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Mechanical laboratory II-ME II-ME310 310 (2011)
Metallography Lab Metallography Metallography of steel and cast iron
Vishal Yadav 0901ME22 C.Kausik B.Reddy
INTRODUCTION : Metallography –
the study of the microstructure of metals using various techniques—has been an invaluable tool for the advancement of science and industry for over one hundred years. Metallography is used to reveal the microstructure of metals, which is affected by alloy composition and processing conditions; including cold working, heat treatment and welding. A finished part's environment can also affect its microstructure microstruc ture and cause problems such as corrosion and decarburization. Analysis of a material's metallographic microstructure aids in determining if the material has been processed correctly and is therefore a critical step for determining product reliability and/or for determining why a material failed.
A. Metallographic Procedure: REQUISITES:
The key to obtaining an accurate interpretation of a microstructure is a properly prepared specimen which is truly representative of the material being examined. The definition of a properly prepared metallographic surface states that the section must meet the following criteria. • Be flat and free from scratches, stains, and other imperfections which tend to mar the surface. • Contain all non-metallic inclusions intact. • Show no chipping or galling of hard and brittle intermetallic compounds. • Be free from all traces of disturbed metal
To ensure achievement of such true surfaces, preparation must be carried out not only with accuracy, but also with a clear understanding of what must be accomplished during each specific stage..
1.Stages of Preparation (Definitions): The most straight-forward approach is to divide the entire process into a logical series of stages involved and the purpose of same.
Stage 1-Sectioning: The removal of a representative sample from the parent piece. This was done using a hacksaw which is made of secondary-hardened tool steel. Although the blade is significantly flexible, it is very hard and can fracture violently if the direction of the stroke deviates much from the plane of the cut.
Stage 2-Mounting : Small samples are generally mounted in plastic for convenience in handling and to protect the edges of the specimen being prepared. Compression-type molding is commonly applied to encase specimens in 1 to 1.5 inch diameter plugs of a hard polymer. Compression molding materials are classified as either thermosetting or thermoplastic:
Stage 3-Coarse Grinding :
The purpose of the coarse grinding stage is to generate the initial flat surface necessary necessary for the subsequent grinding and polishing steps. As a result of sectioning and grinding, the material may get cold worked to a considerable depth with a resultant transition zone of deformed material between the surface and the undistorted metal. Course grinding can be accomplished accomplished either wet or dry using 80 to 180 grit electrically powered disks or belts, but care must be taken to avoid significant heating of the sample. The final objective is to obtain a flat surface free from all previous tool marks and cold working due to specimen cutting. An important factor throughout the Coarse Grinding and Fine Grinding Stages is that the scratches be uniform in size and parallel to each other in any one grinding stage. Proper grinding involves rotation of the sample by 90o between stages while the grinding angle must be held constant during the grinding at any one stage.
Stage 4-Medium and Fine Grinding
Medium and Fine Grinding of metallurgical samples are closely allied with the Coarse Grinding which precedes them. Each stage of metallographic sample preparation must be carefully performed; the entire process is designed to produce a scratch free surface by employing a series of successively finer abrasives. Failure to be careful in any stage will result in an unsatisfactory sample.Movement from one stage to the next should only proceed when all of the scratches from the preceding stage are completely removed. Manual Fine Grinding is performed by drawing the specimens in one direction across the surface of the water lubricated abrasive paper. (Back to front is recommended) recommended) Use of backward and forward motion motion is less less desirable because there is a tendency tendency to rock rock the sample, sample, producing a curved curved rather than a flat surface.To surface.To monitor progress, progress, each fine grinding step should be performed in a direction off-angle with with respect to the previous previous step.
