What is the difference between bellow and non bellow type psv.

What is the difference between Toughness and Strength of a material

Difference between Toughness and Strength of a material.
Importance of Toughness and Strength while selecting a material.

Intergranular Corrosion

 We can understand from the name, it s the corrosion between grains (intergranular means between grains),i.e. corrosion at grain boundaries. This type of corrosion occurs under certain environment condition, In which there is localized corrosion at grain boundary with relatively little corrosion of grains. This process results in deterioration of bonding between grains and metal thus disintegrates losing the strength.

Cause of Intergranular Corrosion (IGC):
Under certain environment condition these are the possible cause for IGC.
      (i) Presence of impurities at grain boundary.
      (ii) More content of one alloying element at grain boundary area.
      (iii) Depletion of one alloying element at grain boundary area.

Intergranular Corrosion (IGC) in Austenitic stainless Steel:
 As we know that Chromium is added in ordinary steel to impart corrosion resistance to steel(Why?). Generally minimum 12% Chromium is added to make stainless steel. If by any means Cr content decreases, steel become susceptible to corrosion.

why Chromium content decreases in austenitic stainless steel (SS 304) at grain boundary?

 When austenitic stainless steel are heated in a temperature rage of approximately 480-780 degree centigrade, it forms Chromium carbide Cr23C6 along the grain boundary & this chromium carbide precipitates out of solid solution. thus Chromium is removed from solid solution. This causes depletion of Cr in area adjacent to grain boundary, subsequently corrosion resistance decreases.
This high temperature range may occur during heat treatment and welding.

This is why SS304L is preferred than SS304 where welding is required.

 We know that SS304L has 0.03% max carbon (quite low to that of SS304 which has 0.08% max carbon). Lesser carbon means lesser possibility of  to form Chromium Carbide. Thus lesser possibility IGC.







 

What is the difference between SS316 and SS316L

The difference between SS316 and SS316L is that of suffix 'L'. This L stands for low carbon.

                            Chemical composition of SS316 and SS316L

                       Cmax    P       S         Si      Mn     Cr        Ni      Mo

     SS316       0.08   0.045   0.03   0.75   2.0   16-18   10-14   2-3
     SS316L     0.03   0.045   0.03   0.75   2.0   16-18   10-14   2-3

From the above table we can see that SS316L has a 0.03% max carbon where as SS316 has a 0.08% max carbon. The low carbon in SS316L improves its wediability at high temperature. In a corrosive environment,SS316L used to minimize intergranulor corrosion.

What is the difference between SS304 and SS316

                          

Both SS304 and SS316 are austenitic stainless steels.

         CHEMICAL COMPOSITION OF SS304 AND SS316               

              C       Mn       P         S        Si           Cr              Ni             Mo

        

SS304  0.08    2.0    0.045   0.03    1.0    18.0-20.0     8.0-10.0        -

SS316  0.08    2.0    0.045   0.03    1.0    16.0-18.0    10.0-14.0    2.0-3.0 

 From the above chemical composition chart we can see the main difference between SS304 and SS316 is that SS316 contains 2%-3% molybdenum and SS304 has no molybdenum.Now the question arises that we are paying double money for SS316 only for that 2.5% 'Moly'.The answer is yes, because this  "molybdenum" is added to improve the corrosion resistance to chlorides (like sea water).That is why in coastal area plants SS316 is used.

Why addition of molybdenum improves resistance against chloride corrosion?

Role of Mechanical Maintenance Engineer

The role of a mechanical maintenance engineer has become very important aspect in production industries(eg.fertilizer industry, chemical industry, petroleum industry,cement industry etc).It can be understood with a very simple equation,

Failure of machine is directly proportional to production loss

And here comes the challenging role of maintenance engineer to prevent break down of machines.

Whenever we are sick, we consult doctor to get cured. Doctor examines us and if required refer for pathologycal tests,finally based on all the reports medicines are given.

Similarly a mechanical maintenance engineer is the DOCTOR of MACHINE. Maintenance engineer looks,listens and feels the machine to under the reason of machines sickness(i.e. high vibration,abnormal noise, high temperature etc ).

To give correct diagnosys vast knowledge of material science, vibration analysis,bearings,corrosion protection, rotodynamics, paint technology, rotory machines eg pumps, turbines, bucket elevators etc is required.

In this blog we will discuss our day to day maintenance problems and their troubleshooting.