Analysis of notch sensitivity factor for ss420 and ss431 over en24

Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
17 – 19, July 2014, Mysore, Karnataka, India
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 5, Issue 9, September (2014), pp. 109-114
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IJMET
© I A E M E
ANALYSIS OF NOTCH SENSITIVITY FACTOR FOR SS420 AND SS431
OVER EN24
Bindu S1, S.K Verma2, Nagaraj P.M3
1P G Student, Siddaganga Institute of Technology, Tumkur, Karnataka, India
2Scientist-E, Central Manufacturing Technology of Institute, Bangalore, Karnataka, India
3Assistant Professor, Siddaganga institute of technology, Tumkur, Karnataka, India
109
ABSTRACT
Tempered martensitic stainless steel has a number of attractive features including swelling resistance, high
thermal loading capacities and low activation variants. Hydraulic tensioning rod made of SS420 material has designed
factor of safety 1.2. This imposes stringent quality requirement on raw material and heat treatment for this part. This part
has got internal threads and is assembled with stud and subjected to tensile load. It is observed that this part fails at the
end of the thread. End of the thread acts as a notch. Such a notch can be said to have an elastic stress concentration factor.
Notch sensitivity ratio is calculated. Fracture occurs when the applied tensile stress exceeds critical stress over a critical
distance. Alternatively this part is made of SS431 material. The fracture behavior of stainless steel SS420 and SS431
under consideration of notch sensitivity and heat treatment are studied using finite element analysis and experimental
methods. EN24 material will be considered as the ideal material. The experimental plans are conceived to simulate the
real situation failures of (stud tensioner) tensioning rod under considered parameters. The outcome of these experiments
is to predict the ultimate tensile properties of above considered materials. In addition to this, stress concentration levels
will be predicted using finite element simulations. Tensile tests are carried out on universal testing machine. Notch
sensitivity factor for both material are calculated. Fractorography tests are conducted to obtain suitable results. Material
with low notch sensitivity is considered for manufacturing sector.
Key words: Martensitic Stainless Steel, Failure Analysis, Heat Treatment, Hardness, Material, Finite Element Analysis.
1. INTRODUCTION
Failure analysis is an approach of engineering technology determining various causes of equipment or
component failure. Failure causes through structural loading, wear, corrosion, and latent defects etc. The aim of a failure
analysis is to know the root cause of the failure so as to prevent similar failures in the future. In this study part named
tensioning rod leads to failure. This part is made of SS420 martensitic stainless steel. Tensioning rod and stud of 20+
numbers will be assembled circularly in a fashion of nut and bolt in hydraulic tensioner which will be used to close the
lid of pressure vessels. Tensioning rod and stud assembled by thread contact and hydraulic tensile load has been applied
to tensioning rod to tighten and loosen the nuts by suitable pressure. But at the end of the thread, failure occurs because
of unengaged threads at the end. End of the thread will be considered as notch. Tensioning rod as the major aspect in our
research we consider all the parameters of tensioning rod for failure detection such as selection of material, heat
treatment cycles, notch effect.
The literature showed that, Martensitic stainless steels are resistant to radiation damage such as swelling. G.R.
Odette, G.E. Lucas [1] investigated that Coupled measurements of tensile and sharp notch were conducted on a heat of

