SCC of 4140 5

[Gem1]

Hi All,

I've heard anecdotal evidence that 4140 steel suffers from Stress Corrosion Cracking in coal conveyor applications. Can anyone provide any information about this?

I assume that 4140T would be worse than 4140R, but does R suffer from severe SCC?

 

[metengr]

I have not seen it, and we have a number of coal plants across the US. Have seen fatigue failures where 4140 is used in shafts, but that is about it.

 

[Guest102023]

I would take the reports seriously if there were at least one published report about the problem. With all the coal conveyors and all the 4140 used in the world, if it were a real problem, it would have been recognized by now.
Start by googling, but I'm sure metengr is right about fatigue - this equipment surely sees a lot of mechanical abuse.

 

[TVP]

In general, alloy steels that are quenched and tempered to a martensitic microstructure, e.g. 4130, 4140, 4340, 8740, etc., are considered to have the following resistance to SCC:

Low --> UTS > 180 ksi (1240 MPa, 39 HRC).
Good --> UTS 160-180 ksi (1100-1240 MPa, 34-39 HRC)
Excellent --> UTS < 160 ksi (1100 MPa, 34 HRC)

SCC for martensitic steels is most commonly associated with environments with chloride ions such as seawater/offshore, automobiles in contact with de-icing salts, etc.

 

[blacksmith37]

My speculation is that there is some S in the coal; that makes significantly corrosive mine drain waters. Various corosives (especially H2S) cause Hydrogen to enter steel and may SCC any steel above HRC 22 (refer to MR 01-75). Carefully processed Cr :Mo steels( eg C110 oil casing) can be resistant to about HRC 32 in severe H2S.
Bottom line , keep your strength as low as fatigue stresses permit.

 

[Gem1]

Thanks everyone, it doesn't sound like there is much evidence to suggest that 4140 should be discounted completly. I'll go with the R classification to reduce the likelihood of problems.

 

[JAWright]

Secondary hardening alloys tempered at ~900F to precipitate M2C strengthening carbides generally have much higher SCC resistance than steels tempered at lower temperature (~500F). These alloys have higher alloy content, so you pay for higher performance. One of the lower alloy content, yet highest performance, grades in this class is new from QuesTek & Latrobe Specialty Steel, called Ferrium M54

http://www.questek.com/ferrium-m54.html

Jim