304 Stainless Steel Permeability Check 3

[Cloya14]

All,

I have a customer that wants all their stainless steel pressure vessels to be below 2.0 mew magnetic permeability. Problem is that they were fabricated without production knowing of this requirement and consequently they are all failing. I do know of the proper steps to prevent this from happening but i dont know how to fix it after its been welded. I have done some research and it looks like the best solution is to anneal at 1900 deg. F and let air cool to bring down ferrite levels. Im looking for any other solutions and to see if anybody else has had this issue before. Thanks in advance.

Christian

 

[metengr]

Having been involved with pressure vessel fabrication, the problem is the cold work introduced during fabrication, and ferrite level in the weld metal(s). You are correct that the material should have been solution annealed after forming, welding is acceptable without solution anneal.

See the excerpt below from the Carpenter web site;

Austenitic (nonmagnetic) Stainless Steels
All austenitic stainless steels are paramagnetic (nonmagnetic) in the fully austenitic condition as occurs in well-annealed alloys. The DC magnetic permeabilities range from 1.003 to 1.005 when measured at magnetizing forces of 200 oersteds (16k A/m). The permeability increases with cold work due to deformation-induced martensite, a ferromagnetic phase. For certain grades such as Types 302 and 304, the increase in magnetic permeability can be appreciable, resulting in these grades being weakly ferromagnetic in the heavily cold-worked condition. The susceptibility of a particular grade to becoming ferromagnetic when heavily cold worked depends on the stability of the austenite, which, in turn, depends on chemical composition and homogeneity. This is described in the article "Stability of Austenite in Stainless Steels" by C. B. Post and W. S. Eberly, published in "Transactions of the American Society for Metals," volume 39, (1947), pages 868 to 890.

The effect of cold work on magnetic permeability is illustrated for several austenitic stainless steels in Figure 1. The relationship between ultimate tensile strength and magnetic permeability is shown in Figure 2. The rise in permeability correlates well with the increase in tensile strength or work-hardening behavior, which is another measure of austenite stability. The differing performance between grades is a reflection of their composition. In particular, nickel increases austenite stability, thereby decreasing the work-hardening rate and the rate of increase of magnetic permeability. Consequently, the higher nickel grades, such as Carpenter Stainless No. 10 (Type 384), exhibit lower magnetic permeabilities than the lower nickel grades such as Project 70+® Type 304/304L when cold worked in equivalent amounts. The high-manganese, high-nitrogen alloys, such as Carpenter 18Cr-2Ni-12Mn, are also noted for maintaining low permeability after heavy deformation.

The magnetic permeabilities achievable in austenitic stainless steels are very low compared with conventional magnetic materials such as silicon-iron alloys. Therefore, their non-magnetic behavior is more of a concern. Certain uses such as housings and components for magnetic detection equipment used for security, measuring and control purposes require that the steel be nonmagnetic. That is because the presence of even weakly ferromagnetic parts can adversely affect performance. Unless the austenitic stainless steel parts are used in the annealed condition and are not subjected to deformation during use, a higher nickel grade would be a prudent choice assuming it offered the appropriate corrosion resistance and strength.

For a given grade, the magnetic permeability can vary significantly depending on the chemistry and degree of cold work of the steel. Often a particular lot of an "unstable" grade such as Type 304 can perform satisfactorily. If the magnetic permeability of an austenitic stainless steel is of particular concern, it can be measured by relatively simple means as described in ASTM Standard Method A342.

 

[MagBen]

very well said! metengr.
just to supplement: besides residual ferrite and Martensitic transformation, strain can contribute to ferromagnetism. The so-called SIF (strain induced ferromag) is petty known to academic, but less known to industry. the increase in hte number of Fe-Fe nearest neighbors both in the anti-phase boundary (APB) coupled dislocation particles and in APB tubes. in case of SIF, a stress relief anneal would do the trick.

 

[EdStainless]

It takes rather severe forming for SIF to set in in.
If this is L grade material you might get away with using a lower temp than full solution anneal (and still not sensitize it).
But when it comes to ferrite in the welds a real solution anneal is the only thing that will help.

SIF in the plate material is not really a big deal. Unless it is very severe it really has no impact. However the ferrite in the welds is another issues. The ferrite that forms on solidification in 3xx alloys is very lean in chemistry, so it has much lower pitting resistance than the bulk material does. In even a mildly acidic environments (cane syrup, orange juice, shampoo) the weld ferrite will be selectively attacked leading to perforation and failures along welds.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[MagBen]

for residual ferrite, solution anneal,
for martensite, reversible temp is about 650C depending on cold work and composition.
for SIF, stress relief anneal.
the SIF I referred has nothing to do with martensitic transformation. Strain alone can cause ferromagnetism. Changing relative permeability from 1 mew to 2 mew is a tiny bit of ferromagnetism induced(hysteresis loop is still a straight line, only the slop change a bit), and may not need much of strain. I experienced measurable mew increase caused only by thermal strain (quenching)! note that no martensite was observed in this situation, also, that was not 3xx SS.