How does an OEM select the proper reduction ratio? 2

[awhicker84]

Hi all,

I'm asking this question as a rotating equipment OEM: How do I choose the proper reduction ratio for a given material? Should I (as the OEM) even specify this at all?

The current problem we have is a shaft that we forged where the materials lab tells us that the grain size is larger than expected on a martensitic stainless steel (410 Cond II). We asked for a forging ratio of 3:1. The lab tells us they would have expected 4:1 or 5:1.

Should OEM's stay out of this debate and let the forging vendors take care of the reduction ratio? I've been reading some literature on the topic, but am in no way an expert. We have old forging specs that have been maintained over the years as technology improved. Some things on the spec have remained the same over the years. I'm wondering if this 3:1 ratio was a carry over and no one on our side understood forging ratios well enough to change it.

Thanks for any guidance,

Andy

 

[EdStainless]

I would spec properties and grain size (and dictate where in forging those are measured) and leave the process up to the forge shop.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

The forging (work) ratio should be specified by the OEM working with a reputable forging shop. I will warn you that the interpretation of work reduction ratio is all over the map for shops based on my experience in conducting audits over the years.

 

[awhicker84]

Thanks for your responses.

When going to a forging vendor to start the discussion, do I need give them stresses the part sees? We are aiming for 'high quality' shaft material, but beyond that I'm not sure what is possible. Perhaps, as is most cases, it comes down mainly to cost / benefit. The vendor may be able to do large reductions, but the cost rises steeper than the benefits of a more reduced shaft forging.

I'll get a hold of the forging vendor to discuss. I was trying to learn more before I pick up the phone.

This service does see high fatigue stress and an 4.5 pH acid.

 

[EdStainless]

Well 410 isn't known either for fatigue properties or corrosion resistance.
I would look at the design and stress distributions.
Is it worth paying more to more uniform properties?
Are there specific portions of the part that you are more concerned about?
I would arrange a visit with them, sometimes a picture is worth a lot more than a thousand words.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

The forging shop will not care about the stresses in the part. They are concerned with four important parameters - size or shape from forging, chemical composition and heat treatment to achieve minimum specified mechanical properties, and acceptance criteria for defects (internal and external).

Beyond this information, specific details require an engineering procurement specification issued by you. I would bring your forging specifications which you have used in the past and build off of these.

Forging shops are busy and turn our product forms each day. You may or may not receive the treatment you anticipate, which means you may need to hire outside expertise to finalize a forging procurement specification.

 

[awhicker84]

Thanks again for your responses.

1) We do have a particular area that we are concerned with. The rotor is highly stressed in one location more than others. We have created a low stress concentration radius in this area, but the overall design can't be changed. In other words, this will always be a highly stressed area.

2) This sounds like I need to talk with a metallurgist to act as an in between for the OEM and the forging vendor.

3) We have a failed shaft that showed signs of intergranular corrosion. That's interesting because the literature I've looked at so far shows this is as an austenite problem. You would think our martensitic stainless steel would avoid this problem. However, and I'm still waiting on the official lab results, the lab seems to indicate that there 'wasn't as much martensite' as they normally see in 410. Anyway, after reading everything I have, I feel comfortable that 'true' 410 wouldn't have any intergranular corrosion. We are also look into coating the area that failed. There's a big rabbit hole to go down with this shaft. For reasons out of my control, we are stuck with a 410 forging again. I would like to look into a different material for future shafts, but it is what it is right now.

4) These shaft failures are very expensive (100k in material and machining for new plus downtime for the customer). We need to solve the issue. I think I'm getting closer. High quality is absolutely necessary.

 

[MagBen]

Martensitic SS can have intergranular corrosion issue, if lots of carbide is formed at grain-boundaries, resulting in Cr depletion adjacent to the GB. carbide can happen at hardening when slow cooling and/or at high tempering temperature (ferrite + carbide). In general, if you got a low impact, your corrosion worsens.

