EFFECT OF SEVERE PLASTIC DEFORMATION ON MECHANICAL PROPERTIES OF WELDED ST37-2 STEEL

Cold treatment techniques are used to enhance the mechanical properties of metal alloys, whose most important characteristics are strength, roughness, and microstructure. The aim of this research is to test the effect of Conventional Shot Peening (CSP) and Severe Shot Peening (SSP) on the mechanical properties of ST37-2 steel. The results of the experiments showed enhancements in surface roughness and tensile strength. However, shot peening decreased the ductility of the metal and caused changes in its microstructure that are indicated in the XRF and XRD tests. Results’ data are provided as an original contribution to the literature while they are compared with the existing data. Keywords-ST37-2; shot peening; severe plastic deformation

Additionally, iron has several technical properties that makes it, and its most famous alloy steel, one of the preferred elements to work with [1].
Due to its good weldability and ability to provide high performance in terms of hardness, ductility and strength, low carbon steel is used to manufacture different parts of heavy machinery and vehicles [2]. The usage of low carbon steel in this industry is due to several advantages, including ease of forming, high toughness, cost and strength [3]. Low carbon steel is extensively used in construction as main elements, jointing components, and reinforcements for concrete elements. Various structures are created with low carbon steel, such as transmission towers, rail tracks and industrial buildings. Due to it is ability to be rolled, angles and sections are created with low carbon steel, in addition to sheets and bars [4]. the weight of the tool is reduced with the use of less steel material. Low carbon steel is highly conductive for electricity and heat.
ST37-2 is a low carbon mild steel that is mostly used as a structural metal due to several performance criteria it possesses. The alloy has excellent weldability and good ductility, which qualified it to be used in many applications, including shelters and water vessels, and can be shaped for several purposes as angles, strips, sheets, and plates.
ST37-2 is a low carbon steel used for structural purposes, and it is also non-alloy in its standard form processed through hot rolling. It has a relative density of 7.85 kg/dm 3 , according to volumetric mass calculations.
Surface treatment using cold techniques is widely used to enhance the mechanical properties of the metal alloys [5]. Shot peening is one of these processes that has its impact on the surface roughness, residual stresses, microstructure and folding of the metal [6]. The effects of plastic deformations resulting from welding or shot peening can be beneficial or have adverse effects on its strength and ductility [7]. Severe plastic deformation (SPD) is formed in metals through processes, such as hydrostatic extrusion, which performs deformations in the metal at low temperatures, in comparison with other techniques. SPD results into a fine crystalline structure, that differs from the crystallographic structure of the original metal or alloy, through forming micrometric and submicrometric sub-grains in the coarse grain of the original material [8]. The advantages of SPD on performance and mechanical properties through its ability to achieve deformations in the microstructure through fine grains, which reflects on the performance results of hardness and yield stress to saturation levels [9]. However, SPD disadvantages are embodied mainly in the decreased ductility, decreasing the metal ability to undergo plastic deformation under stress [10].

PROBLEM, SCOPE AND METHODOLOGY
There were many studies that discussed the positive effects of steel cold treatments, especially shot peening on its mechanical properties [11]. The process of shot peening impose compressive stresses on the surface of the metal, which is faced back with a tensile stress from the inner layers. The status of equilibrium between the two forces creates residual stress that increases the hardness of the metal. Furthermore, the effect of shot peening eliminates failures caused by stress corrosion and fatigue that originate at the surface of the material and propagates [12].
Several steel types have been tested for the effect of shot peening on them, including stainless steel [13], steel sheets [12], high strength QT CrV steel [6], high strength QT CrMo steel [14], case-hardended CrNiMo steel [15], medium carbon CrNiMo steel [16], Dual-phase steel [17], and many other steel alloys. Nonetheless, the majority of these studies are focused on testing the fatigue resistance, frequently studying the microstructure, and rarely addressing strength criteria. Furthermore, there are almost no studies that tested one of the most used structural steels in many industries, which is ST37-2. Therefore, the current study bridges this gap through studying the effect of different shot peening intensities in ST37-2.

