How many PWHT can be done on a carbon steel? 1

[VinceFabspec]

Hi;
I want to get many (up to three) PWHT (1 hour stress relief at 1150°F or per ASME Sect.8 div.1 - UCS-56) on a pressure vessel shell (SA-516-70N) without deterioration of the mechanical properties below a specified limit.
-Is there a variable (chemical composition, metal structure, dimensions, holding time of the PWHT, etc) which does influence the repeatability of a PWHT (stress relief)?
-How many times can I repeat the same PWHT?
Regards

 

[metengr]

If notch toughness is not a concern,you can repeat PWHT as many times as required as long as you qualify the weld procedure with the aggregate PWHT time.

 

[VinceFabspec]

Thanks, I also got the Mittal Pressure Vessel Brochure thanks to another posting on this site. I find the Larson-Miller Parameter and the mechanical properties (tensile and CVN) relations very interesting.

 

[SJones]

You could also, of course, specify that the inspection documents (MTR) provided by the supplier of the A516 certify the material properties after simulation of the heat treatment cycle(s) you wish to subject the steel to. As per ASME IX, 407.2, the minimum requirement would be that the welding procedure test coupon receives 80% of the proposed aggregate time at temperature.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[TomEun]

As Steve mentioned, the repeated time of PWHT is not important, but the total accumulated holding time (hours) are controlled as an essential variable in ASME Sec IX or other pressure vessel codes.

In your case, I doubt how much you can utilize the relation between Larson-Miller Parameter/TS and CVN because they may not have consistent relation. The CVN toughness may be increased up to the condition removed the residual stress effectively, but the continuous PWHT after that may decrease TS below than the MSTS (minimum specified tensile strength) and/or toughness as well.

 

[unclesyd]

Not taking away form any of the above posts but in a practical sense we saw no appreciable deterioration by cycling to PWHT temperatures. The materials were A106, A105, A283, A53, and A517. As stated before our polymer process requires that all process components , mainly piping, be cleaned by heat or pyrolysis. In the early nineties teats were undertaken to ascertain if any damage, metallurgically, had been done to our piping and components by repeated cycling to 1000F-1100F or the current 900F. Our original cleaning procedure was done in a car bottom furnace that operated at 1050F with times dependent on the loads but never less than 6 hrs. A change to a pyrolysis cleaning process reduced the max temperature to 900F where 2 hrs at temperature was nominal. Our tests were conducted on samples taken from process piping or filter housings that were to be modified or replaced that had undergone at least 50 cleaning cycles . There were approximately 50 tensile specimens and approximately 20 Charpy specimens used for RT testing.
The values for both Tensile, Impact and Hardness were all within published values for the associated steels. There was nothing amiss microscopically.
Additional coupons were to be undergo 100 cycles by piggybacking on the cart used in a small pyrolysis furnace max temperature of 900F. The physicals, both longitudinal and traverse, were to be compared to the actual MTR's but fate intervened and the only parameter tested was hardness. There was no change in the reported hardness values.
The last several years this piping has undergone extensive modifications due to change in product lines. After modification the piping was cycled through two cleaning cycles. The pipe was modified and tested with the full concurrence of the Insurance carrier. The high pressure piping was tested using a factor of 1.5 which was 3750 psig. Certain items were tested at 70 psig @ 700F.

 

[ulyssess]

The second and third PWHT will have a marginal influence on mechanical properties of SA516-70. Compsared to the values in normalized condition the main loss in tensile strength and impact values will occur with your first PWHT.

Based on company-internal neuronal network with some 10000 values for SA516-70, a forecast for a 1 inch plate is:
a = average decrease in tensile strength in ksi
b = average increase in transition temperature (20 ftlbs Charpy-V transverse) in °F
(all values compared to the normalized condition)
1st PWHT : a = 2.8 b = 22
2nd PWHT : a = 3.0 b = 23
3rd PWHT : a = 3.3 b = 23

Influences on mechanical properties for the first PWHT:
Chemical composition has only a small influence for SA516-70.
Material thickness (due to metal structure) has a significant influence on the losses. For an identical PWHT (1150°F, 1h):
0.5 inch: a = 3.6
2.0 inches: a = 2.2
4.0 inches: a = 1.7
But generally, for thicker plates, longer holding times and slower heating and cooling rates are required by Code, which partly compensates this effect. For 1150°F and holding, heating and colling according UCS 56:
0.5 inch: a = 3.2
1 inch: a = 2.8
2 inches: a = 2.5
4 inches: a = 2.3
Holding time surely has an influence for the first PWHT as showed on the examples.
The main influence is temperature.

For Charpy-V-values, the tendency is the same, but more difficult to illustrate, as the shape of Charpy-V-curves changes with material thickness and PWHT conditions, but also other parameters like cleanliness and residual contents (sulphur, phophorus, nitrogen, hydrogen,...) have an influence. As ilustrated in the Arcelor Mittal Paper, for a given temperature and thickness, it is possible to illustrate the decrase of impact values with PWHT. But Charpy-values strongly depend on the chemical composition chosen by the manufacurer. A plate with C = 0,16% and Mn = 1,45% has significant higher values then a plate with C = 0,28 and Mn = 1,0%.

 

[stanweld]

I would be more concerned about the cumulative effect of multiple PWHTs on deposited weld metal if improperly selected. The lower carbon content of many of the filler metals today provide lower tensile and yield stress after exposure to prolonged standard or higher temperature PWHT.