BLOG ENTRY - 7


Hello guys!!


In today’s class we learnt more about metals and their properties, Reactions achieved between metals and more about steel in its different compositions. 


The galvanic relationships between metals

the table consists of a list of metals from Anodic, least noble metals (left) to cathodic metals, most noble metals (right). 


Galvanic action


When two metals are in contact, the one that is higher in the galvanic series will become the anode ( a positively charged electrode) and will corrode. 


The current flows from the positive (high end of the table, anodes) to the negative (low end of the table, cathodes). 

For an example, when iron and copper are in contact, iron will become the anode and will quickly corrode. 


Co-efficient of thermal expansion


Metals undergo changes, like expand and contract like any other materials, due to the changes in temperature. These changes are quantified as co-efficients. 


Ratios that gives amount of change per length of material per degree of change in temperature. 


Tensile/ yield strength 


A material reaches a yield point when it’s undergoes an amount of stress, and displays strain hardening. With further stress, we are able to find the amount of resistance it has to withstand breaking in the amount of stress it experiences, also called as the tensile/yield strength of the material. After this point it’s shows signs of deformation, known as Necking. With further strain, the material completely deforms, and fails to be the material for such a purpose. 


Solid melts to become a liquid, the amount of time and temperature required gives us the melting point of the solid to turn into a liquid,which then evaporates to become gas. The temperature at which when liquid is heated to evaporate into a gaseous state.When gas condenses, it becomes liquid and when liquid freezes it turn into a solid again. Solid can attain its gaseous state without turning into liquid at a certain temperature by the process of sublimation. 


Reflectivity 


When a light beam encounters a material, it’s radiation can either be absorbed or reflected by the surface. 


p(y) = Gr(y)/Gi(y) 


p is the reflectivity 

Y is the wavelength of the light 

Gr is the reflected radiation 

Gi is the incident radiation 


Steel reaches the plasticity zone (deformation of the material) when it undergoes a certain amount of stress and strain. There are different types of deformation, 


For example, oil canning of steel sheets. 

The deformation is seen as waviness of the otherwise flat sheet of steel. It’s is visible only when the sun is positioned at certain angles. 

It is the result of the stresses within the material. 


How can oil canning be prevented? 

⁃ design - choice of material 

⁃ Thicker/high quality of the material 

⁃ Light coloured/ less glossy finish 

⁃ Backer rod 

⁃ Skilled labour 


Essentially tools for the Joinery details of the steel construction:


Steel is usually welded or soldered together( joined using heat to metal and attach one part to another part which then becomes cold and forms an unbreakable form) to form a strong bond. 


Bolt


Heat-treated bolts have extra strength - high strength bolts. Due to shear resistance opposed to their tightness, they prevent the slippage of the two materials. 


Pin


Transfers vertical and horizontal shear loads and can not resist any bending or rotational forces. They are firm connections, which do not move/ rotate even thought they might look like they can. A lapped joint connection.


Mechanical Fastner 


A device used to mechanically join two or more objects together. Creates non-permanent joints which can be removed or dis-mantled. 


Rivet 


Creates permanent joints. Mainly used for fastening sheets and shaped rolled metals.

Used in double- cover plate joints.


Compositions of steel


1. Steel alloys with varies other metals 


2. Stainless steel - an alloy steel with 20.5% of chromium, with nickel and molybdenum. Contains varying amounts of carbon, silicon and manganese. 


3. Carbon percentages in steel differing it’s properties and usage as a material: a) low/mild steel - up to .25% carbon b) medium steel - .25-.60% of carbon c) high/hard steel - .6-1.5% of carbon 


Higher the carbon content = higher hardness/strength 


4. Tools 


Different types of stainless steel 


Ferritic - 

⁃ Iron-Chromium alloy with body-centered cubic crystal (BCC). 

⁃ Magnetic

⁃ Strengthened by cold working and not hardened by heat treatments.


Martensitic 

⁃ extremely strong and tough 

⁃ Comparatively not as corrosion-resistant

⁃ Highly machinable 

⁃ Magnetic 

⁃ Heat-treatable 


Austenitic 


⁃ Highly corrosion-resistant 

⁃ Non-magnetic 

⁃ Non-heat treatable 

⁃ Highly weldable


Duplex Stainless Steel 


⁃ two-phase microstructure consisting of grains of ferritic+ austensitic stainless steel.

⁃ Doubly strong as ferritic/ austensitic stainless steel 


Precipitation- Hardening Stainless Steel (PH) 


⁃ possess Ultra high strength due to precipitation hardening. 


Tool


Contains various amounts of tungsten, cobalt, molybdenum and vanadium to 

⁃ increase the heat and wear resistance 

⁃ Durability of the metal

⁃ Makes it ideal for cutting and drilling




Signing off

Tejashri Kumaresan

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