Warping of 4140 after induction hardening 4

[NETDB]

Hello,
We are induction hardening rotors made of SAE 4140 and we are having problems with warping. Most of the rotors are long and have small diameter(for example 70mm diameter and 1000mm long), but it also happens in the ones with smaller D/L ratio.

The material is recieved as bar stock and is machined as recieved (no previous heat treatment after rolling) and after machined in the final dimension is induction hardened. The warping seems to be completely random, it happens in some of the rotors, in others machined from the same bar and same parameters it doesn't.
Sometimes warping is also observed before heat treatment. In these cases the component is straightened in a press and then induction hardened.

I have some ideas to try to minimize warping.
To the ones warping before induction hardening:
-Stress relieving treatment before or after straightening in the press (still haven't decided which would be best).
To the ones warping after/during induction hardening, one of the following:
-Pre heat the part to ~500 °C and then run the induction hardening cicle (to reduce thermal shock);
-Reheat the part after the hardening to "temper"/stress relieve - (wich could be a problem since we need a high hardess of ~55HRC)

Would anyone with more experience in this field have some input on my ideas?
Thanks!

 

[EdStainless]

Have you looked into buying the bar all stress relived?
That is a common approach.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Maui]

Adding a stress relieving operation after rough machining would likely be beneficial. After the finish machining has been performed send them outside for heat treatment. The outside resource should heat them in an atmosphere controlled austenitizing furnace where the carbon content of the furnace can be properly controlled followed by roller-die quenching in oil. They should come out consistently straight and hard.

Maui

 

[NETDB]

EdStainless
We haven't done that for this material, but we have for D6 (due to avoiding microcracks) and the warping still occurs. I believe that the machining processes add too much stress to the material.

Maui
The stress relieving operation after machining will be tested, but we don't want the full quenching (only in the surface, that is why we use induction hardening).

 

[Tmoose]

What is the maximum "warping" that occurs?
What is the maximum that can be allowed?
Are there bearing journals or other features at the ends that are the reference for measuring "warp?"
Are you doing the induction hardening in house?
Is the hardening done in a complete ring around the roll at once, proceeding along the length, like this"

https://www.youtube.com/watch?v=JotsOu2YeOk

I assume the finished rotors are "warped" to look like this -

http://wiki.dtonline.org/images/thumb/d/dc/BentBeam.png/300px-BentBeam.png

 

[NETDB]

Tmoose
The maximum warping allowed depends on the version of the rotor, but usually is around 0,5mm for the longest ones. When they warp it can go up to 1mm.
We do the induction hardening in house, and the coil is the "single turn" type.

http://revistaih.com.br/wp-content/uploads/revista/03653-tempera-de-eixos.jpg

 

[WKTaylor]

NDB1... Huhhhh???? What HT specification are You using????

4140 hardened to 55RHC =300-KSI???? Unreal!!! MIL-H-6875 Lists max HT for 4140 as 200-to-220 KSI when tempered at 725F.... lower tempering temperatures to reach 300-KSI seem impractical.

CAUTION.
Per MMPDS-10 Table 2.3.0.2 Maximum Round Diameters for Low-Alloy Steel Bars (Through Hardening to at Least 90 Percent Martensite at Center) ... Maximum Diameter of Round or Equivalent Round, for HT of 4140 to 200--220-KSI is 1.0" [70-mm Dia = 2.76-in Dia, although this relationship changes for hollow cylinders].

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[dbooker630]

You may want to consider a normalize or quench & temper 'pretreatment' prior to induction hardening. If you are IH'ing the as-rolled bar you could have a wide range of microstructures depending upon the rolling mill practice. It sounds like you don't want a high hardness core; if that is the case then temper at a sufficiently high (900F) to bring the bar down to around 38 HRC. Then you can induction harden (you will get a more uniform as-quenched microstructure) and temper to 55 HRC.

 

[Tmoose]

FWIW here are some residual stress concentrations claimed for a 40 mm bar made of SAE 1045 and induction case hardened to RC 55.

https://www.efatigue.com/training/Chapter_8.pdf

page 22. Fig 8.9 is reportedly from some ASTM source.

 

[NETDB]

Taylor,
That's because it is not tempered, only hardened. This was already defined when I joined the company and seems that never caused problems (because the part doesn't suffer as much mechanical stresses as it suffers with abrasion). The problem is the deformation that happens randomly.

Normalizing before hardening or buying the bar already normalized seem a good ideia as well.

 

[Maui]

Why aren't you using a case carburizing steel for this application instead of 4140?

 

[WKTaylor]

NDB1... OK... so You ARE INDUCTION-CASE-HARDENING the finish-machined rotor shaft... got-it... I've been working with too many mechanics/engineers recently who abuse the terms tempering and hardening... my apologies.

What is the 4140 raw material specification?

What is the original HT process specification?

What is the production [core] temper?

What is the case hardening specification?

Case and nitride surface hardening induces surface compressive stresses. Obviously, warping implies that internal VS surface stress fields are uneven/unbalanced [probably raw stock and finish-machined parts not stress relieved per EdStainless and Maui]; and/or there is some aspect of the case hardening process that is now uneven/irregular [process problems].

Also, a problem that I have encountered, that drives-me nuts...

IF a fab process is stable for 'awhile', then suddenly becomes erratic, it is often a case of contract-re-engineering... changing material/supplier/equipment to a 'lower/lowest bidder'... to save a buck on raw stock. The original material/supplier develops a process that is costly but provides stable and reliable material... then management steps-in and insists on lower [material, equipment/labor] costs 'to be more competitive, etc'. The resulting 'chaos' becomes an engineering problem. Gaaaaaaaaa!

Maui... I agree that 4140 is not the most ideal LAS for case hardening; however it has a long-history in aerospace for this purpose.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]