Carburizing problems

[ataurino916]

I need some help in the procedural process method of carburizing 400 series stainless steel material. Material is purchased in the annealed condition. Roughed out, heat treated to condition "T" or "H" as applicable per drawing requirements, machined to drawing details, carburized at shoulder bearing face per drawing (parts manufactured is a stem with acme and unified threads)and then final grinding on the diameter and bearing face. Problems arising are pitting and cracking like indications on the threads and other areas of the part. Our vendors that do this process claim they stop off all areas except the nitrided area so no other surface will be case hardened. Threads are distorting and chipping, not accepting gages after treatment. Would a 40 hour cycle be appropriate?

Threads ACME/Unified are single-pointed method machined.

Any help, in ways to manufacture?

1 part is 410 SST, other is 416 is accordance with QQ-S-763 / SAE-AMS-QQS763.

 

[israelkk]

Why not machine the the thread after nitriding if the threaded area is masked? The H and T condition still allow machining.

 

[ataurino916]

It seems the nitriding process is hardening all over even when other areas are stopped off, thus the material could not be threaded or machined.

 

[mfgenggear]

Just a note your header states Carburizing not Nitriding

These are two diffrent process.

What is the core hardness requirements.
what is the case depth and hardness of the Nitrided areas.

Nitriding 440 stainless will not achieve a very deep case.
minimal stock may be removed post nitride.

Your acme & threads will require thread grinding prior to
nitriding, it must be held closer tighter than the final print requirements, and be advised nitride is hard and will crack if not properly ground with the correct wheel type & grade. Nitrided surfaces need special handling because it's like glass and will chip & spall easily.

if you can provide the case depth requirement. If it is deep enough you may be able to touch up the acme after nitride but it takes special handling.

I believe your supplier did not correctly manufacture your parts.

How ever stating that let me say this, 440 stainless is not the easiest Material to work with controlling distortion “especially if it requires a high core hardness”. If it distorts it is not easy to straighten with out cracking. It must be re-annealed to do so.
If it is Nitrided at the concurrent operation as core hardening. it is a possibility your parts are scrap.

Depending the core hardness will dictate if parts can be machined pre core harden operation or post core harden operation,
I like the parts to be core harden at approximately at 30-40 HRc, Reason for ease of post machining. Then nitride and any finish close tolerances.

Without seeing the actual print dimensions and requirements
It is difficult to advise the exact blue print issues.

 

[israelkk]

mfgenggear

The alloys mentioned by ataurino are 410 or 416 which is free cutting version of 410 and not 440 which is a whole different alloy usually used for ball bearings (440C).It can be quenched and tempered to 58RC miniumim without thE need for nitriding.

416 can be purchased as heat treated to T or H condition from the factory per ASTM A 582 thereby saving the need to heat treat it prior to nitriding.

 

[mfgenggear]

Israelk

Thanks for pointing that out, I missed that,

however nitiding stainless is not the best.
and the machining points are same, except distortion is not a issue as with 440.

Ataurino

My mistake

416 SS should not be a problem.
It should be easy to semi finish machine prior to nitride
and finish grind (if case depth permits)any close tolerances after nitride.
If machining is not to aggressive (This is the key)
It should not distort during nitride(Because it will stress relieve during nitride.)Therefore distort.

Handling still applies as it will chip and spall easily.
if ground after nitride great care in grinding is required as specified before.
Magnaflux/ flo penetrant is required for cracks during processing.

grinding the thread should not be necessary.
If the case is shallow the ACME may require grinding before Nitride.

Hope this Helps

 

[ataurino916]

later on I'll attach the dwg.