1144 Steel Hardness Problem 1

[mtnz]

Hi Guys,

I am currently having some problem in our endothermic gas furnace for oil quench. The parts do not get HRC 50-52 Hardness. They stay at HRC 45 after quench usually.

We have Nitrogen, Endothermic gas and natural gas for the atmosphere of the furnace. Nitrogen kicks in when we put the parts in and stays on until we reach 1400 degrees. Then Natural and endothermic gas get turned on.

1)What is the best combination of gas in the furnace? (We have 100 psi endothermic gas and 5 psi natural gas in the furnace, air gas ratio in the generator is 2.8)
2)What would be the cause/causes for parts not getting hard enough?

I am trying to figure out but I thought I could get some help..

Thanks!

 

[dbooker630]

I suggest checking surface hardness, core hardness, and microstructure on one of your parts. It could be a quenching issue as much as a heating or atmosphere issue.

 

[bobjustbob]

What is the material?
Edit: I really should read the title of the post, I missed that you gave the material. Have you checked the material coming that you have the carbon content that you expect?

What is the purpose of the atmosphere? Are you just trying to prevent decarburization or are you trying to add carbon to the surface to form a case?

Is this a batch process? Are you cooling your furnace chamber below 1400°F between batches?

What temperature are you quenching from?

Have you checked your microstructure?

Have you achieved your target hardness before and are now not getting it or is this a new process?

The endothermic gas makes up the bulk of your atmosphere. The natural is enrichment gas to get you to the carbon potential that you want. In most cases you use a dew point control or oxygen probe to measure the carbon potential that controls the natural gas addition. There are so many different factors that go into an atmosphere like the tightness of your furnace and how often it is opened that you don't usually set a natural gas flow. You set the endo flow to the size of your furnace so you have enough changeover. There are exceptions for some older designs that I am aware of. PSI really doesn't mean a whole lot although I don't think you have 100psi of endo coming out of your generator. Flow is CFH is more illustrative of your process, but that really does not mean much unless you also give the size of your furnace. Even that really does not matter as long as you are keeping a good carbon potential.

Your parts are not getting the hardness either because you don't have enough hardening agent (carbon most likely), poor quench, or you are not fully austenatizing. With the limited information I would guess it is a material problem.

 

[Metaldoctor]

I suspect it may be related to the Quench process. 1144 is basically a carbon steel, and may harden better in a water quench.

 

[mtnz]

Hi bobjustbob,

Thank you for your time and respond. We are trying to harden the parts to 50-52 hrc range usually. The furnace is set to 1575 F degrees, oil temp. is set to 110 but after 2-3 times using it, it goes up to 140 degrees. Do you think that the oil being 140 degrees might affect it? It is an Endo gas furnace and we have nitrogen, endothermic gas and natural gas in the atmosphere.

We do not cool our chamber below 1400 between batches but every time we open the door and load the parts in, the temp goes down to 1200-1300 degrees. that time nitrogen kicks in until 1450 degrees and after that it turns off and Endothermic and Natural gas kicks in. Endothermic gas is set to 100 psi while natural gas is set to 5 psi for the furnace. Gas/air ratio is usually 2.8 for the gas generator.

We have couple problems lately. We sent our dew point analyzer to get it calibrated so we do not have it at the moment. We can run the furnace 2-3 times a day and get the parts hard enough however after that we cannot harden them more than 45 HRC.. Our furnace is 30 years old and it has old school gauges on them to keep track for gas level. One thing I realized is that we changed the oil in the glass tube last week and after couple weeks they became black.. I believe there is some sort of dirt in the gas pipes even if we cleaned them 2 weeks ago. Do you think it might affect the process?

You said we might not have enough carbon in the furnace. What am I supposed to do to increase the carbon level in the furnace? The operator is getting frustrated so that I need to find a way to solve this issue... I would appreciate if you help me understand what is going on..


Thanks!