PURPOSE OF THE STUDY AND QUESTIONS
The main aim of the current research is to test the effect of SPD through electric arc welding and shot peening on the mechanical and microstructure of ST37-2. It is expected for these processes to affect the microstructure and homogeneity of the specimens, which leads to altering their mechanical properties. There are several objectives that are achieved through the course of this research:  Study the basic processes, physical properties, and mechanical properties of steel, which are potentially affected by cold treatment methods.
 Understand the metallurgy properties of steels, and low carbon steels specifically.
 Study steel classification and designation systems.
 Analyze the mechanical properties and microstructure characterization of ST37-2, and its equivalent in different standards, through surveying the literature for studies that provided experimental data, descriptions and findings on the studied alloy.
 Understand the shot peening mechanism and assess its impact on the surface properties of the affected metal, as well as the impact on the internal layers and their reaction to the treatment.
 Provide a review of the most significant applications of low carbon steels, in addition to their advantages and disadvantages.
 Prepare ST37-2 samples with permanent plastic deformations through cutting and welding, in addition to the application of two types of shot peening: one shot peening type for each sample.
 Assess the mechanical properties through a series of tests for tensile strength, toughness, and hardness.
 Assess the microstructure characterization by Scanning Electron Microscopy (SEM) and optical image microscope, in addition to chemical composition using Energy Disruptive Spectroscopy (EDS).
 Analyse the obtained tested results and discuss their implications in conjunction with the results of similar literature.

ORGANIZATION OF THE STUDY
The research is primarily divided into two main parts: theoretical framework and literature review, and experimental application. For the fulfilment of these parts, the thesis is divided into five chapters:  Introduction: a brief review of the research subject is carried out, and the research problem, scope and used methodology is identified for further development of the study. Thereafter, the main aim of the study, as well as the objective of each research phase are provided.
 Literature review: a study of the history of iron and steel mining, production, and manufacturing, followed by two sections focusing on the physical and mechanical properties of carbon steels, in addition to the metallurgical properties of steels and their alloys. The designation systems used to identity each steel type with a unique number is provided for further understanding. A review of the literature is performed to cover studies that included ST37-2 in their experiments. A section is provided on shot peening and its mechanism.
Finally, a section on the applications of low carbon steels is provided to understand their advantages and disadvantages.

