MATERIAL SCIENCE Knowledge of materials and material science plays an important role in an Engineering Industries. Right type of material shall be selected for a particular service to get required life of the equipment. The selection and use of proper materials, cost effectiveness, minimum failure in the designed life are the direct responsibilities of the Engineers in an Industry. An engineer must have knowledge of nature and behavior of materials, its limitations for manufacturing, chemical compositions, mechanical properties for fabrication and effects of its alloying elements for the required service applications. Materials are selected on the basis of service requirements and the ease of its manufacturing/fabrication but simultaneously the economics is always taken into consideration to limit the cost of the project. Thus the selected material for a particular service must stand up to dimensional stability and process of fabrication, service corrosion, required hardness, strength & toughness, heat resistance, fatigue & creep, machineability, weldability,  heat treatments etc. All these properties depends upon the chemical composition, macrostructure and microstructure of the material. As per ASME, materials are divided in two major groups called Ferrous and Non-ferrous material depending upon their alloying elements.

1)     Ferrous materials are those in which main or base constituent is Iron.

2)     Non-Ferrous material may or may not contain iron at all. FERROUS MATERIALS:More than 50% of the industrial requirements are fulfilled by ferrous materials in one form or other. Ferrous material, which contains Iron as principal element, can be in two forms cast or forged. In casting, molten

metal of required composition is poured in the moulds to get the required shape.In case of forging, at first the metal is caste as raw working material and then it is cold or hot worked to  get the actual required shape with improve mechanical properties.Ferrous material can further be divided into following categories.

Cast Iron, Wrought iron, Carbon Steel, Alloy Steels and

Stainless steel. (Stainless steel is a special type of Alloy Steel which renders special property of corrosion resistance.) NON-FERROUS MATERIALS:All materials other than ferrous material are known as non-ferrous materials. They are equally important to industries and in many critical services these materials are used. Many alloys in which iron is present in form of alloying element only are also covered in non-ferrous materials because the main category of that alloy belongs to Non-ferrous materials.Nickle alloys, aluminum alloys etc are the example of non-ferrous materials. In Industries various type of materials are used in different services. These materials are broadly divided into two main groups.

1)                 Base metal e.g. Iron, Copper, Aluminium etc.

2)                 Alloy e.g. Brass, Stainless Steel, Steel, CastIron  etc.

 Base Metals: - Base metals are used directly for the applications where their property fulfills the requirements. They do contain some elements in

their composition but those are impurities which remains in their matrix during manufacturing.Alloy :-         An alloy is a homogenous mixture of two or more metal (which are called alloying elements) which are added in specific proportion to get required physical properties. e.g. in Stainless steel nickel, chromium, carbon, manganese, silicon are added in specific proportion to get required properties. Every alloying element has its own properties and effect on the properties of the Alloy. Most used alloy in various industries is in forms is Steels. SteelSteel  is an alloy of mainly Iron and Carbon,  which contains carbon upto 1.5%. The percentage of carbon exerts the most influence on properties of steel. Depending on the percentage of carbon in steel, its grades are defined as low carbon, medium carbon or carbon steels. Other alloying elements are further added to enhance its properties in different grades.  Classification of   Steels Steels can be classified into different groups by a variety of different systems depending on:

The composition, such as carbon steels, low-alloy steels or stainless steels.

The manufacturing methods, such as open hearth, basic oxygen process, or electric furnace methods.

The finishing method, such as casting, forging, hot rolling or cold rolling

The product form, such as bar, plate, sheet, strip, tubing or structural shape

The de-oxidation practice, such as killed, semi-killed, capped or rimmed steel

The microstructure, such as ferritic, pearlitic and martensitic The required strength level, as specified in ASTM standards

Based on heat treatment, such as annealed, quenched and tempered etc.

Quality descriptors, such as Industrial and commercial quality. Carbon steel:

Carbon steel are classified according to variations in carbon which have the greatest effect on mechanical properties. With increasing percentage of carbon content hardness and strength of steel tends to increase. As such, carbon steels are generally categorized according to percentage of carbon content. Generally speaking, carbon steels contain up to 2% total alloying elements and can be subdivided into low-carbon steels, medium-carbon steels, carbon steels and high-carbon steels.

