Heat Treat 4340 3

[awol]

I have a bar of 4340 we have heat treated and nitrided after machining. The bar is 60" long, 1-5/8" OD with a 1" ID. We require a minimum tempering temperature of 1000F. The heat treat vendor hangs these bars to reduce distortion but has trouble maintaining a Rc of 38-40. With this configuration, is there a process to consistently obtain 38-40 Rc? We have lowered the Rc requirements to 36-40 Rc but are beginning to experience component failure. Failed bars have the correct nitride hardness but the base material (the 4340 ) checks 36-37 Rc, the minimum we just allowed.
Maybe if the batch count was lower, the bars could have a consistently higher Rc? Maybe a special tempering process?

 

[unclesyd]

Your design is very close if the lost of a couple of points is causing failure.

What is the failure mechanism of the hollow bars?
What is Temperature are you quenching from? Look at quenching from a few degrees higher in temperature. And as you state get the batch size down and watch the quench oil temperature and agitation.
Is the 1000̊F etched in stone, like it being the nitriding temperature? Unless there is a serious reason at temper at 900̊F might work.
When are you nitriding and what is the process?

 

[unclesyd]

My old hardening curves go to 1525°F. This should give the 2 points you say you need for your part.

 

[israelkk]

Is it fatigue or static failure? If it is a fatigue failure you can try to shot peening the bar. If it is a static failure then relying on the 2 RC points is a risky approach. The next batch of material may have minimum properties tensile properties and you may face problems.

You should check the actual failed alloy properties with respect to the minimum spec. If the failed alloys tensile strength is in the minimum spec then the 2 RC can help but, this is a very low safety factor. If the failed alloy is stronger than the minimum spec then you are in trouble.

 

[Carburize]

A number of mills produce various 4340 "Modified" grades with alloys all at the top end of the range and with boosted Mo levels which may give you the increased hardenability you need to temper sufficiently above the nitriding temperature and maintain core hardness. A further increase in alloy content up to 300M is also a possible material option.

 

[TVP]

unclesyd and Carburize provided good information. Austenitizing at as high of a temperature as possible will improve the hardness. Austenitizing at 800 C (1475 F) will produce low hardness, around 340 HB (36-37 HRC), when tempered at 540 C (1000 F). Austenitizing at 845 C (1550 F) and tempering at 540 C (1000 F) should produce higher hardness, more in the range of 360 HB (39 HRC). Also, you should be very precise in how you order this steel. Is it 4340 according to SAE J404? 4340H according to J1268? SAE AMS6415? The latter 2 standards will offer better hardenability than generic 4340 according to J404.

 

[awol]

Great responses from all! Here are the answers of the questions that were asked and also my proposed 'next step'

The bars are centerless ground after HT and then Ion Nitrided to a N-15 scale 85 hardness. The smooth bars are used as a jack. The mechanism grabs the bar and raises and lowers it with weight. We have had great success with this and other sizes as long as the base material is hard enough. Yes, 2 points on the Rc scale is a highly marginal Safety Factor. Extensive field use has proven repeatable and reliable when manufactured correctly. Nitriding temp is 950F, thus the reason for the 1000F minimum temper. The supplier cannot provide exact 4340 spec. They can provide a list of specifications the bars qualify to. 4340 CF Normalized, aircraft-quality. (I will be folllowing this lead further) The 300M is a great suggestion but more expensive. A 79% increase in raw material and probably more machining time. Still, I need a corrosion protection, either hard chrome or again nitriding with the 300M.

I have received additional information to make me wary of the current HT vendor. Low furnace maintenance and virtually no thermocouple replacement. I found another vendor that claims to take 'plain' 4340 and get 39+ Rc consistently with computer-controlled furnaces. Going to give it a try.
If this doesn't work, the 300M is the next logical step. Many thanks to all for the quick and valid responses. -awol