PROPERTIES OF STEEL
For over 3286 years, steel has made a major contribution to human development, e.g., in tools for cultivating the soil and processing stone and almost all other materials, as a construction material for steel and reinforced concrete structures, in transport technology, for the generation and distribution of energy, for the fabrication of machinery and equipment (including equipment for the manufacture of plastics), in the household, and in medicine. It remains, for the foreseeable future, by far the most important material for the maintenance and improvement of our quality of life [18].
The outstanding importance of steel is the result of its ready availability and its versatility. The earth's crust contains ca. 5 wt.% iron, making it the fourth most abundant element after oxygen (46%), silicon (28%), and aluminum (8%). Rich deposits of iron ores are available in many parts of the world. Moreover, the free energy required to isolate iron from its oxide ores is less than half of that required for aluminum [18].
The versatility of steel is due to the polymorphism of the iron crystal and its ability to alloy with other elements, forming solid solutions or compounds. The microstructure of steel in a finished component can be adjusted by means of the chemical composition, the forming conditions, and a wide variety of possible heat treatments.
The attainable tensile strength ranges from ca. 300 N/mm 2 for deep drawing sheet steel (e.g., for automotive body parts that are difficult to draw) to >2000 N/mm 2 for critical components in aircraft. Tensile strengths as high as 2600 N/mm 2 are achieved in 0.15 mm diameter drawn wire for steel cord used in radial tires [19].
Cryogenic steels with high strength and good toughness at very low temperatures are used for the transport and storage of liquefied gases at temperatures of ≤ 200 °C.
Other steels with good properties at temperatures of 650 -700 °C and above are used in power station equipment and gas turbines [20].
Highly developed soft magnetic steels are essential in the construction of transformers. Steel is also used to make permanent magnets. Non-magnetizable and here steel has an advantage over competing materials [1].
Modern knowledge of controlling the micro-structure of steel, and hence its properties, offer opportunities to match steel products to new sets of requirements [1].
Unlike brick or concrete buildings, steel structures can be dismantled relatively easily. Furthermore, almost 100 % of the steel can be re-covered from steelcontaining products and can be re-melted to yield steels of similar or higher quality.
In this respect, iron and steel are superior to all competitive materials [1].
The great importance of steel in the world's economy is also exemplified by production figures. In the early 1900s, total world production of steel was less than 35106t/a. In 1940, it was 140x10 6 t/a. The figures for the period after 1950 ( Figure   2.1) indicate a surprisingly large growth in world crude steel production after World War II. Up to the mid-1970s, this was mostly due to those developed countries with the greatest rate of economic growth, such as Japan, the six founding countries of the European Community, and also the former Soviet Union. In the United States, growth had already ceased by the mid-1960s due to market saturation [21].
A rapid increase in steel production also took place in some Latin American countries, continuing until the mid-1980s. In many countries in Asia, Africa, and the Middle East, new steel industries were built up, or existing capacity was increased.
The developments in some Asian countries during the last 16 years are remarkable (    [22]. Steel is an alloy of iron with varying amounts of carbon content (from 0.5 to 1.5%).
Steel, being an alloy and therefore not a pure element, is not technically a metal but a variation of one instead which results in them having similar characteristics, thus during this webpage we talk about steel as a metal and explain a lot of steel due to referring to metal. We know that one of the properties of a metal (ex. steel ) is that it contains a crystalline structure, which means that the atoms which are in the solid stage are arranged in regular [23], repeating patterns.
The smallest group of atoms which defines the atomic arrangement in a crystal is   Alloy steel is steel that is alloyed with a variety of "impure" elements in total amounts between 1.0% and 50% by weight. This is done to improve the mechanical property of the steel. Alloy steels are broken down into two groups namely: lowalloy and high-alloy steels. The difference between the two is somewhat arbitrary: Smith and Hashemi [25] define the difference at 4.0%, while DeGarmo, et al. [26], define it at 8.0%. Most commonly, the phrase "alloy steel" refers to low-alloy steels.
Strictly speaking, every steel is an alloy, but not all steels are called "alloy steels".
The simplest steels are iron (Fe) alloyed with carbon (C) (about 0.1% to 1%, depending on type). However, the term "alloy steel" is the standard term referring to steels with other alloying elements added deliberately in addition to the carbon. The most common alloyants of steel are manganese, nickel, chromium, molybdenum, vanadium, silicon, and boron [24].

Physical Properties of Steel
Steel is a term given to iron with carbon content, as well as a maximum silicon content of 0.5% and maximum manganese content of 1.5%. Therefore, there are four types of carbon steel depending on their carbon content [27]:  Less than 0.15% carbon content (dead mild steel).
The average density of steel is 7.9 times heavier than water, which makes its relative density 7,900 kg/ m 3 . It also has a melting point higher than most metals at 1,510 °C,

