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Stainless Steels

These are stain less as these have a minimum of 11 to 12% Cr which is

having more affinity for oxygen than iron, forms a very thin, continuous

 protective and stable oxide (Cr 2O3) film on the surface. Thus Cr imparts

corrosion and oxidation resistance and pleasing appearance. Ni. Mo and Mn

enhance strength and improve corrosion resistance. Based on amount and

type of alloying elements these steels are classified into 5 types.

Ferritic: (% Cr – 17 × %C) > 12.7 i.e usually 17 to 26 % Cr. These alloys

are ferritic in structure up to the  melting point. These can’t be heat treated.

Cold working is the only strengthening mechanism. These may containsmall additions of Mn, Si, Ni, Al, Mo and Ti. Carbon is kept very low to

increase toughness to avoid sensitization. Mo is added to improve resistance

to pitting corrosion; Nb and Ta are added to stabilize against intergranular 

corrosion. These are not susceptible to stress corrosion cracking, thus used

in chemical plants. These are used as heat-resisting elements in the making

of furnace components. Less expensive, good machinability, higher thermal

conductivity, low thermal expansion than austenitic. These steels show

ductile to brittle transition. Ridging or roping, temper embrittlement, sigma

 phase and grain growth in HAZ are few problems associated with these

steels.

 Martensitic: (% Cr – 17 × %C) ≤ 12.7. Heat treatable steels (as these are

austenitic at tem ps 950 - 1000°C). As Cr content is more, air cooling is

enough to form martensite which is not brittle due to low carbon content.

(1) Low carbon high strength martensitic types are developed to ensure

good weldability, formability and impact toughness. Tempering temperature

of 440 – 550°C should be avoided (temper embrittlement). These are used

in petrochemical, chemical plants, compressors, discs, aircraft structural and

engine parts, propeller shafts. (2) High carbon high hardness martensitic

types (poor eldability, formability and ductility) are used in cutlery, surgical

instruments, springs, high quality ball bearings, razor blades, cool hammers.

 Austenitic: (17 – 18% Cr, 8 – 10% Ni, C < 0.03%) these contain sufficient

amount of austenitic stabilizers Ni, Mn or N so that steels are austenitic

even at RT. To avoid intergranular corrosion and weld decay ‘C’ should be

less than 0.03%. Mo is added to improve pitting resistance and sulphuric

acid corrosion. Nb and Ti are added to take care of weld decay. Excellent

formability, high work hardening (low stacking fault energy), large uniform

elongation, non-magnetic, no ductile to brittle transition (tough even at verylow temperatures; cryogenic applications). These are not susceptible to

temper embrittlement, but shows reduced ductility in temperature range 750

 – 950°C due to brittle intermetallic formation (sigma phase). These are

susceptible to stress corrosion cracking. These can be strengthened by cold

working or solid solution strengthening. These are used in chemical

industry, house hold, sanitary, biomedical, architectural, food industries,

nuclear and marine applications.

 Duplex :  these are developed to utilize combination of properties of ferrite

and austenite (Toughness, weldability with strength and localized corrosion

resistance). These are stronger than austenitic steels. Presence of δ-ferrite

cause grain-refinement of austenite hence increases strength. Further 

refinement is obtained by using controlled rolling at 900 - 950°C which

results in very fine dispersion of ferrite and austenite grains. These steels

exhibit superplasticity (500% elongation at 950°C). Good corrosionresistance similar to Austenitic type. Not susceptible to stress corrosion

cracking and free from intergranular corrosion. These have ductile to brittle

transition temperature. These suffer from temper embrittlement and sigma

 phase embrittlement.

 Precipitation hardenable: these offer combination of properties but

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expensive and difficult to hot-process. These are used particularly for high

temperature applications such as power-plant. Strengthening is because of 

 precipitates like Ni3Al, Ni3Ti or Ni3Mo etc. on ageing. These are

susceptible to hydrogen embrittlement. The matrix can be (1) martensitic

(2) semi-austenitic (3) austenitic.

Labels: New in GATE 2012 

What is New in GATE 2012?

Application Process:

In GATE 2012, candidates need to register and fill the application ONLINE only by

accessing the zonal GATE websites of IISc and seven IITs. The application process

is complete only when a print out of the filled ONLINE application with the

candidate's signature and a good quality photo affixed in the appropriate place is

received by the respective GATE office along with necessary documents, if any, on

or before 24 October 2011. Please note that sale of application forms through banks

and GATE office counters has been discontinued.

Downloadable Admit Card:

Sending Admit cards by post has been discontinued from this year. Admit cards can

only be downloaded from the zonal GATE websites from 2nd January 2012. Bring

the admit card to the test center along with at least one original (not photocopied /

scanned copy) and valid (not expired) photo identification.

 

Use of black ink ball point pen:

The use of pencils to darken the bubbles in the answer sheet has been discontinued

from this year. Candidates should use only black ink ball point pen for darkening of the bubbles in the answer sheet. Since bubbles darkened by the black ink ball point

pen cannot be erased, candidates should darken the bubbles in the answer sheet

very carefully.

ONLINE examination in two additional papers:

In GATE 2011, the papers with codes GG, TF, AE and MN had ONLINE

examination. In GATE 2012, two additional papers, AR and AG, will also have

ONLINE examination. The ONLINE examination will be conducted in two sessions

on Sunday, January 29, 2012.

Forenoon session  (09:00 hrs to 12:00 hrs): AR, GG and TF.

Afternoon session  (14:00 hrs to 17:00 hrs): AE, AG and MN.

Numerical answer type questions in ONLINE papers:

In the ONLINE papers (AE, AG, AR, GG, MN and TF), the question paper wil l

consist of questions of multiple choice type and questions of numerical answer 

type. For multiple choice type questions, each question will have four choices for 

the answer. For numerical answer type questions, each question will have a number 

as the answer. The number of numerical answer type questions may vary between

5 and 10 in each question paper.

Pre-final year students:

Pre-final year students are not eligible to write GATE 2012. For details, refer to

eligibility for GATE examination.

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