ASME BPVC/B31.3 vs. 1.25Cr-0.5Mo material@low temperature

[Chumpes]

Dear all,

It is my company's standard practice to limit the use of 1.25Cr-0.5Mo material for pressure vessels with applications at minimum temperature above -18°C, due to the difficulties that vessel manufacturers have reaching impact test energies with this material and welded assemblies at such low temperature.

ASME VIII div.1 section UCS-66 and figure UCS-66 provide :
- note (2)(b) : "Curve B applies to [except for cast steels] all materials of curve A if produced to fine grain practice and normalized which are not listed in curves C and D below"
- note (2)(c) "Curve B applies to all pipe, fittings, forgings and tubing not listed in curves C and D"

ASME B31.3 section 323.2.2 and table 323.2.2 provide :
with MDMT > -29°C : no impact test requirements for base metal, weld metal and heat affected zone

I thus have the following question :
Why does the ASME BPVC and ASME B31.3 cover the use of 1.25Cr-0.5Mo material without impact test down to -29°C for pressure vessels and piping sytstems, knowing that 1.25Cr0.5Mo material is unable of sustaining temperatures below -18°C ?

thank you in advance

regards

 

[metengr]

Because the problem is that when the stress tables were developed, the metal temperature listed was for allowable stress only and not notch toughness. There is a movement within BPV II to revise the metal temperature in Part D as being only applicable to stress lines and not notch toughness.

 

[weldstan]

35 years ago we were able to obtain this material impact tested at -20F with 30 ft-lbs min absorbed energy. Base metal thickness was over 2" thick. Client was applying heavy wall hydrocracker technology to 2" and over 1 1/4 Cr applications even though the MDMT was >> -20F.

 

[Chumpes]

thanks for your answers
I understands that the stress related mechanics (ASME stress tables) allow for the use of 1.25Cr-0.5Mo material at low temperature (above -29°C without impact test, below -29°C with impact test), but also that this material cannot pass impact test at such a low temperatures.

Then I will limit the use of 1.25Cr-0.5Mo to MDMT above -18°C, and ask for impact tests at MDMT for base material and welding procedure coupons.

In your opinion and because the impact toughness of 1.25Cr-0.5Mo is not well decribed in ASME, what do you think would be the limit temperature above which impact testing is not necessary for base metal and welding procedure ? ASME B31.3 says -29°C but i think i cannot use this value. On which normative document / construction code can I base the definition of that temperature limit?

thx & regards

 

[metengr]

Chumpes;
That was my point earlier in the thread. The metal temperatures stated in ASME Section II, Part D do NOT apply to impact testing only stress lines. Over the years, user's have assumed that the reference to -20 deg F to 100 deg F metal temperature also applied to impact testing exemption.

If this material is intended to be used below room temperature (non-boiler), impact testing should be performed rather than assuming -29 deg C.

 

[pradipgoswami]

Hi Chumpes;

You may take a look at API-RP-934C & 934E as additional resource.Both of these RPs are on 11/4CR-1/2Mo Steel Pressure Vessels for hydroprocessing services.The toughness requirements are as follows:-
The minimum CVN impact values shall be 40 ft-lbs(54 Joules) average of three specimens at 0 °F (–18 °C) and 20 ft-lbs (27 Joules) minimum for a single specimen
at 0 °F (–18 °C).
Charpy impact as stated above would be difficult to meet @ -29 Deg C.

Looking at the requirements of Table 323.2.2 & Fig. 323.2.2A, the requirements are more for C-Mn Steel.Possibly 1.25Cr-1Mo steel does not fit in to these curves.

Thanks.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299