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4140 steel forging yield strength value problem

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Dear Sirs,

I am working in closed-die steel forging company. Nowadays, we have problem about the production of connection rod

Details are as follows:

Forging Equipment: 6300 mkg Lasco Counterblow hammer
Raw Mateial: SAE 4140
Forging Temperature:~ 950-1000 C

1st question: What must be the hardnees value as forged in air cooled condition?

For the heat treatment,

First, the part was austenized at about ~880 C during 40 minutes and quenched in 90 C oil bath.

Then tempering was performed at about ~ 550 C during 1 hour.

After heat treatment;

Surface Hardness was 34 HRC => This is OK

We have cut a tensile test sample from the body of the part as being perpendicular to the forging direction.The section that we have taken test sample was 20 mm and the diameter of the gage length region after turning was 10 mm.

When we performed tensile testing;

Tensile Strength was 1100 MPa => This is OK( the range of this value must be between 1000 to 1150 MPa)

Yield Strength was 513 MPA=> This is not OK. This value must be greater than 650 MPa

2nd question :Can anyone has a suggestion to provide yield strength greater than 650 MPa without causing deviation of tensile strength from the tolerance range 1000 to 1150 MPa ?

Attached the picture of finished connecting rod and drawing of tensile test specimen are available.

Thanks for your assist in advance.




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My first question: Is your raw material cold finished or hot rolled? A little research shows the end product will have different properties. I'm no expert in metallurgy but I know how to do research.

Do you have access to a book by William E. Bryson called Heat Treatment, Selecton, and Application of Tool Steels?

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Based on the data I have it looks like you should get an air cooled hardness around BH 300 in a 20 mm section. The low yield is difficult to understand. I wonder if you have a retained austenite issue? Have you done any micrographs to look at your grain structure? Does you oil quench have any agitation/circulation?

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Kinda guessing on my part but your soak time at 880 degrees centigrade seems long to me. I would suggest the possibility of enlarged grain size and you might try a shorter soak time and see if your values move in the right direction. Trial and error testing is the way that the guidelines were first developed and can still be refined for a specific process/part.

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It sounds like the part probably isn't completely quenching out to martensite. A properly quenched and tempered alloy steel should have a yield to tensile strength ratio of greater than 90%. It looks like your connecting rod is closer to 50%; I'd say your quench severity needs to be increased to assure that you are getting a fully martensitic microstructure. Like Tim mentioned, maybe an agitated quench or else a colder quench bath. I also think a metallographic section would be the best method to confirm whats going on.

If the problem is on numerous parts or this is the first time making the parts, it may be the process parameters. If you've been making the part for years and are having isolated problems, then it may be worth going through logs and checking temperature records, etc and also it would probably be worth confirming the chemical composition of the stock.

Good Luck

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There are several things that can affect the yeild strength-grain size, martensite/bainite/ferrite content and the presence of austenite formed during tempering. The best way to start your investigation is to look at the microstructure. This will tell you if you have an effective quenching process or not and should also reveal if there is any austenite present, which I think is unlikely. I had a recent experience in my own shop where the yield/tensile ratio was in the 50% range, but the cause of too high a temper which resulted in the formation of some austenite during this step. I would suggest that you consider adding a normalizing step which will help refine the grains and that in turn will result in some increase in the yield strength. I do not think your hold times are excessive for the part size, expecially if you have grain refining elements such as aluminum or vanadium in the steel. It is possible the time in the quench is too short, but this is often a balancing act between obtianing the desired micorstructure and properties with not cracking the part.


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