Replacing cast iron with 4140

[dgallup]

We buy a helical pinion gear to our customers drawing which we assemble into our product. The customer material spec is for an alloy gray cast iron, 3.1 to 3.6% carbon, 2.1 to 2.8% silicon & 1.15 to 1.50% chromium. There are other elemental requirements as well but they are all lower concentrations.

I don't know the loads but it runs at 300 to 3000 RPM & is splash lubricated by motor oil.

A gear supplier has suggested using 4140 instead of gray cast iron. It would be heat treated to the same hardness. I am not a gear engineer and obviously the customer would have to approve any change. I would just like some opinions on this proposed change. How would the two materials compare in strength, wear, etc. I don't want to take this to the customer if it is obviously a bad idea.

 

[CoryPad]

Based on your description, the gearset must be lightly loaded. Gray iron has a uniform wear characteristic due to the graphite flakes in the microstructure. What is the mating part material and treatment? The proposed steel part could have lower wear, but may introduce higher wear to the mating piece. The steel part should have higher static and fatigue strength. Why are you looking to replace the existing material? Cost, availability, other?

 

[dgallup]

I don't know the mating part material but I suspect it is also cast iron, probably a different grade.

Our current supplier has gone out of business, trying to find a new source.

 

[tbuelna]

dgallup,

You did not give many details of your gears. In general, a higher strength material is beneficial when the pinion gear is a smaller diameter than its mate. This helps to equalize the tooth bending capabilities of the two gears.

The only benefits to using gray iron for gears are cost and wear properties under marginal lubrication conditions. As CoryPad noted, the carbon in gray iron is in the form of graphite flakes, which makes it wear (and machine) very nicely. But those graphite flakes also make gray iron very prone to fracture. So its fatigue properties suck.

However, if the existing gear material was giving adequate performance, I don't see why you would want to change material. High quality gray iron stock is available in continuous cast bar form, from many suppliers. It would be less expensive and would machine much nicer than 4140. So you might want to suggest that material to your vendor.

Hope that helps.
Terry

 

[dgallup]

Thanks for the responses. The supplier that suggested using 4140 said he has to buy a minimum of 3 tons of cast iron which would last several years at our current usage. We are going to look for other potential suppliers as our customer has no interest in qualifying a new material.

 

[EdDanzer]

Your supplier can buy cut pieces of cast iron bar from several different distributers. A $50.00 minimum may be the least they will sell per invoice but you don’t have to buy 3 tons.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[Tmoose]

Yeah, the first supplier it seems pretty likely either has a lack of imagination, lack of experience, or a hunk of 4140 in stock. In any case talking to others would be smart.

Dan T

 

[dgallup]

Thanks guys. We are sticking with cast iron. Found a supplier who has a grade that exactly matches our customers spec.