HT-9, which is a 12% Cr martensitic stainless steel. By this, it was found that fracture can be controlled by critical stress-critical
distance criteria. The fracture stress is 2400 MPa, and the critical distance is 55 μm which is consistent compared
to austenite grain size. George K. haritos, Theodore Nicholas, David B.Lanning [2] investigated for cylindrical
specimens of same V-notches having stress concentration factor Kt, approximately 2.78.The loading technique for
obtaining constant life, haigh diagram was implemented on each specimen. Experimental tests were carried out at stress
ratios of 0.1, 0.5, and 0.8 for bar and plate forgings Results obtained that there is a definite notch size effect in the bar
within the range of notch sizes tested, little or no such effect exists in the plate. G.R. Odette[3] explained the cleavage
fracture toughness and cleavage to microvoid transition. Stress occurs over above critical area. Cleavage includes high
toughness particularly in shallow cracks and small size scales .fracture in the transition mode occurs when strains of
process zone exceeds the critical level. Material property with finite element calculations helps in predicting the effects
of size, geometry, loading rate and failure structures of the sub sized specimens. M.A.Neri, R.Colas [4] investigated for
set of cutting blades made of martensitic stainless steel for preparing sausages. Failure analyses were performed and the
blade failure was due to fatigue and impact .it was concluded that blade failure was due to course carbide. According to
nuclear regulatory guide 1.65 [5] of materials and inspection, material for reactor vessel closure studs, for high-strength,
large-diameter bolting, care must be taken to make over fracture, and bolting materials should have to posses toughness
throughout the reactor operating cycle. Control of the stud bolt tempering is very important. Tempering procedures on
grades of steel such as American Iron and Steel Institute 4140 and 4340, which is equivalent to EN24 are preferred.
These steels are approved by ASME as bolting materials.
2. MODELING AND ANALYSIS OF TENSIONING ROD AND STUD
2.1 Introduction
To measure the stress concentration of tensioning rod, the component has to be modeled first, as per the specification 3D
solid model is ,made to study the detailing of the drawings, this was performed on Unigraphics software.
Fig.1: Solid model assembly of tensioning rod stud
110
2.2 Finite element process
1. Building the solid model by Unigraphics.
2. Generating the finite element model.
3. Setting up boundary conditions.
4. Applying loads.
5. Analysis.
The detailed finite element meshes are established with tetra mesh. The mesh reflects the connectivity of
elements. The boundary conditions include constraining the structure i.e. one side of the model will be fixed and the
pressure of 129 Mpa has been applied on the opposite side of the model. The results of the stress concentration are shown
in figure 2 and figure3.

Fig.2: Stress distribution in tensioning rod stud assembly.
Fig.3: Maximum stress induced at the end of thread with 921.63 Mpa
Fig 4: Maximum stress induced at the end of thread with 789.06 Mpa
111
Input Values:
a. SS420 and SS431
1. Desired stress: 800 Mpa
2. Obtained maximum stress: 921.63 Mpa
3. Analysis input:
a. young’s modulus: 2.e+005
b. Poisson’s ratio: 0.28
c. Bulk modulus: 1.6667e+005
From the above analysis it was concluded that stress concentration was found in SS420 and SS431 material.
b. EN24

112
1. Desired stress: 800 Mpa
2. Obtained maximum stress: 789.06 Mpa
3. Analysis input:
a. young’s modulus: 2.e+009
b. Poisson’s ratio: 0.3
c. Bulk modulus: 1.57e+006
The maximum stress obtained for EN24 material is 789.06 Mpa which is below than the desired stress and full
strength of material will be attained, which is shown in figure 4
3. EXPERIMENTATION OF COMPONENT
3.1 Fabrication
Fig 4: Fabrication of Stud tensioner
Once the drawing has been approved next step is to manufacture the prototypes. They are manufactured in 3 sets
of SS420, SS431 and EN24 each at different tempering conditions. After fabrication process the components are been
subjected to ultrasonic cleaning for removing the dirt and chips that are found in between the threads. Components were
inserted into IPA solution and get vibrated to remove the dirt.
3.2 Experimental setup
Stud tensioner is subjected to tensile load on Universal Testing Machine, one side of the component is made to
fix and the other side is made to pull. At the maximum load point material get fails. If it is failed below desired tensile
strength of the material then stress concentration will be found, if the material is taking the full strength then the material
does not concentrate any stress.
The figure-5 below shows the component failure at the area that was predicted by finite element analysis.
Fig 5: Failure of the component
Tensile Notch sensitivity ratio is calculated using below formula
Snet(for notched specimen at maximum load)
NSR =
Su (for unnotched specimen at maximum load) (1)
3.2.1 Tempering at 620ºC-32HRC

