Cracking problems in Heat treating 4340

[umreng]

I have some critical service pivot pins in a heavily loaded piece of plant equipment for which I am having spare pins manufactured.
My original equipment supplier furnished details of the pins and associated parts at time of order placement several years ago. The drawings include the required material, heat treating specification, finish dimensions and plating.
One of the pins cracked during flame hardening, and the heat treater says the specification is at fault. The original equipment supplier says the specification is without fault.
I am looking for some informed opinions from experienced Metallurgists.
The specifications are as follows:
Material: AISI 4340, 12" diameter x 34" long (approx.)
Through harden to a mid-radius yield strength of 148,000 psi yield strength and flame harden to a surface hardness of 45-50 Rockwell C.
Finish diameter is 300mm j6 at 32 micro-inch average roughness after plating.
Plating is to be Nedox FM-5 by Magnaplate.
Constructive comments are requested and will be very welcome.

 

[Carburize]

4340 is very hardenable and if the quench part of the flame hardening cycle is too severe then quench cracks are a distinct possibility. Induction hardening with a polymer quench might be a more controlled surface hardening procedure than flame.

 

[arunmrao]

I suppose that the pins are normalised before hardening. During quenching I have experienced cracking problems with polymer media. I changed to oil quenching and the problem was resolved. Hope this might be helpful.

 

[unclesyd]

I think they only way to go is initially quench the part in a sufficiant quanity of agitated oil. I would use a very minimum of 200 gals of oil with a cooling coil. This also holds for the flame hardening aspect of your heat treatment. I would also immediately double temper the pin after the initial heat treatment.

How is the material purchased, peeled bar, forging,etc?
Does the bar have any NDT checks prior to heat treating?
Can you post your flame hardening procedure?

 

[stanweld]

The specification requirements are acheivable and Unclesyd's recommendations should provide the process controls to meet the specs without cracking.

It would be easier to answer your question if you provided more details regarding the heat treatments performed. For initial heat treating to obtain 148 KSI Y.S in the center of the 12" diameter bar, what were austenitizing temperature, quench media and tempering temperature. What was the "flame hardening" process methods and quench media and its application methods.

 

[Ricko]

I concur that more details are in order. It is important to know "exactly" what the drawing requirements are. I'm particularly interested in what the draw temperature was.
Another point of clarification is in order -- What was the nature of the crack? Was this classical quench cracking, or was the crack a linear line, perhaps indicative of a seam or near surface inclusion band? Did the cracking occur in the vicinity of one or more stress raisers or geometry changes, such as a hole, retaining ring groove or step change in diameter? I'd also like to know what aspect of the specification the heat treater blames. Is he scrambling to avoid blame or does he have an intelligent and legitimate complaint. And perhaps the most important question is, "If he had fault with the drawing, why didn't he raise the issue prior to performing the work?"

 

[umreng]

Thank you all very much for the prompt responses and technical insight & information. The following is in reply to the questions which have been raised by the responders:
1. My original post of July 29 included all of the information on the drawing. Quench medium, draw temperature, etc. were not specified on the drawing (or on the purchase order & attachments).
2. I will attempt to make a photograph of the crack available, as it is most descriptive. This will be addressed in a subsequent post.
3. I will list the detailed steps that were used in the heat treatment processes in a later post. I am in process of gathering this detailed information and do not yet have all of the information.

 

[metman]

I concur with most of what has been said but in particular would like to reiterate Ricko's comment in that you should ask the heat treater exactly what it is they say is at fault with the specicification. Also I would be interested to know how they are monitoring the austenitizing temperature during flame hardening.

Ricko,
The drawing calls out a final surface hardness which dictates the draw temperature depending on as quenched hardness after flame and quench.

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