Stage 5-Mechanical Polishing
Polishing involves the use of abrasives, suspended in a water solution, on a cloth-covered electrically powered wheel. wheel. Diamond Diamond abrasives provide provide the best, best, and most expensive, compounds compounds utilized utilized in polishing;standard polishing;standard sized aluminum aluminum oxide oxide powders are applied for general general use purposes. purposes. Following Following the final 600 grit fine-grinding stage, the sample MUST be washed and carefully dried before proceeding to the first polishing stage! At the polishing stages, even hard dust particles in the air which settles settles on the polishing cloth can cause unwanted scratching of the specimen! Careful washing of the specimen and the operator's hands must be carried out prior to each stage of polishing. polishing.
The polishers consist of rotating discs covered with soft cloth impregnated with diamond particles (3 and 1/3 micron size) and an oily lubricant. Begin with the 3 micron grade and continue polishing until the grinding scratches have been removed. It is of vital vital importance importance that the sample is is thoroughly cleaned using using soapy water, followed by alcohol, and dried before moving onto the final 1/3 micron stage. Any contamination contaminat ion of the 1/3 micron polishing disc disc will make make it impossible impossible to achieve a satisfactory polish.
Stage 6-Etching: The purpose of etching is two-fold. Grinding and polishing polishing operations produce a highly highly deformed, thin layer on the the surface which is removed chemically during etching. Secondly,, the etchant attacks the surface Secondly surface with preference preference for those those sites sites with with the the highest energy, leading to surface relief which allows different crystal orientations, grain boundaries, precipitates, precipitates, phases and defects defects to be distinguishe distinguished d in reflected reflected light microscopy. microscopy. For iron iron & steel steel the etchant etchant used composed of 1-5 parts of Nitric Acid & 100 parts of Alcohol.
2.Heat Treatement Process: Quenching – It is the rapid cooling of a workpiece to obtain certain material material properties. proper ties. It prevents low-tempe low-temperature rature processes, processes, such as phase transformati transformations, ons, from occurring occurring by only only providing a narrow window of time time in which which the reaction reaction is both thermodynam thermodynamically ically favorable favora ble and kinetically accessible. For instance, it can reduce crystallinity and thereby increase toughness of both alloys and plastics (produced through polymerization). polymerization). Annealing – It is a heat treatment wherein a material is altered, causing changes in its properties such such as strength strength and hardness. It It is a process that produces conditions conditions by by heating to above the recrystallization recrystallization temperature, maintaining a suitable temperature, temperature, and then cooling. Annealing is used to induce ductility, soften material, relieve internal stresses, refine the structure by making it homogene homogeneous, ous, and improve cold working properties. Normalising – Normalization is an annealing process in which a metal is cooled in air after heating in order to relieve relieve stress. stress. It can also be referred referred to as: Heating a ferrous alloy to a suitable temperature temperature above the transformation temperature temperature range and cooling in air to a temperature substantially substantially below the transformation range. This process is typically confined to hardenable steel. It is used to refine grains which have been deformed through cold work, and can improve ductility and toughness of the steel. It involves heating the steel to just above its upper upper critical critical point. It is soaked for a short short period then then allowed to cool in air. Small grains are formed which give a much harder and tougher metal with normal tensile strength and not the maximum ductility achieved by annealing. It eliminates columnar grains and dendritic segregation that sometimes occurs during casting. Normalizing improves machinability of a component and provides dimensional stability if subjected to further heat treatment processes.
B.Iron Carbon Phase Diagram :When carbon in small quantities is added to iron, ‘Steel’ is obtained. Since the influence of carbon on mechanical properties of iron is much larger than other alloying elements. The atomic diameter of carbon is less than the interstices between iron atoms and the carbon goes into solid solution of iron. As carbon dissolves in the interstices, it distorts the original crystal lattice of iron. This mechanical distortion of crystal lattice interferes with the external applied strain to the crystal lattice, by mechanically mecha nically blocking the th e dislocation of the th e crystal lattices. In other words, they provide mechanical strength. Obviously adding more and more carbon to iron (upto solubility of iron) results results in more and more distortion of the crystal lattices and hence provides increased mechanical strength. However, solubility of more carbon influences negatively with another important property of iron called the ‘ductility’ (ability of iron to undergo large plastic deformation). deformation). The a-iron or ferrite is very soft and it flows plastically.. Hence we see that when plastically when more carbon carbon is added, enhanced enhanced mechanical mechanical strength strength is obtained, but ductility is reduced. Increase in carbon content is not the only way, and certainly not the desirable way to get increased strength of steels. More amount of carbon causes problems during the welding process. We will see later, how both mechanical strength and ductility of steel could be improved even with low carbon content. The ironcarbon equilibrium diagram is a plot of transformation of iron with respect to carbon content and temperature. temperature. This diagram diagram is also called called Fe-Fe Fe-Fe carbon phase diagram (Fig.1).
