Re: [MW:31483] brittle fracture CS 516 Gr.70N

Dear Ramon,

Regarding Oxygen level, you cannot measure it in the shop (it's not like GTAW/GMAW) but rather, you have to remove samples from deposited weld and send it to the lab.

It is well established that low oxygen level is essential for better toughness test results (typically less than 300 ppm but should not be too low). Flux composition plays a major role in oxygen contamination and as in steel-making, welding engineers try to lower the Oxygen content. Although using Basicity Index has been criticised by many, using basic fluxes (with Basicity Index > 1) could usually guarantee Oxygen contents of between 200-350 ppm and lead to desirable toughness test results. 

But remember, these apply when you have this problem in the weld metal (not in the HAZ). 

Many times, high heat input also could create coarse grain regions in HAZ, especially below the cap pass if the voltage is high and the wide cap pass makes the weld cross-section look like a mushroom. I have seen this one so many times. It has nothing to do with the Oxygen level and you will also see very low toughness test results (all because of high Heat Input).

Regards

Ramin  Kondori

Sr. QA/QC & Welding Engineer

-----------------------------------------------------------

PG-Dip. in Welding Engineering (IWE  AT  0070)

BSc. in Civil Engineering (IUT)

BGAS Painting Inspector

ASNT Level I&II

                        

On Sat, Sep 19, 2020 at 7:50 AM Customer Care <bricenori@hotmail.com> wrote:

 

Dear Ramin,

 

I extend the comment on the brittle fracture at -45 C of a SA-516 Gr.70N carbon steel with SAW process, using Flux / wire: F7A6-EM12K.

The findings were:

Manufacture of five pressure vessels, with shell thickness  32 mm /

Destination an Olefins Plant, propane refrigeration section.

By ASME VIII, Div. 1 impact test was required according to UG-84 / Minimum value 20 ft-lb per type of steel.

Charpy test: three 55x10x10 mm samples, absorbed energy was measured at -45C / values ​​of 4-6 ft-lb

Given these out-of-range values, we polished the samples and evaluate in the metallographic microscope: microstructure massive of coarse-grained ferrite and Widmanstatten ferrite was found. Ideally, for reliable service results, the welding forms an acicular ferrite-like structure, for excellent toughness against thermal shock at low temperatures.

The other faces of the fractured samples were observed in the electron microscope-SEM: topography was found with a high cleavage transgranular fracture, typical of a brittle fracture with total absence of dimples and with 60% of charred areas. Burned welding.

Investigating a little more, it was learned that the workshop did use the correct flux-wire combination F7A6-EM12K, and the qualified WPS, but made a mistake: in the night shift work and to speed up the manufacturing, it abused excess heat input, modifying the variables forward speed and voltage on the SAW machine, thereby violating the qualified WPS.

The Licensor, wanted to test in another Laboratory, extracted a longitudinal weld segment. It set up your Charpy tests, with results of 2-3 ft-lbs.

The pressure vessels were repaired on site, to meet quality assurances.

Other remarks:

The quality certificate for SA-516 Gr.70N steel plates indicated Charpy values ​​of 92 ft-lbs., average.

Mr. Pradip, has made his technical observations to this case (LinkedIn), among others the optimal value of heat input for the SAW process in welds with toughness tests, and recommends being attentive to the oxygen levels during the fusion and cooling process of the weld.

I am left with one question: How can a Welding Engineer or Welding Inspector detect or measure oxygen levels in a shop weld and if this variable should be annotated on the WPS qualified?

 

Regards,

Ramon Briceno

 

Sent from Mail for Windows 10

 

From: Ramin Kondori
Sent: Thursday, September 17, 2020 2:25 AM
To: materials-welding@googlegroups.com
Subject: Re: [MW:31463] brittle fracture CS 516 Gr.70N

 

Dear Ramon,

 

Could you share the test results (the lab roport)...?

Has the samples failed in weld metal or HAZ...?

Additionally, please provide us with other test results (tensile test, macro, etc.).

 

Details are very important here.

 

Regards 

Ramin  Kondori

Sr. QA/QC & Welding Engineer

-----------------------------------------------------------

PG-Dip. in Welding Engineering (IWE  AT  0070)

BSc. in Civil Engineering (IUT)

BGAS Painting Inspector

ASNT Level I&II

                        

 

 

On Mon, Sep 14, 2020 at 7:04 AM Customer Care <bricenori@hotmail.com> wrote:

Dear Fiends,

I have a question about how excessive heat input affects SAW type welding, in a steel SA 516 Gr. 70 N. That is, a material subjected to Charpy test at -45C, and gave brittle fracture with absorbed energy between 4 to 6 lb-ft.

The WPS / PQR clearly rated, but shop production welding gave these results on pressure vessels.

My certainties indicate that coarse-grained ferrite and widmanstatten ferrite could be formed during the cooling of the weld (highly brittle), and not the expected acicular ferrite, which is the appropriate structure for excellent ductile fracture results in this type of steel.

 

I open the forum,

 

Thanks for your attention.

 

Regards,

Ramon Briceno

Sent from Mail for Windows 10

 

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