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Up Topic Welding Industry / Technical Discussions / F6NM Martensitic Stainless Welding/Heat
- - By Richard V. Roch (**) Date 03-14-2006 15:39
Anyone out there has any experience with this type of material. We will use alloy 625 conusumable. We will heat treat to bring hardness requirements to 23 HRC which is what NACE requires. I hear its very difficult to achieve the hardness requirements. Any help would be appreciated.
Thanks in advance
Parent - - By GRoberts (***) Date 03-14-2006 18:16
We have a lot of experience welding the similar casting grade CA6NM. Some of the keys are making sure base metal carbon is below .03%, which is less than the specification allows. Also double temper PWHT. Start by cooling to below 100F, then heat to 1250F, cool to below 100F again, and re-temper at 1125F. Each cycle should be at least 6 hours. We don't have too many problems when we do that.
Parent - - By Richard V. Roch (**) Date 03-14-2006 20:31
I appreciate to info. What pre-heat temperatures do you normally heat up to. We are doing 400 deg. F
Thanks again
Parent - - By GRoberts (***) Date 03-14-2006 21:26
Depends on the code. For the low carbon CA6NM castings, we do not always preheat. 60F is fine for a lot of applications. 200F is specified by some codes, and is more than enough for us.
Parent - - By Richard V. Roch (**) Date 03-14-2006 21:38
Thanks again.
Parent - - By - Date 03-14-2006 22:51
Preheat of the low carbon martensitics are not normally required, but the PWHT temperatures GRoberts suggested are essential for achieving the NACE hardness requirements.

Chuck
Parent - By Richard V. Roch (**) Date 03-15-2006 12:37
Thank you Gentlemen.
Parent - - By Bill M (***) Date 03-15-2006 13:00
To help me understand- is the problem the resulting hardness is too high?
Parent - - By - Date 03-15-2006 14:02
Bill,
The purpose of the PWHT of the martensitic weldments is to temper or anneal the weld metal and HAZ to decrease the hardness, improve toughness, and to relieve any residual stresses incurred during the welding process. In the particular case of the low carbon martensitic materials NACE requires a maximum hardness of 23 Rockwell C for the F-6NM and 22 Rockwell C for the CA-6NM. These NACE requirements are to avoid cracking while in service. So, Bill, without the PWHT process the hardness in the weldment would be quite high with a greater possibility of cracking while in service.

Chuck
Parent - - By Richard V. Roch (**) Date 03-15-2006 14:25
Gentlemen, in the ISO 15156-3:2003 Technical Corrigendum 2, it states the CA6NM or F6NM shall undergo a single or double cycle PWHT. Is it possible to achieve the NACE hardness requirements doing a single cycle at between 580 to 621 C?
Thanks
Parent - - By GRoberts (***) Date 03-15-2006 14:31
Definately not. The higher temperature is needed to soften the martensite enough to get to the lower hardness. The problem after the 1250F though, is that is tecnically and intercritical PWHT, so some of the martensite transforms into austenite. Then when you cool to near room temperature, the austenite re-transforms into untempered martensite. The second temper tempers this new martensite. So you basically end up with some martensite that is very soft due to the 1250F PWHT, and some that is a little harder because it only saw 1125F after its last transformation. If you just do 1125F, the martensite will just be too hard.
Parent - - By Richard V. Roch (**) Date 03-15-2006 14:47
Thanks, we were going to try a sample coupon with the single cycle, but it appears this may be a waste of time and money. I've also read some data that suggests pre-heat at 300 F. Would the pre-heat have any effect one way or the other.
Thanks
Parent - By GRoberts (***) Date 03-16-2006 20:23
I don't think it will affect hardness since this is an air-hardenable material. The only reason for a higher preheat in this family of materials is for more crack sensetive alloys.
Parent - - By - Date 03-16-2006 23:34
I agree with GRoberts. Remember, in martensitic alloys it is the carbon content that determines the preheat temperature. The F-6NM is a low carbon material. I think the 300F preheat you suggested will have virtuall no impact on the final hardness after the double PWHT that is required by NACE.
Parent - By Richard V. Roch (**) Date 04-06-2006 21:47
As I stated in the earlier posting. The NACE contends that you can do either a single cycle or a dual cycle PWHT. Where is this coming from?
Parent - By Bill M (***) Date 03-16-2006 16:40
thx chuck!
Parent - By WELDTECH2112 Date 04-03-2006 13:46
I read that you have a lot of experience of welding thease materials.
Do you also do cladding with Stelitt 6 to A182 F6NM and
what is you experience with this such as preaheat and PWHT.
Requirement API 6A / Nace.
Up Topic Welding Industry / Technical Discussions / F6NM Martensitic Stainless Welding/Heat

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