Machining of 316L VIM-VAR 3

[Tatoz]

We have to make several components from above material using turning and milling machines. Due to the use of parts into aerospace industry 316 Vim-Var has a higher purity grade with no Sulfur Mg and Fe and in order to achieve this higher grade the material is melt twice. VIM stands for Vacuum Induction Melting and Var is for Vacuum Arc Remelt, what we don't know is how good this SS is for machining. Thanks for your support.

 

[TVP]

Austenitic stainless steels that do not have sulfur or other elements added to improve machinability are more difficult to machine than specially formulated machining grades, such as Project 70+® Type 316/316L from Carpenter. There are a number of websites where you can obtain information on machining stainless steel, including the Machining Zone on Carpenter's website:

http://www.cartech.com/machining_zone/index.html

http://www.ssina.com/SSINA2002/SSINAsite/publications/index.htm

http://www.ctemag.com/article/viewArticle.asp?ARTICLE_ID=129

http://www.mmsonline.com/articles/0199scan1.html

http://metalworking.penton.com/default.asp?Section=charts

http://www.production-machining.com/Scripts/Search/PMPastArticles.idq

 

[TEV]

Tatoz:

What do you mean by no sulfur Mg and Fe? Do you mean Sulfur, Manganese and Selenium?

We machine low sulfur, low inclusion VIM/VAR stainless steel every day. The Sulfur in 316 is specified at a maximum of 0.030%, and the producers add Sulfur back in for product intended for fabrication by machining, as Sulfur significantly improves machinability. The sulfur exists in the stainless as precipitates with Manganese, and these precipitates have a lubrication and chip-breaking effect during machining. But the precipitates also are sites for initiation of corrosion on end grains and have negative effects.

When you specify a low sulfur stainless, Sulfur in the range about 0.001%, you will find that you need to reduce the machining speeds approximately 30%. Do not reduce your feed rate too much, however, as the stainless will develop a work-hardened surface which is very difficult to machine. Use a good quality machining fluid, and plenty of it. Preferably carbide tooling, but if you use tool steel, use a grade with good hot hardness characteristics. The chips will tend to be stringy and will require periodic removal to avoid damaging the part.

Be aware that Sulfur also has a tremendous effect on welding, also. As you reduce the sulfur to very low levels, the depth to width ratio of your weld decreases markedly and more heat input is required.

If I can help further, let me know.

 

[Tatoz]

Thanks TVP and TEV four your helpful replies.
The chemical spec for this material is as follows:

Sulfur content: 0.001-0.004%
Aluminum: 0.02 max
Calcium: 0.02 max
Copper: 0.25 max
Niobium: 0.05 max
Selenium: 0.02 max
Titanium: 0.02 max

We're quoting several parts made out of this material and it's important for us to know how we can trust toolwear on a turning machine, due to the large number of parts required we would need to run the machine 24h a day.
Tolerances on diameters and lenghts are really narrow (in the range of 0.001&quot and also roughness is critical (ra 16). Carbide tooling is a must both on inserts and drills.

TEV, I've seen you machine this material, what's your advise for cutting speed and toolwear? We know machining of regular 316, 304, 303, 420, 17-4ph, Stavax, how would compare VIM-VAR with one of these.

I really appreciate your support. Thanks.

 

[mcguire]

Guys
Are you aware that Ugine, the big French stainless producer, has developed a free-machining stainless that employs a non-sulfide inclusion to enhance machinability. This eliminates the need for high sulfur levels which are not a good thing.
Why would you ever pay the exorbitant premium for VIM-VAR when you're glomming it up with sulfur anyway. AOD stainless is tremendously clean if you let it be. I think a very good salesman has visited you.

 

[TEV]

Tatoz:

You have seen me machine this material? Where?

I recognize the spec. It is the SEMI F20 specification for high purity gas delivery systems for semiconductor fabs. It is obsolete, and a new version in being balloted right now. The new version will not specify a production process (i.e. will not require VIM-VAR), but will have inclusion ratings the same as the old spec. The Sulfur content will be 0.003 to 0.010% (this will drive McGuire up the wall!), IF the spec passes balloting. The Japanese (and my company) would rather the spec have no minimum Sulfur level. The new sulfur level is for ease of welding to the tubing that is used in the industry, which is currently specified at 0.005 to 0.012% sulfur. If the sulfur mismatch between the parts being welded together is too large (> 0.007 percentage points), the weld becomes asymmetric and full penetration is not achieved to the root. There are some companies in this field who have publicly stated that they cannot weld 316L that has less than 0.005% sulfur, but we do it every day.

The sulfur level is a compromise between weldability and inclusion levels. The inclusion ratings are quite stringent, which rules out the Ugine material recommended by McGuire, and will generally require a secondary refining process such as VAR to achieve. The secondary process is not specified in the new spec, however, only the inclusion requirements. The producer needs to make a decision how to best achieve the spec with his technology and processes.

I have several recommendations for you:

1. Confirm with your customer that he really wants this level of sulfur. Most companies using the old spec requested an exception to the sulfur level, requesting something similar to the new spec being balloted. Is he aware of the welding issue?
2. I cannot tell you what your tool life will be. You will have to generate some experience on this. As I mentioned, speeds will be about 30% lower, feeds maybe a bit less, compared to standard machining quality 316.
3. If you are aware of us, is there any interest in our working with you on this?

 

[mcguire]

TEV You are right. The sulfur does drive me nuts, but there is a new treatment for austenitic stainless that leaves the sulfur in, but removes the corrosion danger.
The work will be published later this year and there will be product in the market for HX, semiconductor, and biotech uses.