cut, and speed
influence the strain rate imposed by the cutting tools [5]. This suggested that machining is an
456 V. Senthilkumar, K. Lenin Vol.10, No.5
attractive process for producing nanocrystalline materials. In the present study 304 stainless steel
was taken and suitable machining parameters were chosen to produce nanostructured chips.
The chemical composition of the 304 Stainless Steel (SS) specimen taken for the present
investigation is given in Table 1. For machining the specimen a tungsten carbide cutting tool
with negative rake angle was used under orthogonal cutting condition, the machining parameters
used in the present investigation is given in Table 2. Machining was carried out on a CNC lathe
with sinumeric control. The chips so produced under different cutting conditions were
subsequently analyzed for their microstructural evolution and crystal characteristics.
Table 1: Composition of 304 stainless steel
304SS C Mn Si P S Cr No N
0.08 2 0.75 0.045 0.03 20 10.5 0.1
Table 2: Machining parameters used in the present study
Depth of cut, mmSpeed
Feed Rate,
rev/mm Rake
0.5 80,90 and 100 0.25 -6
X-ray diffraction analysis was carried out on the polished samples of Al-TiC composites in as
sintered and extruded conditions using automated Rigaku Ultima III XRD. The samples were
exposed to Cu Ka radiation (k = 1.54056A ˚) at a scanning speed of 20/min. The Bragg angle and
the values of the interplanar spacing d obtained were subsequently matched with the standard
values AISI 304 stainless steel.
3.1 Microstructure Evolution during Machining
The SEM picture of the AISI304 SS machined chip is shown in Fig 1. It is clearly evident that
machined chip microstructures are refined in the submicrometer level due to large strain
deformation imposed by the cutting tool at the machining parameters as for a cutting speed of
100 rpm.
Vol.10, No.5 Synthesis and Characterization of Ultrafine Grained 304 Stainless Steel 457
3.2 Crystallite Size Reduction
XRD pattern of machined chips under varying cutting speeds are shown in Fig.2. The peak
widths obtained in an X-ray diffraction pattern is related to the size of crystallites that composes
the material. The size of the crystalline was determined by measuring the peak width and
substituted in the Scherer equation. Crystalline size of the chips was found to be 66, 86.1 and
71.8 nm respectively for the cutting speeds of 80, 90 and 100 rpm.
Fig. 1 SEM micrograph of Ultrafine grained chip after machining
Fig.2 XRD pattern of machined chip of AISI 304 SS at different cutting speed
458 V. Senthilkumar, K. Lenin Vol.10, No.5
3.3 Effect of Plastic Strain, Strain Rate on Crystallite Size
Machining is a typical deformation process involving large strain and high-strain-rates. The
microstructures and the mechanical properties of the machined chips are directly related to the
degree of plastic deformation, the understanding of the plastic deformation behavior of the
workpiece during the process is very important for the determination of the process conditions.
The geometry of machining in its simplest manifestation, i.e. plane strain machining has been
shown in Fig.3, is characterized by a sharp, wedge-shaped tool that removes a preset depth of
material, the undeformed chip thickness, by moving in a direction perpendicular to the cutting
edge. Chip formation occurs by concentrated shear within a narrow, primary deformation zone
often idealized as the ‘shear plane’. The geometry of the primary deformation zone and shear
strain are determined by the shear angle and the rake angle.
Fig.3 Schematic of plane strain machining with other parameters
The shear angle is calculated using the mathematical formulation as follows [6].
arctg (1)
where γ is the tool rake angle. The plastic strain was calculated using the following method:
sin21 2
X (2)
Finally, the plastic strain rate can be found with the following formula:
Vol.10, No.5 Synthesis and Characterization of Ultrafine Grained 304 Stainless Steel 459
where λ is the chip compression ratio, Vc is the cutting speed, is the cutting shear angle and Δx
is the elemental chip thickness. From the above mathematical expressions (1), (2) and (3) plastic
strain and strain rate were calculated and shown in Table 3. The crystallite size calculated from
Braggs formula using XRD peak width is also given in Table 3.
Table 3: Machining parameters calculated for different cutting speeds
Cutting speed, rpm
Plastic Strain
Strain rate, sec-1
Crystallite size, n m
80 2.04 399.576 66.02
90 1.94 415.237 86.1
100 1.86 433.236 71.84
The effect of cutting speed on deformation characteristics such as plastic strain and crystalline
size were given in Fig. 4(a). From the above graph it was found that as the cutting speed
increases the crystallite size increases initially (66.02 nm to 86.1 nm) due to temperature rise
which causes the grain growth, however, further increase in cutting speed increases the strain
rate and plastic strain (severe plastic deformation) as shown in Fig. 4(b) resulting in decrease in
crystallite size (86.1 nm to 71.84).
Fig. 4 (a) Effect of plastic strain on crystallite size
460 V. Senthilkumar, K. Lenin Vol.10, No.5
Fig. 4(b) Effect of cutting speed on strain rate and crystallite size
Present investigation demonstrated that the synthesis of nanostructured AISI 304 stainless steel is
possible through severe plastic deformation in machining. The effect of cutting speed on
deformation characteristics such as plastic strain and strain rate has been studied. Further,
investigation reveals that the combined effect of plastic strain and strain rate is critical to the
reduction of crystallite size.
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