Low-carbon steels contain up to 0.30% C. The largest category of this class of steel is rolled products (sheet or strip), usually in the cold-rolled and annealed condition. The carbon content for these high-formability steels is very low, less than 0.10% C, with up to 0.4% Mn. Typical uses are in automobile body panels, tin plate, and wire products.Medium-carbon steels contains carbon from 0.30 to 0.60% and the manganese from 0.60 to 1.65%. Increasing the carbon content to approximately 0.5% with an accompanying increase in manganese allows medium carbon steels to be used in the quenched and tempered condition. The uses of medium carbon-manganese steels include shafts, axles, gears, crankshafts, couplings and forge pipe fittings. Steels in the 0.40 to 0.60% C range are also used for rails, railway wheels and rail axles. Carbon steels contain carbon from 0.60 to 1.00% C with manganese contents ranging from 0.30 to 0.90%. High-carbon steels are used for making soft springs, wires for wire ropes etc.As a group, carbon steels are by far the most frequently used steels. More than 85% of the steel produced is carbon steel.High carbon steels contain carbon more than 1.00% with manganese contents ranging from 0.30 to 1.00%. High-carbon steels are used as spring materials, shoes of crane crawlers and high-strength wires

Low-alloy SteelsLow-alloy steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels as a result of additions of alloying elements such as nickel, chromium, and molybdenum. Total quantity of alloying elements can range from 2.07% up to levels just below that of stainless steels, which contain a minimum of 10% Cr.

For many low-alloy steels, the primary function of the alloying elements is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions.

Chromium-molybdenum heat-resistant steels contain 0.5 to 9% Cr and 0.5 to 1.0% Mo. The carbon content is usually below 0.2%. The chromium provides improved oxidation and corrosion resistance, and the molybdenum increases strength at elevated temperatures.Stainless SteelsStainless steels are iron-based alloys containing at least 10.5% Cr. Few stainless steels contain more than 30% Cr or less than 50% Fe. They achieve their stainless characteristics through the formation of an invisible and adherent chromium-rich oxide surface film. This oxide forms itself in the presence of oxygen. Other elements which are added to improve properties include nickel, molybdenum, copper, titanium, niobium, aluminum, silicon, nitrogen, and selenium. Carbon is normally present in amounts ranging from less than 0.03% to over 1.0% in certain martensitic grades. The selection of particular grade of stainless steels may be based on corrosion resistance, fabrication characteristics, availability, mechanical properties in specific service conditions/temperature ranges and product cost. However, corrosion resistance and mechanical properties are usually the most important factors in selecting a grade for a given application.

Classification of Stainless Steels

Stainless steels are commonly divided into five groups:

1)     Martensitic stainless steels, 2)    Ferritic stainless steels, 3)    Austenitic stainless steels, 4)    Duplex (ferritic-austenitic) stainless steels, and 5)    Precipitation-hardened stainless steels.Various grades of Stainless steels are in general available in the form of plate, sheet, strip, shapes, foil, bar, wire, , pipes, tubes, pipe & Tube fittings,  semi-finished products in Forging, Casting etc.

Effect of alloying elements on steel Chromium (Cr) :- Chromium is the single alloying element which renders stainless properties to the steel. A minimum of 11% Cr is needed to be added to steels, at which a continuous stable and inert (passive) Chromium oxide film forms on the surface which is resistance to both wet and dry corrosion. This increases the high temperature stability and strength of material. Carbon (C) :- Primary alloying element. It is essential for formation of martensite during hardening. Carbon exerts the most influence on ferrous material. It increases tensile strength and hardness and decreases ductility and weldabitity. Generally without carbon no ferrous material (except maragin steel) exists. Carbon has a major effect on steel properties.  Carbon is the primary hardening element in steel.  Hardness and tensile strength increases as carbon content increases. Ductility and weldability decrease with increasing carbon. 

Sulfur (S) :- Sulfer contents are considered as impurity. This reduces the tensile strength at high temperature and improves the machinability.Phosphorus (P):- Ii is also considered as impurity. It can cause brittleness at low temperature. 

Nickel (Ni) :-        Nickel is added to increase tensile strength, yield strength, heat resistance & creep resistance. Addition of Nickle has effect on Co-officient of thermal expansion also. 

