Carburizing Hot Work Steels

[mewhg]

I have been working with a heat treater that does nitriding. In our discussions he said that stainless steels take his nitriding process very well. I have an application where the part I am considering nitriding would be subject to impact, sometimes in very cold outside environments and I have not found a martinsitic stainless to my liking with good impact properties (the heat treater says the stainless should have nickel to take the nitriding well).

Well, this is a round about way to asking my question. What do you think might happen if I took a hot work steel such as H-13 and removed most of the carbon to make it like a carburizing alloy? Then carburized the steel to produce a hard case and then applied a nitriding process on top of the hard case. This way the case would not soften since its tempering temperature would be above the nitriding temperature, there would be a ductile core, a case about 50 RC on top of the core that would provide residual compressive stress and the case would have its own "case" that would be slick and extremly hard.

Any thoughts or comments????

thanks,
Bill

 

[metengr]

In lieu of going this complicated route, why don't you consider using a quenched and tempered AISI Type 4340 alloy steel, and nitride it. You can achieve the best of both worlds - increased surface hardness thru nitriding, with a quenched and tempered martensite core that can exhibit good toughness.

 

[mewhg]

Metengr,

Thanks for the reply. I might try and go that route also. These are investment castings that I can prototype any steel I want too. Does 4340 take nitriding well?

 

[metengr]

Yes it does.
See FAQ330-1075

 

[TVP]

mewhg,

FYI, there are carburizing alloys that are meant for higher temperature applications, meaning they have improved temper resistance. Carbonitriding is also a possibility if you want very high surface hardness. Timken Latrobe and Allvac both offer high performance carburizing grades:

http://www.timken.com/products/specialtysteel/engineering/tech_info/bearingsteel.asp

http://www.allvac.com/allvac/pages/Steel/VascoX2M.htm

 

[mewhg]

TVP,
I have looked at the Timken website before and will again.....the only trouble is I need to use the alloy by investmet casting.

thanks
Bill

 

[TVP]

Bill,

Sorry about that-- I noticed the investment casting but then forgot about it. An alloy steel like 4340 would probably be best.

 

[mewhg]

I looked at 4340 a bit and the tempering temperature is below the 1100F nitriding temperature. If I heat treated the 4340 to lets say 55RC and then nitrided it the base steel would lose hardness, correct?

If I want a specific core hardness and then a nitrided case I will need a steel with a tempering temperature above 1100F.

Do I have this right?
thanks
Bill

 

[metengr]

You are not clear on all your stated objectives with your investment cast part. If you nitrided a quenched and tempered 4340 at 55 HRc, yes, you will soften or temper the 4340 core material during nitriding because the original tempering temperature is below the nitriding temperature.

I am going on limited information here by saying that if you want to have good impact properties during cold weather you should have a core hardness of around 25-30 HRc. This hardness range will balance good toughness and suitable strength. The tempering temperature to achieve a 25-30 HRc core hardness range will be above the nitriding temperature so you should be ok to nitride after your initial quench and temper heat treatment.