Dear Mack:
I don't know if when you said "excessive tool wear" you are comparing it with other steels or with austenitic stainless steels without plasma cutting, please confirme it me. For the moment I can tell you the following:
As you said, the austenitic S.S. are not hardened by heat treatment but they can be hardened by Cold Working because the austenite can be transformed in martensite (wich is very hard) by deformation; for other side, the austenitic S.S. have a low thermal conductivity wich increase the temperature at the tool face affecting its life. These two aspects show that the Austenitic SS has a very lower machinability than other kind of steels. In order to avoid a work hardened surface during the machining you must mantain a positive feed (continuous) or, in some cases, increase the feed and reduce the speed (excessive cutting speeds result in tool wear or tool failure); you should be avoid interrupted cuts or a succession of thin cuts; drills should not be allowed to dwell during the cutting because cause GLAZING on the bottom of the hole, wich makes difficult restart (when you relieve chip congestion, drills must be backed quickly and reinserted at full speed); tools must be kept as sharp as possible in order to obtain the longest tool life. For the temperature aspect, you may use a cutting fluid directed to the cutting area for effective heat removal. Remember, is good to have high feed and low speed.
The difficulties involved is reamming austenitic SS are most often caused by previous operations, for example, if the feed in drilling is too light, the hole can be severely work hardened and can resist the cutting by de reamer.
I hope this information can help you in any way.
Jorge Giraldo
Medellín, Colombia
Upon review of my original question, I see that I can narrow the scope. I pose this revised question. Does the application of heat from a plasma torch change the machining characteristics of austenitic stainless steel.
By D_MacK
Date 06-19-2001 02:01
Thank you Jorge Giraldo.
Are you drilling and reaming holes that still have dross from the plasma cutting? If so then you trying to cut away some extremely hard and abrasive material. I'm not a metallurgist but information I've seen indicates that nitrogen from the plasma cutting process reacts with metals to generate very hard compounds. However, the depth should be limited.
Maybe you can grind some of that away before you begin machining? Even if the problem is from carbide precipitation, grinding may help.
CHGuilford
CORRECTION TO MY LAST POST:
The main mechanism involved in austenitic SS hardened by cold work is not the austenite to martensite transformation, like I said, but the multiplication of dislocations (at atomic level) and the block among them. The former can be presented only in some alloys other than yours. The other things in my post doesn't change.
I'm sorry, My Mistake.
Jorge Giraldo
Medellín, Colombia
There is one main reason that I know about that can cause machining problems in austenitic stainless steels that have been heated as with a thermal cutting or welding process, and that is carbide precipitation. (there are others, but they usually take an extended thermal cycle) I have experienced this problem also when a 1/2" to 3/4" plate was plasma cut, and subarc welded to form a circle. The area around the plasma cut was harder to machine, and the area around the weld was almost impossible.
When austenitic stainless steel is exposed to temperatures within the range of 750F to 1550F, chromium carbides precipitate. This is usually viewed as a corrosion problem, as it steals the chromium from solution and prevents it from forming the protective oxide that gives stainless steel its good corrosion properties. A lesser known problem that this causes is difficulty in machining, as the chromium carbide particles are very hard. It is my belief that it takes much less precipitation to affect machining than it takes to affect corrosion restance, although I have no data to back this up, but it seems that way to me from experience.
There are several ways to avoid or reduce the problem I will list below. I hope they can help you.
1. Take a look at your material. Is the non "L" grade of stainless that you are machining harder than the "L" grade? If so, see if you can switch exclusively to the "L" grade of material. The less carbon the material has, the better from the precipitation point of view.
2. If possible, do any or as much machining before plasma cutting as you can.
3. Have the plasma cutting done on a water table, if it isn't already. This will cause the plate to cool through the detrimental range more quickly, and thus decrease the size of or eliminate the precipitation zone.
4. If none of these work, you could have the pieces cut with a water jet, which would have no heat affected zone at all, and gives superior edge finishes. However, it is slower and more expensive.
Before you go and change everything though, you might do a hardness survey of the area to see if carbides are really your problem. I don't know if a macro hardness test such as brinell, or a micro hardness test such as vickers would suit this application best, but if the hardness is uniform you may not be getting carbides and would need to look elsewhere for your problem. If the readings get harder as you approach an area that has been heated to the precipitation range, then you know what your problem is.
Greg Roberts
Tacoma, WA