Molybdenum (Mo.) :- It increases strength at elevated temperature, hardness at good strain. Molybdenum, when added to stainless steels, it enhances the properties (passivity) of the passive film and thereby increases the resistance to corrosion at high temperature.

Aluminum (Al):- Aluminum helps in controlling the grain size and it works as deoxidizer.

Titanium(Ti) :-Titanium is an high temperature stabilising alloying elements i.e. it is a strong carbide former which forms titanium carbide. It helps in reducing chromium depletion at higher temperature. Titanium is

used to retard grain growth and thus improve toughness. Titanium is also used to

achieve improvements in inclusion characteristics.  Titanium causes sulfide

inclusions to be globular rather than elongated thus improving toughness and

ductility in transverse bending.

 Tungsten (W) :-It increases hardness by refining the grain structure and forming tungsten carbide in the matrix.Niobium(Nb.) (Columbium) :-Niobium is also a high temperature stabilising elements. It helps in making sound forging of the stabilised grade of stainless steels. Niobium  increases the yield strength and, to a lesser degree, the tensile strength of carbon steel. The addition of small amounts of Niobium can significantly increase the yield strength of steels.  Niobium can also have a moderate precipitation strengthening effect. Its main contributions are to form precipitates above the transformation temperature, and to retard the recrystallization of austenite, thus promoting a fine-grain microstructure having improved strength and toughnessVanadium increases the yield strength and the tensile strength of carbon steel. The addition of small amounts of Vanadium can significantly

increase the strength of steels.  Vanadium is one of the primary contributors to precipitation strengthening in microalloyed steels.   When thermomechanical processing is properly controlled the ferrite grain size is refined and there is a corresponding increase in toughness.  The impact transition temperature also increases when vanadium is added.Nickel and chromium

Nickel steels are noted for their strength, ductility and toughness, while chromium steels are characterized by their, corrosion resistance,  hardness and resistance to wear. The combination of nickel and chromium produces steels having all these properties, some intensified, without the disadvantages associated with the simple alloys. The depth of hardening is increased, and with 4.5% nickel, 1.25% chromium and 0.35% carbon the steel can be hardened simply by cooling in air.

Ni-Cr-Mo steels are widely used for ordinance, turbine rotors and other large equipments and articles, since molybdenum tends to minimize temper brittleness and reduces mass effect. Molybdenum is also a constituent in some high-speed steels, magnet alloys, heat-resisting and corrosion-resisting steels.

Manganese (Mn) :- Addition of Manganese improves the ductility and hardness of the Steels. All commercial steels contain 0.3 - 0.8% of manganese, to reduce oxides and to counter act the harmful influence of iron sulfide. Any manganese in excess of these requirements partially dissolves in the iron and partly form Mn3C which occurs with the Fe3C. There is a tendency now a days to increase the manganese content and reduce the carbon content in order to get a steel with improved ductility having all other physical properties same.

The manganese content is also increased in certain alloy steels, with  reduction or elimination of expensive nickel, in order to reduce costs. Steels with 0.3-0.4% carbon, 1.3-1.6% manganese and 0.3% molybdenum have replaced 3% nickel steel for some purposes.

Silicon (Si) :-It increased elasticity and strength and decreases weldabilty and forgeability, Silicon is added to casting to improve the fluidity of molten metal. Silicon and Manganise are introduced to de-oxidize the steel. These elements combine with Oxygen of ferrous oxide and form their oxides in preference and go over the slag in the form of oxides. This improves the properties of steel. It is called killing and the steel thus produced is called killed steel.  Aluminum is also used as good de-oxidiser (Oxygen killer) agent and to control the grain size. Addition of silicon in castings, minimizes the possibility of porosities (Blow Holes).Titanuim and Zierconuim are strong de-oxidiser and they can only be used in fully killed steels. Niobium is one of the strong carbide formers and stabilizers of ferrite in the matrix. It is also works as a grain refiner. At high temperature where creep is a problem coarser grain materials have advantages but at normal temp fine grain provides improved strength.