Mechanical Properties of Steel
The variation of the mechanical properties of steel mainly depend on its composition, processing conditions and manufacturing methods. In engineering design, it is essential to understand several aspects of the mechanical properties of steel in order to enable its usage for the suitable elements that can produce the required performance and efficiency. Thus, the understanding of the mechanical properties of steel include its compressive and tensile strengths, toughness, hardness, flexibility, ductility, malleability, and weldability. Other factors affect the mechanical properties of steel, including its heat treatment and the alloyant metals that are used in its composition [28]. The addition of alloys, such as vanadium and manganese, has the ability to increase the strength of steel; however, other properties are negatively affected, such as toughness and ductility [29]. The addition of nickel enhances toughness, while the elimination of Sulphur improves ductility. Due to the high impact of the chemical composition of steel on its mechanical properties, it is crucial to create the most suitable balance between all the elements for the achievement of the desired properties. There is a close connection between the influence of heat treatment and the chemical composition of steel, as well as the mechanical processing techniques that are used during heat treatment [30]. It was found that these factors highly affect all the mechanical properties including strength. The forming and rolling of steel play a major role in its strength, as studies show that yield strength is reduced with the thickness of the material. There are five main methods of heat treatment that are used for steel: as-rolled, normalized, normalizedrolled, thermomechanically-rolled (TMR) and quenched and tempered (Q&T) [31], as shown in Figure 2 [31].
When the steel is allowed to cool naturally, after being heated to 1200 °C, it is classified under as-rolled steel. When heated to 900 °C, kept under that temperature for a particular time, then cooled under the room temperature, the steel is classified as normalized, which has enhanced toughness due to the refinement of the grain size.
Similarly, the normalized-rolled steel is kept at 900 °C. It is essential to enhance the tensile strength in the steel for a higher performance but without affecting the necessary properties for ductility and toughness, which are best exhibited with low carbon steels that are treated to the finest grains. Such a result can be achieved through TMR steel that is rolled at 700 °C with higher force. When the steel is normalized at 900 °C and quenched/ cooled quickly, a steel with high hardness and strength is produced. Nonetheless, the quenching process reduces the toughness of the steel, which requires the material to be reheated for a maintained temperature of 600 °C and let it naturally reach to room temperature. The previously described process is named quenching and tempering that are used in conjunction to balance hardness, increase strength, and increase the toughness of the steel [31].
The mechanical properties of metals generally, and steel specifically, are presented through several parameters. The general stress-strain curve, shown in Figure 2  General stress-strain curves for steel before (left) and after (right) heat treatment [28].
During the design of engineering elements from steel, ductility, toughness, fatigue, modulus of elasticity and stress values are the most parameters that are focused on.
Due to the reverse relationship between the strength and other parameters, it is imperative to balance the requirements of the components to obtain the desired outcomes and know its design limits. The strength of the steel is governed by the amount of load it is able to carry without having any deformations. The yield strength is measured to the extent the component is bearing the load within its elastic range. Thereafter, a permanent and irreversible deformation occurs when this load is exceeded, while the component fractures at the ultimate tensile strength value [30].
The extent of stretching within the component is defined as strain, which is part of the plastic deformation that occurs after exceeding the yield strength. The ductility of the steel is measured through its ability to stretch without cracking during the plastic phase, which decreases with the increase of the strength of the steel. Ductility also depends on the carbon content and the heat treatment of the steel, as shown in Figure   2.6, as increased carbon contents lead to cracks during welding and heat treatment.
Based on the results of the tensile test, the modulus of elasticity is derived from the ratio of stress to strain, while heat treatment does not affect the value [32].  Another mechanical property of steel is its ability to resist impact, which indicates its behavior for cracking or fracture. The parameter measured in this test is the toughness of the steel when it is exposed to impact loadings or low temperatures and able to maintain its integrity without cracking. Therefore, the energy required to cause failure in the steel under s specific temperature is called toughness. For measurement, Charpy test is used to measure toughness, which is in reverse relationship with the strength of the steel specimen. Carbon content is a vital factor in reducing the toughness of the steel. As shown in Figure 2.8, the increase in carbon content of two heat treated steel specimens led to significant reduction in their toughness [33].  [33].
The ability of the material to sustain its surface integrity, when rubbed with another material under a specific loading, is measured through a wear test. There are several factors that are considered during the wear test, including corrosion, impact, abrasion and gouging.
The wear test is also a measurement of the steel hardness, as harder steel has the ability to resist wear better than softer counterparts. Steel with higher carbon content and higher strength performs better under the wear test, as these parameters are correlated to increased hardness. The steel has also to have sufficient toughness with high hardness to avoid permanent failure and cracking [34].
Loads below the yield strength value is applied to steel at high temperature to measure its creep, which is its ability to resist stretching permanently under these conditions. Thus, creep is measured as an elongation value with respect to the applied temperature and load, while failure is termed as stress rupture. Generally, steels with higher carbon content and added alloyants have better creep performance.
Oxidation is one of the important factors that determine the creep performance of steel, which is also linked to the corrosion behavior of the material. Corrosion tests can be performed on steel with the application of different weathering conditions, such as oxygen, pH, temperature, and chlorine [34].