The important metallurgical terms, used in the diagram, are presented below. Ferrite (α): Virtually pure iron with body centered cubic crystal structure (bcc). It is stable at all temperatures upto 9100C. The carbon solubility in ferrite depends upon the temperature; the maximum being 0.02% at 723C. Cementite: Iron carbide (Fe3C), a compound iron and carbon containing 6.67% carbon by weight. Pearlite: A fine mixture of ferrite and cementite arranged in lamellar form. It is stable at all temperatures below 723C. Austenite (γ): Austenite is a face centred cubic structure (fcc). It is stable at temperatures above 723oC depending upon carbon content. It can dissolve upto 2% carbon. The maximum solubility solubility of carbon in the form of Fe3C in iron is 6.67%. Addition of carbon to iron beyond this percentage percentage would would result in in formation of free carbon or graphite graphite in iron. iron. At 6.67% of carbon, iron transforms completely completely into cementite or Fe3C (Iron Carbide). Generally carbon content in structural steels is in the range of 0.12-0.25%. Upto 2% carbon, we get a structure of ferrite + pearlite or pearlite + cementite depending upon whether carbon content is less than 0.8% or beyond 0.8%. Beyond 2% carbon in iron, brittle cast iron is formed.
Fig1. Iron Carbon phase diagram
The iron-iron carbide portion of the phase diagram that is of interest to structural engineers is shown in Fig.1. The phase diagram is divided into two parts called “hypoeutectoid steels” (steels with carbon content to the left of eutectoid point *0.8% carbon+) and “hyper eutectoid steels” which have carbon content to the right of the eutectoid point. It is seen Metallography of steel and cast iron from the figure that iron containing very low percentage of carbon (0.002%) called very low carbon steels will have 100% ferrite microstructure (grains or crystals of ferrite with with irregular boundaries) boundaries) as shown in Fig 1. Ferrite is soft and ductile ductile with very low mechanical strength. This microstructure at ambient temperature has a mixture mixture of what is known as ‘pearlite and ferrite’ as can be seen in Fig. 1. Hence we see that ordinary structural structura l steels have a pearlite + ferrite microstructure. microstructure. However, it is important to note that steel of 0.20% carbon ends up in pearlite + ferrite microstructure, only when it is cooled very slowly from higher temperature during manufacture. When the rate of cooling is faster, the normal pearlite+ ferrite microstruct microstructure ure may not form, instead instead some other microstructure microstructure called called bainite or martensite martensite may may result.
C. Microstructure Analys s:
1. Microstructure analysis of
When carbon content in iron is < 2% , then we generally consider it as steel. ild steel is the most common form of steel because i s price is relatively low while it provides mater al properties that are acceptable for many applicati ns. Mild steel contains 0.16–0.29% carbon; therefore, it is neither brittle nor ductile ductile.. Mild steel as a relatively low tensile strength, but it is heap and malleable; surface hardness can be increas d through carburizing. The steel has two major constituents, which are ferrite and pearlite The properties of the steel dep nds upon the microstructur microstructure. e. Decreasing the size of the grains and decreasing the amount of pearlite improves the strength, ductility ductility and the tough ess of the steel. The inclusions can also affect affect the to ghness. For example, they can encourag e ductile fracture. Mild steel is a very versatile a nd useful material. It can be machined and worked into complex shapes, has low cost and good mechanical properties.
