Improving steel with inhomogenous microstructure and material banding?
Improving steel with inhomogenous microstructure and material banding?
(OP)
This is probably basic stuff for a lot of you guys here but I'm having a hard time finding clarification. I am analyzing a failed ratchet handle made from steel grade AISI 5046. Hardness and mechanical properties meet spec. There are no signs of mechanical damage or any other defects that might have led to failure. The only possible negative factors I've found for this part are that the material structure is relatively inhomogeneous (mixed tempered martensite and bainite) and there is heavy banding/segregation throughout. The wild card is possible hydrogen embrittlement as this part was plated with Ni-Cr and demonstrates a prominent brittle fracture surface (intergranular cracking).
1. My first question is related to the inhomogeneous microstructure. This tool went through normalization (1400 F for 45 minutes with air cool) after hot forging. If normalization was carried out properly, would this homogenize the structure? If no then what would achieve this?
2. Regarding the heavy banding/segregation, where exactly does this come from and what would remove it? If not normalization then could austenitizing take it out?
I would really appreciate any guidance.
1. My first question is related to the inhomogeneous microstructure. This tool went through normalization (1400 F for 45 minutes with air cool) after hot forging. If normalization was carried out properly, would this homogenize the structure? If no then what would achieve this?
2. Regarding the heavy banding/segregation, where exactly does this come from and what would remove it? If not normalization then could austenitizing take it out?
I would really appreciate any guidance.





RE: Improving steel with inhomogenous microstructure and material banding?
It is caused by the fact that as the steel is solidifying, the alloying elements are rejected by the solid into the liquid. The last to solidify contains higher amounts of the alloying elements (such as Manganese and Chromium) than the first material to solidify. Hot working the steel (rolling, forging) tends to physically mix the lean and rich regions, but you still end up with banding. Carbon can diffuse at high tempeartures, so you can homoginize the carbon levels with hormalizing or other high temperature treatments, but the Manganese and Chromium diffuse much more slowly.
rp
RE: Improving steel with inhomogenous microstructure and material banding?
In your second question, you probably meant going to a higher temperature. (Both normalizing and austenitizing transform the steel to austenite; both are above the A1.) 1400F does sound low for normalizing. (We use 1850F on a different alloy.) Yes, heat turns on the diffusion coefficients although it may bring other problems.
You may try the steel producer for advice. Also how and in what environment did the parts fail?
RE: Improving steel with inhomogenous microstructure and material banding?
Is it safe to say then that normalization, if done correctly, cannot remove or greatly reduce the banding?
What about my issue of inhomogeneous microstructure following heat treating (mixed tempered martensite and bainite)? How could the structure have been better homogenized?
What type of negative effects would banding have on the steel properties such as with my failed part, which fractured during torque overload testing. It's also worth noting that this was the only sample that failed from the batch.
RE: Improving steel with inhomogenous microstructure and material banding?
RE: Improving steel with inhomogenous microstructure and material banding?
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RE: Improving steel with inhomogenous microstructure and material banding?
I have one last question on using actual heat treatment to reduce the banding and homogenize the structure. If normalizing wasn't up to par after forging, wouldn't the austenitizing (with proper temperature and soak time) step of heat treating catch those inconsistencies and result in a refined and homogeneous structure? Is it possible?
Sorry for such basic questions but I need to understand this.
RE: Improving steel with inhomogenous microstructure and material banding?
No. A normalization heat treatment provides the basics for obtaining consistent grain size but not for eliminating segrgation because the austenitizing temperature is too low. This is why there is a separate homgenization heat treatment at around 2000 deg F for hours.
RE: Improving steel with inhomogenous microstructure and material banding?
I will provide a very short description of this part's forming cycle as it relates to heating:
1. Normalize at 1400 F for 45 minutes with air cool after hot forging.
2. Hardening/austenitizing at 1575 deg F for 45 minutes.
3. Tempering at 750 deg F for 60 minutes.
Where should this separate homogenization treatment at 2000 deg F be? I've never seen this step carried out on any tools we make.
RE: Improving steel with inhomogenous microstructure and material banding?
For components that I have seen fabricated, a homgenization treatment was done prior to foring or forming. So, for your siutation, prior to forging the billet would be held for an homgenization treatment, temperature dropped and into forging.
RE: Improving steel with inhomogenous microstructure and material banding?
You left out some importing steps
0.2 teeming of liquid steel from ladle to ingot (or continuous casting mold), Temp ~2700F
0.4 Reheating ingot/bloom/billet for rolling; Temp ~2300F
0.8 Reheating billet for forging; Temp ~ 1900-2200F
and, your step 2 needs a cooling method (air cool, oil quench, etc...)
Generally, a homogenization treatment, if used, is between 0.2 and 0.4. Homogenization treatments are much more common for castings.
The 1400 F normalizing temperature is low for this alloy. It should have been around 1625F, but this would not have affected the banding.
My point is that the material was likely exposed to homogenization temperatures during the hot-rolling process, but that did not remove the banding. It is pretty much set in the material and, short of remelting it, you aren't going to get rid of it.
You can reduce the effects if you can quench the material at a fast enough rate so the lean bands transform to martensite rather than bainite, but with this carbon content, you are likely to encounter a cracking problem.
For this particular lot, a higher tempering temperature, settling for a lower hardness, but better toughness, may be the best you can do.
To avoid banding, you will need better control of your steel supply. Steels with high manganese are very susceptible to banding (5046 has a manganese content of 0.75-1.00, which is relatively high). Reducing the teeming temperature and increasing the solidificaiton rate will also help. Higher rolling reduction also helps. Unfortunately, all of these increase the cost of the steel, so it might be better to find a way to live with they banding instead of eliminating it.
rp
RE: Improving steel with inhomogenous microstructure and material banding?