STEEL ALLOY SYSTEMS AND DESIGNATION
Steel alloys are classified and designated through the SAE system, which uses a system of four digits to assign the chemical composition of the alloy. The SAE/ AISI  XXBXX for added boron between 0.0005% and 0.003%, which is used to improve hardenability.
 XXLXX for added lead between 0.15% and 0.35%, which is used to improve machinability.
 MXXXX for merchant quality steel, which is hot rolled used in non-critical elements.
 EXXXX for electrical-furnace steel  XXXXH for hardenability requirement

PROPERTIES OF ST37-2 ALLOY
ST37-2, also designated as S235JR under European standards (Table 2.3 shows the designations of ST37-2 according to different standards), is a low carbon mild steel that is mostly used as a structural metal due to several performance criteria it possesses. The alloy has excellent weldability and good ductility, which qualified it to be used in many applications, including shelters and water vessels, and can be shaped for several purposes as angles, strips, sheets, and plates. ST37-2 is a low carbon steel used for structural purposes, and it is also non-alloy in its standard form processed through hot rolling. It has a relative density of 7.85 kg/dm 3 , according to volumetric mass calculations [35]. Impact strength of ST37-2, and its equivalents, is only checked under special requests from the designers through the deoxidization method, where G1 method is used for rimming steel but the usage of G2 method is  The chemical composition of ST37-2 also differs based on the designating standards.
Rahbar and Zakeri [36] performed a chemical composition analysis using a Ladle analysis for a 20 mm specimen of ST37-2 and the results are presented in Table 2.5.      The Carbon Equivalent Value (CEV), carbon content, shape of product, dimensions of product, service conditions and manufacturing conditions are all factors that affect the weldability of carbon steels. Since ST37-2 has less than 0.35% carbon.
Therefore, it is considered as a low carbon steel. Due to this carbon content and with a CEV less than 0.45%, ST37-2 requires no treatment for welding and its weldability is considered to be excellent. If segregation zones are faced in welding, it is recommended that ST37-2 be selected as a rimmed type [35].
Several studies in the literature have experimented with the mechanical and microstructural properties of ST37-2. Depending on the country of origin of the study, the designation used was different from one study to another. However, most of the studies addressed the impact of impact and heat on ST37-2, which are caused by the processing, workmanship and operations performed on it. Table 2.6 provides a summary of the studies that included ST37-2 in their experiments, as well as brief of the outcomes of the research.

Machinery and Vehicles
Due to its good weldability and ability to provide high performance in terms of hardness, ductility and strength, low carbon steel is used to manufacture different parts of heavy machinery and vehicles [2]. Figure 2.12 shows the different parts that are manufactured out of low carbon steel or mild steel for vehicles. The usage of low carbon steel in this industry is due to several advantages [3]:  The easiness to shape carbon steel into the different panels used for to assemble the vehicle.
 The higher toughness and ductility associated with the low carbon content.
 Low carbon steel is economically feasible especially for mass manufacturing  High tensile strength providing the necessary strength for structural components of the vehicle.
Poor corrosion resistance is the disadvantage of low carbon steel. Therefore, using it in machinery and vehicle manufacturing requires protecting it with paint coating to separate it from weather conditions [46].

Construction and Pipelines
Low carbon steel is extensively used in construction as main elements, jointing components, and reinforcements for concrete elements [4]. Various structures are created with low carbon steel, such as transmission towers, rail tracks and industrial buildings. Due to it is ability to be rolled, angles and sections are created with low carbon steel, in addition to sheets and bars [47].

Tools and Cookware
The functional and crucial components of hand tools that are used in manufacturing and construction are made from low carbon steel. The ability to obtain the best performance at lower thicknesses made low carbon steel the prime choice for designers of the tools. Subsequently, the weight of the tool is reduced with the use of less steel material. Low carbon steel is highly conductive for electricity and heat.
Thus, isolation components, such as wood, plastic, and rubber, are used to substitute this disadvantage. However, the poor corrosion resistance of mild steels remains an issue, which requires durable coating [48].

