Temperbead Repair CA-6NM 1

[Metaljon]

I am prepared a PQR test report for qualification of a temper-bead butt repair on CA-6NM martensitic material to ASME IX. As part of the testing we are performing tensile, bend tests, macro and hardness. The WPS specifies a temper-bead repair using a minimum preheat 120 C. The weld metal is E410NiMo-26. The thickness of the plate is 18mm. For a hardness, the maximum HAZ hardness is 373 HV. The weld metal hardness ranges from 325 to 348 HV. I was expecting to achieve maximum of 350 HV. Weld is good quality with no defects. Are these hardness results acceptable to ASME IX?

 

[Guest102023]

ASME will say nothing about hardness, but NACE and API might, and your spec certainly does. You haven't done too badly, but you might have achieved a little lower.

A characteristic of 410NiMo welds is that the deposit is generally harder than the hardest part of the HAZ, the more usual target of temper bead welding. (I will assume that 309 filler is not an option, although it is frequently used when PWHT is not possible.)

When it comes to preheat of CA-6NM, sometimes more is less. For 18mm plate, minimum preheat could actually be a little lower, say 100°C. The key thing is to strictly control the interpass to not more than 260°C. Due to extreme suppression of the Ms temperature, mainly by nickel, overheating during welding can result in extreme hardness after cooling due to austenite transforming in bulk to untempered martensite - disastrous for toughness and risk of hydrogen cracking. ALL welding of CA-6NM benefits from a temper bead approach, which is why electrode diameter and bead size should be limited (5/32"Ø for SMAW, 1/16"Ø for FCAW). Stringer beads only.

Have you examined the deposit chemistry of the electrode (as per AWS A5.4)? Back in the day, building hydro turbines with seld sections up to 6" thick, I ordered hardness testing with every heat/lot of FCAW wire.

Remember to qualify each welder to the same procedure and hardness limit. Consider welding with excess reinforcement, such that the untempered final passes are completely removed.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[Metaljon]

Thanks for the input on this repair. The repair is a turbine runner for a hydro station with blades and PWHT is not an option. I did consider 309 option as there is a large crack in the casing and 309 would give good ductility but decided to go with the 410. I restricted pre-heating to 120 C in the temper-bead process and was aware of the need to control interpass to below 260 C limit. Yes stringer beads only were used and small electrode size. I have tested the weld composition as per AWS A5.4 and I get C 0.047, Si 0.27, Mn 0.87, Cr 10.8, Mo 0.48, Ni 4.3. The Chromium is a little low but overall acceptable. Good point about welding with excess reinforcement and then grinding off excess. I will add that recommendation in the final repair procedure.

 

[metengr]

ASME Section IX does not provide acceptance criteria for hardness because it is a service code book and not a construction code book. Hardness acceptance is either based on construction code requirements or by client engineering specification for the job. I would say the hardness looks very reasonable for this application.

 

[Guest102023]

Don't worry about the chromium too much, I don't consider CA-6NM to be truly stainless anyway (just watch what happens when you store your turbine out in the yard for a month). You want carbon to be as low as humanly possible - 410NiMo will never be understrength. Nitrogen is a bad actor for hardness, so maintain a tight arc with SMAW.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[Metaljon]

Just completed the mechanical tests and OK for tensile but fails the face bend test to ASME IX. The root bend was good, the two face bend tests failed. I did not specify in the procedure about excess reinforcement and grinding off the cap and this may have affected the result for the face bend test. I need to repeat the qualification. As this is hydro station and subject to water loading would a 309 electrode be a better selection than 410?

 

[weldstan]

It would provide greater ductility. With your weld metal and HAZ hardness, there is no way that you could pass a standard bend test.

 

[metengr]

You also have the option of using a different bend radius per Section IX (QW-466.1) knowing the ductility of the bend will not achieve 20% or more elongation for a standard bend test.

 

[Guest102023]

Given the 18mm thickness I surmise you are repairing a fatigue crack on the blade trailing edge near the band. Therefore your two service criteria are tensile strength (which determines fatigue strength), and cavitation resistance. 309 will come reasonably close to CA-6NM in tensile strength, but cavitation, not so much. If so far you haven't had cavitation issues at that location, 309 might well be sufficient - that is a judgment call based on past history.

For hydro turbine equipment you are not forced (unless by client specification) to meet the ASME bend test criterion. Even if you do everything possible to get the hardness lower, I think passing the Section IX criterion is borderline at best. I suggest a 90° acceptance criterion would suffice. 309 might not make bend test problems disappear completely, due to the abrupt hardness step at the fusion line.

Did you qualify on 3/8" thickness? Because probably face bends will be more difficult than side bends.

Just by the way, anyone know of CA-6NM turbines being repaired with duplex stainless? Strength would be good, but I've no idea of cavitation performance.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[Duwe6]

Use the [allowable] larger bend radius in Sect IX. And brimstoner gave some good advice - 309 is excellent for repairs on high-strength castings and forgings. Believe 309 was developed to weld foot-thick armor-plate on naval vessels.