Cast Iron :-The term cast iron, like steel, identifies a large family of ferrous alloys. Alloys of carbon in iron, in which carbon content exceed 2.14% are called Cast iron. Cast irons are multi component ferrous alloys. They contain major (iron, carbon, silicon)and  alloying (>0.01%) elements. Other alloying elements are added for getting the improved properties for specific services.Cast Iron is generally weak in tensile load and cannot be used for making items like fasteners, shaft, rotor etc. It is brittle in nature. But cast iron has good compression strength, lubrication retaining properties and damepning properties. Therefore many machinery parts are made of cast iron only which require vibration dampning, continuous lubrication etc. Based on requirement, different grades of cast irons have been developed which have improved properties.

Cast iron has higher carbon and silicon contents than steel. Because of the higher carbon content, the structure of cast iron, as compared to that of steel, exhibits a rich carbon phase. Depending primarily on composition, cooling rate and melt treatment, various grade of cast irons can be made.

There are many grades of cast iron. e.g. white cast iron, Grey cast iron, Nodulor cast iron and Maleable cast iron. Each cast iron has specific properties and  is used for specific requirement.

The cast iron contain greater percentage of carbon ( 2 ~ 5%) than in steel. Carbon in cast Iron may be found in two different forms. One is in total chemically combined form and another is in independent form. When all carbon present in the matrix is in chemically combined condition with iron forming (Fe3C) iron carbide in the form of “cementite”, it is called “ white cast iron” . When all percentage of carbon is not chemically combined with Iron and also exists as free carbon or graphite flakes, it is called “grey cast iron”.

 Grey Cast Iron :- When all percentage of carbon is not chemically combined with Iron and some percentage of carbon exists as free carbon or graphite flakes, it is called grey cast iron. It is machineable and have very good dampening property.White Cast Iron :- When all carbon present in the matrix is in chemically combined condition with iron, forming (Fe3C) iron carbide in the form of “cementite”, it is called white cast iron. The presence of cementite or iron-carbide makes this metal hard and brittle and wear resistance. This is also machineable and it has high temperature resistance property.  Ductile or Nodular Cast Iron :- When small quantity of Magnesium or Cerinum is added to cast iron the graphite content gets converted into nodular or spherodial form and gets dispersed throughout the matrix. This improves the fluidity, and thus excellent castings are made. Nodular cast iron has good strength, high toughness, wear resistance and good impact strength.Alloy Cast Iron:- Some alloying elements are added to cast iron to overcome certain inherent deficiencies and to improve mechanical properties more suitable for special purpose. To increase strength, wear resistance, corrosion resistance, hardness etc. Nickel, Chromium, Molybdenum , Vanadium  etc. are added forming various grades of alloy cast iron. Cast Carbon SteelsCarbon steel is a generalized name given to steels containing only carbon as the principal alloying element. Other elements are present in small quantities. Important elements added for de-oxidation and other properties are Silicon and Manganese in cast carbon steels typically range from 0.25 to about 0.80% Si, and 0.50 to about 1.00% Mn respectively. Carbon steels can be classified according to their carbon content into three broad groups:

Low-carbon steels: < 0.20% C Medium-carbon steels: 0.20 to 0.50% C High-carbon steels: > 0.50% C

Carbon steel castings are produced in different grades of properties depending on the specific requirements. Accordingly composition and heat treatment shall be selected to achieve specific combinations of properties, which includes hardness, strength, ductility, fatigue resistance, and toughness. Although there are certain other properties specific to particular grades but it is important to recognize the inter relationships for all grades for these general properties. Cast Stainless Steels

Stainless steels are a class of chromium-containing steels widely used for their corrosion resistance properties in corrosive environments and for service at cryogenic and elevated temperatures. Stainless steels are distinguished from other steels by the enhanced corrosion and oxidation resistance created by chromium additions. Chromium imparts passivity to ferrous alloys when present in amounts of more than about 11%, particularly if conditions are strongly oxidizing. Consequently, steels with more than 10 or 12% Cr are sometimes defined as stainless steels.