Metallurgical Properties of Steel Alloys
There are various elements that are used to create steal alloys in order to improvement different performance criteria, such as corrosion resistance and thermal resistance. The ratio of the alloying elements does not exceed 5% wt.; however, their impact on the alloy strength or hardness. Alloyants are also added in higher percentages reaching to 20% wt. to enhance performance criteria related to thermal stability and corrosion [49]. Table 2.7 shows the different alloyants added to steel and the enhanced properties through their inclusion in the composition of the alloy.  [50]. Manganese is one of the most added elements to steel, which is used to avoid the development of iron sulfide leading to hot shortness defections [51], enhance mechanical properties [52], and improve hardenability to use of less brutal quenching to reach martensite [53]. If Manganese is added between 0.6% wt. and 0.8% wt. acicular ferrite is formed, which improves the tensile strength of steel.
Acicular ferrite, also known as Widmanstatten structure, is ferrite-layer-like thin intergranular structure, which takes the shape of needles that follow different orientations, as shown in Figure 2.13. Toughness is increased with the addition of 0.6% wt. to 1.8% wt. of manganese, as well as the presence of acicular ferrite [50].
The yield strength increase is associated with the increase in acicular ferrite's dislocation density and fine grain. Thermal inputs through the welding process, especially above 20 kJ/cm contributes into the disappearance of acicular ferrite [54].
Proeutectoid ferrite, shown in Figure 2.13, is mixed with pearlite, which is a mixture of ferrite and Fe3C, if the carbon content is less than 0.55% wt., while crystallographic networks appear enveloping the grains of pearlite if the carbon content is increased up to 0.85% wt. [50]. In deoxidized steels, silicon is used as an energetic deoxidizer, which removes the excess oxygen that dissolves in molten steel and develops blowholes during its solidification through reacting with the oxygen to form SiO2 [56], as shown by the below equation: Si + O2 → SiO2 The replacement of the base metal is avoided through retaining higher amount of oxygen by low concentrations of silicon. A comparison between silicon and other deoxidization elements, including aluminum, calcium, and manganese, showed that silicon had the highest ability to reduce the amount of oxygen [56].
Nonetheless, the use of other deoxidization elements like aluminum has other benefits by hindering grain growth and solidifying the steel leading to grain size refinement [57]. Additionally, the nitrogen content is improved in the steel through the addition of aluminum [58]. Moreover, the grains of steel are recrystallized, as shown in Figure 2.15, especially during steel annealing, with the presence of aluminum nitride, which enhances the mechanical properties of the steel through eliminating internal stresses, increasing ductility, and reducing hardness.   The ferrite phase is hardened, and yield strength is increased in steel through the rapid formation nitrides with iron through the addition of nitrogen [59]. At high temperatures, the atmospheric nitrogen at the surface of the steel is interstitially diffused into steel through nitrogen infusion. Iron nitride is apparent in treated steel samples through its platelet precipitates, as shown in Figure 2.17. The increase in nitrogen concentration leads to the increase in surface hardness from 7.5 GPa to 14.4 GPa with a linear relationship, after exposing steel to nitrogen for 36 hours [59].  The machinability of steel is enhanced through the addition of phosphorus and Sulphur [63]. Cold-forming processes and drawability in high-strength steel is enhanced by adding phosphorus, while machining performance is enhanced through the addition of Sulphur. For instance, the increase of tool travel by 33% between 1018 steel and 1117 steel is only achieved due to Sulphur content [64]. Table 2.8 provides a summary of all the discussed elements and their implications on steel microstructure and properties.

EEFECTS OF ARC WELDING ON STEEL MATERIAL PROPERTIES
Welding imposes several changes on the metal through the changes in temperature and electrical current that is used for the process. It is argued that changes in the mechanical and microstructure properties are imposed on specimens through arc welding [65]. Therefore, this part of the study reviews a few studies to understand these changes in low carbon steel. with the use of E7016 as an electrode.