 

[AidanMc]

I'm with dbooker630, can you section a part and investigate the microstructure? If not, can you find a lab that can. That should help narrow down where the problem is.

 

[bobjustbob]

So the first couple of loads quench out fine and then you start having problems? That seems like a quench issue. I would start charting the oil temperature before and after the quench and see if that correlates with low hardness. How do you cool your quench oil? Are your heat exchangers plugged up and not working? Have you checked your oil for cleanliness?

Have you sectioned a part to look at the microstructure to see if you are decarburizing the parts? Something could be causing your atmosphere to change during the day allowing for decarb on the surface. I assume that you are grinding a bit off of the surface before taking your hardness measurement, have you tried taking more off of the samples that failed to see if it hardens up any?


Can you check chemistry? Are you seeing different results between incoming steel lots? If you can check chemistry I would check the failed pieces against your incoming certs. If you cannot check chemistry and microstructure on-sight, I wouild gather a few good pieces and bad pieces and have them checked for chemistry and get some photos of the microstructure from an outside lab.

Bob

 

[EdStainless]

It could also be a combination of heating quench and degrading atmosphere.
You need to get some metallography done so that you know if there is surface decarb of if this is a bulk issue.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[mtnz]

I tried again today and the parts "again" did not get hardened thru 50-52 HRC. First load came out around 45 and the second load came out around 40... I do not understand why it is happening. We used to have a problem with hard parts couple months ago and now we do have soft parts and we cannot harden them enough...

What can I do to add more carbon to the parts?
Does it do anything with the gas combination?
What is the best temperature for oil (ours is set up 110)?

Thanks!

 

[bobjustbob]

Talk to your quench oil supplier. Different oils are made to work best at different temperatures. Different oils are designed to quench at different speeds to balance quench and distortion. If you have not been checking your oil it may be contaminated. Your supplier should be able to tell you the correct temperature and have it tested.

To understand what is happening you need to start with the basics. Base metal chemistry and microstructure. On base metal chemistry, first you need to check that you are receiving material within your specification. Most important is going to be carbon and manganese. If the base metal chemistry that you are receiving is correct to your specification, you need to check the chemistry of good parts compared to bad parts. Your specification could be too wide and lot to lot variation is the cause. If this is the case you need to tighten your specification. If the chemistry is within spec and you do not see a difference between the parts that meet hardness and the parts that don't then you can eliminate that as a cause. The other track is to look at microstructure. That will tell you if you are damaging your surface by decarburization, if you are not fully austenatizing, if you are having a delayed quench issue, etc.. Without looking at your microstructure it is difficult to diagnose a problem. You should be looking at both base metal chemistry and microstructure at the same time.

Bob

 

[mtnz]

Hi Bob,

Thank you for helping out! We just received our dew point analyzer and the dew point of our endothermic gas generator is 68... Is there a way to decrease it to 35-40? Is it anything to do with air/gas ratio which is 2.8 at the moment?

Thanks!

 

[dbooker630]

mtnz,
Before adjusting the ratio, which by enriching will reduce the dewpoint, you may also want to examine the catalyst which makes the endothermic reaction work. If the catalyst is degraded it will need replacement.

 

[dbooker630]

Also need to check core hardness on your parts. If it is significantly higher than the surface then you are on the right track with the atmosphere.

 

[bobjustbob]

Adjusting your RX generator is not something that can be done through a message board. If you don't have the knowledge in your facility to diagnose the problem it is best to contact the generator manufacturer. Have you established that you are de-carburizing the parts through a microstructure check?

Bob

 

[Maui]

Why are you willing to make changes now when you have not yet identified the source of the problem? It is pointless to alter your process if you don't know what to change or how much to change it. Send your underhardened part out to a competent laboratory and have them analyze it. If there is an obvious problem they should be able to identify it. After you get the results back then you can make the proper changes to your process.

 

[swall]

As Metaldoctor suggested, you are dealing with a grade of steel that is intended for water quenching or brine quenching. You will not meet your target hardness by oil quenching this grade.