Martensite content in 304SS 3

[280474]

Dear All,

Could anybody give information/literature link regarding the Martensite transformation in Austenitic SS (grade 304)due to cryogenic services..?
As we know this martensite content can reduce the toughness strength of parent material and become more brittle that can be easily to crack in cryogenic temperature.
In the last measurement the highest content of martensite was found 15%wt. As our sister company experience, they found a few crack on their LNG loading arm (SS304)and investegiated and found the martensite content is more than 33%wt.
Therefore I need some literature and or sharing information to find the method for calculating and prediction the martensite transformation content of SS304 caused by rapid cooling down and subzero temperature treatment.

Regards,

 

[TEV]

Chapter 4 of the book "Handbook of Stainless Steels" edited by Peckner and Bernstein (McGraw-Hill, 1977) is THE best review of wrought austenitic stainlesses, written by Charlie Novak. The martensite transformation is addressed starting on page 4-23 of this chapter.

Also, see Chapter 2 of the book "Austenitic Stainless Steels - Microstructure and Mechanical Properties" by P. Marshall (Elsevier, 1984). This discusses the martensite transformation in some detail and gives equations (with references) for temperature induced and deformation induced martensite transformations as a function of composition.

 

[mcguire]

If you don't have that book, a great one, you'll have trouble finding it and it's out of print. The answer to your specific question, however, isn't in it. 304 is considered metastable austenite, meaning it's not supposed to transform to martensite at all unless it undergoes deformation. I know of nothing published about purely thermally ( cooling) induced martensite formation in 304. If someone else has something more than anecdotal, I would love to hear it.
For cryogenic purposes the 200 series is preferred because its higher alloy content makes it more resistant to martensite formation. 21-6-9 is optimal.

 

[280474]

Guys,
Thanks for the information. Another question, is the content growth of martensite in SS304 depending on time..? What I mean, once we measured the content, let's say 10%wt and found it due to rapid cooling down as per analysis. If the cooling down is still in the same rate, will the content of martensite increase and sometimes it will become, let's say 33%wt..?
This is my concern, since we've been measured the content during this 10 years and found it remain at 10-15%wt at same cooling down rate as per operating requirement.

Regrads,

 

[mcguire]

The amount of martensite will depend only on the lowest temperature encountered ( if no deformation occurs ). It does not depend on the rate of cooling , time at temperature, or number of thermal cycles.

 

[TEV]

280474: If you work in stainless steels, I highly recommend the "Handbook of Stainless Steels". Yes, it's out of print, but there are 5 copies available on Amazon.

McGuire: Thermally induced martensite formation in austenitic stainlesses is discussed in detail with references in the book I referenced by Marshall. I don't know about the validity of the discussed work. I have never been able to induce martensite thermally, but I sure have by deformation.

 

[280474]

Guys,

Let me describe my problem,
We've been applied Loading Arm system with 304 SS material for transferring LNG product to the LNG Tanker. To prevent thermal shock-stress due to extrem temperature differences,
Loading arm must be cooled down from ambient to LNG temperature (-165 deg.Celcius) within 1 hour 45 minutes (76 deg.C / hour). This must be done before the arm is ready to load LNG product. To load LNG to the 125000 m3 LNG tanker, it is required around 12 hours.
According to many references that martensite transformation from Austenitic SS such as 304 grade can be occured as a result of :
1. Rapid cooling down
2. Subzero treatment below Martensite start point of 304 ss, in this case arm is operated in -165 deg.C for 12 hours continously.
3. Deformation.

Since there is no deformation indication found on the arm, will the arm with 304 ss material partially transform into martensite phase caused by its cool down rate (76 deg.C/hour) and its service temperature (-165 deg.C)..?

Our last 6 years survey record for martensite content shows that there are 10-15% martensite content and it remains stable during last six years since the first measurement.

Regards,

 

[mcguire]

280474
How are you measuring martensite content? You couldn't be measuring ferrite could you? Is this wrought 304? I find the 33% hard to fathom. I also find a crack curious. Was there plastic deformation? corrosion?

 

[280474]

Mcguire,

Martensite Content has been measured by using Induction Magnetic Technique.In the last measurement (June 2002) carried out by loading arm's manufacturer, ferrite content are 14% in welded area and 12% in the base material. According to the manufacturer that it does not reach the excess level of ferrite content.
Actually I am not sure whether it's wrought 304 or what..? What I know the material is pipe 16" ASTM A312 type 304 SS.
As I mentioned it is found no palstic deformation and or corrosion.

RGDs

 

[mcguire]

Ok...ferrite isn't martensite, so this is something that came from the original manufacturing process. Do some hardness measurements in the affected area. If it is under RC20, or Rb100, it's just ferrite.
I don't know why a manufacturer should consider 12% ferrite other than excessive. Is this centrifugally cast pipe? Only a casting could have that much ferrite. With this amount of ferrite, however,the ferrite may be a continuous phase with much less toughness than the austenite you thought you would have. At cryogenic temperatures the ferrite is totally lacking in toughness and this may be the underlying reason for failure.
The manufacturer is seriously deficient in thinking that large amounts of ferrite are acceptable for cryogenic service.

 

[TVP]

Just to be clear, you are measuring ferrite content, not martensite content. The ferrite is due to solidification of the welds. mcguire is correct. That amount of ferrite is excessive. You should be specifying that the LNG loading arm be supplied to you with essentially no ferrite, as it will seriously degrade the fracture toughness at cryogenic temperatures. Post-weld heat treating in the range of 1900 F is a requirement in ASTM A 312, and a similar procedure should be used on the LNG loading arm, provided that heat treating is feasible (size, mass, etc.).