Al and Nb influence on low alloyed forged steels

[magna76]

Hello folks,

At the moment I am investigating surface defects of large forgings (up to 50 tonnes). The forgings are used in several types of applications. The common denominator is that they are rotor shafts, forged to cylindrical shape and then machined. What influence has Al and Nb in low alloyed steels? Al works as a desoxidation agent when added ni small amounts, but if you add more it refines the austenitic grain size... (AlN). More than that I do not know... Could Al cause the material to crack when it is deformed at hight temperatures? >1100 degrees C?

 

[metengr]

magna76;
What is the specific aluminum content in the alloy you are dealing with because I am familiar with large steam turbine forgings? There can be many causes for surface defects in forgings.

 

[dbooker630]

Nb is often used as a microalloying agent to avoid post-forge heat treat when combined with controlled cooling. I always preferred Nb over V for high temp 2300F forging for its resistance to grain growth.

 

[magna76]

The Al-content is between 0.020 and 0.040 percent. Nb 0.015-0.025 and C about 0.45. Theres also some Mn, 0.70. I believe the problem has something to do with either tension because the material shrinks differently from the surface and invards during forging or that maybe the Al has some influence on the grain boundaries, reducing the intergranular cohesion... The forging reductions are quite large from ingot to final product (>3:1). The cracks are almost always longitudal.

There has also been some cracks already in the ingots, appearing before forging. But most of them appear at forging.

Im often down in the forgeshop to see at which surface temperatures these cracks occur, but always when im down there the product comes out perfectly fine ...

Next week ill be doing some metallografhic examination of material from a rotor with cracks... Some hints what to look for?

Most of our products have specified material compositions which we are not allowed to modify, due to customers demands. So a process change is the only way to solve this problem...

Thanks for your help

 

[CoryPad]

You are looking at the wrong factors - amount of material concentration for Al and Nb < 0.04 mass percent will not have much influence on cracking.

Factors to consider include chemical segregation (although I wouldn't expect Al & Nb to be a problem), residual stresses, actual metal temperature during forming, cooling rate, strain imparted during forging, and many others.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[metengr]

Based on my experience with ASTM Material Specifications for large turbine forgings, I would agree with Corypad.

I believe it is wise to evaluate the cause of the cracks via metallographic examination. I would also perform some SEM-EDS analysis of the crack surfaces to check elemental segregation.

 

[redpicker]

A 3:1 reduction from ingot to final product is not large. In fact, would be considered insufficient by some and the bare minimum for a forged product by most.

I won't mention any specific shops, but there is at least one US open die forge shop that specifically avoids aluminum in the steels they forge, reportedly because it leads to internal cracking. They deoxidize with Silicon and kill with Vanadium. I get concerned about grain size when I have to deal with their products because of the lack of Aluminum (these concerns would be much greater with marginal reductions). I have never experienced a problem (I've always had them normalized prior to Q&T), but I have heard of some. Good microstructure, good hardness, but poor impacts.

While I don't think Columbium (Nb?) is effective as a deoxidizer, it does a good job as a grain refiner. Maybe you could have some success by specifying vanadium killed, Columbium treated. If these elements are specified by your customers, you may not be able do this. If not, maybe you can work with your steel supplier.

Having worked in an Open Die shop, I was surprised to learn of what I said above since I had never experienced cracking issues that could not be attributed to other causes (too cold, too much reduction at one time, unsound centers of ingots, etc...). The nameless shop mentioned above would melt their own steel, while we would only re-heat ingots produced by commercial melt-shops, so maybe there was something in our reheating procedures that reduced the problem.

rp

 

[magna76]

Thanks for all the tips.

A correction. The cracks are not longitudal but transverse.
The products made of this specific steel "cracks" in the surface. If we forge similar products in other materials there is no cracks, so the problem is specific for this material.

More investigation have led me closer to the solution of the problem though... Some time ago we lowered the forging temp and the forgings with these kind of problems seem to have increased after this. The cracks often start at the corners of the forged square, that later is worked to octagonal shapen and then roundforged. All this implies that the surface temperature is to low for this specific material... The investigation continues!

 

[Flesh]

One more point of view, could you be overheating the billet thus causing a burst? This could be ruled out quite easily by looking at a metallographic sample of the crack region.