T22 Type IV Cracking & Post Weld Treatment 2

[alexx]

For T22 material, according to ASME Section I Table PW-39, it is not necessary to post weld heat treat provided a minimum pre-heat of 300 F is used along with a few other requirements.

My concern is if the lack of post weld heat treatment in this case will adversely affect the resistance to Type IV cracking. In other words, will the failure rate be higher or the same if the joint is not post weld heat treated.

Tubing is used in a HRSG. We are trying to avoid post heat treatment if at all possible. ASME says it is OK but we want to make sure we dont have problems down the road.

Can anyone help?

thanks,

Alex

 

[metengr]

Yes. Normally, the factors that drive this type of creep damage mechanism are tensile stresses from thermal/mechanical forces in service and a fine grained tempered martensite microstructure in the base metal heat affected zone (HAZ). When you weld on T22 material given the proper preheat requirements and wall thickness constraints, the rate of cooling is such that you produce a mixture of bainite + ferrite/chromium carbide structure in the HAZ. You can get isolated islands of martensite. However, in comparison to higher chromium alloys that have increased air hardenability, you will not have the conditions that would result in Type IV cracking like T9 or T91 alloys, which develop close to 100% martensite upon cooling after welding.

 

[metengr]

One additional comment on this subject is that the service stress in fabricated (welded) components is probably equally as important as the highly susceptible fine grained region of the base metal HAZ after welding and subcritical PWHT.

The European Creep Collaborative Committee (ECCC) has done extensive work into identifying a threshold bending stress that results in Type IV creep damage. It is my opinion that the reason for the increased susceptibility and reports of problems with the advanced ferritic materials, like Grade 91, is that designers take advantage of the higher allowable stresses to reduce wall thickness but don’t go beyond this to evaluate in service thermal/mechanical bending stresses.

Unfortunately, thinner wall components can also be subjected to much higher thermal/mechanical bending stresses at structural attachments (hangers) and at nozzle penetrations. The thinner wall components must be evaluated in terms external flexibility during design. The work horse alloys like T11 and T22 have been used successfully for many, many years in boiler construction partly because these alloys are more forgiving (as mentioned in the previous post) and have much lower allowable stress values, which means thicker wall components.

 

[eyec]

ASME says it is OK but

When you weld on T22 material given the proper preheat requirements and wall thickness constraints,

make sure you meet these requirements, especially if you are near the headers. additionally, i suggest any preheat be done by controlled means and not temp sticks/torch.

 

[alexx]

Thank you metengr for your detailed explanation. It has been a big help.

Also thanks eyec for the suggestion near the headers

Alex