2. Microstructure analysis of ast iron:
When carbon content in iron is > 2% , then we generally consider as cast iro . A typical chemical composition to obtain a graphitic microstructure is 2.5 to 4.0% carbon and 1 to % silicon. Silicon is important to making grey iron a s opposed to white cast iron, because silicon is a graphite stabilizing element in cast iron, which mea s it helps the alloy produce graphite instead of ir on carbides. Another factor affecting graphitization is the solidification rate; the slower the rate, the gr ater the tendency for graphite to form. A moderate cooling rate forms a more pearlitic matrix, whil a slow cooling rate forms a more ferritic matrix. T o achieve a fully ferritic matrix the alloy must be annealed. Rapid cooling partly or completely su presses graphitization and leads to formation o cementite, which is called white iron . The graphite t kes on the shape shape of a three dimensional dimensional flake. flake. In two dimensions, as a polished surface will appear under a microscop microscope, e, the graphite flakes appe r as fine lines. The graphite has no appreciable stre gth, so they can be treated as voids.The tips o the flakes act as pre existing notches notches and so it is britt le.
3.Comparison between ash-pol ished and etched specimen:
The microstructure between et hed mild steel and unetched mild steel does n t change. Etching is only to make the microstructure visible. Ash polished Mild steel
Grain boundaries are not visible in case of ash polished specimen. Grain boundaries are clearly visible in c ase of etched specimen.
Etched Mild steel
4.Comparison between etched Mild steel and Cast iron:
The light coloured region of the microstructure is the ferrite. The grain boundari s between the ferrite grains can be seen quite clearly. he dark regions are the pearlite. It is made up f om a fine mixture of ferrite and iron carbide, which an be seen as a "wormy" texture.You can also see small spots within the ferrite grains. These are inc usions or impurities such as oxides and sulphides.Etchant used is 2% Nital.
Grey cast iron, containing graphite flakes in a matrix which is pearlitic. pearlitic. The The la ellar structure of the pearlite can be resolved, appe ring to consist of alternating layers of cementite and ferrite. The speckled white regions represent a phosphide eutectic. Etchant used is 2% Nital. 5.Comparison of quenched mil d steel and cast iron:
normalizing temperatures and th en rapidly cooled (quenched) in water to the criti cal temperature. The critical temper ture is dependent on the carbon content, but as a general rule is lower as the carbon conte t increases. This results in a martensitic structur . Thus
quenched steel is extremely hard but brittle, brittle, usually too too brittle for practical purpo purposes. These internal stresses cause stress cra ks on the surface. Quenched steel is approximat ly three to four-fold harder than normali ed steel.
Cast iron except tempered at 95 °C after quenching results in the formati n of graphite flakes in a matrix of tempered mart ensite. 6.Comparison of normalised
ild steel and cast iron:
Mild steel. Mild steel normalized by austenitizi ng at 950 °C and air cooling. We obtain a pearlite which looks li e a coarse grain structure in a ferrite matrix.
The temperature range for norm lizing gray iron is approximately 885 to 925°C .No rmalizing generally produces affine pearlitic mat rix that combines good wear resistance ith reasonable machinability machinability and an excellent response to induction or flame hardening, pr ovided that the cooling rate is fast enough and th
hardenability is sufficient for the section thickness. 7.Comparison of annealed mild steel and cast iron:
Mild steel annealed by austenitizing at 950°C and cooling slowly in a furnace. The structure consists of coarse lamellar lamellar pearlite (dark) in a matrix matrix of ferrite (light). (light).
If the microstructure of gray iron contains massive carbide particles, higher annealing temperatures are necessary. Annealing may simply serve to convert massive carbide to pearlite and graphite. To break down massive massive carbide carbide with reasonable reasonable speed, tempe temperatures ratures of at least 870°C are required. required. With each additional 55 °C increment in holding temperature, the rate of carbide decomposition doubles. Consequently, it is general practice to employ holding temperatures of 900 to 955°C.
Ref: ASM Metal hand book volume 09: Metallography and Microstructure.