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Kumar, Ravi Ranjan

Machine-Learning Driven Modeling and Optimization of Independent Cultivation of Oleaginous Yeast and Microalga for Biodiesel Production Leading Towards Developing a Closed-Loop Integrated Symbiotic Ecosystem

Singh, Vasundhara

ABSTRACT 
ON 
STUDY ON HOT DEFORMATION BEHAVIOUR AND WELDABILITY OF 
EN30B/EN25 STEEL GRADES 
Submitted by 
VASUNDHARA SINGH 
Research Scholar 
17MT91R12 
 
UNDER THE SUPERVISION 
 OF 
PROF. GOUR GOPAL ROY 
(INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR) 
ASSOC. PRAKASH SRIRANGAM 
(UNIVERISTY OF WARWICK, UK) 
 
 
METALLURGICAL AND MATERIALS ENGINEERING DEPARTMENT 
 
INDIAN INSTITUTE OF TECHNOLOGY 
KHARAGPUR-721302 
JANUARY 2024 
  
 
ABSTRACT 
The thesis aims to investigate two medium carbon low alloy steel (MCLA) grades, EN30B and 
EN25, on two major aspects, namely the hot deformation behaviour and joining characteristics 
using sophisticated electron beam welding. These MCLA grades contain Ni in the range of 2.4 
to 4% along with Mn, Cr, Mo and are primarily used for producing heavy machineries in 
mining industries like excavators and shovels, besides their application in aerospace and 
automobile industries.   
Although several studies have been reported on the hot deformation behavior of various 
MCLA, studies on the hot deformation behavior of EN30B and EN25 steels are conspicuously 
absent in the literature.   It is also observed that the hot deformation behavior of various MCLA 
grades is different and a vital function of alloying elements and their amount.  Therefore, each 
steel grade possesses unique processing maps identifying safe deformation zones.  Therefore, 
developing constitutive equations for flow stress vs. strain rate, temperature relationship, and 
processing maps for these European grades was found to be essential and form one of the 
objectives of the present study.  Additionally, inclusions in steel influence the dynamic 
response of the material under hot deformation and the evolution of the safe deformation zone. 
However, a study correlating the evolution of inclusion with the material’s hot deformation 
dynamic response has rarely been reported for MCLA steel, especially for EN30B and EN25 
steel.  
 
Although the welding issue for medium carbon steel is well known, the welding behavior of 
MCLA, especially EN30B and EN25 steels, needs to be reported better in the literature.  
Electron beam welding (EBW), characterized by low heat input and contamination-free 
welding, has not been attempted for EN30B and EN25 for sophisticated joining.   Therefore, 
EBW of EN30B and EN25 also form another objective of the present study.   
The hot deformation behaviour of EN30B and EN25B steel grades have been studied using the 
Gleeble® simulator 3800. The experimental flow stress values were analysed to generate the 
processing map based on dynamic material modelling reported in the literature. Such maps 
identified the safe deformation zone in the two-dimensional domain of strain rates and 
temperatures at various strain levels with maximum deformation up to 70%. At a comparatively 
lower strain, the safe zone locates in the high temperature and high strain rates regime for both 
the steel grades. At higher strain, the safe zone moves to lower strain rates and moderate to 
high-temperature regime for EN30B steel, while it moves further to higher temperature and 
slightly expands to lower strain rates for EN25 steel. The development of such safe deformation 
zones has been explained based on the evolution of microstructure, estimated DRX, inclusions, 
and precipitates during deformation. At lower deformation levels, microstructural examination 
indicated lack of mobile dislocations, which reduced the deformation ability for microstructural 
change at low strain rates. At a higher deformation level, which generates several dislocations 
and also defects for nucleation of DRX, safe zones move to lower strain rates and high 
temperatures, which allows sufficient time and thermal activation for dislocation movement 
and rearrangement to form strain-free grains through DRX. At higher strains, the instability 
region also expands, especially at high strain rates, due to the formation of deformation bands 
leading to inhomogeneous deformation, crack formation, and failure.   
The non-metallic inclusions (NMIs) analysis was carried out by electrolytic extraction and 
subsequently analyzed by SEM and XCT. The results demonstrated that the count and size of 
the inclusions decreased at high strain rates and temperatures, corroborating the location of the 
safe deformation zone. In fact, at the higher strain levels, the competition between the favorable 
and unfavorable deformation, assisted by the presence of second phases in the matrix 
determines the location of the safe zone. Cracks originated at complex inclusions where Al2O3 
and MnS were embedded and propagated through unstable plastic deformation zone in the 
processing map at lower temperatures and higher strain rates. Crack propagation was restricted 
in the safe deformation zone, attributed to the higher volume fraction of the dynamically 
recrystallized grains. 
Butt welding of EN30B and EN25 steel grades was performed using electron beam welding 
with or, without beam oscillation at different oscillation beam diameters. The evolution of non-
metallic inclusions, microstructures, and mechanical properties of these steel grades have been 
investigated. Beam oscillation is an useful tool to develop a fine surface finish, and weld with 
minimal defects, attributed to generating churning action and heat mixing in the weld seam. 
Some common features that were observed for welding of both the steel grades are: (i) beam 
oscillation promoted equiaxed grain in the center region of the fusion zone, attributed to 
breaking of unidimensional temperature gradient and heat mixing in the weld seam. ii) The 
retained austenite in the weld was higher in weld produced with beam oscillation, especially 
with higher oscillation diameter, and became minimum for weld produced without beam 
oscillation, attributed to decreasing cooling rate and strain induced transformation. iii) The 
application of beam oscillation resulted in a significant decrease in the size of the non-metallic 
inclusions, attributed to rapid cooling. Residual stresses were tensile in the fusion zone, 
irrespective of beam oscillation. Weld produced with beam oscillation was characterized with 
high amount of dislocations, lattice strains, residual stress, hardness, and wear resistance.  In 
the case of EN30B steel, weld strength was lower than base metal; however, the weld's strength 
and ductility improved with beam oscillation. In case of EN25 steel, the tensile sample braked 
in the base metal, indicating good strength of all welds. 
 


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