Holes and threads in tool steel vs. heat treatment

[MichaelP]

I'd like to know the general approach to holes and threads management in parts to be heat hardened.
The steels of interest are: W1, O1, A2, D2, S7, 4140.

What happens to holes, internal and external threads after heat treatment? Do all the above mentioned steels display the same behavior (for example, do holes in all of them become smaller after heat treatment)? Yes, I know that W1 and O1 are much worse in terms or distortion than A2 or D2, but is the dimentional change goes to the same direction?

Do you have any issue with not protecting the threads (other than wrapping parts on foil or using atmospheric control furnace)?

Do you routinely use oversized taps or undersized dies if hardening will be done? How much of a dimentional change can you expect in holes from, let's say, .125 up to about .750"? How do you handle routine press fit situations? I understand that grinding and lapping are options for certain cases, but how often do you really do in routine press fit tasks with hardening?

Thank you for sharing your knowledge and hands on experience.

Mike

 

[toolmaker96]

Built tools from a huge variety of tool steel. Everything from H-13 to D-2, we have never given the threads a second thought. Only time we have had an issue is with case hardening. In which case you may want to use an oversize tap.

 

[RJT]

We heat treat an average of 25 pcs per week of all variety of tool steel (usually vacuum heat treated). The simple answer is there is no gaurantee. Small parts (1sq in) very little change (.0001 -.0002) . The larger the part and the more machining on it, the more distortion. General rule up to .001 growth or shrinkage per inch is possible in A-2, D-2, S-7. I've seen 2 parts 10 inches long made form the same plate heat treated at the same time, one grow .007" one shrink .003" so plan accordingly. We very rarely have any problem with tapped holes in vacuum heat treating. Case hardening is a crap shoot. Stress relieving after rough machining helps. We jig grind or wire EDM all close tolerance holes after heat treating.
www.progtool.net

 

[juergenwt]

Holes in O-1 will get bigger (mostly). A-2 or D-2 no change. A good heat treater can usually make adjustment when O-1 is used. We took advantage of this when building dies - in the good old day's. 1018 case hardend will shrink on I.D's and grow on O.D's.

 

[smalltime]

What RJT said.

We heat treat our own stuff, usually A2, D2 and CPM15V. I'll always drill one size over for the tap drill size, but I never use an oversize tap. We do small stuff as a rule, so we're talking #10 is the biggest we tap. Mostly I'm doing #4s and #2s.

The heat treat house I used when I worked in Toledo Oh. always packed the tapped holes with a "brine" solution. Kind of like Kosher salt mixed with saltpeter and water. The feeling was that this helped the threads keep from gaining austinite and reduced the hardness of them. I'm sure others know WAY more about this than I do.

Maybe they can chime in......

 

[Cole2534]

Re: Brine solution

I'd like hear more about the packing of the holes.

 

[77ironhead]

we heat treat literally hundreds of parts weekly where I currently work, the majority in-house; some in foil wrap, some in atmoshpere-controlled. Tapped holes range from 4-40 to 1/2-13, with the random NPT just to keep it interesting. We tap both with cut and form taps, and typically- if the go/no go was right before heat treat, it'll be right after.

As far as press-fit tolerances, it's about 50-50 whether material is left for finish honing after heat treat- but I can't answer for sure whether they (engineering) decide that based on material or part dimensioning in the production departments. Speaking for myself, I usually leave .0005/.001 material to allow for finish honing after heat treat, I'm a 'better safe than sorry' kind of guy, especially when I've got sometimes 20 or 30 hours (or more) tied up in a part.

 

[gbent]

For case hardening we always noted on the purchase order to the heat treater to pack the holes. This prevents hardening the threads. I believe they used a clay.

 

[cash]

We heat treat A-2, D-2 and S-7 every day in house in our vacuum furnaces.

It all depends on the thread size and the heat treater.

Are you doing this in house or sending the heat treat out?

 

[MichaelP]

Are you doing this in house or sending the heat treat out?

In house (in my garage.I'm a HSMer with a heat treat furnace).