Reduction ratio is pretty much dependent on ingot size from melting. It is what it is, you do not have much room to tweak, so unlikely it will be cost related.

Not so often, but I did see once in a while a spec that requests a MIN reduction ratio

 

[awhicker84]

I thought the definition of IC was Chrome Carbide formation at the GB? The galvanization that takes place afterward is the second step in the process.

However, my understanding was that martensite formations have a stronger hold on the carbon atoms than austenite. Regardless, I have tried to find research / experience with IC on martensite over the past few days and I can't really find anything too useful. The other weird thing about this case: we had over an inch of stock (on the diameter) from finish forge to finish machined. However, we found IC at the crack location after the shaft failed. Why would IC form after final machining? Sensitization Temperature for 410 is way higher than this process (~200 F). The only contributing factors I see are: Acyrlic acid (pH = 4.5) and excessive amounts of carbon that builds up in this area. Maybe this excess carbon is promoting chrome carbide growth?

Like I said, this is a rabbit hole that I'm still digging down...

Would a higher reduction ratio result in a shaft less likely to succumb to IC or would it matter? I'm assuming Reduction Ratio has an impact on TS and Hardness.

Sorry if I'm all over the place, I'm juggling multiple complex jobs right now and I haven't always had enough time to focus on this one issue as I would like.

Thanks and cheers,

 

[Tmoose]

I believe (brochure cowboy here) Shot peening properly done can have a strong positive effect on intergranular corrosion, even after you get the metallurgy right.

The brochures have proven right on more mundane topics like simple fatigue resistance.

 

[Maui]

Andy,

The grain size you end up with in the finished bar is dependent on several different factors including

1.) The temperature of the melt prior to teeming the steel into the ingot molds (assuming this is ingot cast material). The melted steel is heated to a temperature somewhere above the liquidus temperature in order to ensure that it will not begin to solidify prior to or during the teeming process. If this was not the case you would be pouring lumpy steel into your ingot molds. The temperature in excess of the liquidus temperature is called the superheat. Usually it runs in the range of about 75F to 180F for various steel alloys. If the melt is too hot going into the molds, it can take longer for the steel to cool down and solidify. And this can result in large grain sizes and pronounced segregation.
2.)The ingot size and geometry used to cast the material. Typically big end up ingot molds for stainless steel are tall and skinny compared to the molds used for tool steels for example.
3.) The temperatures that the material is heated to and the time at temperature during the forging process. Higher temperatures tend to promote grain growth, as do longer soak times.
4.) The extent and manner in which the cross-section of the steel is reduced during forging, i.e. the pass sequence. One of the main purposes of forging is to break up and refine the cast ingot structure. The pass sequence that is used is important because you want the material to move though the entire cross-section during forging. If you don't work the centers properly, then a whole host of different problems can result. And large grain size can be one of them. An overall reduction ratio of at least 4:1 is usually specified.

There are other considerations at play here too, and it would be in your best interest to hire a metallurgist to help you sort it out.

Maui

 

[mrfailure]

Is the question you are really want to have answered regarding specification of the reduction ratio during forging, or are you actually trying to figure out why one or more shafts failed? I was particularly curious about the statement

the lab seems to indicate that there 'wasn't as much martensite' as they normally see in 410.

I would have thought Condition 2 would have been quenched and tempered to achieve mechanical properties and result in a full martensitic structure. Do you think heat treatment might be of concern? What did the lab say regarding the failure mode (for example, did it fail under fatigue)?

 

[EdStainless]

IGC is when the chrome carbides form at the grain boundaries and leave a Cr depleted region along them.
It is this depleted region that is attacked.
When the carbides form at the lower end of the temp range you will not get enough Cr diffusion to restore the local Cr levels and IGA is a real risk.

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P.E. Metallurgy, Plymouth Tube

 

[awhicker84]

We're trying to figure out both: how the single shaft failed and how to define reduction ratio.