Stainless steel castings are usually classified as corrosion-resistant castings or some times heat-resistant, used in the services where temperature range from 450 to 650oC. The usual distinction between heat-resistant and corrosion-resistant cast steels is based on carbon content and stablising element in the composition. ForgingFor Services in which parts are exposed to tensile strength or transmission of torque etc then forging are used. Forging may be in carbon steel, Stainless steel or Alloy Steel.Forging is the direct compression-type process and it is probably the oldest method of metal forming. It involves the application of a compressive stress, either by impact or by pressure, which exceeds the

flow stress of the metal. The stress can be applied quickly or slowly. The process can be carried out in hot or cold conditions, choice of temperature being decided by factors such as ease and cheapness of formation. Requirement of production quantity, imparting certain mechanical properties or surface finish are the overriding factor. There are two kinds of forging process, impact forging and press forging. In the former, the load is applied by impact, and deformation takes place over a very short time. Press forging, on the other hand, involves the gradual build up of pressure to cause the metal to yield. The time of application is relatively long. Over 90% of forging processes are hot. Impact forging can be further subdivided into three types:

Smith forging, Drop forging, Upset forging.

Structure and Properties of ForgingsForgings are invariably produced by the hot-working process and this helps in controlling the resultant grain structure and hence properties of the forgings. There are, however, important differences in forgings produced by different techniques.

The fact that the impact forges applies a stress for a very short duration compared to the press forge, results in totally different grain structures in the product. In the case of impact forge, the mechanical working is concentrated on the surface layers. Removal of the stress after the blow results in metal relaxation before the effect of the blow could have penetrated up to the center.

Impact forging of a large "as cast" piece of metal at high temperature will result in a very inhomogeneous structure, the outside layers showing a typical hot-worked structure whilst the center is still as cast. Any attempt to achieve greater penetration by increasing the impact load usually leads to internal cracking. Impact forging is therefore limited to relatively small work pieces.

Press forging invariably results in total penetration of the effect of the applied stress up to the center of the work piece. The process is generally less severe on the metal than impact. The end result is a more homogeneous product having very high quality. Since the process is much slower and the equipment used is much larger, press-forged articles are more expensive than impact forged components.

MARAGING STEEL :- Maraging steel is an iron based steel alloy which do not contain Carbon. It is known for possessing superior strength without loosing malleability. Iron with various proportions of alloying elements like Cobalt, Nickel as strengthening agents are added with Molybdenum and Titanium. It possess. -        High strength and tonghness-        Easy Machinable-        Can be easily nitrided-        Resist corrosion and crack propagation. The common grades contain 17 ~ 19 % Nickel, 8 ~ 12 % Cobalt 3 ~ 5 % Molybdenum and 0.2 ~ 1.6% Titanium. HEAT TREATEMENT OF MATERIAL :-The specific properties of steels or alloys (Ferrous or Non ferruns) may be changed as required by following a heating and cooling cycle for that material under definite conditions. Desired  results are obtained by heating in a temperature range where a phase or combination of phases are stable. This process of following a cycle of heating and cooling of material to impart specific properties in the material is called heat treatment. The structure of steel is composed of two variables.

1)     Grain structure –The arrangement of atoms in a metal2)     Grain size - the size of the individual crystals of the metal

Steels can exhibit a wide variety of properties depending on their grain structure and grain sizes as well as the phases of its micro-constituents

present in its matrix. The matrix structures of materials depend on the heat treatment cycle followed after the formation process. Some times a minor addition of alloying element changes the entire cycle of heat treatment as the matrix body structure gets changed. The basis for consideration of heat treatment of steels is the iron-carbon (Fe-C) phase diagram. During heat treatment of the metals, it is impossible to cool the specimen at a uniform rate through out. The surface of the specimen will always cool more rapidly than the interior region. Therefore the Austenite will transform over a range of temperatures, yielding a possible variation of microstructure and properties of the material within the shape. The successful heat treatment of steels is to produce a predominantly Martensitic microstructure throughout the cross section. This depends upon following factors:

1)     The composition of the alloy2)     The type and character of the quenching medium3)     The size and shape of the specimen

By way of various heat treatment cycles, internal stresses, which may have been developed due to formation, can be removed, grain size may be refined, hard and soft surfaces may be produced. In many cases it helps in improving the tensile strength and toughness also. Final results of heat treatment depends upon the rate of heating, final temperature of soaking, period of soaking, rate of cooling, quenching media etc. Specific cycles are designed by the metallurgist to get the best results in the required services. Generally heat treatment is done for the following purposes.1.       To Impart hard surface2.       To Improve machineablity3.       To refine grain size4.       To relieve internal stresses5.       To improve wear resistance6.       To improve tensile strength7.       To Improve magnitic or electrical properties8.                 To Improve softness and mleability

