Class test 4 - materials

Question 1

Describe how galvanic corrosion occurs in metals?

Answer 1

*When two dissimilar materials are in contact with each other and exposed to a conductive solution or electrolyte, a potential (voltage) is set up between these two metals and a current flows.
The corrosion rate on the less corrosion resistant metal is usually increased and attack on the more resistant material is decreased, as compared to the behavior of these metals when they are not in contact. The less resistant metal becomes anodic and the more resistant metal cathodic.

Question 2

Describe how stress corrosion cracking occurs?

Answer 2

Stress corrosion cracking requires the following (adverse) factors for development, the presence of a:
*Tensile Stress
*The presence of a corrosive environment, eg hydrogen, liquid metal, ammonia etc
*A minimum temperature (usually associated with brittle fracture in carbon steels)

Question 3

Explain how sensitisation occurs in stainless steels and list three methods for prevention of sensitisation.?

Answer 3

*When stainless steels are heated or cooled through the temperature range 500°c to 850°c, carbon, in solution in the steel, forms carbides with the chromium
precipitates out as small particles.
*These carbides form preferentially at the grain boundaries and cause the interior of the grain to be depleted of chromium.
* The lower chromium results in a loss of corrosion resistance and the steel is sensitised

Question 4

In accordance with the WRC Diagram, calculate the ferrite number from the following information. ?

Answer 4

C- 0.02, Cr- 18.96, Ni - 12.24, Mo - 2.38, Cb - 0.04, N- 0.06
Cr eq = 18.96 + 2.38 + 0.03 = 21.37
Nieq = 0.70 +12.24 + 1.20 = 14.14
Answer + - 7 FERRITE NUMBER

Question 5

Carbon contamination requires careful control when welding stainless steels, name four precautions that must be taken to avoid contamination. ?

Answer 5

a) Use stainless steel brushes
b) Use tools reserved for stainless steel
c) Use aluinium oxide grinding wheels reserved only for stainless steel
d) Do not use co 2for shielding

Question 5

A martensitic stainless steel requires welding to the highest quality level and a procedure has to be developed in order to meet this requirement, compile this procedure in a logical sequence so the requirements of the above are obtained ?

Answer 5

*Preheat and interpass temperatures must be carried out during the welding operation to ensure that hydrogen has been diffused out to the atmosphere, mainly to aviod cracking
*When the welding has been completed the weld must be cooled below the mf line this is to make sure there is no retained austenite as hydrogen diffusion from
austenite is extremely slow and hydrogen cracking may occur.
*Tempering of the material will be required and this will give the material greater toughness and ductility.

Question 6

Explain the procedure when welding carbon steel to stainless steel. 4]

Answer 6

*Butter the carbon steel with a 309 electrode first to form a layer of austenitic weld metal then complete the weld as a conventional stainless weld.
*buttered carbon steel may need to be stress relieved prior to completing the joint. *In this way carbon steel can be stress relieved without exposing the stainless steel to a heat treatment that may sensitise it.