MECHANISM OF SHOT PEENING
Shot peeing is performed on various types of materials through bombarding their surface with small spherical bodies, and it is a type of cold working processes [12].
The spherical bodies act as a tiny peening hammer when they strike the surface, which cause simples and indentations. As a result of the process, fibers below the surface attempt restoring the surface to the original shape, which cause compression stresses at each point-of-work. A more complex residual stresses are formed with the overlap of dimples at the same or close points. At compressively stressed zones, it is unlikely for cracks to develop or disseminate. Through simulating the compressive stresses with shot peeing, failures caused by stress corrosion and fatigue that originate at the surface of the material are avoided [70]. The local compressive strength induced by shot peeing is equal to at least half of the material's yield strength, and hardness increases with the process. Several failures are treated through shot peeing, including fretting, cavitation erosion, galling, stress by corrosion cracking, fatigue, cracking simulated by hydrogen [71].
The stresses that are maintained within the material without the application of external loads, after manufacturing processes are finalized, are called residual stresses, which vary into compressive and tensile forces. The compressive stresses created by shot peening are the most influential in obtaining the benefits of the treatment. Figure 2.19 shows the changes of the profile of residual compressive stress that is resulting from shot peeing [72]. From the above figure [72]:  Tsmax is the maximum value of the simulated tensile stress. An equilibrium is created through an offset tensile stress at the core that balances the compressive stress at the surface, while its value should not be too high to the extent of creating internal failures.
 Csmax is the shot peeing maximum compressive strength as it is the highest at the surface.
 SS is the stress measured at the surface (surface stress).
 d is the cross point of the compressive stress towards the tensile stress reaction at the neutral axis.
The magnitude of the induced residual stress is another way to describe Csmax, where its variation does not highly influence magnitude as long as the used shots has hardness values that are equal or more than the hardness value of the shot peened material. As shown in Figure 2.20, the magnitude of Csmax is at least 1.5 times the yield strength of the material and it is a function of the shot peened material [73]. Figure 2.20. Relationship between tensile strength of steel and residual stressed induced by shot peeing [73].
The changes in the shot peeing parameters determines the depth of the compressive layer. The above Figure 2.24 illustrates the relationship between the intensity of shot peeing that changes with the type of material used and the depth of the compressive layer [73].
In specimens with smooth surfaces, shot peeing on the lower and upper surfaces exerts no external load, as shown in Figure 2.21. The stress distribution curve illustrates the equilibrium created between the tensile and compressive stressed, in the case of no external loads applied, which means that the stress distribution areas below and above the surface must be equal with the total moment equal to zero for the areas [74].   In notched specimens, effective stress is increased and highly concentrated at the surface, as illustrated through the distribution of stress with a Kt = 2 bending load in Figure 2.23 [73].   [73].

MATERIAL
The alloy used in this experiment is ST37-2 (a typical metal content percentages are shown in Table 3.1). The alloy has a maximum carbon content of 0.2% and nitrogen content of 0.011%. The tensile strength of the alloy ranges between 360 and 460 MPa, yields at 235 MPa with a minimum elongation of 25% [75].

SAMPLE PREPARATION
After cleaning the steel plates, samples were initially prepared with specific dimensions. Each two plates were welded with an arc welding device to form a single plate, as shown in Figure 3.1. The welding slag was removed with an electric grinding machine. Using a plasma cut, two specimens of welded plates were cut into several samples, according to the schematic shown in Figure 3.2. A group of specimens were prepared for tensile testing, while another group was prepared for fatigue testing. The sample dimensions followed the ASTM specifications of the testing specimens. Figure 3.3 shows the specimens for the tensile testing.   Four out of the six specimens were shot peened with two Almen intensities, A12-14 (CSP) and A28-30 (SSP). The shot peening conditions are shown in Table 3.2. The process of shot peening involves striking the samples with tiny particles with a specific pressure that act as a hammer on a small area of the surface in order to create indentations or dimples [77].
The processes energize the particles with kinetic energy that is absorbed by the metal surface [78]. As a reaction, the metal surface absorbs the energy and attempts to restore its original condition [79]. The main target of using shot peening is to increase the hardness and fatigue resistance of the samples, while reducing tension [80].      respectively. The results show a significant increase in roughness between the untreated metal and the CSP cases, while more increase in roughness can be observed in SSP cases. These results are expected and were shown similarly in the literature. Omari, et al. [81] increased hardness and roughness for engine bladed with shot peening. Liu, et al. [82] showed that the increase in shot peening severity increased surface roughness in ZK60 alloy.

TENSILE TEST
The force required to break a specimen and its elongation are measured through a tensile strength test according to ASTM D-638, ASTM D-3039 and ASTM C-297 standards. The test outputs are shown in Table 4.2 below.  [83].
The results also show that severe shot peening (SSP) with type A28-30 showed enhanced tensile strength in comparison with conventional shot peening (CSP), which confirms the results of the literature in this aspect [84]. Generally, shot peening showed enhanced strengths in comparison with samples that were untreated [81].

