How do I through harden AISI 4340? 3

[radcad]

I have a tool that is used to pre-set ferrules on tubing. It's made from A2 tool steel and hardened. The problem is that the A2 is brittle and after about 6 to 20 uses it breaks. I was looking into using a differnt material such as 4340 and have it through hardened but I'm not quite sure how to spec out the heat treatment. The tool has threads on one end so it is seeing about 15-20 ft/lbs torque when setting the ferrules. Can I get some advise from the metallurgists out there?

 

[Metalguy]

For starters, how big is this tool, and how hard do you think it needs to be? Any impact/shock loading?

 

[unclesyd]

We made all our tools of this type out of a Chisel Steel, mainly S-7. To save heat treating if we could buy a tool from made from S-7 we would grind it to our size. We used Bearcat by Timken-Latrobe.

If your tool isn’t too big you can purchase preheatreated 4340.

 

[radcad]

Thanks for responding. The tool is 3/4" square x 2" lg. It's turned down on one end to 9/16 thread 1/2" long. It has a .281 dia. blind hole in the turned down end 1/2" deep. No shock loading, just the tightening and loosening of the nut on the end. It needs to be 55-60 HRc.

 

[metengr]

The hardness requirement of 55 to 60 HRc will probably not be met using AISI 4340. In the quench and temper condition, you are looking at a max hardness of 53 HRc. This steel has excellent hardenability, strength and fatigue resistance.

In reviewing your post, I am presuming that the high hardness requirement is to resist plastic deformation during service, in lieu of wear resistance.

If this is correct, the following heat treatment specification for AISI 4340 should probably work;

1. For your application you will need to have the bar in the normalized condition for machining prior to heat treatment.

2. Heat the tool to 1525-1550 deg F and hold for 1 hour at temperature. Make sure this operation is performed in a protective atmosphere to prevent decarburization.

3. Oil quench to about 150 deg F.

4. Immediately temper at 450 deg F for 1 hour.

This temper should provide you with mechanical properties that can resist plastic deformation in your application.

If the hardness requirement must be met for wear or other reasons, you would need to switch to a tool steel like an M2 or other comparable grade. I have included a web site by Carpenter that summarizes some of the various tool steel grades and selection criteria;

http://www.crucibleservice.com/eselector/general/generaltitle.html

 

[Maui]

There could be a number of reasons why the tool that you are using is not performing well. Where is it breaking? Does it break in the same place every time? It's possible that the premature failure is the result of a design problem, and not material related. On the other hand, it could be that the material selected for this application does not possess the combination of hardness and toughness necessary to meet your requirements. A2 is a general purpose tool steel that would typically work well in this type of application, provided the heat treatment that is specified is appropriate and carried out properly. Do you know in detail what heat treatment was that this tool received? For example, do you know what type of heat teating furnace was used, the austenitizing temperature, hold time at temperature, method of quenching, tempering temperature and number of tempers, etc? It is possible that the tool may be in an undertempered condition, which would result in a reduced level of toughness. This in turn would cause the tool to be more susceptible to breakage.

If you are looking for an alternative material which is capable of 55-60 HRC and has better toughness and wear resistance than A2 you may want to consider using Crucible CPM 3V. By the way, the website provided in the post above by metengr is for Crucible, not Carpenter. The address for the data sheets on CPM 3V is

http://www.crucibleservice.com/datash/ds3Vv5b.pdf

If you have any questions regarding how to properly heat treat this grade, post them here and I will respond. Good luck.



Maui

 

[funnelguy]

I think there's a problem in either design or heat treating of this tool. 15-20 ft-lbs of torque on a 9/16 thread should be a cakewalk for ANY tool steel, IMO.

I think the transition from the threads to the square section is too abrupt (and probably full of sharp corners...). This coupled with the .281 hole ending at the same depth as the end of the external threads is asking for trouble. You are going from a 3/4 x 3/4 cross section to an annular section of approx .134 sq in. A-2 is really forgiving, but I'm thinking quench cracks.

Run the threaded section 1/2" beyond the bottom of the .281 hole and radius both the thread undercut and the transition to the shoulder at the 3/4 square section. I'd go at least .020" under the minor diameter for the thread undercut. Making the part out of S-7 would guarantee serviceability at some expense in life, IMO. My 2c...

 

[radcad]

I really do appreciate all the responses. The tool is breaking at the same point every time (at the base of the thread where the undercut is). The heat treatment is as follows. Preheat thoroughly at 1450 deg F. Then raise to 1725-1800 deg F. Quench in still air or dry blast at 125-175 deg F. Then temper imediately. I haven't spoke with our vendor yet to see what kind of oven they use or how long they treat it. But I plan on doing that today.
Thanks

 

[funnelguy]

Face off the end of a grade 8 bolt and drill your .281 hole. You'll get plenty of parts before the threads wear out. Cheap and quick.

You really cannot have 2 major section changes and sharp corners in the same plane for quenching. That needs to be changed.