Special heat treatments have been designed and named by the metallurgist for getting the properties enhanced in the materials. These are in general performed after some steps of forming. ANNEALING: - Annealing is one of the most widely used process in the heat treatment of Iron or steel. According to American society of Material Testing (ASTM) annealing is a generic term denoting a heat treatment that consist of heating and holding at a suitable temperature followed by cooling at a suitable rate in a suitable quenching medium primarily to soften the material. NORMALIZING :-This heat treatment is generally followed on the materials which are formed by forging at high temperature. It consist of heating ferrous alloys to a suitable temperature, keep it there for specific time called soaking, followed by cooling at specific rate for refining grain size of a steel. This heat treatment is must for all forging formations. TEMPERING:- In the as quenched state the martensite formed is not only very hard but also very brittle and can not be worked further in any application. Also many internal stresses gets introduce during the quenching operation. Temperring is used to enhance the ductility and toughness of martensite and relieve the internal stresses. In tempering steel is re-heated to specified low temperature below its critical temperature for a certain period of time and then cooled at a prescribed rate. AUSTEMPERING:- Austempering is also known as Austeninting is the heat treatment process in which steel or iron alloy is heated to a temperature of 205°C ~  500°C above the critical temperature and it is kept at that temperature for sufficient time, un till it is all ferrite and pearlite are converted into austenite and then cooling at a definite rate for specific grades of steels. 

HARDENING :- Hardening is the heat treatment for making the steel hard at the outer surface while the core remains soft. The steel is heated above the critical temperature and held at that temperature for considerable time to ensure thorough penetration of the temperature in side the metal and then cooled at a specific fast rate. Hardness of the surface achieved depends upon the volume of the cooling bath, media of the cooling etc. STRESS RELIEVING:- Stress relieving is the most frequently and widely done heat treatment in any industry. A specific heating and cooling cycle is designed to relieve internal stresses developed by forming, forging, welding or any hot work. Rate of heating, temperature of soaking, time of soaking, loading and un-loading temperatures depends upon the grade of material and thickness of the job.          There are certain other heat treatment process also e.g. CARBURIZING, CYANIDING, NITRIDING, FLAME HARDNING, which are done to obtain various surface hardness for specific purpose. These heat treatments are more related to manufacturing industries. QUENCHING:- Quenching is the operation of rapid cooling of work piece from high temperature to ambient in a liquid or gaseous media. In this operation intermediate transformations are avoided by not giving sufficient time at intermediate temperatures. For achieving this the volume of the quenching media shall be sufficient with respect to the work piece. There are various Specific gaseous or liquid medium used for specific purposes. Media plays important role in achieving the specific properties after the heat treatment.Some important terms which are used frequently in industries: GRAIN SIZE:- When material mass starts solidifying from molten state, the solidification starts simultaneously at  numerous locations. The grains formed early in the solid mass gets sufficient time and heat from the surrounding to grow in its vicinity and on completion of  solidification the

material is composed of various sizes of grains. These grains are visible under metallurgical microscope. The number of grains per unit area is called the grain size of that material. According to American Society for Testing Material (ASTM) the grain size of a steel is specified by the number of grains per square area which are visible under magnification of 100 times.More the grains per square inch area or higher grain no (ASTM 6  ~ 9) means fine grain structure and less the grains per square inch area or less ASTM grain No. (ASTM 1  ~5) means coarse grain structure.Grain No or Grain size in steel is one of the most influential factors for mechanical properties and application of service. Fine grain denotes good strength and coarse grain indicates weak materials. For high pressure and high temperature application grain structure is very important.          CREEP :- Material exposed to high temperature and pressure for long time may develop permanent plastic deformation which can leads to its failure. This permanent plastic deformation is called Creep. It is measured in percentage as variation in size w.r.t. original size. Material used in this type of condition should have high creep resistance. Some high temperature resisting material are Inconel 600, Inconnel 800, HK4O, IN519 etc  in which percentage of  Chromium ranges from 20 ~ 35% and Nichel 25 ~ 70 % and Carbon 0.06 ~ 0.50%. There are certain other alloying and micro alloying elements also as Silicon, Niobium, Cobalt, Aluminum etc. which are added  to enhance the specific properties as per requirement. HYDROGEN EMBRITTLEMENT :- Hydrogen embrittlement is a service related failure or deterioration of material. It involves the ingress of hydrogen atoms into the matrix occupies the space available in the matrix. When large number of such ingress takes place the matrix become densely occupied and thus hardness of the material gets increased. Atoms ingress in the matrix starts forming molecules and thus the space available in the matrix become more and more congested. The

