welding of martensitic SS steam turbine nozzles
welding of martensitic SS steam turbine nozzles
(OP)
Dear all
We have a steam turbine nozzle block that is made from a martensitic stainless steel (similar to 422). The nozzle assembly is fabricated from individual nozzles which are welded together. The weld material is a nickel based alloy believed to be Inconel 182.
We are currently investigating a repair of the nozzle assembly. My question arises to the reasons why a nickel based alloy was used in the fabrication, rather than a 410 martensitic stainless bearing in mind it is possible to PWHT.
Any comments would be appreciated.
Regards
We have a steam turbine nozzle block that is made from a martensitic stainless steel (similar to 422). The nozzle assembly is fabricated from individual nozzles which are welded together. The weld material is a nickel based alloy believed to be Inconel 182.
We are currently investigating a repair of the nozzle assembly. My question arises to the reasons why a nickel based alloy was used in the fabrication, rather than a 410 martensitic stainless bearing in mind it is possible to PWHT.
Any comments would be appreciated.
Regards





RE: welding of martensitic SS steam turbine nozzles
RE: welding of martensitic SS steam turbine nozzles
RE: welding of martensitic SS steam turbine nozzles
I think that all the procedures that you see call for a 1150F PWHT because that is what the base material specification calls for, and is the most common reccomended temperature. However, we do double tempers with this filler metal when required to be 23HRC or less, with the first temper at 1225-1275F, and the second temper at 1050-1150F. Of course there is nothing wrong with the standard 410 filler metal except it is more crack sensitive. What base metal specification are you working with?
RE: welding of martensitic SS steam turbine nozzles
The material is a Parsons propriety alloy with composition of C 0.07-0.15, Mn 0.3-1, Cr 11.5-13, Mo 0.40-0.80, V 0.10-0.25.
We have tried the 410 filler with a minimum preheat of 250C which we maintained for 2 hours after completion. There was no cracking on cooling and a hardness traverse indicated a maximum of 430 HV before PWHT.
Thanks for you assistance
RE: welding of martensitic SS steam turbine nozzles
I am working with CA6NM alloy with low carbon content (<.03%) I am trying to determine the PWHT required after welding with ER410NiMo. The maximum hardness required is 250 BHN. As you rightly mentioned in one of your replies, the ER410NiMo calls for a PWHT with temperatures not exceeding 1150F. Would you still recommend a double tempering HT? Also would the lower carbon content help gain the 250 BHN max requirement?
The welding to be done on these castings is mainly for repair purposes.
Regards
RE: welding of martensitic SS steam turbine nozzles
RE: welding of martensitic SS steam turbine nozzles
- Decreases the possibility of delta ferrite formation
- Lowers the martensite start (Ms) temperature by about 125F. This is partially offset by the lower carbon specified for the 12Cr-4Ni composition
- The lower critical temperature Ac1 is lowered approximately 345F, below 1200F
The first of these effects, elimination of delta ferrite is beneficial as it results in a homogeneous martensitic structure, free of islands of weaker, less tough delta ferrite hence better fatigue properties. However, the delta ferrite as grain boundary films has the advantage of reducing creep crack propagation rate.
The extent of lowering the Ms temperature is important because it controls the final structure obtained. It is desirable to have an Ms temperature above 400F as the transformation temperature range (Ms to Mf) is a least 300F. From this point of view the 410NiMo is anyway less of a problem than the 422SS base material, anyway.
Lowering the Ac1 temperature can be a problem as it limits the use of high stress-relief temperature.
If the Ac1 temperature is exceeded during the stress relief, austenite is reformed which transforms to untempered martensite upon cooling. The result is an increase in strength and hardness with a consequential loss in ductility.
A solution to the problem is a double PWHT, a higher one followed by a second one that doesn't exceed 1150F (which takes care of the re-transformed austenite). In this particular case is important to determine if the higher PWHT temperature employed for the 422ss is not over-tempering the 410NiMo.
In conclusion, Inco 182 may be a good choice as long as undermatching of the properties of the base material is not a problem. This may be a solution for small repairs.
For a matching filler material, Thermanit MTS4 from Bohler may be the right option. The potential for hydrogen cracking is higher than with Inco 82, obviously.
The hardness of 410NiMo PWHT-ed to 1150F is around 260-320HB.
RE: welding of martensitic SS steam turbine nozzles
I used E410NiMo-16 covered welding electrode to weld 1" thick plates and a double tempering PWHT ( 1250F, AC and 1150F,hold 8h, slow cool @100F/h). Does any one have a clue as to why the hardness was not consistant?
Is the 410NiMo capable of attaining a hardness lower than 250BHN?
Thank you.
RE: welding of martensitic SS steam turbine nozzles
Frequently suppliers will offer to supply purchasers with fabrications which are welded under the processes and controls you outline but with no guarantee on final hardness because of the problems.
RE: welding of martensitic SS steam turbine nozzles
RE: welding of martensitic SS steam turbine nozzles
The heat treatment was like this,
Heat to 1250F, hold 4 hours
Air Cool to room temperature
Heat to 1150F, hold 8 hours
Slow cool @100F/H to 600F, Allow to cool in furnace without openning the furnace door. Final temperature - 250F when removed from furnace.
The filler metal has .03%C
Also, this weld test sample was previously treated once with an exactly same double tempering cycle. I am unable to understand why the base metal and the filler metal react so differently to the heat treatment when the composition is very identical.
The NACE MR0175 says that the hardness permissible is 255BHN when measured using a Brinel Hardness tester which has been empirically determined to be equivalent to 23HRc.
Thank you
RE: welding of martensitic SS steam turbine nozzles
RE: welding of martensitic SS steam turbine nozzles
The Parsons alloy is an X20 derivitive. Bohelr Thyssen manufactures matching SMAW electrodes under trade name MTS 4 and bare wire fillers under MTS 4 Si. PWHT is 1400 F for 4 hrs. Recommended preheat is 450 F.
RE: welding of martensitic SS steam turbine nozzles
Carburize is also correct in the NACE MRO-175 lists 23HRC as the hardness limit, any other hardness scale would be a conversion from HRC. I haven't used Brinell on a weld, but HRC is easier to meet than vickers or HRA/HRB.
RE: welding of martensitic SS steam turbine nozzles
Here's what I did, after welding CA6NM plates with ER410NiMo welding rods, I PWHTed it at
1250F for 4 hours,
Air cooled to room temperature.
tempered at 1125F for 8 hours,
furnace cooled at ~150F/hr to about 300F and then in air to room temperature.
The result, hardness in base metal: 241-250BHN, weld: 255 - 270BHN. The hardness in the weld never came down. Do you have access to cooling curves of this alloy?
Thank you.
RE: welding of martensitic SS steam turbine nozzles
At this point, it looks like you might have at least 3 options left to try.
1. Keep fiddling with the current heat treatment. Trying a longer time at 1250F, or cooling colder between cycles. I have also heard a rumor that waiting 24 hours between cycles helps, but never tried it. I have recently (in the last few weeks) tried a double 1125F, and it did improve the toughness, so it may affect hardness too. I also tried 1150/1125, but it didn't make a tougness difference compared to 1125/1125. This would be after the 1250F of course.
2. Find a filler metal with lower carbon.
3. Try a different heat treatment, such as Nomalization, followed by a combo of 1250/1125.
If you have any of your previous test plates left though, can you experiment with the heat treatment on remnants of those tests instead of welding new ones? It could save a lot of time & $$.