Hi,
I have problems with the qualification of a welding procedure for CA-6NM casting repair.
The test is done on a 1.5 in thick plate and up to now, we are not able to meet the impact requirement (50J @ 32F) in the HAZ and weld.
We are using stick electrode E410NiMo with a heat input under 25 Kj/in, a preheat of 300F and a maximum interpass of 400F. We bake the electrodes at 750F before using it. Prior to welding, the material is in the Q&T(1125F) condition. After welding, we let cool to room temperature followed by a temper at 1075F for 7.5 hours.
I looked on my references and on the net and the way i see it, we are doing everything by the book except for one thing. Actually, the final temper is followed by a slow cooling in oven as required by our customer. I found that the cooling has to be rapid (air cool) and it will be our next move but i want to make sure that this time will the good one.
Do you think this change will have a significant effect on impact test? Can someone explain me what is happening during this cooling. Is there something else we should modify in our procedure? A lot of people are waiting for this procedure and your help would be really appreciated. Regards.
By -
Date 02-09-2006 22:25
When welding CA-6NM, the 410NiMo filler is certainly the filler metal of choice. Normally a preheat is not required, but your 3ooF preheat in this case will not, in my opinion, have a bearing on your impact tests. But, a subcritical PWHT is recommended to improve mechanical properties. Prior to the subcritical PWHT, the weldment should be slowly cooled to room temperature to achieve austenitic/martensitic transformation. Even cooliing to 0 degrees F, if possible, will virtually assure this transformation will take place, since the Mf temperature is believed to be below the freezing point. This transformation to martensite prior to heat treatment should minimize the amount of untempered martensite after heat treatment, and produce a lower hardness. If the weldment is used for corrosion service, the Specification of NACE often require the weldment not to exceed a hardness of 22 Rockwell C to avoid crackiing in service. To meet this particular requirement, a double intercritical and subcritical (1250F and 1140F) tempering treatment is employed. One more thing that I did not understand is the reason for heating the welding rods to 740F before using them.
I have faced the same challenge in the past. Our customer wanted 50ft-lb instead of 50 joules. I can't remember off the top of my head whether it was 0F or 32F though. It would not make a big difference either way because the toughness of CA6NM is not very sensitive to temperature variations in that temperature range.
A couple of comments on what you are doing first:
I do not think 1075F is hot enough of a tempering temperature. Your hold time is good. A lot of people do not go for a long enough hold time to really soften the material. I think you need to temper at 1125F. It is normal practice for CA6NM to PWHT at the tempering temperature, even though that is not reccommended for most other materials. You also do not need to heat the CA6NM that hot for preheat. We generally keep our carbon low, in both the base and filler metal, and have been welding CA6NM for many years with room temp preheat. At little doesn't hurt, but I think you may actually be better off with lower. 200F is very common. Heat input under 25kJ/in is not that critical either. We do not have a problem with 40kJ/in or even higher.
When qualifying our procedure for a similar application, we found one thing that really helped.
Work with a manufacturer of electrodes that has a lot of experience with this specific material in meeting toughness and NACE requirements. We have been getting our critical SMAW and FCAW 410NiMo material from Devasco International in Houston, TX for years. They have a lot of experience making 410NiMo. Other manufacturers have problems meeting the HRC 22 maximum requirement for NACE after a double temper, but Devasco routinely is able to achieve harnesses under 20HRC. Never had a hardness problem. We were actually using FCAW wire for the welding process in this case, but that usually only makes it harder to pass CVNs. A lot of the big manufacturers have been making 410NiMo that runs really nice for years, but it doesn't get the properties.
I don't think the cooling to freezing point hurts anything, as mentioned by Mr. Meadows, but we have never had to do it to get the properties we need. Also, air cooling is preferred, but I think the other things mentioned are more important.
Thanks for your answers,
Cmeadows
Our application do not require to meet NACE maximum hardness requirement. The double temper technique (intercritical & subcritical) would probably gives better impact results but at the same time, I think this treatment would be detrimental to the 110Ksi min. tensile requirement.
We heat the stick electrodes to make sure we do not put hydrogen in the weld. The electrodes came from a sealed container and were kept in oven after opening but we wanted to put all the chances on our side. The 740F is probably overkilled as the electrode datasheet specify 500F.
Groberts
I think that you have a good point concerning the temper temperature. We set the temperature at 1075F because we wanted it to be 50F lower than the previous temper (1125F). However, the way I understand it, the principal is to make sure the final temper is done at a temperature lower than the critical temperature (around 1175F for CA-6NM). I will try to convince my people to do it at 1125F for the next test.
We were told that some electrodes for CA-6NM were better than others but I did not think there could be a significative difference between electrodes of the same AWS classification. I will take good note of your comment.
Thanks for your help gentlemen
Our problem finally ended-up in a strange manner. It was found that the impact requirement was not 50 J but 50 J/cm². This mean 40 J as the specimen is 0.8 cm². We had 41 J so the procedure was approuved as is. I did not have the chance to implement your good advises.
Many thanks anyway. It was a good occasion to learn about this material. It was new for me to have an impact requirement in J/cm². Is it an usual way to specify it?