molecules of the Hydrogen has tendency to get librated from the congetion and thus they exert sufficient pressure on the grain boundries to generate fishers. These fishers when gets accumulated becomes crack. Hydrogen embrittlement can seriously reduce the ductility and load bearing capacity and can cause catastrophic brittle failure at stresses much below the yield stresses. At high temperature the diffused hydrogen on the grain boundaries combines with the carbon in the vicinity and in presence of iron form methane gas (CH4). The molecule size of the methane is large as compared to hydrogen atom. Thus the methane gas builds up enormous pressure on the grain boundaries and thus fishers and cracks are initiated. Hydrogen embrittlement does not affect all materials equally. Some materials are suitable for the service while others are not. Nelson curve gives the required help for selecting the materials for particularly in hydrogen service. Failures of Metal ComponentsIn industries the failure analysis is equally important for the engineers. This gives the idea for the maintenance engineers about the performance of the designed part in the particular service. Root causes of the failure shall be analyzed and shall be recorded for future use for the elimination of similar mistakes. While designing modern equipments to operate in severe environments, a designer is confronted with many complex problems in selecting and evaluating materials, for expected loadings and design stresses.  Materials selection must not be confined to a small group of metals for outstanding resistance in one characteristic, such as inertness to the environment in chemical processing. However, many other factors must also be considered such as strength, toughness, mode of fabrication and wear resistance etc. before selection and design.Many elements of fracture are used to describe and categorize the types of fractures encountered in the laboratory and in service. These elements include loading conditions, rate of crack growth, and macroscopic and microscopic appearance of fracture surfaces.

Failure analysis often finds itself useful to classify fractures on a macroscopic scale as ductile fractures, brittle fractures, fatigue fractures and fractures resulting from the combined effects of stress and environment. The last group includes stress-corrosion cracking and liquid-metal embrittlement, interstitial embrittlement, corrosion fatigue and stress rupture.Detailed analysis of failures encountered in developing a prototype (or in a service component) is vital before appropriate changes can be made in design or material to assure a reliable product.Ductile Fractures:Ductile fractures are characterized by tearing of metal accompanied by appreciable gross plastic deformation and expenditure of considerable energy during the failure. Brittle Fractures:Brittle fractures are characterized by rapid crack propagation with less expenditure of energy than with ductile fractures and without appreciable gross plastic deformation. Fatigue Fractures:Fatigue fractures result from cyclic loading, and appear brittle on a macroscopic scale. They are characterized by incremental propagation of cracks until the cross section has been reduced to where it can no longer support the maximum applied load and fast fracture ensues.

 Although all cares are taken while designing or manufacturing equipments but failures do takes place. In general, service failures may arise from many causes. For mechanical equipment, these causes might be classified into three categories as follows:

Design inadequacies:- Many times while designing small things are overlooked and these small things become the source of failure. Sharp corners or abnormal stress-raisers, inadequate fasteners, wrong material or heat treatment, unforeseen conditions of service, and lack of accurate stress analysis may be such causes of failures.

Processing and fabrication:- About half of the failures are due to metallurgical process, which are either not followed properly or not performed at all while fabricating. Factors such as quench cracks, improper heat treatment, forging or casting defects, nonmetallic inclusions etc are some examples; the other half failures are due to minor mistakes done during fabrication such as  Joint misalignments, weld flaws, improper machining or assembly, grinding cracks, cold straightening etc.

Environmental and service deterioration:- These factors are part of the service in which equipment is performing. These include overloads, change of parameters, chemical attack, wear, corrosion, diffusion, and improper maintenance.

A "failure" usually occurs as:

1. Fracture (Breakage)

2. Excessive deformation (over heating, bending)

3. Material deterioration (Carburizing, metal dusting)

These facts are basic information for any engineer in an industry.