Is porosity common in 440C rod stock?

[sromkee1]

I am (swiss screw) machining some .0469 pins out of 1/8" 440C bar stock, then hardening to 58RC. I have found porosity in the last 2 lots of Carpenter material. Is this a common problem with 440C?

I am considering switching to 440A drawn to size (.0469)to eliminate this defect. The yield strength is almost as high, and it is more available as wire than 440C...any thoughts?

 

[unclesyd]

We made any number of parts from 440C hardened to 60 Rc minimum. We used it diver blades, pins and a lot of different points. I've have never been aware of any problem with the steel. We also made a lot micro tone blades from 440C. The of said blades is very critical as any break in the edge would cause the blade to tear out material being sliced. The smallest part we made what we call a plug. 0.009" x 0.015" on a 1 1/2" 1mm diameter points.

I would get in touch with the Carpenter Technical Service with your problem. It will help a lot if you have the Details of the product where you are seeing the porosity.

Addenda:
I have many many knife blades larger dicer blades and have never seen a problem. All my blades were machined ground with edge made on an grinder using a 20" wheel. I inspected the knife under 60 power microscope.

 

[TVP]

I agree with unclesyd, you need to discuss this directly with your supplier(s). In general, "porosity" does not exist in cold finished stainless steel bars in the way that castings have porosity due to entrapped gas and solidification shrinkage. Is it possible that you are observing some other defect, such as carbide banding or sulfide stringers (elongated non-metallic inclusions)?

 

[swall]

Actually, I have seen a centerline leakage in a small drive shaft made from 440C. It was most likely "pipe", i.e. centerline shrinkage carried over from the ingot that did not heal during rolling. The defect was found when a pressure test of the fuel control showed fuel leaking out the center of the exposed end of the shaft. We only found one.

 

[sromkee1]

Thanks swall. I suspect that this is exactly the defect.

This is the second lot where this has been observed, and the first lot was much more pronounced. We were careful to use a completely different batch of material, expecting not to see this again.

I must confess, that I did not believe that this was porosity at first. After several thousand dollars for micro x ray (sorting for good parts), mounting and sectioning (all done by a very prestigious firm), I have seen enough to be convinced.

I also seem to recall my EDM machinist complaining about 440C porosity and preferring to use other alloys. There is nothing like a hole in your finished mold.

 

[mcguire]

If they attempted to continuously cast this material, this would be the expected result as well.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[unclesyd]

Another possibility is what you are seeing is quench cracks. This is a common problem with small diameter parts. These cracks can be short, measured in thousand's, or the whole length of the part. Heat treating a small part require special care in prevent staking, too large a batch if done in batch furnace, and the handling of the parts while hardening.
Are you are cooling?
Any time between hardening and tempering.

 

[sromkee1]

Would quench cracks appear to run along the axis of the draw?

 

[unclesyd]

Yes,
They can either broken or continuous.

 

[mcguire]

You should consider using tool steel produced via powder metallurgy, which Crucible developed. One gets excellent homogeneity and, therefore, freedom from these macro-segregation problems.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[prdave00]

Mcguire,

Is the Crucible technology equivalent to Carpenter's micro-melt technology? Just curious. I like being up on all the latest technologies.

Thanks,
Dave

 

[mcguire]

Dave, they look equivalent to me. It's really good in that one is not limited in alloy design by issues like castability and segregation. Look up Crucible's alloys on their website.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[prdave00]

I read the blurbs from Crucible & Carpenter about thier respective processes and they appear almost identical as you surmised. Links below.

http://www.cartech.com/about.aspx?id=3394

http://www.crucibleservice.com/eselector/general/generalpart3.html

As for how this relates to the OP is that Carpenter offers Micro-Melt® 440C Alloy (UNS S44004). Not sure if this would be of interest to sromkee1.

 

[sromkee1]

Would quench cracks affect only a small portion of the lot?

I checked on the PM options, and at our present volume it is a non-starter, way too expensive. Too much further processing is required.

Wire (like 440A) is commonly available in standard grades, at ~$.03/ft. This would get me to final diameter and surface finish. It also would make a simple tensile test capable of discrimation for these typs of defects, whereas a tensile test on the 1/8" rod is not.

 

[lgearhart]

Interesting that others are also having problems with CarTech lots of 440C. One of the ways we've dealt with this for applications where we can't tolerate pipe is to buy VAR or ESR remelted grades. Since the solidification is from the bottom up, rather than from the sides in, the propensity to form pipe is greatly reduced. (Actually, I should express it stronger than that. I don't think pipe happens at all in VAR or ESR processed steels. I'll be curious if any of the learned bunch on this forum corrects me...)

VAR (AMS 5618) is harder to find than air melt (AMS 5630), but when you do is a lot less expensive than the P/M grades.