CA6NM-F6NM

Date 04-18-2008 17:14

I'm looking for some current industry consensus on the PWHT of these alloys. Please see questions below. And please, feel free to open the discussion to martensitic SS's as a whole.
1) What is the most common PWHT regimes currently being used?
2) What sort of hardnesses are you seeing with a single cycle at what temp and time, and a double cycle at what temp and time?
3) Do you find interim cooling temps critical?
4) Anybody doing sub room temp cooling?
5) Are you having trouble making NACE maximums?
6) Do you specify carbon maximums to help you hit NACE?
7) Has alternative conversions from Rockwell to Vickers been considered (for example a conversion that yields 275 Vickers instead of 253 Vickers)or am I already behind times here?
8) Has anybody encountered Ni limitations to facilitate specific PWHT responses?

thanks

By G.S.Crisi (****)

Date 04-18-2008 20:07

Question nº 7.
The equivalence of hardness units (for example Rockwell to Vickers, as you say, or Brinell to Rockwell) is only approximate. There are a quite a few equivalence tables around (I've got 2 or 3 in my personal library), and the good and honest ones always state that they're only approximate and are to be used for information purposes, never for precise calculations.
The best table I know of is the one contained on the SAE Handbook.
Giovanni S. Crisi
Sao Paulo - Brazil

By js55

Date 04-18-2008 20:31

Giovanni,
What generated that particular question was an article I read on F6NM (always a dangerous thing reading articles) wherein they stated that the ASTM E140 conversion isn't accurate for this alloy. The scatter actualy fits the Hays-Patrick conversion data better. And given the historical problems with getting hardness down to NACE maximums (23HRC converts to 253 HV10) that perhaps a re-evaluation of the conversion was in order, making the max now actually a Vickers 275 and not a 253.
Now the thing is, this article is about, 10 years old so I did not know where the industry was at this point with thsi alloy family.
Not to mention my copy of MR0175 is an older version as well.

By G.S.Crisi (****)

Date 04-22-2008 17:19

Just to confirm what I've said above:
one conversion table that I've got at home says that 23 HRC is equivalent to 257 Vickers; another one that I've also got at home says that 23 HRC is equivalent to 253 Vickers.
I must have another table at home but I couldn't find it.
Giovanni S. Crisi

By G.S.Crisi (****)

Date 04-23-2008 23:46

I've just looked at a conversion table existing here at the metallurgical lab of the Engineering School of Mackenzie University, and it says that a HRC hardness of 23 is equivalent to a Vickers hardness of 251.
The table was issued by the Wilson Instrument Division of American Chain and Cable Company (ACCO).
So, you see, three different tables and three different equivalences.
Giovanni S. Crisi

By GRoberts (***)

Date 04-22-2008 00:43

I'll generally be talking about CA6NM, but I don't see why it wouldn't apply to F6NM as well.

1- There are generally two types of PWHT, as you probably already know. CA6NM Class A get a PWHT at 1125+/-F. CA6NM class B gets a PWHT at 1250+/-F, followed by cooling and then re-heating to 1125+/-F. The cooling before the 1250F and 1125F are very important. I haven't seen any research regarding how far to cool, but MIL specs say room temperature, and ASTM says to cool below 200F. Also some people specify a slightly higher temperature than 1250F, such as 1265F.

2- The target of the double temper is obviously to get below 23 HRC. You can do that on a regular basis if you know what you are doing. If not, it can be hard to get. A typical single temper PWHT might yield you results in the 23-28 HRC range.

3- Interim cooling is definately critical, but the actual temps seem to be debated.

4- No personal experiance with this.

5- Not generally. If you use GMAW/GTAW, it seems to be easier. If you use FCAW or SMAW, you need to use a manufacturer that has good knowledge and regular practice in meeting the NACE maximums- that is as important if not more than the C level, because not only is the C level of the consumable important, but the flux formulation as well.

6- You do have to have a lower carbon content max than any of the AWS specs call for. I think most people call for a .03% maximum because it is easier for the filler metal manufacturers to meet, but a well known and highly respected material expert at a major manufacturer is a proponent for .02% max C. (I think he even has dropped by this BB a few times)

7- A alternate conversion is required, but I got out of the CA6NM business before I had to do much vickers, so I didn't have to deal with that specific one. The excellent metallurgist I worked with developed her own data for converting brinell to HRC (for base metal), as the standard conversions are not accurate.

8- I haven't seen any particular Ni limitations. Obviuosly the intent would be to raise the upper and lower critical temperatures to allow for a higher PWHT temperature. The trade-off will of course be reduced toughness.

By js55

Date 04-22-2008 13:18

Most excellent Greg.
Thank you