113
a. SS420
Snet Su NSR
985.67 N/mm² 1030 N/mm²
b. SS431
Snet Su NSR
925.20 N/mm² 1030 N/mm²
c. EN24
Snet Su NSR
1037.5 N/mm² 1030 N/mm²
3.2.2 Tempering at 475ºC-42HRC
a. SS420
Snet Su NSR
1257.9 N/mm² 1350 N/mm²

b. SS431
Snet Su NSR
1066.8 N/mm² 1350 N/mm²
c. EN24
Snet Su NSR
1355.5 N/mm² 1030 N/mm²
4. RESULTS AND DISCUSSION
Table 1: NSR values at different tempering temperatures
620ºC 650ºC
SS420 NSR 0.956 0.931
SS431 NSR 0.898 0.790
EN24 NSR 1.0072 1.0041
From the above NSR values it can be said that Notch sensitivity is found both in SS420 and SS431 materials. It
is more in case of SS431 comparatively. Due to the 12-14 % chromium content in the composition of the material, so the
NSR values of both materials are lesser than 1. Whereas, in case of EN24 material chromium content is 0.44%, which is
much lesser in other two materials. Even though it is a non corrosive resistance material, it behaves with a very high
toughening resistance, so it is most recommended to use EN24 material for manufacturing sector.SS420 and SS431
materials failed due to brittle fracture and EN24 material failed due to ductile fracture.
Fractography tests were conducted to know the behavior of the grains and to analyze the reason for the
failure.[6]
(a) (b)
Fig 6: Effect of test temperature on a SS420 that was heat treated to a hardness of 32 HRC (a) and 42 HRC (b)
and tensile tests were conducted failure occurs by intergranular fracture

(a) (b)
Fig 7: Effect of test temperature on a SS431 that was heat treated to a hardness of 32 HRC (a) and 42 HRC (b)
and tensile tests were conducted. Brittle fracture has been occurred
(a) (b)
Fig 8: Effect of test temperature on EN24 that was heat treated to a hardness of 32 HRC (a) and 42 HRC (b) and
tensile tests were conducted. Fractures consisting entirely of dimple rupture
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5. CONCLUSION
This study was conducted on a failed hydraulic tensioning used in stud tensioner. Finite element analysis and
mechanical testing results identified the material of the failed stud bolt as SS420 and SS431 martensitic stainless steel.
Crack initiation occurred at maximum stress concentration points. The fracture surface is characteristic of failure caused
by tensile stress and a small amount of cyclic load. The tensioning rod fractured below the critical stress of the material.
Since martensitic stainless steel SS420 and SS431 has low toughness. High-strength, low-alloy reactor stud bolting
material as EN24 is most preferable
6. REFERENCE
[1] G.R.Odette, G.E.Lucas, R.Maiti, and J.W. Sheckherd, The Micromechanical Mechanisms Of Cleavage
Fracture In Martensitic Stainless Steels, Journal of Nuclear Materials 122123 (1984) 442447 North-Holland,
Amsterdam.
[2] George K. Haritos, Theodore Nicholas, David B. Lanning, Notch size effects in HCF behaviour of Ti-6Al-4V,
International Journal of Fatigue 21 (1999) 643–65.
[3] G.R.Odette, On the ductile to brittle transition in martensitic stainless steels -Mechanisms, models and structural
implications, Journal of Nuclear Materials 212-215 (1994) 45.
[4] M.A. Neri, R. Cola, Analysis of a martensitic stainless steel that failed due to the presence of coarse carbide,
Materials Characterization 47 (2001) 283– 289.
[5] Nuclear Regulatory Guide, Materials and inspections for reactor vessel closure studs, Draft was issued as DG-
1211, dated April 2009.
[6] Fractography, ASME handbook, Volume 12, The material information company

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Analysis of notch sensitivity factor for ss420 and ss431 over en24