MICROHARDNESS
Vickers microhardness test is conducted through applying a load of 1 kilogram for 15 seconds by a ball indicator. Values are measured through a calibrated optical microscope for the stress in MPa, as shown in Table 4.4 and Figure 4.7. As shown in results, shot peening increased microhardness from its original value of 150 for ST37-2 from 21.1% to 47.6% by average [85]. Another study showed that microhardness for low carbon steel has an average value of 92, which proves that the obtained hardness values in the current study are a huge enhancement [86]. A28 -30

XRF ANALYSIS
X-ray fluorescence (XRF) is a microscopy method that is used to measure the concentration of metals in a specified area. The method depend on the interaction between the x-ray arrays with the matter through dislodging electrons from atoms and measuring the energy resulting to correlate it with a specific element [87]. As seen from the results of the XRF analysis in the current research (Table 4.5), the mass percentage of each metal component has been changed through shot peening, while the analysis depth ranged between 0.0009 to 0.0355 mm.

XRD ANALYSIS
In order to identify crystalline material in the samples that were treated with shot peening, an x-ray powder diffraction (XRD) analysis is used.

OPTICAL MICROSCOPE
Two specimens were investigated using optical microscope to study the microstructure and mechanical changes of the shot peening treatment. Images are presented at 50 μm for comparison. Shot peening with CSP A12-14 Almen intensity, γ : Austenite (Fe-Cr-Ni) α : Ferrite (Fe-C) γ α α Figure 4.9, affected a thin layer with minimum influence, while shot peening with SSP A28-30 Almen intensity, Figure 4.10, had a higher intensive influence with a thicker layer. In the SSP case, an intrinsic structure is observed. The ferrite and perlite phases are clear with SSP A28-30 [88]. Further images are provided in

SCANNING ELECTRON MICROSCOPY (SEM) OBSERVATIONS
The specimens treated with shot peening are tested with scanning electron microscopy. Both A12-18, Figure 4.13, and A28-30, Figure 4.14, Almen intensities yielded plastic deformations as illustrated. Disturbances are increased in the SSP case, while it created more intense bumps and pits. Layer thickness is increased by 24.4% with the SSP case in comparison with the CSP case. The homogeneity of the alloy does not seem to be affected in both samples by shot peening; however, the increase in hardness as confirmed by the hardness test is reflected by the formation of connected deformed structures [89].

CHAPTER 5 CONCLUSIONS
ST37-2is a low carbon mild steel that is mostly used as a structural metal due to several performance criteria it possesses. The alloy has excellent weldability and good ductility, which qualified it to be used in many applications, including shelters and water vessels, and can be shaped for several purposes as angles, strips, sheets, and plates. ST37-2 is a low carbon steel used for structural purposes, and it is also non-alloy in its standard form processed through hot rolling. It has a relative density of 7.85 kg/dm 3 , according to volumetric mass calculations.
Surface treatment using cold techniques is widely used to enhance the mechanical properties of the metal alloys. Shot peening is one of these processes that has its impact on the surface roughness, residual stresses, microstructure and folding of the metal. The effects of plastic deformations resulting from welding or shot peening can be beneficial or have adverse effects on its strength and ductility. Severe plastic deformation (SPD) is formed in metals through processes, such as hydrostatic extrusion, which performs deformations in the metal at low temperatures, in comparison with other techniques. SPD results into a fine crystalline structure, that differs from the crystallographic structure of the original metal or alloy, through forming micrometric and submicrometric sub-grains in the coarse grain of the original material. The advantages of SPD on performance and mechanical properties through its ability to achieve deformations in the microstructure through fine grains, which reflects on the performance results of hardness and yield stress to saturation levels. However, SPD disadvantages are embodied mainly in the decreased ductility, decreasing the metal ability to undergo plastic deformation under stress.
The main aim of the current research is to test the effect of SPD through electric arc welding and shot peening on the mechanical and microstructure of ST37-2. It is expected for these processes to affect the microstructure and homogeneity of the specimens, which leads to altering their mechanical properties. Six samples were prepared in order to test the mechanical properties and perform microstructure characterization. Two